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MAGNETIC AND CRISTALLIZATION STUDIES OF
AMORPHOUS RIBBONS WITH Cr CONTENT
H. Kadiri, C. Djega-Mariadassou, P. Rougier, J. Dormann, A. Berrada, P.
Renaudin
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, decembre 1988
MAGNETIC AND CRISTALLIZATION STUDTES OF AMORPHOUS RIBBONS
WITH Cr CONTENT
H. Kadiri (I), C. Djega-Mariadassou ('), P. Rougier (2), J. L. Dormann ('), A. Berrada (I) and P. Renaudin (')
(I) Laboratoire de Physique des Mate'riaux, Universite' Mohamed V, B P 1014 Rabat, Maroc
(2) Laboratoire de Magne'tisme, CNRS, 921 95 Meudon- Cedex, France
Abstract. - Amorphous ribbons with general formula Fel-y-,Cr, (PCSi),
,
(0.004<
y<
0.12), (0.08<
z<
0.22) have been studied. Cr seems responsible for the decrease of saturation moment, Curie temperature and mean hyperline field. The hyperline field distribution exhibits some particular structure when Cr content is increased. A low Avrami exponent has been found from crystallization studies.Introduction
Iron based amorphous alloys are suitable for mag- netic and mechanical applications. But the use of these alloys can be limited by their corrosion resis- tance which can be highly increased by Cr addition. Few studies are devoted t o the magnetic properties of this kind of addition [I-31 although interesting features can occur owing t o the antiferromagnetic coupling be- tween Fe and Cr. In this paper we present the results obtained on amorphous alloys with medium addition of Cr.
Experimental
The amorphous state of the sample was checked by X-Ray diffraction. Nominal composition for samples A, B, C, D is reported in table I. The magnetic mea- surements from 4 K up t o 300 K have been performed in a field range 0-17 KG with the field parallel to the sample plane. From 300 K up to 900 K an applied field of 100 Oe and a heating rate of 10 K/mn allowed the determination of the Curie temperature and of the crystallization temperatures. Mossbauer studies were performed from 1.4 K up to 300 K. The Mossbauer spectra were fitted using a distribution P ( H ) with a line-width of 0.30 mms-l, a quadrupole splitting equal to zero and with a unique isomer shift value
[4]. The crystallization rate has been obtained from
Mossbauer measurements at 300 K after annealing in a furnace under vacuum at 723 K with annealing time varying in the range 10 mu-66 H. It has been com- pared to that obtained by the Joule heating method
[5] where the samples annealed as above mentionned were submitted to an increasing electrical power up to the whole crystallization of the samples.
Results and discussion
Results are summarized in table I. All samples show a well defined Curie temperature T, with a decrease in agreement with those measured on similar compounds 11, 21. The Curie temperature does not sensitively de- pend on the nature of the non-metal element the rapid decrease with Cr content points out the existence of a critical concentration (- 23 at % Cr) where the fer- romagnetic behaviour dissappears. The atomic effec- tive moment per iron atom is very sensitive too t o the Cr concentration and decreases when Cr content in- creases. The hyperfine field distribution corroborates the drastic effect of Chromium (Fig. 1). In the con- centration range studied three types of hyperfine field distributions can be deduced: i) for the lowest Cr con- tent (sample A) one symmetrical peak appears with
H,, = 260 KG a t 77 K; ii) for intermediate Cr con- tent (sample B, C) the main peak is quite symetric
H,,, = 255 and 250 kG. Satellites peaks clearly ap-
pear with one first maximum at 85 kG and a relative
Table I. - Composition, C u r i e temperature T, (K)
,
crystallization temperatures T,,,
T,, (K) and effective mag- netic moment per i r o n atom at 300 K ( p ~ ) f o r studied samples.Sample
A Fe74Cr4PsCl1Si1,6 485 690 815 1.44
B Fe7zCrsPsCllSi1,6 410 830 1 3 2
C J?e72CrsPgC1oSi0,3 375 730 780 1.05
D FessCrllPsCloSil,g 344 750 0.75
JOURNAL DE PHYSIQUE
For samples B, C, D, a broad distribution spreading from 0 over 385 kG could give too a good agreement ratio of the Mossbauer fits with anglle 8 different from that found for the distribution prerien4ed in figure 1.
As the solutions presented in figure 11 for samples B, C, D are in continuity with the univotlue distribution of sample A we prefered those solutio~ls rather than the 'broad distribution.
The possibility of a reentrant transition a t 7 K for sample C must not be excluded. Taking into account the effect of the 8 value upon the inaximum value of the strongest field distribution, we observed a slight increase of H,, beginning near 7 K (at 40 K and
10 K, H,, = 248 and 249 kG respectively; when at 1.4 K H,, = 255 kG). For sample D such transition is not easy t o detect because of the complexity of the field distribution.
The susceptibility measurements at high temperaure Fig. 1. - P ( H ) distribution versus H for sample A,
*,
C , allows to define two crystallisation temperature T,,D, at 77 K. and T,, (Tab. I) corresponding to the beginning of
two steps of the crystallisation process. The depen- abundance of 10-20 % ; iii) for the highest Cr content
(sample D) the mean peak splits into two and the satel- lite peaks too. One may define a mean value of H,,
equal to 220 kG. The maximum of the lowest peaks is very close to that of the second case.
These facts can be related t o the existence of various Cr neighbours appearing with increasing Cr content; lowest peaks may correspond to Fe atom with sub- stantial number of Cr neighbours. One must be how- ever circumspect in the interpretation of the lowest peaks as they can be related too t o the approximation used in the calculation of the hyperfine field distribu- tion. Nevertheless, such satellites detected previously in Fe32Ni3,jCr14P 12B6 compounds (3) have been ex- plained as resulting from Fe atoms with high number of Cr neighbours. The decrease versus Cr content of
the mean maximum of hyperfine field distribution cor- responding t o the main peak at low temperature is similar to that found for the magnetic moment, show- ing that the exchange interaction between iron atoms decreases when x increases.
One must emphasize the difficulty inherent to the determination of the maxima of the hyperfine field dis- tribution particularly at highest Cr content: a mean orientation O of the magnetic moment relative to the
surface sample has been detected for all samples. This mean orientation which varies versus temperature be-
low 300 K is moreover very sensitive to the historic cooling of the sample below 300 K, probably in con- nexion with thermal stress effects which lead to a be- ginning of crystallization after several cycles. However, for sample A, the field distribution deduced from the Mossbauer temperature study was unambiguously as- sociated to a unique value of 8 at each temperature.
dence of the amorphous to crystalline transformation
versus annealing time at 725 K has been analyzed by
Joule effect method using the Johnlson-Mehr-Avrami equation. A mean exponent of 0,9 up to 66 hours an- nealing was obtained. Similar results were deduced- from the Mossbauer spectra. This rather low value is consistent with a mechanism of cryr:tallization involv- ing nucleation a t the heterogeneity boundary. After 66 hours annealing, the samples are completely crys- tallized. When the annealing time is increased up to 7 days, the Mossbauer spectra revleal at 300 K, the presence of a paramagnetic fraction which main part splits into several 6 line-patterns at 4 K. This fact may be interpreted by bulk magnetic phases present in the samples with low ordering temperature. The presence of small superparamagnetic clusters cannot however be ruled out.
Acknowledgment
The authors are indebted to Mr Benoit de Guillebon from Centre de Recherche St. Gobain, Pont B Mousson (France) for providing the samples.
[I] Henry, M., Bourrous, M., Varret, F. and Fournier, P., J. Mater. Sci. 19 (1984) 1000.
[2] Olivier, M., Strom-Olsen, J. 0. and Altounian,
Z., Phys. Rev. B 35 (1987) 33.
[3] Chien, C. L., Phys. Rev. B 19 (1979) 81.
[4] Le Caer, G. and Dubois, J.
Mr.,
J. Phys. E 12(1979) 1083. .
