Exchange bias in ferrimagnetic–antiferromagnetic nanocomposite produced by mechanical attrition

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  • om

    ravorty e,

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    27 January 2009Available online 5 February 2009

    Keywords:

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    size

    sis

    Exchange bias varying from 140 to 10Oe was observed. This is explained as arising due to

    ferrimagneticantiferromagnetic coupling at the nanointerfaces between the two phases. Analysis of

    data shows a pronounced increase in the effective anisotropy constant as the milling time is increased.

    wing insity r

    with ferromagnetantiferromagnet (FMAFM) interfaces [3]. EB

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    The phase transformation of the powder was studied by X-ray

    ARTICLE IN PRESS

    Contents lists availabl

    .els

    an

    Journal of Magnetism and Magnetic Materials 321 (2009) 22692275transmission electron microscope. Magnetic measurements werehas been studied in nanoparticles (np) of metal (core)metaloxide (shell) structure of transition metals or their alloys [4,5]. Sofar various experimental methods have been used to synthesize

    diffraction by taking out the sample after few hours of milling andrecording the diffraction pattern using a Philips X-ray diffract-ometer (PW1130) consisting of a PW1710 controller. The micro-structures of the samples were investigated by a JEM2010

    carried out using a Quantum Design SQUID magnetometer in thetemperature range 5300K.

    Corresponding author.

    E-mail address: mlsdc@iacs.res.in (D. Chakravorty).0304-88

    doi:10.1structure can be centred about a non-zero magnetic eld.Exchange bias (EB) has been extensively studied in thin lms

    volume was subjected to grinding operation in a FritschPulverisette 5 Planetary ball Mill under ordinary atmosphere.and the antiferromagnet produces a ferromagnetic behavior withgood stability and high anisotropy. In such a structure, theanisotropy may be unidirectional a feature not found in ferro-magnets. This phenomenon is called exchange bias, because thehysteresis loop associated with the ferromagnet/antiferromagnet

    2. Experimental

    Magnetite powder of 99% purity was procured fromAldrich Chemicals. The powder taken in steel vials of[1]. Most applications depend on thermal stability of the magneticorder in the nanoparticles. Magnetization reversal in an assemblyof magnetic nanoparticles determines the stability of storedinformation and limits the ultimate storage density. Exchangecoupling (exchange bias) induced at the interface betweenferromagnetic and antiferromagnetic systems can provide anextra source of anisotropy leading to magnetization stability [2].In heterostructures, exchange coupling between a ferromagnet

    mechanical attrition is an useful technique for making nanos-tructured materials in bulk quantity [9]. Our earlier work hasshown that nanointerface between Fe-based oxides can begenerated by subjecting magnetite powder to mechanical grind-ing [10]. We have now extended this investigation to delineate themagnetic particles of these composites. The latter exhibitexchange bias behavior. The details are reported in this paper.Coreshell

    Nanocomposite

    Exchange bias

    Mechanical attrition

    1. Introduction

    Magnetic nanoparticles are of gropotential application in ultra high-de53/$ - see front matter & 2009 Elsevier B.V. A

    016/j.jmmm.2009.01.037nterest because of theirecording and medicine

    core (FM)shell (AFM) interface structure to observe exchangebias, e.g. inert-gas-condensation [6], plasma-gas-condensation[7], vapour deposition technique [4,8], etc. These methods areexpensive and not suitable for bulk production. On the other hand,& 2009 Elsevier B.V. All rights reserved.Exchange bias in ferrimagneticantiferrby mechanical attrition

    P. Hajra a, S. Basu b, S. Dutta c, P. Brahma d, D. Chaka Department of Physics, Sammilani Mahavidyalaya, Kolkata 700075, Indiab Department of Physics, National Institute of Technology, Durgapur 713209, Indiac Rammohan College, 102/1 Raja Rammohan Roy Sarani, Kolkata 700009, Indiad Gurudas College, Department of Physics, Kolkata 700054, Indiae DST Unit on Nanoscience, Indian Association for the Cultivation of Science, Kolkata 7

    a r t i c l e i n f o

    Article history:

    Received 24 December 2008

    Received in revised form

    a b s t r a c t

    Nanoparticles comprising

    around 9nm were synthe

    magnetization and hystere

    journal homepage: www

    Journal of Magnetismll rights reserved.2, India

    magnetite (Fe3O4) corehematite (a-Fe2O3) shell with mean diameterd by mechanical grinding. Zero-eld-cooled (ZFC) and eld-cooled (FC)

    loop measurements were carried out over the temperature range 5300K.agnetic nanocomposite produced

    e at ScienceDirect

    evier.com/locate/jmmm

    d Magnetic Materials

  • 3. Structural analysis

    X-ray data analysed by Rietvelt method showed a phasetransformation from Fe3O4 to a-Fe2O3 as the duration of grindingwas increased. The detailed X-ray data were reported earlier [10].The crystalline phases and volume fractions thereof in differentsamples are summarized in Table 1. Fig. 1(a) is a typicaltransmission electron micrograph for a specimen subjected to amilling operation for 6 h. Fig. 1(b) is the electron diffractionpattern obtained from Fig. 1(a). The interplanar spacing deter-mined from the diffraction rings conrmed the presence of Fe3O4and a-Fe2O3 phases, respectively. Similar results were obtainedfor samples milled for different periods. The particle sizes in thesespecimens were analysed by tting the histograms of particlesizes (estimated from transmission electronic microscopy) to alognormal distribution function. Table 1 gives the extracted valuesof the median diameter x and geometric standard deviation s fordifferent samples. It can be seen that the mean diameter does notchange much as a function of grinding time. This can berationalized as follows. As the grinding duration is increased theprecursor particles get reduced in size. However, under ambientcondition the reactive surface of Fe3O4 particles become oxidizedto a-Fe2O3. The two processes tend to keep the average particlediameter unchanged. Fig. 1(c) shows the high-resolution electronmicrograph for a specimen subjected to grinding for 6 h. It isevident that there is a coreshell conguration with the coreregion showing a lattice spacing of 0.241nm. The latter corre-sponds to the plane (2 22) of magnetite (Fe3O4) phase. The shellregion shows a lattice spacing of 0.251nm, which corresponds tothe plane (110) of hematite (a-Fe2O3) phase. The core diameter of

    the particle concerned is estimated as 10nm and the shellthickness as 3.3nm.

    The mechanism of shell formation can be explained as follows.a-Fe2O3 phase grows on the surfaces of Fe3O4 particles. Thecloseness of interplanar spacings of Fe3O4 (222) and a-Fe2O3(110) planes helps this surface growth because of a low value ofinterfacial energy between the two phases. The shell is made up ofnanosized a-Fe2O3 particles. The diffraction spots (Fig. 1(b))forming rings conrm the presence of these particles. The poresin between the a-Fe2O3 particles ensure diffusion of atmosphericoxygen to bring about the phase transformation even for longgrinding duration.

    4. Temperature dependence of low eld magnetization

    Fig. 2(a) shows eld-cooled (FC) and zero-eld-cooled (ZFC)

    temperature, so that above the latter the magnetization relaxes

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    P. Hajra et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 226922752270Table 1Summary of parameters extracted from X-ray diffraction data and transmission

    electron microscopy.

    Grinding

    period (h)

    X-ray analysis TEM analysis

    Phases

    present

    Volume

    fraction

    Particle diameter

    x (nm)

    Geometric standard

    deviation (s)

    0 Fe3O4 1.0

    2 Fe3O4 0.6 8.0 1.6

    a-Fe2O3 0.46 Fe3O4 0.1 10.0 1.3

    a-Fe2O3 0.910 Fe3O4 0.05 9.0 1.4

    a-Fe2O3 0.95Fig. 1. (a) Transmission electron micrograph for a specimen subjected to a milling opresolution electron micrograph for a specimen grinding for 6h.superparamagnetically in the time scale of measurement. It isseen from Fig. 2(a) that ZFC magnetization does not show amaximum and ZFC and FC branches remain open up to roomtemperature (300K). Thus, the system shows a broad range ofblocking temperatures consistent with the particle size distribu-tion. Also, interparticle dipolar interactions are operative whichhinders the appearance of a maximum in the magnetization vs.temperature curves [11]. Fig. 2(b) shows the sample after 10hgrinding which also has similar characteristics. Particle size of theball-milled specimens in the present study being well withinsuperparamagnetic region [12], SP blocking temperature (TB) isexpected to occur in the low-temperature side of the ZFC curvewhereas Fe3O4 nanocore remains ferrimagnetic up to at leastroom temperature (300K). We believe that an extra anisotropy(Kint) is induced at ferrimagnetic (FerriM) (Fe3O4)/AFM (a-Fe2O3)interface such that, KintVbKBT, where V is the particle volume andKB is Boltzmanns constant.

    5. Temperature dependence of high eld magnetization

    Magnetic hysteresis loops were measured at different tem-peratures for both the ZFC and FC samples from 300K in 50kOemagnetization curves as a function of temperature measured in50Oe for specimens subjected to 2 and 6h grinding, respectively.A peak in the ZFC curve is generally associated with the meansuperparamagnetic blocking temperature (TB). The temperature atwhich the FC and ZFC branches meet is the maximum blockingeration for 6 h. (b) Electron diffraction pattern obtained from Fig. 1(a). (c) High-

  • ARTICLE IN PRESS

    MaP. Hajra et al. / Journal of Magnetism andbeing the maximum extend of magnetic eld. The loops measuredat 10K are shown in Figs. 3 and 4 for specimens subjected to 2 and6h grinding, respectively. Horizontal loop shift of about 10 and108Oe could be extracted for specimens subjected to 2 and 6hgrinding, respectively. No loop shift was found for unmilledmagnetite. Horizontal shift of the loops has been termed asexchange bias eld [HE (HC2HC1)/2], where HC1 and HC2 are theabsolute values of positive and negative coercive elds. Theobserved exchange bias in the present study can be understoodwith the help of a simple phenomenological model as follows. a-Fe2O3 displays unusual magnetic properties [13]. It crystallizes ina corundum type structure and is an antiferromagnet with a Neeltemperature of 950K. Between 263 and 950K, the Fe3+ spins arealigned perpendicular to the c-axis in a weak ferromagnetic statecaused by an exchange interaction that gives rise to a slight spincanting. At TM 263K (Morin transition), the spins undergo areorientation which makes them lie along the c-axis in a pureantiferromagnetic phase. Fe3O4 crystallizes with a spinel structure[14] and strong antiferromagnetic coupling between Fe octahedral

    Fig. 2. (a) Field-cooled (FC) and zero-eld-cooled (ZFC) magnetization M (emu/gm) curves as a function of temperature T(K) measured in 50Oe for specimens

    subjected to 2 and 6h. (b) Field-cooled (FC) and zero-eld-cooled (ZFC)

    magnetization M (emu/gm) curves as a function of temperature T(K) measured

    in 50Oe for specimens subjected to milling operation for 10h.and tetrahedral sites yields a high curie temperature TC 858K.Bulk Fe3O4 is magnetically soft. In present case all milledspecimens were eld-cooled from 300K. As a result Fe3O4 (core)spins align along the eld-cooled direction. When the tempera-ture is lowered through the Morin temperature of AFM (a-Fe2O3)spins next to the FerriM align ferromagnetically due to exchangeinteraction at the interface. The other AFM spin planes alignantiferromagnetically to produce zero net magnetization. Whenthe eld is reversed, the FerriM spins start to rotate, while AFMspins remain unchanged owing to its large anisotropy. The latterprevents FerriM spins in turning away from eld cooling direction.Thus, the eld needed to reverse completely FerriM phase (core)will be larger if it is in contact with an AFM phase, because anextra eld is needed to overcome the microscopic torque, whichacts at the nanointerface. However, once the eld is rotated backto original direction, the FerriM spins will start to rotate at asmaller eld, due to interaction with the FM spins.

    Temperature dependence of the exchange bias (HE) for allmilled specimens is shown in Fig. 5. We note a very low HE value(10Oe) for sample subjected to 2h milling. A very thin andpoorly crystalline a-Fe2O3 shell is formed over Fe3O4 particles ofdiameter 8nm after 2 h milling. This is believed to cause suchlow exchange bias effect [15]. On the other hand, as the durationof milling progresses there will be an increase of shell thickness.These coreshell-structured magnetic particles also get embeddedwithin the a-Fe2O3 matrix as the duration of milling is increased.Morphology of the composite is not further modied beyond 6hmilling and consequently identical exchange bias characteristic isobserved for specimens after 10h grinding. The dispersion ofmagnetic nanoparticles in AFM a-Fe2O3 matrix enhances thedensity of nanointerfaces and results in enhancement of exchangebias as observed for samples subjected to longer milling periods incomparison to those with shorter grinding. The exchange bias HEhas been shown [16] to vary as HE sint/MFMtFM where sint is theinterfacial energy, tFM is the thickness of the FM core and MFM isthe magnetization. This can be explained as arising due to adecrease in tFM, because the X-ray data indicate a lowering ofvolume fraction of the FM core. Loop shift (exchange bias) usuallydiminishes with temperature, eventually vanishing at the transi-tion temperature (TN) of the AFM phase [2]. In the present study,loop shift does not vanish as the Neel temperature of the AFM (a-Fe2O3) is far beyond the room temperature (300K). It should benoted here that two humps are observed around 50 and 130K inZFC M(T) curves of all specimens (obtained by differentiating themagnetization with temperature) as shown in Fig. 2(a and b). Thisbehavior can be associated with sharp fall of the correspondingexchange bias (HE) around 50 and 130K when the samples areheated from 10K under eld-cooled condition. This change isfurther enhanced due to onset of Morin temperature. Mostprobably 50 and 130K temperatures indicate the onset of spinfreezing [17] and charge-ordering transition [18] (Verwey transi-tion), respectively.

    Spin-wave theory [19,20] describes low-temperature behaviorof magnetic materials. The spin-wave energy can be expressed by

    hwq E0 Dq2 Fq4 . . . . . . (1)where E05Dq

    2 is the effective energy arising from dipoledipoleinteraction, q is the wave vector of spin wave, and D and F arespin-wave stiffness constants. Thus, it would be interesting toexamine the low-temperature behavior of systemwith this type ofmagnetic ordering.

    In general the temperature dependence of magnetization wellbelow the Curie temperature arises due to spin-wave uctuation

    gnetic Materials 321 (2009) 22692275 2271as rst described by Bloch [21]

    MT MS01 BTn (2)

  • ARTICLE IN PRESS

    MaP. Hajra et al. / Journal of Magnetism and2272where MS(0) is the saturation magnetization at zero temperature,B is the Bloch constant and n is the Bloch exponent. For bulkmaterial n 3/2 and Eq. (2) becomes

    MT MS01 BT3=2 (3)

    The parameter B is related to the spin-wave stiffness...

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