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Magnetic properties of gadolinium silicide thin films produced by different fabrication processes
ELSEVIER
Thin Solid Films 278 ( 1996) 140-143
Magnetic properties of gadolinium silicide thin films produced by different fabrication processes C. Pescher a, J. Pierre b, A. Ermolieff ‘T*, C.
Vannuffel aFisons Instruments, 85 Avenue Aristidi Briand, 94110 Arcueil, France
’
b CNRS, Laboratoire de Magnt?isme Louis N&l, 25 Avenue des Martyrs, BP166, 38042 Grenoble Cedex 9, France ’ LETI, CEA, Technologies Avancles DOPTKPM, I7 Avenue des Martyrs, 38054 Grenoble Cedex 9, France Received 23 March 1995; accepted 4 October 1995
Abstract The magnetic and transport properties of heavy rare earth silicide GdSi2_x thin films were investigated as a function of temperature and duration of the annealing. For films annealed for a short time at low temperature, two magnetic transitions occur which may be attributed to the presence of ordered and disordered phases in the film. For films annealed at higher temperatures, only one magnetic transition is observed at the NCel temperature, which is higher than that for bulk silicide. Keywords:
Magnetic properties and measurements; Magnetic structures; Siiicides
1. Introduction Rare earth (RE) silicides are of interest due to their potential applications in silicon technology; heavy RE silicides may be obtained as epitaxial layers on Si( 111) surfaces with a stoichiometry close to RE,Si,. The aim is to obtain better films and interfaces for diodes, IR detectors and X-ray mirrors. In this paper, we investigate the resistivity and magnetic properties of gadolinium silicide films as a function of the fabrication process. 2. Experimenta
detaiIs
Gadolinium silicide films were obtained by co-evaporation of Si and Gd (Si/Gd ratio, approximately 1.7) onto an Si( 111) wafer in an ultrahigh vacuum chamber [ 11. Samples clipped onto a heated sample holder were heated in situ by thermal conduction. The experimental methods used are described in Ref. [2]. Heat treatments were performed on three different layers. Sample A was annealed at 650 “C for 40 min and then at 740 “C for 15 min, sample B at 740 “C for 15 min and sample C at 450 “C for 15 min. The samples were free of oxide as shown by X-ray photoelectron spectroscopy (XPS) , and their composition was homogeneous as observed by secondary ion mass spectroscopy [ 11. * Corresponding author. 0040.6090/96/$15.00 0 1996 Elsevier Science S.A. All rights resewed SSDI0040-6090(95)08149-6
Films, approximately 10 nm thick, were analysed in situ by low energy electron diffraction (LEED) and ex situ by transmission electron microscopy (TEM) . Resistivity measurements were performed on the films below 100 K using the classical four-probe alternating current method in order to avoid spurious voltage variations from thermoelectric phenomena and resistive contacts with Cu-Be needles. The current-voltage characteristics of the silicide-intrinsic silicon interface were measured for sample A by applying silver contacts on both sides of the sample. The curve obtained (Fig. 1) is characteristic of a Schottky diode with a low barrier height (0.3 V) .
3. Results 3.1. Compound structures LEED analysis results in two different types of pattern depending on the annealing temperature [ 11. Samples A and B present a sharp fifiR30’ pattern, characteristic of a silicide having a hexagonal AlB2 structure, with vacancies arranged in a hexagonal superstructure. The C diffraction pattern reveals a 1 * 1 type reconstruction. It is characteristic of a hexagonal AlB, silicide [ 1] , the silicide having no Si vacancy.
C. Pescher et al. /Thin Solid Films 278 (1996) 140-143
0.5
0.3
3.2. Resistivity measurements
I-
‘.’
_,_ ., ._. .._ ._ ._.-.. . .
a g _r 0.1
‘.
..i .
.-..
I----0.1 t---_-L -1.0
I
I
---_--
__c__-_---
1
-0.5
141
0
0.5
1.0
V( volts)
Fig. 1. Current vs. voltage for a Gd,Si,-Si
diode at 20 “C.
From the resistivity measurements, it is possible to deduce the residual resistivity p. due to the interaction of conduction electrons with impurity atoms or mechanical stress in the lattice, and also the magnetic resistivity p,,, due to the crystal field and spin disorder. The values obtained are summarized in Table 1. The residual resistivity is lower for films prepared at higher temperatures. The high temperature may decrease the crystalline defect density, as shown in the micrographs. The magnetic ordering temperatures can be determined from the first derivative of the resistivity curves (Figs. 4-6). Two well-separated anomalies are observed for samples B and C, whereas only one anomaly or two closely located anomalies occur for sample A. The characteristic temperatures are given in Table 2; the upper temperature is always Table 1 Residual ( po) and magnetic
(pm) resistivities
( p,fI cm) of thin films
Film
Pa
Pm
A B C
15.25 59 108.5
8.75 14 15.5
Sample A
0.3
0.2 Fig. 2. Transmission
electron micrographof film A with diffraction pattern. 0.1
0 -0
20
40 Temperature
60
60
(K)
Fig. 4. First derivative of the resistivity curve vs. temperature
Fig. 3. Transmission
electron micrograph
of film C with diffraction
for sample A.
pattern.
A TEM study on layers A and C was performed (Fig. 2 and Fig. 3). Diffraction superlattice reflections similar to those found for other RE silicides [ 3,4] strongly suggest the ordering of, vacancies in the sublattice of GdSi,. A more detailed TEM study has been reported elsewhere [ 51,
0
”
“#
“““““‘I 20
40 Temperature
60
‘.“‘I
60
(K)
Fig. 5. First derivative of the resistivity curve vs. temperature
for sample B.
C. Pescher et al. /Thin Solid Films 278 (1996) 140-143
142
Sample C 0.4 -
0.2 -
a ?
: O.l-
dd 8
b A@“7
,-“(a 0
’ 20
’
’ ’ a 40 Temperature (K)
0 60
Fig. 6. First derivative of the resistivity curve vs. temperature Table 2 Critical temperatures
a
’
. 60
for sample C.
(K) of thin films
Film
TN
TX
A B C
50 49 55
40 41
close to 50 K and is obviously the Ntel temperature. The knee on the curves around 15 K is not related to a magnetic transition, but to the rate of thermal population of excited magnetic levels.
4. Discussion Magnetic and transport measurements have been performed previously in bulk silicides. Ordering of the orthorhombic GdSi (FeB-type) phase takes place at 50 K. The hexagonal GdSi,,, phase has a Ntel point near 33 K, whereas the ordering of the orthorhombic GdSi,., phase occurs at 25 K [ 6,7] and presents a modification of its magnetic structure near 23 K. The high transition temperature observed is close to that of bulk GdSi, whereas the low transition temperature is closer to that of hexagonal GdSi1.65; thus a first explanation would be that these two phases are present in samples B and C. Indeed, XPS experiments [ 1,2] have shown that the GdSi compound is present in several gadolinium silicide thin films. However, it has been unambiguously shown [2] that the GdSi,, monolayer formed at the interface between the Si wafer and silicide disappears on annealing. However, if the hypothesis of the presence of GdSi were true, no GdSi,.,, phase would be present in sample A, whereas this sample is the most homogeneous with the best hexagonal GdSi,,67 characteristics. It appears that the Nobel temperature of the GdSi, phases ( 1.6
Ho are antiferromagnetic, with a frustrated magnetic structure; magnetic interactions cannot be simultaneously fulfilled, leading to a reduction in the Nkel temperature. The corresponding hexagonal phases do not exhibit such a frustration effect and their Nkel temperatures are higher. Other reasons for a change in the NCel temperature include: the stabilization of the hexagonal phase over a range of compositions not allowed for the bulk silicide; the strains occurring in the epitaxial layers, which may modify the c/a ratio of the crystallographic parameters, and thus the interactions; the occurrence of a higher density of states at the Fermi level due to the regular ordering of vacancies: vacancy ordering is accompanied by a minimization of the overall electron energy due to sharper structures in the band density of states than for disordered structures. The occurrence of a second transition temperature around 40 K for samples B and C can be attributed to two origins: the existence of two different ordering temperatures TN1 and TN2 corresponding to two crystallographic phases (with different compositions or different types of vacancy order), or a magnetic structural transformation at temperature TX in the case of a unique phase. The ordering temperature is strongly dependent on the position and arrangement of the vacancies. In the tetragonal phases of CeSi,.s6 [8], CeGe2_, [9] and PrGe,,, [lo], ordered and disordered phases coexist. It has been shown that the ordered and disordered phases of germanides have rather different ordering temperatures. In the case of a single crystallographic phase, the second anomaly at TX may correspond to a transition temperature from a non-commensurate magnetic structure to a commensurate magnetic structure. Such a transformation has been observed in orthorhombic GdSi,,, [ 61 and hexagonal TbSi,,,,
[Ill. We now compare our resistivity data with the theoretical predictions for a simple collinear antiferromagnet. Yamada and Takada [ 121 proposed a theory which relies on a mean field calculation, taking into account longitudinal and transverse spin fluctuations. The relevance of such a model for the description of the magnetic resistivity and magnetoresistance in RE silicides was discussed in Ref. [ 71. The reduced magnetic resistivity R(T) lR( TN) vs. T/T, was calculated by subtracting the phonon contribution corresponding to a Debye temperature of 350 K. The temperature dependence is given in Fig. 7 for samples A, B and C, and compared with the variation calculated within the framework of YamadaTakada theory. For each sample, TN was taken as the temperature of the higher anomaly. It appears that the resistivities of the three samples are higher than the theoretical prediction. Sample A, which was annealed at 650 “C for 40 min and then at 740 “C for 15 min, may have a structure with ordered vacancies. Thus only one crystallographic phase and one critical temperature may exist. The experimental curve is in good agreement with the theoretical curve, although slightly above it. The same result was also found for orthorhombic
C. Pescher et al. /Thin Solid Films 278 (I 996) 140-143
0.6
143
and disordered phases coexisting in the specimen. Sample A, annealed at a low temperature for a long time and then at a higher temperature for a short time, has the same LEED diagram as sample B. However, its residual resistivity is lower and its thermal dependence is close to that of a collinear antiferromagnet, which leads to the conclusion that this film contains very few disordered domains. However, the value of the NCel temperature encountered in this film is larger than that for bulk silicides, a phenomenon which is not completely understood at present.
Acknowledgements
Fig. 7. Reduced magnetic resistivity R(T) /R( TN) for films A, B and C vs. TIT, compared with the theoretical curve (broken line) of Yamada and Takada [12].
GdSi,., and hexagonal GdSi,,,, polycrystalline samples [7]. This can be attributed to a non-collinear or modulated structure occurring below the NCel point. Another reason for the discrepancy may be the fact that the present model is a mean field model: Yamada and Takada [ 121 found a higher resistivity at low temperature when spin waves were taken into account: however, this model is valid only for non-interacting spin waves, i.e. for temperatures much smaller than TN. For the other two samples, annealed at two different temperatures for a short time ( 15 min), there are probably two different structures, ordered and disordered, with two different Ntel temperatures TN, and TN2.
5. Conclusions The transport properties of gadolinium silicide thin films depend on the temperature and duration of the annealing. The resistivities of samples B and C, annealed at two different temperatures for a short time, appear to be similar in spite of differences in their LEED diagrams. They are characterized by two Ntel temperatures, which may correspond to ordered
The authors are grateful to J.Y. Veuillen the fabrication of the GdSi thin films.
and T.N. Tan for
References [ 11 C. Pescher, A. Ermolieff, J.Y. Veuillen and T.N. Tan, Solid State Commun., submitted for publication. [21 C. Pescher, These de Doctorat, Institut National Polytechnique de Grenoble, 3 February, 1995. [3] T.L. Lee, L.J. Chen and F.R. Chen, J. Appl. Phys., 33 (1992) 2089. [4] F.H. Kaatz, W.R. Graham and J. Van der Spiegel, Appl. Phys. Letr., 62 (15) (1993) 1748. [5] C. Vannuffel, Proc. Inst. Phys. Conf on Microscopy of Semiconductor Materials, Oxford, 2&23 March, 199.5, Vol. 146, 1995, p. 537. [6] S. Auffret, J. Pierre, B. Lambert-Andron, R. Madar, E. Houssay, D. Schmitt and E. Siaud, Physica B, I73 ( 199 1) 265, and references cited therein. [7] J. Pierre, S. Auffret, J.A. Chroboczek and T.T.A. Nguyen, .I. Phys. Condensed Matter, 6 (1994) 79. [81 R. Madar, E. Houssay, A. Rouault, J.P. Senateur, B. Lambert, C. Meneau d' Anterroches, J. Pierre and J. Pelissier, J. Mater. Res., 5 ( 10) (1990) 2126. [91 B. Lambert-Andron, J. Pierre, B. Chenevier, R. Madar, N. Boutarek and J. Rodriguez-Carvajal,J. Phys. Condensed Matter, 6 ( 1994) 8725. [ 101 B. Lambert-Andron, N. Boutarek, J. Pierre and R. Madar, J. Alloys Compounds, 203 ( 1994) 1. [ 11I P. Schobinger-Papamantellos and K.H.J. Buschow, J. Less Common Met., 146 (1989) 279. [ 121 H. Yamada and S. Takada, J. Phys. Sot. Jpn., 34 ( 1975) 51.
Thin Solid Films 278 ( 1996) 140-143
Magnetic properties of gadolinium silicide thin films produced by different fabrication processes C. Pescher a, J. Pierre b, A. Ermolieff ‘T*, C.
Vannuffel aFisons Instruments, 85 Avenue Aristidi Briand, 94110 Arcueil, France
’
b CNRS, Laboratoire de Magnt?isme Louis N&l, 25 Avenue des Martyrs, BP166, 38042 Grenoble Cedex 9, France ’ LETI, CEA, Technologies Avancles DOPTKPM, I7 Avenue des Martyrs, 38054 Grenoble Cedex 9, France Received 23 March 1995; accepted 4 October 1995
Abstract The magnetic and transport properties of heavy rare earth silicide GdSi2_x thin films were investigated as a function of temperature and duration of the annealing. For films annealed for a short time at low temperature, two magnetic transitions occur which may be attributed to the presence of ordered and disordered phases in the film. For films annealed at higher temperatures, only one magnetic transition is observed at the NCel temperature, which is higher than that for bulk silicide. Keywords:
Magnetic properties and measurements; Magnetic structures; Siiicides
1. Introduction Rare earth (RE) silicides are of interest due to their potential applications in silicon technology; heavy RE silicides may be obtained as epitaxial layers on Si( 111) surfaces with a stoichiometry close to RE,Si,. The aim is to obtain better films and interfaces for diodes, IR detectors and X-ray mirrors. In this paper, we investigate the resistivity and magnetic properties of gadolinium silicide films as a function of the fabrication process. 2. Experimenta
detaiIs
Gadolinium silicide films were obtained by co-evaporation of Si and Gd (Si/Gd ratio, approximately 1.7) onto an Si( 111) wafer in an ultrahigh vacuum chamber [ 11. Samples clipped onto a heated sample holder were heated in situ by thermal conduction. The experimental methods used are described in Ref. [2]. Heat treatments were performed on three different layers. Sample A was annealed at 650 “C for 40 min and then at 740 “C for 15 min, sample B at 740 “C for 15 min and sample C at 450 “C for 15 min. The samples were free of oxide as shown by X-ray photoelectron spectroscopy (XPS) , and their composition was homogeneous as observed by secondary ion mass spectroscopy [ 11. * Corresponding author. 0040.6090/96/$15.00 0 1996 Elsevier Science S.A. All rights resewed SSDI0040-6090(95)08149-6
Films, approximately 10 nm thick, were analysed in situ by low energy electron diffraction (LEED) and ex situ by transmission electron microscopy (TEM) . Resistivity measurements were performed on the films below 100 K using the classical four-probe alternating current method in order to avoid spurious voltage variations from thermoelectric phenomena and resistive contacts with Cu-Be needles. The current-voltage characteristics of the silicide-intrinsic silicon interface were measured for sample A by applying silver contacts on both sides of the sample. The curve obtained (Fig. 1) is characteristic of a Schottky diode with a low barrier height (0.3 V) .
3. Results 3.1. Compound structures LEED analysis results in two different types of pattern depending on the annealing temperature [ 11. Samples A and B present a sharp fifiR30’ pattern, characteristic of a silicide having a hexagonal AlB2 structure, with vacancies arranged in a hexagonal superstructure. The C diffraction pattern reveals a 1 * 1 type reconstruction. It is characteristic of a hexagonal AlB, silicide [ 1] , the silicide having no Si vacancy.
C. Pescher et al. /Thin Solid Films 278 (1996) 140-143
0.5
0.3
3.2. Resistivity measurements
I-
‘.’
_,_ ., ._. .._ ._ ._.-.. . .
a g _r 0.1
‘.
..i .
.-..
I----0.1 t---_-L -1.0
I
I
---_--
__c__-_---
1
-0.5
141
0
0.5
1.0
V( volts)
Fig. 1. Current vs. voltage for a Gd,Si,-Si
diode at 20 “C.
From the resistivity measurements, it is possible to deduce the residual resistivity p. due to the interaction of conduction electrons with impurity atoms or mechanical stress in the lattice, and also the magnetic resistivity p,,, due to the crystal field and spin disorder. The values obtained are summarized in Table 1. The residual resistivity is lower for films prepared at higher temperatures. The high temperature may decrease the crystalline defect density, as shown in the micrographs. The magnetic ordering temperatures can be determined from the first derivative of the resistivity curves (Figs. 4-6). Two well-separated anomalies are observed for samples B and C, whereas only one anomaly or two closely located anomalies occur for sample A. The characteristic temperatures are given in Table 2; the upper temperature is always Table 1 Residual ( po) and magnetic
(pm) resistivities
( p,fI cm) of thin films
Film
Pa
Pm
A B C
15.25 59 108.5
8.75 14 15.5
Sample A
0.3
0.2 Fig. 2. Transmission
electron micrographof film A with diffraction pattern. 0.1
0 -0
20
40 Temperature
60
60
(K)
Fig. 4. First derivative of the resistivity curve vs. temperature
Fig. 3. Transmission
electron micrograph
of film C with diffraction
for sample A.
pattern.
A TEM study on layers A and C was performed (Fig. 2 and Fig. 3). Diffraction superlattice reflections similar to those found for other RE silicides [ 3,4] strongly suggest the ordering of, vacancies in the sublattice of GdSi,. A more detailed TEM study has been reported elsewhere [ 51,
0
”
“#
“““““‘I 20
40 Temperature
60
‘.“‘I
60
(K)
Fig. 5. First derivative of the resistivity curve vs. temperature
for sample B.
C. Pescher et al. /Thin Solid Films 278 (1996) 140-143
142
Sample C 0.4 -
0.2 -
a ?
: O.l-
dd 8
b A@“7
,-“(a 0
’ 20
’
’ ’ a 40 Temperature (K)
0 60
Fig. 6. First derivative of the resistivity curve vs. temperature Table 2 Critical temperatures
a
’
. 60
for sample C.
(K) of thin films
Film
TN
TX
A B C
50 49 55
40 41
close to 50 K and is obviously the Ntel temperature. The knee on the curves around 15 K is not related to a magnetic transition, but to the rate of thermal population of excited magnetic levels.
4. Discussion Magnetic and transport measurements have been performed previously in bulk silicides. Ordering of the orthorhombic GdSi (FeB-type) phase takes place at 50 K. The hexagonal GdSi,,, phase has a Ntel point near 33 K, whereas the ordering of the orthorhombic GdSi,., phase occurs at 25 K [ 6,7] and presents a modification of its magnetic structure near 23 K. The high transition temperature observed is close to that of bulk GdSi, whereas the low transition temperature is closer to that of hexagonal GdSi1.65; thus a first explanation would be that these two phases are present in samples B and C. Indeed, XPS experiments [ 1,2] have shown that the GdSi compound is present in several gadolinium silicide thin films. However, it has been unambiguously shown [2] that the GdSi,, monolayer formed at the interface between the Si wafer and silicide disappears on annealing. However, if the hypothesis of the presence of GdSi were true, no GdSi,.,, phase would be present in sample A, whereas this sample is the most homogeneous with the best hexagonal GdSi,,67 characteristics. It appears that the Nobel temperature of the GdSi, phases ( 1.6
Ho are antiferromagnetic, with a frustrated magnetic structure; magnetic interactions cannot be simultaneously fulfilled, leading to a reduction in the Nkel temperature. The corresponding hexagonal phases do not exhibit such a frustration effect and their Nkel temperatures are higher. Other reasons for a change in the NCel temperature include: the stabilization of the hexagonal phase over a range of compositions not allowed for the bulk silicide; the strains occurring in the epitaxial layers, which may modify the c/a ratio of the crystallographic parameters, and thus the interactions; the occurrence of a higher density of states at the Fermi level due to the regular ordering of vacancies: vacancy ordering is accompanied by a minimization of the overall electron energy due to sharper structures in the band density of states than for disordered structures. The occurrence of a second transition temperature around 40 K for samples B and C can be attributed to two origins: the existence of two different ordering temperatures TN1 and TN2 corresponding to two crystallographic phases (with different compositions or different types of vacancy order), or a magnetic structural transformation at temperature TX in the case of a unique phase. The ordering temperature is strongly dependent on the position and arrangement of the vacancies. In the tetragonal phases of CeSi,.s6 [8], CeGe2_, [9] and PrGe,,, [lo], ordered and disordered phases coexist. It has been shown that the ordered and disordered phases of germanides have rather different ordering temperatures. In the case of a single crystallographic phase, the second anomaly at TX may correspond to a transition temperature from a non-commensurate magnetic structure to a commensurate magnetic structure. Such a transformation has been observed in orthorhombic GdSi,,, [ 61 and hexagonal TbSi,,,,
[Ill. We now compare our resistivity data with the theoretical predictions for a simple collinear antiferromagnet. Yamada and Takada [ 121 proposed a theory which relies on a mean field calculation, taking into account longitudinal and transverse spin fluctuations. The relevance of such a model for the description of the magnetic resistivity and magnetoresistance in RE silicides was discussed in Ref. [ 71. The reduced magnetic resistivity R(T) lR( TN) vs. T/T, was calculated by subtracting the phonon contribution corresponding to a Debye temperature of 350 K. The temperature dependence is given in Fig. 7 for samples A, B and C, and compared with the variation calculated within the framework of YamadaTakada theory. For each sample, TN was taken as the temperature of the higher anomaly. It appears that the resistivities of the three samples are higher than the theoretical prediction. Sample A, which was annealed at 650 “C for 40 min and then at 740 “C for 15 min, may have a structure with ordered vacancies. Thus only one crystallographic phase and one critical temperature may exist. The experimental curve is in good agreement with the theoretical curve, although slightly above it. The same result was also found for orthorhombic
C. Pescher et al. /Thin Solid Films 278 (I 996) 140-143
0.6
143
and disordered phases coexisting in the specimen. Sample A, annealed at a low temperature for a long time and then at a higher temperature for a short time, has the same LEED diagram as sample B. However, its residual resistivity is lower and its thermal dependence is close to that of a collinear antiferromagnet, which leads to the conclusion that this film contains very few disordered domains. However, the value of the NCel temperature encountered in this film is larger than that for bulk silicides, a phenomenon which is not completely understood at present.
Acknowledgements
Fig. 7. Reduced magnetic resistivity R(T) /R( TN) for films A, B and C vs. TIT, compared with the theoretical curve (broken line) of Yamada and Takada [12].
GdSi,., and hexagonal GdSi,,,, polycrystalline samples [7]. This can be attributed to a non-collinear or modulated structure occurring below the NCel point. Another reason for the discrepancy may be the fact that the present model is a mean field model: Yamada and Takada [ 121 found a higher resistivity at low temperature when spin waves were taken into account: however, this model is valid only for non-interacting spin waves, i.e. for temperatures much smaller than TN. For the other two samples, annealed at two different temperatures for a short time ( 15 min), there are probably two different structures, ordered and disordered, with two different Ntel temperatures TN, and TN2.
5. Conclusions The transport properties of gadolinium silicide thin films depend on the temperature and duration of the annealing. The resistivities of samples B and C, annealed at two different temperatures for a short time, appear to be similar in spite of differences in their LEED diagrams. They are characterized by two Ntel temperatures, which may correspond to ordered
The authors are grateful to J.Y. Veuillen the fabrication of the GdSi thin films.
and T.N. Tan for
References [ 11 C. Pescher, A. Ermolieff, J.Y. Veuillen and T.N. Tan, Solid State Commun., submitted for publication. [21 C. Pescher, These de Doctorat, Institut National Polytechnique de Grenoble, 3 February, 1995. [3] T.L. Lee, L.J. Chen and F.R. Chen, J. Appl. Phys., 33 (1992) 2089. [4] F.H. Kaatz, W.R. Graham and J. Van der Spiegel, Appl. Phys. Letr., 62 (15) (1993) 1748. [5] C. Vannuffel, Proc. Inst. Phys. Conf on Microscopy of Semiconductor Materials, Oxford, 2&23 March, 199.5, Vol. 146, 1995, p. 537. [6] S. Auffret, J. Pierre, B. Lambert-Andron, R. Madar, E. Houssay, D. Schmitt and E. Siaud, Physica B, I73 ( 199 1) 265, and references cited therein. [7] J. Pierre, S. Auffret, J.A. Chroboczek and T.T.A. Nguyen, .I. Phys. Condensed Matter, 6 (1994) 79. [81 R. Madar, E. Houssay, A. Rouault, J.P. Senateur, B. Lambert, C. Meneau d' Anterroches, J. Pierre and J. Pelissier, J. Mater. Res., 5 ( 10) (1990) 2126. [91 B. Lambert-Andron, J. Pierre, B. Chenevier, R. Madar, N. Boutarek and J. Rodriguez-Carvajal,J. Phys. Condensed Matter, 6 ( 1994) 8725. [ 101 B. Lambert-Andron, N. Boutarek, J. Pierre and R. Madar, J. Alloys Compounds, 203 ( 1994) 1. [ 11I P. Schobinger-Papamantellos and K.H.J. Buschow, J. Less Common Met., 146 (1989) 279. [ 121 H. Yamada and S. Takada, J. Phys. Sot. Jpn., 34 ( 1975) 51.