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Magnetization studies of amorphous Fe80 − xTmxB20 alloys
ELSEVIER
~1~ Journalof magnetism ,444 and magnutlc materials
Journal of Magnetism and Magnetic Materials 153 (1996) 132-134
Magnetization studies of amorphousFes0_xTmxB20 alloys M. Slimani a,b, M. Hamedoun
a,
H. Lassri c, S. Sayouri
a, * ,
R. Krishnan d
a Laboratoire de Physique du Solide, D~partement de Physique, Facult~ des Sciences, B.P. 1796, Fbs Atlas, Morocco b Laboratoire de Chimie du Solide E.N.S., Ben souda F~s, Morocco c Laboratoire de Physique des Mat~riaux et de Micro~lectronique, Facult~ des Sciences, Ain chok, Casablanca, Morocco a Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, C.N.R.S., 1 Place A. Briand, F-92195 Meudon, France
Received 21 December 1994; revised 21 July 1995
Abstract We have carried out magnetic studies at fields up to 18 kOe of melt- spun amorphous Fes0_xTmxB20 alloys. Magnetization and Curie temperatures were investigated. We have extracted the anisotropy constant from the coercivity.
1. Introduction In the last few years, studies of amorphous alloys based on rare earth metals have become very intense [1-4] because of their potentially useful physical properties in technological applications, and the fundamentally very important random magnetic anisotropy and magnetic exchange. In order to study the influence of the addition of Tm on the various magnetic properties of amorphous F e - B alloys, we prepared some amorphous alloys in the system Fes0_xTmxB20 (0 < x < 15).
2. Experimental The amorphous Fes0_xTmxB20 alloys (0 < x < 15) in the form of ribbons were prepared by the usual melt-spinning technique in an inert argon atmosphere. The amorphous structure was characterised by X-ray diffraction using Co K s radiation. The compositions of the alloys were determined
* Corresponding author.
by electron probe analysis. The magnetic moment was measured using a vibrating sample magnetometer (VSM) with a maximum applied field of 18 kOe, and in the temperature range 4-300 K. The Curie temperature T~ was determined from the evolution of the magnetic moment in a weak field (100 Oe) and in the temperature range 300 < T < 800 K.
3. Results and discussion For all the samples studied, magnetic saturation could be obtained with H < 1.8 T at all temperatures. Fig. 1 shows some typical results at 10 K. At the same temperature, 10 K, the magnetic moment of the alloy, /Za, decreases with increasing Tm concentration, indicating the antiparallel coupling between the Fe and Tm moments (Fig. 2). An extrapolation shows that at x = 20 a magnetic compensation would occur. The magnetic moment of Tm (/ZTm) was calculated from the alloy moment as follows [5]. The alloy m o m e n t ]£a can be written as /£a = 1( 8 0 -- X) "]£Fe -- X"
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)005 18-8
p.wml/100.
(1)
M. Slimani et al. / Journal of Magnetism and Magnetic Materials 153 (1996) 132-134
133
80(:
140
x=4
5o
:
,¥ 60C
120
3o~ ,_u
100
% 2O0
7r60
.
.
40
_
:
I}
~,
2O
0
}
L
@
lb
~
~,
=.
~
X (%)
1'0
1~
10 16
i~
Fig. 3. Tm concentration dependences of the critical temperature Tc and the exchange constant A at I0 K.
_=x=16
~5
~
18
H(k~)
Fig. 1. Field dependence of the magnetization M in F e - T m - B alloys at 10 K for different concentrations.
For low Tm concentrations ( x < 4), the Fe moment is not perturbed. So taking the value of/xr~ 2.06/z B obtained from the alloy with x = 0, and substituting in Eq. (1), it is possible to determine /~Tm for x = 4. The calculated moment at 10 K is found to be 6.2/z B, which is smaller than the theoretical value of 7/z B, suggesting a conical spin structure of Tm. This phenomenon is the result of the strong random anisotropy of Tm and the antiferromagnetic JFe-Tm interaction which normally lead to a sperimagnetic structure [6]. Now assuming /"['Tm= 6"2/XB for other compositions, the calculated /ZFe is found to be practically the same, namely ( 2 . 0 4 _ 0.02)/zB, and hence independent of Tm concentration in the range x = 0-15. This consistency can be explained by the small hybridization effects arising from Tm. //'Fe is =
,10K
1 K4oc d 6
He
20 A3Ms '
where d is the length over which the local axes show a correlation. We have assumed d = 1 nm, which is normally used in the literature [8]. The exchange constant A can be obtained from the mean field model proposed by Hasegawa [9] and from the Curie
tOO(
• 300 IK
t5
reduced by the charge transfer phenomenon arising from B, as has been observed in F e - B alloys. The variation of the Curie temperature T~ with x is also somewhat similar to that of the magnetic moment (Fig. 3). The decrease in Tc could be caused by the weakening of Fe-Fe interaction and the results are characteristic of an antiferromagnetic interaction between Tm and Fe atoms which is well known. Alben and Becker [7] have developed a theory which relates the coercivity to the fundamental parameters and one can write
sO(
6O(
)
~.~ 20(
Oq
5
~0
15
20
25
30
x¢1,)
Fig. 2. Tm concentration dependence of the alloy magnetic moment /z a at 10 and 3 0 0 K.
2
2
6
8 X (%)
/O
12
14
~6
Fig. 4. Concentration dependence of the coercivity H c at 10 K.
134
M. Slirnani et al./Journal of Magnetism and Magnetic Materials 153 (1996) 132-134
temperature using the relation proposed by Heiman et al. [10]. We have the following relation: A = CSFe K B T c / 4 ( S F e + 1)rFe_Fe,
where C ( = 8 0 - x) is the concentration of Fe in at%, SFe is the spin of Fe and rze_Fe is the average distance between the Fe atoms. We found that the exchange constant A decreases from 37 × 10 -8 to 17 × 10 -8 when x is increased from 0 to 15. From the experimental H e and M S values (Fig. 4) we have calculated the random local anisotropy constant KIo~; the results are given in Table 1. The local magnetic anisotropy in amorphous transition metal alloys was studied by a perturbation treatment on the basis of a semi-empirical self-consistent HartreeFock calculation [11]. The found value of Kloc is of the order of 107 e r g / c m 3. Recently, we have studied Table 1
0 4 8 12 15
A (10-8 erg/cm)
Ms (emu/g)
Kioc (106 erg/cm 3)
36.97 32.67 26.12 20.62 17.30
190 140.43 99.27 57.78 36.92
7.97 8.29 9.29 11.82 12.61
the magnetic properties of amorphous F e 7 2 _ x Y x HosB20 alloys [12]. Using Chudnovsky's model, we have determined Kloc (assuming d = 1 nm). The calculated values agree fairly well with those obtained from the experimental H c values using the model of Alben and Becker [7].
References [1] B.Y. Gu, H.R. Zhai and B.G. Shen, Phys. Rev. B 42 (1990) 10648. [2] J. Tejada, B. Martinez, A. Labarta, R. Grossinger, H. Sassik, M. Vasquez and A. Hernando, Phys. Rev. B 42 (1990) 898. [3] J. Tejada, B. Martinez, A. Labarta and E.M. Chudnovsky, Phys. Rev. B 44 (1991) 7698. [4] H. Lassri and R. Krishnan, J. Magn. Magn. Mater. 104 (1992) 157. [5] R. Krishnan, H. Lassri and J. Teillet, J. Magn. Magn. Mater. 98 (1991) 155. [6] J.M.D. Coey, J. Appl. Phys. 49 (1978) 1646. [7] R. Alben and J. Becker, J. Appl. Phys. 49 (1978) 3. [8] H. Lassri, L. Driouch and R. Krishnan, J. Appl. Phys. 75 (1988) 5770. [9] R. Hasegawa, J. Appl. Phys. 45 (1974) 3109. [10] N. Heiman, K. Lee, R. Potter and S. Kirpatrich, J. Appl. Phys. 47 (1976) 2634. [11] C. El Sasser et al., J. Phys. F: Metal Phys. 18 (1988) 2463. [12] R. Krishnan, H. Lassri and L. Driouch, J. Magn. Magn. Mater. 140-144 (1995) 355.
~1~ Journalof magnetism ,444 and magnutlc materials
Journal of Magnetism and Magnetic Materials 153 (1996) 132-134
Magnetization studies of amorphousFes0_xTmxB20 alloys M. Slimani a,b, M. Hamedoun
a,
H. Lassri c, S. Sayouri
a, * ,
R. Krishnan d
a Laboratoire de Physique du Solide, D~partement de Physique, Facult~ des Sciences, B.P. 1796, Fbs Atlas, Morocco b Laboratoire de Chimie du Solide E.N.S., Ben souda F~s, Morocco c Laboratoire de Physique des Mat~riaux et de Micro~lectronique, Facult~ des Sciences, Ain chok, Casablanca, Morocco a Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, C.N.R.S., 1 Place A. Briand, F-92195 Meudon, France
Received 21 December 1994; revised 21 July 1995
Abstract We have carried out magnetic studies at fields up to 18 kOe of melt- spun amorphous Fes0_xTmxB20 alloys. Magnetization and Curie temperatures were investigated. We have extracted the anisotropy constant from the coercivity.
1. Introduction In the last few years, studies of amorphous alloys based on rare earth metals have become very intense [1-4] because of their potentially useful physical properties in technological applications, and the fundamentally very important random magnetic anisotropy and magnetic exchange. In order to study the influence of the addition of Tm on the various magnetic properties of amorphous F e - B alloys, we prepared some amorphous alloys in the system Fes0_xTmxB20 (0 < x < 15).
2. Experimental The amorphous Fes0_xTmxB20 alloys (0 < x < 15) in the form of ribbons were prepared by the usual melt-spinning technique in an inert argon atmosphere. The amorphous structure was characterised by X-ray diffraction using Co K s radiation. The compositions of the alloys were determined
* Corresponding author.
by electron probe analysis. The magnetic moment was measured using a vibrating sample magnetometer (VSM) with a maximum applied field of 18 kOe, and in the temperature range 4-300 K. The Curie temperature T~ was determined from the evolution of the magnetic moment in a weak field (100 Oe) and in the temperature range 300 < T < 800 K.
3. Results and discussion For all the samples studied, magnetic saturation could be obtained with H < 1.8 T at all temperatures. Fig. 1 shows some typical results at 10 K. At the same temperature, 10 K, the magnetic moment of the alloy, /Za, decreases with increasing Tm concentration, indicating the antiparallel coupling between the Fe and Tm moments (Fig. 2). An extrapolation shows that at x = 20 a magnetic compensation would occur. The magnetic moment of Tm (/ZTm) was calculated from the alloy moment as follows [5]. The alloy m o m e n t ]£a can be written as /£a = 1( 8 0 -- X) "]£Fe -- X"
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)005 18-8
p.wml/100.
(1)
M. Slimani et al. / Journal of Magnetism and Magnetic Materials 153 (1996) 132-134
133
80(:
140
x=4
5o
:
,¥ 60C
120
3o~ ,_u
100
% 2O0
7r60
.
.
40
_
:
I}
~,
2O
0
}
L
@
lb
~
~,
=.
~
X (%)
1'0
1~
10 16
i~
Fig. 3. Tm concentration dependences of the critical temperature Tc and the exchange constant A at I0 K.
_=x=16
~5
~
18
H(k~)
Fig. 1. Field dependence of the magnetization M in F e - T m - B alloys at 10 K for different concentrations.
For low Tm concentrations ( x < 4), the Fe moment is not perturbed. So taking the value of/xr~ 2.06/z B obtained from the alloy with x = 0, and substituting in Eq. (1), it is possible to determine /~Tm for x = 4. The calculated moment at 10 K is found to be 6.2/z B, which is smaller than the theoretical value of 7/z B, suggesting a conical spin structure of Tm. This phenomenon is the result of the strong random anisotropy of Tm and the antiferromagnetic JFe-Tm interaction which normally lead to a sperimagnetic structure [6]. Now assuming /"['Tm= 6"2/XB for other compositions, the calculated /ZFe is found to be practically the same, namely ( 2 . 0 4 _ 0.02)/zB, and hence independent of Tm concentration in the range x = 0-15. This consistency can be explained by the small hybridization effects arising from Tm. //'Fe is =
,10K
1 K4oc d 6
He
20 A3Ms '
where d is the length over which the local axes show a correlation. We have assumed d = 1 nm, which is normally used in the literature [8]. The exchange constant A can be obtained from the mean field model proposed by Hasegawa [9] and from the Curie
tOO(
• 300 IK
t5
reduced by the charge transfer phenomenon arising from B, as has been observed in F e - B alloys. The variation of the Curie temperature T~ with x is also somewhat similar to that of the magnetic moment (Fig. 3). The decrease in Tc could be caused by the weakening of Fe-Fe interaction and the results are characteristic of an antiferromagnetic interaction between Tm and Fe atoms which is well known. Alben and Becker [7] have developed a theory which relates the coercivity to the fundamental parameters and one can write
sO(
6O(
)
~.~ 20(
Oq
5
~0
15
20
25
30
x¢1,)
Fig. 2. Tm concentration dependence of the alloy magnetic moment /z a at 10 and 3 0 0 K.
2
2
6
8 X (%)
/O
12
14
~6
Fig. 4. Concentration dependence of the coercivity H c at 10 K.
134
M. Slirnani et al./Journal of Magnetism and Magnetic Materials 153 (1996) 132-134
temperature using the relation proposed by Heiman et al. [10]. We have the following relation: A = CSFe K B T c / 4 ( S F e + 1)rFe_Fe,
where C ( = 8 0 - x) is the concentration of Fe in at%, SFe is the spin of Fe and rze_Fe is the average distance between the Fe atoms. We found that the exchange constant A decreases from 37 × 10 -8 to 17 × 10 -8 when x is increased from 0 to 15. From the experimental H e and M S values (Fig. 4) we have calculated the random local anisotropy constant KIo~; the results are given in Table 1. The local magnetic anisotropy in amorphous transition metal alloys was studied by a perturbation treatment on the basis of a semi-empirical self-consistent HartreeFock calculation [11]. The found value of Kloc is of the order of 107 e r g / c m 3. Recently, we have studied Table 1
0 4 8 12 15
A (10-8 erg/cm)
Ms (emu/g)
Kioc (106 erg/cm 3)
36.97 32.67 26.12 20.62 17.30
190 140.43 99.27 57.78 36.92
7.97 8.29 9.29 11.82 12.61
the magnetic properties of amorphous F e 7 2 _ x Y x HosB20 alloys [12]. Using Chudnovsky's model, we have determined Kloc (assuming d = 1 nm). The calculated values agree fairly well with those obtained from the experimental H c values using the model of Alben and Becker [7].
References [1] B.Y. Gu, H.R. Zhai and B.G. Shen, Phys. Rev. B 42 (1990) 10648. [2] J. Tejada, B. Martinez, A. Labarta, R. Grossinger, H. Sassik, M. Vasquez and A. Hernando, Phys. Rev. B 42 (1990) 898. [3] J. Tejada, B. Martinez, A. Labarta and E.M. Chudnovsky, Phys. Rev. B 44 (1991) 7698. [4] H. Lassri and R. Krishnan, J. Magn. Magn. Mater. 104 (1992) 157. [5] R. Krishnan, H. Lassri and J. Teillet, J. Magn. Magn. Mater. 98 (1991) 155. [6] J.M.D. Coey, J. Appl. Phys. 49 (1978) 1646. [7] R. Alben and J. Becker, J. Appl. Phys. 49 (1978) 3. [8] H. Lassri, L. Driouch and R. Krishnan, J. Appl. Phys. 75 (1988) 5770. [9] R. Hasegawa, J. Appl. Phys. 45 (1974) 3109. [10] N. Heiman, K. Lee, R. Potter and S. Kirpatrich, J. Appl. Phys. 47 (1976) 2634. [11] C. El Sasser et al., J. Phys. F: Metal Phys. 18 (1988) 2463. [12] R. Krishnan, H. Lassri and L. Driouch, J. Magn. Magn. Mater. 140-144 (1995) 355.