Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Fe
particles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300
K range. The results are found to be consistent with a spin-glasslike state of the oxide layer
inducing, through exchange interaction with the ferromagnetic core, a shift of the field cooled
hysteresis loops at temperatures below the freezing at approximately 50 K.Dirección General de Investigación y Desarrollo. Gobierno de España-CICYT MAT95- 1042-C02-02Dirección General de Investigación y Desarrollo. Gobierno de España-PB96-0863-C02-02.Comisión Europea-ERBFMBI-CT95-0534

Bianco, Lucia del

Bianco, Lucia del (Coordinador)

Conde Amiano, Alejandro

Conde Amiano, Clara Francisca

Fernández, A.

Hernando, A.

Multigner, M.

Prados, Clara

Sánchez López, Juan Carlos

English

idUS. Depósito de Investigación Universidad de Sevilla

JOURNAL OF APPLIED PHYSICS VOLUME 84, NUMBER 4 15 AUGUST 1998Evidence of spin disorder at the surface–core interface of oxygenpassivated Fe nanoparticlesL. Del Bianco,a) A. Hernando, M. Multigner, and C. PradosInstituto de Magnetismo Aplicado, UCM-RENFE, P.O. Box 155, Las Rozas, Madrid 28230, SpainJ. C. Sa´nchez-Lo´pez and A. Ferna´ndezDepartamento de Quı´mica Inorga´nica, Instituto de Ciencia de Materiales de Sevilla, Centro deInvestigaciones Cientifı´cas Isla de la Cartuja, Avda. A. Vespuccio, s/n. Sevilla 41092, SpainC. F. Conde and A. CondeDepartamento de Fı´sica de la Materia Condensada, Universidad de Sevilla-Instituto de Ciencia deMateriales de Sevilla, P.O. Box 1065, Sevilla 41080, Spain~Received 10 February 1998; accepted for publication 1 May 1998!Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Feparticles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300K range. The results are found to be consistent with a spin-glasslike state of the oxide layerinducing, through exchange interaction with the ferromagnetic core, a shift of the field cooledhysteresis loops at temperatures below the freezing at approximately 50 K. © 1998 AmericanInstitute of Physics. @S0021-8979~98!07315-0#INTRODUCTIONThe study of the magnetic properties of fine particles hasbeen traditionally urged by both a technological and theoret-ical interest connected with the possibility of developing abetter understanding of magnetic phenomena related to sizeeffects. In recent times, new methods of synthesis ~inert gascondensation, layer deposition, ultrarapid quenching, me-chanical attrition, aerosol! have been used to fabricate mag-netic systems with characteristic dimensions on a nanometerscale. Innovative nanostructured systems with excellent softor hard magnetic properties1,2 or with giant magnetoresis-tance response3 have been created, which at present play animportant role in modern technology and research.That was also the occasion for a renewed interest in thefield of magnetism of ultrafine particles, probably in view oftheir potential applications in high density magnetic record-ing or as ferrofluids, which has invariably led to the discov-ery of unexpected magnetic behavior strictly depending onthe reduced dimensional regime.4A number of research works has been published duringthe last few years dealing with the magnetic behavior ofnanometric Fe, Co, and Ni nanocrystalline particles, mainlyprepared by the inert gas condensation method, and present-ing different degrees of surface oxidation.5–9 In this article,once more attention has been focused on the low temperaturemagnetic behavior of oxide coated ultrafine Fe particles anda different explanation, based on the spin-glasslike state ofthe oxide layer, is proposed to account for the observed re-sults.a!Author to whom correspondence should be addressed; on leave from: Di-partimento di Fisica, Universita` di Bologna and INFM, Viale Berti Pichat6/2, 40126 Bologna, Italy.2180021-8979/98/84(4)/2189/4/$15.00Downloaded 20 Feb 2003 to 192.167.227.94. Redistribution subject EXPERIMENTNanocrystalline Fe particles have been prepared byevaporation of iron in a tungsten boat at 1773 K. The pres-sure of gas phase during evaporation was 133 Pa ~1 Torr!helium.Through interatomic collision with the inert gas atoms,the evaporated atoms lose kinetic energy and condense as anultrafine powder that accumulates on a cold finger.10 Thepassivation was achieved by dosing oxygen ~266 Pa for 10min! before opening the chamber to air. The loose powderwas smoothly stripped off and stored in air.Figure 1~a! shows a transmission electron microscope~TEM! micrograph of the passivated ultrafine powder, whichhas been carried out through a Philips CM200 microscopeworking at 200 kV. For the TEM examination the powderwas previously dispersed in ethanol by sonication anddropped on a conventional carbon coated copper grid. Thematerial consists of nanometer sized particles nearly spheri-cal ~15–40 nm in diameter! presenting a core–shell struc-ture. The dark inner core, visible in the micrograph, corre-sponds to metallic iron, whereas the surrounding light layeris expected to be the oxide phase.X-ray diffraction ~XRD! analysis of the as-prepared ul-trafine powder @Fig. 1~b!# reveals the presence of pure a-Feand some small and very broad features at the position of thepeaks for g-Fe2O3 and Fe3O4 ~it is not possible to differen-tiate between these two phases by XRD as their lattice pa-rameters are very similar!. These small peaks can be due to apoor crystallization of these compounds in the thin passiva-tion coating. The presence of oxide species at the surface ofthe passivated nanocrystalline powders was also fully con-firmed by x-ray photoelectron spectroscopy.11The magnetic behavior of the ultrafine powder has beeninvestigated through a commercial semiconducting quantuminterference device ~SQUID! magnetometer supplying a9 © 1998 American Institute of Physicsto AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp2190 J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Del Bianco et al.magnetic field up to 55 kOe and operating in the 5–300 Ktemperature range.RESULTS AND DISCUSSIONIn Fig. 2, the temperature dependence of the magnetiza-tion, measured at an applied field H550 kOe is shown. Fullsquare curves have been measured after zero-field coolingand open circles corresponds to the field cooling ~at 50 kOe!FIG. 1. TEM micrograph ~a! and x-ray diffraction spectrum ~b! of the oxy-gen passivated ultrafine Fe powder.FIG. 2. Zero-field cooled ~full squares! and field cooled ~open circles! tem-perature dependence of magnetization for the ultrafine Fe powder ~appliedfield H550 kOe!.Downloaded 20 Feb 2003 to 192.167.227.94. Redistribution subject procedure. Up to approximately 30–50 K, the magnetizationin the zero-field cooling curve is seen to rise and then de-creases regularly up to room temperature.In Figs. 3~a! and 3~b!, hysteresis loops measured at atemperature of 5 K according to the conventional zero-fieldcooled ~ZFC! and field cooled ~FC! methods are displayed.The field applied at 300 K for the field cooling procedurewas of 20 kOe. It has been observed that the ZFC loop issymmetric about the origin, whereas after field cooling, ashift of the loop is experienced. The measurements havebeen repeated at different measuring temperatures Tm , fol-lowing the same procedure. In Fig. 4, the coercivity (Hc) ofthe FC loops ~curve a! and the shift ~curve b! between ZFCand FC loops is reported as a function of Tm . It is notewor-thy that the loop shift decreases with increasing Tm and van-ishes near 50 K, namely where the maximum in the magne-tization against the temperature curve is observed ~Fig. 2!.The coercivity also decreases abruptly with temperature go-ing from about 1500 Oe at 5 K to 200 Oe at room tempera-ture and following a temperature dependence similar to theloop shift.It is well known that a shift of a FC hysteresis loop maybe originated by an exchange anisotropy effect determinedby an exchange coupling between a ferromagnetic and anantiferromagnetic ~or ferrimagnetic! phase in intimate con-tact so that the exchange interaction can propagate throughthe interface.12 A necessary requirement for the establish-ment of exchange anisotropy is that the field cooling is per-FIG. 3. Hysteresis loops measured at Tm55 K after cooling in zero field ~a!and after cooling from room temperature in a field H520 kOe ~b!. In theinset: central part of the loop.to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp2191J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Del Bianco et al.formed across the Ne´el temperature of the antiferromagnetic.Owing to the particular structural configuration of the Feparticles, an exchange anisotropy effect was expected. How-ever, the magnetic order–disorder transition temperatures ofg-Fe2O3 (;1020 K) and Fe3O4 ~858 K!, namely, the Fe ox-ides identified by XRD, and also of FeO ~198 K! are consid-erably higher than the temperature above which the loopshift disappears.On the other hand, further evidence of the only neces-sity, in order to observe the shift, to go through the criticaltemperature of 50 K during the field cooling procedure, hasbeen obtained by performing a FC loop at 5 K, taking care toapply the field at 60 K. In this case too, a shift of the FC loopof the same entity as in Fig. 3~b! has been induced.We propose that the shift of the hysteresis loop is deter-mined by an exchange interaction between the ferromagneticcore of the ultrafine particles and the oxide surface with spin-glasslike magnetic behavior and freezing temperature T f550 K. In this hypothesis, the application of the field at Thigher than T f forces the magnetization of the particle coreto lie along the field direction. Due to the exchange couplingwith the core spins, the spins at the oxide layer assume apreferential orientation which becomes frozen below T f . Inturn, below T f , the presence of the surface frozen spins fa-vors the ferromagnetic core of the particle being magnetizedin the field cooling direction, resulting in a shift of the hys-teresis loops.13One characteristic of a spin glass is to possess multiplestable configurations of the frozen state. The application of astrong field, at a temperature lower than freezing, can forcethe spins to align along the field direction. As a matter offact, the increase of the magnetization with temperature up toabout 50 K ~Fig. 2! can be accounted for by invoking ther-mally activated transitions toward the spin configuration ~fa-vored by the presence of the magnetic field! in the oxidecoating of the Fe particles.Accordingly, in order to support the hypothesis of aspin-glasslike character of the oxide layer, an hysteresis loophas been measured at 5 K, after field cooling from 20 K. Infact, for the above considerations, it was expected that thefield cooling procedure, even if carried out starting from aFIG. 4. Coercivity ~curve a! and shift ~curve b! of the hysteresis loops,measured after cooling in a field H520 kOe, as a function of the measuringtemperature.Downloaded 20 Feb 2003 to 192.167.227.94. Redistribution subject temperature lower than T f , could select a spin configurationand lead therefore to a displacement of the loop at 5 K. Aminor ~with respect to the case of field cooling from 300 K!but considerable shift of about 150 Oe has been indeed ob-served.With regard to the temperature dependence of the coer-civity @Fig. 4~a!#, a similar trend ~even if relative to ZFCloops! for Fe particles prepared in the same way, was alreadyobserved by Gangopadhyay et al.14 and explained through a‘‘shell–core’’ model based on the presence, at the particlesurface, of an oxide coating in the form of very small crys-tallites, superparamagnetic above a blocking temperature of30–40 K.However, the high coercivity value at 5 K and the strongdecrease with increasing temperature @Fig. 4~a!# may beequally well accounted for considering that the presence ofthe frozen surface constitutes a strong hindrance to the rever-sal of the ferromagnetic phase. Moreover, it must be consid-ered that, owing to the small size of the Fe core, some par-ticles may be superparamagnetic at room temperature, whichwould result in a further decrease of Hc . This last hypothesishas been indeed confirmed by performing an usual ZFC-FCmagnetization against temperature measurement at an ap-plied field H5800 Oe ~Fig. 5!. Note that this weak field, incomparison with that used in Fig. 2, allows us to obtaininformation about the possible superparamagnetic behaviorof the Fe cores, whereas the result of Fig. 2 can be correlatedwith the magnetic behavior of the oxide coating. It is note-worthy that the ZFC magnetization in Fig. 4 does not show adefined maximum and that the ZFC and FC branches remainseparated up to practically room temperature. This behavioris compatible with a wide size distribution of the particlecores which, therefore, enter the superparamagnetic regimeat different temperatures.Mo¨ssbauer spectroscopy studies by Haneda andMorrish15 and by Linderoth et al.16 on surface oxidized Fenanoparticles revealed that the surface layer consisted ofvery small crystallites with spinel structure and that a largespin canting characterized the oxide phase.A canting of the spins was also observed by Kodamaet al.17 at the surface of ferrite nanoparticles. In that case too,it was found that the presence of the canted spins, arisingFIG. 5. Zero-field cooled ~circles! and field cooled ~triangles! magnetizationas a function of temperature at an applied field H5800 Oe.to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp2192 J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Del Bianco et al.from reduced coordination and broken exchange bonds be-tween surface spins, was compatible with a spin-glasslikebehaviour of the surface itself. It should be remarked that thebehavior of the surface becomes relevant due to the veryreduced dimensions of the particles so that the number ofatoms at the surface is comparable to the number of atoms atthe core. Recently, computer modeling of the influence ofsurface disorder and spin uncompensation on the magneticproperties of nanoparticles and on the exchange anisotropyof thin films has been reported.18In our case, assuming that the oxide layer is made up ofsmall crystallites, it follows that a high percentage of thetotal number of spins of the oxide is located at the crystallitessurface, more properly indicated as the interfacial or grainboundary region,19 as well as at the oxide–Fe interfacewhich would therefore be ~in our opinion! the ultimate re-sponsibility of the observed exchange anisotropy effect.CONCLUSIONIn conclusion, an approach has been proposed in order toexplain the low temperature magnetic behavior ~temperaturedependence of magnetization and coercivity! of oxide coatedultrafine Fe particles, based on the spin-glasslike nature ofthe oxide layer. The occurrence of an exchange anisotropyeffect between the ferromagnetic core and the surface resultsin a shift of the FC hysteresis loops below 50 K, correspond-ing to the freezing temperature of the spin-glass phase.It is worth noting that the possibility of observing theabove described effects is strictly connected with obtainingsystems modulated on a length scale in the nanometer re-gime. In fact, it has been proposed that the spin-glasslikestate of the oxide surface is favored by the very reduceddimensions of its constituting crystallites and, moreover, it isclear that the exchange interaction would not be strongenough to affect considerably the magnetic behavior if theparticle core and the oxide layer were not extended on ashort and comparable scale length.Downloaded 20 Feb 2003 to 192.167.227.94. Redistribution subject View publication statsACKNOWLEDGMENTSThe authors acknowledge Project Nos. CICYT MAT95-1042-C02-02 and PGC PB96-0863-C02-02. LDB acknowl-edges the European Commission for financial support ~GrantNo. ERBFMBI-CT95-0534!.1 Y. Yoshizawa, S. Oguma, and K. Yamauchi, J. Appl. Phys. 64, 6044~1988!.2 E. F. Kneller and R. Hawig, IEEE Trans. Magn. MAG-27, 3588 ~1991!.3 S. S. Parkin, N. More, and K. P. Roche, Phys. Rev. Lett. 64, 2304 ~1990!.4 See for example, Magnetic Properties of Fine Particles, edited by J. L.Dormann and D. Fiorani ~North-Holland, Amsterdam, 1992!.5 W. Wagner, H. Van Swygenhoven, H. J. Ho¨fler, and A. Wiedenmann,Nanostruct. Mater. 6, 929 ~1995!.6 W. Gong, H. Li, Z. Zhao, and J. Chen, J. Appl. Phys. 69, 5119 ~1991!.7 S. Gangopadhyay, G. C. Hadjipanayis, S. I. Shah, V. Papaefthymiou, andA. Kostikas, J. Appl. Phys. 70, 5888 ~1991!.8 J. Lo¨ffler, H. Van Swygenhoven, W. Wagner, J. Meier, B. Doudin, and J.Ph. Ansermet, Nanostruct. Mater. 9, 523 ~1997!.9 H. Kisker, T. Gessmann, R. Wu¨rschum, H. Kronmu¨ller, and H. E.Schaefer, Nanostruct. Mater. 6, 925 ~1995!.10 H. Gleiter, Prog. Mater. Sci. 33, 223 ~1989!.11 J. C. Sa´nchez-Lo´pez and A. Ferna´ndez, Proceedings of Mechanically Al-loyed, Metastable and Nanocrystalline Materials, ISMANAM-97, editedby M. D. Baro´ and S. Surinach ~Trans. Tech. Publications Ltd, Ammers-dorf, Switzerland, 1997!, pp. 827–832.12 B. D. Cullity, in Introduction to Magnetic Materials ~Addison-Wesley,London, 1972!, p. 423.13 J. A. Mydosh, in Spin Glasses: An Experimental Introduction ~Taylor andFrancis, London, 1993!, p. 91.14 S. Gangopadhyay, G. C. Hadjipanayis, B. Dale, C. M. Sorensen, and K. J.Klabunde, Nanostruct. Mater. 1, 77 ~1992!.15 K. Haneda and A. H. Morrish, Surf. Sci. 77, 584 ~1978!.16 S. Linderoth, S. Morup, and M. D. Bentzon, J. Mater. Sci. 30, 3142~1995!.17 R. H. Kodama, A. E. Berkowitz, E. J. McNiff, and S. Foner, J. Appl. Phys.81, 5552 ~1997!.18 R. H. Kodama, S. A. Makhlouf, and A. E. Berkowitz, Phys. Rev. Lett. 79,1393 ~1997!; K. Takano, R. H. Kodama, A. E. Berkowitz, W. Cao, and G.Thomas, ibid. 79, 1130 ~1997!.19 L. Del Bianco, A. Hernando, E. Bonetti, and E. Navarro, Phys. Rev. B 56,8894 ~1997!.to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp

Journal of Applied Physics

Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles

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Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles

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idUS. Depósito de Investigación Universidad de Sevilla