【精品论文】Size-dependent scaling of exchange bias in NiFe2O4NiO nanogranular systems synthesized by a phase separation method.doc

精品论文size-dependent scaling of exchange bias in nife2o4/nio nanogranular systems synthesized by a phase separation method5tian zhaoming, huang shuai(huazhong university of science & technology, wuhan 430074)abstract: exchange bias (eb) effect has been studied in a series of nanogranular systems of ferrimagnetic (ferri) nife2o4 nanoparticles embedded into antiferromagnetic (afm) nio matrix, synthesized by a phase pprecipitation method from diluted ni(1-x)fexo3 (x=0.09) oxides. for these10systems, the crystalline size (dnfo) of nife2o4 ranging from 3 nm to 55 nm has been obtainedwith thermal treated at different temperatures from 550oc to 1000oc. magnetization measurementshows that, both exchange bias field (heb) and vertical magnetization shifts (mshift) can be exhibited below 250 k after field cooling procedure. the heb and mshift decrease monotonically with crystalline size, and their behavior strongly depend on the crystalline size of nife2o4 nanoparticles.15linear relationship between heb and mshift is observed for systems with smaller sizes (dnfo8nm), reveals a straightforward correlation between them. this phenomenon is ascribed to the interfacial exchange coupling between ferri nife2o4 clusters and spin-glass-like (sgl) phases, where the frozen uncompensated spins in sgl phases play critical role of in inducing eb effect. as dnfo is above 12 nm, the dependence of heb on mshift deviates from the linear relationship, which is discussed in20terms of the superimposed contribution from the exchange coupling between ferri nife2o4 core with the sgl phase, and the exchange coupling between ferri nife2o4 core and afm nio phases at the interfaces.keywords: exchange bias; phase precipitation; uncompensated spins250introductionexchange bias (eb) refers to the shift of the hysteresis loop along the magnetic field axis, which is typically observed in exchange interacting ferromagnetic/antiferromagnetic (fm/afm) materials.1 this effect was firstly discovered in 1956 by meiklejohn and bean when studying co particles coated with a layer of afm coo.2 since then, eb effect has been observed in other30inhomogeneous materials involving a ferrimagnet (fi) (e.g., fi/afm and fi/fm)3,4 or a spin-glass (sg) phase (e.g., fm/sg, afm/sg and fi/sg).5-7 generally, this effect has been extensively investigated mainly in multilayer film systems due to its technological applications in magnetic random access memories, spin valves or magnetic tunnel junctions.7,8 during the past decade, eb effect in nanoparticle systems are also acquiring attention, because the fm-afm exchange35interactions can be useful to beat the superparamagnetic limit of fm nanoparticles,9 a criticalbottleneck for ultrahigh density of magnetic data storage application. moreover, the study of eb effect in reduced dimensions is also interesting from a fundamental point of view, because fm clusters are three-dimensional nanometer-scale inclusions in the afm matrix in these systems in contrast to multilayer films. furthermore, it has allowed researchers to get deeper insight into the40characteristics and microscopic mechanism of this phenomenon, especially provide insight on finite size effect at nanoscale level.although there are many studies on the scaling effect of eb in magnetic nanostructures, controversial results have been reported. some authors reported that the eb field (heb) is enhanced with size reduction of afm phase,10-12 whereas others observed the opposite45trend.13,14 these discrepancies have been attributed to the different factors which impact theinterfacial coupling, such as domain formation, interfacial roughness, impurities, and grainfoundations: this work was supported by the by the foundation from the ministry of the national education(grant no. 20090142120069)brief author introduction:spintronic materials. e-mail: - 13 -shapes, etc,10-16 a full understanding of this phenomenon still remains elusive. in addition, most studies are focused on the film systems, size-dependent eb effect in granular systems that confines fm nanoparticles to afm matrix is less investigated.50in this work, we focus on the investigation of size-dependent scaling of eb effect in the nanogranular system composed of ferri nife2o4 nanoparticles embedded within afm nio matrix, synthesized by a phase precipitation method from fe-doped nio matrix. both eb fields (heb) and vertical magnetization shift (mshift) can be observed in these systems, and their behavior strongly depend on the particle size of nife2o4 nanoparticles. two possible exchange coupling55interactions are proposed to explain this size-dependent scaling behavior.1experimental detailsthe (1-x)nio/xnife2o4(x=0.05) nanogranular systems were prepared by a high-temperature phase precipitation method from fe-doped nio matrix with x=0.09, similar to the description in our previous literature.17 the precursory powders were sintered at different temperatures from60550oc to 1000oc to produce the nio/nife2o4 nanocomposites with different particle sizes. for convenience, the samples are labeled as s1, s2, s3, s4, s5, s6, and s7 for samples sintered at550oc for 3 h, 600oc for 1 h, 600oc for 3 h, 650oc for 3 h, 700oc for 3 h, 800oc for 3 h, and1000oc for 1 h, respectively. the exact values for the synthesized nanogranular systems with different crystalline sizes are given in table 1.65the microstructure of the samples was characterized by x-ray diffraction (xrd). the micrographs of the samples were investigated by transmission electron microscopy (tem). selected-area electron diffraction (saed), in conjunction with high-resolution transmission electron microscope (hrtem) analysis, was used to determine the local crystallographic phases. the magnetic properties were performed using a commercial physical properties70measurements system (ppms, quantum design). temperature dependence of magnetizations with different magnetic fields were measured on both zero-field cooled (zfc) and field cooled (fc) processes in a range 10kt350k. magnetization versus time measurements were made by first cooling in zero field to measuring temperatures, then applying the applied field to 40 koe and magnetization was measured every 20 s for 7200 s. after that, the field75was ramped to -40 koe, and measured again for 7200 s. for the eb measurements, the samplewas first cooled from 350k to the measuring temperatures, and then magnetic hysteresis loops were subsequently recorded.intensity (a.u)(220)*(111) (202)* *(311)(222)ts=1000oc ts=800oc ts=700octs=650oc ts=600octs=550oc20 30 40 50 60 70 802 (degree)80figure 1 the xrd patterns of (1-x)nio/xnife2o4 (x=0.05) nanocomposites sintered at different temperatures.figure 2. the tem and saed micrographs of samples (a) s1, (b) s3, (c) s5, (d) s6, (c) s7, (d) the hrtem image for s3.9(a)6m (emu/g)30-3-6-9310s1s3s5s6-8-4048h (k oe)m600(b)s 15hcm (emu/g)sh (oe)40010c20050003060grain size (nm)85figure 3 (a) room temperature magnetic hysteresis for samples s1(), s3 (), s5 (), and s6 (). (b) variationof the coercivity (hc) and magnetization (ms) with nife2o4 crystalline size (dnfo).0.350.30m (emu/g)0.25(a) s1zfc fc0.80.6(b) s2zfc fc0.203.00 100 200 3000.40 100 200 300m (emu/g)2.5(c) s3zfc fc8.07.5(d) s5zfc fc2.07.00 100 200 3000 100 200 300m (emu/g)9.89.1(e) s6zfc 17fc16(f) s7zfc fc8.40 100 200 300t (k)150 100 200 300t (k)figure 4. temperature dependence of zfc and fc magnetizations at 10 koe for samples (a) s1, (b) s2, (c) s3, (c)90s4, (c) s6, and (d) s7.1.2(a) h=1 koe(b) h=5 koem (emu/g)m (emu/g)0.60.3m (emu/g)(c) h=20 koe0 100 200 300t (k)(d) h=40 koe0 100 200 300t (k)3.3m (emu/g)3.02.7figure 5. temperature dependence of zfc and fc magnetizations at different magnetic fields for sample s3.1200 h 2/3 (oe)2/38004000200 240 280 320 360t (k)figure 6. field dependence of the transition tirr, as determined by the separation of zfc and fc curves for sample95s3, showing the at lines.3.28h= -40 koeh= 40 koe3.27 m (emu/g)3.26zfct=10 k3.2503691215t (103 s)figure 7. time dependence of the magnetization in applied fields of alternated sign after a zfc process for samples3.1.00.5m (emu/g)0.0-0.5-1.0(a) s1zfc fc-0.9(b) s2zfc fc-1.8-40 -20 0 20404m (emu/g)20-2(c) s3zfc fc-4-40 -20 0 20408 (e) s540m (emu/g)-4zfc fc630-3-620100-10-40 -20 0 20 40(d) s4zfc fc-40-30-20-10 0 10 20 30 40(f) s7zfc fc-8-20 -15 -10 -5 0 5 10 15 20h (k oe)-20 -10 -5 0 5 10h (k oe)100figure 8. the zfc and fc hysteresis loops measured at 10 k after field cooling in 30 koe from 350 k for samples (a) s1, (b) s2, (c) s3, (d) s4, (e) s5,and (f) s7.33.42s3.3h (koe)eb1m (emu/g)ebh3.2sm00 10 20 30 403.1h (koe)fcfigure 9 cooling field dependence of hc and ms after field cooling from 350 k for s3.6000h (oe)4000(a) s2s3s4s5s7eb20000(emu/g)0.30.2(b) s2s3s4s5s7mshift0.10.00 100 200 300t (k)105figure 10. temperature dependence of heb (a) and mshift (b) for samples with different crystalline sizes.6000h (oe)4000(a)30002000(b)eb200001000s2s3012000.0 0.1 0.2 0.3(c)4000.0 0.1 0.2(d)h (oe)eb6000200s4s500.00 0.070.140.00 0.01 0.02mshift(emu/g)mshift(emu/g)figure. 11 the relationship between heb and mshift for samples (a) s2, (b) s3, (c) s4, and (d) s5.110115120table 1. description of the synthesis conditions for the nife2o4/nio nanogranular systems and crystalline sizes determined by xrd.1251301351401451501552results and discussionfigure 1 show the x-ray diffraction (xrd) patterns for the samples sintered at different temperatures from 550oc to 1000oc, respectively. for sample sintered at 550oc, all diffraction patterns can be indexed to the cubic structure of nio phase, suggests that the nife2o4 nanoparticles have not precipitated from the diluted ni1-xfexo (x=0.09) oxides during the thermaltreatment process. as sintering temperature increases to 600oc, two sets of different patternscorresponding to nife2o4 and nio phases can be found, confirming the coexistence of ferri nife2o4 and afm nio phases. as sintering temperature increases, the diffraction peaks corresponding to nife2o4 and nio phases become intensive and narrowing, indicative of their particle size growth. the average crystallite sizes for nife2o4 and nio phases are estimated according to the (111) (35.5o) and the (202) (43.1o) peaks using the scherrer formula. theaverage diameter (dnfo) of nife2o4 phases ranging from 3 nm to 55 nm can be obtained with different thermal treatment temperatures, as listed in table 1.to evaluate the morphology and particle sizes of the nife2o4/nio nanogranular systems, the tem and hrtem images are shown in figure. 2. from fig. 2(a-e), the average particle size of nio nanoparticles can be estimated, which increases from 10 nm to 140 nm as the sintering temperature increases from 550oc to 1000oc, follows the same trend as the crystalline size from xrd result. from the inset of fig. 2(a), the entire saed patterns can be indexed by pure nio phase for sample s1. this reveals that the fe cations are incorporated into the nio host matrix instead of the formation of nife2o4 nanoparticles. as sintering temperature increases up to 600 oc, the saed pattern becomes a mixture of the diffraction rings from both cubic nio phase and spinel nife2o4 phases, demonstrating the formation of nio/nife2o4 nanocomposites. as for the formation of these binary-phase herostructural systems, we argue that fe cations are reacted with ni cations to form ferri nife2o4 phase during high temperature thermal treatment process, leaving the other nio particles as the afm matrix. that means that, the diluted afm ni1-xfexo (x=0.09) oxides have separated into ferri nife2o4 and afm nio phases. further increasing sintering temperature, particle size of nife2o4 phase grows larger. to get more information on the interfacial microstructure, a typical high-resolution tem (hrtem) image of the sample s3 is shown in fig.2 (f), it reveals that the nife2o4 particles have a diameter about 82 nm surrounded by the nio phase. in addition, a structure disordered region is evidenced at the nife2o4/nio interfaces, extends to a few atomic layers from the ferri nife2o4 cores. the formation of this160165170175180185190structural disorder layers can be correlated with the broken of chemical bond, chemical intermixing and amorphous structural defects at interfaces, similar to the recent reporting in ni/nio and nife2o4 nanoparticles. 18,19room temperature magnetic properties for all samples are present in fig. 3(a). linear fieldmagnetization dependence is observed for sample s1, confirming no nife2o4 nanoparticles precipitated from nio matrix after thermal treated at 550oc. considering that the nife2o4 nanoparticles are precipitated from ni1-xfexo matrix, they should experience nucleation and growth process during the thermal treatment procedure.20 when the sample is thermal treated at550oc, the nife2o4 clusters do not nucleate or the particle size is small that long ranged fm order is thermal unstable. thus, no fm hysteresis loops appear. as sintering temperature increases up to600oc, the nucleated nife2o4 nanoparticles grow large that fm hysteresis loop is found for s2. infig. 3 (b), the evolution of saturated magnetization (ms) and coercivity (ec) with crystalline size (dnfo) is displayed. the magnetization increases monotonically with increasing crystalline size, while the coercivity follows a nonmonotonic size dependence and approaches maxima for dnfo15 nm. this nonmonotonic behavior can be explained in terms of the competition between the interfacial exchange coupling and thermal fluctuation effect. 21, 22 as dnfo decreases from55nm to 15 nm, smaller particles increase the interfacial exchange coupling energy and these provide extra torque on fm spin rotation, enhance the coercivity. further decreasing crystalline size below 15 nm, the thermal energy becomes sufficient to overcome the ferri anisotropy energy of nife2o4 nanoparticles. in this case, ferri order is thermal unstable and gradually transit to superparamagnetic state, thus smaller coercivity is observed.the zero-field cooled (zfc) and field cooled (fc) magnetizations for the nife2o4/nio nanogranular systems with different sizes are measured, as shown in fig. 4. for the smaller ones, zfc and fc magnetization curves show distinct divergence below the irreversibility temperaturetirr, defined as the irreversible magnetization m=mfc-mzfc becomes different from zero. thismagnetic irreversibility confirms the existence of ferri/afm exchange coupling, generally observed in spin disorder systems such as spin glassy and cluster glassy system 23,24. as crystalline size increases, this divergence becomes smaller, and fully coincides when dnfo is above 32 nm. meanwhile, the blocking temperature given by the peak of zfc magnetization curves shifts to lower temperatures, correlated with the reduction of magnetic disorder at interfaces.to clarify the magnetic state at low temperatures, the zfc and fc magnetizations with different magnetic fields are illustrated in fig. 5 for sample s3, as a representative. the irreversibility temperature tirr shifts to low temperatures with the increase of magnetic fields. moreover, the field dependence of the freezing temperature tirr was examined using thealmeida-thouless (at) line:253h at (tirr ) / j (1 tirr / tf ) 2 ,(1)195200where tf is the zero-field spin-glass freezing temperature and j is the width of the distribution of exchange interaction. as shown in fig. 6, tirr varies linearly with h2/3. this clearly shows that present data satisfy the at line, manifested a sgl state at low temperatures as reported in coo nanoparticles.26 on extrapolating the at line back to zero field, it gives the spin-glass transition temperature tf365 k.high field relaxation processes in alternating fields provide another evidence of the sgl behavior. as seen from fig. 7, an upward creep in the magne