Magnetic properties of granular films Fe-Al2O3

Journal of Magnetism and Magnetic Materials 31-34 (1983) 947-948

947

MAGNETIC PROPERTIES OF GRANULAR FILMS Fe-A1203 D. F I O R A N I

*, J.L. T H O L E N C E

+ a n d J.L. D O R M A N N

* CRTBT, CNRS, BP 166X, 38042 Grenoble Cedex, France + Laboratoire de Magnetisme, 1 Place A. Briand, 92190 Meudon, France

The frequency dependence (17 Hz ~
X'aJ:. 1,.,.I

1. Introduction Granular films are composite materials consisting of finely dispersed metallic grains in a generally amorphous matrix. Above a blocking temperature T B, giving a maximum in the ac susceptibility, these materials exhibit superparamagnetic behaviour. Recently, the dynamical properties of granular F e - A I 2 0 3 films have been the object of interest with special regard to the analogies exhibited with respect to spin glass behaviour, such as obedience to a Fulcher law [1,2]. These properties have been found to be strongly concentration dependent. In this paper we report a study of the magnetic properties of a granular F e - A I 2 0 3 film with an iron content of 46% (sample $12A) [2]. Results of measurements of ac susceptibility and of remanent magnetization (time and temperature evolution) are discussed.

X (, ,1 V'(Hz) Tip(K)tO.1K 17 [34.4 198 36.4 2103 38D

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2. Results and discussion The granular film Fe-A1203 (S 12A) was prepared by cosputtering Fe and A1203. The substrate was rotated during the sputtering in order to ensure a better homogeneity. The composition of the sample was checked by electron microprobe analysis (0.46 Fe; 0.45 A1; 0.09 O-Ar). 2.1. A C susceptibility measurements

A C susceptibility measurements were performed using a mutual inductance bridge in the frequency range 17 Hz ~< v < 2103 Hz. The temperature dependence of X~c at different frequencies is reported in fig. 1: X I exhibits a well defined maximum at TB; a maximum in X" is observed below T B. The dc susceptibility (Xfc = M / H ) measured after field cooling ( H = 240 Oe) continues to increase slowly below T B. Mrssbauer spectra (v = 5 x 10 7 Hz) give a blocking temperature between T = 50 K and T = 40 K, where the * On leave from I.T.S.E., Area della Ricerca di Roma, PB 10, 00016 Monterotondo Stazione, Rome, Italy. 0304-8853/83/0000-0000/$03.00

20

30

40

50

T(K)

Fig. 1. The temperature dependence of the ac susceptibility for three frequencies is composed to the dc susceptibility. The imaginary part, X", is also shown for a frequency of 198 Hz.

two temperatures correspond to the appearance of a six-lines pattern superposed o n a paramagnetic spectrum and a 100%'magnetically-ordered spectrum, respectively. The frequency dependence of the blocking temperature follows an Arrhenius law ~"=~'o e x p ( E a / K T ) , but the parameters deduced are unphysical (~'0 = 1 0 - 2 2 s;" E J K = 1630 K). Although the range of temperature is limited a clear deviation from a Curie-Weiss behaviour up to temperature well above T B [TB(v = 17 Hz) = 34.4 K] it observed in the X- ~ vs. T plots. The maximum in X~c therefore

© 1983 N o r t h - H o l l a n d

D. Fiorani et al. / Properties of granular films Fe-A1203

948 I

I

I

~

I

I

I

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-

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1

2

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0

I

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20

30

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Fig. 2. Hysteresis cycle at T = 4.2 K after zero field cooling from T :~ TB.

Fig. 3. Field dependence of IRM and TRM at T = 4.2 K. The remanent magnetization (RM) being measured 60s after switching off the field.

does not define a transition to a pure paramagnetic state, probably because of the inhomogeneity of the sample (distribution in particle size) and of interactions between particles above T a.

time and temperature evolution of T R M s are associated in the same unique variable K T l o g ( t / , o ) . This model has given a satisfactory description in the case of some spin glasses, such as CuMn, for which the following expression was found in the temperature range Tt/3 <~ T 2 T f / 3 [3]: T R M s ( T , t) = T R M s ( 0 ) e x p [ - ( T / T ) l n ( t / , o)]. The time and temperature evolution of F e - A I 2 0 3 granular films (S12A) can be described by the above reported equation from which a characteristic relaxation time ~0 = 10- ]3 s is deduced, giving a satisfactory superposition of the data in terms of T ln(t/,o). The field dependence of I R M and T R M was studied at T = 4.2 K (fig. 3). T R M increases with the field up to a constant value, as expected for a series of independent and additive relaxation processes, differently from what is observed in canonical spin glasses, exhibiting a maximum.

2.2. Remanent magnetization measurements Irreversibility properties have been observed in the granular F e - A 1 2 0 3 film. At T = 4 . 2 K ( T < < T a ) a regular and symmetric hysteresis cycle was observed after zero-field cooling from T >> T B (fig. 2) with I R M (isothermal remanent magnetization) - M s / 2 , as expected for single domaine ferromagnetic particles with relaxation time larger than the measuring time. The time evolution of saturated I R M s and T R M s (thermoremanent magnetization) has been studied at T = 4.2 K. A power law R M tx t -'~ is followed as observed in some spin glasses. The temperature dependence of T R M s has been measured after field cooling down to the lowest temperature ( T = 4.2 K) and then warming (large steps of temperature) after removing the field. The thermoremanent magnetization remains well above T B indicating the presence of some particles still blocked above the temperature of the susceptibility maximum. An exponential law T R M s ec e - b r is observed with a change of slope at the blocking temperature. If a thermally activated blocking model of doublewell potentials, governed by an Arrhenius law, with a distribution of energy barriers P ( W ) is assumed, the

One of the authors (D.F.) is indebted to the " C e n t r e de Recherches sur les tr~s basses temperatures", for its hospitality ( N A T O fellowship).

References [1] J.L. Dormann, P. Gibart, G. Suran, J.L. Tholence and C. Sella, J. Magn. Magn. Mat. 15-18 (1980) 1121. [2] D. Fiorani, J.L. Tholence and J.L. Dormann, Physica 107B (1981) 643. [3] J.J. Prejan and J. Souletie, J. de Phys. 41 (1980) 1335.