RESEARCH PAPER

Suppression and enhancement of the ferromagneticresponse in Fe-doped ZnO nanoparticles by calcinationof organic nitrogen, phosphorus, and sulfur compounds

D. Ortega • J. C. Hernandez-Garrido •

C. Blanco-Andujar • J. S. Garitaonandia

Received: 7 June 2013 / Accepted: 7 November 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The current study reports on the prepara-

tion of *10–13 nm ZnO nanoparticles homoge-

neously doped with Fe2? ions ((Zn0.97Fe0.03)O) and

the manipulation of their ferromagnetic response at

room temperature with an appropriate post-process-

ing. The homogeneous spatial distribution of iron is

studied at atomic column level through high-resolu-

tion transmission electron microscopy, high-angle

annular dark field, and electron energy loss spectros-

copy. Magnetization isotherms show a ferromagnetic

feature within the low field region exhibiting temper-

ature dependence. X-ray diffraction and analytical

microscopy measurements are compatible with a

homogeneous distribution of Fe2? over the ZnO

lattice, and discard the formation of iron or iron oxide

clusters that could account for the low field ferromag-

netic signal. From the temperature dependence of the

susceptibility, it is inferred that Fe2? cations do not

order magnetically. Induced defects upon calcination

of different organic molecules over the (Zn0.97

Fe0.03)O nanoparticles in aerobic conditions lead to a

significant modification of the magnetic properties,

even suppressing the room temperature ferromagnetic

signal originally observed. More specifically, when

the heat treatment is carried out in the presence of

dodecylamine, the original room temperature ferro-

magnetic signal is canceled, whereas an enhancement

on the same ferromagnetic contribution is observed

when using trioctylphosphine oxide. No significant

differences have been found after calcinating 1-dode-

canethiol-doped ZnO nanoparticles.Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-013-2120-5) contains supple-mentary material, which is available to authorized users.

D. Ortega

Instituto Madrileno de Estudios Avanzados en

Nanociencia (IMDEA-Nanociencia),

Cantoblanco, 28049 Madrid, Spain

D. Ortega (&)

Institute of Biomedical Engineering, University College

London, London WC1E 6BT, UK

e-mail: daniel.ortega@imdea.org

J. C. Hernandez-Garrido

Departamento de Ciencia de los Materiales e Ingenierıa

Metalurgica y Quımica Inorganica, Facultad de Ciencias,

Universidad de Cadiz, Campus Rıo San Pedro,

11510 Puerto Real, Cadiz, Spain

C. Blanco-Andujar

Department of Physics and Astronomy, University

College London, London WC1E 6BT, UK

J. S. Garitaonandia

Zientzia eta Teknologia Fakultatea, Euskal Herriko

Unibertsitatea, Bilbao, Spain

123

J Nanopart Res (2013) 15:2120

DOI 10.1007/s11051-013-2120-5

Keywords Zinc oxide � Diluted magnetic

oxides � Nanoparticles � Ferromagnetism �Nanostructure processing

Introduction

The possibility of having functional magnetic semi-

conductors at room temperature or above is undoubt-

edly a major challenge in solid state physics (Kennedy

and Norman 2005). Encouraged by the seminal work

of Dietl et al., where theoretic predictions of room

temperature ferromagnetism in p-type semiconductors

such as ZnO were presented (Dietl et al. 2000),

experimental confirmations were not long in coming,

and the following year the first observation of room

temperature ferromagnetism in transparent transition

metal-doped TiO2 was made by Matsumoto et al.

(Matsumoto et al. 2001). The so-called diluted mag-

netic semiconductors (DMS) constitute the near future

of spintronics (Awschalom and Flatte 2007), since

they combine both magnetic and electronic dopants to

allow the propagation of spin-polarized currents

without the necessity of continuously applying a

magnetic field. It has to be noted that the onset of a

room-temperature ferromagnetic order in semicon-

ductors has been considered a genuine nano phenom-

enon, as it had only been observed in nanoparticles and

thin films, but reports on bulk materials exhibiting the

same phenomenology do also exist (Han et al. 2002).

The plethora of reports accumulated during the last

few years on ferromagnetism in some diluted mag-

netic oxides still claims for a more general and

satisfactory explanation, as the well-established the-

ories of ferromagnetic order are unable to provide a

solid ground for it. A charge-transfer model has been

proposed for Fe-doped TiO2 thin films (Coey et al.

2010b), whereas a wandering axis ferromagnet model

appears to explain the ferromagnetic order in indium

SnO2 films, where the magnetization is considered to

be confined at grain boundaries (Coey et al. 2010a).

Although in many cases the appearance of a temper-

ature-dependent hysteresis is usually linked to ferro-

magnetic impurities (Laiho et al. 2010; Coey 2006), in

the case of Co-doped ZnO nanoparticles, XMCD and

EELS measurements clearly demonstrate an intrinsic

ferromagnetic behavior (Zhang et al. 2009).

The vast majority of the previous research in this

field has been devoted to the changes in the magnetic

properties of ZnO derived from the incorporation of

several transition metals as dopants (Pan et al. 2008;

Ogale 2010). However, transition metal doping has

been rendered as a non-universal condition to observe

room temperature ferromagnetism by many reports

(Ortega et al. 2012; Garcıa et al. 2007; Chen et al.

2009), yielding prominence to structural defects as the

more likely causing factor (Potzger and Zhou 2009).

The role of defects has been put forward both

theoretically and experimentally, giving rise to several

models that successfully encompass a good deal of the

data obtained from doped and/or undoped oxides in a

more general fashion (Uchino and Yoko 2012; Coey

et al. 2010a, b).

A better understanding of the magnetic order in

nanoscale diluted magnetic oxides is inevitably tied to

the ability of preparing high quality nanostructures

and finding the extent to which these can be manip-

ulated to finally have the materials with a real

technological interest. The current study is concerned

with these two crucial aspects. We have focused on the

study of (1) the magnetic properties of ZnO nanopar-

ticles when doped with 3 % Fe following a wet

chemical process and (2) the subsequent changes

introduced by capping the doped nanoparticles with

three different molecules (dodecylamine, 1-dodecane-

thiol, and trioctylphosphine) followed by heat treat-

ment in aerobic conditions. We show that

ferromagnetism in (Zn0.97Fe0.03)O nanoparticles can

be either enhanced or suppressed depending on the

nature of the calcined capping molecules.

Experimental

The core chemical method used in this study has been,

with a certain variation, the hydrolysis and condensa-

tion of zinc acetate solutions in dimethylsulfoxide

(DMSO) through alkaline activation formerly envis-

aged by Norberg et al. (2004). Unlike thermal

decomposition or pulsed-laser deposition, this method

is particularly interesting because it is easier to control

the oxidation state of the dopants, as no high

temperatures are involved. Briefly, a 0.552 M tetra-

methylammonium hydroxide pentahydrate (TMAH,

Fluka, purum C95 %) solution in ethanol (Sigma-

Aldrich, anhydrous) was added dropwise to a 0.003 M

Fe(OAc)2 (Aldrich, 95 %) and 0.097 M Zn(OAc)2

(Fluka, ACS reagent C99 %) solution in dimethyl

Page 2 of 10 J Nanopart Res (2013) 15:2120

123

sulfoxide (DMSO, Fluka, purum C95 %) at 25 �C

under vigorous stirring. Assuming that all the iron is

incorporated into the ZnO nanoparticles during the

precipitation, these amounts provide a nominal 3 % Fe

doping, which represents a good balance between a

cation percentage well below the percolation threshold

and a significant ferromagnetic signal in the final

nanoparticles. The resulting solution was then kept at

50 �C in a heater for 4 days. The nanoparticles were

precipitated with ethyl acetate (Sigma-Aldrich, ACS

reagent C99.5 %) and resuspended in ethanol three

times. For the capped series, a suitable amount of three

different coating molecules—dodecylamine (DDA,

Fluka, purum, 98 %), trioctylphosphine oxide (TOPO,

Aldrich, technical grade, 90 %), and 1-dodecanethiol

(DDT, Fluka, purum C97 %)—were added to the

precipitated particles followed by vigorous stirring at

150 �C for 1 hour. After three washing steps, the

nanoparticles were submitted to heat treatment at

500 �C in aerobic conditions.

Iron oxidation state was determined to be ?2 by the

1,10-phenanthroline assay carried out in aliquots taken

from the resulting colloids. A subsequent treatment

with hydroxylamine hydrochloride, which reduces

any Fe3? present in samples to Fe2?, did not change

the outcome of the initial 1,10-phenanthroline assay.

This is in agreement with the accumulated results on

nanoparticles synthesized through wet chemical meth-

ods, where Fe is usually found in the form of Fe2?

(George et al. 2010).

X-ray diffractometry was carried out with a X-ray

diffractometer PanAlytical, using CoKa radiation

k = 1.789010 A and a zero-background Si holder.

Conventional transmission electron microscopy

(TEM) imaging was carried out with a JEOL JEM

1200-EX transmission electron microscope operated

at an acceleration voltage of 120 kV, whereas high-

resolution (HRTEM), high-angle annular dark field

(HAADF), electron energy loss spectroscopy (EELS)

and X-ray energy dispersive (EDX) spectroscopy were

performed with a field emission gun JEOL JEM-

2010F electron microscope operated at 200 kV—

point to point resolution 0.19 nm—and equipped with

a Gatan Imaging Filter 2000 with 0.8 eV energy

resolution. A quantum design hybrid superconducting

quantum interference device-vibrating sample mag-

netometer (SQUID-VSM) was used for the magnetic

measurements. Field intensity values were also cor-

rected using a dysprosium oxide standard sample to

discard any anomalies induced by flux trapping and

remanence in the superconducting magnet of the

magnetometer.

Results and discussion

Fe2? doped ZnO nanoparticles

Indexing and intensity ratios from a representative

room temperature XRD pattern in Fig. 1 evidence the

presence of solely hexagonal ZnO with a wurtzite-type

structure (JCPDS card No. 01-079-0206). The notice-

able peak broadening is consistent with the small size

of the nanoparticles (12.18 ± 0.08 nm, Fig. S1,

electronic supplementary material), well below 0.5

microns, which is an approximate threshold below

which broadening starts to appear independently from

other factors like instrumental effects and lattice

strain. The occurrence of other phases with compatible

chemical composition, such as zinc ferrite ZnFe2O4,

was initially discarded by this technique. Neverthe-

less, high-resolution and analytical microscopy exper-

iments described below were carried out to gain

further insight on the structural features of the

nanoparticles under study.

The structural analysis through HRTEM images

(Fig. 2a and Figs. S2a, b in the electronic supplemen-

tary material) allows us to confirm the wurtzite-type

structure previously found by XRD, as illustrated by

the corresponding digital diffraction patterns (DDPs)

Fig. 1 Room temperature XRD of the as obtained

(Zn0.97Fe0.03)O nanoparticles. Labels correspond to P63/mc

ZnO

J Nanopart Res (2013) 15:2120 Page 3 of 10

123

indexed for the space group P63/mc (see Fig. 2b, c).

The micrographs also give account of the stepped and

slightly asymmetric particle surface found in the

samples. For this structural analysis, as for the XRD

analysis, other chemically compatible phases were

considered for more accurate crystallographic phase

identification. The closest possible compound,

ZnFe2O4 (space group Fd-3 m), roughly matches the

symmetry of some DDPs, but it is far from the

experimental intensity and structure factor ratios.

Nonetheless, the precision of the information obtained

with this technique about the spatial distribution of Fe

atoms within the particle is limited. The use of TEM-

based spectroscopic techniques, like EELS and/or

EDX, seems to be more suitable to address this

particular aim. Fig. 2d shows the results of a scanning

transmission electron microscopy (STEM)-EELS

study performed in the so-called spectrum line mode.

In this mode, a series of EEL spectra is acquired at

each pixel within a predefined line over the material.

By analyzing the complete set of spectra, qualitative

and quantitative element distribution can be obtained.

Fig. 2e shows EELS spectra acquired by means of the

spectrum line mode containing signals corresponding

to Zn (L2,3 edges), O (K edge), and Fe (L2,3 edges).

The relative composition extracted from this profile

showed no clustering of Fe atoms (see Fig. 2f) with an

average composition of 3.1 at.% Fe. Further quanti-

tative analyses were carried out by EDX, choosing

point measurements over a number of random loca-

tions throughout the samples. These measurements

confirmed an average composition of 2.7 at.% Fe,

very close to that of the initial precursor employed

during the synthesis, which denotes a good retention of

Fig. 2 HRTEM image (a) and crystallographic analysis (b and

c) of Fe2?:ZnO nanoparticles. d STEM image with an EELS

Spectrum-Line. e Representative EELS spectra showing the

occurrence of Fe and Zn signals. f Compositional profile

showing a roughly homogenous distribution of Fe

Page 4 of 10 J Nanopart Res (2013) 15:2120

123

the dopant within the ZnO lattice. Moreover, the

observed Fe2? homogeneous distribution and the

negligible lattice parameters distortion upon doping

match the results from first principles calculations

demonstrating the stabilization that Fe2? ions bring to

the wurtzite structure of ZnO (Xiao et al. 2011).

Raw magnetization isotherms at 200 and 300 K

(Fig. 3a) showed a ferromagnetic-like feature within

the low field region superimposed to a diamagnetic

signal (Fig. 3a, inset). At 50 and 100 K, the curves

showed a hysteretic feature coupled to a paramag-

netic-like signal (Fig. 3a and inset). Finally, the

ferromagnetic signal featured a very small coercivity

at 10 K and the onset of an approach-to-saturation

behavior was evidenced throughout the whole field

range (Fig. 3a). Given the small measured moment,

the resulting data is prone to interferences—

understood as magnetic contributions different from

that of the sample itself—coming from the magne-

tometer sample mounting elements (containers, hold-

ers, etc.); hence magnetization data were corrected by

subtracting the magnetization curves of an empty

sample container at each selected temperature. The

resulting corrected data is plotted in Fig. 3b. The most

remarkable difference with respect to the raw magne-

tization data is the disappearance of the clear diamag-

netic signal previously observed at 200 and 300 K

(Fig. 3b inset). Since the sample and sample holder/

container magnetic moments may be of the same order

of magnitude, with this correction only the doping-

induced behavior is obtained, as the removal of the

diamagnetism coming from the sample mounting

components is implicit to the chosen processing

method. In addition, a paramagnetic contribution that

may be in principle attributed to isolated Fe2? cations

without magnetic neighbors is also evident after the

above correction. The samples are not superparamag-

netic, as their magnetization curves do not scale with

H/T (Ortega 2011; Bedanta and Kleemann 2009).

Zero-field and field cooled magnetization curves

(ZFC/FC) are shown in Fig. 4a. The ZFC branch reveals

the occurrence of what could be in principle regarded as

a blocking process within the 225–280 K temperature

range. Nonetheless, this situation arises from the

diamagnetic contribution to the overall magnetization,

which remains constant within a narrow temperature

range in the upper end. In relation to other general

aspects commonly found in nanoparticulated materials,

no evidence of a freezing or glassy state transition

temperature is observed. A paramagnetic dependence at

low temperatures and irreversibility between both

curves are also observed. Moreover, in Fig. 4b the FC

magnetization follows the Curie law, which represents

the natural tendency of both ZFC and FC branches when

the sample is cooled at an infinitely slow rate.

Having previously discarded the presence of iron

clusters or Fe3? cations, and taking into account that

Fe2? concentration (0.03) is under the percolation

threshold of the lattice, i. e. &2/Z = 0.18 where Z is

the cation coordination number, Fe2? may be found in

the ZnO lattice as: (1) isolated ions, (2) pairs or dimers,

and (3) clusters of three or more cations. In the cases (1)

and (2), ions are expected to contribute to the overall

magnetization with their own paramagnetic suscepti-

bility, whereas in case (3) paramagnetic ions do not

necessarily order magnetically. The temperature

Fig. 3 Hysteresis loops of (Zn0.97Fe0.03)O nanoparticles at

selected temperatures, featuring a uncorrected data, and

b corrected data. Insets detail of the low field region at the

same scale for comparison

J Nanopart Res (2013) 15:2120 Page 5 of 10

123

dependence of the magnetic susceptibility for isolated

ions is expected to follow a Curie law of the form:

v ¼ xC=T ð1Þ

where C is the Curie constant and x the fraction of

metal cations in the sample. Equation (1) does not hold

for dimers or small clusters, for which a Curie–Weiss

type equation should be used instead :

v ¼ yC

T � hð2Þ

where y represents the fraction of ions forming dimers

and the Weiss constant h is typically of the order -10 to

-100 K (Coey 2011). The theoretic magnetic moment

value of the high-spin state of the free Fe2? ion (spin

momentum S = 2 and orbital momentum L = 2) is

6.71 lB and that of the spin-only ion (S = 2, L = 0) is

4.90 lB (Rhee et al. 2011). The expected C value can be

calculated from the mean-field expression:

C ¼ l0N g2lBS Sþ 1ð Þ=3kBT ð3Þ

where l0 is the permeability of the free space

(4p 9 10-7 T m A-1), N is the number of cations

per unit volume, g the Lande factor and kB the

Boltzmann constant (1.3807 9 10-23 J K-1). On the

one hand, the linear fit of the high-field susceptibility

data from the inverse temperature dependence of the

plot (Fig. 4b) yields the experimental C value by

virtue of Eq. (1). This experimental value from

susceptibility data (1.08910-4 K m3 mol-1) is

acceptably consistent with the g = 2 value of the

Curie constant calculated from Eq. (3)

(1.65910-4 K m3 mol-1) (Flokstra et al. 1973), indi-

cating that iron is not magnetically ordered in these

samples. Additionally, the best linear fit is obtained for

x = 1. On the other hand, fitting the high-field

susceptibility data to Eq. (2) results in unreasonably

high C values and dimer fractions above 1, thus Fe2? is

mostly present in the form of isolated cations.

Based on the results obtained in Fe-doped TiO2 thin

films, Coey et al. developed a model for charge-

transfer ferromagnetism (Coey et al. 2010b). The key

element is the charge transfer mechanism between a

charge reservoir—formed by Fe2? and Fe3? cations—

and a spin-split defect band. In other reports dealing

with similar systems (Kataoka et al. 2010), room

temperature ferromagnetism is explained in terms of

the antiferromagnetic coupling of Fe3? ions in

unequivalent positions. Other models are reviewed

elsewhere (Kittilstved and Gamelin 2006). Given the

absence of Fe3? cations in our samples, and hence the

nonexistent Fe-based charge reservoir, we hypothe-

size that there is a charge-transfer from the ZnO

valence band to Fe2? ions, following a similar

polarization than that observed in Co-doped ZnO

(Schwartz et al. 2003). Even if the dopants were

mainly located at the surface of the nanoparticles, the

coupling between spin-polarized O 2p states can take

place through several atomic layers, giving rise to an

extended ferromagnetic coupling beyond the surface

(Chen et al. 2012). In relation to the way in which Fe2?

would couple to oxygen interstitials, recent first

principles calculations have shown that an antiferro-

magnetic coupling is stable at the lowest temperatures,

Fig. 4 a ZFC/FC curves obtained under a probing field

l0H = 10-2 T, and b molar susceptibility versus 1/T plot

showing the best linear fit of the high-field data (*0.1 B 1/

T \ 0.2)

Page 6 of 10 J Nanopart Res (2013) 15:2120

123

changing to ferromagnetic as temperature increases up

to room temperature (Xiao et al. 2013).

Calcined Fe2? doped ZnO nanoparticles

In order to study the role of induced defects in the

stability of the room temperature ferromagnetism

shown by these Fe2?-doped ZnO nanoparticles, we

coated three different aliquots from the same batch

with three different molecules—namely DDA, DDT

and TOPO—followed by a heat treatment in aerobic

conditions. Room temperature XRD patterns for the

complete series correspond to a ZnO wurtzite-type

structure, keeping the expected intensity ratio between

peaks (Fig. 5). As for the case of the uncoated

nanoparticles, a noticeable peak broadening due to

the small particle size is observed. These results

discard any phase separation or spinodal decomposi-

tion previously reported for other diluted oxide

systems (Moreno et al. 2002; Jamet et al. 2006).

While DDT (Fig. 6a) and TOPO-coated nanoparticles

(Fig. 6b) have a relatively similar average size of

10.36 ± 0.08 and 9.53 ± 0.07 nm respectively, the

DDA-coated ones (Fig. 6c) are slightly bigger, with

12.83 ± 0.10 nm (Fig. S1, electronic supplementary

material). All samples show the same irregular shape.

The nature of the coating molecules visibly affects

the magnetic behavior of Fe2?:ZnO nanoparticles

upon calcination, as demonstrated by the room

temperature hysteresis loops in Fig. 7. From a purely

qualitative point of view, calcination of DDA-coated

particles suppresses the ferromagnetic response pre-

viously observed in uncoated particles, whereas DDT-

coated nanoparticles still preserve a small ferromag-

netic contribution at low fields, approximately similar

to that of the uncoated sample. In the case of TOPO-

coated nanoparticles, calcination enhances the ferro-

magnetic contribution to the overall magnetization,

giving rise to a small coercivity of 10.2 mT and

triplicating the magnetic moment of the uncoated

sample. This magnetization enhancement cannot be

attributed to the formation of any ferrimagnetic iron

oxides due to the aerobic heat treatment, since the

same phenomenon should then be observed in the

other samples. Moreover, XRD diffractograms do not

give account of any other phases different than ZnO.

In previous reports, Kittilstved et al. experimentally

verified that calcination of amines activate room

temperature ferromagnetism in Mn2?:ZnO thin films,

while the use of phosphines yield paramagnetic films

(Kittilstved and Gamelin 2005). The incorporation of

p-type defects (like the shallow acceptor NO2-) via

nitrogen introduction in the ZnO lattice was suggested

to explain the ferromagnetic behavior. These results

are opposing views to that of ours, since the presence

of Fe2? leads to a donor-mediated doping mechanism

(n-type) due to its extra d electron. This implies that

the ferromagnetic order cannot be established in

Fe2?:ZnO nanoparticles via charge delocalization by

using acceptors derived from the aerobical calcination

of DDA molecules, resulting in a clear paramagnetic

behavior (Fig. 7). On the other hand, we have

previously shown that the ferromagnetic response in

TOPO-coated ZnO nanoparticles without transition

metal doping may be the result of: (1) charge transfer

of three electrons from P atoms to Zn vacancies as a

consequence of the formation of a PZn–2VZn com-

plex—with PZn designating Zn substitution by P and

VZn a Zn vacancy—or (2) spin splitting impurity states

formed by P atoms and their nearest O atoms at the

valence band (Ortega et al. 2012). The oxygen-rich

conditions used in the present experiments are known

to stabilize the PZn–2VZn complex, lowering its

formation energy (Lee et al. 2008). Both paths are a

Fig. 5 Room temperature XRD patterns of the DDT, TOPO,

and DDA-capped and subsequently calcined series of

(Zn0.97Fe0.03)O nanoparticles. The uppermost reference pattern

corresponds to uncoated (Zn0.97Fe0.03)O nanoparticles taken

from the original batch. Intensities are not normalized

J Nanopart Res (2013) 15:2120 Page 7 of 10

123

direct consequence of P-doping due to the thermal

decomposition products of TOPO molecules, such as

P2O5, which may act as either donors or acceptors.

Since (Zn0.97Fe0.03)O nanoparticles were already

ferromagnetic before the heat treatment, the available

data does not prove whether there is a synergistic

effect between Fe-doping and TOPO calcination or a

superposition of the effects associated to both the

treatments.

Conclusions

We have presented the synthesis and characterization

of homogeneously doped (Zn0.97Fe0.03)O nanoparti-

cles by a wet chemical route. High-resolution and

analytical electron microscopy results show that: (1)

the doping process proceeds without inducing appre-

ciable changes in the original ZnO cell parameters, (2)

Fe is present in the same oxidation state than its

precursor (Fe2?), and (2) Fe is homogeneously

distributed over the ZnO lattice without forming

clusters or aggregates.

The data obtained from the structural and magnetic

characterization suggests that the nanoparticles are not

uniformly magnetized despite the homogeneous dis-

tribution of Fe in the samples. The magnetic properties

of these (Zn0.98Fe0.03)O nanoparticles cannot be

explained by the standard paradigm based on the

interaction of localized magnetic moments connected

through a Heisenberg exchange integral. Our results

demonstrate that Fe2? suffices to induce roomFig. 6 Conventional TEM images of a DDT, b TOPO, and

c DDA-coated Fe:ZnO nanoparticles

Fig. 7 Room temperature hysteresis loops for DDT, TOPO,

and DDA-coated (Zn0.97Fe0.03)O nanoparticles after aerobic

calcination at 500 �C

Page 8 of 10 J Nanopart Res (2013) 15:2120

123

temperature ferromagnetism in ZnO nanoparticles,

unlike other studies in which the presence of Fe3?,

either experimentally detected or just assumed in the

case of theoretic calculations, is needed to observe a

ferromagnetic-like behavior in these type of com-

pounds (Ganguli et al. 2009; Johnson et al. 2010; Coey

et al. 2010b; Laiho et al. 2010).

Post-processing of several aliquots from the same

batch with different organic coatings and subsequent

heat treatment at 500 �C in aerobic conditions dem-

onstrates how the ferromagnetic response of these

nanoparticles can be further manipulated. No remark-

able magnetization changes take place in DDT-coated

Fe2?:ZnO nanoparticles after the aerobic calcination.

When DDA-coated Fe2?:ZnO nanoparticles are sub-

mitted to the heat treatment, the original room

temperature ferromagnetic signal is canceled, whereas

an enhancement on the same ferromagnetic contribu-

tion is observed when using TOPO as capping agent.

Similarly to the case of undoped ZnO ferromagnetic

nanoparticles, the latter observation is due to either the

charge transfer from P to Zn vacancies or spin splitting

impurity states formed by P atoms and their nearest O

atoms at the ZnO valence band.

Acknowledgments The authors wish to thank the Spanish

Ministry of Science and Technology for financial support

through the project MAT2009-14398.

References

Awschalom DD, Flatte ME (2007) Challenges for semicon-

ductor spintronics. Nat Phys 3(3):153–159

Bedanta S, Kleemann W (2009) Supermagnetism. J Phys D

Appl Phys 42(1):013001

Chen SJ, Suzuki K, Garitaonandia JS (2009) Room temperature

ferromagnetism in nanostructured ZnO-Al system. Appl

Phys Lett 95(17):172507. doi:10.1063/1.3254224

Chen S, Medhekar NV, Garitaonandia J, Suzuki K (2012) Sur-

face charge transfer induced ferromagnetism in nano-

structured ZnO/Al. J Phys Chem C 116(15):8541–8547.

doi:10.1021/jp211523f

Coey JMD (2006) Dilute magnetic oxides. Curr Opin Solid State

Mater Sci 10(2):83–92

Coey JMD (2011) Magnetism in dilute oxides. In: Tsymbal EY,

Zutic I (eds) Handbook of spin transport and magnetism.

CRC Press, Boca Raton, p 408

Coey JMD, Mlack JT, Venkatesan M, Stamenov P (2010a)

Magnetization process in dilute magnetic oxides. IEEE

Trans Magn 46(6):2501–2503. doi:10.1109/tmag.2010.

2041910

Coey JMD, Stamenov P, Gunning RD, Venkatesan M, Paul K

(2010b) Ferromagnetism in defect-ridden oxides and

related materials. New J Phys 12:053025. doi:10.1088/

1367-2630/12/5/053025

Dietl T, Ohno H, Matsukura F, Cibert J, Ferrand D (2000) Zener

model description of ferromagnetism in zinc-blende mag-

netic semiconductors. Science 287(5455):1019–1022.

doi:10.1126/science.287 5455.1019

Flokstra J, Meijer HC, Bots GJC, Verheij WA, Van der Marel

LC (1973) Investigation of paramagnetic saturation in

lanthanum manganese nitrate. Physica 63(2):288–296

Ganguli N, Dasgupta I, Sanyal B (2009) The making of ferro-

magnetic Fe doped ZnO nanoclusters. Appl Phys Lett

94(19):192503

Garcıa MA, Merino JM, Pinel EF, Quesada A, De La Venta J,

Gonzalez MLR, Castro GR, Crespo P, Llopis J, Gonzalez-

Calbet JM, Hernando A (2007) Magnetic properties of ZnO

nanoparticles. Nano Lett 7(6):1489–1494

George S, Pokhrel S, Xia T, Gilbert B, Ji Z, Schowalter M,

Rosenauer A, Damoiseaux R, Bradley KA, Madler L, Nel

AE (2010) Use of a rapid cytotoxicity screening approach

to engineer a safer zinc oxide nanoparticle through iron

doping. ACS Nano 4(1):15–29. doi:10.1021/nn901503q

Han SJ, Song JW, Yang CH, Park SH, Park JH, Jeong YH, Rhie

KW (2002) A key to room-temperature ferromagnetism in

Fe-doped ZnO: Cu. Appl Phys Lett 81(22):4212–4214.

doi:10.1063/1.1525885

Jamet M, Barski A, Devillers T, Poydenot V, Dujardin R, Bayle-

Guillemaud P, Rothman J, Bellet-Amalric E, Marty A,

Cibert J, Mattana R, Tatarenko S (2006) High-Curie-tem-

perature ferromagnetism in self-organized Ge1-xMnx

nanocolumns. Nat Mater 5(8):653–659

Johnson LM, Thurber A, Anghel J, Sabetian M, Engelhard MH,

Tenne DA, Hanna CB, Punnoose A (2010) Transition

metal dopants essential for producing ferromagnetism in

metal oxide nanoparticles. Phys Rev B 82(5):054419

Kataoka T, Kobayashi M, Sakamoto Y, Song GS, Fujimori A,

Chang FH, Lin HJ, Huang DJ, Chen CT, Ohkochi T,

Takeda Y, Okane T, Saitoh Y, Yamagami H, Tanaka A,

Mandal SK, Nath TK, Karmakar D, Dasgupta I (2010)

Electronic structure and magnetism of the diluted magnetic

semiconductor Fe-doped ZnO nanoparticles. J Appl Phys

107(3):033717–033718

Kennedy D, Norman C (2005) What don’t we know? Science

309(5731):75. doi:10.1126/science.309.5731.75

Kittilstved KR, Gamelin DR (2005) Activation of high-Tc fer-

romagnetism in Mn2?-doped ZnO using amines. J Am

Chem Soc 127(15):5292–5293

Kittilstved KR, Gamelin DR (2006) Manipulating polar ferro-

magnetism in transition-metal-doped ZnO: why manga-

nese is different from cobalt (invited). J Appl Phys

99(8):08M112

Laiho R, Ojala I, Vlasenko L (2010) Percolation of ferromag-

netism in ZnO codoped with Fe and Mg. J Appl Phys

108(5):053915

Lee WJ, Kang J, Chang KJ (2008) p-Type doping and com-

pensation in ZnO. J Korean Phys Soc 53(1):196–201

Matsumoto Y, Murakami M, Shono T, Hasegawa T, Fukumura

T, Kawasaki M, Ahmet P, Chikyow T, Koshihara S, Koi-

numa H (2001) Room-temperature ferromagnetism in

J Nanopart Res (2013) 15:2120 Page 9 of 10

123

transparent transition metal-doped titanium dioxide. Sci-

ence 291(5505):854–856

Moreno M, Trampert A, Jenichen B, Daweritz L, Ploog KH

(2002) Correlation of structure and magnetism in GaAs

with embedded Mn(Ga)As magnetic nanoclusters. J Appl

Phys 92(8):4672–4677

Norberg NS, Kittilstved KR, Amonette JE, Kukkadapu RK,

Schwartz DA, Gamelin DR (2004) Synthesis of colloidal

Mn2?:ZnO quantum dots and high-Tc ferromagnetic

nanocrystalline thin films. J Am Chem Soc 126(30):

9387–9398

Ogale SB (2010) Dilute doping, defects, and ferromagnetism in

metal oxide systems. Adv Mater 22(29):3125–3155.

doi:10.1002/adma.200903891

Ortega D (2011) Structure and magnetism in magnetic nano-

particles. In: Thanh NTK (ed) Magnetic nanoparticles:

from fabrication to biomedical and clinical applications.

CRC Press, Boca Raton, FL, pp 3–44

Ortega D, Chen SJ, Suzuki K, Garitaonandia JS (2012) Room

temperature spontaneous magnetization in calcined trioc-

tylphosphine-ZnO nanoparticles. J Appl Phys 111(7):

07C314. doi:10.1063/1.3677674

Pan F, Song C, Liu XJ, Yang YC, Zeng F (2008) Ferromagne-

tism and possible application in spintronics of transition-

metal-doped ZnO films. Mater Sci Eng R Rep 62(1):1–35

Potzger K, Zhou S (2009) Non-DMS related ferromagnetism in

transition metal doped zinc oxide. Phys Status Solidi B

246(6):1147–1167. doi:10.1002/pssb.200844272

Rhee CH, Lee IK, Moon SJ, Kim SJ, Kim CS (2011) Neutron

diffraction and Mossbauer studies of LiFePO4. J Korean

Phys Soc 58(3):472–475

Schwartz DA, Norberg NS, Nguyen QP, Parker JM, Gamelin

DR (2003) Magnetic quantum dots: synthesis, spectros-

copy, and magnetism of Co 2?- and Ni2?-doped ZnO

nanocrystals. J Am Chem Soc 125(43):13205–13218

Uchino T, Yoko T (2012) Symmetry and nonstoichiometry as

possible origins of ferromagnetism in nanoscale oxides.

Phys Rev B 85(1):012407

Xiao J, Kuc A, Pokhrel S, Schowalter M, Parlapalli S, Rosenauer A,

Frauenheim T, Madler L, Pettersson LGM, Heine T (2011)

Evidence for Fe2? in wurtzite coordination: iron doping sta-

bilizes ZnO nanoparticles. Small 7(20):2879–2886. doi:10.

1002/smll.201100963

Xiao J, Frauenheim T, Heine T, Kuc A (2013) Temperature-

mediated magnetism in Fe-doped ZnO semiconductors.

J Phys Chem C 117(10):5338–5342. doi:10.1021/jp400429s

Zhang ZH, Wang X, Xu JB, Muller S, Ronning C, Li Q (2009)

Evidence of intrinsic ferromagnetism in individual dilute

magnetic semiconducting nanostructures. Nat Nanotech

4(8):523–527. doi:10.1038/nnano.2009.181

Page 10 of 10 J Nanopart Res (2013) 15:2120

123