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Magnetoelastic phenomena in UNiGa1
Journal of Magnetism and Magnetic Materials 184 (1998) 369—371
Magnetoelastic phenomena in UNiGa1 K. Prokes\ !,",*, F. Honda#, G. Oomi#, L. Havela#, A.V. Andreev", V. Sechovsky´", A.A. Menovsky!, F.R. de Boer! ! Van der Waals-Zeeman Institute, University of Amsterdam, 1018 XE Amsterdam, The Netherlands " Department of Metal Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic # Department of Mechanical Engineering and Materials Science, Kumamoto University, Kumamoto 860, Japan
Abstract Results of dilatometric studies on UNiGa single crystals with respect to temperature, magnetic field and hydrostatic pressure are presented. Similar to the magnetism and the electrical-transport properties, also the elastic and magnetoelastic properties are strongly anisotropic. The linear thermal expansion coefficient a along the a-axis (a ) is always positive, a whereas a is negative up to 70 K. Sharp anomalies in both a and a are observed at the magnetic phase transitions. The c a c antiferromagnetic ordering at low temperatures is accompanied by a considerable linear spontaneous magnetostriction of different signs along the a- and c-axis. In the analysis and discussion of these results in conjunction with magnetic and other electronic properties, the anisotropic 5f-ligand hybridization and related exchange interactions are considered as principal mechanisms. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Uranium compounds; Thermal expansion; Magnetoelastic phenomena
UNiGa, one of the most extensively studied equiatomic ternary compounds [1], crystallizes in the hexagonal ZrNiAl-type of structure [2]. The shortest U—U distance is found within the basal plane and amounts to 349 pm. The c-axis separation (402 pm) gives the next-nearest U—U distance. Bulk magnetic measurements of single crystals revealed magnetic ordering below ¹ "39 K and N
* Corresponding author. Tel.: #42 2 21911367; fax: #42 2 21911351; e-mail: [email protected]. 1 Originally accepted for publication in the Proceedings of ICM’97 but not included therein by the Conference ruling on presentation.
three additional magnetic phase transitions within a region of 5 K below ¹ . A relatively low magnetic N field of 1—1.5 T applied along the c-axis induces metamagnetic transitions from zero-field antiferromagnetic structures to an uncompensated antiferromagnetic and/or to a ferromagnetic phase. All the magnetic structures are collinear, with the magnetic moments parallel to the c-axis. The structures consist of ferromagnetic basal-plane sheets which are coupled along c in various ways determining the magnetic periodicity. The preparation of the UNiGa single crystal used in the present investigations is described in Ref. [3]. The thermal expansion along the principal axes was measured with a parallel-plate
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K. Prokes\ et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 369—371
capacitance method [4] in the temperature range 1.5—210 K on a cube-shaped sample. The temperature dependence of the linear thermal-expansion coefficients, a"¸~1(*¸/*¹), along the a- and caxis (a and a , respectively), are shown in Fig. 1 a c together with the coefficient of the volume expansion. As can be seen, the thermal expansion of UNiGa is highly anisotropic. Along the a-axis, the lattice monotonously expands with increasing temperature (a is always positive). At low tempera atures we find a considerable shrinking of the c-axis with increasing temperature. Around 70 K, a (¹) c changes sign and at higher temperatures the lattice expands also in the c direction. Several sharp anomalies observed between 34 and 39 K on both a (¹) and a (¹) curves reflect magnetic phase a c transitions. Since the absolute value of a is approxc imately twice as large as a , the coefficient of the a volume expansion a at low temperatures is rather V small. At the low-temperature limit, a (¹), a (¹) and a c also a (¹) can be fitted by a straight line and the V effective electronic Gru¨neissen parameter C "4.8 % can be determined. Although this value is about one order of magnitude smaller than for typical heavy-fermion compounds, it is enhanced by a factor of 2 with respect to normal metals. Measurements under external hydrostatic pressures up to 2.2 GPa were performed on a sample on which also the magnetostriction data reported in Ref. [5] have been taken. Prior to the experiments, the sample was exposed to an additional heat treatment at 250°C for 16 h in order to remove internal stresses. This treatment leads to magnetic phasetransition temperatures which are about 1.5 K higher than before. The linear thermal expansion and the magnetostriction was measured by a strain gauge glued on the single crystal which was placed under the hydrostatic-pressure in a magnetic field applied along the c-axis. Fig. 2a shows the linear thermal expansion along the c-axis in zero magnetic field and in the temperature interval between 30 and 45 K. The data taken at several pressures between 0.1 and 2.2 GPa reveal that (a) the phases 1 and 2 (see figure caption for identification of the phases) are destabilised at pressures above 0.6 GPa, (b) the transition temperatures ¹ and ¹ are 7?3 3?4 reduced at a rate of 1.1 and 2.6 KGPa~1, respec-
Fig. 1. Temperature dependence of the linear thermal expansion coefficients along the a-axis (n) and along the c-axis (#). The coefficients of the volume expansion a /3 is given by the V solid line.
tively and (c) at pressures *1.8 GPa a new AF phase is induced instead of phase 3. The thermal expansion along the c-axis in a magnetic field of 1.5 T at various pressures up to 2.2 GPa is shown in Fig. 2b. The sharp increase of (*¸/¸) (and the simultaneous decrease of (*¸/¸) ) c a at ambient pressure reflects the magnetic phase transition from a paramagnetic to the uncompensated AF phase 5. Opposite and more pronounced effects are observed at lower temperatures for the transition to the ferromagnetic phase 6. The application of pressure apparently enhances the AF coupling which is documented by the gradually expanding existence range of phase 5. The field dependence of the magnetostriction under ambient and high pressure is also strongly anisotropic. The metamagnetic transition (from phase 4 to phase 6) is accompanied by a large magnetostrictive effect. The crystal shrinks along the c-axis and expands in the basal plane. From the linear magnetostriction values of j "2.3]10~4 a and j "!3.1]10~4 (at ambient pressure) a volc ume magnetostriction u is calculated to be 1.5]10~4. These values are reduced in 1 GPa to j "1.7]10~4, j "!2.6]10~4 yielding u" a c 0.7]10~4.
K. Prokes\ et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 369—371
371
Fig. 2. Temperature dependences of the change of the relative length along the c-axis of UNiGa in zero magnetic field (a-left) and in 1.5 T (b-right) at (I) ambient pressure, (II—VII) pressures of 0.3, 0.6, 1.0, 1.4, 1.8 and 2.2 GPa. The magnetic phases are denoted as follows: 1 — incommensurate AF structure with q"$(0, 0, d) and d&0.36; 2 — AF with q"$(0, 0, 1/3) and stacking (# 0 !); 3 — AF phase with stacking the (##!#!!#!), q"$(0, 0, 1/8), $(0, 0, 3/8); 4 — AF phase with stacking (##!#!!), q"(0, 0, 1/2),$(0, 0, 1/3),$(0, 0, 1/6); 5 — uncompensated AF phase with the stacking (##!), q"$(0, 0, 1/3); 6 — ferromagnetic phase with q"$(0, 0, 0); 7 — paramagnetic phase.
On the one hand, it is evident from the enhanced c-value of the low-temperature specific heat of 43 mJ/mol K2 [1—3] that in UNiGa the 5f states are present at the Fermi level. On the other hand, the high-temperature Curie—Weiss behaviour with an appreciable effective moment close to 3.0 l /U B [2, 3] and the ordered U moment of 1.4 l /U obB served by neutron diffraction [2, 3] for all magnetic phases suggest that the 5f states in UNiGa are more localized that in most of the other isostructural UTX compounds. The strong uniaxial magnetocrystalline anisotropy in UNiGa locks the uranium moments along the c-axis and leads to an Ising-like behaviour. The anisotropy apparently originates from a considerable uranium 5f-orbital moment and from the strong ferromagnetic interaction in the U—T planes. The interaction along the c-axis is considerably weaker and provides various (antiferromagnetic) couplings of the ferromagnetic basalplane sheets. Because the value of the ordered U moment is independent of the actual magnetic structure at ambient pressure and under hydrostatic pressures up to 0.9 GPa [3], we can definitely conclude that the pressure effects in UNiGa can be
primarily attributed to lattice variations between individual magnetic phases, arising due to the strong spin—orbit interaction, i.e. not to pressure effects on the size of the U moments. Acknowledgement This research was supported by the Stichting voor Fundamenteel Onderzoek deer Materie (FOM) and by the Grant Agency of The Czech Republic (no. 202/96/0207).
References [1] K. Prokes\ , E. Bru¨ck, F.R. de Boer, M. Miha´lik, A.A. Menovsky, P. Burlet, J.M. Mignot, L. Havela, V. Sechovsky´, J. Appl. Phys. 79 (1996) 6396 and references therein. [2] V. Sechovsky´, L. Havela, in: E.P. Wolfarth, K.H.J. Buschow (Eds.), Ferromagnetic Materials, vol. 4, ch. 4, NorthHolland, Amsterdam, 1988, p. 309. [3] K. Prokes\ , Ph.D. Thesis, University of Amsterdam, 1997. [4] G.H. White, Cryogenics 1 (1961) 151. [5] A.V. Andreev, M.I. Bartashevich, T. Goto, J. Alloys Compd. 219 (1995) 267.
Magnetoelastic phenomena in UNiGa1 K. Prokes\ !,",*, F. Honda#, G. Oomi#, L. Havela#, A.V. Andreev", V. Sechovsky´", A.A. Menovsky!, F.R. de Boer! ! Van der Waals-Zeeman Institute, University of Amsterdam, 1018 XE Amsterdam, The Netherlands " Department of Metal Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic # Department of Mechanical Engineering and Materials Science, Kumamoto University, Kumamoto 860, Japan
Abstract Results of dilatometric studies on UNiGa single crystals with respect to temperature, magnetic field and hydrostatic pressure are presented. Similar to the magnetism and the electrical-transport properties, also the elastic and magnetoelastic properties are strongly anisotropic. The linear thermal expansion coefficient a along the a-axis (a ) is always positive, a whereas a is negative up to 70 K. Sharp anomalies in both a and a are observed at the magnetic phase transitions. The c a c antiferromagnetic ordering at low temperatures is accompanied by a considerable linear spontaneous magnetostriction of different signs along the a- and c-axis. In the analysis and discussion of these results in conjunction with magnetic and other electronic properties, the anisotropic 5f-ligand hybridization and related exchange interactions are considered as principal mechanisms. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Uranium compounds; Thermal expansion; Magnetoelastic phenomena
UNiGa, one of the most extensively studied equiatomic ternary compounds [1], crystallizes in the hexagonal ZrNiAl-type of structure [2]. The shortest U—U distance is found within the basal plane and amounts to 349 pm. The c-axis separation (402 pm) gives the next-nearest U—U distance. Bulk magnetic measurements of single crystals revealed magnetic ordering below ¹ "39 K and N
* Corresponding author. Tel.: #42 2 21911367; fax: #42 2 21911351; e-mail: [email protected]. 1 Originally accepted for publication in the Proceedings of ICM’97 but not included therein by the Conference ruling on presentation.
three additional magnetic phase transitions within a region of 5 K below ¹ . A relatively low magnetic N field of 1—1.5 T applied along the c-axis induces metamagnetic transitions from zero-field antiferromagnetic structures to an uncompensated antiferromagnetic and/or to a ferromagnetic phase. All the magnetic structures are collinear, with the magnetic moments parallel to the c-axis. The structures consist of ferromagnetic basal-plane sheets which are coupled along c in various ways determining the magnetic periodicity. The preparation of the UNiGa single crystal used in the present investigations is described in Ref. [3]. The thermal expansion along the principal axes was measured with a parallel-plate
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 2 5 4 - 0
370
K. Prokes\ et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 369—371
capacitance method [4] in the temperature range 1.5—210 K on a cube-shaped sample. The temperature dependence of the linear thermal-expansion coefficients, a"¸~1(*¸/*¹), along the a- and caxis (a and a , respectively), are shown in Fig. 1 a c together with the coefficient of the volume expansion. As can be seen, the thermal expansion of UNiGa is highly anisotropic. Along the a-axis, the lattice monotonously expands with increasing temperature (a is always positive). At low tempera atures we find a considerable shrinking of the c-axis with increasing temperature. Around 70 K, a (¹) c changes sign and at higher temperatures the lattice expands also in the c direction. Several sharp anomalies observed between 34 and 39 K on both a (¹) and a (¹) curves reflect magnetic phase a c transitions. Since the absolute value of a is approxc imately twice as large as a , the coefficient of the a volume expansion a at low temperatures is rather V small. At the low-temperature limit, a (¹), a (¹) and a c also a (¹) can be fitted by a straight line and the V effective electronic Gru¨neissen parameter C "4.8 % can be determined. Although this value is about one order of magnitude smaller than for typical heavy-fermion compounds, it is enhanced by a factor of 2 with respect to normal metals. Measurements under external hydrostatic pressures up to 2.2 GPa were performed on a sample on which also the magnetostriction data reported in Ref. [5] have been taken. Prior to the experiments, the sample was exposed to an additional heat treatment at 250°C for 16 h in order to remove internal stresses. This treatment leads to magnetic phasetransition temperatures which are about 1.5 K higher than before. The linear thermal expansion and the magnetostriction was measured by a strain gauge glued on the single crystal which was placed under the hydrostatic-pressure in a magnetic field applied along the c-axis. Fig. 2a shows the linear thermal expansion along the c-axis in zero magnetic field and in the temperature interval between 30 and 45 K. The data taken at several pressures between 0.1 and 2.2 GPa reveal that (a) the phases 1 and 2 (see figure caption for identification of the phases) are destabilised at pressures above 0.6 GPa, (b) the transition temperatures ¹ and ¹ are 7?3 3?4 reduced at a rate of 1.1 and 2.6 KGPa~1, respec-
Fig. 1. Temperature dependence of the linear thermal expansion coefficients along the a-axis (n) and along the c-axis (#). The coefficients of the volume expansion a /3 is given by the V solid line.
tively and (c) at pressures *1.8 GPa a new AF phase is induced instead of phase 3. The thermal expansion along the c-axis in a magnetic field of 1.5 T at various pressures up to 2.2 GPa is shown in Fig. 2b. The sharp increase of (*¸/¸) (and the simultaneous decrease of (*¸/¸) ) c a at ambient pressure reflects the magnetic phase transition from a paramagnetic to the uncompensated AF phase 5. Opposite and more pronounced effects are observed at lower temperatures for the transition to the ferromagnetic phase 6. The application of pressure apparently enhances the AF coupling which is documented by the gradually expanding existence range of phase 5. The field dependence of the magnetostriction under ambient and high pressure is also strongly anisotropic. The metamagnetic transition (from phase 4 to phase 6) is accompanied by a large magnetostrictive effect. The crystal shrinks along the c-axis and expands in the basal plane. From the linear magnetostriction values of j "2.3]10~4 a and j "!3.1]10~4 (at ambient pressure) a volc ume magnetostriction u is calculated to be 1.5]10~4. These values are reduced in 1 GPa to j "1.7]10~4, j "!2.6]10~4 yielding u" a c 0.7]10~4.
K. Prokes\ et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 369—371
371
Fig. 2. Temperature dependences of the change of the relative length along the c-axis of UNiGa in zero magnetic field (a-left) and in 1.5 T (b-right) at (I) ambient pressure, (II—VII) pressures of 0.3, 0.6, 1.0, 1.4, 1.8 and 2.2 GPa. The magnetic phases are denoted as follows: 1 — incommensurate AF structure with q"$(0, 0, d) and d&0.36; 2 — AF with q"$(0, 0, 1/3) and stacking (# 0 !); 3 — AF phase with stacking the (##!#!!#!), q"$(0, 0, 1/8), $(0, 0, 3/8); 4 — AF phase with stacking (##!#!!), q"(0, 0, 1/2),$(0, 0, 1/3),$(0, 0, 1/6); 5 — uncompensated AF phase with the stacking (##!), q"$(0, 0, 1/3); 6 — ferromagnetic phase with q"$(0, 0, 0); 7 — paramagnetic phase.
On the one hand, it is evident from the enhanced c-value of the low-temperature specific heat of 43 mJ/mol K2 [1—3] that in UNiGa the 5f states are present at the Fermi level. On the other hand, the high-temperature Curie—Weiss behaviour with an appreciable effective moment close to 3.0 l /U B [2, 3] and the ordered U moment of 1.4 l /U obB served by neutron diffraction [2, 3] for all magnetic phases suggest that the 5f states in UNiGa are more localized that in most of the other isostructural UTX compounds. The strong uniaxial magnetocrystalline anisotropy in UNiGa locks the uranium moments along the c-axis and leads to an Ising-like behaviour. The anisotropy apparently originates from a considerable uranium 5f-orbital moment and from the strong ferromagnetic interaction in the U—T planes. The interaction along the c-axis is considerably weaker and provides various (antiferromagnetic) couplings of the ferromagnetic basalplane sheets. Because the value of the ordered U moment is independent of the actual magnetic structure at ambient pressure and under hydrostatic pressures up to 0.9 GPa [3], we can definitely conclude that the pressure effects in UNiGa can be
primarily attributed to lattice variations between individual magnetic phases, arising due to the strong spin—orbit interaction, i.e. not to pressure effects on the size of the U moments. Acknowledgement This research was supported by the Stichting voor Fundamenteel Onderzoek deer Materie (FOM) and by the Grant Agency of The Czech Republic (no. 202/96/0207).
References [1] K. Prokes\ , E. Bru¨ck, F.R. de Boer, M. Miha´lik, A.A. Menovsky, P. Burlet, J.M. Mignot, L. Havela, V. Sechovsky´, J. Appl. Phys. 79 (1996) 6396 and references therein. [2] V. Sechovsky´, L. Havela, in: E.P. Wolfarth, K.H.J. Buschow (Eds.), Ferromagnetic Materials, vol. 4, ch. 4, NorthHolland, Amsterdam, 1988, p. 309. [3] K. Prokes\ , Ph.D. Thesis, University of Amsterdam, 1997. [4] G.H. White, Cryogenics 1 (1961) 151. [5] A.V. Andreev, M.I. Bartashevich, T. Goto, J. Alloys Compd. 219 (1995) 267.