N . T - i
~ i journal of -.~_" magnetism
and magnetic
~ H materials
ELSEVIER Journal of Magnetism and Magnetic Materials 163 (1996) 339-344
Influence of small addition of Er on the magnetic properties of amorphous Co80ErxB20_ x alloys
H. Lassri a , * , A. Qachaou b, A. Belayachi c, A. Itri c, N. Hassanal/n c, A. Berrada c, M. E1 Y a ma ni c, R. Krishnan d
a Laboratoire de Physique des Mat£riaux et de Micro-£lectronique, Facult£ des Sciences, B.P. 5366, Ain Chok, Casablanca, Morocco b Laboratoire de Physique du Solide, Facultd des Sciences, K£nitra, Morocco
c Laboratoire de Physique Des Matdriaux, Facultd des Sciences, B.P. 1014, Rabat, Morocco d Laboratoire de Magn£tisme et d'Optique de Versailles, CNRS, URA 1531, 92195 Meudon, France
Received 5 October 1995; revised 8 February 1996
Abstract
The influence of the addition of a few atomic percent of Er on the magnetic propeities of amorphous Co-B alloys has been investigated. With increasing Er content (decreasing B content), the magnetic moment of the Co atom increases up to x = 4. The temperature dependence of the magnetization of the samples has been studied. Spin echo nuclear magnetic resonance measurements were carded out at 4.2 K.
Keywords: Rare earth doping; Magnetization - temperature dependence; Amorphous systems - alloys; Substitution effect
1. Introduction
It is well known that C o - B amorphous alloys have good soft magnetic properties. Amorphous tran- sition metal (TM)-metalloid (ME) alloys have in- voked considerable scientific activity. However, studies of rare earth doped T M - M E metallic glasses are very recent [1-4]. In order to study the influence of the addition of few atomic percent erbium on the magnetic properties of C o - B alloys, we prepared amorphous Cos0ErxB20_ x (0 < x < 4) alloys and in- vestigated their magnetic properties and nuclear magnetic resonance spin echo. We have substituted
* Corresponding author.
Er in place of the metalloid B for two reasons: (i) rare earth atoms can also stabilize the amorphous state, (ii) in this way the Co content is kept constant.
2. Experimental details
A m o r p h o u s C 0 8 0 E r x B 2 0 _ x (0 < X < 4) alloys were prepared by conventional melt spinning in a pure argon atmosphere in the form of ribbons about 2 mm wide and 3 0 - 4 0 Ixm thick. The amorphous state of the ribbons was checked by X-ray diffraction and the chemical composition was determined by inductively coupled plasma (ICP) analysis.
The magnetization ( M ) was measured using a vibration sample magnetometer in the temperature 0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved.
PII S 0 3 0 4 - 8 8 5 3 ( 9 6 ) 0 0 3 4 4 - 7
340 H. Lassri et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 339-344
range from 4.2 to 300 K under applied fields up to 18 kOe.
Nuclear magnetic resonance (NMR) measure- ments were carried out at 4.2 K using a home built frequency spin-echo spectrometer. The RF field on the exciting coil was parallel to the ribbon plane.
3. Results and discussion
3.1. Magnetic studies
3.1.1. Concentration dependence of the magnetiza- tion
For all the samples studied technical magnetic saturation could be achieved with an applied field of 18 kOe at all temperatures. Fig. 1 shows the Er concentration dependence of the magnetization M at 4.2 and 300 K. M decreases linearly with addition of Er at both temperatures. As the Co concentration is constant, this decrease in M clearly arises from the antiferromagnetic interaction between Co and Er mo- ments. We have reported recently magnetic studies under high magnetic fields, on amorphous C80_xErxB20 (0 < x < 9) alloys [5]. From these stud- ies the magnetic moment of Er atom was determined to be 7 /~B at 4 K a value which we have assumed for the present study. This moment is smaller than the theoretical value of 9 /x B. This reduction in Er moment, a feature which is generally encountered for rare earth atoms in amorphous alloys, arises from the
110 ~ r x B z o - x
.~"~1°0~ ~ ~ ~ . . ~
~N cz T = 4 . 2 K "~
90- ~" T = 2 9 5 K
80 o.o 1.b e.'o 3.b 4.'o--
X(Er)
Fig: 1. Concentration dependence of magnetization at 4.2 and 295 K.
(2O-x) (B)
2 0 1 9 1 8 1 7 1 6
1 . 4 0 - - - - J - __ _ ~ r
Co~oErxBZ0-x
1 . 3 5
o
~u 1.30
1.25
0.0 1 .'0 2 J0 3 .'0 4 .'0
X(Er)
Fig. 2. The cobalt moment as a function of Er concentration at 4.2 K.
random local anisotropy of Er atoms which tends to align the Er moment along random directions.
It is known that due to the hybridization of 5 d - 3 d orbitals, the Co moment decreases with the increase in Er concentration. Furthermore, in our case, the addition of boron, as is well known, also leads to a decrease in the Co moment due to the charge transfer between 3d orbitals and p orbitals of B. Because of the antiferromagnetic alignment of the magnetic mo- ments of Er and Co atoms, we can write:
---- 180 o - -
--180/~Co - X / ~ E r - C(20 -- x) 1/100, (1) where /X~o = 1.7 /x B, C is the loss factor in Co moment per B atom. C was found to be 1.77/z B by fitting the data. When x = 0, the previous equation describes very well the Co moment dependence in C o - B as function of boron concentration and it is in agreement with the results of Tange et al. [6]. The Er concentration dependence of the Co moment (/~co) expressed in Bohr magnetons at 4.2 K is shown in Fig. 2 from which we can deduce that /Zco increases with increasing Er content (decreasing B content) in the series studied. The change of /ZCo in amorphous CosoErxB2o_ x alloys may be due to the influence of both B and Er. This increase shows that the decrease in /ZCo caused by Er is offset by the diminution of B.
The latter has a stronger effect on /ZCo.
H. Lassri et el./Journal of Magnetism and Magnetic Materials 163 (1996) 339-344 341
3.1.2. Temperature dependence of magnetization
The temperature dependence of the magnetization of the samples was studied at H = 18 kOe and some typical results are shown in Fig. 3. It is seen that with increasing Er content as the temperature is lowered, M shows first a broad peak then starts decreasing. This decrease in M ( = / x a) is due to an increase in the magnetization o f the sub-network of Er. For the concentrations studied here the compen- sation of moments does not occur.
The temperature dependence of the saturation magnetization o f amorphous CosoErxB20_ x alloys can be described in terms of molecular field theory [7-9]. The total saturation magnetization/.L a to a first approximation can be written as
/.z a = M = Mco -- MEt
= NtzB[SOgcoSco- (20-- X) gErJZr]/lO0, (2)
where N is the number of atoms per unit volume, gi
(i = Co, Er), is the Land6 factor and we take gco = 2.2 and gzr = 1.2. The sub-network magnetizations S¢o and Jet are assumed to follow Brillouin func- tions:
S c o ( T ) = < S c o ( 0 ) ) B s c o [ gcoSco P~BHco(T)/kT], (3) J e r ( T ) = ( J e r ( 0 ) ) B j e r [ gerJerlZBHer(T)/kT],
(4)
CosoErxBzo-x 110
9 0 o x = l
J c~ x = 2
D X : 3
<> x = 4
80 o 16o z6o aoo
T(K)
Fig. 3. Temperature dependence of the magnetization for different concentrations of Er.
1.5
CoaoEr4B~s
0.5
7 o o o g x p r :
o . o z~o 500
. . . . . T ( K ) - 0 . 5
Fig. 4. Temperature dependence of the magnetization calculated from the molecular field theory.
where Sco and ( J e r ( 0 ) ) are the Co spin momentum and Er effective total angular momentum, respec- tively. The calculations are based on an effective moment g/x B (JEr(0)) of about 7 / x R.
The molecular fields Hco(T) and HEr(T) are
given by
Hco(T) = 2 JCo_Co Zco_coSco(T)/gco
q- 2 JCo-Er ZCo-Er( gEr - 1) J E t ( T )
/gco tZu , (5)
H E r ( T ) = 2 J E r - C o Z E r - C o S c o ( T ) ( gEr -- 1 ) / g E r / Z B + 2 JEr-Er ZEr-Er( gEr -- 1) 2 J E r ( T )
/ g E r / d ' B , (6)
where Jco-co, Jco-Er and Jer-Zr are the exchange integrals for C o - C o , C o - E r and E r - E r interactions, respectively, zij (i, j = Co, Er) is the number of nearest neighbors o f the atom j for the atom i.
The values o f the parameters Sco, Jco-co, Jco-Er and Jzr-Zr were determined as functions of Er con- tent such that Eqs. (2)-(6), fitted the experimental data of the temperature dependencies of the magneti- zation. It is seen that Jco-co decreases from 19.3 ×
10 -22 J for x = 1 to 17.1 × 10 -22 J for x = 4 . JCo-Er and Jer-Er are found to be 2.1 × 10 -22 J and 0.2 X 10 -22 J, respectively. A typical example is shown in Fig. 4 for the sample with x = 4. It is seen
342 H. Lassri et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 339-344
I
CoaoErxBeo-x
----4/
100 150 200
Frequency MHz Fig. 5. Resonance spectra for CosoErlBa9 at T = 4.2 K.
that the experimental points align well with the calculated curve. We also calculated the temperature dependence of/XCo and ]J'Er'
3.2. NMR studies
We carried out spin echo NMR studies at 4.2 K for all the samples. Fig. 5 shows the 59Co nuclear resonance spectra for the alloy with x = 1. It is evident that the spectra of the amorphous alloys are characterized by a broad distribution of hyperfine field. For all samples with (0.5 < x < 4) a shoulder at 218 MHz is found near the value of pure crys- talline cobalt. This shoulder is probably due to small components of pure cobalt fcc phase in the amor- phous matrix (the hcp phase is illustrated by the cusp around 227 MHz). The existence of inhomogeneities in this sample is also confirmed by X-ray diffraction.
Following Kobayashi et al. [10] we assume that the Co hyperfine field (Hhy p) is determined by the mo- ment of the atom itself and those which are the nearest neighbors to it, (Hhy p > is given by:
<Hhyp> = O ~ l o c H - / 3 E f C n n ,
where /Zlo c is the moment of the Co atom concerned, /Znn is the moment of each of nearest neighbor atoms to it and a and /3 are the constants of proportional- ity. E means the summation for all nearest neighbor atoms. Its recalled that the Co resonance frequency (RF) in MHz is equal to 1.0054Hny p (kOe).
According to the standard theory of domain walls
the parameters which correspond to the wall thick- ness and energy density are:
~ ( a / K u ) 1/2,
( AK ) 1/2,
where A is the exchange constant and K u is the coherent anisotropy constant. The interesting points concerning the series of amorphous CosoErxB20_ x (0 < x < 4) are the following:
1. Good soft magnetic properties.
2. Anisotropy/exchange ratio in our alloys is rela- tively small.
Consequently, the wall thickness is large. The fluctuation in the position of the nearest neighbor atoms and the nature of the polarization at long range can be contributed over the broad frequency range from 70 to 225 MHz needed for the amor- phous Co80ErxB20_ x alloys.
The mean center of gravity of the spectra in- creases with increasing Er content up to x = 4. This increase in resonance frequency (hence < n h y p > ) is due to an increase in the cobalt moment. Thus the concentration dependence of (RF> can be written as (RF> = (160 + 7.89x) MHz. The variation of (RF>
with Er content is shown on Fig. 6 and the values extrapolated to x = 0 are in good agreement with those published for Cos0B20 [11-13].
The hyperfine field distribution causes the broad- ening of the spectra. The width of the hyperfine field
1 9 0 -
~ 8
~170
160
CosoErxBzo-x
150 O. O 1 .~0 2.'0 3.'0 4,'0
X(Er)
Fig. 6. Variation of resonance frequency versus Er content.
185 Co0oErxBz0-~
N
175-
¢d
z eT*
165-
H. Lassri et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 339-344 343
155 2.4 2/6 2/8 3.'0
ZB
Fig. 7. Variation of resonance frequency versus Z s.
at a reduction of the Co frequency by A f = 37 MHz per B neighbour. Consequently, each B atom substi- tuted by Er atom restores Hhy p. This explains why the center of this spectrum is at a higher frequency than that observed for the Co80B20 sample.
Fig. 8 shows a linear relationship between the nuclear resonance frequency and the mean magnetic moment (/xco) per Co atom for the CosoErxB20_x samples investigated. The corresponding hyperfine coupling constant is 130 M H z / / x B, which is nearly the same as in amorphous C o - B alloys [11]. The main result of our measurements is the reduction of the mean value of the hyperfine field nhy p with increasing boron content, in agreement with the trend observed in magnetization measurements.
distribution in C o - B can be explained by the distri- bution of the metalloid and Er atoms around the metal atoms in the amorphous structure. The mean number ( Z s ) of metalloid neighbors of a Co atom at a concentration x is given by
20 - x
(ZB) Z 8 0 + x
where Z is the mean number of Co neighbors of a B atom (taken as 12).
The reduction of the nuclear resonance frequency as function of ( Z B) would indicate a destruction of the localized hyperfine field (Fig. 7). Thus, we arrive
1.8 1.6 1.4 1.2 1.0 0.8 :::k~ 0.6 0.4 0.2
C o o o E r x B /
/ / /
D Exp. d a t a
• C o fcc
0.0 ~'o 16o 16o z6o
Frequency M H z
Fig. 8. Variation of resonance frequency with /.Lco for Cos0ErxB20_ x (0 < x _< 4).
4. Conclusion
The saturation magnetization has been analyzed in terms of mean field theory. The exchange interac- tions Jco-co, Jco-zr and JEr-Zr were evaluated.
From low temperature magnetization and nuclear magnetic resonance (spin-echo) studies, we have shown that in amorphous Cos0ErxB2o_ x (0 < x < 4) alloys the cobalt moment increases with increasing Er content. The main result of our study is the increase in the mean value of the Co resonance frequency with increasing Er content up to x = 4.
We have obtained a linear relationship between the resonance frequency and the magnetic moment per Co atom.
Acknowledgements
We gratefully acknowledge Dr. K. Le Dang from I.E.F. of Orsay (France) for obtaining the spin echo NMR spectra.
References
[1] N.C. Koon, C.M. Williams and B.N. Das, J. Appl. Phys. 52 (1981) 2535.
[2] R. Kxishnan and H. Lassri, Solid State Commun. 69 (1986) 803.
344 H. Lassri et a l . / Journal of Magnetism and Magnetic Materials 163 (1996) 339-344 [3] R. Krishnan and H. Lassri, Solid State Commun. 73 (1990)
467.
[4] R. Krishnan, O. El Marrakchi and H. Lassri, Solid State Commun. 77 (1991) 567.
[5] R.J. Radwanski, J.J.M. Franse, R. Krishnan and H. Lassri; J.
Magn. Magn. Mater. 119 (1993) 221.
[6] H. Tange, Y. Tanaka, T. Kaminori and M. Qoto, J. de Phys.
Coll. C-8 49 (1988) 1283.
[7] R. Hasegawa, B.E. Argyle and L.J. Tao, A:.I.P. Conf. Proc.
24 (1975) 110.
[8] J.F. Herbst and J.J. Croat, J. Appl. Phys. 53 (1982) 4304,
[9] R. Krishnan, O. E1 Marrakshi, H. Lassri and P. Rougier, J.
Appl. Phys. 73 (1993) 7599.
[10] S. Kobayashi, K. Asayama and J. Itoh, J. Phys. Soc. Jpn. 21 (1966) 65.
[11] W. Boehner, H. Ltitgemeir and W. Zinn, J. Magn. Magn.
Mater. 62 (1986) 152.
[12] P. Panissod, A. Qachaou and J. Durand, Nucl. Instr. and Meth. 199 (1982) 231.
[13] R. Krishnan, K. Le Dang, P. Veillet and V.R.V. Ramanan, J.
Appl. Phys. 57 (1985) 15.
