Journal of Magnetism and Magnetic Materi& 140-144 W95) 383-384
Magneticpropertiesof y-Fe@, smallpartic by spy pyrolysis A. Lhpez a, F.J. Lizam al*, T. GonziIez-Carreiis b,
.Bv Moaaks b, GJ,
a I.C.M.A. Wniuersidad dc Zaragoza-CSKA 50015 Zaragoza, Spain b Institute de Cienciu k Materiales (CSIC), 286% Madrti Spain
Abstract Small particles of y-Fe,O, synthesized by spray pyrolysis have been studied by ac susceptibility. nanometric crystallites resulting in dense and hollow spherical aggregates. The results show the superparamagnetic to blocked state. The anisotropy and magnetic moment per aggregate are less than for SI the same dimensions.
The great interest in magnetic recording of y-Fe,O, particles has brought the emergence of new preparation methods [1,2], that allow to synthesize from single particles to aggregates of different geometries. By varying synthesis conditions it is possible to get different sizes, compositions and degrees of aggregation [3], leading to a broad range of magnetic properties. Until now, the magnetic behaviour of aggregates is less understood than in the case of isolated particles. Other authors have studied previously the magnetic properties of aggregates of irregular shapes [4,S]. This paper presents ac susceptibility results on y-Fe,03 aggregates of, in this case, well-defined shapes, whose fmt characterization was done by other techniques [2]. The influence of the size and the geometry of the aggregates is discussed in terms of xa&T, d. y-Fe20, small particles were synthesized by spray pyrolysis. Particles with a medium diameter of 180 f 90 nm, spherical in shape, were obtained using nitrate solutions iike precursors (see Fig. l(a)). They are made out of very small crystallites of around 6 mn. Hollow spherical particles with a medium diameter of 1’70 + 90 nm, where formed when a solution of Fe(H) ammonium citrate was used as precursor. In this case small crystallites of around 4 nm are aggregated forming a shell (see Fig. l(b)). The X-ray diffractograms and the electron diffraction patterns showed the nanocrystalline nature of these sampls~. The ac susceptibility has been measured at 119 and 1190 Hz by an induction method and with an exciting ac field of 10 Oe.
* Corresponding author. Fax: (34) 76 512932; em&
[email protected].
broad maximum
Fig. I. TEM micmgraphs of the y-Fe,OJ dense spherical particks lb) hoSlow particles. of nanocrystallites.
0304-SSS3/95/$09.50 8 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00793-4
Fig. 2. Te
rame ~~~~oce
of ,y, for the dense spherical
flies isdiierent. The high temperahas been fitted to a
Curie-Weiss law.
e momentsobtained
presenceof non-collinear spin structures on the surface of a nanocrystallittwill decreasethe specificmagnetization. The exchangeinteractionbetweencontiguouscrystallites could affect the arrangementof spins on the contactsurfacesand lead to different valuesof the effectivemoment mdf. Actually the averagenumber n of fmt-near neighhourschangesfrom one sampleto another.The aumberof small crystallitesthat are at a superficial site (internal or external) is estimatedio be around 80% for the hollow spheres and 1% for the dense ones. Consequently,h decreasesfrom the densesphericalto the hollow spherical particles. Different distributions of activation energiesand thus different ~~0’) curves,may also be obtainedfrom different magnetizationreversal modes within the aggregates. The number n and the geometry of the particle seemto affect the reversal modes. Hence, changesin n could induce shifts of the average blocking temperature Tn, However,it is just probablethat differentsize distributions producethe TB differencesobserved. In conclusion,the presentwork makesa descriptionof the magnetic properties of an exotic sort of aggregates with well-defined shapes.The ac susceptibility results corroboratepreviousMiissbauerdata and show superparamagnetismwith a lower blocking temperaturescompared to single particles of the same size. From the effective momentsand from the magneticrelaxation observedwe suggestthat the magneticentities responsiblefor the behaviour of both samplesare not the denseor the hollow particlesbut the very small nanocrystallites.Nevertheless it appearsthe interaction plays a crucial role. The novel geometryof theseparticlesmight provide a way of tailoring the n parameterin order to vary the degreeof interaction betweennanocrystallites. Acknowledgements:The authorsare thankful to DGA under project PITO892,CICYT project MAT 93/0789, as well asto Fundaci6nDomingo Martinezfor their financial support. References [l] M.P. Morales, C. de JuIiBn, J.M. Gotilez and C.J. Sema, J. Mater. Res. 9 (1994) 135. i2] T. Gor&lez-Carreho, M.P. Morales, M. Gracia and C.J. Sema, Mat. Ldt. 18 (1993) 151. [3] K.Lhima. M. Wada and A. Kate,J. Mater.Res5 &BO) [4; S. &uni,
G. Gavoille and R. G&din,
J. Magn. Magn.
Mater.123(1993)175. in: Studiesof MagneticProperties of FineParticlesandtheir Relevance to MaterialsScience,eds. J,L.DormannandD. Fiorani(Elsevier,11)92)p. 199. [6] K HanedaandAH. Morrish,PhaseTransitions 24-26 (1930) 661. [51 E. Tronc and J.P. Jolivet,
lIouow spherical