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Evolution of magnetic nanophases of Ni embedded in Al2O3 (0 0 1) matrixby X-ray magnetic circular dichroism

P. Thakur a,⇑, Ravi Kumar b,1, J.C. Cezar a, N.B. Brookes a, Aditya Sharma c, S.K. Arora d, S. Gautam e,Arvind Kumar f, K.H. Chae e, I.V. Shvets d

a European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, Franceb Centre for Materials Science and Engineering, National Institute of Technology, Hamirpur, 177 005 HP, Indiac Material Science Research Laboratory, Department of Physics, S.V. College, Aligarh, 202 001 UP, Indiad CRANN, School of Physics, Trinity College Dublin, Dublin 2, Irelande Nano Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Koreaf Beant College of Engineering & Technology, Gurdaspur 143521, India

a r t i c l e i n f o

Article history:Received 11 August 2010In final form 10 November 2010Available online 13 November 2010

a b s t r a c t

We report on the element-specific magnetic characterization of Ni nanoparticles (NPs) embedded inAl2O3 (0 0 1) matrix prepared by ion-implantation (Ni:5 � 1016–2 � 1017 ions cm�2) technique. TheX-ray diffraction studies reveal the coexistence of fcc Ni NPs of average size �8–19 nm along with theother NiOx phases. X-ray magnetic circular dichroism (XMCD) experiments demonstrate the competingnature of magnetic interactions between the Ni NPs and NiOx phases. The ferromagnetism at 10 K (forall the Ni implants) is related to the combined contribution of both phases, while the room temperaturesuperparamagnetism at higher fluence is dominated by Ni NPs phase as confirmed by Ni L3,2 XMCDmeasurements.

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A common characteristic of metal/metal oxide nanostructuredmagnetic materials [so-called magnetic nanoparticles (NPs)] isthe coexistence of two or more phases, magnetically and/or struc-turally different, which are modulated on a length scale of the orderof a nanometer. Interactions between different magnetic phases ofNPs have attracted both fundamental and technological researchinterest, motivated by the challenge of discovering and understand-ing phenomena related to the finite-size effects, surface/interfaceeffects, including symmetry breaking and lattice-host exchangeinteractions [1–3]. There has been a great deal of recent interestin the incorporation of metal or metal oxide NPs into dielectricand semiconductor matrices to form nanocomposites [4–12]. MetalNPs in dielectric matrix could have potential applications in nonlin-ear optics and high density magnetic storage devices or magneticsensors [13,14]. Various synthesis routes of ferromagnetic NPs fab-rication are being currently employed and continuously improved;the aim is to improve the control over the size distribution, surfacemorphology, and stability of nanostructures. Negative-ion implan-tation is one of the promising method to fabricate metal NPs,because of little surface charging, high-purity process and goodcontrollability in fluence, spatial position and depth [15]. In spiteof huge efforts in the fabrication and characterization techniques

of metal NPs, there are many unanswered questions concerningthe development of magnetic order on a microscopic scale. In thiscontext, the spin-dependent coupling mechanisms and the pres-ence of surrounding medium (electronic environment/chargetransfer) play an important role in determining their magneticproperties.

In this Letter, we present the X-ray absorption (XAS) and X-raymagnetic circular dichroism (XMCD) characterizations of Ni NPsembedded in the sapphire (Al2O3-0 0 1) matrix prepared by Niion beam implantation. We study the systematic evolutions ofmagnetic phases of Ni NPs as a function of different Ni fluencesand their magnetic response in the presence of other NiOx phases.

Implantation of 80 keV Ni� ions (current density of1.5 lA cm�2) was performed in a Al2O3 matrix at room tempera-ture (RT) in a vacuum chamber of 1.3 � 10�7 Torr using source ofnegative ions by Cesium sputtering at Inter University AcceleratorCentre, New Delhi. The implantation fluence was varied in therange of 5 � 1016 to 2 � 1017 ions cm�2. Then the implanted sam-ples were post-annealed in air at 600 �C for 4 h to recover fromradiation/structural damage caused by ion-implantation. Accord-ing to Monte-Carlo ion-range simulation code SRIM2006, theprojected range and straggling of Ni ions of 80 keV are �38 and�11 nm, respectively, in Al2O3 matrix. The formation of the Ninanostructures was observed initially by high-resolution X-ray dif-fraction (HRXRD) with k = 1.5425 Å, at the bending magnet 10BXRS KIST-PAL beamline of the Pohang Accelerator Laboratory(PAL). Figure 1 shows the HRXRD patterns (in log scale) for all

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⇑ Corresponding author.E-mail address: thakur@esrf.fr (P. Thakur).

1 On extraordinary leave from Inter University Accelerator Center, New Delhi 110067, India.

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the implants (Ni: 5 � 1016–2 � 1017 ions cm�2) along with the bareAl2O3 substrate. As evident, a characteristic peak at 2h = 44.5� cor-responding to the fcc phase of Ni(1 1 1) is clearly observed. In addi-tion to the fcc Ni(1 1 1) phase, pattern also shows the presence ofother Ni oxide phases (NiOx) [16], which are at variance with Nifluences (marked by symbol ‘*’ in the Figure 1). It is well knownthat the crystal structure of Ni and its NiOx phases is same and onlythe lattice parameters are different, therefore, Ni/NiOx Bragg‘sreflections occur at different 2h values. From the diffraction profileanalysis of the Ni (1 1 1) peak, an average size of Ni nanostructures(Ni: 5 � 1016–2 � 1017 ions cm�2) was calculated to be �8–19 nmby using Scherrer relation (D = 0.9k/b Cosh, where b is the fullwidth at half maximum of 2h in radians). We have also estimatedthe average size of the other NiOx phases as to be �4–5 nm for allthe samples. It is interesting to observe that there is no major in-crease in its value with the increase in implantation dose.

To understand the bulk magnetic properties of Ni NPs, the iso-thermal magnetization hysteresis measurements were performedat different temperatures (10–300 K) using a vibrating samplemagnetometer (Quantum Design – Physical Property Measure-ments System, USA) with a sensitivity of 5 � 10�7 emu. Figure 2shows the magnetization versus magnetic field (M–H) curve forall the implants at 10 K (top panel) and 300 K (bottom panel).The diamagnetic contribution from the Al2O3 was subtracted fromthe measured data by performing the magnetization of the un-im-planted Al2O3 sample with similar dimensions at RT. The M-H datapresented in the Figure 2 is normalized with respect to the higherimplantation fluence (Ni:2 � 1017 ions cm�2). As evident, samplesexhibits a well defined magnetization hysteresis and show ferro-magnetic behavior at 10 K and their magnetic response increaseswith the Ni fluences. However, the magnetic properties of the im-plants at RT is very intriguing, the lower fluence samples show aweak ferromagnetism with observable coercive force and magneticremanence, while the higher fluence (Ni:2 � 1017 ions cm�2) sam-ple exhibits a superparamagnetic like behavior. It is recalled that

the surface morphology and magnetic domain formation of allthe implants was also investigated by atomic force microscopy(AFM)/magnetic force microscopy (MFM) using Digital Nano-scope-III. As a representative, insets in the Figure 2 display theAFM (bottom panel) and MFM (top panel) images taken at RT forthe higher fluence (Ni:2 � 1017 ions cm�2) sample. A surfaceroughness was estimated to be �13 nm (through AFM), whileMFM image (taken at the 30 nm lift height of the tip) shows thecorresponding magnetic contrast of the implants. It is worth men-tioning that before annealing no AFM/MFM structure was observedfor any implantation dose. The observed MFM image can be char-acterized by darker regions adjacent to the brighter regions in thenanometer scale and the magnetic regions are seems to be distrib-uted uniformly along the plane of the Al2O3 matrix. The averagecontrast between dark and bright areas is measured by means ofsection analysis [17]. From the analysis of the MFM data, we havecalculated the corresponding root mean square (RMS) phase shiftof the as-implanted/annealed samples. The value of phase shift isfound to be increased systematically from �1.01o (as-implantedsamples) to �1.75o–2.54o (Ni:5 � 1016–2 � 1017 ions cm�2) forthe annealed samples, which undoubtedly indicates the formationof magnetic nanostructure of the Ni implants at the surface of theAl2O3 matrix. Since all the samples exhibit fluence/temperaturedependent magnetic properties, we have carried out the zero-field-cooling (ZFC) and field-cooling (FC) magnetization as a func-tion of the temperature (5–300 K). Figure 3 shows the ZFC and FCcurves of the Ni implants (Ni:5 � 1016 and 2 � 1017 ions cm�2)recorded with an applied magnetic field of l0H = 10 mT. In theseZFC/FC curves, the temperature at the position with maximummagnetization is known as the blocking temperature (TB), abovewhich the sample loses all its hysteric response. As evident, thehigher fluence sample shows a superparamagnetic behavior aboveTB � 180 K, while the low fluence samples are ferromagnetic at RT.This can be related to the competing nature of magnetic interac-tions between fcc Ni(1 1 1) NPs and NiOx phases. We will show

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Figure 1. HRXRD pattern of all the implants (Ni: 5 � 1016–2 � 1017 ions cm�2) inAl2O3 matrix. A characteristic peak at 2h = 44.5� corresponding to the fcc phase ofNi(1 1 1) is clearly visible along with other NiOx peaks marked with symbol ‘�’.

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in the following that the fluence-dependent destabilization ofmagnetism in the Ni NPs, and the presence of other NiOx phasesare playing an important role in determining their magneticproperties.

In order to investigate the structural/chemical environment ofNi NPs and its subsequent influence on the magnetic behavior withincreasing fluence, we have carried out XAS and XMCD measure-ments at the Ni L3,2 edge. The XAS/XMCD experiments were per-formed at the ESRF‘s ID08 beamline, which uses an APPLE II typeundulator giving �100% linear/circular polarization. All scans wererecorded simultaneously in both total electron yield (TEY) andtotal fluorescence yield (TFY) modes, ensuring both surface (TEY)and bulk (TFY) sensitivities. The spectra were normalized to inci-dent photon flux and the base pressure of the experimental cham-ber was better than 3 � 10�10 Torr. The samples were aligned at anangle of 45o between the surface normal and the incident beam.Since Ni NPs are embedded in the insulating matrix (Al2O3), thesurface charging effect of the samples was severe in the TEY mode.Moreover, to avoid contributions from the surface contaminations(air exposure etc.), all the spectra presented here are in the TFYmode. It should be noted that the absence of any surface contribu-tion to the TFY mode spectra was further confirmed by performingXAS/XMCD experiments of the in situ sputtered samples with anAr+ ion beam (1 keV, 3.5 lA, 30 min.). Figure 4 shows the circularlypolarized XAS spectra (l+ and l�) in the photon energy region ofthe Ni L3,2 absorption collected in TFY mode for the lower fluence(Ni:5 � 1016 ions cm�2) sample. A similar XAS spectra wereobserved for the other implants (not shown here). The correspond-ing XMCD spectra for all the implants at 10 and 300 K, defined as

l+–l�, as shown in the Figure 4 are collected with an applied mag-netic field of ±5 T generated by a split coil superconducting mag-net. Here, l+(l�) refers to the absorption coefficient for thephoton helicity parallel (antiparallel) to the Ni 3d majority spindirection. As a result of spin–orbit coupling in the 2p state, thespectra display two prominent features in the energy range of845–860 eV (marked by A–C) and 865–875 eV (marked by D, E),respectively, corresponding to the L3 (2p3/2 ? 3d) and L2 (2p1/

2 ? 3d) absorptions. As a guide, the reference XAS (top panel)and XMCD (bottom panel) spectra of metallic Ni thin films col-lected under the same experimental conditions are also shown inthe Figure 4 for comparison. The XMCD spectrum of pure NiOphase collected at 10 K is also shown in the lower panel of the Fig-ure 4. The observed XAS spectra of the implants (Ni: 5 � 1016–2 � 1017 ions cm�2) are closely resemble to the ionic multipletstructure of Ni [18], however XMCD spectral features are differentto that of either pure Ni metal or NiOx [18]. An inspection of XMCDspectra at the Ni L3 edge shows that the peak A is due to metallicphase of the Ni NPs, while peaks B and C corresponds to the NiOx

phases in the samples (similarly at the Ni L2 edge, peak D is dueto the Ni NPs and peak E corresponds to the NiOx phases). It shouldbe noted that all the XMCD spectra presented in the Figure 4 arenormalized with respect to the Ni L3 edge jump (peak B). At 10 K,it is clear that the XMCD signal of the peak A (associated to theNi NPs) increases systematically with the Ni ion fluences, whilethe magnetic signal of the peaks B and C (associated to the otherNiOx phases) is almost constant. The magnetic behavior of thesepeaks is reversed at RT as compared to that of low temperature(XMCD signal due to the peak A is almost constant, while the

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magnetic signal of the peaks B and C decreases with the Ni ion flu-ences). These results suggest that the magnetic properties of theimplants are strongly depending upon the Ni ion fluences andcan be correlated to the fluence-dependent magnetic contributionsof the Ni NPs and NiOx phases towards the total magnetic momentof the system.

In order to see the systematic evolution of these magnetic nano-phases as a function of Ni fluences, we have performed the XMCDmeasurements at different temperatures (10–300 K). Figure 5shows the temperature dependence of XMCD data collected inTFY mode for all the implants, while inset in the Figure 5 (bottompanel) displays the hysteresis curve of the peak A (associated to theNi NPs phase) at 10 K for the higher fluence sample. A similar hys-teresis curve was obtained at the peak B (associated to the otherNiOx phases), which demonstrate that the NiOx phases at low tem-perature have ferromagnetic components in all the implants (datanot shown here). From the Figure 5, it is clearly seen that the mag-netic contribution of NiOx phases for the higher fluence (Ni:2 � 1017 ions cm�2) sample continuously decreases with tempera-ture and then almost disappear at RT. At low fluences (Ni: 5 � 1016

and 7 � 1016 ions cm�2), samples still show an observable mag-netic contribution of the NiOx phases at RT. Since the higher flu-ence sample shows a superparamagnetic behavior at RT withbulk magnetometry (see Figures 2 and 3) having a blocking tem-perature of TB � 180 K, therefore, it is reasonable to consider thatthis superparamagnetism is dominated by the Ni NPs phase apartfrom small contributions from the other NiOx phases that maynot be accessible within the sensitivity (smaller than 0.1% of the3d metal) of XAS/XMCD spectroscopy. In other words, the thick-ness (volume) of NiOx at higher fluence is not enough to pin themoments of Ni NPs. As the remanence of the implants at RT de-creases with increasing Ni fluence during implantation (increasingfcc-Ni particle size), this destabilization of the magnetism in Ni NPscan be attributed to a fluence-dependent NiOx blocking tempera-ture, above which magnetization is unstable [19]. We believe that

at low fluences the NiOx phase is blocked and stabilizes the mag-netic moment of adjacent Ni NPs, while at high fluence the NiOx

is above the blocking temperature. These results were further con-firmed by performing RT XMCD hysteresis loops at the peak B indi-cating a loss of NiOx blocking with increasing Ni fluence (data is notshown here due to poor signal to noise ratio in the TFY mode). An-other possible scenario is that at higher fluence the amorphizationof the surrounding Al2O3 matrix may reduce the effective anisot-ropy of the Ni NPs leading to a reduction of the blocking tempera-ture. A similar amorphization effect at higher fluence has beenpredicted by Xiang et al. in this system [6]. However, we have usedhere different synthesis conditions like; (a) the use of negative Niions during implantation (it helps to avoid surface charging effectand good controllability in fluence) and (b) the post-annealing ofthe samples in air at 600 �C for 4 h, which have indeed helped torecover from the radiation/structural damage caused by the ionimplantation as such no amorphization effects can be seen fromthe HRXRD and AFM/MFM data. Thus the embedded Ni NPs exhib-its a fluence-dependent transition from ferromagnetic to super-paramagnetic state and the NiOx phases are playing an importantrole in determining their magnetic properties. It is worth notingthat the observed magnetic properties of Ni NPs and NiOx can beattributed to the presence of either (i) individual Ni and NiOx

particles in the samples, or (ii) Ni–NiOx particles in a core–shellmorphology with the NiOx acting as a passivating layer [20]. How-ever, our magnetic and XMCD data do not support (i) due to thefluence/temperature dependent magnetic interactions betweenNi NPs and NiOx phases. Moreover, the hysteresis loops displayedin the Figure 2, and in the inset of Figure 4 show that these loopsare symmetric about zero field (without any exchange bias) indi-cating the absence of Ni NPs with core–shell Ni–NiOx morphology.These exchange bias effects were checked in both the FC/ZFC mag-netization conditions. However, core–shell morphology withoutany exchange bias in this system can not be completely ruledout. In our opinion, such peculiarities of magnetic phenomenonin this system demands further XMCD experiment on the similarsystems, especially metal NPs embedded in dielectric/insulatingoxide matrices, to model these complex magnetic interactions be-tween Ni NPs and NiOx phases.

In summary, the element-specific magnetic properties of the NiNPs embedded in Al2O3 (0 0 1) matrix prepared by ion-implanta-tion technique have been investigated by performing XAS/XMCDmeasurements at the Ni L3,2 edge. It is observed that all the im-plants exhibits ferromagnetism at 10 K due to magnetic contribu-tion of both the phases (Ni NPs and NiOx), while the roomtemperature superparamagnetism at higher fluence is dominatedby Ni NPs phase. This destabilization of the magnetism in Ni NPsis attributed to a fluence-dependent NiOx blocking temperature.Our XMCD data provide a distinct ‘fingerprint’ for the complexmagnetic interactions between the nanophases of Ni NPs and NiOx.From a methodological point of view, we illustrate how the XMCDmay be used as a suitable tool to study the embedded metal NPsinto dielectric/insulating matrices and to distinguish the magneticcontribution of different phases.

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Figure 5. Temperature dependence of Ni L3,2 edge XMCD spectra. Inset (bottompanel): hysteresis curve of the peak A (associated to the Ni NPs phase) at 10 K forthe higher fluence (Ni: 2 � 1017 ions cm�2) sample.

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