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High pressure and polarized neutron scattering study of magnetic excitations in CeSb
Physica B 156 & 157 (1989) North-Holland, Amsterdam
798-800
HIGH PRESSURE AND POLARIZED NEUTRON MAGNETIC EXCITATIONS IN CeSb
SCATTERING
STUDY OF
L.P. REGNAULT’, J.L. JACOUD’, C. VETTIER*, T. CHA’ITOPADHYAY’, J. ROSSAT-MIGNOD’, T. SUZUKI’, T. KASUYAR and 0. VOGT4 ‘Centre d’Etudes Nucliaires, DRFISPh-MDN, 38041 Grenoble Cedex. France ‘Institut Laue-Langevin, l_%X, 38042 Grenoble Cedex. France ‘Department of Physics, Tohoku University, Sendai 980, Japan ‘Lab. Fiir Festkiirperphysik, ETH, CH-8093, Ziirich. Switzerland
Inelastic neutron scattering experiments have been performed using the full polarization analysis to determine unambiguously the nature of the non-magnetic planes in CeSb. The effect of a hydrostatic pressure on the magnetic excitation spectrum has been studied at 10 kbar and 30 kbar.
1. Introduction Among cerium compounds, the monopnictide CeSb exhibits highly unusual magnetic properties [l, 21. The main typical properties discovered in this system are a very small crystalline electric field (CEF) splitting (‘3 meV), a large anisotropy along the cube edge directions, a very complex magnetic phase diagram, unusual spectra of magnetic excitations and a large sensitivity to an applied hydrostatic pressure. In particular non-magnetic (P) planes have been introduced to explain the H-T and P-T phase diagrams, coexisting with well ordered ferromagnetic (F) planes [ 1, 21. Extensive theoretical calculations have been carried out to understand these complex magnetic properties [3,4]. Nevertheless a microscopic interpretation of both the origin and the nature of the P-planes is not available. Previous experiments have been interpreted by assuming that the non-magnetic planes were in fact paramagnetic planes [l]. In order to clarify this point we have undertaken inelastic polarized neutron scattering experiments under field and more conventional INS experiments under pressure. 2. Nature of the P-planes The dynamical
behaviour
of CeSb in the AFP
0921-4526/89/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
and FP-phases is of great interest since it may reveal the nature of the P-planes. Experiments have been performed at H = 18 kOe, T = 6.5 K in the AFF2 phase (+ + - -) without P-planes and at H = 18 kOe, T = 15 K in the FP2 phase (+ +00) with half P-planes. The vertical field was applied along the c-axis. We used the full polarization analysis technique to measure both the spin-flip (SF) and non-spin-flip (NSF) contributions. These experiments have been performed on the 3-axis spectrometer IN20 installed at ILL, using incident neutrons of fixed ki = 2.662 A-’ (FWHM = 1 meV). The sample was oriented with the a* and 6* axis in the equatorial plane. Both Heusler monochromator and analyser were used, with the polarization field on sample vertical, i.e. perpendicular to the scattering vector. Fig. 1 shows the SF and NSF components recorded at 15 K and 6.5 K for a scattering vector Q = (0, -2.1,O) near the zone center. At higher temperature, the SF contribution gives evidence for two peaks at energies 3.5 meV and 4.9 meV, whereas the NSF contribution displays only a single peak centered around 3.5 meV, with an intensity roughly equal to that observed for the SF contribution. Only a SF contribution around 4.5 meV is observed at 6.5 K. Therefore this mode is unambiguously ascribed to magnetic excitations within the F-planes. The mode at 4.9 meV which is detected in the SF channel is B.V.
L. P. Regnault et al. I Magnetic excitation in CeSb
~~~~
&
‘0
Z20 11
1.9 27
35
4.3 5.1 5.9 '
Energy
1.1 1.9 27
35
4.3 5.1 19
CmeV)
Fig. 1. Spin-flip and non-spin-flip contributions observedat T= 15K and T= 6 K for an applied magnetic field H = 18 kOe at Q = (0, -2.1,O).
then attributed to a magnetic excitation within the F-planes, slightly renormalized by the applied magnetic field. Therefore the mode at 3.5 meV appears as a new excitation, not present in the low temperature phase without P-planes. This new excitation is unambiguously ascribed to a magnetic excitation within the P-planes. Because the polarization axis in our experiment was perpendicular to the scattering vector Q, one expects Z,, = Z,,, in the case of a paramagnetic scattering and ZNsF= 0 in the case of a fully saturated ground state with the magnetic moments along the polarization axis. The fact that the former relation is verified experimentally for the mode at 3.5 meV is a direct proof of the parumagnetic nature of the P-planes in CeSb. The same kind of measurements performed at the zone boundary Q = (0, 1, 0) reveal the existence of a single peak structure both in the SF (at 3.8 meV) and NSF (at 3.5 meV) channels, with an intensity ratio 2/l. The first excitation originates partly from the F-planes, whereas the second one is ascribed to the P-planes and is found to have almost no wave vector dependence. The absence of dispersion is another argument for the paramagnetic nature of the P-planes.
799
10 kbar and V= 30 mm3 at 30 kbar) placed in a clamp system designed to be inserted in a temperature controlled cryostat (T = 1.5-50 K). We have measured the dispersion curves both along the [lOO] and [llO] directions of the reciprocal space. The dispersion curves deduced from the analysis of energy scans at different scattering vectors are reported in fig. 2. The main pressure effect is to induce a large and uniform shift in energy of the whole dispersion curve which varies approximately linearly with the pressure: with CY= 0.15 meV/kbar =I o,(O) + aP, w,(P) = 1.7klkbar. The dispersion curve itself is little changed by pressure, in sharp contrast with the strong dependence of the transition temperature dT,IdP - 0.3-0.6 K/kbar [2]. This behaviour reflects the strong anisotropy of the interactions existing in CeSb, which do not have cubic symmetry. Within a ferromagnetic plane the dispersion relation can be described by a relation of type [2,5]: oq 2: A + Mf Zxx(q), with A = A,, + Mz I”’ ( q = 0), in which A,, is the CEF splitting, M, and M, the matrix elements of J, and J, and where Zaa( q) are the Fourier components of the (anisotropic) interaction tensor. We conclude from our experiments that Z”“(q) is only slightly affected by the pressure whereas Z”(O) (i.e. TN) is strongly increased by the pressure. In other words pressure affects only the mean field part of the interaction. We have tried to measure the dependence with P of
3. High pressure inelastic neutron scattering INS experiments at 10 kbar and 30 kbar have been carried out on the high flux 3-axis spectrometer IN8 at ILL. The hydrostatic pressure was applied on a single crystal (V- 70 mm3 at
1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 a alonq~lO] d alonq~0Cj
Fig. 2. Magnetic excitation spectra along the [lOO] and Ill01 directions at T = 1.5 K as a function of an applied hydrostatic pressure.
800
L. P. Regnault
et al.
I Magnetic
the CEF splitting A, in a paramagnetic region. At P 2: 0 and T = 26 K, a broadened peak centered around 2.9 meV is observed with an intrinsic width ~1.5 meV, in agreement with previous results [2]. At P = 10 kbar, no well defined excitation could be observed. Only a broad contribution is seen, with intensity at energies smaller than 3 meV, indicating a broadening and probably also a lowering of the CEF excitation. No accurate data have been obtained at 30 kbar due to the important elastic background induced by the pressure cell.
excitation
in CeSb
References [l] J. Rossat-Mignod, P. Burlet, S. Quezel, J.M. Effantin, D. Delacote, H. Bartholin, 0. Vogt and D. Ravot, J. Magn. Magn. Mat. 31-34 (1983) 398. [2] J. Rossat-Mignod, J.M. Effantin, P. Burlet, T. Chattopadhyay, L.P. Regnault. H. Bartholin. C. Vettier, 0. Vogt. D. Ravot and J.C. Achart, J. Magn. Magn. Mat. 52 (1985) 111. [3] H. Takahashi and T. Kasuya, J. Phys. C 18 (1985) 2697. [4] P. Thayamballi. D. Yang and B.R. Cooper, Phys. Rev. B 29 (1984). [S] B. Halg and A. Furrer, J. Appl. Phys. 55 (1984) 1860.
798-800
HIGH PRESSURE AND POLARIZED NEUTRON MAGNETIC EXCITATIONS IN CeSb
SCATTERING
STUDY OF
L.P. REGNAULT’, J.L. JACOUD’, C. VETTIER*, T. CHA’ITOPADHYAY’, J. ROSSAT-MIGNOD’, T. SUZUKI’, T. KASUYAR and 0. VOGT4 ‘Centre d’Etudes Nucliaires, DRFISPh-MDN, 38041 Grenoble Cedex. France ‘Institut Laue-Langevin, l_%X, 38042 Grenoble Cedex. France ‘Department of Physics, Tohoku University, Sendai 980, Japan ‘Lab. Fiir Festkiirperphysik, ETH, CH-8093, Ziirich. Switzerland
Inelastic neutron scattering experiments have been performed using the full polarization analysis to determine unambiguously the nature of the non-magnetic planes in CeSb. The effect of a hydrostatic pressure on the magnetic excitation spectrum has been studied at 10 kbar and 30 kbar.
1. Introduction Among cerium compounds, the monopnictide CeSb exhibits highly unusual magnetic properties [l, 21. The main typical properties discovered in this system are a very small crystalline electric field (CEF) splitting (‘3 meV), a large anisotropy along the cube edge directions, a very complex magnetic phase diagram, unusual spectra of magnetic excitations and a large sensitivity to an applied hydrostatic pressure. In particular non-magnetic (P) planes have been introduced to explain the H-T and P-T phase diagrams, coexisting with well ordered ferromagnetic (F) planes [ 1, 21. Extensive theoretical calculations have been carried out to understand these complex magnetic properties [3,4]. Nevertheless a microscopic interpretation of both the origin and the nature of the P-planes is not available. Previous experiments have been interpreted by assuming that the non-magnetic planes were in fact paramagnetic planes [l]. In order to clarify this point we have undertaken inelastic polarized neutron scattering experiments under field and more conventional INS experiments under pressure. 2. Nature of the P-planes The dynamical
behaviour
of CeSb in the AFP
0921-4526/89/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
and FP-phases is of great interest since it may reveal the nature of the P-planes. Experiments have been performed at H = 18 kOe, T = 6.5 K in the AFF2 phase (+ + - -) without P-planes and at H = 18 kOe, T = 15 K in the FP2 phase (+ +00) with half P-planes. The vertical field was applied along the c-axis. We used the full polarization analysis technique to measure both the spin-flip (SF) and non-spin-flip (NSF) contributions. These experiments have been performed on the 3-axis spectrometer IN20 installed at ILL, using incident neutrons of fixed ki = 2.662 A-’ (FWHM = 1 meV). The sample was oriented with the a* and 6* axis in the equatorial plane. Both Heusler monochromator and analyser were used, with the polarization field on sample vertical, i.e. perpendicular to the scattering vector. Fig. 1 shows the SF and NSF components recorded at 15 K and 6.5 K for a scattering vector Q = (0, -2.1,O) near the zone center. At higher temperature, the SF contribution gives evidence for two peaks at energies 3.5 meV and 4.9 meV, whereas the NSF contribution displays only a single peak centered around 3.5 meV, with an intensity roughly equal to that observed for the SF contribution. Only a SF contribution around 4.5 meV is observed at 6.5 K. Therefore this mode is unambiguously ascribed to magnetic excitations within the F-planes. The mode at 4.9 meV which is detected in the SF channel is B.V.
L. P. Regnault et al. I Magnetic excitation in CeSb
~~~~
&
‘0
Z20 11
1.9 27
35
4.3 5.1 5.9 '
Energy
1.1 1.9 27
35
4.3 5.1 19
CmeV)
Fig. 1. Spin-flip and non-spin-flip contributions observedat T= 15K and T= 6 K for an applied magnetic field H = 18 kOe at Q = (0, -2.1,O).
then attributed to a magnetic excitation within the F-planes, slightly renormalized by the applied magnetic field. Therefore the mode at 3.5 meV appears as a new excitation, not present in the low temperature phase without P-planes. This new excitation is unambiguously ascribed to a magnetic excitation within the P-planes. Because the polarization axis in our experiment was perpendicular to the scattering vector Q, one expects Z,, = Z,,, in the case of a paramagnetic scattering and ZNsF= 0 in the case of a fully saturated ground state with the magnetic moments along the polarization axis. The fact that the former relation is verified experimentally for the mode at 3.5 meV is a direct proof of the parumagnetic nature of the P-planes in CeSb. The same kind of measurements performed at the zone boundary Q = (0, 1, 0) reveal the existence of a single peak structure both in the SF (at 3.8 meV) and NSF (at 3.5 meV) channels, with an intensity ratio 2/l. The first excitation originates partly from the F-planes, whereas the second one is ascribed to the P-planes and is found to have almost no wave vector dependence. The absence of dispersion is another argument for the paramagnetic nature of the P-planes.
799
10 kbar and V= 30 mm3 at 30 kbar) placed in a clamp system designed to be inserted in a temperature controlled cryostat (T = 1.5-50 K). We have measured the dispersion curves both along the [lOO] and [llO] directions of the reciprocal space. The dispersion curves deduced from the analysis of energy scans at different scattering vectors are reported in fig. 2. The main pressure effect is to induce a large and uniform shift in energy of the whole dispersion curve which varies approximately linearly with the pressure: with CY= 0.15 meV/kbar =I o,(O) + aP, w,(P) = 1.7klkbar. The dispersion curve itself is little changed by pressure, in sharp contrast with the strong dependence of the transition temperature dT,IdP - 0.3-0.6 K/kbar [2]. This behaviour reflects the strong anisotropy of the interactions existing in CeSb, which do not have cubic symmetry. Within a ferromagnetic plane the dispersion relation can be described by a relation of type [2,5]: oq 2: A + Mf Zxx(q), with A = A,, + Mz I”’ ( q = 0), in which A,, is the CEF splitting, M, and M, the matrix elements of J, and J, and where Zaa( q) are the Fourier components of the (anisotropic) interaction tensor. We conclude from our experiments that Z”“(q) is only slightly affected by the pressure whereas Z”(O) (i.e. TN) is strongly increased by the pressure. In other words pressure affects only the mean field part of the interaction. We have tried to measure the dependence with P of
3. High pressure inelastic neutron scattering INS experiments at 10 kbar and 30 kbar have been carried out on the high flux 3-axis spectrometer IN8 at ILL. The hydrostatic pressure was applied on a single crystal (V- 70 mm3 at
1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 a alonq~lO] d alonq~0Cj
Fig. 2. Magnetic excitation spectra along the [lOO] and Ill01 directions at T = 1.5 K as a function of an applied hydrostatic pressure.
800
L. P. Regnault
et al.
I Magnetic
the CEF splitting A, in a paramagnetic region. At P 2: 0 and T = 26 K, a broadened peak centered around 2.9 meV is observed with an intrinsic width ~1.5 meV, in agreement with previous results [2]. At P = 10 kbar, no well defined excitation could be observed. Only a broad contribution is seen, with intensity at energies smaller than 3 meV, indicating a broadening and probably also a lowering of the CEF excitation. No accurate data have been obtained at 30 kbar due to the important elastic background induced by the pressure cell.
excitation
in CeSb
References [l] J. Rossat-Mignod, P. Burlet, S. Quezel, J.M. Effantin, D. Delacote, H. Bartholin, 0. Vogt and D. Ravot, J. Magn. Magn. Mat. 31-34 (1983) 398. [2] J. Rossat-Mignod, J.M. Effantin, P. Burlet, T. Chattopadhyay, L.P. Regnault. H. Bartholin. C. Vettier, 0. Vogt. D. Ravot and J.C. Achart, J. Magn. Magn. Mat. 52 (1985) 111. [3] H. Takahashi and T. Kasuya, J. Phys. C 18 (1985) 2697. [4] P. Thayamballi. D. Yang and B.R. Cooper, Phys. Rev. B 29 (1984). [S] B. Halg and A. Furrer, J. Appl. Phys. 55 (1984) 1860.