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Neutron scattering studies of magnetic superconductors
N E U T R O N S C A T T E R I N G S T U D I E S OF MAGNETIC S U P E R C O N D U C T O R S J. W. L Y N N Department of Physics, University of Maryland*, College Park, Md. 20742 and National Bureau of Standards, Washington, DC 20234, USA
and R. N. S H E L T O N Ames Laboratory-USDOE and Department of Physics, lowa State University**, Ames, 1.4 50011, USA
Measurements have been carried out on a series of rare earth Chevrel-phase superconductors. In the selenide materials well defined crystal field transitions have been observed, which can be understood to a first approximation on the basis of a cubic crystal field with a magnetic ground state. In HoMo6S s, on the other hand, no crystal field excitations have been observed over a wide range of energies. Diffraction data show that essentially the full free-ion m o m e n t is readily induced in HoMotSs, but that in ErMotSe s less than half the free-ion m o m e n t is induced at T = 5 K a n d H = 70 kOe. The induced-moment data on H o M o t S s can be readily interpreted on the basis of one Ho atom per unit cell, whereas for ErMotSe~ this appears not to be the case. These data also demonstrate that the only significant magnetic impurity phases in these samples are (RE)202Se, a n d these are typically a few percent or less in volume.
The ternary Chevrel-phase superconductors, R M o 6 X s (R-rare earth; X - - - S e , S), exhibit a variety of unique and interesting phenomen/t related to the interplay between magnetism and superconductivity [1]. In all these materials the magnetic ordering temperatures are typically 1 K or less, so that we may anticipate that the crystal field splittings of the rare earth ions may dominate the magnetic energies and thus determine the magnetic (or non-magnetic) properties at low temperatures. To investigate these materials we have carried out neutron scattering experiments on ErMo6Ses, TbMo6Ses, HoMo6Se s and HoMo6S s as a function of temperature and magnetic field. The measurements were performed on triple-axis spectrometers at the National Bureau of Standards research reactor. Fig. 1 shows data on HoMo6Se s at 4 K. An excitation is clearly seen at 4.58 meV, and there is additional scattering at low energies which, under higher resolution, can be identified as another excitation at 1.06 meV. This scattering has been uniquely identified as crystal field in origin since: (i) the energy of the scattering is independent of the wavevector transfer K; (ii) the intensity decreases with increasing temperature, in contrast to Bose excitations whose intensity increases with temperature; and (iii) the intensity as a function of
K quantitatively follows the magnetic form factor f(K). The observance of crystal-field transitions in HoMo6Se 8 is characteristic of the selenide materials, in which crystal field transitions have been found in all the compounds studied to date [2]. The overall crystal-field level scheme can be described to a first approximation by a cubic crystal field; additional splittings due to the perturbations from cubic symmetry which are present may be treated as second-order effects. A cubic crystal field is 1750
HoMo6Se 8
I~l: 14o ~,-'
1500
Ef =14.80 meV T=4K
1250
o I000
go
(..)
750
5OO
÷ 25O
•
0 I,'" - 2.0
*Work supported by the NSF, D M R 79-00908. **Work supported by the US Department of Energy, contract No. W-7405-Eng 82, Division of Basic Energy Sciences, .43(01-02-02-2.
#
#
I 0.0
....
2.5
•
o°O
~o° .......
5.0
~*°
7.5
E (meV)
Fig. 1. Observed crystal field scatter;rag at low temperatures in HoMo~Se 8. U n d e r high resolution the scattering at low energies is identified as a crystal field transition at 1.06 meV.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1577-1578 ©North Holland
1577
J. W. Lynn, R. N. She#on/Neutron scattering studies of magnetic superconductors
1578
{{
1500
T=I5K
,t
l
f
~
L
500
I000
G) I-Z
400
0
o
[
171 =1.6,& -~
,t o
]
600
HoMo6S 8
500
E
t
300
t
"
t 0
Z-o
"-.'..~,'9~"---'--.--"V" 5.0
I0.0
15.0
"Background"
20.0
200
o. . . .
- ~--
E (meV)
Fig. 2. Spectrum at low temperatures for HoMo6S s, showing that there are no crystal field transitions in this energy range. There are also no transitions observed in the peak at E = 0 under high resolution (98 #eV FWHM).
I
o
1
[ 0
indeed capable of explaining the data in each case, and the ground states are found to be magnetic in agreement with specific heat, susceptibility and previous neutron diffraction data. In contrast to the behavior observed in the selenide materials, fig. 2 shows data for HoMo6S 8. N o crystal field transitions are observed over this energy range. We conclude from this that either the strength of the crystal field is more than an order of magnitude larger or smaller than the selenide, or that the linewidths are an order of magnitude larger. To determine if the crystal field transitions are unresolved in the peak at E = 0, high resolution (0.098 meV F W H M ) measurements were carried out; only the scattering at E = 0 was observed with a width that was solely instrumental in origin. To establish that the crystal-field splittings were not still unresolved, inelastic measurements were performed in a magnetic field. The intensity of the scattering at E = 0 decreased by a factor of three, but still no (Zeeman) transition was observed. The Ho ion therefore does not behave as a J = 8 free ion. We remark that essentially the full moment [3] was induced with a field of 20 kOe or
[
. . . . .
0
i L IO
~ 20
I " 50
I 40
~ 50
1 60
] 70
H (kOe)
Fig. 3. Intensity of the {110} Bragg peak as a function of applied magnetic field for ErMo6Se s. The observed increase in intensity is due to the induced magnetic moment.
larger at 4.5 K, while in the selenide materials the moments induced are considerably smaller than the free-ion values. Fig. 3 shows the peak intensity of the (110) Bragg peak as a function of field for ErMo6Se s. The scattering at H = 0 is due to the nuclear Bragg peak plus background. Comparison of the induced magnetic intensity with the nuclear intensity gives an induced m o m e n t /~z = 3.9 /~B at 70 kOe. Similar data on HoMo6Se 8 gave a moment of 5.9 ~B. Further details of these measurements will be reported elsewhere.
References [1] O. Fischer, Appl. Phys. 16 (1978) 1. [2] J. W. Lynn and R. N. Shelton, J. Appl. Phys. 50 (1979) 1984. [3] J. W. Lynn, D. E. Moncton, W. Thomlinson, G. Shirane and R. N. Shelton, Solid State Commun. 26 (1978) 493.
and R. N. S H E L T O N Ames Laboratory-USDOE and Department of Physics, lowa State University**, Ames, 1.4 50011, USA
Measurements have been carried out on a series of rare earth Chevrel-phase superconductors. In the selenide materials well defined crystal field transitions have been observed, which can be understood to a first approximation on the basis of a cubic crystal field with a magnetic ground state. In HoMo6S s, on the other hand, no crystal field excitations have been observed over a wide range of energies. Diffraction data show that essentially the full free-ion m o m e n t is readily induced in HoMotSs, but that in ErMotSe s less than half the free-ion m o m e n t is induced at T = 5 K a n d H = 70 kOe. The induced-moment data on H o M o t S s can be readily interpreted on the basis of one Ho atom per unit cell, whereas for ErMotSe~ this appears not to be the case. These data also demonstrate that the only significant magnetic impurity phases in these samples are (RE)202Se, a n d these are typically a few percent or less in volume.
The ternary Chevrel-phase superconductors, R M o 6 X s (R-rare earth; X - - - S e , S), exhibit a variety of unique and interesting phenomen/t related to the interplay between magnetism and superconductivity [1]. In all these materials the magnetic ordering temperatures are typically 1 K or less, so that we may anticipate that the crystal field splittings of the rare earth ions may dominate the magnetic energies and thus determine the magnetic (or non-magnetic) properties at low temperatures. To investigate these materials we have carried out neutron scattering experiments on ErMo6Ses, TbMo6Ses, HoMo6Se s and HoMo6S s as a function of temperature and magnetic field. The measurements were performed on triple-axis spectrometers at the National Bureau of Standards research reactor. Fig. 1 shows data on HoMo6Se s at 4 K. An excitation is clearly seen at 4.58 meV, and there is additional scattering at low energies which, under higher resolution, can be identified as another excitation at 1.06 meV. This scattering has been uniquely identified as crystal field in origin since: (i) the energy of the scattering is independent of the wavevector transfer K; (ii) the intensity decreases with increasing temperature, in contrast to Bose excitations whose intensity increases with temperature; and (iii) the intensity as a function of
K quantitatively follows the magnetic form factor f(K). The observance of crystal-field transitions in HoMo6Se 8 is characteristic of the selenide materials, in which crystal field transitions have been found in all the compounds studied to date [2]. The overall crystal-field level scheme can be described to a first approximation by a cubic crystal field; additional splittings due to the perturbations from cubic symmetry which are present may be treated as second-order effects. A cubic crystal field is 1750
HoMo6Se 8
I~l: 14o ~,-'
1500
Ef =14.80 meV T=4K
1250
o I000
go
(..)
750
5OO
÷ 25O
•
0 I,'" - 2.0
*Work supported by the NSF, D M R 79-00908. **Work supported by the US Department of Energy, contract No. W-7405-Eng 82, Division of Basic Energy Sciences, .43(01-02-02-2.
#
#
I 0.0
....
2.5
•
o°O
~o° .......
5.0
~*°
7.5
E (meV)
Fig. 1. Observed crystal field scatter;rag at low temperatures in HoMo~Se 8. U n d e r high resolution the scattering at low energies is identified as a crystal field transition at 1.06 meV.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1577-1578 ©North Holland
1577
J. W. Lynn, R. N. She#on/Neutron scattering studies of magnetic superconductors
1578
{{
1500
T=I5K
,t
l
f
~
L
500
I000
G) I-Z
400
0
o
[
171 =1.6,& -~
,t o
]
600
HoMo6S 8
500
E
t
300
t
"
t 0
Z-o
"-.'..~,'9~"---'--.--"V" 5.0
I0.0
15.0
"Background"
20.0
200
o. . . .
- ~--
E (meV)
Fig. 2. Spectrum at low temperatures for HoMo6S s, showing that there are no crystal field transitions in this energy range. There are also no transitions observed in the peak at E = 0 under high resolution (98 #eV FWHM).
I
o
1
[ 0
indeed capable of explaining the data in each case, and the ground states are found to be magnetic in agreement with specific heat, susceptibility and previous neutron diffraction data. In contrast to the behavior observed in the selenide materials, fig. 2 shows data for HoMo6S 8. N o crystal field transitions are observed over this energy range. We conclude from this that either the strength of the crystal field is more than an order of magnitude larger or smaller than the selenide, or that the linewidths are an order of magnitude larger. To determine if the crystal field transitions are unresolved in the peak at E = 0, high resolution (0.098 meV F W H M ) measurements were carried out; only the scattering at E = 0 was observed with a width that was solely instrumental in origin. To establish that the crystal-field splittings were not still unresolved, inelastic measurements were performed in a magnetic field. The intensity of the scattering at E = 0 decreased by a factor of three, but still no (Zeeman) transition was observed. The Ho ion therefore does not behave as a J = 8 free ion. We remark that essentially the full moment [3] was induced with a field of 20 kOe or
[
. . . . .
0
i L IO
~ 20
I " 50
I 40
~ 50
1 60
] 70
H (kOe)
Fig. 3. Intensity of the {110} Bragg peak as a function of applied magnetic field for ErMo6Se s. The observed increase in intensity is due to the induced magnetic moment.
larger at 4.5 K, while in the selenide materials the moments induced are considerably smaller than the free-ion values. Fig. 3 shows the peak intensity of the (110) Bragg peak as a function of field for ErMo6Se s. The scattering at H = 0 is due to the nuclear Bragg peak plus background. Comparison of the induced magnetic intensity with the nuclear intensity gives an induced m o m e n t /~z = 3.9 /~B at 70 kOe. Similar data on HoMo6Se 8 gave a moment of 5.9 ~B. Further details of these measurements will be reported elsewhere.
References [1] O. Fischer, Appl. Phys. 16 (1978) 1. [2] J. W. Lynn and R. N. Shelton, J. Appl. Phys. 50 (1979) 1984. [3] J. W. Lynn, D. E. Moncton, W. Thomlinson, G. Shirane and R. N. Shelton, Solid State Commun. 26 (1978) 493.