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NMR study of 59Co in (Fe1−xCox)7S8
Journal of Magnetism and Magnetic Materials 1114-1117 ( 19921 1961- 1962 North-! lolland
10/11
NMR study of 5'Co in (Fe _,Co,)TSs M. Sato ", T. K m n i m u r a ", T. S h i n o h a r a h and T. Sato ~' " Coih'ge Of General i-ducation, 7"ohokt~ Unirersit.v, Sendai 080, Japao t, Institute for Materials Research. Tohoku University, Sendai oSO. Japan NMR of 5uCo in the pseudo-binary compotmds (Fe I ,Co, )TSs with a NiAs-type structure has been i 'easui'cd under zero field and external fields up to 3 T. As x increases, tile internal field of 5'~('o de,.'l'eascs abruptly near x = 1).5 ;.tnd becomes zero around a magnetic critical concentration (x -.- 11.61. Tile resulls indicate that the Co atom possesses a magnetic moment for x < 11.6 and the moments of the ('o and Fc atoms tire absent for x > 11.6. NiAs-typc compouods MX (M: 3d-transition clement, X: clement of the Vb and Vib groups in the periodic table) can be classified into three groups according to their lattice p a r a m e t e r and magnetism [I]: [high c-l] (M = Ti and V), [high c-ll] (M = Cr, Mn and Fe) and [low c] (M = Ce an't NI). Tile lattice p a r a m e ter c of the [low c] group is about 10% smaller than that of thc [high c] groups, though a is nearly thc same. Thc mcmbcrs of thc [high c-ll] group havc a comparatively high transition temperature and are referred to as ",;trongly magnetic". The c o m p o u n d s of the o t h e r two groups arc "'weakly magnetic". The d i s a p p c a r a n c c of the strong magnctism of the [low c] group is likely to bc duc to a collapse of the magnetic m o m e n t s caused by the reduction of c. in the pseudobinary systems consisting of mixtures of strongly magnetic and weakly magnetic compounds, there are two distinctive types of behavior around the magnetic critical concentration: one is a spin-glass-like behavior at low t e m p e r a t u r e and the o t h e r a simple dis,.ppcarance of the strong magnetism. T b s e distinctive behaviors a r o u n d the magnetic critical concentration d e p e n d on w h e t h e r the weakly magnetic constituent is a m e m b e r of thc [high c-l] or [low c] group. (Fc i .,C0,)7S s ((I ~
the hump on the Ifigh-frequency side is nol detected. For x = 11.5, the peak is observed around 22 MHz but its intensity is very. weak. In FevSs i x = 0), there are four types of Fc sites resulting from the ordered a r r a n g e m e n t of vacancies in the Fc sites. From the results of M6ssbaucr mcasurcments [3], the value,,, of the internal field of 57Fe at each site are in the range from - 2 3 to - 3 1 T (i.e. the frequency range from 30 to 44 MHz). This implies a possibility that the peaks in fig. I come from 57Fe. However, spin-echo signals arc absent for x < 0.2 and the N M R sensitivity of 57Fe relative to 5'~Co is very small. Therefore, those peaks arc considered to come from 5'~Co rather than 57Fc. in order !o confirm the origin of those peaks, field-swept spectra wcrc measured at 22.0 M t t z and 5.11 K. For x = (I.4, a broad peak was observed at 1 T as shown in fig. 2. From the z,:ro-ficld n'lcasuremctltS, the value of internal field of 5'~('o ix estimated as - 3 . 2 q'. and the resonance field as 1.() T. This value is in g~,od agreement with obscrvalion. Thus, the obscr~'ctt p,:aks in the zero-field spectra arc attributed to '"Co. The bfot~,dncss of the peak ix
i
i
(Fe, _~Co~) 7SB T=4.2K
A 4-)
oe°I
c •
S
o
x=0.4
eool •
i
~o
•
>0 ,1o if) Z
,.°,g~,"
'"01 i..--
~°.
.,,--0.2
2"
.~
20
°. o 40 FREQUENCY (MHz)
oo (30
Fig. I. Zero-field spectra for (Fe I ,Co, 17S~ (x = 0.2 and (I.41
0312-8853/92/$115.00 ~s:~1992 - Elsevier Science Publishers B.V. All rights rcsei~'ed
at 4.2 K.
M. Sato et al. / NMR study of 5°Co in (Fe t _..,Co.,)7S s
1962 A
i
(Fe1_,,CcL) 7Ss
£.
~.
• .
I-
,%
•"~e :.,~.
....
#
ttll Z ~J t-" Z
T=5. OK x=o. 4
N 0 0
1
3
2
NAGNETIC FIELO (T)
Fig. 2. Field-swept spectrum for (Fe t _xCOa)7S8( x 22 MHz and 5 K.
= 0.4)
at
likely to be due to the presence of various configurations of Fe, Co and cation vacancies in these compounds. As shown in fig. 3, the internal field estimated from the peak fl'equency is nearly constant for 0.2 ~
,
,
,
,
,
,
,
,
,
800
-5 A V
A
and decreases abruptly around x = 0.5. These values indicate that the Co atoms possess relatively small magnetic m o m e n t s up to x = 0 . 5 (cf. - 6 . 2 T and 0.85tz n tot" ferromagnetic CoS, [4]). Thc temperature depcndcnce of the Kl,:ght shift of 5'~Co at 22.0 MHz was measured to insure the absence of a magnetic moment for x >1 0.6. For x = 0.8 and 1, the Knight shift ( , . - 1 . 7 + 0.5% as y / 2 ' r r = 10.03 M H z / T ) of 5'~Co is almost temperature-independent. Therefore, both the magnetic m o m e n t s of Fe and Co atoms are absent in this concentration range. For x = 0.6 ( T c = 30 K), the Knight shift is nearly equal to those for x = 0.8 and 1 at temperatures above 50 K. The Knight shift at 5 K is somewhat large compared with that at 50 K. These results indicate that the Co atoms do not have a magnetic m o m e n t for x >_ 0.6. For (Fel_xCOx)7S s, the results of N M R measuremerits show that the Co atom possesses a magnetic moment for x < 0.6 and the m o m e n t of the Co atom collapses lk~r x > 0.6. The Fe atom, like the Co atom, does not have a magnetic m o m e n t for x > 0.6. The concentration d e p e n d e n c e of the magnetic moments, obtained in this system, confirms that the simple disappearance of strong magnetism at x ,-, 0.6 is due to the moment collapse caused by the large reduction of the lattice p a r a m e t e r c [2].
600 ILl E3 ....I 1,1 Lt.
400 w O.
_I
This work was supportcd by a Grant-in-Aid o1' Scientific Research from the Ministry, of Education, Science and Culture, Japan.
LLI
Z U.I
LLI 7
200 ~
References
t.3
-1
0 I
0
0.5
1
0
x
Fig. 3. Internal field and Curie temperature versus x in (Fe t ,Co,)7S s. x" internal fiehJ (vertical bar: width at the half height o1' a peak), e: Curie temperature.
[1] T. Kamimura, J. de Phys. 49 (1088) C8-191. [2] M. Sato, T, Kamimura, T. Shinohara and T. Sato, J. Magn. Magn. Mater. 9t)&91 (1990) 179. [3] J.R. Go~selin, M.G. Townsend, R.J. l'remblay and A.H. Webster, Mater. Res. Bull. 10 (1975)41 [4] N. lnoue, H. Yasuoka, M. Matsui and K. Adachi, J. Phys. Soc. Jpn. 50 (1981) !181.
10/11
NMR study of 5'Co in (Fe _,Co,)TSs M. Sato ", T. K m n i m u r a ", T. S h i n o h a r a h and T. Sato ~' " Coih'ge Of General i-ducation, 7"ohokt~ Unirersit.v, Sendai 080, Japao t, Institute for Materials Research. Tohoku University, Sendai oSO. Japan NMR of 5uCo in the pseudo-binary compotmds (Fe I ,Co, )TSs with a NiAs-type structure has been i 'easui'cd under zero field and external fields up to 3 T. As x increases, tile internal field of 5'~('o de,.'l'eascs abruptly near x = 1).5 ;.tnd becomes zero around a magnetic critical concentration (x -.- 11.61. Tile resulls indicate that the Co atom possesses a magnetic moment for x < 11.6 and the moments of the ('o and Fc atoms tire absent for x > 11.6. NiAs-typc compouods MX (M: 3d-transition clement, X: clement of the Vb and Vib groups in the periodic table) can be classified into three groups according to their lattice p a r a m e t e r and magnetism [I]: [high c-l] (M = Ti and V), [high c-ll] (M = Cr, Mn and Fe) and [low c] (M = Ce an't NI). Tile lattice p a r a m e ter c of the [low c] group is about 10% smaller than that of thc [high c] groups, though a is nearly thc same. Thc mcmbcrs of thc [high c-ll] group havc a comparatively high transition temperature and are referred to as ",;trongly magnetic". The c o m p o u n d s of the o t h e r two groups arc "'weakly magnetic". The d i s a p p c a r a n c c of the strong magnctism of the [low c] group is likely to bc duc to a collapse of the magnetic m o m e n t s caused by the reduction of c. in the pseudobinary systems consisting of mixtures of strongly magnetic and weakly magnetic compounds, there are two distinctive types of behavior around the magnetic critical concentration: one is a spin-glass-like behavior at low t e m p e r a t u r e and the o t h e r a simple dis,.ppcarance of the strong magnetism. T b s e distinctive behaviors a r o u n d the magnetic critical concentration d e p e n d on w h e t h e r the weakly magnetic constituent is a m e m b e r of thc [high c-l] or [low c] group. (Fc i .,C0,)7S s ((I ~
the hump on the Ifigh-frequency side is nol detected. For x = 11.5, the peak is observed around 22 MHz but its intensity is very. weak. In FevSs i x = 0), there are four types of Fc sites resulting from the ordered a r r a n g e m e n t of vacancies in the Fc sites. From the results of M6ssbaucr mcasurcments [3], the value,,, of the internal field of 57Fe at each site are in the range from - 2 3 to - 3 1 T (i.e. the frequency range from 30 to 44 MHz). This implies a possibility that the peaks in fig. I come from 57Fe. However, spin-echo signals arc absent for x < 0.2 and the N M R sensitivity of 57Fe relative to 5'~Co is very small. Therefore, those peaks arc considered to come from 5'~Co rather than 57Fc. in order !o confirm the origin of those peaks, field-swept spectra wcrc measured at 22.0 M t t z and 5.11 K. For x = (I.4, a broad peak was observed at 1 T as shown in fig. 2. From the z,:ro-ficld n'lcasuremctltS, the value of internal field of 5'~('o ix estimated as - 3 . 2 q'. and the resonance field as 1.() T. This value is in g~,od agreement with obscrvalion. Thus, the obscr~'ctt p,:aks in the zero-field spectra arc attributed to '"Co. The bfot~,dncss of the peak ix
i
i
(Fe, _~Co~) 7SB T=4.2K
A 4-)
oe°I
c •
S
o
x=0.4
eool •
i
~o
•
>0 ,1o if) Z
,.°,g~,"
'"01 i..--
~°.
.,,--0.2
2"
.~
20
°. o 40 FREQUENCY (MHz)
oo (30
Fig. I. Zero-field spectra for (Fe I ,Co, 17S~ (x = 0.2 and (I.41
0312-8853/92/$115.00 ~s:~1992 - Elsevier Science Publishers B.V. All rights rcsei~'ed
at 4.2 K.
M. Sato et al. / NMR study of 5°Co in (Fe t _..,Co.,)7S s
1962 A
i
(Fe1_,,CcL) 7Ss
£.
~.
• .
I-
,%
•"~e :.,~.
....
#
ttll Z ~J t-" Z
T=5. OK x=o. 4
N 0 0
1
3
2
NAGNETIC FIELO (T)
Fig. 2. Field-swept spectrum for (Fe t _xCOa)7S8( x 22 MHz and 5 K.
= 0.4)
at
likely to be due to the presence of various configurations of Fe, Co and cation vacancies in these compounds. As shown in fig. 3, the internal field estimated from the peak fl'equency is nearly constant for 0.2 ~
,
,
,
,
,
,
,
,
,
800
-5 A V
A
and decreases abruptly around x = 0.5. These values indicate that the Co atoms possess relatively small magnetic m o m e n t s up to x = 0 . 5 (cf. - 6 . 2 T and 0.85tz n tot" ferromagnetic CoS, [4]). Thc temperature depcndcnce of the Kl,:ght shift of 5'~Co at 22.0 MHz was measured to insure the absence of a magnetic moment for x >1 0.6. For x = 0.8 and 1, the Knight shift ( , . - 1 . 7 + 0.5% as y / 2 ' r r = 10.03 M H z / T ) of 5'~Co is almost temperature-independent. Therefore, both the magnetic m o m e n t s of Fe and Co atoms are absent in this concentration range. For x = 0.6 ( T c = 30 K), the Knight shift is nearly equal to those for x = 0.8 and 1 at temperatures above 50 K. The Knight shift at 5 K is somewhat large compared with that at 50 K. These results indicate that the Co atoms do not have a magnetic m o m e n t for x >_ 0.6. For (Fel_xCOx)7S s, the results of N M R measuremerits show that the Co atom possesses a magnetic moment for x < 0.6 and the m o m e n t of the Co atom collapses lk~r x > 0.6. The Fe atom, like the Co atom, does not have a magnetic m o m e n t for x > 0.6. The concentration d e p e n d e n c e of the magnetic moments, obtained in this system, confirms that the simple disappearance of strong magnetism at x ,-, 0.6 is due to the moment collapse caused by the large reduction of the lattice p a r a m e t e r c [2].
600 ILl E3 ....I 1,1 Lt.
400 w O.
_I
This work was supportcd by a Grant-in-Aid o1' Scientific Research from the Ministry, of Education, Science and Culture, Japan.
LLI
Z U.I
LLI 7
200 ~
References
t.3
-1
0 I
0
0.5
1
0
x
Fig. 3. Internal field and Curie temperature versus x in (Fe t ,Co,)7S s. x" internal fiehJ (vertical bar: width at the half height o1' a peak), e: Curie temperature.
[1] T. Kamimura, J. de Phys. 49 (1088) C8-191. [2] M. Sato, T, Kamimura, T. Shinohara and T. Sato, J. Magn. Magn. Mater. 9t)&91 (1990) 179. [3] J.R. Go~selin, M.G. Townsend, R.J. l'remblay and A.H. Webster, Mater. Res. Bull. 10 (1975)41 [4] N. lnoue, H. Yasuoka, M. Matsui and K. Adachi, J. Phys. Soc. Jpn. 50 (1981) !181.