0 Elsevier, Paris
T,,Gd,B,, amorphous allays (l=Fe, Co) Ann. Chim. Sci. Mat, 1999,24, pp. 509-514
MAGNETIC PROPERTIES AND EXCHANGE INTERACTIONS IN T, GdJm AMORPHOUS ALLOYS (T=Fe, Co)
A. HASSINI, A. BOUHDADA, M. AYADI
Laboratoire de Physique des Matiriaux et de MicroBlectronique, Universiti Hassau II, Facult6 des Sciences &n Chock, Route d’E1 Jadida, Km 8, Casablanca, Maroc
Abstract - The magnetization and Curie temperature of amorphous TsO-xGdxBrO alloys (01x124) were investigated. The Gd moment is found to be 7~s in agreement with the theoretical value at 4.2 K. This would indicate a collinear spin structure for Gd. A mean field model has been used to explain the temperature dependence of the magnetization. This analysis allows to determine the spin value of T and the effective exchange interactions between T-T and T-Gd atoms which are found correlated with the T moment.
R&urn6 - PropriMs magnktiques et interaction d’khanges dans les alliages amorphes Ts&d,B~o (T = Fe et Co). L’aimantation et la temperature de Curie dans les alliages amorphes Tso-xGdXB20 (01x124) ont Cte etudiees. A 4.2 K le moment de Gd est de 7 pa, en accord avec la valeur theorique a la mkne tempCrature, ce qui indique une structure de moment colineaire. En utilisant un modele de champ moyen, nous avons pu deduire les interactions d’kchanges entre les atomes T-T et T-Gd et d&tire la variation de l’aimantation en fonction de la temperature. Ces interactions sont correlees avec le moment du T.
1. INTRODUCTION
Amorphous alloys containing small amounts of rare earth metals are of interest due to their industrial applications. These alloys are magnetically soft and appear useful as magnetostrictive device materials. When crystallized they develop very large hysteresis, with high intrinsic coercive force suggesting possible uses as permanent magnets. The alloys are also interesting from a fundamental point of view as they offer the possibility to study various aspects of 3d and 4f magnetism. On the contrary to crystalline materials these aspects can be investigated
_*_ wt. : A. HASSINI, Laboratoire de Physique des Mat&iaux et de @xo&ctronique, Universit.6 Hassan II, Facult6 des Sciences &n Chock, Route d’E1 Jadida, Km 8, Casablanca, Maroc
510 A. Hassini et al.
versus a continuous concentration of the rare earth metal as well as relation with the low local symmetry that is a characteristic for the amorphous state. Rare earth metals with spin orbit moments are known to give rise to large random magnetic anisotropy in amorphous state [l].
Therefore we wanted to study the magnetic behaviour of Gd which is interesting because, as well known, it has no spin orbit coupling. In this work we describe the results of magnetic studies of amorphous TsaexGd,BzO alloys versus temperature in the range 4.2-300 K, under magnetic fields up to 1.8 tesla, only.
2. EXPERIMENTAL
Amorphous Tso.,Gd,Bzo ribbons with OM24, were prepared under an argon atmosphere by the single roller quenching technique. The purity of the starting materials was 99.99 % for B, Gd, and 99.999 % for Fe and Co. Argon ejection pressure of 2 to 5 kPa and a substrate speed of 35 m/s were employed. The melt ejecting tubes were made of quartz glass with an ejecting orifice about 0.4 mm in diameter. The ribbon samples were about 30 pm thick with different widths varying from about 3 to 6 mm. X-ray diffraction was used to check the amorphous structure. The exact chemical composition of the samples was determined by’ electron probe microanalysis. The magnetization was measured by vibrating sample magnetometer from 4.2 to 900 K in magnetic fields up to 1.8 tesla. Curie temperature was determined in applied field of about 0.0 1 tesla.
3. RESULTS AND DISCUSSION
For all the samples studied, magnetic saturation was recorded for HI1 tesla at all temperatures. The concentration dependence of the magnetization (pa in pa at 4.2 K) and the Curie temperature (Tc in K) are shown in&ures I and 2. There is a linear decrease in magnetization for W_x124 and the magnetization compensation is reached for hmp close to 10 and 16 at % for the alloys with Co and Fe, respectively.
LGdx B*0
T = 4.2 K n T=Fe l T=Co
Gd ( at. % )
0-
0 5 10 15 20 25
Gd ( at. % ) Figure 1. The Gd concentration Figure 2. The Gd concentration
dependences of pW dependences of Tc.
8 600
H” 400
LGdx B 20
n T=Fe
800 a T=Ca
T8,Gd,B20 amorphous alloys (l=Fe, Co) 511
The variation of Tc versus x is somewhat similar also to that of the magnetic moment pL,.
The above results are characteristic of the antiferromagnetic interaction between Gd and T atoms which is well known in metal systems. The moment pa can be written as:
b= 1 @O-x) ,-h-x,.&, 1 /loo (1)
For small concentration (x I 8) of Gd, the moment of T is not modified. So taking the value of kt.r obtained from the alloy with x=0 and substituting in the equation (1) it is possible to determine pod.
n T=Fe . T=Co
0.0 I
0 5 10 15 20 25
Gd ( at. % )
Figure 3. The Gd concentration dependences of p-r.
At 4.2K the calculated moment is found to be 7pn which is in agreement with the theoretical value g&$&n. This would indicates a collinear spin structure for Gd. Then using this value of k&J we calculate the value of pr for other Gd-rich compositions. It is found that )1r decreases when the Gd concentration increases (Figure 3). This decrease is attributed on one hand to the increased filling of 3d bands of T by the sp electrons from B since now the relative concentration of B with respect to T increases, and on the other hand, to the hybridization of the 5d and the 3d orbitals.
The temperature dependence of saturation magnetization of the samples was studied and the results are shown infigures 4 and 5. For the samples with x<~, the magnetization shows a decrease as the temperature decreases. However, for x>x-,r a compensation occurs at T=T,,, and for T<T,,, the alloy magnetization increases again due to the contribution from Gd moments.
512 A. Hassini et al.
h V zi=
P 0.9
0.6
0.3
0.0 _..
0 100 200 300 400 500 600
T(K)
0.4 h V P
*- 0.2
0.0
0 100 200 300 400 500 600 700 T(K)
Figure 4. The temperature dependence of the Figure 5. The temperature dependence of magnetization for the FesO,GdXBzo alloys. the magnetization for the CosO-xGdxBZO alloy.
The calculated curves are shown as solid lines The calculated curves are shown as solid lines In the past the mean field model has been used by several authors to calculate the temperature dependence of magnetization in many amorphous rare earth (R)-transition metal (T) alloys [2,3]. We have performed such an analysis of the temperature dependence of the magnetization in the amorphous Tso-XGdXBzo alloys. The equation (1) can be rewritten as
(2) where ST and SGd are the T spin and Gd spin values respectively, pa is the Bohr magneton and g is the Lande factor (i = T, Gd). S,(T) and Sod(T) are assumed to be expressed by the Brillouin function
Si (T)=Si (O)J% (&QhJWBT)
The molecular field Hr and Hod are given by
HGd=2(JGdT%dTST+JGdGdzGdGdSGd)/(gGd~B) (6)
Here, Jrr, Jrod and Jodod are. the exchange constants for T-T, T-Gd and Gd-Gd interactions, respectively; and zi (i, j=T, Gd) is the number of nearest neighbors of the atom j for the atom i.
The values of the parameters Sr, Jrr, Jrod and JGaGd were determined as a function of the Gd content in such a way that equations (2-7), fits the experimental data of the temperature dependence of the magnetization p.=. A typical example is shown infigures 4 and 5 for alloys with different concentrations. It is seen that the experimental points align well with the calculated curves.
T,,,Gd,B,, amorphous alloys (T=Fe, Co)
0 I I
0 1 2
PTT( PJ
Figure 6. Exchange interaction JTG~ as function of PT
513
We find that the predicted Tc and the corresponding experimental values agree within about 2%. It is seen that Jrr and JodT increase when the T concentration and hence l.rr decrease, where pr was calculated knowing the moment of Gd and that of the alloy (table I). Figure 6 shows the variation of exchange interaction Jrod with transition metal moment PT. A similar increase in JR~
has been reported in intermetallic compounds and amorphous alloys as well [4-61. The 3d-5d interactions depend critically on 3d-5d hybridization according to Brooks et al [7]. Therefore the increase in Jodr would indicate an increase in the 3d-5d hybridization when the T concentration relative to Gd is decreased.
Table I. Some fnndamental parameters calculated from the mean field model at 4.2 IS.
%30B20 0.57 - 198.0 - 820
C%.2Gd7.&2o 0.51 17.0 168.0 2 715
C%&dmB2o 0.38 19.5 167.0 1 470
4. CONCLUSION
The magnetic properties of Tsa,Gd,Bzs alloys and their temperature dependence were investigated with respect to their composition. The saturation magnetization was analysed in terms of the mean field model. The T moment, the Curie temperature and exchange interactions Jm and
514 A. Hassini et a/.
JrGd were evaluated. Therefore the increase of J rGd when increasing the Gd content should result of an extension of the 3d-Sd band hybridation.
5. REFERENCES
[l] R. Harris, M. Pilschke and M. J. Zukermann, Phys. Rev. Lett., 3 1 (1973) 160.
[2] R. Hasegawa, B.E. Argyle and M.L.J. Tao, AIP. Conf. Proc., 24 (1975) 110.
[3] A. Gangulee and R.L. Kobliska, J. Appl. Phys., 49 (1978) 4169.
[4] N.H. Due, Phys. Stat. Sol., 164 (1991) 545.
[S] N.H. Due, T.D. Hien and D. Givord, J. Magn. Magn. Mater., 104-107 (1991) 1344.
[6] S. Ishio, N. Obara, S. Negami, T. Myazaki, T. Kamimori, H. Tange and M. Goto, J. Magn.
Magn. Mater., 119 (1993) 271.
[7] M.S.S. Brooks, L. Nordstom and P. Johanson, J. Phys. Condens. Matter, 3 (1991) 2357.
(Article recu le 06/08/98, sous forme definitive le 17/11/98.)
