This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

This content was downloaded by: sgoumri

IP Address: 109.171.129.98

This content was downloaded on 30/10/2013 at 08:41

Please note that terms and conditions apply.

The origin of magnetism in transition metal-doped ZrO2 thin films: experiment and theory

View the table of contents for this issue, or go to the journal homepage for more

2013 J. Phys.: Condens. Matter 25 436003

(http://iopscience.iop.org/0953-8984/25/43/436003)

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 25 (2013) 436003 (7pp) doi:10.1088/0953-8984/25/43/436003

The origin of magnetism in transitionmetal-doped ZrO2 thin films: experimentand theory

Nguyen Hoa Hong1, Mohammed Benali Kanoun2,Souraya Goumri-Said3, Jae-Hee Song1, Ekaterina Chikoidze4,Yves Dumont4, Antoine Ruyter5 and Makio Kurisu6

1 Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 151-747, Korea2 KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST),Thuwal 23955-6900, Saudi Arabia3 Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST),Thuwal 23955-6900, Saudi Arabia4 Groupe d’Etudes de la Matiere Condensee (GEMaC), UMR 8635-CNRS, Universite de VersaillesSt Quentin en Yvelines, 45 Avenue des Etats-Unis. F-78035 Versailles Cedex, France5 Laboratoire GREMAN, EMA Pole, UMR 7347 CNRS—Universite F Rabelais, Parc de Grandmont,F-37200 Tours, France6 Department of Physics, Graduate School of Science and Engineering, Ehime University, Matsuyama790-8577, Japan

E-mail: nguyenhong@snu.ac.kr and Mohammed.Kanoun@kaust.edu.sa

Received 19 June 2013, in final form 3 September 2013Published 4 October 2013Online at stacks.iop.org/JPhysCM/25/436003

AbstractWe have investigated the magnetic properties of Fe/Co/Ni-doped ZrO2 laser ablated thin filmsin comparison with the known results of Mn-doped ZrO2, which is thought to be a promisingmaterial for spintronics applications. It is found that doping with a transition metal can induceroom temperature ferromagnetism in ‘fake’ diamond. Theoretical analysis based on densityfunctional theory confirms the experimental measurements, by revealing that the magneticmoments of Mn- and Ni-doped ZrO2 thin films are much larger than that of Fe- or Co-dopedZrO2 thin films. Most importantly, our calculations confirm that Mn- and Ni-doped ZrO2 showa ferromagnetic ground state in comparison to Co- and Fe-doped ZrO2, which favor anantiferromagnetic ground state.

(Some figures may appear in colour only in the online journal)

1. Introduction

Materials for spintronics applications should ideally have bothcharge and spin that can be manipulated at room temperature.After the theoretical suggestion by Dietl in 2000 [1] fordoping Mn into ZnO in order to create magnetic p-typesemiconductors with magnetic interaction via holes, manygroups have conducted many studies on oxides such asZnO, TiO2, SnO2 and HfO2 [2]. Even though they couldbe made ferromagnetic, the mechanism seems to be verydifferent from what was expected: in many cases, it is

proved that transition-metal doping does not play a key role,but the defects/oxygen vacancies should be the source ofmagnetism [3–8]. If the observed ferromagnetism (FM) isindeed due to defects, then it is very difficult to control andmanipulate them to use in applications, so that materialswith ‘real’ intrinsic FM are still well searched. In 2007,Ostanin et al, based on their ab initio electronic structurecalculations, suggested that Mn-doped cubic zirconia, whichis usually known as synthetic diamond, might be a quitepromising candidate for spintronics [9]. They predicted thatMn-doped ZrO2 ceramics could be ferromagnetic above

10953-8984/13/436003+07$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

J. Phys.: Condens. Matter 25 (2013) 436003 N H Hong et al

Figure 1. Representation of the fully relaxed theoretical structure TM-doped ZrO2 used in ab initio simulations.

500 K; the Mn concentration can exceed 40 at.%, and thehigh-TC FM is due to oxygen vacancy defects, and thusfacilitates the Mn impurities to distribute on the Zr fccsublattice [9]. These authors also predicted that similar resultscould be found in Fe- and Co-doped ZrO2. Theoretically,Mn-stabilized ZrO2 is considered as a promising futurespintronics material. However, technically, it is not easyto obtain well stabilized transition metal (TM)-doped ZrO2compounds. Recently, we have obtained certain results onMn-doped ZrO2. Experimentally, it is shown that Mn-dopedZrO2 ceramics are not ferromagnetic, but the nanometer-sizedMn-doped ZrO2 thin films grown on LaAlO3 substrates canbe ferromagnets with TC above 400 K, due to confinementeffects and the special oxygen atmosphere that exists onlyin low dimension materials. The largest saturated magneticmoment (Ms) is huge for the 5% Mn-doped ZrO2 films,and it decreases as the Mn content increases [10]. Theintrinsic FM is strongly associated with the cubic structureof Mn-doped ZrO2, and the Mn–Mn interactions via oxygenintermediates are important. No electrical conductivity isobserved. Mn-doped ZrO2 thin films can be truly excellentcandidates for spintronics applications [10].

In order to check whether this behavior is universalor not, we have carried out detailed experimental andtheoretical studies of Fe/Co/Ni-doped ZrO2, in comparisonwith Mn-doped ZrO2, with the hope that we can shed a lighton fruitful directions for searching for ideal candidates forapplications.

2. Experimental details

TM0.05Zr0.95O2 targets (where TM = Fe, Co, Ni,and Mn) were prepared by a sol–gel method (5% wasexperimentally found to be the optimal concentration forthose dopants). 100 nm thick TM-doped ZrO2 films weredeposited by the pulsed laser deposition (PLD) technique(248 nm KrF excimer laser, repetition rate of 10 Hz) on (001)LaAlO3 (LAO) substrates. The partial oxygen pressure (PO2 )was 10−4 Torr, and the energy density was 2 J cm−2. The

substrate temperature was kept at 600 ◦C. After deposition,all films were annealed at 450 ◦C with the same PO2 asduring the growth for 30 min, and then cooled down slowlyto room temperature under a PO2 of 20 mTorr. All films weredeposited on substrates with the same size (3.5 mm× 3.5 mmor 5 mm × 5 mm, depending on the purpose). Thestructural analysis was carried out by x-ray diffraction (XRD)with Cu Kα radiation. The magnetic measurements wereperformed by a vibrating sample magnetometry (VSM) on aQuantum Design Inc. PPMS-9T platform under a magneticfield applied parallel to the film plane from 0 to 2 T, inthe temperature range from 380 K down to 5 K, and byan atomic force microscope (Cypher-Bruker) operated atroom temperature in zero field. For the VSM measurements,magnetic moment of films and bare LaAlO3 substrates ofexactly the same dimensions were measured under the sameconditions in order to get rid of systematic errors. For theAFM and MFM measurements, the images were taken for anarea of 3.5 µm× 3.5 µm.

3. Computational approach

The TM-doped ZrO2 systems were modeled with supercellbuilt of 2× 2× 1 containing eight conventional cells (namely,16 Zr and 32 O atoms), which corresponds to the dopinglevel of 6.25%, approaching the experimental concentration,as shown in figure 1). Our first-principles calculationswere carried out by using spin-polarized density functionaltheory (DFT) as implemented in Vienna ab initio simulationpackage (VASP) code [11]. The projector augmented wave(PAW) [12] potentials are used to represent the interactionbetween the valence electrons and the core. The generalizedgradient approximation (GGA) in Perdew–Burke–Ernzerhof(PBE) is adopted to describe the exchange–correlationinteractions [13]. A kinetic energy cutoff is set to 520 eV forplane waves included basis set. For sampling the irreduciblewedge of the Brillouin zone, we use the k-point grids of7 × 7 × 13 for cubic supercells. In all calculations, theatomic positions are optimized until the residual forces are

2

J. Phys.: Condens. Matter 25 (2013) 436003 N H Hong et al

Figure 2. XRD patterns for (a) Y; (b) Ni; (c) Fe; and (d) Co-doped ZrO2 thin films (5% of TM; thickness: 100 nm, substrate: LaAlO3).Note that the spectrum of Y-doped ZrO2 film is only shown as a guide for the eye.

smaller than 0.001 eV A−1. To study the magnetic propertiesof TM-doped ZrO2, we carried out spin-polarized calculationsincluding both ferromagnetic (FM) and antiferromagnetic(AFM) configurations.

4. Results and discussion

XRD patterns are shown in figure 2. Very similar to whatwe obtained in Mn-doped ZrO2 thin films [10], all otherTM-doped ZrO2 thin films grown on LaAlO3 substrates aresingle cubic phase and well oriented. Normally, Yttriumdoping is known to very well stabilize the ZrO2 structure,therefore a XRD spectrum of Y-doped ZrO2 is also presentedin figure 2(a) in order to guide the eye for the XRD spectra forour films of Fe–Co–Ni-doped ZrO2 shown in figures 2(b)–(d).For all the TM-doped ZrO2 films, no alien phase is present.

Magnetization versus magnetic field taken at 300 K for5% Mn–Fe–Co–Ni-doped ZrO2 films is shown in figures 3(a)and (b). Note that the data shown are for bare films, aftersubtracting the diamagnetic signals of the LaAlO3 substrates.A curve of magnetization versus temperature taken at 0.5 Tfor the Ni-doped ZrO2 film is shown in figure 3(c) asrepresentative. As one can see, all films show a ferromagneticbehavior at room temperature. However, in comparison to thecase of doping with either Fe or Co, doping with Mn and Niresults in larger magnetic moments (Mn doping ZrO2 has Msthree times larger, while Ni- and Fe-doped ZrO2 has Ms abouttwo times larger than that of Co-doped ZrO2 films). If we tryto attribute the observed FM- to TM-doping, i.e. expressingthe magnetic moment per TM atom, it will lead to the valueof Ms as 13.8 µB/Mn [10]; 0.9 µB/Co; 1.63 µB/Fe, and2.9µB/Ni. To have the magnetization of the sample in the unitof Bohr magnetons per TM atom, one must normalize basedon the number of TM cations thought to be in the sample. Notethat this value of magnetization in Bohr magnetons per TM is

useful only for reference, because it may mislead when theoxides are not real diluted magnetic semiconductors, and theTM cations are not the only source of the induced magnetism.

The intrinsic nature of high TC FM in our TM-dopedZrO2 films is strongly confirmed by MFM measurements.We used an MESP cantilever, which was magnetizedperpendicular to the film plane. The representative Ni-dopedZrO2 film gave clear magnetic signals at room temperature.The roughness of the film is estimated as 1.27 nm. Figure 4(a)shows the topography image taken on the area of 3.5 µm ×3.5 µm. The corresponding phase of MFM (recorded witha lift height of 20 nm), is shown in figure 4(b). Note thatthe MFM image is completely different from the topographyimage, therefore the recorded magnetic signals are certainlyreal, and it is not due to any surface effect. Detectedperpendicular magnetic signals have confirmed the strong FM,which was observed from magnetization measurements. Thedifference in absolute values of the phase is large, going from−0.07◦ to 0.07◦, and it confirms the real signals of a strongFM at room temperature.

In order to investigate the magnetic interaction, wesubstitute one Zn atom by either Mn, Fe Co or Ni, giving riseto a doping level of 6.25%. To determine the magnetic state,the total energy is calculated while the spin of the TM atomsis set parallel or antiparallel, representing ferromagnetismor antiferromagnetism. We define this difference as 1E =EAFM − EFM. Here E is the total energy of a given phaseand negative value for 1E means that AFM is more stablethan the FM phase. We find that Mn- and Ni-doped ZrO2favor the FM phase as their ground state, where FM stateenergy is 0.141 eV and 0.034 eV lower than the AFM stateenergy for Mn and Ni, respectively. On the other hand, Fe-and Co-doped ZrO2 do prefer AFM coupling, where the AFMstate energy is 0.162 and 0.232 eV lower than the FM stateenergy, respectively. Hence, we turn to calculate the magnetic

3

J. Phys.: Condens. Matter 25 (2013) 436003 N H Hong et al

Figure 3. Magnetization versus magnetic field taken at 300 K for (a) Mn/Fe/Co and (b) Ni-doped ZrO2 films (signals of substrates weresubtracted); and (c) versus temperature taken at 0.5 T for Ni-doped ZrO2 films. Field was applied parallel to the film plane.

moment of TM-doped ZrO2. The total magnetic momentson Mn, Fe, Co, and Ni atoms are, respectively, 3.241 µB,0.124 µB, 0.861 µB, and 2.938 µB, and mainly arise fromtheir 3d orbitals 3.211µB, 0.168µB, 1.212µB, and 1.600µB.Note that the theoretical value found for Ni-doping case isthe same as the experimental value reported earlier (2.9 µB).The hybridization between TM-3d and its neighboring O-2porbitals reduces the magnetic moment of a free TM atom.The induced magnetic moments in the nearest-neighbor O porbitals are −0.041 µB, −0.004 µB, 0.028 µB, and 0.077 µBfor Mn, Fe, Co, and Ni, respectively. We also find that the Zrneighbors of TM atoms show smaller magnetic moments of0.011 µB, 0.01 µB, 0.007 µB and −0.004 µB.

To elucidate the effect of TM-doping on the electronicstructure and to obtain information on the role of eachindividual orbital and atom, we address in figures 5(a)–(d), thedensity of states (DOSs) calculated by GGA+U calculationswith the Coulomb interaction parameter U = 3.0 eV, and anexchange interaction parameter, J = 0.87 eV for Mn, Fe, Co

and Ni 3d electrons [14]. Figure 5 provides the total and partialDOSs of the FM calculations for TM-doped ZrO2. It can beseen from DOS and PDOS that the Mn-doped ZrO2 shows alarger majority and partially filled spin DOS. Simultaneously,the minority-spin DOS are fully occupied minority-spin stateunder the Fermi level (EF). For the minority-spin, thereare two adjacent double-splitting peaks above the top ofthe valence band in the minority-spin gap, which can beassociated with the eg–t2g splitting that might be expectedfrom a simple crystal field model. Additionally, the Mn-dopedZrO2 shows double t2g peaks around the EF, indicating anenhanced half-metallicity. In the case of Fe-doped ZrO2, themajority states of Fe atoms lie at the top of the valence band,and the unoccupied minority states of Fe also merge into theconduction band. Moreover, the crystal field splitting of thespin-minority d orbitals, which are between the eg and the t2gstates, are situated above the Fermi level. Thus, the Fe stateshave partially filled 3d manifolds and contribute to the DOSat the Fermi level, leading to half-metallicity. In contrast, the

4

J. Phys.: Condens. Matter 25 (2013) 436003 N H Hong et al

Figure 4. Topography image (a) and the corresponding MFM image (b) of the same area of 3.5 µm× 3.5 µm for the 5% Ni-doped ZrO2film. The tip was magnetized perpendicular to the film plane.

Figure 5. The total and partial spin-polarized DOSs for ferromagnetic TM-doped ZrO2: (a): Mn; (b) Fe; (c) Co; and (d) Ni. Spin up andspin down correspond to positive and negative values, respectively. The vertical dashed line denotes the Fermi level.

5

J. Phys.: Condens. Matter 25 (2013) 436003 N H Hong et al

Figure 6. Localization of spin-polarized electrons for the representative case of Fe-doped ZrO2.

eg minority-spin states of Co atoms are partially occupiedwhile the t2g unoccupied minority states appear within theband gap, making the system metallic. For the Ni-doped ZrO2,we can find that the unoccupied minority t2g states of the Niatoms are separated from the completely filled minority egbelow the Fermi levels, so that for this system, it is clearlyinsulating. In addition, due to the decomposition of the densityof states shown in figure 5, the substitutional transition metalinduced impurity level contains a large contribution of theTM-3d and its nearest neighboring O-2p states. It is clearthat the p–d hybridization mechanism is responsible for thepredicted ferromagnetism.

The spin density contour is very important because itcompletes the DOS and magnetic moment calculations. Infigure 6, we display the contours in a plane (110) wherethe magnetic dopant is located at the center. We have shownthe spin density contours for the Fe dopant but the samebehavior is observed for all the considered magnetic dopants.The form of the contours around the oxygen atoms showstheir polarization, even when the spin-polarized electronsare localized at the TM atom. There are slight contoursaround Zr atoms, which confirm the main conclusion of themagnetic moments and DOS calculations: the polarizationis stronger on O than on Zr atoms. One may conclude thatthe spin-polarization mechanism is similar to what had beenobserved in the recent studies on Sc-doped ZnO thin films [15]and Gd-doped ZnO [16].

5. Conclusion

In this work, we have experimentally and theoreticallyinvestigated the origin of magnetization of laser ablatedtransition-metal doped ZrO2 thin films, which are supposedto be promising materials for spintronics. Certainly, it isshown that doping with a transition metal can induce roomtemperature ferromagnetism in ZrO2 in thin film form. It is

found that the magnetic moments of Mn- and Ni-doped ZrO2thin films are much larger than that of Fe- and Co-dopedZrO2 thin films, indicating that (Mn, Ni) doped ZrO2 certainlyfavors a ferromagnetic ground state while Co- and Fe-dopedZrO2 favor an antiferromagnetic ground state. This findingmay help to direct the materials research in targeting the rightcandidates for spintronics.

Acknowledgments

The authors thank A T Raghavender for some assistancein making targets by sol–gel method, and C K Park forthe Mn-doped ZrO2 samples. We gratefully acknowledgeproject 3348-20120033 of the National Research Foundationof Korea and Ehime University Project for the Promotionof International Relations between SNU and EU for theirfinancial supports. We also acknowledge financial supportfrom the Ile de France region for magnetic measurements(‘NOVATECS’ C’Nano IdF project no. IF-08-1453/R). ARacknowledges P Paruch group at the DPMC of GenevaUniversity for assistance in AFM measurements.

References

[1] Dietl T, Ohno H, Matsukura F, Cibert J and Ferrand D 2000Science 287 12019

[2] Hong N H 2006 J. Magn. Magn. Mater. 303 338[3] Venkatesan M, Fitzgerald C B and Coey J M D 2004 Nature

430 630[4] Hong N H, Sakai J, Poirot N and Brize V 2006 Phys. Rev. B

73 132404[5] Yoon S D, Chen Y, Yang A, Goodrich T L, Zuo X,

Arena D A, Ziemer K, Vittoria C and Harris V G 2006J. Phys.: Condens. Matter 18 L355–61

[6] Hong N H, Sakai J and Brize V 2007 J. Phys.: Condens.Matter 19 036219

[7] Sundaresan A, Bhagavi R, Rangarajan N, Siddesh U andRao C N R 2006 Phys. Rev. B 74 161306(R)

6

J. Phys.: Condens. Matter 25 (2013) 436003 N H Hong et al

[8] Hong N H, Poirot N and Sakai J 2008 Phys. Rev. B 77 33205[9] Ostanin S, Ernst A, Sandratskii L M, Bruno P, Dane M,

Hughes I D, Staunton J B, Hergert W, Mertig I andKudrnovsky J 2007 Phys. Rev. Lett. 98 016101

[10] Hong N H, Park C-K, Raghavender A T, Ruyter A,Chikoidze E and Dumont Y 2012 J. Magn. Magn. Mater.324 3013

[11] Kresse G and Furthmuller J 1996 Phys. Rev. B 54 11169[12] Blochl P E 1994 Phys. Rev. B 50 17953

[13] Perdew J P, Burke K and Ernzerhof M 1996 Phys. Rev. Lett.77 3865

[14] Park M S, Kwon S K and Min B I 2002 Phys. Rev. B65 161201(R)

[15] Kanoun M B, Goumri-Said S, Manchon A andSchwingenschlogl U 2012 Appl. Phys. Lett. 100 222406

[16] Bantounas I, Goumri-Said S, Kanoun M B, Manchon A,Roqan I and Schwingenschlogl U 2011 J. Appl. Phys.109 083929

7