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Local magnetization of Fe atoms in γ Fe-Ni alloys
LOCAL
MAGNETIZATION
J. HESSE,
OF Fe ATOMS IN y Fe-Ni
J. B. MULLER,
Instituf A fir Physik,
B. WIECHMANN
Technische Universit&,
ALLOYS
and H. ULLRICH
3300 Braunschweig,
Fed. Rep. Germany
Results of 57Fe Mossbauer effect measurements on y Fe-Ni alloys show that the exchange interaction between Fe-Fe neighbor atom pairs is temperature and concentration dependent. Additionally, we observe an anomalous temperature behaviour of the isomer shift in Invar alloys.
We observe quite different 57Fe Mossbauer patterns for iron and for nickel rich y Fe-Ni alloys. This remarkably can be seen in the courses of the parameter h, with the temperature [l]. The parameter h, describes the change of the magnetic field on the site of a 57Fe nucleus by replacing a neighboring Ni atom by a Fe one [2-41. A reduction of h, leads to a narrowing of the absorption lines of the Mossbauer spectra, an increase respectively spreads the linewiths (fig. 1). The distribution of hyperfine fields in Fe-Ni alloys reveals the same effect as hi [5]. The model applied by us, however, leads back the different behaviour of the hyperfine fields on the influence of the neighboring atoms. Eliminating the direct contribution of the conduction electrons to the
I
field at the nucleus one gets the indirect influence via the exchange interaction on the local magnetization of the central atom [l]. A model considering a noncollinear spinstructure developed by Sidorov and Doroshenko [6, 71 allows us computing the indirect influence of the first neighbors on the local magnetization of the central Fe atom (fig. 2). In comparison with the measured indirect part of h,, there is an agreement only for the Ni-rich alloys [l]. We conclude that the exchange integral JFeFe in Fe rich alloys is remarkably dependent on temperature and concentration. An increase of the exchange integral with the temperature stabilizes the ferromagnetism. This leads to flat magnetization curves (fig. 3). Three physical mechanisms can be responsible for flat courses of the magnetization curves: 1) weak itinerant ferromagnetism, 2) spatial fluctuations of the effective exchange integral (environment effects) [8],
0 x=40 0
AG
0.0
-0.10
!
.
-015,. Fe-xat%N;
-0zoj 00
:
:
!
:
02
0.4
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:_ IO
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:
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02
04
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T, Fig. 1. Temperature dependencies of h, for three different y Fe-Ni alloys plotted versus the reduced temperature T/ TCuric. h,(O) = 1 T for all alloys presented here.
Journal of Magnetism
and Magnetic Materials
15-18 (1980) 1195-l
Fig. 2. Calculated differences in the local Fe atom magnetizations Au = (Bs(p) - Bs(p3)/(p - p2 as a function of reduced temperature z. p = mean number and p = actual number of first Fe neighbor atoms. Assumed values: J,,r,i/J,i,i = 0.93, JFcFc/JNiNi= - 0.05, SF, = t and & = f , B = Brillouin function.
196 BNorth Holland
11%
11%
J. Hose
et al./Local
magnetization
of Fe afomr iny Fe-Ni
400 '.
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-OF
04..
-02.. -03.. -04
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0.4
06
08
qn
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600
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T 77
.
%%$
.
.
.
x=59
0
x=33
II
A:
q 0 0
t
10 L,, T,
Fig. 3. Molecular field calculation of the spontanous magnetization for a ferromagnet (S = t) with an exchange interaction J depending linearly (parameter p) on temperature.
3) temperature dependent exchange integrals. Possible reasons are: a) changes in the interatomic distances (thermal expansion, vibration of the atoms), b) changes in the noncollinear spinstructure with temperature, c) thermal excited fluctuations of the electronic state (local spin fluctuations) of the Fe atom [ 1, 9, lo], known as the 2y-hypothesis of Weiss [ 1 I]. We also performed measurements of the 57Fe isomer shift in y Fe-Ni alloys as a function of temperature. Fig. 4A shows the centre of gravity shift us for two alloys. The course of us essentially is provided by the second order Doppler shift. In comparison to the Fe 59 at% Ni alloy the measurement of the Fe 33 at% Ni sample shows an anomaly (fig. 4B). We found those anomalies only for Invar alloys. A change of the isomer shift with the temperature can stand for an alteration of the electronic state of the Fe atoms. The authors would like to express sincere gratitude to Prof. Ch. Schwink and Prof. H. Bromer. The financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Fig. 4. A: Centre of gravity shift es of the Mossbauer spectra as a function of temperature for two y Fe-Ni alloys. Only the Invar alloy shows an anomalous behaviour. B: The difference Au, = t&x = 59) - os(x = 33) as a function of temperature. It is assumed, that the second order Doppler shift is eliminated in this way.
References [l] J. Hesse, B. Wiechmann and J. B. Miiller, J. Magn. Magn. Mat. 10 (1979) 252. (21 J. Hesse and J. B. Miiller, Solid State Commun. 22 (1977) 637. [3] L. Billard and A. Chamberod, Solid State Commun. 17 (1975) 113. [4] J. Hesse, J. B. Miller and B. Wiechmann, J. de Phys. 40 (1979) C2-161. [5] S. Tomiyoshi, H. Yamamoto and H. Watanabe, J. Phys. Sot. Japan 30 (1971) 1605. [6] S. K. Sidorov and A. V. Doroshenko, Fiz. Met. Met. 18 (1964) 811. [7] A. 2. Menshikov, J. Magn. Magn. Mat. 10 (1979) 205. [8] K. Handrich, Phys. Stat. Sol. 32 (1969) K55. [9] S. Chikazumi, J. Magn. Magn. Mat. 10 (1979) 113. (lo] H. Miwa, J. Magn. Magn. Mat. 10 (1979) 223. [ 111 R. J. Weiss, Proc. Roy. Sot. 82 (1963) 28 1. [I21 H. H. Ettwig, W. Bendick and W. Pepperhoff, Verhandl. DPG 1 (1978) M64 236.
MAGNETIZATION
J. HESSE,
OF Fe ATOMS IN y Fe-Ni
J. B. MULLER,
Instituf A fir Physik,
B. WIECHMANN
Technische Universit&,
ALLOYS
and H. ULLRICH
3300 Braunschweig,
Fed. Rep. Germany
Results of 57Fe Mossbauer effect measurements on y Fe-Ni alloys show that the exchange interaction between Fe-Fe neighbor atom pairs is temperature and concentration dependent. Additionally, we observe an anomalous temperature behaviour of the isomer shift in Invar alloys.
We observe quite different 57Fe Mossbauer patterns for iron and for nickel rich y Fe-Ni alloys. This remarkably can be seen in the courses of the parameter h, with the temperature [l]. The parameter h, describes the change of the magnetic field on the site of a 57Fe nucleus by replacing a neighboring Ni atom by a Fe one [2-41. A reduction of h, leads to a narrowing of the absorption lines of the Mossbauer spectra, an increase respectively spreads the linewiths (fig. 1). The distribution of hyperfine fields in Fe-Ni alloys reveals the same effect as hi [5]. The model applied by us, however, leads back the different behaviour of the hyperfine fields on the influence of the neighboring atoms. Eliminating the direct contribution of the conduction electrons to the
I
field at the nucleus one gets the indirect influence via the exchange interaction on the local magnetization of the central atom [l]. A model considering a noncollinear spinstructure developed by Sidorov and Doroshenko [6, 71 allows us computing the indirect influence of the first neighbors on the local magnetization of the central Fe atom (fig. 2). In comparison with the measured indirect part of h,, there is an agreement only for the Ni-rich alloys [l]. We conclude that the exchange integral JFeFe in Fe rich alloys is remarkably dependent on temperature and concentration. An increase of the exchange integral with the temperature stabilizes the ferromagnetism. This leads to flat magnetization curves (fig. 3). Three physical mechanisms can be responsible for flat courses of the magnetization curves: 1) weak itinerant ferromagnetism, 2) spatial fluctuations of the effective exchange integral (environment effects) [8],
0 x=40 0
AG
0.0
-0.10
!
.
-015,. Fe-xat%N;
-0zoj 00
:
:
!
:
02
0.4
06
08
:_ IO
T 7
:
01
02
04
C!6
I)6
T
10
T, Fig. 1. Temperature dependencies of h, for three different y Fe-Ni alloys plotted versus the reduced temperature T/ TCuric. h,(O) = 1 T for all alloys presented here.
Journal of Magnetism
and Magnetic Materials
15-18 (1980) 1195-l
Fig. 2. Calculated differences in the local Fe atom magnetizations Au = (Bs(p) - Bs(p3)/(p - p2 as a function of reduced temperature z. p = mean number and p = actual number of first Fe neighbor atoms. Assumed values: J,,r,i/J,i,i = 0.93, JFcFc/JNiNi= - 0.05, SF, = t and & = f , B = Brillouin function.
196 BNorth Holland
11%
11%
J. Hose
et al./Local
magnetization
of Fe afomr iny Fe-Ni
400 '.
06..
-OF
04..
-02.. -03.. -04
-0.5
00’
o%o
I a2
0.4
06
08
qn
,.
%
"0
800
600
-+
..
T 77
.
%%$
.
.
.
x=59
0
x=33
II
A:
q 0 0
t
10 L,, T,
Fig. 3. Molecular field calculation of the spontanous magnetization for a ferromagnet (S = t) with an exchange interaction J depending linearly (parameter p) on temperature.
3) temperature dependent exchange integrals. Possible reasons are: a) changes in the interatomic distances (thermal expansion, vibration of the atoms), b) changes in the noncollinear spinstructure with temperature, c) thermal excited fluctuations of the electronic state (local spin fluctuations) of the Fe atom [ 1, 9, lo], known as the 2y-hypothesis of Weiss [ 1 I]. We also performed measurements of the 57Fe isomer shift in y Fe-Ni alloys as a function of temperature. Fig. 4A shows the centre of gravity shift us for two alloys. The course of us essentially is provided by the second order Doppler shift. In comparison to the Fe 59 at% Ni alloy the measurement of the Fe 33 at% Ni sample shows an anomaly (fig. 4B). We found those anomalies only for Invar alloys. A change of the isomer shift with the temperature can stand for an alteration of the electronic state of the Fe atoms. The authors would like to express sincere gratitude to Prof. Ch. Schwink and Prof. H. Bromer. The financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Fig. 4. A: Centre of gravity shift es of the Mossbauer spectra as a function of temperature for two y Fe-Ni alloys. Only the Invar alloy shows an anomalous behaviour. B: The difference Au, = t&x = 59) - os(x = 33) as a function of temperature. It is assumed, that the second order Doppler shift is eliminated in this way.
References [l] J. Hesse, B. Wiechmann and J. B. Miiller, J. Magn. Magn. Mat. 10 (1979) 252. (21 J. Hesse and J. B. Miiller, Solid State Commun. 22 (1977) 637. [3] L. Billard and A. Chamberod, Solid State Commun. 17 (1975) 113. [4] J. Hesse, J. B. Miller and B. Wiechmann, J. de Phys. 40 (1979) C2-161. [5] S. Tomiyoshi, H. Yamamoto and H. Watanabe, J. Phys. Sot. Japan 30 (1971) 1605. [6] S. K. Sidorov and A. V. Doroshenko, Fiz. Met. Met. 18 (1964) 811. [7] A. 2. Menshikov, J. Magn. Magn. Mat. 10 (1979) 205. [8] K. Handrich, Phys. Stat. Sol. 32 (1969) K55. [9] S. Chikazumi, J. Magn. Magn. Mat. 10 (1979) 113. (lo] H. Miwa, J. Magn. Magn. Mat. 10 (1979) 223. [ 111 R. J. Weiss, Proc. Roy. Sot. 82 (1963) 28 1. [I21 H. H. Ettwig, W. Bendick and W. Pepperhoff, Verhandl. DPG 1 (1978) M64 236.