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77Se NMR study of CeSe
ELSEVIER
Physica B 230-232 (1997) 133-135
77Se NMR study of CeSe K. Hiraoka a'*, K. Kojima b, T. Shinohara °, T. Hihara d, P. Wachter e aFaculty of Engineering, Oita University, Oita 870-11, Japan bFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan ¢Institutefor Materials Research, Tohoku University, Sendai 980-77, Japan °Department of General Education, Fukuyama University, Fukuyama 729-02, Japan eLaboratoriumfur FestkSrperphysik, ETH Ziirich, 8093 Ziirich, Switzerland
Abstract The 77Se Knight shift K(T) in CeSe exhibits a Curie-Weiss-like behavior above 20K, and a plot of K(T) against susceptibility yields a hyperfine field of 11.0 kOe/#B. The nuclear spin-lattice relaxation rate T ~-1 is independent of temperature at high temperatures. The NMR results suggest the localized nature of Ce 4f electrons. Keywords: CeSe; NMR; Knight shift; Nuclear spin relaxation
1. Introduction
2. Experimental
CeSe, having the NaCl-type crystal structure, is a type II-antiferromagnet with the N6el temperature TN = 5.6K [1, 2]. The lowest-lying J multiplet 2F5/2 of a Ce 3+ free ion is split into a ground F7 doublet and an excited F8 quartet by the crystalline electric field. The energy splitting between the F7 and F8 states corresponds to about 100K [1]. The saturation magnetic moment is considerably reduced below the value of 0.71pB expected from the F 7 state [2]. The low-temperature electrical resistivity, in its magnetic part, exhibits a In T dependence [3]. This, as well as the reduced moment, suggests that CeSe is a dense K o n d o system. In this work the Knight shift K and the nuclear spin-lattice relaxation rate T i -1 of 77Se have been measured as a function of temperature in order to investigate magnetic properties microscopically.
N M R measurements were made on a powder sample prepared from a polycrystalline ingot, which was obtained by melting pre-reacted materials in a tungsten crucible at ETH. A phasecoherent pulsed N M R apparatus at the IMR of T o h o k u University was used, and the nuclear spin-lattice relaxation rate was measured by the saturation recovery method with comb pulses. Magnetization measurements were made on a single-crystal sample with a moving-sample magnetometer described in Ref. [4].
* Corresponding author.
3. Results and discussion Fig. 1 shows the temperature dependences of the susceptibility X and its inverse X-1, which were obtained by the magnetization measurements at B = 7T. These are almost same as the results obtained by Hulliger et al. [1], except that TN is lowered to 4.2 K at 7 T.
0921-4526/97/$17.00 Copyright © 1997 ElsevierScienceB.V. All rights reserved PI1 S092 1-4526(96)00569-8
134
K. Hiraoka et aL /Physica B 230-232 (1997) 133-135 0.025
.
,
•
,
.
,
.
,
.
,
.
0.35
,
400
CeSe
0.30
0.020
~ 200
~ O.OLO &
0.20 0.15
'7 100 ~e
0.005
77Se NMR in CeSe B=6.3369 T
0.25
300 ~,
-~ o.o15
. . . . . . . . . . . .
0.10 0.05
50 100 150 200 250 300
' 5'0 ' 100' 150 ' 200 ' 250 ' 300 Temperature(K)
Temperature(K) Fig. 1. Temperature dependences of the magnetic susceptibility and its inverse in CeSe.
Fig. 3. Temperature dependence of the half-maximum width do of 77Se N M R line.
(emu/mol) e~
77Se NMR in CeSe B=6.3369 T T=50 K
0
0.005
3to
.% • •
0.010
B=6.3369T
° ~ ~
t
0.020
/"
3°Io CeSe ~ 2.5 I-
"
0.015
~
/ o
1.5F
o
.
.
,
"
/
T=225 K
•o
!k i
51.0
5;.5
52.0 52.5 Frequency(Mttz)
53.0
0
50
100
150 200
250
300
Temperature(K)
Fig. 2. 77Se N M R frequency spectra in CeSe.
Fig. 4. Temperature dependence of the 77Se Knight shift K (open circle) and the K versus X plot (solid circle).
Fig. 2 shows typical frequency spectra of the VVSe NMR, which were obtained by the frequency sweep at 6.3 T. The NMR line is not symmetrical, and extends to lower frequency. In Fig. 3, its half maximum width At) is shown as a function of temperature. The line width decreases nearly proportional to z(T) with increasing temperature, suggesting that the line broadening is primarily due to the Ce 4f moments including the demagnetizing field (its contribution to At~ is 0.09 MHz at 20 K). Fig. 4 shows the temperature dependence of the Knight shift K(T) obtained at 6.3 T. K(T) follows a Curie-Weiss law between 20 and 300 K, except for the value at 10 K, close to TN. In Fig. 4, where K(T) is plotted against z(T), the K versus g curve is
linear above about 20K. Using the relation K = A h f g / N # B , where Ahf is the transferred hyperfine coupling constant, N the Avogadro number, and #B the Bohr magneton, we obtain 77Ahf =ll.0kOe/#B from the slope of the K versus Z plot. It is interesting to compare the obtained value of 77Ahf with the values of 75Ahf for 75As in CeAs, which has the NaC1 structure and is a dense Kondo system with a low carrier concentration [5]. The values of 75Ahf in CeAs are ascribed to the polarization of As 4s electrons by the Ce 4f moments and are found to be 2.3 and 20kOe/#B in the/'7 and/'8 states, respectively, I-6]. The charge cloud of the F 7 doublet is extended along the [1 1 1] direction, avoiding nearest neighbor As ions,
K. Hiraoka et al. /Physica B 230-232 (1997) 133-135 •
,
.
,
•
,
,
,
,
,
240 200
77Se N M R CeSe
160 0
B=6.3369T
120 80
40 0
5'0 ' 100 ' 150 ' 200 ' 250 ' 300 Temperature(K)
135
was rather difficult to m e a s u r e T 1 1 because of weak NMR signals. The data are few especially at low temperatures, but T~-1 seems to be independent of temperature within experimental errors, as in the localized spin systems. In summary, the results of the 77Se NMR in CeSe show the Knight shift exhibiting a temperature variation of Curie-Weiss-like type and are nearly temperature-independent T1. These suggest localized nature of Ce 4f moments. No NMR behavior clearly attributed to the dense Kondo effect was observed in the experimental temperature range.
Fig. 5. Temperature dependence of the nuclear spin-lattice relaxation rate Ti -1 for 77Se.
One of the authors (K. H.) thanks K. Mattenberger for preparing the samples.
whereas that of the F8 quartet is extended toward nearest neighbor As ions. This leads to the large difference between 75Ahf(F7) and 75Ahf(_F8). The observed value of 77Ahf in CeSe, which expected to be also due to the polarization of 4s electrons, is much smaller than 75Ahf(F8). This is consistent with the experimental result [-1, 7] that Ce 3 + for CeSe is in the ground F7 state. (Here, we take into account a difference in the 4s electron spin density at the nucleus for Se and As ions by using the 4s atomic hyperfine fields for Se and As atoms; HA(Se)/HA(AS)
References
= 1.3 [ 8 ] . )
Fig. 5 shows the temperature dependence of the nuclear spin-lattice relaxation rate T ~-1 for 77Se. It
[1] F. Hulliger, B. Natterer and H.R. Ott, J. Magn. Magn. Mater. 6 (1978) 87. [2] H.R. Ott, J.K. Kjems and F. Hulliger, Phys. Rev. Lett. 42 (1979) 1378. [3] J. Schoenes and F. Hulliger, J. Magn. Magn. Mater. 63-64 (1987) 43. [4] F. Hulliger and H.R. Ott, Z. Physik B 29 (1978) 47. ['5] Y.S. Kwon, Y. Haga, O. Nakamura, T. Suzuki and T. Kasuya, Physica B 171 (1991) 324. ['6] K. Hiraoka, Y. Kasamatsu, K. Kojima and T. Hihara, Physica B 186-188 (1993) 535. [7] A. D6nni, A. Furrer, P. Fischer and F. Hulliger, Physica B 186-188 (1993) 541. [8] R.E. Watson and L.H. Bennett, Phys. Rev. B 15 (1977) 502.
Physica B 230-232 (1997) 133-135
77Se NMR study of CeSe K. Hiraoka a'*, K. Kojima b, T. Shinohara °, T. Hihara d, P. Wachter e aFaculty of Engineering, Oita University, Oita 870-11, Japan bFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan ¢Institutefor Materials Research, Tohoku University, Sendai 980-77, Japan °Department of General Education, Fukuyama University, Fukuyama 729-02, Japan eLaboratoriumfur FestkSrperphysik, ETH Ziirich, 8093 Ziirich, Switzerland
Abstract The 77Se Knight shift K(T) in CeSe exhibits a Curie-Weiss-like behavior above 20K, and a plot of K(T) against susceptibility yields a hyperfine field of 11.0 kOe/#B. The nuclear spin-lattice relaxation rate T ~-1 is independent of temperature at high temperatures. The NMR results suggest the localized nature of Ce 4f electrons. Keywords: CeSe; NMR; Knight shift; Nuclear spin relaxation
1. Introduction
2. Experimental
CeSe, having the NaCl-type crystal structure, is a type II-antiferromagnet with the N6el temperature TN = 5.6K [1, 2]. The lowest-lying J multiplet 2F5/2 of a Ce 3+ free ion is split into a ground F7 doublet and an excited F8 quartet by the crystalline electric field. The energy splitting between the F7 and F8 states corresponds to about 100K [1]. The saturation magnetic moment is considerably reduced below the value of 0.71pB expected from the F 7 state [2]. The low-temperature electrical resistivity, in its magnetic part, exhibits a In T dependence [3]. This, as well as the reduced moment, suggests that CeSe is a dense K o n d o system. In this work the Knight shift K and the nuclear spin-lattice relaxation rate T i -1 of 77Se have been measured as a function of temperature in order to investigate magnetic properties microscopically.
N M R measurements were made on a powder sample prepared from a polycrystalline ingot, which was obtained by melting pre-reacted materials in a tungsten crucible at ETH. A phasecoherent pulsed N M R apparatus at the IMR of T o h o k u University was used, and the nuclear spin-lattice relaxation rate was measured by the saturation recovery method with comb pulses. Magnetization measurements were made on a single-crystal sample with a moving-sample magnetometer described in Ref. [4].
* Corresponding author.
3. Results and discussion Fig. 1 shows the temperature dependences of the susceptibility X and its inverse X-1, which were obtained by the magnetization measurements at B = 7T. These are almost same as the results obtained by Hulliger et al. [1], except that TN is lowered to 4.2 K at 7 T.
0921-4526/97/$17.00 Copyright © 1997 ElsevierScienceB.V. All rights reserved PI1 S092 1-4526(96)00569-8
134
K. Hiraoka et aL /Physica B 230-232 (1997) 133-135 0.025
.
,
•
,
.
,
.
,
.
,
.
0.35
,
400
CeSe
0.30
0.020
~ 200
~ O.OLO &
0.20 0.15
'7 100 ~e
0.005
77Se NMR in CeSe B=6.3369 T
0.25
300 ~,
-~ o.o15
. . . . . . . . . . . .
0.10 0.05
50 100 150 200 250 300
' 5'0 ' 100' 150 ' 200 ' 250 ' 300 Temperature(K)
Temperature(K) Fig. 1. Temperature dependences of the magnetic susceptibility and its inverse in CeSe.
Fig. 3. Temperature dependence of the half-maximum width do of 77Se N M R line.
(emu/mol) e~
77Se NMR in CeSe B=6.3369 T T=50 K
0
0.005
3to
.% • •
0.010
B=6.3369T
° ~ ~
t
0.020
/"
3°Io CeSe ~ 2.5 I-
"
0.015
~
/ o
1.5F
o
.
.
,
"
/
T=225 K
•o
!k i
51.0
5;.5
52.0 52.5 Frequency(Mttz)
53.0
0
50
100
150 200
250
300
Temperature(K)
Fig. 2. 77Se N M R frequency spectra in CeSe.
Fig. 4. Temperature dependence of the 77Se Knight shift K (open circle) and the K versus X plot (solid circle).
Fig. 2 shows typical frequency spectra of the VVSe NMR, which were obtained by the frequency sweep at 6.3 T. The NMR line is not symmetrical, and extends to lower frequency. In Fig. 3, its half maximum width At) is shown as a function of temperature. The line width decreases nearly proportional to z(T) with increasing temperature, suggesting that the line broadening is primarily due to the Ce 4f moments including the demagnetizing field (its contribution to At~ is 0.09 MHz at 20 K). Fig. 4 shows the temperature dependence of the Knight shift K(T) obtained at 6.3 T. K(T) follows a Curie-Weiss law between 20 and 300 K, except for the value at 10 K, close to TN. In Fig. 4, where K(T) is plotted against z(T), the K versus g curve is
linear above about 20K. Using the relation K = A h f g / N # B , where Ahf is the transferred hyperfine coupling constant, N the Avogadro number, and #B the Bohr magneton, we obtain 77Ahf =ll.0kOe/#B from the slope of the K versus Z plot. It is interesting to compare the obtained value of 77Ahf with the values of 75Ahf for 75As in CeAs, which has the NaC1 structure and is a dense Kondo system with a low carrier concentration [5]. The values of 75Ahf in CeAs are ascribed to the polarization of As 4s electrons by the Ce 4f moments and are found to be 2.3 and 20kOe/#B in the/'7 and/'8 states, respectively, I-6]. The charge cloud of the F 7 doublet is extended along the [1 1 1] direction, avoiding nearest neighbor As ions,
K. Hiraoka et al. /Physica B 230-232 (1997) 133-135 •
,
.
,
•
,
,
,
,
,
240 200
77Se N M R CeSe
160 0
B=6.3369T
120 80
40 0
5'0 ' 100 ' 150 ' 200 ' 250 ' 300 Temperature(K)
135
was rather difficult to m e a s u r e T 1 1 because of weak NMR signals. The data are few especially at low temperatures, but T~-1 seems to be independent of temperature within experimental errors, as in the localized spin systems. In summary, the results of the 77Se NMR in CeSe show the Knight shift exhibiting a temperature variation of Curie-Weiss-like type and are nearly temperature-independent T1. These suggest localized nature of Ce 4f moments. No NMR behavior clearly attributed to the dense Kondo effect was observed in the experimental temperature range.
Fig. 5. Temperature dependence of the nuclear spin-lattice relaxation rate Ti -1 for 77Se.
One of the authors (K. H.) thanks K. Mattenberger for preparing the samples.
whereas that of the F8 quartet is extended toward nearest neighbor As ions. This leads to the large difference between 75Ahf(F7) and 75Ahf(_F8). The observed value of 77Ahf in CeSe, which expected to be also due to the polarization of 4s electrons, is much smaller than 75Ahf(F8). This is consistent with the experimental result [-1, 7] that Ce 3 + for CeSe is in the ground F7 state. (Here, we take into account a difference in the 4s electron spin density at the nucleus for Se and As ions by using the 4s atomic hyperfine fields for Se and As atoms; HA(Se)/HA(AS)
References
= 1.3 [ 8 ] . )
Fig. 5 shows the temperature dependence of the nuclear spin-lattice relaxation rate T ~-1 for 77Se. It
[1] F. Hulliger, B. Natterer and H.R. Ott, J. Magn. Magn. Mater. 6 (1978) 87. [2] H.R. Ott, J.K. Kjems and F. Hulliger, Phys. Rev. Lett. 42 (1979) 1378. [3] J. Schoenes and F. Hulliger, J. Magn. Magn. Mater. 63-64 (1987) 43. [4] F. Hulliger and H.R. Ott, Z. Physik B 29 (1978) 47. ['5] Y.S. Kwon, Y. Haga, O. Nakamura, T. Suzuki and T. Kasuya, Physica B 171 (1991) 324. ['6] K. Hiraoka, Y. Kasamatsu, K. Kojima and T. Hihara, Physica B 186-188 (1993) 535. [7] A. D6nni, A. Furrer, P. Fischer and F. Hulliger, Physica B 186-188 (1993) 541. [8] R.E. Watson and L.H. Bennett, Phys. Rev. B 15 (1977) 502.