High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloyLakhan Bainsla, K. G. Suresh, A. K. Nigam, M. Manivel Raja, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, andK. Hono Citation: Journal of Applied Physics 116, 203902 (2014); doi: 10.1063/1.4902831 View online: http://dx.doi.org/10.1063/1.4902831 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Large magnetoresistance in current-perpendicular-to-plane pseudospin valve using a Co 2 Fe ( Ge 0.5 Ga 0.5 )Heusler alloy Appl. Phys. Lett. 98, 152501 (2011); 10.1063/1.3576923 Low-temperature grown quaternary Heusler-compound Co2Mn1−x Fe x Si films on Ge(111) J. Appl. Phys. 109, 07B113 (2011); 10.1063/1.3563039 The effect of substitution of Fe with Cr on the giant magnetoresistance of current-perpendicular-to-plane spinvalves with Co2FeSi Heusler alloy J. Appl. Phys. 109, 043901 (2011); 10.1063/1.3549722 Enhanced Curie temperature and spin polarization in Mn 4 Fe Ge 3 Appl. Phys. Lett. 91, 142505 (2007); 10.1063/1.2794425 A magnetic and structural study of Mn, Co, and Ni substituted Fe 3 Ge 2 hexagonal germanides J. Appl. Phys. 84, 401 (1998); 10.1063/1.368080

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High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

Lakhan Bainsla,1,2 K. G. Suresh,1,a) A. K. Nigam,3 M. Manivel Raja,4

B. S. D. Ch. S. Varaprasad,2 Y. K. Takahashi,2 and K. Hono2

1Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India2Magnetic Materials Unit, National Institute for Materials Science, Tsukuba 305-0047, Japan3Department of Condensed Matter and Materials Science, Tata Institute of Fundamental Research,Mumbai 400005, India4Defence Metallurgical Research Laboratory, Hyderabad 500058, India

(Received 20 August 2014; accepted 15 November 2014; published online 25 November 2014)

We report the structure, magnetic property, and spin polarization of CoFeMnGe equiatomic

quaternary Heusler alloy. The alloy was found to crystallize in the cubic Heusler structure

(prototype LiMgPdSn) with considerable amount of DO3 disorder. Thermal analysis result

indicated the Curie temperature is about 750 K without any other phase transformation up to

melting temperature. The magnetization value was close to that predicted by the Slater-Pauling

curve. Current spin polarization of P¼ 0.70 6 0.01 was deduced using point contact andreev reflec-

tion measurements. The temperature dependence of electrical resistivity has been fitted in the tem-

perature range of 5–300 K in order to check for the half metallic behavior. Considering the high

spin polarization and Curie temperature, this material appears to be promising for spintronic appli-

cations. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4902831]

INTRODUCTION

Co-based Heusler alloys have attracted a lot of attention

due to their theoretically predicted half-metallic nature and

experimentally observed high spin polarization values with

high Curie temperatures.1–6 Half-metallic ferromagnets

(HMF) have applications as spin polarized current sources for

magnetic tunneling junctions,7 current-perpendicular-to-plane

giantmagnetoresistive (CPP-GMR) devices,8,9 spin injectors

to semiconductors,10 and lateral spin valves.11 Recently

reported Co2Fe(Ga0.5Ge0.5),3 and Co2(Fe0.4Mn0.6)Si (Ref. 7)

show rather high spin polarization at room temperature; how-

ever, their spin transport properties at room temperature are

substantially degraded. Because of this, further exploration of

half-metallic Heusler alloys is strongly desired.

In ternary Heusler alloys [XYZ (half Heusler) and X2YZ(full Heusler)], X and Y are transition and Z is a main group

element. Full Heusler alloys have the L21 structure (space

group No. 225) consisting of four interpenetrating sub-

lattices. If each of the four sublattices is occupied by different

atoms, a quaternary Heusler structure with different symme-

try (space group no. 216) is obtained, i.e., LiMgPdSn

type.12,13 Depending on the occupation of the different lattice

sites, three different type of structure is possible for the

LiMgPdSn structure. To the best of our knowledge, only a

few quaternary Heusler alloys with 1:1:1:1 stoichiometry

have been explored.14–16

Heusler alloys are known to exhibit tunable electronic

and magnetic properties depending on their valance electron

number. The usual method to conceive new quaternary

Heusler alloys has been to combine two ternary Heusler

alloys such as Co2FeGe (Ref. 17) and Co2MnGe,18 which

results in the quaternary CoFeMnGe (CFMG) alloy.14 This

alloy has been predicted to have a half-metallic nature with a

high Curie temperature.14 Recently, some other quaternary

Heusler alloys, CoFeMnX (X¼Al, Si, Ga, Ge)14,15 have

been synthesized successfully. Ab initio calculations as well

as experimental observations indicate that there is great pos-

sibility of achieving half-metallicity in these alloys. Klaer

et al.15 have predicted half-metallic behavior in the case of

CFMG. For applications, such quaternary Heusler alloys

with 1:1:1:1 stoichiometry have advantage over pseudo-

ternary alloys such as Co2Fe(Ga0.5Ge0.5), because the ran-

dom distribution of Ga and Ge in Co2Fe(Ga0.5Ge0.5) leads to

an additional disorder scattering and thus the spin diffusion

length becomes very short, only in the order of a few nm.19

On the other hand, the devices based on equiatomic quater-

nary Heusler alloys are expected to have lower power dissi-

pations. In the light of these, we have investigated the

structural, magnetic, transport, and current spin polarization

properties of equiatomic CFMG quaternary Heusler alloy.

EXPERIMENTAL DETAILS

The polycrystalline bulk sample of an equiatomic

CFMG alloy was prepared by arc melting of stoichiometric

quantities of constituent elements in an inert atmosphere.

The ingot was melted several times to increase the chemical

homogeneity and the final weight loss was less than 1%. To

increase the homogeneity, the as-cast alloy was annealed

under vacuum for 14 days at 1073 K and then quenched in

cold water. The crystal structure was investigated by x-ray

diffraction with Cu Ka radiation at room temperature.

Scanning electron microscopy (SEM) images were taken

using JSM-7600F FEG-SEM. 57Fe M€ossbauer spectra at

room temperature were recorded using a constant accelera-

tion spectrometer with 25 mCi 57Co(Rh) radioactive source.

The spectrometer was calibrated by using natural iron foil of

25 lm before measuring the sample. The obtained spectra

a)Author to whom correspondence should be addressed. Electronic mail:

suresh@phy.iitb.ac.in

0021-8979/2014/116(20)/203902/6/$30.00 VC 2014 AIP Publishing LLC116, 203902-1

JOURNAL OF APPLIED PHYSICS 116, 203902 (2014)

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were analyzed using PCMOS-II least-square fitting program.

Curie temperature and the structural transition temperature

(if any) were probed by using the differential thermal analy-

sis (DTA) with a heating rate of 20 K min�1. Magnetization

measurements at 3 K and 300 K were performed using a

vibrating sample magnetometer (VSM) attached to a physi-

cal property measurement system (PPMS, Quantum design).

Current spin polarization measurements were done by using

the point contact Andreev reflection (PCAR) technique.20

Sharp Nb tips prepared by electrochemical polishing were

used to make point contacts with the sample. Spin polariza-

tion of the conduction electrons was obtained by fitting the

normalized conductance G(V)/Gn curves to the modified

Blonder-Tinkham-Klapwijk (BTK) model.21 A multiple pa-

rameter least squares fitting was carried out to deduce spin

polarization (P) using dimensionless interfacial scattering

parameter (Z), superconducting band gap (D), and P as

variables.

RESULTS AND DISCUSSION

Fig. 1 shows the Rietveld refinement of powder XRD

pattern recorded at room temperature. The lattice parameter

of the alloy is found to be 5.75 A, which is in agreement with

the reported value of CFMG.14 As one can see, the superlat-

tice reflections (111) and (200) are very weak for this alloy.

When Z element is from the same period of the periodic sys-

tem as the transition metals, it is very difficult to find out cor-

rect structure unambiguously by using x-ray or neutron

diffraction data. It is reported that (111) and (200) reflections

were not observed in the XRD patterns of the Co2FeZ(Z¼Al, Si, Ga, Ge) samples, but the extended x-ray absorp-

tion fine structure technique showed the existence of L21

order.17 It may be noted that the structural ordering is a nec-

essary requirement for high spin polarization.22 In order to

make a rough estimate of chemical order present in the alloy,

we calculated the I200/I220 and I111/I220 intensity ratios. The

superlattice reflections (111) and (200) are proportional to

the order parameters as S2 and S2(1–2a)2, where S¼ ((I200/

I220)Exp./(I200/I220)Theory)1/2 and S(1–2a)¼ ((I100/I220)Exp./

(I100/I220)Theory)1/2.23 In the case of A2 disorder, S¼ 0 and

a¼ 0, for complete B2 order S¼ 1 and a¼ 0.5. For a well or-

dered cubic phase, S¼ 1 and a¼ 0 are expected. Therefore,

the x-ray diffraction analysis gives an indication that the

alloy has a well ordered cubic Heusler structure (LiMgPdSn

type).

To further investigate the structural order, we performed57Fe M€ossbauer spectroscopic measurements at room tem-

perature as shown in Fig. 2. The experimental spectrum has

been fitted with two sextets having hyperfine magnetic field

values of 285 and 104 kOe and relative intensities of 67%

and 33%, respectively (fitted parameters are given in Table

I). The quadrupole splitting and isomer shift values are very

small. The data have been analyzed by taking the type II

LiMgPdSn structure, where Co, Mn, Fe, and Ge occupy X,

X0, Y, and Z sites, respectively, in quaternary Heusler

alloy.13,15 It is reported from theoretical calculations that

quaternary CoFeMnSi Heusler alloy can be half-metallic for

type I and type II structure.13 Two sextets namely S1 and S2

are found to be essential for obtaining good fitting. In a well

ordered atomic arrangement, Fe atoms must occupy the Ysites with cubic symmetry (Oh), which will result in a single

sextet because there is only one crystallographic site for Fe.

FIG. 1. Rietveld refinement of the powder x-ray diffraction pattern of

CoFeMnGe collected at the room temperature. The inset shows the

expanded part of the low angle region.

FIG. 2. 57Fe M€ossbauer spectra of CoFeMnGe collected at room

temperature.

TABLE I. Hyperfine field (Hhf), quadrupole splitting (DEq), isomer shift

(d), line width, and relative intensity of M€ossbauer spectra collected at room

temperature.

Mossbauer parameters

CFMG

Sub

spectrum

Hhf

(kOe)

Quadrupole

shift (DEq)

(mm/s)

Isomer

shift d(mm/s)

Relative

intensity

(%)

Phase

identified

S1 285 �0.011 �0.003 67 L21 (Y-site)

S2 104 �0.115 0.179 33 DO3 (X-site)

203902-2 Bainsla et al. J. Appl. Phys. 116, 203902 (2014)

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The presence of the second sextet could be attributed to the

occupation of Fe atoms at X or Z sites, thereby indicating

some amount of structural disorder. When Fe occupies Zsite, hyperfine magnetic field is expected not to change much

as the number of magnetic near neighbours is similar for Yand Z sites, whereas when it goes to X site, a large decrease

in hyperfine magnetic field is expected as this has the highest

number of non-magnetic near neighbours. The experimen-

tally observed values of hyperfine magnetic fields (285, 104

kOe) clearly indicate that the Fe also occupy X, X0 sites,

resulting in DO3 type disorder. The sub-spectra S1 and S2 are

ascribed to the ordered LiMgPdSn type phase and DO3

phases, respectively. The intensity of S1 (67%) is found to be

higher as compared to S2 (33%), which means that the struc-

ture is reasonably ordered at room temperature. It is pre-

dicted earlier that high spin polarization can be achieved in

the materials with the presence of structural disorder.24 The

values of the hyperfine field (Hhf) and isomer shift for spectra

S1 are comparable to those obtained for other Co-based

Heusler alloys.25,26 Quadrupole shift value is almost zero,

which is in accordance with the cubic symmetry of the local

Fe environment. The SEM image, shown in Fig. 3, reveals

the contrast that supports the presence of two phases.

Therefore, the presence of an ordered cubic Heusler phase

and disordered DO3 phase seen in the M€ossbauer spectrum is

verified by the SEM image.

In order to investigate the structural stability with tem-

perature, we performed DTA in the temperature range from

400 K to 1450 K as shown in Fig. 4. In the DTA curve, there

are only two minima, one is near TC and the other close to Tm

(melting temperature), which suggests there is no structural

transition in the CFMG alloy up to the melting temperature.

Hence, the alloy is ordered on solidification, i.e., it is an inter-

metallic compound, which is good from the point of view of

applications. Curie temperature of �750 K obtained from the

DTA curve agrees very well with the report by Alijani et al.14

Isothermal magnetization measurements have been done

at various temperatures up to a field of 640 kOe and are

shown in Fig. 5. The M-H curves show the characteristics of

a soft ferromagnet. Saturation magnetization of 4.2 lB/f.u.

and 3.9lB/f.u. is observed at 3 K and 300 K, respectively,

which are in good agreement with earlier observations.14,15

According to the Slater-Pauling rule,27 the value of satura-

tion magnetization is 4 lB/f.u. The experimental value is

higher than the theoretical value, which may be due to the

presence of the D03 structural disorder as observed in

M€ossbauer spectroscopy.

FIG. 3. SEM image of the CoFeMnGe alloy at room temperature.

FIG. 4. Temperature variation of the DTA curve of CoFeMnGe in the tem-

perature range of 400–1450 K.

FIG. 5. Isothermal magnetization curves of CoFeMnGe at 3 K and 300 K.

Solid line denotes the magnetization value obtained from the Slater-Pauling

rule.

203902-3 Bainsla et al. J. Appl. Phys. 116, 203902 (2014)

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Spin polarization of the sample was measured by using

the PCAR technique. In the measurement of spin polariza-

tion by PCAR, one measures the conductance curves across

the ferromagnetic (FM)/superconductor (SC) point contact.

Since spin polarization (PC) measured by the PCAR is the

transport spin polarization and hence it is much more impor-

tant from the application point of view in spintronic devices

such as CPP-GMR devices. Figs. 6(a)–6(c) show the normal-

ized conductance curves with typical bias dependence. In

Figs. 6(a)–6(c), open circles denote the experimental data

and the solid lines are fit to the data by modified BTK

model.21 As one can see, the shape of the curves changes

near the superconducting band gap (D) with change in inter-

facial scattering parameter (Z). We fitted the measured data

using modified BTK model by keeping P, D, and Z as varia-

bles. Since there is no proximity effect observed in the

PCAR spectra, we assumed D¼D1¼D2. The values of P, D,

and Z with the best fitting (least v2 value) are shown in the

figures. The conductance curves become more flat near D for

low Z values. This can be easily observed by closely looking

at Figs. 6(a) [with Z¼ 0.13] and 6(c) [Z¼ 0.29]. D values

obtained from the best fit is lower than the bulk supercon-

ducting band gap of Nb (1.5 meV), which is attributed to the

multiple contacts which can give rise to the suppression of

the band gap as reported for Nb/Cu by Clowes et al.28 The

value of current spin polarization is estimated by obtaining

the conductance curves for Z¼ 0, but in our case the lowest

value of Z was 0.13 and hence the current spin polarization

was estimated by linear fitting of the experimental data and

extrapolating it to Z¼ 0. Fig. 6(d) shows the P vs. Z curve

with extrapolation down to Z¼ 0.

The current spin polarization value of 0.70 6 0.01 is

deduced from the P vs. Z plot. The high value of spin polar-

ization obtained for this bulk alloy is comparable to that

obtained for bulk Co2Fe(Ga0.5Ge0.5) (P¼ 0.69 6 0.02),3

which has been proved to be one of the best FM Heusler

alloys for CPP-GMR devices.9,11,19,29 This is the first report

FIG. 6. PCAR normalized conductance curves in CoFeMnGe measured at 4.2 K using Nb as a superconducting tip. Solid lines are fit to the experimental data

by modified BTK model and open circles are experimental data. (d) P vs. Z plots with extrapolation down to Z¼ 0.

FIG. 7. Electrical resistivity measurements under the field of 0 and 50 kOe

in the temperature range of 5–300 K (left hand side scale). Residual resistiv-

ity ratio (RRR) vs. temperature curve (right hand side scale).

203902-4 Bainsla et al. J. Appl. Phys. 116, 203902 (2014)

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103.21.126.77 On: Wed, 26 Nov 2014 04:40:53

on the experimental measurement of the spin polarization in

equiatomic quaternary Heusler alloys.

Electrical resistivity measurements have been done in

0 and 50 kOe in the temperature range of 5–300 K as

shown in Fig. 7. Resistivity value of 3.1 lXm is found for

CFMG, which is relatively high as compared to that of

Co2Fe(Ga0.5Ge0.5) (0.6 lXm) alloy,19 which may be due to

the presence of larger DO3 disorder. Zero field resistivity

has been normalized with respect to the value at 5 K and

plotted in Fig. 6 (right hand side scale). The residual resis-

tivity ratio [q (T)/q5K] at 300 K is found to be low (1.02),

which confirms the large disorder in the alloy. A minimum

in the resistivity curve is found near 40 K, which is fol-

lowed by an upturn at lower temperatures. Such a feature

has been reported in many Heusler alloys30 and is typically

attributed to the disorder enhanced coherent scattering of

conduction electrons.31 At least in some alloys, such a min-

imum has been explained on the basis of Kondo effect.32

Resistivity behavior has been analyzed in the temperature

range of 5–300 K by considering different scattering mech-

anisms. Typically in half-metallic Heusler alloys, the

electron-magnon contribution is absent at low tempera-

tures. As the temperature increases, the half metallicity

trend decreases and the electron-magnon contribution

appears.33,34 Therefore, the major contributions to the re-

sistivity in a half metallic ferromagnet arise due to (i)

electron-electron (T2 dependence),35 (ii) electron-phonon

(T dependence),35 and (iii) double magnon scattering (T9/2

at low temperatures and T7/2 at high temperatures).36,37 In

addition, a T1/2 dependence reflects the contribution due to

the disorder.30 With these points in mind, the resistivity

data have been fitted in two different temperature regions,

i.e., between 5 and 85 K and between 85 and 300 K. In

the low temperature region, the resistivity fits well with the

relation, q(T)¼ q0-AT1/2þBT2þCT9/2, while in the

high temperature region, the best fit is given by q (T)¼q0

þDTþET7/2. In the low temperature regime, T1/2 term

dominates over the other two contributions due to the pres-

ence of disorder. This also suggests that the minimum seen

in the q (T) plot is mostly due to the disorder. In the high

temperature regime, the linear dependence due to the

electron-phonon contribution dominates, but along with a

weak negative contribution from the T7/2 term. Thus, the

resistivity data show the possibility of half-metallicity

even at temperatures as high as 300 K. The values of vari-

ous parameters obtained from the fits are given in Table II.

Therefore, it is clear that both PCAR and the resistivity

data indicate the half metallic ferromagnetic nature of this

alloy.

CONCLUSIONS

In conclusion, the quaternary equiatomic Heusler alloy

CoFeMnGe was found to exist in the cubic Heusler structure

(LiMgPdSn type) with considerable amount of DO3 disorder.

However, no structural order-disorder transition was detected

up to the melting temperature. Magnetization values roughly

agree with those calculated by Slater-Pauling rule. The value

of the current spin polarization was deduced to be 0.70 6

0.01 by PCAR measurements at 4 K, which is one of the

highest values for ternary or quaternary Heusler alloys

reported so far.5,6,38 Electrical resistivity measurements

show the signature of half-metallicity for the material even

at high temperatures. Therefore, this material appears to be

promising to be used in spintronic devices such as GMR,

which need high current spin polarization to achieve high

MR ratios.

ACKNOWLEDGMENTS

One of the authors, Lakhan Bainsla would like to thank

UGC, Government of India for granting senior research

fellowship (SRF). K.G.S. thanks ISRO cell, IITB for funding

the work. The authors also thank D. Buddhikot for his help

in the resistivity measurements.

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TABLE II. Analysis of the zero field electrical resistivity in the temperature range of 5 K to 300 K.

q (5 K) q (300 K) RRR Fitting parameters

3.09 lXm 3.16 lXm 1.02 Region I Region II

q0

(lXm)

A (lXm K�1/2)

� 10�3

B (lXm K�2)

� 10�6

C (lXm K�9/2)

� 10�11

q0

(lXm)

D (lXm K�1)

� 10�4

E (lXm K�7/2)

� 10�11

3.09 2.29 2.72 1.04 3.06 4.35 �5.69

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