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Correlation between oxygen vacancies and magnetism in Mn-doped Y2O3nanocrystals investigated by defect engineering techniquesT. S. Wu, Y. C. Chen, Y. F. Shiu, H. J. Peng, S. L. Chang et al. Citation: Appl. Phys. Lett. 101, 022408 (2012); doi: 10.1063/1.4732094 View online: http://dx.doi.org/10.1063/1.4732094 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i2 Published by the American Institute of Physics. Related Articles

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Correlation between oxygen vacancies and magnetism in Mn-doped Y2O3

nanocrystals investigated by defect engineering techniques

T. S. Wu,1 Y. C. Chen,1 Y. F. Shiu,1 H. J. Peng,1 S. L. Chang,1,2 H. Y. Lee,3 P. P. Chu,3

C. W. Hsu,4 L. J. Chou,4 C. W. Pao,2 J. F. Lee,2 J. Kwo,1,5 M. Hong,6 and Y. L. Soo1,2,a)

1 Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan2 National Synchrotron Radiation Research Center, Hsinchu, Taiwan3 Department of Chemistry, National Central University, Jhongli 32001, Taiwan4

 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan5Center of Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan6 Department of Physics, National Taiwan University, Taipei 10617, Taiwan

(Received 21 March 2012; accepted 13 June 2012; published online 11 July 2012)

Defect engineering techniques have been employed to generate and remove oxygen vacancy

defects in nanoparticles of Y2O3:Mn diluted magnetic oxide (DMO). These samples were prepared

by thermal decomposition method followed by a series of thermal annealing in oxygen and

forming gas. The x-ray absorption analysis reveals that O vacancies surrounding Mn and Y atoms

were appreciably increased by forming-gas-annealing and decreased by oxygen-annealing,

accompanied by enhanced and reduced saturation magnetization as demonstrated by magnetic

measurements, respectively. Our results demonstrate strong correlation between magnetism and O

vacancies and therefore strongly support the bound magnetic polaron model for these high-k

DMOs.VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4732094]

The advance of spintronic technology has aroused con-

siderable interest in the study of diluted magnetic semicon-

ductors (DMS) and diluted magnetic oxides (DMO). Of 

special interest in the present research are the magnetic-

ion-doped high-k dielectric systems. Unlike DMS where

itinerant carriers are responsible for mediating exchange

interaction between magnetic ions, these insulating materials

are highly resistive, and therefore such carrier-mediated fer-

romagnetism cannot account for their magnetic ordering.

One of the most promising models proposed for DMO sys-

tems is the bound magnetic polaron (BMP) model, in whichoxygen vacancies play a central role for their ferromagnet-

ism.1,2 Regardless of the great interest of the BMP model,

many reports have suggested that ferromagnetism in DMO

may arise from magnetic ions clustering on grain boundaries

instead of ordered dopant moments inside the hosts.3,4 We

have previously investigated the annealing effects on

Co-doped Y2O3 nanocrystals and demonstrated the impor-

tance of grain boundaries in affecting the ferr omagnetism of 

DMOs without invalidating the BMP model.5 However, to

warrant the usefulness of DMOs for spintronic applications,

further investigation for the validity of the BMP model using

DMO samples free of grain-boundary effects and possible

ferromagnetic clusters is needed. To this end, lower anneal-ing temperature around 350 C and Mn dopant, which is anti-

ferromagnetic in its metal state, has been adopted in the

present work. Oxygen and forming gas were alternately used

in such low-temperature thermal annealing to vary the oxy-

gen vacancies in Mn-doped Y2O3 nanocrystals. Various ex-

perimental techniques such as x-ray diffraction (XRD), high-

resolution transmission electron microscopy (HRTEM), and

x-ray absorption fine structures (XAFS) were employed to

probe the structures of the material system at different scales

and then compared with the magnetic results obtain from

superconducting quantum interference device (SQUID)

measurements. It is worth noting that the high surface to vol-

ume ratio of nanoparticles makes it easier to engineer oxygen

vacancy defects (OVD) into or out of the DMO hosts. There-

fore, nanocrystals of DMO is an ideal platform for investi-

gating the effects of OVD and the validity of BMP model in

the ferromagnetism of DMO insulators using defect engi-

neering techniques.6

Samples of Mn-doped Y2O3 nanoparticles were synthe-sized using a thermal decomposition method.7 A slurry of 

Y(acac)3ÁxH2O (3 mmole) (acac¼ acetylacetonate) and

Mn(acac)3 in oleylamine solvent (30 ml) was vigorously

stirred under nitrogen purge for 30 min to remove oxygen

and moisture, which may lead to formation of manganese

oxide, and then heated to 200 C for 120 min in nitrogen.

The resulting reacting mixture was cooled down to room

temperature to form a light gray suspension. A white precipi-

tates were collected from the suspension by centrifugation

and repeatedly washed with a mixture of deionized water 

and ethanol. A white suspension was prepared by adding

ether to the white precipitates and then sonicated for 20 min

to form a clear solution. Finally, the solution is centrifugedat 5000 rpm for an hour to remove insoluble components and

then dried in an oven to form a white powder which can be

easily re-dispersed in many organic solvents such as

dichloromethane. To engineer oxygen vacancy defects into

and out of the samples, the as-made powders (sample M0)

were sequentially annealed at 350 C for 30 min in oxygen

(sample M1), 300C for 30 min in forming gas (5%H2/ 

95%N2) (sample M2), and 300C for 30min in oxygen

again (sample M3).

The long-range-order structures for all samples were

determined from XRD patterns as plotted in Fig. 1. The

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

soo@phys.nthu.edu.tw.

0003-6951/2012/101(2)/022408/5/$30.00 VC 2012 American Institute of Physics101, 022408-1

APPLIED PHYSICS LETTERS 101, 022408 (2012)

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HRTEM shown in Fig. 2 reveal the formation of nano-sized

particles in the as-made powder and agglomeration of nano-particles in the annealed samples. X-ray absorption near-

edge fine structure (XANES) at Mn K-edge were measured

for each samples, as shown in Fig. 3, to estimate the effective

valency of the Mn dopant in Y2O3 host. Local structures sur-

rounding Y atoms in the host and Mn dopant atoms were

probed by extended x-ray absorption fine structure (EXAFS)

technique. The XANES and EXAFS measurements were

performed at beamline BL07A of Taiwan Light Source at

National Synchrotron Radiation Research Center (NSRRC)

in Taiwan. Conventional fluorescence mode of detection was

adopted using Lytle fluorescence detector for all samples.8

An established data reduction method was used to

extract the EXAFS functions from the raw experimental

data.9 The EXAFS v functions of the Y K-edge and Mn

K-edge EXAFS are then Fourier-transformed into real space

and plotted as fine lines in Figs. 4 and 5, respectively. Local

structural parameters were quantitatively extracted from the

EXAFS functions using an improved curve-fitting procedure

with back scattering amplitude and phase shifts functions

obtained from the FEFF software.8,10 The amplitude reduction

factor S02 representing the central atom shake-up and shake-

off effects and the mean free path of photoelectrons k were

set to be 0.8 and 10 A for Y data and 0.8 and 10A for Mn

data as determined in previous papers.8,11 The final values of 

fitting parameters for the Y K-edge and Mn K-edge EXAFS

are listed in Tables II and III, respectively. To study the cor-

relation between structures and magnetic properties, satura-

tion magnetization per Mn atom for all powder samples was

measured using a quantum design SQUID magnetometer.

The M-H curves for these samples are plotted in Fig. 6. The

doping level of the samples was determined by inductively

coupled plasma mass spectrometry (ICPMS).

As shown in Fig. 1, the XRD data indicate that the as-

grown sample M0 is largely amorphous with only two humps

FIG. 1. X-ray powder diffraction patterns for the Mn doped (upper) samples

and standard reflection pattern of cubic Y2O3 (JCPDS #89-5592). Curves

have been shifted vertically for the sake of clarity.

FIG. 2. TEM micrographs for (a) sample M1, (b) sample M2, and (c) sam-ple M3. (d) A typical electron diffraction pattern.

FIG. 3. (a) Mn K-edge XANES spectra of sam-

ple M1 and model compounds. (b) Mn K-edge

XANES spectra for Mn-doped Y2O3 samples.

022408-2 Wu et al. Appl. Phys. Lett. 101, 022408 (2012)

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arising from the background. After annealing, the XRD pat-

terns for samples M1, M2, and M3 match well with that of 

cubic Y2O3 at the (211), (222), (400), (440), and (622) Bragg

peaks. The XRD measurements demonstrate a highly crystal-

line nature of these Mn-doped Y2O3 nanoparticles and

preliminarily exclude substantial formation of manganese

oxide in the samples. The electron diffraction (SEAD) of the

nanoparticles also show strong Y2O3 ring patterns due to the(222), (400), (440), and (622) planes as shown in Fig. 2(d).

The crystallite sizes listed in Table I were determined from

the four major Bragg peaks in the XRD data by using both

Scherrer equation and Williamson-Hall plot. From the

HRTEM micrographs in Figs. 2(a) – 2(c), we can see that the

nanoparticles slightly agglomerate after annealing. However,

the particle sizes show no dramatic change from the average

value around 10 nm during the annealing process as esti-

mated by both Scherrer equation and Williamson-Hall plots,

which is in good agreement with the TEM results. The TEM

micrographs also reveal that nanoparticles in all samples are

in the form of single crystals and have nearly spherical

shapes.

As demonstrated in Fig. 3(a), the XANES spectra of all

Mn doped samples show distinctly different features from

those of Mn metal, MnO, Mn3O4, Mn2O3, and MnO2. This

indicates that the Mn atoms have most likely been incorpo-

rated into the Y2O3 host instead of forming separated metal

or oxide phases. As shown in Fig. 3(b), the Mn absorption

edge of sample M1 shifts to higher energy compared to that

of M0 indicating an increase of Mn valency due to oxygen-

atmosphere annealing of the as-made M0. The edge of sam-

ple M2 however shifts back to the M0 value showing that

annealing in forming gas can indeed effectively reduce the

valency of Mn in the sample. Finally, annealing M2 in oxy-gen gas shifts the Mn edge and valency of the resulting sam-

ple M3 to the oxidized M1 values again. In the absence of 

phase separation and long-range-order structural changes as

FIG. 4. Y K-edge EXAFS data for Mn-doped Y2O3 samples. Fine lines: ex-

perimental; Coarse lines: curve fitting. Curves have been shifted vertically

for the sake of clarity.

FIG. 5. Mn K-edge EXAFS data for Mn-doped Y2O3 samples. Fine lines:

experimental; Coarse lines: curve fitting. Curves have been shifted vertically

for the sake of clarity.

FIG. 6. M-H curves for Mn-doped samples measured at 10K. Inset: a

zoom-in image at low-field region.

TABLE I. Average diameters of Y2O3 nanoparticles determined from XRD

data.

Diameter (nm)

Sample Scherrer equation Williamson-Hall plot

M1 10.4 9.0

M2 10.2 8.9

M3 9.2 9.4

022408-3 Wu et al. Appl. Phys. Lett. 101, 022408 (2012)

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revealed by the XRD data, the increase and decrease of Mn

valency may indicate decrease and increase of OVD concen-tration surrounding Mn atoms due to annealing in oxygen

and forming gas, respectively. Such reasoning is to be con-

firmed by the EXAFS analysis described below.

The local structures surrounding Y and Mn probed by

EXAFS also show appreciable variations due to different gas

annealing processes. As shown in Fig. 4 and Table II, the as-

made sample M0 has only one oxygen near neighboring shell

around Y at distance 2.33A with coordination number 

7.360.1, consistent with its largely amorphous nature

exhibited by XRD. After thermal annealing at 300–350 C,

more distant neighboring shells start to show up in the

EXAFS data. The oxygen-ambience-annealed sample M1

shows a nearest O neighboring shell with a lowered coordi-

nation number of 6.36 0.4 and a shortened distance of 

2.2666 0.005 A, as well as two Y near shells of coordination

number 6.76 0.6 and 4.560.9 at 3.5076 0.005 A and

4.0316 0.005 A, respectively. The decreased oxygen-shell

coordination number and shortened Y–O bond length as

compared to those of the as-made sample M0 indicate that

hydroxides in the as-made sample has been largely trans-

formed into oxides in M1 due to oxygen-ambience anneal-

ing. This initial annealing also has drastically improved the

crystallinity of the sample such that two more distant shells

appear in the EXAFS data of M1, which shows clear XRD

pattern of Y2O3, as compared to the one-shell-only EXAFSof the largely amorphous M0. When sample M1 is further 

annealed in forming gas ambience, the resulting sample M2

has a nearest neighboring shell with a decreased number of 

5.46 0.4 O atoms at 2.2686 0.005 A and two Y near shells

of coordination number 5.760.6 and 4.360.9 at 3.515

6 0.005 A and 4.0356 0.005 A. The decreased coordination

numbers indicate that a large number of OVDs have been

engineered into M2 by forming-gas annealing. Finally, after 

annealing sample M2 in oxygen ambience again, the result-

ant sample M3 shows a nearest O shell with an increased

coordination number of 5.66 0.3 at distance 2.263

6 0.005 A and two Y near shells of coordination number 5.86 0.6 and 6.36 1.2 at 3.50260.005 A and 4.028

6 0.005 A. We note that the final oxygen annealing has re-

moved OVDs in M2 to yield EXAFS parameters excellently

consistent with those calculated from the Y2O3 structure.

In contrast to the relatively rich features in the Y K-edge

EXAFS data, the Fourier transforms of Mn K-edge EXAFS

show only one pronounced peak representing the first O shell

for all samples investigated. As shown in Table III, the

Mn–O bond length for all Mn-doped samples is around

1.89 A which is very different from the Y–O bond length of 

around 2.27 A. This indicates that Mn atoms in all Mn-doped

samples most likely occupy interstitial sites in the samples.

Similar to the systematical variation of the coordination

number for the nearest (O) shell surrounding Y atoms, the

number of nearest O neighboring atoms surrounding Mn was

also increased by oxygen-ambience annealing and decreased

by forming-gas-ambience annealing, in good agreement with

the Mn K-edge XANES results. As listed in Table III, the

nearest (O) shell coordination number for the as-made sam-

ple M0 is 3.260.2. After annealing in oxygen atmosphere,

such number was increased to 4.360.2 in sample M1.

Annealing M1 in forming gas brought the number down to

3.46 0.2 in sample M2. The second oxygen-ambience

annealing finally increased the oxygen-shell coordination

number back to 4.46

0.2 in sample M3. It is worth notingthat we can easily engineer OVDs into the immediate vicin-

ity around both Mn dopant atoms and Y constituent atoms in

these nanocrystal systems by means of forming-gas-ambi-

ence annealing at a moderate temperature around 300C

without causing apparent surface-bound migration of dopant

atoms. On the other hand, removal of OVDs can also be

achieved by oxygen-ambience annealing at the same temper-

ature. Therefore, we have demonstrated an effective defect

engineering method for these doped nanocrystal systems.

In Fig. 6, we show the M-H curves measured by SQUID

at temperature 10 K with applied magnetic field up to a max-

imum of 5 T for the Mn-doped samples. The M-H curve of 

an undoped Y2O3 nanoparticle sample shows a diamagneticcurve indicating that the magnetic ordering in the Mn-doped

samples is predominantly due to the dopant atoms. These

curves exhibit a clear dependence of average Mn magnetic

moment on the annealing procedures. The magnetic moment

per dopant atom systematically increases and decreases

when the samples are annealed in forming gas and oxygen,

respectively. The XANES spectra and EXAFS analysis have

demonstrated that oxygen atoms surrounding the Mn dopant

atoms in Y2O3 nanoparticle host can be effectively removed

by forming-gas annealing and replenished by oxygen anneal-

ing leading to increased and decreased concentration of 

OVDs, respectively. In the framework of the bound magnetic

polaron model, increased number of oxygen vacancies

TABLE III. Parameters of local structure around Mn atoms obtained from

curve-fitting of the Mn K-edge EXAFS.

Sample Bond N R (A) r2

(10À3

A2

) D E (eV)

M0 Mn–O 3.26 0.2 1.90160.005 2.86 0.5 1.06 0.8

M1 Mn–O 4.36 0.2 1.89060.005 4.76 0.5 2.66 0.7

M2 Mn–O 3.46 0.2 1.8946 0.005 4.06 0.5 0.76 0.8

M3 Mn–O 4.46 0.2 1.8926 0.005 5.36 0.5 1.96 0.8

TABLE II. Parameters of local structure around Y atoms obtained from

curve-fitting of the Y K-edge EXAFS. N is the coordination number. R is

the bond length. r2 is the Debye-Waller-like factor serving as a measure of 

local disorder. DE0 is the difference between the zero kinetic energy value

of the sample and that of the theoretical model used in FEFF. Uncertainties

were estimated by the double-minimum residue (2v2) method.

Sample Bond N R (A) r2 (10À3

A2

) D E (eV)

M0 Y–O 7.360.1 2.3266 0.005 9.36 0.3 À4.46 0.5

M1 Y–O 6.360.4 2.2666 0.005 8.46 0.6 À6.860.5

Y–Y 6.760.6 3.5076 0.005 7.26 0.4 À10.16 0.5

Y–Y 4.560.9 4.0316 0.005 6.36 0.5 À0.661.6

M2 Y–O 5.460.4 2.2686 0.005 7.56 0.6 À7.265.0

Y–Y 5.760.6 3.5156 0.005 7.06 0.4 À10.76 0.5

Y–Y 4.360.9 4.0356 0.005 7.26 5.0 À1.761.3

M3 Y–O 5.660.3 2.2636 0.005 7.36 0.6 À7.565.0

Y–Y 5.860.6 3.5026 0.005 6.56 0.6 À10.96 0.5

Y–Y 6.361.2 4.0286 0.005 8.46 0.5 À1.061.4

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surrounding Mn atoms can enhance the ferromagnetic

exchange interaction between Mn atoms. Therefore, the

decreased (increased) number of oxygen neighboring atoms

surrounding Mn can explain for the increased (decreased)

magnetic moment per dopant atom after annealing in a form-

ing gas (oxygen gas) atmosphere. To calculate the magnetic

moment per Mn atom, the Mn concentration in the Mn-

doped samples was determined by ICP-MS measurements to

be around 7.37 at. %. Possibility of substantial Fe contamina-tion from our stainless steel devices was also ruled out by

ICP-MS. The Fe concentration was found to be as small

as 440 ppm.

As a side remark, we note that electron-mediated mag-

netism has recently been reported for Co-doped TiO2.12

However, the band gap of our high-k Y2O3 system is nearly

twice as large as that of the semiconducting TiO2. It is thus

much more difficult to generate itinerant electrons required

for the carrier-mediated magnetism in our samples. We have

repeatedly carried out electric measurements on pellets com-

pressed from our nanocrystal Y2O3:Mn powders, as well as

one from nanocrystal TiO2. The observed resistivity of our 

Y2O3:Mn pellets turned out to be at least two orders of mag-

nitude larger than that measured from the TiO2 pellet. It is

worth noting that annealing in forming gas does not lead to

appreciable resistivity change in our samples. Furthermore,

electric measurements on Y2O3:Co thin films grown by mo-

lecular beam epitaxy also yield the same conclusion. There-

fore, the electron-mediated mechanism reported for the

semiconducting Co-doped TiO2 is unlikely the correct model

for our highly resistive DMO systems. The BMP model, on

the other hand, suits the observed magnetic behaviors rather 

well. We also note that a non-ferromagnetic model for hys-

teresis loops of  semiconducting Zn1ÀxCoxO DMO was pro-

posed recently.

13

More in-depth experimental works arerequired to investigate the validity of such model in our insu-

lating Y2O3:Mn systems in the future.

In conclusion, we observed promising magnetic order-

ing of Mn-doped Y2O3 nanoparticle samples prepared by a

thermal decomposition method. Employing the HRTEM,

XRD, EXAFS, and SQUID techniques, we have monitored

the variations in particle size, long-range-order crystal struc-

tures, short-range-order chemical environments surrounding

Y and Mn, and magnetic moment per dopant atom in the

samples, respectively. When annealed in oxygen or forming

gas atmosphere, the nanoparticles can be effectively oxidized

or reduced as showed in the XAFS analysis. We have dem-

onstrated engineering of OVDs using atmosphere annealing.

As suggested by the SQUID results, the increased number of 

oxygen vacancies surrounding Mn atoms has led to the

increase of magnetization after forming gas annealing. On

the other hand, the decreased number of oxygen vacancies

surrounding Mn atoms has led to the decrease of magnetiza-tion after oxygen gas annealing. Our experimental work has

demonstrated effective control of OVDs in Mn-doped Y2O3

nanoparticles using defect engineering techniques and the

results have lent a strong support for the validity of bound

magnetic polaron model in these high-k DMO dielectric

systems.

The present research has been supported by National

Science Council, Taiwan (Project No. 100-2112-M-007-015-

MY3) and by Academia Sinica (Project No. AS-98-TP-

A03).

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