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Antiferromagnetism of La2CuO4: Mössbauer effect of 57Fe doped in La2CuO4
Physica B 149 (1988) 91-94 North-Holland, Amsterdam
ANTIFERROMAGNETISM OF LazCuO4: MOSSBAUER EFFECT OF STFe DOPED IN La2CuO 4 Y. NISHIHARA, K. OKA and H. UNOKI Electrotechnical Laboratory, Tsukuba Research Center, lbaraki 305, Japan
M6ssbauer effect of 57Fe doped in antiferromagnetic La2CuO 4 was measured between 77 K and room temperature. Iron atoms occupy the Cu site. Most of them have an electronic configuration of the high-spin Fe 3+. Below 220 K the M6ssbauer spectrum broadens owing to the magnetic ordering. The hyperfine field of iron is 395 kOe at 77 K. From the analysis of the quadrupole splitting it is concluded that the magnetic moment is perpendicular to the c-axis of the tetragonal crystal.
1. Introduction After the discovery of the new high-T c superconductor [1], La2CuO 4 has attracted much attention as the starting material for a discussion of the physics of superconductivity [2-4]. Magnetic and electric properties have been studied intensively by many workers [5-8]. The neutron diffraction [6, 7] and M6ssbauer [8] experiments have revealed that La2CuO 4 has antiferromagnetic ordering below -220 K. The magnetic susceptibility shows a cusp around the magnetic ordering temperature. The electrical resistivity increases with decreasing temperature. The slope of the resistivity becomes small below the temperature where the susceptibility shows the cusp [8]. These results show that the intrinsic properties of La2CuO 4 are semiconducting and antiferromagnetic. We have measured the M6ssbauer effect of 57Fe doped in La2CuO 4 to investigate the magnetic structure of L a 2 f u O 4 and the electronic state of iron in La2CuO 4. This paper reports the results of the magnetic hyperfine field and the quadrupole splitting of iron. A discussion of the magnetic structure of La2CuO 4 will be given using these results.
2. Experimental results The sample o f La2Cu0.99557Fe0.00504 was prepared from La20 3 (99.99%), CuO (99.99%) and
90% e n r i c h e d 57Fe20 3. The mixture of these materials was heated in an alumina crucible. The final heat treatment is an annealing at ll00°C over 10 h. After the annealing it was cooled at a rate around 30°C/h. The sample was confirmed by X-ray diffraction to be a single-phase material with the orthorhombically distorted K2NiF 4 structure. M6ssbauer ~ e c t r a were measured with a source of 10 mCi Co in Rh. The velocity was calibrated with the spectrum of metallic iron. The temperature dependences of magnetic susceptibility and electrical resistivity of this sample are given in ref. [8]. The susceptibility is about three times as large as that of pure La2CuO 4. However, the overall feature of the temperature variation is essentially similar to that of the pure one. The cusp of the susceptibility appears around 220 K. The slope of resistivity also changes around this temperature. Fig. 1 shows the M6ssbauer spectra of 57Fe doped in L a 2 f u O 4. The spectrum is a quadrupole doublet above 250 K. The isomer shift relative to metallic iron is 0.305--_ 0.002mm/s and the quadrupole splitting (=e2qQ/2) 1.558 _+ 0.005 mm/s at room temperature. A broadening of the spectrum is observed below 220 K. The splitting of the hyperfine sextet is not clear in the spectrum at 200 and 220 K. As shown in the figure, we can observe a broadened hyperfine sextet at 77 K. The broadening is significant for outer lines. The average hyperfine field is 395-+ 1 kOe. The broadening of the outer lines sug-
0378-4363/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
92
Y. Nishihara et al. / Antiferromagnetism o f La2CuO 4 I
I
I
I
[
I
i
i
the cusp of the susceptibility and the change of the slope of resistivity, the hyperfine field disappears above 220K. The results show that the magnetic state of La2CuO 4 is antiferromagnetic and that the ordering temperature agrees with the cusp of the susceptibility around 220 K.
0
290K o~
2
Z 0
L a 2 CL10,99557Fe0.005 O4
3. D i s c u s s i o n
(X
nO crnO 0
3.1. Electronic state of iron
..
-10
I
I
-8
-6
I
77K
I
I
I
-4 -2 0 2 4 V E L O C I T Y (mm/s)
6
8
I
I
]
10
Fig. 1. M6ssbauer spectra of SVFe doped in La2CuO 4.
gests a distribution of the hyperfine field of - 3 8 k O e . This broadening may come from a local environment effect due to oxygen vacancy or a distribution of spin density depending on the magnetic structure. More work will be necessary to make clear the origin of the broadening. The effective quadrupole splitting is - 0 . 8 7 --0.02 m m / s at 77 K. The hyperfine field of iron varies with temperature, as shown in fig. 2. Corresponding to I
I
I
SrLaFeO 4 also belongs to the K2NiF4-type structure. The magnetic moment of Fe 3+ ions is 4.23pm and orders antiferromagnetically below 3 8 0 K [9]. We prepared a single crystal of SrLaFeO 4 and measured the M6ssbauer spectrum. Fig. 3 shows the M6ssbauer spectrum of the powdered sample at room temperature. The hyperfine field is 291 kOe and the effective quadrupole splitting - 0 . 5 7 mm/s. The hyperfine field extrapolated to low temperatures is - - 4 0 0 k O e and nearly equal to the value of that in LazCuO 4. Comparing with the results of SrLaFeO 4, we find that irons in L a z C u O 4 occupy Cu site and have an electronic configuration of Fe 3+. In the spectrum of SrLaFeO 4 we can observe a strong single absorption line together with the hyperfine sextet. The intensity of this single line is nearly 20% of the total absorption intensity. The X-ray diffraction pattern shows no extra phase other than the K2NiF 4 phase. Therefore, we find that this single absorption line does not
SrLaFeO 4
Z o
~- 2 0 0
's "-,~',',".,
,,',,,,,~ ,,*'~"
z
o
ha. n0 rn
0
I 0
I 200 T (K)
1
2
'I
~
"" "
•
°° °,,
•
.~.
3
_~ 400
-10
I
I
I
I
]
t
I
l
[
-8
-6
-4
-2
0
2
4
6
8
VELOCITY
Fig. 2. Temperature dependence of the magnetic hyperfine field of Fe in La2CuO 4.
(ram/s)
Fig. 3. M6ssbauer spectrum of SrLaFeO4.
10
93
Y. Nishihara et al. / Antiferromagnetism of La~CuO4
originate from irons in a crystallographically different phase but from irons in a different electronic state in the same crystal phase. The isomer shift of this state is - 0 . 0 8 m m / s relative to metallic iron. This value falls in the distribution of the isomer shift of the low-spin Fe 3÷ or Fe 2+ [10]. The valence of iron cannot be determined from this value. However, since this iron has no magnetic moment, we can conclude that the valence of this iron is 2 + . The isomer shift of the hyperfine sextet is 0.69 mm/s. This value corresponds to the isomer shift of the high spin Fe 3÷ [10]. Therefore, in this sample nearly 20% of iron is in the low-spin Fe 2+ and others in the high-spin Fe 3+. The M r s s b a u e r spectrum reported by Soubeyroux et al. [9] shows that all the irons are in the state of the high-spin Fe 3÷. The population of the high-spin Fe 3+ and the lowspin Fe 2+ seems to depend on the condition of the sample preparation. The Mrssbauer spectrum of iron in La2CuO 4 also suggests the presence of two kinds of iron states, as shown in fig. 1. The slight asymmetry of the doublet at room temperature suggests an absorption due to irons other than the high-spin Fe 3÷. The contribution of iron other than the high-spin Fe 3+ is observed also in the spectrum at 77K. Taking into account the results of SrLaFeO4, we can conclude that this contribution originates from the absorption of the lowspin Fe 2+. The contribution of Fe 2+ is estimated to be nearly 10% of the total absorption intensity. 3.2. Magnetic structure
The quadrupole splitting of the Mrssbauer spectrum depends on the angle 0 between the symmetry axis of electric field gradient and the direction of internal magnetic field. When the quadrupole splitting is far smaller than the magnetic splitting, the quadrupole splitting varies as 2 (3cos 0-1)/2. Therefore, by comparing the value of the quadrupole splitting at 77 K with the value at paramagnetic state, we can obtain information on the magnetic structre of La2CuO 4. Fig. 4 shows the quadrupole splitting ( e Z q Q / 2 ) of 57Fe in La2CuO 4. The sign of the quadrupole
I
'
I
~eZqQ/2
E ca
.=_ 0,. (/)
,,observed
6)
"5
-1
value
Hhyp J- tetragonal C axis -
"10
o -2
L
I
0
t
200 T
400
(K)
Fig. 4. Quadrupole splitting of Fe in
La 2C uO 4.
splitting cannot be determined from the spectrum at the paramagnetic region. In SrLaFeO 4 and CaLaFeO4, however, from the relation between the magnetic structure [9, 11] and the quadrupole splitting we can determine the sign of the quadrupole splitting in the paramagnetic region. The spin direction is perpendicular to the c-axis of the tetragonal structure. Since the principal axis of electric field gradient is parallel to the c-axis, the quadrupole splitting in the antiferromagnetic state has the absolute value of half of that in the paramagnetic state. The sign is different between the paramagnetic and antiferromagnetic states. The negative sign in the antiferromagnetic state [9, 12] leads to the conclusion that the sign of the quadrupole splitting is positive in the paramagnetic region. Since La2CuO 4 has the same crystal structure as SrLaFeO 4 and C a L a F e O 4, we find that the quadrupole splitting of La2CuO 4 is also positive in the paramagnetic state. Thus, we plotted the positive values in fig. 4. Table I Values of quadrupole splitting (e2qQ/2) (3cos2 0 - 1)/2, where 0 is the angle between the principal axis of electric field gradient and the direction of the magnetic hyperfine field. Quadrupole splitting (mm/s) Para (O = 0°) Antiferro (0 = 90°) LazCuO4(Fe) SrLaFeO4 CaLaFeO4 [12]
1.558 -+0.005 1.07 -+0.01 -1.0
-0.87 -+0.02 -0.57 -+0.02 - - 0.45
94
Y. Nishihara et al. / Antiferromagnetism o f La2CuO 4
T h e values o f the q u a d r u p o l e splitting are listed in table I. T h e relation b e t w e e n the quadrupole splitting at 77 K (antiferromagnetic state) and r o o m t e m p e r a t u r e ( p a r a m a g n e t i c state) in L a 2 C u O 4 is similar to those in S r L a F e O 4 and C a L a F e O 4. This result shows that the spin direction is p e r p e n d i c u l a r to the tetragonal c-axis also in L a 2 C u O 4. This m a g n e t i c structure is consistent with the result of the n e u t r o n diffraction e x p e r i m e n t [6]. T h e o p e n circle in fig. 4 is the estimated value assuming this magnetic structure.
4. Conclusion T h e M 6 s s b a u e r effect of 57Fe d o p e d in L a 2 C u O 4 revealed that L a 2 C u O 4 is in the antif e r r o m a g n e t i c state below 220 K. T h e irons occupy C u sites. T h e electronic state of nearly 90% of irons is the high-spin Fe 3÷. T h e state o f others m a y be the low-spin Fe 2*. Analysis of the quadrupole splitting shows that the spin direction is p e r p e n d i c u l a r to the tetragonal c-axis. L a 2 C u O 4 has a m a g n e t i c structure similar to that o f S r L a F e O 4 and C a L a F e O 4. One
of the
authors
(H.U.)
dedicates
this
p a p e r to Professor M a s a o Shimizu on the occas i o n o f his r e t i r e m e n t f r o m N a g o y a University.
References [1] J.G. Bednorz and K.A. Mfiller, Z. Phys. B 64 (1986) 189. [2] P.W. Anderson, Science 235 (1987) 1196. [3] Y. Hasegawa and H. Fukuyama, Japan. J. Appl. Phys. 26 (1987) L332. [4] T. Fujita, Y. Aoki, Y. Maeno, J. Sakurai, H. Fukuba and H. Fujii, Japan. J. Appl. Phys. 26 (1987) L368. [5] P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.E. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phys. Rev. Lett. 58 (1987) 2482. [6] D. Vaknin, S.K. Sinha, D.E. Moncton, D.C. Johnston, J.M. Newsam, C.R. Safinya and H.E. King, Jr., Phys. Rev. Lett. 58 (1987) 2802. [7] B.X. Yang, S. Mitsuda, G. Shirane, Y. Yamaguchi, H. Yarnauchi and Y. Synono, J. Phys. Soc. Japan 56 (1987) 2283. [8] Y. Nishihara, M. Tokumoto, K. Murata and H. Unoki, Japan. Appl. Phys. 26 (1987). [9] J.L. Soubeyroux, P. Courbin, L. Fournes, D. Fruchart and G. Le Flem, J. Solid State Chem. 31 (1980) 313. [10] J.G. Stevens and R.S. Preston, M6ssbauer Effect Data Index-1970, J.G. Stevens and V.E. Stevens, eds. (Plenum, New York, 1972), p. 16. [11] N.T. Dinh, M. Vlasse, M. Perrin and G. Le Flem, J. Solid State Chem. 32 (1980) 1. [12] J. Fava, M. Danot, N.T. Dinh, A. Daoudi and G. Le Flem, Solid State Commun. 22 (1977) 733.
ANTIFERROMAGNETISM OF LazCuO4: MOSSBAUER EFFECT OF STFe DOPED IN La2CuO 4 Y. NISHIHARA, K. OKA and H. UNOKI Electrotechnical Laboratory, Tsukuba Research Center, lbaraki 305, Japan
M6ssbauer effect of 57Fe doped in antiferromagnetic La2CuO 4 was measured between 77 K and room temperature. Iron atoms occupy the Cu site. Most of them have an electronic configuration of the high-spin Fe 3+. Below 220 K the M6ssbauer spectrum broadens owing to the magnetic ordering. The hyperfine field of iron is 395 kOe at 77 K. From the analysis of the quadrupole splitting it is concluded that the magnetic moment is perpendicular to the c-axis of the tetragonal crystal.
1. Introduction After the discovery of the new high-T c superconductor [1], La2CuO 4 has attracted much attention as the starting material for a discussion of the physics of superconductivity [2-4]. Magnetic and electric properties have been studied intensively by many workers [5-8]. The neutron diffraction [6, 7] and M6ssbauer [8] experiments have revealed that La2CuO 4 has antiferromagnetic ordering below -220 K. The magnetic susceptibility shows a cusp around the magnetic ordering temperature. The electrical resistivity increases with decreasing temperature. The slope of the resistivity becomes small below the temperature where the susceptibility shows the cusp [8]. These results show that the intrinsic properties of La2CuO 4 are semiconducting and antiferromagnetic. We have measured the M6ssbauer effect of 57Fe doped in La2CuO 4 to investigate the magnetic structure of L a 2 f u O 4 and the electronic state of iron in La2CuO 4. This paper reports the results of the magnetic hyperfine field and the quadrupole splitting of iron. A discussion of the magnetic structure of La2CuO 4 will be given using these results.
2. Experimental results The sample o f La2Cu0.99557Fe0.00504 was prepared from La20 3 (99.99%), CuO (99.99%) and
90% e n r i c h e d 57Fe20 3. The mixture of these materials was heated in an alumina crucible. The final heat treatment is an annealing at ll00°C over 10 h. After the annealing it was cooled at a rate around 30°C/h. The sample was confirmed by X-ray diffraction to be a single-phase material with the orthorhombically distorted K2NiF 4 structure. M6ssbauer ~ e c t r a were measured with a source of 10 mCi Co in Rh. The velocity was calibrated with the spectrum of metallic iron. The temperature dependences of magnetic susceptibility and electrical resistivity of this sample are given in ref. [8]. The susceptibility is about three times as large as that of pure La2CuO 4. However, the overall feature of the temperature variation is essentially similar to that of the pure one. The cusp of the susceptibility appears around 220 K. The slope of resistivity also changes around this temperature. Fig. 1 shows the M6ssbauer spectra of 57Fe doped in L a 2 f u O 4. The spectrum is a quadrupole doublet above 250 K. The isomer shift relative to metallic iron is 0.305--_ 0.002mm/s and the quadrupole splitting (=e2qQ/2) 1.558 _+ 0.005 mm/s at room temperature. A broadening of the spectrum is observed below 220 K. The splitting of the hyperfine sextet is not clear in the spectrum at 200 and 220 K. As shown in the figure, we can observe a broadened hyperfine sextet at 77 K. The broadening is significant for outer lines. The average hyperfine field is 395-+ 1 kOe. The broadening of the outer lines sug-
0378-4363/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
92
Y. Nishihara et al. / Antiferromagnetism o f La2CuO 4 I
I
I
I
[
I
i
i
the cusp of the susceptibility and the change of the slope of resistivity, the hyperfine field disappears above 220K. The results show that the magnetic state of La2CuO 4 is antiferromagnetic and that the ordering temperature agrees with the cusp of the susceptibility around 220 K.
0
290K o~
2
Z 0
L a 2 CL10,99557Fe0.005 O4
3. D i s c u s s i o n
(X
nO crnO 0
3.1. Electronic state of iron
..
-10
I
I
-8
-6
I
77K
I
I
I
-4 -2 0 2 4 V E L O C I T Y (mm/s)
6
8
I
I
]
10
Fig. 1. M6ssbauer spectra of SVFe doped in La2CuO 4.
gests a distribution of the hyperfine field of - 3 8 k O e . This broadening may come from a local environment effect due to oxygen vacancy or a distribution of spin density depending on the magnetic structure. More work will be necessary to make clear the origin of the broadening. The effective quadrupole splitting is - 0 . 8 7 --0.02 m m / s at 77 K. The hyperfine field of iron varies with temperature, as shown in fig. 2. Corresponding to I
I
I
SrLaFeO 4 also belongs to the K2NiF4-type structure. The magnetic moment of Fe 3+ ions is 4.23pm and orders antiferromagnetically below 3 8 0 K [9]. We prepared a single crystal of SrLaFeO 4 and measured the M6ssbauer spectrum. Fig. 3 shows the M6ssbauer spectrum of the powdered sample at room temperature. The hyperfine field is 291 kOe and the effective quadrupole splitting - 0 . 5 7 mm/s. The hyperfine field extrapolated to low temperatures is - - 4 0 0 k O e and nearly equal to the value of that in LazCuO 4. Comparing with the results of SrLaFeO 4, we find that irons in L a z C u O 4 occupy Cu site and have an electronic configuration of Fe 3+. In the spectrum of SrLaFeO 4 we can observe a strong single absorption line together with the hyperfine sextet. The intensity of this single line is nearly 20% of the total absorption intensity. The X-ray diffraction pattern shows no extra phase other than the K2NiF 4 phase. Therefore, we find that this single absorption line does not
SrLaFeO 4
Z o
~- 2 0 0
's "-,~',',".,
,,',,,,,~ ,,*'~"
z
o
ha. n0 rn
0
I 0
I 200 T (K)
1
2
'I
~
"" "
•
°° °,,
•
.~.
3
_~ 400
-10
I
I
I
I
]
t
I
l
[
-8
-6
-4
-2
0
2
4
6
8
VELOCITY
Fig. 2. Temperature dependence of the magnetic hyperfine field of Fe in La2CuO 4.
(ram/s)
Fig. 3. M6ssbauer spectrum of SrLaFeO4.
10
93
Y. Nishihara et al. / Antiferromagnetism of La~CuO4
originate from irons in a crystallographically different phase but from irons in a different electronic state in the same crystal phase. The isomer shift of this state is - 0 . 0 8 m m / s relative to metallic iron. This value falls in the distribution of the isomer shift of the low-spin Fe 3÷ or Fe 2+ [10]. The valence of iron cannot be determined from this value. However, since this iron has no magnetic moment, we can conclude that the valence of this iron is 2 + . The isomer shift of the hyperfine sextet is 0.69 mm/s. This value corresponds to the isomer shift of the high spin Fe 3÷ [10]. Therefore, in this sample nearly 20% of iron is in the low-spin Fe 2+ and others in the high-spin Fe 3+. The M r s s b a u e r spectrum reported by Soubeyroux et al. [9] shows that all the irons are in the state of the high-spin Fe 3÷. The population of the high-spin Fe 3+ and the lowspin Fe 2+ seems to depend on the condition of the sample preparation. The Mrssbauer spectrum of iron in La2CuO 4 also suggests the presence of two kinds of iron states, as shown in fig. 1. The slight asymmetry of the doublet at room temperature suggests an absorption due to irons other than the high-spin Fe 3÷. The contribution of iron other than the high-spin Fe 3+ is observed also in the spectrum at 77K. Taking into account the results of SrLaFeO4, we can conclude that this contribution originates from the absorption of the lowspin Fe 2+. The contribution of Fe 2+ is estimated to be nearly 10% of the total absorption intensity. 3.2. Magnetic structure
The quadrupole splitting of the Mrssbauer spectrum depends on the angle 0 between the symmetry axis of electric field gradient and the direction of internal magnetic field. When the quadrupole splitting is far smaller than the magnetic splitting, the quadrupole splitting varies as 2 (3cos 0-1)/2. Therefore, by comparing the value of the quadrupole splitting at 77 K with the value at paramagnetic state, we can obtain information on the magnetic structre of La2CuO 4. Fig. 4 shows the quadrupole splitting ( e Z q Q / 2 ) of 57Fe in La2CuO 4. The sign of the quadrupole
I
'
I
~eZqQ/2
E ca
.=_ 0,. (/)
,,observed
6)
"5
-1
value
Hhyp J- tetragonal C axis -
"10
o -2
L
I
0
t
200 T
400
(K)
Fig. 4. Quadrupole splitting of Fe in
La 2C uO 4.
splitting cannot be determined from the spectrum at the paramagnetic region. In SrLaFeO 4 and CaLaFeO4, however, from the relation between the magnetic structure [9, 11] and the quadrupole splitting we can determine the sign of the quadrupole splitting in the paramagnetic region. The spin direction is perpendicular to the c-axis of the tetragonal structure. Since the principal axis of electric field gradient is parallel to the c-axis, the quadrupole splitting in the antiferromagnetic state has the absolute value of half of that in the paramagnetic state. The sign is different between the paramagnetic and antiferromagnetic states. The negative sign in the antiferromagnetic state [9, 12] leads to the conclusion that the sign of the quadrupole splitting is positive in the paramagnetic region. Since La2CuO 4 has the same crystal structure as SrLaFeO 4 and C a L a F e O 4, we find that the quadrupole splitting of La2CuO 4 is also positive in the paramagnetic state. Thus, we plotted the positive values in fig. 4. Table I Values of quadrupole splitting (e2qQ/2) (3cos2 0 - 1)/2, where 0 is the angle between the principal axis of electric field gradient and the direction of the magnetic hyperfine field. Quadrupole splitting (mm/s) Para (O = 0°) Antiferro (0 = 90°) LazCuO4(Fe) SrLaFeO4 CaLaFeO4 [12]
1.558 -+0.005 1.07 -+0.01 -1.0
-0.87 -+0.02 -0.57 -+0.02 - - 0.45
94
Y. Nishihara et al. / Antiferromagnetism o f La2CuO 4
T h e values o f the q u a d r u p o l e splitting are listed in table I. T h e relation b e t w e e n the quadrupole splitting at 77 K (antiferromagnetic state) and r o o m t e m p e r a t u r e ( p a r a m a g n e t i c state) in L a 2 C u O 4 is similar to those in S r L a F e O 4 and C a L a F e O 4. This result shows that the spin direction is p e r p e n d i c u l a r to the tetragonal c-axis also in L a 2 C u O 4. This m a g n e t i c structure is consistent with the result of the n e u t r o n diffraction e x p e r i m e n t [6]. T h e o p e n circle in fig. 4 is the estimated value assuming this magnetic structure.
4. Conclusion T h e M 6 s s b a u e r effect of 57Fe d o p e d in L a 2 C u O 4 revealed that L a 2 C u O 4 is in the antif e r r o m a g n e t i c state below 220 K. T h e irons occupy C u sites. T h e electronic state of nearly 90% of irons is the high-spin Fe 3÷. T h e state o f others m a y be the low-spin Fe 2*. Analysis of the quadrupole splitting shows that the spin direction is p e r p e n d i c u l a r to the tetragonal c-axis. L a 2 C u O 4 has a m a g n e t i c structure similar to that o f S r L a F e O 4 and C a L a F e O 4. One
of the
authors
(H.U.)
dedicates
this
p a p e r to Professor M a s a o Shimizu on the occas i o n o f his r e t i r e m e n t f r o m N a g o y a University.
References [1] J.G. Bednorz and K.A. Mfiller, Z. Phys. B 64 (1986) 189. [2] P.W. Anderson, Science 235 (1987) 1196. [3] Y. Hasegawa and H. Fukuyama, Japan. J. Appl. Phys. 26 (1987) L332. [4] T. Fujita, Y. Aoki, Y. Maeno, J. Sakurai, H. Fukuba and H. Fujii, Japan. J. Appl. Phys. 26 (1987) L368. [5] P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.E. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phys. Rev. Lett. 58 (1987) 2482. [6] D. Vaknin, S.K. Sinha, D.E. Moncton, D.C. Johnston, J.M. Newsam, C.R. Safinya and H.E. King, Jr., Phys. Rev. Lett. 58 (1987) 2802. [7] B.X. Yang, S. Mitsuda, G. Shirane, Y. Yamaguchi, H. Yarnauchi and Y. Synono, J. Phys. Soc. Japan 56 (1987) 2283. [8] Y. Nishihara, M. Tokumoto, K. Murata and H. Unoki, Japan. Appl. Phys. 26 (1987). [9] J.L. Soubeyroux, P. Courbin, L. Fournes, D. Fruchart and G. Le Flem, J. Solid State Chem. 31 (1980) 313. [10] J.G. Stevens and R.S. Preston, M6ssbauer Effect Data Index-1970, J.G. Stevens and V.E. Stevens, eds. (Plenum, New York, 1972), p. 16. [11] N.T. Dinh, M. Vlasse, M. Perrin and G. Le Flem, J. Solid State Chem. 32 (1980) 1. [12] J. Fava, M. Danot, N.T. Dinh, A. Daoudi and G. Le Flem, Solid State Commun. 22 (1977) 733.