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Zero field NMR-spin echo investigation of satellite lines in CuMn SPIN glasses
Journal of Magnetism and Magnetic Materials 28 (1982) 209-213 North-Holland Publishing Company
209
ZERO FIELD NMR- SPIN ECHO INVESTIGATION OF SATELLITE LINES IN CuMn SPIN GLASSES Ch. S C H L O S S E R and H. B R O M E R Institut A fiir Physik, Abt. fiir Angew. Kern- und Neutronenphysik, Technische Universititt Braunschweig, Fed. Rep. Germany
Received 30 March 1982
In 1979 Alloul made zero field NMR measurements of satellite lines in CuMn spin glasses. He pointed out the existence of an enhancement factor and showed that the Mn spins behave like the spins in ferromagnetic domain particles. New independent measurements, presented here, performed especially in order to prove the existence of an enhancement factor and therefore the model description based hereupon, are in agreement with the interpretation of Alloul. The discrepancy between the cluster and the one domain model of spin glasses can well be understood, if one realizes the existence of a subsystem of clusters with dipole interaction and a main system of single spins rigidly bound by RKKY interaction. Within this model the behaviour of the enhancement factor, measured by us, can be interpretated.
1. Introduction By showing the existence of a ferromagnetic enhancement factor ~, Alloul [1,2] showed in 1979 that the M n spins of a frozen spin glass behave like a ferromagnetic single d o m a i n particle. Therefore collective aspects of the frozen state came into discussion. In a multidirectional spin system like the spins in a Bloch wall or a spin glass only a mean value can be measured, which characterizes the behaviour of the whole sample. O w n measurements [3] with ferromagnetic iron powder have shown that the derivation of an enhancement factor ~ from the measurements is not out of the question. Measuring the intensity of the free induction decay or the intensity of spin echoes as a function of the turning angle a = "yHtT, the usual way of deriving ~, the results are different for varying either z or H r As a consequence of a limited excitation bandwidth (Ao~e)l/2 three different enhancement factors have to be defined, being ~(¢), ~(H1) and ~0(At% i/2 --' 0). Ignoring these problems, large errors are possible in deriving ~. The work of Alloul does not show by what procedure 7/has been derived. The above n a m e d uncertainties do not exist if a
Fourier-transformation of the NMR-signals is used, We therefore intended to prove the measurements of Alloul by applying this technique.
2. Experimental procedure N M R spin echoes were measured using a pulse sequence ~'-tv-~, ~" being the pulse duration and tv the time delay between the pulses. The frequency was 11 M H z corresponding to the signal frequency coming from the fourth copper neighbours of M n atoms. The spin echoes were Fourier transformed into the frequency domain. The line intensities were plotted against the turning angle a = ~,~H~'. The enhancement factors ~ were derived from the maxima of the curves. In order to effect a value ~1~ 1 the samples were cooled from T = 150 K to the measuring temperature T = 1.3 K in an external field H c, parallel to the z-axis. Additional to the measurements of ~ in zero field, ~ has been evaluated also by applying low external fields H o and varying their angle 0 the direction of Hc as shown in fig. 1.
0 3 0 4 - 8 8 5 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
210
Ch. Schlosser, H. Brfmer / Satellite lines in CuMn spin glasses
3uM~ 2 . 5 o£ % "
20
No " 9. 7 6 RG
T
-
1.3K
Y
Fig. 1. Geometry of NMR experiments.
i
-i.5
-i
-.5
.5 Ho [ k G ]
3. The single domain model of Alloul First of all let us briefly report Allouls single d o m a i n model. After cooling his sample in a field Hc Alloul found an enhancement factor 7/ which could be described by the simple relation:
Fig. 3. ~ vs. H 0 of CuMn with 2.5 at% Mn after cooling the sample in H¢=9.76 kG. The solid line follows eq. (1) if H a ----1.02 kg.
easily can derive additional rf-fields
n--(Ho+ HA)-' ,
H ~ = IH, I]HL[ s i n O / l U o + n A [ ,
H0 IIHe, H 0 being an external field. H A is a single fitting parameter, which can be interpretated as an anisotropy field acting on the spin system as a whole one. D u r i n g an exciting rf-pulse with H i l l y , the magnetisation ~r, lying in z-direction at the beginning of the pulse, is turned a r o u n d the x-axis by an angle
Since the local fields H L are oriented at r a n d o m one has to integrate over the signal contributions of all spins with varying angle 8, getting an average enhancement factor
"t' ~ Hi/IHo + HAI as shown in fig. 2. Taking into account that all the local magnetic hyperfine fields H L, seen by the neighbouring c o p p e r nuclei, participate in this rotation of ~r, one
~',n'
,~o
6/", ! J
8 ---- ± ( H L , X ).
~=0.9IHLI/IHo + HAI.
(1)
A p p l y i n g a low field H 0 in opposite direction of He, Alloul found H A to consist of two parts as described in eq. (2): - an unidirectional field H d in direction of H c and - an uniaxial field H,x its axis lying parallel to He. HA----Hd+--¢Hax,
with c = sign (Hd + H0).
(2)
In fig. 3 the results of our measurements have been plotted. It shows ~ as a function of the magnetic field H 0 parallel and antiparallel to H c. Applying eq. (1) a value H A = 1.02 k G can be derived giving the solid line in fig. 3. Our own measurements are therefore in full agreement with the one d o m a i n model of Alloul.
p !
i
i
,
4. Synthesis of the single domain and the cluster model Y Fig. 2. Rotation geometry of the domain magnetisation o and and the hyperfine fie•s H L.
U n d o u b t e d l y our measurements confirm the single domain model. U n d o u b t e d l y too the mea-
Ch. Schlosser, H. Br6mer / Satellite lines in CuMn spin glasses
211
Table 1 Portions of the whole M n content of alloys with various M n concentrations C, that build up spherical clusters with different numbers N and of single M n spins. The numbers are calculated assuming a statistical distribution of Mn atoms N
C 1 at%
2 at%
2.5 at%
5 at%
10 at%
1 2 3 4 5 6 7 8
88.6% 10.0% 0.30% 0.026% 0.003% 0.001%
78.5% 16.7% 1.20% 0.20% 0.037% 0.01% 0.003% 0.001%
73.6% 19.0% 1.72% 0.286% 0.078% 0.026 % 0.011% 0.004%
54.0% 23.8% 3.88% 1.23% 0.60% 0.38% 0.292% 0.214%
28.2% 18.0% 4.73% 2.68% 2.11% 2.39% 3.14% 3.71%
]~
98.9%
96.7%
94.7%
84.4%
65.0%
surements of Schwink et al. [4] and Murani [5], for example, confirm the cluster model. It is therefore necessary to understand why different models sufficiently describe the behaviour of CuMn spin glasses and to combine both conceptions. Performing N M R experiments with dilute CuFe alloys (500 ppm), it has been shown by Alloul [6] and by us [7], that the R K K Y interaction is weaker for pairs and triplets than for single spins. Moreover the indirect exchange integral J describing the s - d exchange interaction between the local moments and the conduction electrons decrease quickly with increasing number of magnetic atoms building up clusters of nearest neighbours. Clusters with about five members show only dipolar interaction, which is about ten times smaller than the R K K Y interaction of single Fe moments, we suppose the same behaviour for Mn. In order to get a survey about the distribution of Mn atoms in a statistical alloy, the probability for clusters of different sizes has been calculated for different Mn concentrations and is shown in table 1. Taking a concentration of 5 at% Mn, the concentration of Mn building up clusters consisting of 3-6 atoms is about 6.1%. This is nearly the percentage of Mn spins which build up the remanent magnetization observed in frozen CuMn spin glasses.
Inferring a subsystem of clusters having a different kind of interaction, the interaction being an order of magnitude smaller than that of the single Mn spin majority, one can imagine why the magnetic hysteresis loop is as remarkable and asymmetrical to the external field H 0 = 0 as observed. Taking into account the above considerations, the synthesis of both models is seen by us in the following way: Performing N M R experiments one proves the response of the single domain system. Performing magnetization experiments one proves the response of the whole system. In the region of small magnetic fields H 0 one gets mainly a response of the subsystem of clusters, which is only weakly coupled to the single domain system and which gives rise to the displaced hysteresis loop.
5. Results and discussion
In fig. 3 the enhancement factor, derived from our measurements, has been plotted as a function of an external magnetic field H o, which has been applied in and opposite to the direction of H c. Coming from high fields, ~ increases to a maximum at the field H 0 = - H a. H d is the field at which the remanent magnetization rapidly changes
Ch. Schlosser, H. Br6mer / Satellite lines in CuMn spin glasses
212
its direction. If H A, as Alloul supposes, consists of the two parts H d and ¢Hax (see eq. (2)) the shape of ~(H0) should be symmetrical with respect to H 0 = - - H d, contrary to our measurements. In order to describe our results, we first assume only an unidirectional anisotropy field H A. This assumption has been proved by varying the direction between H 0 and/arc. With fields smaller than H 0 = H d 7} shall be described by
15
12.5
I
EuMn 2. 5 o~.X
Ho " B75 G
lg 7.5 5
T-1.3K 2.5
= 0 . 9 H L / [ Hg sin28 + ( H A + H 0 cos 8)2] 1/2. (3)
H:- g, 7B kG
0 0
30
120
90
80
ANGLE
150
180
(
O
Fig. 4 shows the good agreement between our measurements and the theoretical description by eq. (3). With fields H 0 higher than H d eq. (3) has to be modified by assuming H A to consist of two parts:
Fig. 5. 7} vs. 0 ( ~ Hc,Ho) of C u M n w i t h 2.5 at% M n after c o o l i n g the s a m p l e in H c = 9 . 7 6 kG, H o = 8 7 5 G. The solid line follows eq. (4) if H s = 1.55 k G a n d H c t = - 0 . 5 3 kG.
1. H s which is due to the system of single spins, and 2. H c~ which is due to the subsystem of clusters.
In fig. 5 ,j has been plotted as a function of the angle O ( Z He, H o) at constant field H o = 875 G > H d. The measurements agree well with the solid line calculated by eq. (4). In order to get agree-
H s is considered to be constant in the region of fields H 0 used in our experiments. With fields H 0 > H a the magnetization of the subsystem of clusters changes its direction and therefore the direction of H c l too. The modified equation (3) then reads:
0.9HL/[(tto + +(HS+(Ho+HCl)cosO)2] 1/2.
30 CuMn 2. 5 a£ T-
o H o " g. TBkG
)~ sin2/~ H c'
=
O ~
t~
20
t
lb
u_
• Ho" 7.50kG ~7 Ho - 5. 00 kG
(4)
LU
CuNn 2. 5 :iX
1.3K
25
H.- 300 G
z
12.5
15
\
10 w h
7.5
z
5
H o - 4. 00 kG
10
T-I.3K Ho- 9. 71BRG
2.5 0
30
50
120
90 ANGLE
e
150
1BO
5 -.B
-.6
-.4
-.2
0
( 'I~=~o )
Fig. 4. 7j vs. e ( Z H~,1to) of C u M n w i t h 2.5 at% M n after c o o l i n g the s a m p l e in He = 9.76 kG, H o = 300 G. The solid line follows eq. (3) if H A = 1.02 kG.
Ho [kG] Fig. 6. E n h a n c e m e n t factor ~j of C u M n with 2.5% M n as a function of H o for various values He.
Ch. Schlosser, H. Bri~mer / Satellite lines in CuMn spin glasses
m e n t between theory and experiment the values H c 1 = --0.53 k G and H s = 1.55 k G have been derived. Fig. 6 shows the behaviour of ~(H0) with the variation of He. It is remarkable that the field value H d is equal for all values H e used. Probably H d is destinated by the strength of coupling between the subsystem of clusters and the single spin system. The steepness of the curves and therefore H A is different for all values of He. In order to prove whether H c influences H s or HAcl or both, careful measurements with high precision are necessary.
Acknowledgement The authors acknowledge the financial aid by the Deutsche Forschungsgemeinschaft, B o n n - B a d Godesberg.
213
References [1] [2] [3] [4]
H. Alloul, Phys. Rev. Lett. 42 (1979) 603. H. Alloul, J. Appl. Phys. 50 (1979) 7330. A. Blacha and H. Br6mer, Z. Phys. B41 (1981) 9. Ch. Schwink, K. Emmerich and U. Schulze, Z. Phys. B31 (1978) 385. [5] A.P. Murani, J. Magn. Magn. Mat. 22 (1981) 271 [6] H. Alloul, J. Darville and P. Bernier, J. Phys. F4 (1974) 2050. [7] Ch. Schlosser and H. Br6mer, J. Magn. Magn. Mat. 21 (1980) 239.
209
ZERO FIELD NMR- SPIN ECHO INVESTIGATION OF SATELLITE LINES IN CuMn SPIN GLASSES Ch. S C H L O S S E R and H. B R O M E R Institut A fiir Physik, Abt. fiir Angew. Kern- und Neutronenphysik, Technische Universititt Braunschweig, Fed. Rep. Germany
Received 30 March 1982
In 1979 Alloul made zero field NMR measurements of satellite lines in CuMn spin glasses. He pointed out the existence of an enhancement factor and showed that the Mn spins behave like the spins in ferromagnetic domain particles. New independent measurements, presented here, performed especially in order to prove the existence of an enhancement factor and therefore the model description based hereupon, are in agreement with the interpretation of Alloul. The discrepancy between the cluster and the one domain model of spin glasses can well be understood, if one realizes the existence of a subsystem of clusters with dipole interaction and a main system of single spins rigidly bound by RKKY interaction. Within this model the behaviour of the enhancement factor, measured by us, can be interpretated.
1. Introduction By showing the existence of a ferromagnetic enhancement factor ~, Alloul [1,2] showed in 1979 that the M n spins of a frozen spin glass behave like a ferromagnetic single d o m a i n particle. Therefore collective aspects of the frozen state came into discussion. In a multidirectional spin system like the spins in a Bloch wall or a spin glass only a mean value can be measured, which characterizes the behaviour of the whole sample. O w n measurements [3] with ferromagnetic iron powder have shown that the derivation of an enhancement factor ~ from the measurements is not out of the question. Measuring the intensity of the free induction decay or the intensity of spin echoes as a function of the turning angle a = "yHtT, the usual way of deriving ~, the results are different for varying either z or H r As a consequence of a limited excitation bandwidth (Ao~e)l/2 three different enhancement factors have to be defined, being ~(¢), ~(H1) and ~0(At% i/2 --' 0). Ignoring these problems, large errors are possible in deriving ~. The work of Alloul does not show by what procedure 7/has been derived. The above n a m e d uncertainties do not exist if a
Fourier-transformation of the NMR-signals is used, We therefore intended to prove the measurements of Alloul by applying this technique.
2. Experimental procedure N M R spin echoes were measured using a pulse sequence ~'-tv-~, ~" being the pulse duration and tv the time delay between the pulses. The frequency was 11 M H z corresponding to the signal frequency coming from the fourth copper neighbours of M n atoms. The spin echoes were Fourier transformed into the frequency domain. The line intensities were plotted against the turning angle a = ~,~H~'. The enhancement factors ~ were derived from the maxima of the curves. In order to effect a value ~1~ 1 the samples were cooled from T = 150 K to the measuring temperature T = 1.3 K in an external field H c, parallel to the z-axis. Additional to the measurements of ~ in zero field, ~ has been evaluated also by applying low external fields H o and varying their angle 0 the direction of Hc as shown in fig. 1.
0 3 0 4 - 8 8 5 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
210
Ch. Schlosser, H. Brfmer / Satellite lines in CuMn spin glasses
3uM~ 2 . 5 o£ % "
20
No " 9. 7 6 RG
T
-
1.3K
Y
Fig. 1. Geometry of NMR experiments.
i
-i.5
-i
-.5
.5 Ho [ k G ]
3. The single domain model of Alloul First of all let us briefly report Allouls single d o m a i n model. After cooling his sample in a field Hc Alloul found an enhancement factor 7/ which could be described by the simple relation:
Fig. 3. ~ vs. H 0 of CuMn with 2.5 at% Mn after cooling the sample in H¢=9.76 kG. The solid line follows eq. (1) if H a ----1.02 kg.
easily can derive additional rf-fields
n--(Ho+ HA)-' ,
H ~ = IH, I]HL[ s i n O / l U o + n A [ ,
H0 IIHe, H 0 being an external field. H A is a single fitting parameter, which can be interpretated as an anisotropy field acting on the spin system as a whole one. D u r i n g an exciting rf-pulse with H i l l y , the magnetisation ~r, lying in z-direction at the beginning of the pulse, is turned a r o u n d the x-axis by an angle
Since the local fields H L are oriented at r a n d o m one has to integrate over the signal contributions of all spins with varying angle 8, getting an average enhancement factor
"t' ~ Hi/IHo + HAI as shown in fig. 2. Taking into account that all the local magnetic hyperfine fields H L, seen by the neighbouring c o p p e r nuclei, participate in this rotation of ~r, one
~',n'
,~o
6/", ! J
8 ---- ± ( H L , X ).
~=0.9IHLI/IHo + HAI.
(1)
A p p l y i n g a low field H 0 in opposite direction of He, Alloul found H A to consist of two parts as described in eq. (2): - an unidirectional field H d in direction of H c and - an uniaxial field H,x its axis lying parallel to He. HA----Hd+--¢Hax,
with c = sign (Hd + H0).
(2)
In fig. 3 the results of our measurements have been plotted. It shows ~ as a function of the magnetic field H 0 parallel and antiparallel to H c. Applying eq. (1) a value H A = 1.02 k G can be derived giving the solid line in fig. 3. Our own measurements are therefore in full agreement with the one d o m a i n model of Alloul.
p !
i
i
,
4. Synthesis of the single domain and the cluster model Y Fig. 2. Rotation geometry of the domain magnetisation o and and the hyperfine fie•s H L.
U n d o u b t e d l y our measurements confirm the single domain model. U n d o u b t e d l y too the mea-
Ch. Schlosser, H. Br6mer / Satellite lines in CuMn spin glasses
211
Table 1 Portions of the whole M n content of alloys with various M n concentrations C, that build up spherical clusters with different numbers N and of single M n spins. The numbers are calculated assuming a statistical distribution of Mn atoms N
C 1 at%
2 at%
2.5 at%
5 at%
10 at%
1 2 3 4 5 6 7 8
88.6% 10.0% 0.30% 0.026% 0.003% 0.001%
78.5% 16.7% 1.20% 0.20% 0.037% 0.01% 0.003% 0.001%
73.6% 19.0% 1.72% 0.286% 0.078% 0.026 % 0.011% 0.004%
54.0% 23.8% 3.88% 1.23% 0.60% 0.38% 0.292% 0.214%
28.2% 18.0% 4.73% 2.68% 2.11% 2.39% 3.14% 3.71%
]~
98.9%
96.7%
94.7%
84.4%
65.0%
surements of Schwink et al. [4] and Murani [5], for example, confirm the cluster model. It is therefore necessary to understand why different models sufficiently describe the behaviour of CuMn spin glasses and to combine both conceptions. Performing N M R experiments with dilute CuFe alloys (500 ppm), it has been shown by Alloul [6] and by us [7], that the R K K Y interaction is weaker for pairs and triplets than for single spins. Moreover the indirect exchange integral J describing the s - d exchange interaction between the local moments and the conduction electrons decrease quickly with increasing number of magnetic atoms building up clusters of nearest neighbours. Clusters with about five members show only dipolar interaction, which is about ten times smaller than the R K K Y interaction of single Fe moments, we suppose the same behaviour for Mn. In order to get a survey about the distribution of Mn atoms in a statistical alloy, the probability for clusters of different sizes has been calculated for different Mn concentrations and is shown in table 1. Taking a concentration of 5 at% Mn, the concentration of Mn building up clusters consisting of 3-6 atoms is about 6.1%. This is nearly the percentage of Mn spins which build up the remanent magnetization observed in frozen CuMn spin glasses.
Inferring a subsystem of clusters having a different kind of interaction, the interaction being an order of magnitude smaller than that of the single Mn spin majority, one can imagine why the magnetic hysteresis loop is as remarkable and asymmetrical to the external field H 0 = 0 as observed. Taking into account the above considerations, the synthesis of both models is seen by us in the following way: Performing N M R experiments one proves the response of the single domain system. Performing magnetization experiments one proves the response of the whole system. In the region of small magnetic fields H 0 one gets mainly a response of the subsystem of clusters, which is only weakly coupled to the single domain system and which gives rise to the displaced hysteresis loop.
5. Results and discussion
In fig. 3 the enhancement factor, derived from our measurements, has been plotted as a function of an external magnetic field H o, which has been applied in and opposite to the direction of H c. Coming from high fields, ~ increases to a maximum at the field H 0 = - H a. H d is the field at which the remanent magnetization rapidly changes
Ch. Schlosser, H. Br6mer / Satellite lines in CuMn spin glasses
212
its direction. If H A, as Alloul supposes, consists of the two parts H d and ¢Hax (see eq. (2)) the shape of ~(H0) should be symmetrical with respect to H 0 = - - H d, contrary to our measurements. In order to describe our results, we first assume only an unidirectional anisotropy field H A. This assumption has been proved by varying the direction between H 0 and/arc. With fields smaller than H 0 = H d 7} shall be described by
15
12.5
I
EuMn 2. 5 o~.X
Ho " B75 G
lg 7.5 5
T-1.3K 2.5
= 0 . 9 H L / [ Hg sin28 + ( H A + H 0 cos 8)2] 1/2. (3)
H:- g, 7B kG
0 0
30
120
90
80
ANGLE
150
180
(
O
Fig. 4 shows the good agreement between our measurements and the theoretical description by eq. (3). With fields H 0 higher than H d eq. (3) has to be modified by assuming H A to consist of two parts:
Fig. 5. 7} vs. 0 ( ~ Hc,Ho) of C u M n w i t h 2.5 at% M n after c o o l i n g the s a m p l e in H c = 9 . 7 6 kG, H o = 8 7 5 G. The solid line follows eq. (4) if H s = 1.55 k G a n d H c t = - 0 . 5 3 kG.
1. H s which is due to the system of single spins, and 2. H c~ which is due to the subsystem of clusters.
In fig. 5 ,j has been plotted as a function of the angle O ( Z He, H o) at constant field H o = 875 G > H d. The measurements agree well with the solid line calculated by eq. (4). In order to get agree-
H s is considered to be constant in the region of fields H 0 used in our experiments. With fields H 0 > H a the magnetization of the subsystem of clusters changes its direction and therefore the direction of H c l too. The modified equation (3) then reads:
0.9HL/[(tto + +(HS+(Ho+HCl)cosO)2] 1/2.
30 CuMn 2. 5 a£ T-
o H o " g. TBkG
)~ sin2/~ H c'
=
O ~
t~
20
t
lb
u_
• Ho" 7.50kG ~7 Ho - 5. 00 kG
(4)
LU
CuNn 2. 5 :iX
1.3K
25
H.- 300 G
z
12.5
15
\
10 w h
7.5
z
5
H o - 4. 00 kG
10
T-I.3K Ho- 9. 71BRG
2.5 0
30
50
120
90 ANGLE
e
150
1BO
5 -.B
-.6
-.4
-.2
0
( 'I~=~o )
Fig. 4. 7j vs. e ( Z H~,1to) of C u M n w i t h 2.5 at% M n after c o o l i n g the s a m p l e in He = 9.76 kG, H o = 300 G. The solid line follows eq. (3) if H A = 1.02 kG.
Ho [kG] Fig. 6. E n h a n c e m e n t factor ~j of C u M n with 2.5% M n as a function of H o for various values He.
Ch. Schlosser, H. Bri~mer / Satellite lines in CuMn spin glasses
m e n t between theory and experiment the values H c 1 = --0.53 k G and H s = 1.55 k G have been derived. Fig. 6 shows the behaviour of ~(H0) with the variation of He. It is remarkable that the field value H d is equal for all values H e used. Probably H d is destinated by the strength of coupling between the subsystem of clusters and the single spin system. The steepness of the curves and therefore H A is different for all values of He. In order to prove whether H c influences H s or HAcl or both, careful measurements with high precision are necessary.
Acknowledgement The authors acknowledge the financial aid by the Deutsche Forschungsgemeinschaft, B o n n - B a d Godesberg.
213
References [1] [2] [3] [4]
H. Alloul, Phys. Rev. Lett. 42 (1979) 603. H. Alloul, J. Appl. Phys. 50 (1979) 7330. A. Blacha and H. Br6mer, Z. Phys. B41 (1981) 9. Ch. Schwink, K. Emmerich and U. Schulze, Z. Phys. B31 (1978) 385. [5] A.P. Murani, J. Magn. Magn. Mat. 22 (1981) 271 [6] H. Alloul, J. Darville and P. Bernier, J. Phys. F4 (1974) 2050. [7] Ch. Schlosser and H. Br6mer, J. Magn. Magn. Mat. 21 (1980) 239.