Q Elsevw, Pans
The magnetic properties of Fe-Cr based compounds Ann. Chim. Sci. Mat, 2001,26 (3), pp. 9-16
INFLUENCE OF THE NATURE OF METALLOIDS ON THE MAGNETIC PROPERTIES OF Fe-Cr BASED COMPOUND!3
Abdcsslcm MOUSTAlDE*, Rabo BERRAH@, Sahhcddmc SAYOU’@. Ahmcd HASSIN+, HalumaKAD~.AmalBERRADAc
Laboratoxe de Phys~quc du Solldc, D+rkment de Physlquc. Faculti dcs Sciences, B P 17%, F&s-Atlas, Maroc Laboratoue de Physique des Mathaux et de Mtcrohctroniquc, Facultt des Sc~enccs. B.P. 5366, Ah Chock, Route d’E1 Jad~da, Casablanca, Maroc.
Iabomtoire de Phywquc des Mathaux, Faculti dcs Sc~cnces, B.P 1014, Rabat, Maroc.
Abstract - Magnew prop&~ of amorphous Pe~,_&(PCSi), (0.004 c y < 0.12,0.08 < z c 0.22) alloys were studted. Cr addihon influences drastically these properties. The comparison with a pmvmus study performed on amorphous FeCrBSi shows the role played by the nature of the
inetallods pm&it in these systems. _
Keywords: amorphous ferromagnets- amsotmpy- magneuc properues.
Rt!SUd- Iutluence de la uature des rn&alhUee SW lea propriMs nia&tiques des eumpoa& & hnae de Fe-Cr. Nous nous intkessons & l’&ude des propri&ds magnctiques de la tie d’alhages amorphes Fer_&r&PCSi)z, (0.004 c y c 0.12 , 0.0s < 2 c 0.22), &at& sous forme de rubans par trempe rapide. La substitutmn des atomes de fer par des atomes de Cr influe, de faqon notoim, sur ces prop&&. En outm, nous mettons en 6vidence le tile joti par la nature des m&alloi&s presents dans ces alhages.
Motscles : alliages ferromagnetlques amorphes, amsotropie, propri&& magn&ques.
1. INTRODUC!TION
Because of thev potential for technologrcal applications [ 1,2], amorphous ferromagnets based
on the 3d transition metals (TM) have been extensively studied. Among these studies, a
constderable part has been devoted to the amorphous Fe and Fk-Ni based alloys containing Cr
atoms [3 to 151; Cr alloying, like Ni, has been found to have pronounced effects on the magnettc
Rents : &&he&he SAYOURI. Laboratoue de Physrque du Sohde; D6partement de Physique,
Fact&? des Sciences, BP 17%, F&Atlas. Maroc.
10 A Moustarde et al.
characteristics of these alloys, such as Curie temperature, saturation moment, exchange constant and amsotropy constant. In particular, chrommm has a more destructtve effect on the magnetic ordering m amorphous alloys than in crystalline ones; indeed, because of the hybruhzatton of Cr and Fe 3d orbttals, the presence of chrommm netghbours should have a greater effect on the local iron 3d density of states m an amorphous alloy [la] Moreover, the presence and the nature of the metalloids m these alloys seem to play a certain role, more or less important, on magnetic charactensttcs as revealed by MaJtundar et al [17] spin waves studies on Fe(BC) metalhc glasses.
In these systems the replacement of boron by carbon increases the spm wave stiffness (and hence the room temperature magnetization ). Luborsky et al. [14] reported analogous results on a series of compounds mcludmg Fe(BC). To explam the experimental results, a variety of models have been proposed (charge transfer, (sp)-d hybrtdizauon, Fnedel’s sum rule...)
Competmg mteracttons can be present m dtlute alloys or in concentrated ones. In the first group they result from the oscillatmg long-distance- behavrour of the R.K.K.Y interactions and from the distnbutton of the distances between magnetic impurtttes. In the second group, m the other hand, they gtve rise to short-range ferromagnetic and anttferromagnetic exchange [l&19]. In particular, m amorphous magnets, ferromagnetic ones are those where ferromagnettc exchange dornmates while those where the sign of the exchange interactton between neighbourmg spins is random exhibit spin-glass behaviour. In tiuon to the exchange mteractton, the magnetic anrsotropy plays a mam role m magnetic ordermg m both crystalline and dtsordererd systems. In amorphous ferromagnets, the direction of the amsotropy randomly fluctuates from one magnetic atom to another one [20]. If the amsotropy energy is larger than the exchange energy, then the onentauon of any spm 1s determined by the due&on of local anisotropy [21]. In the hmit of weak random anisotropy, the exchange mteractton favors long-range ferromagneuc order, so there must be ferromagnetic clusters of spins at small dtstances. At large distances, weak fluctuations m spm onentation due to the local random anisotropy destroy long-range order[22-241 Theoretical considerations have been based on two assumpttons; the first assuming that magnetic order is governed by random exchange only [25,26], and the second assuming that magnetic order IS created by random amsottopy in the presence of ferromagnetic exchange [20]. The latter consideration &s been adopted to establish a phenomenological model [27 to 291 to study amorphous ferromagnetism. This model has been used to dertve all mam properues of amorphous magnets [3, 30 to 331.
Recently, crystallization properties and Mossbauer studtes of amorphous Fei_&r@C!Si), have been mveshgated by Kadui et al [lo]. These authors showed the effect of Cr addition on these properttes. In this paper, we consider, agam, the magnettc measurements performed at 10 K [lo], and, to underline the effect of Cr addition and the role played by the metalloids, we have determmed the saturation moments and the amsotropy ( KL) and exchange (A) constants; KL and A were calculated usmg the phenomenologcal model mentioned above. Then we have compared all the results obtamed to those previously reported [3] concerning amorphous Fe-N&r-St-B, focusing our attention on the combined effect of Cr and the metalloids The effect of Ni from magnettc anisotropy point of view will he mported in the near future)
2. RESULTS AND DISCUSSION 2. 1. Maanetro studies
The studred compostttons were Fe&rJ%CiiSii 6, Fe&sPsCi&i 6, Fe72CrsP&i&s. and
Fe.&riiPsCi&i 9; the methods of preparation and X-ray chamcterizauon have been reported in ref.
The magnetic properties of Fe-Cr based compounds 11
[lo]. The vananon of magnetmatron, at 10 K, as a functton of the applied field rangmg from 0 to 1.45 MA/m, 1s shown in j&m-e 1. We observe that the mater& have a htgh suscepttbility. The saturation magnetmatton MO has been detennmed at I-I_ usmg the H-1’2 dependence, which wtll be ~umfied m the next sectton.
I I I I
I 15
-7
l mmmmmmmmmmmmm m
mm
v T=lOK
n xc?4 l Xc-8
17 A xc-9 v X(-ill
75 ; ‘I I
00 04 08 12 16
H(MA/m)
Figure 1. Magnetization curves of the samples, as a functron of the applied field.
The alloy moment c(~ for each sample can be deduced from the mlanon.
I&I = Mom/ NPu (1)
where N is Avogadro’s constant, m the molecular weight of the alloys, and pg the Bohr magneton.
p3d decreases with increasing Cr content @gure 2). The values of MO and p3d of the samples
studted are reported in ruble 1. As discussed pmvtously [3], Cr has a dramatic effect on the
ferromagneusm present m these materials. fable ZZ mcludes, with the aim of doing comparison, the
values of and the exchange and anisotropy constants of amorphous Fe-&r4BtsSis [3] and
Fe-&r&C!i~Sit 6 (this study); the composition of these two compounds differ by the nature of the
metallouls. Let us first compare then MO value (table ZZ) It seems logical to infer that the
12 A. fvlouataide et a/.
difference between the two values may be attrrbuted to the nature of the metahoids present in these systems. The replacement of B atoms by P and C ones diminishes Me. Majumdar et al. [17] have reported a similar result when replacing B atoms by C ones in Fe(BC) metallic glasses. The lowering of Ma suggests an electron transfer from metalloid atoms to TM atoms whtch have the unfilled d band more important m the case of C and P atoms than that of B atoms [34,35].
0. .
2 4 6 8 IO
xcr
Figure 2. The variation of the magnetic moment p3d, the exchange A and the anisotropy KL constants versus Cr concentration.
Table I. Some magnetrc parameters obtained from the approach to saturatron at 10 K.
Samples Mo(Am21Lg)
h&B)A(IO-'3J/m) KL(MJ/m3) I.
I I I 1 I
l+dW&~~S~~6 150.8 1.66 24.3 0.58 0.08
Fe72%PsCllSrl6 140.5 1.53 19.6 0.97 0.18
FenCrsppC~oSlo 3 133.8 1.45 17.4 1.0 0.20
119.3 1.29 14.7 1.2 0.29
The magnetic properhea of Fe-Cr baaed compounds 13
Table II. Comparison of the magnebzation, exchange and anisotropy constant values of amorphous &74cT99c~~!h~ 6 @resent Work) and Pe&r&lZSis [3].
Samples
Fe76Cdh2S18 131
Pe7&&G&16
(thl8 study )
M&m2 ILg) A(IO-‘3J/m) KL(MJ/m3)
160 28.58 2.64
150.8 24.3 0.58
2.2. Random maenetrc anisotropy
The approach to saturauon of the magnettc moment in random amsotropy magnets has been studied by FZUlnle and Kronmtiller [36]. They showed the presence of a II++& term in the saturauon magnetizatron. Chudnovsky and &rota [28,37] have given a phenomenological model to mterpret the approach to saturation. From this model, for applied fields less than the exchange field KX, the magnetic moment IS expected to show a linear dependence on H-l’*. The followmg equauon and relauons descnbe this situation [28,37]
M = Mo(l-(1/15)(H, /H)“*) (2)
H, = H; /H& (3)
H, = 2AlM,Rf (4)
where H, is the random anisotropy field, and R, is the length over which the local axes show a correlation; mdeed, the amsotropy directtons are assumed to be randomly dtstnbuted beyond the character&c length scale R, , where atomtc short-range order takes place. We assumed R, = 10 A [13]. A IS the exchange constant which can be obtamed from the mean field model proposed by Hasegawa [381 and from the Curie temperature using the mlatton proposed by Heiman et al. [39]
A = CmKBT&,., 14(Sm + l)r,, (5)
where C, IS the TM concentratron, S, IS the spin of TM, T, IS the Curie temperature [lo] and the interatonnc distance r,_, IS taken as 2.5A.
Plotting M as function of H-l’* yigure 3). one can deduce MO, the magnetization extrapolated value to H, , and H, (from the slope). Knowing MO and A, H, and consequently H, can be determmed. H, is related to the anisotropy constant by the relation[28,37]
H, = 2K, /MO (6)
14 A. Mouataide et al.
150
h
