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Anomalies of the dependence of the magnetic properties on the chemical composition in amorphous Fe-Si-B alloys
188
Journal of Magnetism and Magnetic Materials 41 (1984) 188-190 North-Holland, Amsterdam
A N O M A L I E S O F T H E D E P E N D E N C E OF T H E M A G N E T I C P R O P E R T I E S O N T H E C H E M I C A L C O M P O S I T I O N I N A M O R P H O U S F e - S i - B ALLOYS W. D M O W S K I , H. M A T Y J A Institute of Materials Science and Engineering, Warsaw Technical University, 02.524 Warsaw, Narbutta 85, Poland
and R. PUT, N I A K Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, AI. Lotnikdw 32/46, Poland
The magnetic properties of FexSig0_xB10 metallic glasses (68 < x ~<83) have been studied. In that group of glassy alloys anomalies of the dependence of the magnetic properties with the Fe content are found. There occurs a maximum of the Curie temperature in the vicinity of x = 75. The saturation magnetic moment varies linearly up to x = 75. For higher Fe content changes of the magnetic moment are small. The spinwavestiffness constant is almost constant for x ~ 75 whereas for x > 75 it decreases strongly. Among the curves describing the reduced saturation magnetizationas a function of the reduced temperature the plot for x = 75 is located highest. It is suggested that there is a gradual transition from normal to Invar-like behaviour in that system. 1. Introduction I n the past ten years a lot of both theoretical and experimental results have been obtained on magnetic metallic glasses. Many series of alloys composed of transition-metal alloys with a wide variety of metalloids have been studied. It is recognized that owing to the topological disorder in these solids the critical behavior and the spinwave excitation spectra are influenced by short-range fluctuations of the exchange energy [1]. The temperature dependence of the magnetization is much stronger in amorphous ferromagnets than in crystalline ones and it is manifested in less convex upward reduced magnetization curves. Fluctuations of the exchange energy are primarily due to distributions of atomic separations, and number and chemical character of the nearest neighbors. The last ones are important in multi-element system containing different metallic elements. It is interesting to compare the magnetic properties of amorphous alloys containing a single metallic element such as F e - B or C o - B glasses. The magnetic properties of glassy C o - B alloys are similar to those of their crystalline counterparts [2]. The magnetic properties of F e - B metallic glasses are quite different f r o m those of C o - B amorphous alloys. The decrease of T~ with increasing magnetic moment per iron atom is opposite to what one
might expect [3]. The reduced magnetization curves of amorphous C o - B alloys are less flat than those for F e - B . For F e - B glassy alloys the curve becomes flatter with increasing Fe content [4]. In F e - B metallic glasses magnetic anomalies have been found similar to those occurring in crystalline F e - N i Invar alloys. The Invar alloys show, in addition to a small thermal expansion coefficient, numerous unusual physical properties such as a large magnetovolume effect, a strong composition dependence of the magnetic moment and a gradual decrease of Tc in the Invar region, a flat reduced magnetization curve and large high-field susceptibility [5]. The aim of this contribution is to show that FexSigo_xBm metallic glasses exhibit gradual transition from normal to Invar-like behaviour and to compare their properties with those of C o - B and F e - B glassy alloys.
2. Experimental Amorphous FexSigo_xBl0 alloys (68 ~ x ~< 83) have been prepared by chill-casting method. The Curie temperatures were determined using a differential scanning calorimeter at a heating rate of 20 K / m i n . The temperature dependence of the magnetization was measured with an applied field of 7.5 kOe by a magnetic balance with a resolution
0304-8853/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
189
W. Dmowski et aL / Anomalous magnetic properties of amorphous Fe- S i - B
of about 10 -4 emu in the temperature range from 78 to 800 K. The sample densities used in the calculations were measured by the Archimedian method.
Tc [
..= ",,,,
[K] i
. Fe-B ",~,
• Fe x S igo.xBlo
700 I~___. ~
D
3. Results and discussion
Figs. 1 and 2 illustrate the composition dependence of the saturation magnetization, magnetic moment per iron atom and the Curie temperature. The saturation magnetization extrapolated to 0 K increases with iron content. However the magnetic moment per iron atom varies linearly only up to x = 75. For x > 75 the changes of the magnetic moment are small. The Curie temperature exhibits a maximum in the vicinity of the composition x = 75. Annealing the samples at high temperature causes a rise of Tc in the amorphous state but does not change the character of the dependence of T~ on composition for those alloys. The decrease of Tc with decreasing iron content for x < 75 may be due to decreasing average coordination number or reduction of the average exchange constant. For x > 75, Tc decreases with increasing iron content as in the case of F e - B alloys. At low temperature demagnetization is connected with thermal excitations of long-wavelength spin-waves. Amorphous ferromagnets as well as crystalline ones obey Bloch's T 3/2 law: = 1 - B T ~ / 2 - C T 5/2.
M(T)/M(O)
2.0 1.9 1.8
~~,[~,, / Fe]
|
190
(1)
I
i
,
,
,
,
,
[meV,~ 2 ] \ \
600
140
.
100 ,X
68 72 7~6 80 8'/,. 88 Fig. 2. The dependence of the Curie temperature and the
spin-wave stiffness constant for FexSigo_xBloand FexBloo_x amorphous alloys. Data for Fe-B alloys from ref. [3].
Assuming the relation between B and D [3] B = 2.612[g#B/M(O)]
( k B / 4 ~ r D ) 3/2,
(2)
the coefficient D was determined. The values of D for F e - S i - B and F e - B metallic glasses are shown in fig. 2. For x up to 75 D is almost constant and it then greatly decreases, indicating destabilization of ferromagnetism. The reduced magnetization curves for three F e - S i - B glasses are shown in fig. 3. The curve for x - - 7 5 is located highest. The curve for x = 83 lies considerably lower than the two others. These results agree well with the findings of Fukunaga et al. [6] for that system. The data shown above suggest that the concentration dependence of the magnetic properties for x < 75 is similar to that observed for C o - B amorphous alloys, and for x > 75 it is similar to
•
Gs[em=/9 ] T=OK ~
\
~'~ 0.80.~
~ s s ~ a l
o
Fe8 7 $i 3 alO~" x ~ x ~
17o 150
O.4
X
76 ' I~ ' e~4 Fig. 1. The concentrationdependenceof the saturation magnetization and magnetic moment per iron atom for Fe~Sigo_~Bm glassy alloys. 6"8
'7"2 '
x ~ Fe69Si21fil 0
0.2 fl
T¢
6.2 d,4 o:e o:8 to Fig. 3. The reduced magnetizationcurves as a function of the reduced temperature for three Fe~,Sigo_xB10glassy alloys.
W. Dmowski et aL / Anomalous magnetic properties of amorphous Fe- S i - B
190
that occurring in F e - B glassy ribbons. A great increase of the forced volume magnetostriction [7] and the pressure dependence of T~ for x > 75 [9] indicate a gradual transition from normal to Invar-like magnetic behavior near x = 75. The relation between dTJdp and T~ for crystalline alloys of 3d metals is derived within the band model of ferromagnetism [8]:
dTc/d p = ( 5 / 3 ) KT~ - ATe.
(3)
The influence of pressure on Tc of crystalline and amorphous Invar alloys is manifested by a large and negative shift of T~, i.e. the second term in the relation (3) prevails (A is a positive constant). The pressure dependence of T~ of some F e - S i - B sam-
die
dp
K GPo-1 0
pies [9] is shown in fig. 4 plotted against T~. The change of its character is evident. The origin of the magnetic anomalies in Invar alloys has not been established [5]. One possibility is a latent antiferromagnetism. This would suggest inhomogenities in the magnetic order connected with iron atoms with a high coordination number for x > 75 in that system as well as in F e - B glassy ribbons.
Acknowledgements W. Dmowski and H. Matyja were supported by grants from the Ministry of Science, Academic Education and Technology under contract MR.I.21.
References 4,0--
~
--
W
•
J X
-2(
,,,1 ."
_~
X rig
[1] [2] [3] [4]
/
' 2~)0' 460' 600 ' 800)c(K ) Fig. 4. The pressure dependenceof the Curie temperature against Tc for crystalline and amorphousalloys. Data for other than Fe-Si-B alloys were taken from ref, [8]. × crystalline Fe-Ni; • amorphous (Ni=Fe~_x)soP1oB10; • amorphous Fe83B17, FessB~5; 0 amorphous Fe72SilsB10, Fe75SilsBlo, FeTsSi12B10.
[5] [6] [7] [8] [91
H. Kronmtiller, J. Appl. Phys. 52 (1981) 1859. R. Hasegawa and R. Ray, J, Appl. Phys. 50 (1979) 1586. R. Hasegawa and R. Ray, Phys. Rev. B20 (1979) 211, C.L. Chien, D. Musser, E.M. Gyorgy, R.C. Sherwood, H.S. Chen, F.E. Luborsky and J.L. Walter, Phys. Rev. B20 (1979) 283. Y. Nakamura, IEEE Trans. Magn. MAG-12 (1976) 278. H. Fukunaga, J. Yamasaki and K. Narita, Japan. J. Appl. Phys. 19 (1980) 2407. W. Dmowski, T. Jagielinski, T. Walecki and H. Matyja, J. Phys. C8 (1980) 127. J. Kamarad, Z. Arnold and H.J.V. Nielsen, J. Magn. Magn. Mat. 23 (1981) 69. J. Kamarad et al., to be published.
Journal of Magnetism and Magnetic Materials 41 (1984) 188-190 North-Holland, Amsterdam
A N O M A L I E S O F T H E D E P E N D E N C E OF T H E M A G N E T I C P R O P E R T I E S O N T H E C H E M I C A L C O M P O S I T I O N I N A M O R P H O U S F e - S i - B ALLOYS W. D M O W S K I , H. M A T Y J A Institute of Materials Science and Engineering, Warsaw Technical University, 02.524 Warsaw, Narbutta 85, Poland
and R. PUT, N I A K Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, AI. Lotnikdw 32/46, Poland
The magnetic properties of FexSig0_xB10 metallic glasses (68 < x ~<83) have been studied. In that group of glassy alloys anomalies of the dependence of the magnetic properties with the Fe content are found. There occurs a maximum of the Curie temperature in the vicinity of x = 75. The saturation magnetic moment varies linearly up to x = 75. For higher Fe content changes of the magnetic moment are small. The spinwavestiffness constant is almost constant for x ~ 75 whereas for x > 75 it decreases strongly. Among the curves describing the reduced saturation magnetizationas a function of the reduced temperature the plot for x = 75 is located highest. It is suggested that there is a gradual transition from normal to Invar-like behaviour in that system. 1. Introduction I n the past ten years a lot of both theoretical and experimental results have been obtained on magnetic metallic glasses. Many series of alloys composed of transition-metal alloys with a wide variety of metalloids have been studied. It is recognized that owing to the topological disorder in these solids the critical behavior and the spinwave excitation spectra are influenced by short-range fluctuations of the exchange energy [1]. The temperature dependence of the magnetization is much stronger in amorphous ferromagnets than in crystalline ones and it is manifested in less convex upward reduced magnetization curves. Fluctuations of the exchange energy are primarily due to distributions of atomic separations, and number and chemical character of the nearest neighbors. The last ones are important in multi-element system containing different metallic elements. It is interesting to compare the magnetic properties of amorphous alloys containing a single metallic element such as F e - B or C o - B glasses. The magnetic properties of glassy C o - B alloys are similar to those of their crystalline counterparts [2]. The magnetic properties of F e - B metallic glasses are quite different f r o m those of C o - B amorphous alloys. The decrease of T~ with increasing magnetic moment per iron atom is opposite to what one
might expect [3]. The reduced magnetization curves of amorphous C o - B alloys are less flat than those for F e - B . For F e - B glassy alloys the curve becomes flatter with increasing Fe content [4]. In F e - B metallic glasses magnetic anomalies have been found similar to those occurring in crystalline F e - N i Invar alloys. The Invar alloys show, in addition to a small thermal expansion coefficient, numerous unusual physical properties such as a large magnetovolume effect, a strong composition dependence of the magnetic moment and a gradual decrease of Tc in the Invar region, a flat reduced magnetization curve and large high-field susceptibility [5]. The aim of this contribution is to show that FexSigo_xBm metallic glasses exhibit gradual transition from normal to Invar-like behaviour and to compare their properties with those of C o - B and F e - B glassy alloys.
2. Experimental Amorphous FexSigo_xBl0 alloys (68 ~ x ~< 83) have been prepared by chill-casting method. The Curie temperatures were determined using a differential scanning calorimeter at a heating rate of 20 K / m i n . The temperature dependence of the magnetization was measured with an applied field of 7.5 kOe by a magnetic balance with a resolution
0304-8853/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
189
W. Dmowski et aL / Anomalous magnetic properties of amorphous Fe- S i - B
of about 10 -4 emu in the temperature range from 78 to 800 K. The sample densities used in the calculations were measured by the Archimedian method.
Tc [
..= ",,,,
[K] i
. Fe-B ",~,
• Fe x S igo.xBlo
700 I~___. ~
D
3. Results and discussion
Figs. 1 and 2 illustrate the composition dependence of the saturation magnetization, magnetic moment per iron atom and the Curie temperature. The saturation magnetization extrapolated to 0 K increases with iron content. However the magnetic moment per iron atom varies linearly only up to x = 75. For x > 75 the changes of the magnetic moment are small. The Curie temperature exhibits a maximum in the vicinity of the composition x = 75. Annealing the samples at high temperature causes a rise of Tc in the amorphous state but does not change the character of the dependence of T~ on composition for those alloys. The decrease of Tc with decreasing iron content for x < 75 may be due to decreasing average coordination number or reduction of the average exchange constant. For x > 75, Tc decreases with increasing iron content as in the case of F e - B alloys. At low temperature demagnetization is connected with thermal excitations of long-wavelength spin-waves. Amorphous ferromagnets as well as crystalline ones obey Bloch's T 3/2 law: = 1 - B T ~ / 2 - C T 5/2.
M(T)/M(O)
2.0 1.9 1.8
~~,[~,, / Fe]
|
190
(1)
I
i
,
,
,
,
,
[meV,~ 2 ] \ \
600
140
.
100 ,X
68 72 7~6 80 8'/,. 88 Fig. 2. The dependence of the Curie temperature and the
spin-wave stiffness constant for FexSigo_xBloand FexBloo_x amorphous alloys. Data for Fe-B alloys from ref. [3].
Assuming the relation between B and D [3] B = 2.612[g#B/M(O)]
( k B / 4 ~ r D ) 3/2,
(2)
the coefficient D was determined. The values of D for F e - S i - B and F e - B metallic glasses are shown in fig. 2. For x up to 75 D is almost constant and it then greatly decreases, indicating destabilization of ferromagnetism. The reduced magnetization curves for three F e - S i - B glasses are shown in fig. 3. The curve for x - - 7 5 is located highest. The curve for x = 83 lies considerably lower than the two others. These results agree well with the findings of Fukunaga et al. [6] for that system. The data shown above suggest that the concentration dependence of the magnetic properties for x < 75 is similar to that observed for C o - B amorphous alloys, and for x > 75 it is similar to
•
Gs[em=/9 ] T=OK ~
\
~'~ 0.80.~
~ s s ~ a l
o
Fe8 7 $i 3 alO~" x ~ x ~
17o 150
O.4
X
76 ' I~ ' e~4 Fig. 1. The concentrationdependenceof the saturation magnetization and magnetic moment per iron atom for Fe~Sigo_~Bm glassy alloys. 6"8
'7"2 '
x ~ Fe69Si21fil 0
0.2 fl
T¢
6.2 d,4 o:e o:8 to Fig. 3. The reduced magnetizationcurves as a function of the reduced temperature for three Fe~,Sigo_xB10glassy alloys.
W. Dmowski et aL / Anomalous magnetic properties of amorphous Fe- S i - B
190
that occurring in F e - B glassy ribbons. A great increase of the forced volume magnetostriction [7] and the pressure dependence of T~ for x > 75 [9] indicate a gradual transition from normal to Invar-like magnetic behavior near x = 75. The relation between dTJdp and T~ for crystalline alloys of 3d metals is derived within the band model of ferromagnetism [8]:
dTc/d p = ( 5 / 3 ) KT~ - ATe.
(3)
The influence of pressure on Tc of crystalline and amorphous Invar alloys is manifested by a large and negative shift of T~, i.e. the second term in the relation (3) prevails (A is a positive constant). The pressure dependence of T~ of some F e - S i - B sam-
die
dp
K GPo-1 0
pies [9] is shown in fig. 4 plotted against T~. The change of its character is evident. The origin of the magnetic anomalies in Invar alloys has not been established [5]. One possibility is a latent antiferromagnetism. This would suggest inhomogenities in the magnetic order connected with iron atoms with a high coordination number for x > 75 in that system as well as in F e - B glassy ribbons.
Acknowledgements W. Dmowski and H. Matyja were supported by grants from the Ministry of Science, Academic Education and Technology under contract MR.I.21.
References 4,0--
~
--
W
•
J X
-2(
,,,1 ."
_~
X rig
[1] [2] [3] [4]
/
' 2~)0' 460' 600 ' 800)c(K ) Fig. 4. The pressure dependenceof the Curie temperature against Tc for crystalline and amorphousalloys. Data for other than Fe-Si-B alloys were taken from ref, [8]. × crystalline Fe-Ni; • amorphous (Ni=Fe~_x)soP1oB10; • amorphous Fe83B17, FessB~5; 0 amorphous Fe72SilsB10, Fe75SilsBlo, FeTsSi12B10.
[5] [6] [7] [8] [91
H. Kronmtiller, J. Appl. Phys. 52 (1981) 1859. R. Hasegawa and R. Ray, J, Appl. Phys. 50 (1979) 1586. R. Hasegawa and R. Ray, Phys. Rev. B20 (1979) 211, C.L. Chien, D. Musser, E.M. Gyorgy, R.C. Sherwood, H.S. Chen, F.E. Luborsky and J.L. Walter, Phys. Rev. B20 (1979) 283. Y. Nakamura, IEEE Trans. Magn. MAG-12 (1976) 278. H. Fukunaga, J. Yamasaki and K. Narita, Japan. J. Appl. Phys. 19 (1980) 2407. W. Dmowski, T. Jagielinski, T. Walecki and H. Matyja, J. Phys. C8 (1980) 127. J. Kamarad, Z. Arnold and H.J.V. Nielsen, J. Magn. Magn. Mat. 23 (1981) 69. J. Kamarad et al., to be published.