Heusler nanoparticles for spintronics and ferromagnetic shape memory alloysChanghai Wang, Judith Meyer, Niclas Teichert, Alexander Auge, Elisabeth Rausch, Benjamin Balke, Andreas

Hütten, Gerhard H. Fecher, and Claudia Felser Citation: Journal of Vacuum Science & Technology B 32, 020802 (2014); doi: 10.1116/1.4866418 View online: http://dx.doi.org/10.1116/1.4866418 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/32/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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REVIEW ARTICLE

Heusler nanoparticles for spintronics and ferromagnetic shapememory alloys

Changhai WangMax Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany

Judith Meyer, Niclas Teichert, and Alexander AugeDepartment of Physics, Bielefeld University, D-33501 Bielefeld, Germany

Elisabeth Rausch and Benjamin BalkeInstitute for Inorganic and Analytical Chemistry, Johannes Gutenberg University, D-55099 Mainz, Germany

Andreas H€uttenDepartment of Physics, Bielefeld University, D-33501 Bielefeld, Germany

Gerhard H. FecherMax Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany

Claudia Felsera)

Institute for Inorganic and Analytical Chemistry, Johannes Gutenberg University, D-55099 Mainz, Germanyand Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany

(Received 31 August 2013; accepted 3 February 2014; published 27 February 2014)

Heusler nanoparticles emerge as a new class of multifunctional materials. In this critical review,

the latest progress in studies on Heusler nanoparticles is summarized. The authors discuss their

structural and physical properties interesting for research fields such as spintronics and

ferromagnetic shape memory alloys. As a young research field, the majority of studies on Heusler

nanoparticles focus on their synthesis, structure, and magnetic characterizations. Important issues

such as size dependent structure, phase transition, magnetic, and spin-related properties are still

open. Further investigations are needed to verify the technical significance of Heusler nanoparticles

for practical applications such as data storage, magnetic sensors, and microactuators. VC 2014American Vacuum Society. [http://dx.doi.org/10.1116/1.4866418]

I. INTRODUCTION

The Heusler compounds, discovered by German engineer

Friz Heusler in 1903,1 refer to ternary intermetallic com-

pounds having the formula X2YZ and a L21 crystal structure.

Usually, X and Y are transition metals and Z is a main group

element. After their discovery for more than one century, the

Heusler compounds are recognized as promising materials

that might provide solutions for future technological

improvements. The robustness of Heusler compounds lies at

the fact that their structural and physical properties can be

tuned in an exotic way, rendering features appealing for a

wide span of research fields including spintronics,2–5 ferro-

magnetic shape memory alloys,6,7 thermoelectrics,8,9 and

topological insulators.10–12

For practical applications, Heusler compounds are domi-

nantly used in the form of thin films. Thin films might be

viewed as two-dimensional materials with the thickness

reduced to the nanometer range. Heusler thin films serve

as building blocks for many spintronic and microelectronic

devices.13–20 Further confining the length scale in three-

dimension results in Heusler nanoparticles. These new low

dimensional Heusler compounds are expected to exhibit

size dependent structural and physical properties that are of

paramount importance not only for basic scientific interests

but also for potential applications. Compared to magnetic

nanoparticles that have been intensively investigated,21–23

the research on ferromagnetic Heusler nanoparticles is an

emergent field. Recently, a few investigations have been dedi-

cated to Heusler nanoparticles. In this short topical review,

we focus on this young research field by showcasing the

scientific significance and technical merits of Heusler

nanoparticles.

The context of the critical review is organized as follows.

In Sec. I, we discuss the structure, phase stability, ferromag-

netism, and magnetic shape memory effect of Heusler nano-

particles with emphases on the size and surface effects. In

Sec. II, theoretical and experimental results on Heusler nano-

particles are discussed in the framework of granular giant

magnetoresistance (g-GMR). Section III deals with the strat-

egies and synthetic approaches of Heusler nanoparticles

developed to date. Studies of Heusler nanophase require

demanding structural and magnetic characterizations. The

analytical tools developed to thoroughly characterize the

long- and short-order structure and magnetic properties of

Heusler nanoparticles are discussed in Sec. IV. Finally, wea)Electronic mail: felser@cpfs.mpg.de

020802-1 J. Vac. Sci. Technol. B 32(2), Mar/Apr 2014 2166-2746/2014/32(2)/020802/13/$30.00 VC 2014 American Vacuum Society 020802-1

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present, in our view, a collection of challenges and plausible

approaches for future studies.

A. Spintronics

The discovery of the GMR effect24,25 gives birth to a new

research field, namely "spintronics." The core concept of

spintronics is to utilize the spin of electrons in addition to

the charge. The advantages of spintronic devices include

nonvolatility, increased data processing speed, large storage

density, and lower energy consumption. To enhance their

competition versus semiconducting devices, new materials

with high spin polarization are essential. The concept of half

metallic ferromagnets (HMF) was first proposed for the

Heusler compound NiMnSb.26 The HMFs exhibit 100% spin

polarization at the Fermi energy since there is a finite density

of states at the Fermi level in one spin direction and a gap in

the other spin direction. The half metallic ferromagnetism

found in many Heusler compounds enables them promising

materials for spintronic devices. A strong technological link

between Heusler compounds and spintronics has been

forged.20 Spintronic devices composed of Heusler thin films

have been populated as magnetic hard disks, magnetic ran-

dom access memories, and magnetic sensors.

Investigations on Heusler nanoparticles have been cata-

lyzed by the importance of size and interfaces in affecting

the structure and properties of new functional devices. Even

though many Heusler compounds have been theoretically

predicted 100% spin polarized at Fermi level, much

decreased values were obtained.27–29 This phenomenon

could be generally related to the structural disorder30,31 and

the surface effects.32 Heusler nanoparticles exhibit all the

above features and are excellent candidates for studying the

crucial factors for spintronic devices. From the viewpoint of

applied physics, many important issues are to be addressed.

How size affects the structure (long range and short range

order) and magnetic properties of Heusler nanoparticles?

What is the critical size of superparamagnetic Heusler nano-

particles? Whether the HMF behavior vanishes for Heusler

nanoparticles? Understanding the behavior of Heusler nano-

particles paves the way to technical innovations in spintronic

devices to meet the increasing requirements of miniature,

high performance, and energy-saving.

Theoretical investigations reveal that Heusler thin films

differ from their bulk counterparts in terms of phase stabil-

ity, grain size, and surface effects. For example, bulk

Co2CrGa exhibits a L21 ordered structure, whereas lower

structure ordering is frequently observed in thin film sam-

ples.33,34 Recently, the effect of grain size of Co2FeSi

Heusler layers on the magnetic properties of IrMn/Co2FeSi

exchange-biased films was investigated. It was found that a

large exchange bias requires a large grain size (15–20 nm) of

Co2FeSi as well as a matched size of IrMn.35 It was con-

cluded by ab initio calculations that the spin polarization

depends strongly on the surface termination and the surface

states appearing in the gap might lead to a loss of the spin

polarization contrary to the bulk related calculations.32 For

example, the CrAl-terminated Co2CrAl surface preserves the

half-metallicity at the Fermi level with a surface spin polar-

ization value �84%.32 The influences of terminating surfaces

states on the phase stability and electronic and magnetic

properties of Co2MnSi thin films were also investigated using

density functional theory calculations.36 We are aware of

only two papers where the structure, electronic, and magnetic

properties of Heusler nanoclusters have been examined theo-

retically.37,38 The Co0.52Mn0.22Ga0.25 clusters exhibit a bulk-

like structure, higher magnetic moment than the bulk value.

The authors attribute the enhanced magnetic moment to the

“cluster size effect,” i.e., the amount of uncompensated spins

is proportional to the surface-to-volume ratio, which is inver-

sely proportional to the cluster size. Furthermore, the p-d

hybridization in Heusler compounds normally reduces the

magnetic moment. On the surface layers, the degree of p-d

hybridization is smaller than in the bulk, and therefore, the

surface magnetic moment becomes larger. As shown in Fig.

1, the electronic structure of the above Heusler clusters exhib-

its extraordinary changes including: (1) from 100% majority

spin polarized to 80% minority spin polarized; (2) the gap in

the bulk minority band is completely filled by the surface

states; and (3) the majority spin states shifted 2 eV to lower

energy compared to the bulk. For Co-Mn-Ge Heusler nano-

clusters, the half-metallicity is found completely destroyed

due to the filling of the energy gap in the minority spin chan-

nel by surface states.38 Similarly, Heusler nanoparticles are

considered to exhibit different magnetic properties and

reduced spin polarization compared to those in the bulk since

a larger fraction of atoms are distributed at the free surfaces,

grain boundaries, and interfaces with decreasing particle size.

One of the most intriguing issues in Heusler nanoparticles

is whether the structure and property of Heusler nanopar-

ticles exhibit size-dependent patterns and whether the high

spin polarization or the HMF feature can be preserved.

Unfortunately, these issues have been less investigated both

theoretically and experimentally. So far, experimental work

on Heusler nanoparticles has mostly focused on their synthe-

sis, structure, and the magnetic properties. Further theoreti-

cal investigations on Heusler nanoparticles are required to

understand Heusler nanocrystals and also shed lights along

the experimental approach.

B. Ferromagnetic shape memory compounds

Many Heusler compounds are ferromagnetic and exhibit

interesting magnetic shape memory properties. The funda-

mental reason for the shape memory phenomenon lies at

phase transitions between the high temperature austenite

phase and the low temperature martensite phase. Martensitic

transformations are first-order phase transformations occur-

ring by cooperative movements of the atoms in the form of

nucleation and growth.39,40 Martensitic phase transformation

can be induced and therefore can be manipulated by thermal

and external field stimuli (such as stresses and magnetic

fields). One of the most investigated shape-memory Heusler

compounds is the material system of Ni-Mn-Z (e.g., Z¼Ga,

Sn, In, and Sb).41–44 For example, a very large magnetic-

field-induced-strain (MFIS) (up to 10%) has been observed

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in Ni-Mn-Ga single crystals.42 The large MFIS is attributed

to a field-induced reorientation of the martensitic

variants.42,45

With an increasing tendency of downsizing in shape-

memory devices (with the aims to obtain optimized proper-

ties of the shape-memory effect and high mechanical

strength),46–48 knowledge on the size effect on the marten-

sitic transformation is crucial. Size effects (in the microme-

ter range) on MFIS of Ni-Mn-Ga have been reviewed

recently.49 To obtain large MFIS in ferromagnetic Heusler

compounds, a large magnetic anisotropy constant and a low

critical stress associated with variant reorientation or twin-

ning are required. Furthermore, the martensite phase should

be ferromagnetic with an appropriately modulated structure

(i.e., 10M tetragonal or 14M orthorhombic). When the

length scale of Heusler compounds decreases to the nanome-

ter range, however, extrinsic parameters such as crystal size

or sample dimension come into play. For example, the sig-

nificantly small MFIS found in polycrystalline Ni-Mn-Ga

has been attributed to the presence of grain boundaries that

hinder the motion of twin boundaries. It was also found that,

when the characteristic sample length (particle diameter,

wire width, film thickness, etc.) is comparable to the grain

size, the grains become surrounded by free space. This

enhances twinning between grains and results in higher

MFIS. In addition, decreasing sample size might increase the

magnetic anisotropy due to a hindrance of magnetization

reorientation, which also favors a large value of MFIS.50 On

the other hand, small size might hinder the formation of dis-

location and twinning disconnections, which is a prerequisite

for martensite nucleation. Therefore, size effects might ei-

ther prevent or promote MFIS at the micrometer scale. Even

though direct experimental evidence is lack, intuitively the

grain-sample size matching strategy derived from

micrometer-sized Heusler compounds might be applicable in

the nanometer range as well.

Whether the shape-memory properties of martensitic

materials can be preserved at the nanoscale has been the

focus of ongoing studies. Figure 2 shows a simulated result

on the correlation of temperature and critical size in shape

memory nanocrystals.51 It appears that a smaller size and a

FIG. 1. (Color online) Construction of 169-atom Co88Mn38Ga43 clusters. The spin-resolved electronic structure near the Fermi levels of bulk Co2MnGa and

Co88Mn38Ga43Heusler clusters. Adapted with permission from Zayak et al., Phys. Rev. B 77, 212401 (2008). Copyright 2008, The American Physical

Society.

FIG. 2. (Color online) Simulated correlation of critical size and temperature

in shape memory nanocrystals. Reprinted with permission from Comput.

Mater. Sci. Dhote et al., 63, 105 (2012). Copyright 2012, Elsevier B. V.

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decrease in temperature favor to suppress the martensitic

transformation. An experimentally derived suppression limit

size is around 50 nm for Ni-Ti shape memory alloys.52 The

motivation and current status of theoretical and experimental

research of nanocrystalline martensitic materials have been

elegantly reviewed.53 The size effect in the martensitic trans-

formation of nanocrystalline martensitic materials has been

theoretically explored in two approaches. In the thermody-

namic framework, various factors that affect the phase equi-

librium are the major concerns. For free nanocrystals, the

total free energy contains additional contributions from the

surfaces. The smaller the crystal size, the more atoms are

affected by the surfaces, leading to a size dependence of the

difference in the total free energy between austenite and

martensite. When the surface energy of austenite is lower

than that of martensite, it causes a critical temperature,

where the Gibbs energies of austenite and martensite in their

stress-free states are equal, to decrease with crystal size.53 In

a similar way, surface stresses also result in a stabilization of

the austenite with decreasing crystal size. With decreasing

crystal size, the transformation strain energy can increas-

ingly contribute to the free energy, hindering martensite

formation.53

In parallel, the kinetic approach focuses on the factors

influencing the heterogeneous nucleation and growth of mar-

tensite. For free nanoparticles, strain energy calculations

indicate that the barrier for the homogeneous nucleation of

martensite increase with decreasing crystal size.43 Under a

given driving force, an austenitic crystal must contain at

least one nucleation site to facilitate its transformation to

martensite. According to the statistical model of nucleation,

the probability of finding such particles exponentially

decreases with crystal size.55 Thus, the nanocrystals smaller

than a critical size might have a negligible probability of

nucleation and remain untransformed even for large driving

forces. The influence of the grain size on the nucleation bar-

rier and critical nucleation size was determined by consider-

ation of grain boundary energy and interface boundary

energy.54 Specifically, if the grain size is below 100 nm, a

rise in the nucleation barrier and a larger critical nucleus was

found.

In the theoretical studies of the shape-memory properties,

nanocrystalline martensitic materials are conveniently

treated as free nanocrystals. In practical applications, how-

ever, martensitic materials normally contain or are integrated

to a large variety of interfaces, which might play a critical

role in affecting the martensitic phase transformation. For

example, it was observed that the martensitic transition of

Ni2MnSn thin films is suppressed in the vicinity of the inter-

faces to MgO substrates.56 Here, interfaces refer those

between the austenitic and the martensitic phases, the

self-organized elastic domains of the martensite, as well as

geometrical constraints of the surrounding matrix.53 With

decreasing crystal size, interface energy increasingly con-

tributes to the total free energy of the martensite. Therefore,

in nanocrystalline materials, the variant structure exhibit fea-

tures that are absent in coarse grains. Nanocrystals normally

possess a finely twinned martensitic substructure.53 A single

variant of martensite might become energetically favorable

as compared to a twinned laminate when the crystal size is

less than some critical value.57 The critical size depends on

the intrinsic features of the materials. In coarse-grained

materials, the requirement of deformation compatibility nor-

mally results in the formation of habit planes. In contrast,

habit planes are not indispensible for nanocrystals since the

twin-related variants of the martensite can elastically accom-

modate their lattice deformation. Twin boundaries of nano-

structures become finer with decreasing crystal size. For

example, in nanostructured NiTi, twins of the martensite can

be as narrow as 1 nm.58

Compared to the well-studied nanocrystalline shape

memory alloys such as Ni-Ti,58–60 Fe-Ni,61 Cu-Al-Ni,62 and

Ni-Ti-Cu,63 only a few experimental and theoretical studies

on the size effect in shape memory Heusler nanocrystals

have been reported.56,64 For example, a stable (up to 4 K)

three-layered premartensitic phase was identified in ball-

milled Ni-Mn-Ga nanoparticles, and its presence was related

to a decrease of martensite transformation temperature.64 In

another study, a thickness dependent martensitic transforma-

tion was detected in the sputtered Ni-Mn-Sn films.56 The

martensitic transformation temperature was found to

decrease together with a larger temperature range with

decreasing film thickness. The transformation is still possible

with a thickness �10 nm.

The martensitic transformations of Heusler clusters have

been investigated by a theoretical approach.65 As shown in

Fig. 3, for Ni2MnGa, the stability of Ni2MnGa Heusler nano-

clusters increases with an increase of the number of atoms.

For smaller Ni-Mn-Ga clusters, the tetragonal transformation

seems being prohibited due to surface effect. The tetragonal

phase is less stable than the cubic one due to the presence of

surfaces. For tetragonal nanoclusters, the surface tensions act

against this configuration and increase the total energy of the

system. For larger nanoclusters, the surface effect becomes

less important, and the tetragonal structure becomes more sta-

ble. It is also proposed that there might be a critical size of the

nanoclusters, at which the two structures are equally stable.

At this size, the cubic-tetragonal transformation can be trig-

gered externally easily.65 It is noteworthy to comment that the

size and surface effects in Heusler clusters and nanoparticles

are closely correlated and cannot be easily separated.

Clearly the effects of crystal size on the martensitic phase

transformation of Heusler shape-memory nanocrystals are to

be further investigated from both the thermodynamic and ki-

netic approaches. It is of extreme importance to examine the

martensitic transformation suppression size of Heusler nano-

crystals at the operating conditions for practical applications.

II. GRANULAR GIANT MAGNETORESISTENCE

The GMR effect is not unique to multilayer structure and

was also observed in granular systems. The g-GMR effect

was discovered in granular systems with magnetic particles

embedded in a metallic matrix.66,67 Recently, a new g-GMR

approach using magnetic nanoparticles dispersed in conduc-

tive water-based gel matrices has been proposed to achieve

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higher sensor performance as compared to conventional g-

GMR devices metallic matrices.68,69

Incorporation of Heusler nanoparticles in the above gran-

ular systems could bring further progress as one would profit

from the qualities of Heusler compounds already known

from layer systems. The unique features of Co2Fe-based

Heusler compounds such as high saturation magnetization,

low magnetocrystalline anisotropies, and large Curie

temperature makes them ideal electrode candidates for GMR

devices. Figure 4(a) shows the calculation results of magnet-

ization reversal and GMR magnitude for Co2FeSi nanopar-

ticles in both superparamagnetic and ferromagnetic ranges.

Under superparamagnetic condition, the magnetization re-

versal of Co2FeSi ascends steeply leading to a rectangular

profile. In the ferromagnetic regime, however, the magnet-

ization reversal of Co2FeSi is extremely narrow and is char-

acterized by a coercivity of about 3 mT. This is attributed to

the low magnetocrystalline anisotropy. The related sensing

amplitude can be calculated using the magnetization reversal

with Eq. (1)

DR

RHextð Þ ¼ AGMR 1� MðHextÞ

Ms

� �2" #

: (1)

Here, AGMR is the GMR effect amplitude, which is

assumed to be 10% for convenience. The resulting GMR sen-

sor characteristics are summarized in Fig. 4(b). Compared to

the ferromagnetic condition, the GMR characteristic for

superparamagnetic Co2FeSi nanoparticles is very narrow.

This indicates that Co2FeSi nanoparticles with size in the

superparamagnetic range are ideal candidate to achieve

higher GMR sensitivity. Due to the very low coercivity of

Co2FeSi nanoparticles, their GMR behavior is slightly hyste-

retic in the ferromagnetic regime and hence also interesting

for sensor applications. When compared to other materials, as

shown in Fig. 5, the GMR characteristic for Co2FeSi nano-

particles in the superparamagnetic range is very narrow due

to a large superparamagnetic limit size and a high magnetiza-

tion. In regard of magnetoresistive sensors, this is of high

technological relevance since it results in high sensing

FIG. 3. (Color online) Dependences of the number of atoms on the binding

energy (a) and the total energy difference (b) between the cubic and the tet-

ragonal structures of the Ni–Mn–Ga nanocrystals. Reprinted with permis-

sion from Zayak et al., J. Appl. Phys. 104, 074307 (2008). Copyright 2008,

American Institute of Physics.

FIG. 4. (Color online) Calculated magnetization reversal (a) and resulting

granular GMR magnitude (b) of Co2FeSi nanoparticles in the superparamag-

netic (red, centered) and ferromagnetic (blue, periphery) regimes.

FIG. 5. (Color online) Calculated room temperature granular GMR magnitude for

noninteracted superparamagnetic nanoparticles of various materials: hcp-Co

(black), Fe3O4 (pink), Fe3Co (blue), and Co2FeSi (red) (from top to bottom).

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capability. Therefore, Co2FeSi Heusler nanoparticles are

promising component for g-GMR sensors.

The preliminary experimental result of g-GMR sensors

using Co2FeGa nanoparticles88 dispersed in a conductive

hydrogel is shown in Fig. 6. An effective sensing amplitude

as high as 120% is obtained. This is far above common val-

ues for granular systems measured at room or even at low

temperatures.67,70 The hysteresis is attributed to the ferro-

magnetic behavior due to bimodal size distribution of

Co2FeGa nanoparticles covering both superparamagnetic

and ferromagnetic regimes. In a recent work, the effect of

gel-like matrices on the amplitude of g-GMR sensors was

studied using Co nanoparticles.69 A high magnetic resonance

(MR) amplitude of 260% has been achieved.

In terms of practical GMR senor applications, g-GMR

using Heusler nanoparticles exhibit unique features including

higher sensitivity, higher reliability, and low cost for large

scale production. Furthermore, the concept of g-GMR can be

extended to tunneling granular MR (g-TMR).71–73 Granular

TMR systems based on Heusler nanoparticles embedded in

insulating matrices are expected to significantly raise the

TMR amplitude. Compared to conventional stack devices,

one advantage of g-TMR systems is their stability. In particu-

lar, the breakdown of the tunneling barrier, which poses prob-

lems in thin films, can be avoided. Although these potential

applications are exciting, several technical challenges must

be met before the g-GMR sensor devices based on Heusler

nanoparticles reach commercialization. For example, a reli-

able synthetic route has to be realized to produce Heusler

nanoparticles of precisely controlled dimension and morphol-

ogy. Also, dispersion media suitable for particle assembly

and compatible for gel-printing are to be developed.

III. SYNTHETIC STRATEGIES

A. Mechanical ball milling

Coarse grain Heusler compounds have been convention-

ally prepared by mechanical milling and post-annealing.

Recently, this method has also been utilized for the fabrica-

tion of Heusler nanoparticles such as Ni2MnGa,64,74,75

Fe2CrAl,76 Zr0.5Hf0.5CoSb0.8Sn0.2,77 Hf0.75Zr0.25NiSn0.99Sb0.01,

78

and MCo1�xFexSnySb1�y (M¼Ti, Zr, and Hf).79 Figure 7(a)

shows the x-ray diffraction (XRD) patterns of the as-milled

TiCo0.85Fe0.15Sb half Heusler nanoparticles as a function of

milling time. After milling for more than 6 h, the half Heusler

phase appears. Longer milling, however, results in some im-

purity phase (for example, as the case of 20 h). Transmission

electron microscope (TEM) reveals the size of Heusler nano-

particles is around 15 nm [see Figs. 7(b) and 7(c)].

Mechanical ball milling method is a top-down based method

and is facile to scale up. Heusler nanoparticles produced by

this approach, however, are easily agglomerated, severely

strained, presented with numerous intermediate phase. For

Heusler nanoparticles dedicated for magnetic shape memory

alloys, the ball milling approach might be not favorable since

substantial amounts of defects and stresses are introduced

that might substantially mediate the nucleation and growth of

martensite.

B. Vapor deposition

Vapor deposition based approaches have been attempted

to prepare the so-called “Pseudo” binary Heusler nanopar-

ticles and nanowires. For example, 30 nm Fe3Si nanopar-

ticles have been fabricated by a physical vapor deposition

method based on sputtering and XRD analysis indicated the

formation of DO3 ordered Fe3Si phase.80 The obtained Fe3Si

Heusler nanoparticles are easily oxidized in air forming iron

oxide layers. In addition, Co3Si (Ref. 81) and Fe3Si nano-

wires82 were prepared by a chemical vapor deposition

(CVD) method. During the CVD reactions, only the most

thermodynamically stable phases are frequently obtained by

the solid-state reactions of Si substrate and metallic precur-

sors. To our best knowledge, this approach to date has not

been attempted for ternary intermetallic nanowires.

C. Chemical preparation

From the perspective of materials chemistry, chemical

preparation of ternary Heusler nanocrystals is challenging.

Besides particle size/morphology control, nominal stoichiom-

etry and ordered Heusler phase are also required. In a ternary

phase system, structural disorder due to lattice mismatch,

immiscibility, and phase separation are frequently encoun-

tered. A colloidal approach was developed for synthesizing

4–7 nm DO3 ordered Fe3Si nanoparticles by reacting the pre-

formed iron nanoparticles with silicon tetrachloride at

220–250 �C.83 A simple coprecipitation approach was used to

synthesize B2 ordered Co2FeAl Heusler nanoparticles. The

particle size distribution is rather wide (50–400 nm) due to a

lack of size control.84 Chemical reduction method has also

been attempted to prepare Heusler nanoparticles like

Co2FeSn nanoparticles. From XRD, the obtained Co2FeSn

nanoparticles were ordered in B2 type structure. This

approach, however, might be only feasible for Heusler com-

pounds containing elements with standard reduction poten-

tials higher than �0.48 V.85

Successful chemical syntheses of ternary Co2FeGa and

Fe2CoGa nanoparticles have been reported in our group.86–90

The formation of carbon coated Co2FeGa nanoparticles

might involve several steps including loading the metal pre-

cursors within silica matrices, high temperature reduction,

and subsequent carbon coating. After solvent removal, the

FIG. 6. (Color online) Measured g-GMR sensing performance of Co2FeGa

nanoparticles dispersed in a hydrogel matrix. The red and blue lines indicate

increasing and decreasing magnetic field, respectively.

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metal loaded silica opals are condensed, and the silica par-

ticles formed pores of specific dimension and morphology.

The metal precursors accommodated in such interparticle

voids are treated by high temperature annealing under H2

atmosphere to form nanoparticles. The graphite layers are

deposited onto the particle surfaces by a chemical vapor dep-

osition process using methane. Free-standing Co2FeGa nano-

particles can be obtained by removing the silica supports

using hydrofluoric acid etching. A schematic illustration of

the chemical synthesis of carbon coated Co2FeGa nanopar-

ticles using silica supports is shown in Fig. 8.

Recently, a facile carbon nanotube-assisted approach to

prepare Co2FeGa nanoparticles was reported.91 In the syn-

thesis, multiwall carbon nanotubes were used as multiple

functions as reaction container, size-confiner, and protecting

shelter. The reduction reactions were conducted at mild tem-

perature with prolonged annealing time and the obtained

Co2FeGa nanoparticles are spherical, around 35 nm in diam-

eter. The formation of Heusler phase was verified by the

presence of 111 and 200 superlattice reflections revealed by

nanobeam electron diffraction. Compared to bulk Co2FeGa,

a pronounced coercive field enhancement with a factor of

FIG. 7. (Color online) XRD patterns (a) of as-milled Co0.85Fe0.15Sb half Heusler nanoparticles as a function of milling time and the TEM micrograph (b), and

histogram (c) corresponding to the 6 h milled sample.

FIG. 8. (Color online) Schematic illustration of the chemical synthesis of free-standing carbon coated Heusler nanoparticles using silica supports.

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�30 was observed (see Fig. 9). This phenomenon is plausi-

bly explained by the assumption that Co2FeGa nanoparticles

fall in a size range corresponding to a multi-to-single domain

transition.

D. Heusler nanowires and nanorods

Ternary Co2FeAl nanowires were first synthesized by an

electrospinning method.92 The width of the nanowires, how-

ever, was in a wide range from 50 nm to 500 nm. The crystal

structure of the nanowires was B2 or A2. Binary transition

metal silicide nanowires such as Co3Si (Ref. 81) and Fe3Si

(Ref. 82) nanowires have been prepared by a CVD method.

The formation of DO3 ordered Fe3Si and Co3Si nanowires

was due to a diffusion driven crystal conversion process

using preformed nanowire precursors.

For the purpose of particle size control, low temperature

methods based on microemulsion and chemical reduction in

liquid media seems appealing. These soft template based

approaches, however, might not be suitable for Heusler

nanoparticles since the formation of Heusler phase normally

requires high temperature annealing. The approaches using

hard templates such as silica and carbon nanotubes has been

prove feasible but is not robust in particle size control. The

key to improve size control might originate from an

enhancement of size confinement of the templates. The

state-of-the-art synthetic methodologies developed so far are

mainly for nanowires composed of metal, semiconductors,

oxide, and chalcogenides.93 The template-assisted synthesis

constitutes a facile and conceptually intuitive approach to

prepare intermetallic nanowires.94–98 The templates (e.g., an-

odic alumina) contain ordered nanochannel structure, within

which the nanowires adopting the pore morphology are

formed. Coupled with electrochemical deposition, the anodic

alumina assisted approach is anticipated to enable better con-

trol on the composition and morphology of Heusler nano-

wires. It is noted that many elements present in Heusler

compounds (e.g., Ni, Mn, Co, Fe, and Ga) can be

electrodeposited.99–104 Furthermore, the vapor–liquid–solid

methods that previously have been utilized mainly for prepar-

ing semiconductor nanowires are also feasible for the synthe-

sis of intermetallic nanowires including Heusler nanowires.93

Recently, we successfully developed a SBA-15 silica

assisted approach to prepare Heusler nanoparticles and nano-

rods. Ever since their successful preparation in the 1990s, the

two dimensionally ordered mesoporous silica [e.g., SBA-15

(Ref. 105)] have been widely utilized as templates for nano-

materials. A large variety of nanoparticles, nanorods, and

nanowires including metal oxides, mono- and binary-metals

have been synthesized using SBA-15 templates.106–110 As

shown in Fig. 10(a), the synthesized SBA-15 silica is highly

ordered with uniform nanopores with a periodicity of 9–10 nm

and a pore size of about 6 nm. Under suitable impregnation

and processing conditions, Co2NiGa Heusler nanoparticles

and nanorods were obtained inside the nanochannels of

SBA-15, as illustrated in Figs. 10(b) and 10(c). One excep-

tional advantage of SBA-15 silica is that the pore diameter can

be tuned in a wide range of 5–30 nm.105 Therefore, this

FIG. 9. Bright field TEM image (a) and corresponding nanobeam diffraction

pattern (b) of individual Co2FeGa particles inside carbon nanotubes viewed

from a [110] zone axis. The hysteresis curves of carbon nanotubes filled

with Co2FeGa Heusler nanoparticles and polycrystalline bulk sample meas-

ured at 5 K and 300 K are shown in (c). Adapted with permission from

Gellesch et al., Cryst. Growth Des. 13, S2707 (2013). Copyright 2013,

American Chemical Society.

FIG. 10. TEM micrographs of synthesized SBA-15 silica templates (a), Co2NiGa Heusler nanoparticles (b) and nanorods (c) prepared based on a SBA-15

assisted approach.

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SBA-15 assisted synthesis route is very promising to obtain

single sized Heusler nanoparticles which are essential to

investigate the size effect of Heusler nanoparticles.

IV. CHARACTERIZATIONS

Demanding characterizations are required for Heusler

nanoparticles to clarify the structural ordering, magnetic and

shape-memory properties. Compared to mono- and binary

metallic nanoparticles, ternary Heusler nanoparticles exhibit

a rather complicated phase structure and therefore are very

demanding in structural characterizations. To unambigu-

ously determine the order type of Heusler nanoparticles is

not a trivial task. Conventional structural analytic tools such

as XRD and TEM alone are not capable to reveal the struc-

ture and more sophisticated structural probes are required.

Furthermore, robust analytical tools are required to reveal

the unique magnetic features (e.g., superparamagnetic size

limit) and shape-memory parameters (e.g., martensite phase

transition suppression critical size) of Heusler nanoparticles.

A. Structural characterizations

Elemental site swapping has been proved a fundamental li-

mitation to realize HMF in Heusler compounds. Furthermore,

less information on the short range order of Heusler nanopar-

ticles is available in the literature. In this section, we summa-

rize some powerful structural probes specifically for Heusler

nanoparticles. Extended x-ray absorption fine structure

(EXAFS) has been proved to be a powerful tool to study the

short range order of Heusler compounds. The short-range

order of Co2MnSi Heusler compounds was first investigated

by EXAFS technique in the 1980s.111,112 Antisite disorder in

Co2MnSi was examined by EXAFS, and its sensitivity was

compared to neutron diffraction.113 EXAFS was employed to

study the local atomic structure of quaternary Co2CrxFe1�xAl

Heusler compounds.114,115 The variations in local atomic

structure of Ni and Mn was correlated to the long range phase

transition of Ni-Mn based half-Heusler compounds.116 Under

conditions when the assignment of A2, B2, and L21 structure

of Co2FeZ (Z¼Ga and Ge) by XRD analysis is ambiguous,

EXAFS fitting provides an alternative way to determine the

correct structure utilizing the intrinsic short-range order

signatures of the concerned long range order structures.

The capability of using EXAFS analysis to differentiate

the crystal structure of Co-Fe based full Heusler compounds

was demonstrated.117 EXAFS method was applied to

Co2�xFe1þxGa (x¼ 1, 2) Heusler nanoparticles to verify the

L21 ordered crystal structure of Co2FeGa nanoparticles89 and

differentiate the X and L21 ordered structure of Fe2CoGa

Heusler nanoparticles.90

An alternative powerful local structural probe for Heusler

nanoparticles is the total x-ray or neutron scattering and pair

distribution function (PDF) technique.118–120 The PDF

derived from the total scattering data reveal the real-space

distribution of interatomic distances. This technique resolves

short and medium range order information from diffuse scat-

tering component and is suitable for small nanoparticles with

limited long range order and a complicate structure.121–123

Given that total scattering techniques have been imple-

mented in characterizing the local structural (up to a few

nanometers) of nanoscale and bulk intermetallic com-

pounds,124 this technique has not been applied to Heusler

nanoparticles. Total x-ray scattering experiments normally

require pine powder samples with an amount of a few milli-

grams and are sealed in capillaries. For Heusler nanopar-

ticles generated by a template-assisted method, the templates

(e.g., SBA-15 silica) have to be etched off to acquire suffi-

cient amount of samples and to minimize the template dif-

fuse scattering signal. Technical difficulty might arise for

intermetallic nanowires prepared by electrochemical deposi-

tion using anodic alumina templates since the amount of

obtained nanowires is conventionally of the order of

micrograms.

Anomalous XRD (AXRD) is another synchrotron-based

powerful technique to quantitatively reveal the atomic disor-

der of Heusler compounds. AXRD has been employed to

precisely estimate the Co-Mn disorder in Co2MnGe thin

films.125 In a followed study on Co2MnGe epitaxial thin

films, a model of anomalous diffraction at Co and Ge edges

was proposed that quantitatively resolve the disorders

between Co-Mn, Co-Ge, and Mn-Ge sites.126 AXRD method

was also used to unambiguously confirm the L21 structure of

Co2FeGa nanoparticles by performing synchrotron based

XRD around the absorption edges of Co and Fe.86,88 AXRD

was also employed to differentiate between the L21 and Xtype structure of Fe2CoGa nanoparticles90 using two differ-

ent x-ray wavelengths from synchrotron radiation facilities.

Furthermore, it is also demonstrated that the Fe-Co disorder

in Co2FeSi can be quantitatively evaluated by conventional

x-ray diffractometer using Cu and Co x-ray sources.127

B. Magnetic characterizations

Size distribution is more meaningful compared to singular

nominal size for Heusler nanoparticles. Therefore, the meas-

ured magnetic responses also exhibit specific distribution

pattern. To correctly interpret the origin of magnetic prop-

erty distributions, it is important to make measurements on

individual nanocrystals as well as to the traditional volume

averaged measurement techniques such as superconducting

quantum interference device and vibrating sample magne-

tometer. As similar with other magnetic nanostructures for

spintronics,128 the magnetization reversal of a single domain

magnetic nanoparticle typically completes in a few nanosec-

onds. Therefore, an ultrafast switching frequency is expected

for single domain Heusler nanoparticles. It is important to

investigate the switching process on a rather short time scale.

Furthermore, electrical resistivity experiments reveal that the

atomic disorder in Heusler alloys can be examined by meas-

uring low temperature resistivity.129,130

It would be more convincing if the results from different

characterizations point to the same conclusion. A combined

approach using AXRD, EXAFS, and 56Fe M€ossbauer spec-

troscopy has been found valuable to distinguish between dif-

ferent structural types in Co2FeZ Heusler nanoparticles.86–90

It is also demonstrated that bulk-sensitive structural probes

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(such as M€ossbauer spectroscopy) can be correlated to local

structural probes (such as TEM) to derive important struc-

tural parameters for Heusler nanoparticles.88

Even though EXAFS technique is capable to probe the

element-specific short range order structure of Heusler com-

pounds, the structure resolving power of EXAFS is compro-

mised if the atomic number difference among various

scatters is less five. Nuclear magnetic resonance (NMR)

probes the local hyperfine magnetic field of the active nuclei,

which strongly depend on the local magnetic and electronic

environments.131 This spectroscopic technique is able to

reveal the next nearest shells of the NMR active nuclei. The

presence of foreign atoms, disorder and the existence of a

different crystal structure all cause variations in the local

field at a nucleus. It is well known that Heusler compounds

exhibit various structure types and some types of disorder

tend to decrease the spin polarization. In many cases, when

the main group element is from the same period of the peri-

odic system as the transition metals, conventional x-ray or

neutron diffraction may not provide enough information to

identify the structure unambiguously.123 The 59Co NMR

spectra of Heusler compounds Co2FeSi (Refs. 132 and 133)

and Co2Fe(Al,Si) (Refs. 134 and 135) are extremely sensi-

tive to the site disorder and help to quantify the amount of

disorder and clearly differentiate between the A2, B2, and

L21 structures.

Microscopic methods such as Lorentz microscopy136 and

electron holography137 are appealing techniques for investi-

gation of Heusler nanoparticles and nanowires. Magnetic

nanostructures can be detected using Lorentz microscopy by

defocusing. The domain structure of Ni2FeGa, Ni2MnGa,

and Co2NiGa can be detected by Lorentz electron micros-

copy.138,139 Nevertheless, magnetic stray fields still remain

undetectable by Lorentz microscopy alone. Electron holog-

raphy is able to retrieve the phase information which is lost

using conventional electron microscopy. Magnetic as well as

electric fields shift the phase of the electrons thus become

available. The magnetic domain structure of Ni2FeGa has

been observed by electron holograph.140 Furthermore, elec-

tron holography in Lorentz mode can be used to directly vis-

ualize the magnetic induction fields and to estimate the

magnitude of magnetic stray field intrinsic of nanowires.

The magnetic induction fields of 40 nm diameter Co-Fe-B

nanowires resolved by electron holography are in a range of

0.5–1.5 T depending on the location of nanowires.97

Another powerful microcopy/spectroscopy technique is an

integration of EELS/STEM with magnetic circular dichroism

(MCD).141–144 MCD works based on the fact that spin-orbit

coupling breaks the degeneracy of core states with different

total angular momentum and the intrinsic or externally

applied magnetic field gives rise to difference in spin

up/down density of available states. Traditionally, MCD phe-

nomenon is probed with x-rays (XMCD) at synchrotron radi-

ation facilities. In the state-of-the-art electron microscopes, it

is possible to obtain microscopic and spectroscopic fine struc-

ture information together with probing the magnetic proper-

ties of magnetic materials at atomic resolution. This unique

electron microscopy based magnetic spectroscopy technique

has been applied to retrieve the dichroic features of magnetite

nanoparticles and Co2MnSi Heusler films.145,146 To date,

Heusler nanoparticles are mostly in the form of fine powders

and exhibit strong ferromagnetism. The Heusler nanopowders

might not suitable for XMCD experiment since they are eas-

ily attracted by the electromagnets. In comparison, the

dichroic EELS/STM technique is free of this problem and is

suitable to study the spin-related magnetic properties of

Heusler nanoparticles.

V. CHALLENGES AND OUTLOOK

A. Heusler nanoparticles

Even though progress has been made in preparing

Heusler nanoparticles, facile and robust synthetic approaches

are to be developed to prepare highly quality Heusler nano-

particles. To our best knowledge, the surface effect of

Heusler nanoparticles on their physical properties has not

been probed both experimentally and theoretically. It is

noted that the requirements for examining the surface effect

of Heusler nanoparticles are very stringent. One option

might be to uniformly embed the single sized Heusler nano-

particles into a variety of matrices possessing various inter-

facing strength with the nanoparticles. Alternatively, surface

engineering might be performed, as conducted for magnetite

nanoparticles aiming to preserve their half metallic charac-

teristics.147 Using this approach, it was demonstrated that

appropriate atomic H adsorption on the magnetite (001) sur-

face reinforced the Fe atom arrangement to a bulklike man-

ner and therefore preserved the half metallicity at the

surface.

To date, the half-metallicity of selected Heusler nanopar-

ticles has been only roughly examined by comparing their

magnetic behavior with the bulk and no spin-related trans-

port measurements has yet been conducted. Given the rapid

developments in nanospintronics, the research on spin-

related functionalities of Heusler nanoparticles are envisaged

to be very active in the near future. In this term, break-

throughs in both material synthesis and characterization

tools of Heusler nanoparticles are required. The new strategy

of “cluster-assembling” facilities simultaneous control of

cluster size, concentration, and intercluster spacing and the

obtained cluster-in-metal system exhibit enhanced magneto-

resistance response.148 Recently, tremendous progress has

been made in nanofabrication technique, the intrinsic spin

polarization of nanowire devices can be directly measured,

for example, on Fe-Co-Si nanowires.149

To the authors’ knowledge, there is to date no systematic

theoretical or experimental investigation on the size-

dependent electronic and magnetic properties of Heusler

nanoparticles. With experimental investigations difficult due

to the quality of the material, theoretical investigations are

extremely valuable to shed lights on the key issues such as

the size effect of Heusler nanoparticles. In recent years,

much theoretical investigations have been conducted to

examine the potential of nanostructures (typically in quasi-

one-dimensional system) for use in spintronics.150–152 As an

example, theoretical studies on Co and Fe nanowires, which

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are either freestanding or encapsulated in carbon nanotubes,

reveal a high degree of spin polarization.150,153 In addition,

first principles theory has been successfully utilized to eluci-

date the size-dependent electronic and magnetic properties

in dilute magnetic semiconducting (DMS) nanocrystals.154

The well studied Ni-Mn-Ga Heusler shape-memory alloys

have some problems for industrial applications such as brit-

tleness, low working temperature, and difficulty for scale-up

production. Therefore, new ferromagnetic shape-memory

Heusler alloys other than Ni-Mn-Z have been investigated

aiming to overcome the above problems and also to explore

new shape-memory functionalities.155,156 The ferromagnetic

shape-memory Heusler alloys also show thermoelastic and

stress-induced martensite transformation as observed in con-

ventional shape-memory alloys. For example, superelastic

properties have been observed in Ni-Fe-Ga,157 Co-Ni-Al,158

and Co-Ni-Ga.159 For nanocrystalline martensitic materials,

thermally and magnetically induced martensitic phase trans-

formation could be suppressed if the crystal size is smaller

than threshold value. The thermal–mechanical and supere-

lasticity of martensitic materials, however, is found to be in-

dependent of crystal size.160,161 Thus, stresses provide a

solution to enable martensite formation even when the grain

size is smaller than the critical size associated with the ther-

mally induced transformation and/or the superparamagnetic

limit. Recently, novel multifunctional properties such as

superelasticity, highs strength, good ductility, and high

energy damping have been found in single ferrous polycrys-

talline ferromagnetic shape-memory alloys.162 Thus, new

Heusler shape-memory nanoparticles that exhibit robust

shape-memory properties and are capable to break the size

limitations are promising candidates for the next generation

ultrasmall medical devices, actuators, and sensors.

B. Heusler nanowires

Unlike the three-dimensionally confined Heusler nanopar-

ticles, Heusler nanowires possess two directions configured

to the nanoscale and one unconfined direction. Due to the

unique distribution of electronic state density, along the di-

ameter direction, Heusler nanowires are expected to exhibit

different spin-polarized electronic, electrical, and magnetic

properties to their bulk counterparts. The length scale of

Heusler nanowires is normally large enough to exhibit crystal

structure and properties similar to the bulk parent materials.

This is advantageous for theoretical studies since a wealth of

knowledge of their bulk properties can be exploited to predict

the properties of Heusler nanowires. Despite the exceptional

anisotropies in their structure and physical properties, no sys-

tematic theoretical study on Heusler nanowires has been

undertaken. Furthermore, from the standpoint of device appli-

cations, Heusler nanowires are more feasible to manipulate

(e.g., positioning, aligning, orienting, and assembling) than

their nanoparticle counterparts.163–166

Another impetus for Heusler nanowires comes from the

recent investigations on the size dependent shape-memory

behavior of nanoscale volumes in shape-memory materi-

als.62,167 Single crystalline Cu-Al-Ni shape-memory pillars

exhibit superelasticity and complete shape recovery at both

the micro- and nanometer scales.62 Deformation studies of

Ni-Mn-Ga micropillars have also been reported and the

force-triggered twinning of microscale Ni-Mn-Ga pillars

was followed by atomic and magnetic force microscopic

observations.168 Atomistic simulations indicate that materi-

als that do not show the shape-memory effect in their bulk

can become shape-memory nanomaterials.169–171 Only nano-

wires made of fcc metals with high twinning tendencies and

with diameters up to a few nanometers show a shape-

memory and pseudoelasticity. The wires can recover axial

elongations of �50%, which are much larger than the value

of 5–8% typical for conventional bulk shape-memory

alloys.169 The atomistic simulation studies show that this

behavior is associated with a reversible lattice reorientation

driven by the surface stress-induced internal stresses and

only occurs at the nanoscale. Here, the crucial consideration

is that the lateral size of the wires must be compatible to the

width of the stable stacking faults of the fcc metals so that

twinning, instead of crystalline slipping, dominates the de-

formation mechanism. The stresses might arise by the dan-

gling atomic bonds at the free surfaces that provide strong

mechanical driving forces for phase transformation.170

Therefore, this class of metallic nanowires are appealing

candidates for future ultrasmall sized sensing and actuation

applications.

ACKNOWLEDGMENTS

Financial support by the DFG is gratefully acknowledged

(Project TP 2.3-A in research unit FOR 1464

“ASPIMATT”). The authors affiliated with Bielefeld

University would like to thank the FOR 945, the SPP1599,

and International Office of BMBF for financial support in

the framework of the project 3, A6, and TUR09/I01,

respectively.

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