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Transport properties across the metamagnetic transition in UCoAl
Physica B 259—261 (1999) 240—241
Transport properties across the metamagnetic transition in UCoAl T.D. Matsuda *, H. Sugawara , Y. Aoki , H. Sato , A.V. Andreev, Y. Shiokawa, V. Sechovsky, L. Havela Department of Physics, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji-shi, Tokyo 192-0397, Japan Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Department of Metal Physics, Charles University, Ke Karlovu 5, 12116 Prague-2, Czech Republic
Abstract We report the results on the magnetoresistance and thermoelectric power in a single crystal of UCoAl exhibiting metamagnetic transition at H (&1 T). The longitudinal (transverse) magnetoresistance shows a step-like increase + (decrease) across H with increasing field. Thermoelectric power also shows a sharp change *S&10 lV/K (at 4.5 K) + across H . From these results, we inferred that the high field state is a 5f-band split ferromagnetic state. 1999 Elsevier + Science B.V. All rights reserved. Keywords: UCoAl; Metamagnetic transition; Transport properties
The uranium intermetallic compound UCoAl is known to have a paramagnetic ground state [1]. This compound has attracted much attention, since it exhibits metamagnetism induced by magnetic field (H) below 1 T for H#c-axis. Recently, Tran et al. have suggested that both ferromagnetic and antiferromagnetic interactions are almost equally important in UCoAl based on the magnetic susceptibility measurements on UCo Ni Al \V V [2]. The origin of the metamagnetic transition (MT) in UCoAl has not yet been elucidated. From the view point of MT from a paramagnetic ground state, it might be of interest to compare the transition with the metamagnetic-like anomaly reported for heavy fermion Ce compounds such as CeRu Si , CeCu and CeNi Ge . Recently, based on the experiment of thermal properties, we have inferred that the origin of the metamagnetic transition in UCoAl is different from that of 4f-electron systems [3]. Here, we report the effect of the MT on the
* Corresponding author. Tel.: #81-426-77-2487; fax: #81426-77-2483; e-mail: [email protected].
magnetoresistance (MR) and thermoelectric power (TEP) in UCoAl. A single crystal of UCoAl was grown by Czochralski pulling method. The raw materials were 3N5-U, 3N>Co, 6N-Al. From the same sample, we prepared several bar-shaped samples (typically 4;1.3;0.5 mm), on which we can measure the transport properties both in the transverse and longitudinal geometries. The resistivity was measured by a conventional DC four-probe method. TEP was measured by the ordinary differential method using AuFe (0.07%)-normal silver thermocouples. Fig. 1 shows the transverse MR(o ) and longitudinal , MR(o ) at 2 and 4.2 K. Across MT, MR(o ) shows an , , increase with increasing H, which agrees with the previously reported data [4] except the lower transition field and the lower resistivity owing to a higher quality of the present sample. In Ref. [4], the increase in MR(o ) is , interpreted as due to the reduction of number of carriers resulting from the 5f-band splitting. In contrast, MR(o ) , in the present experiment shows a clear decrease with increasing H. We believe that to explain these phenomena, additional contributions should be considered
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 0 7 8 - 3
T.D. Matsuda et al. / Physica B 259–261 (1999) 240—241
241
Fig. 2. The field dependence of thermoelectric power in the transverse geometry. Fig. 1. The magnetoresistance in the (a) transverse and (b) longitudinal geometries. The closed circles and open squares denote the data for increasing and decreasing field, respectively.
besides that arising from a simple mechanism such as a Fermi surface change proposed in Ref. [4]. Actually, we must consider at least three contributions to the change of MR(*o) across H . Firstly, the Fermi surface changes + across H as suggested in Ref. [4], since the mean value + of *o, which is defined as *o"(*o #2*o )/3 at , , ¹P0 K, increases across H . Secondly, the field-in+ duced band split ferromagnetic state can be inferred from the resistivity anisotropy, we must take into account another contribution to MR: namely the so-called anisotropic MR that is usually observed in ferromagnetic materials. In particular, it is well known for 3d-ferromagnetic metals and alloys that MR is usually positive for the longitudinal and negative for the transverse geometry due to the anisotropy in the spin—orbit coupling. Thirdly, there exists a magnetic scattering contribution from magnetic fluctuations to *o, which dominates at high temperature and is suppressed above H . + Fig. 2 shows the field dependence of TEP along c-axis at 4.5 K and 10 K. TEP shows a drastic decrease of *S&10 lV/K at 4.5 K and &20 lV/K at 10 K across
H with increasing H. These changes of TEP strongly + indicate the change of the Fermi surface across H . + Concluding the results on MR and TEP, we suggest the following scenario for the metamagnetic transition in UCoAl. In zero field, the 5f-band is located near Fermi level, and it is split in magnetic field. In this process, the Fermi surface change is reflected in the change of TEP, the specific heat coefficient c [3] and MR. The 5f-band split state leads to the anisotropic MR. This is consistent with the results of band structure calculations by Eriksson et al. [5].
References [1] V. Sechovsky, L. Havela, F.R. de Boer, J.J.M. Franse, P.A. Veenhuizen, J. Sebek, J. Stehno, A.V. Andreev, Physica 142B (1986) 283 and references therein. [2] V.H. Tran, R. Troc, H. Noe¨l, J. Alloys and Compounds 262—263 (1997) 471. [3] T.D. Matsuda et al., to be published. [4] A.V. Kolomiets, L. Havela, V. Sechovsky, J. Appl. Phys. 83 (1998) 6435. [5] O. Eriksson, B. Johansson, M.S.S. Brooks, J. Phys. Condens. Matter 1 (1989) 4005.
Transport properties across the metamagnetic transition in UCoAl T.D. Matsuda *, H. Sugawara , Y. Aoki , H. Sato , A.V. Andreev, Y. Shiokawa, V. Sechovsky, L. Havela Department of Physics, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji-shi, Tokyo 192-0397, Japan Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Department of Metal Physics, Charles University, Ke Karlovu 5, 12116 Prague-2, Czech Republic
Abstract We report the results on the magnetoresistance and thermoelectric power in a single crystal of UCoAl exhibiting metamagnetic transition at H (&1 T). The longitudinal (transverse) magnetoresistance shows a step-like increase + (decrease) across H with increasing field. Thermoelectric power also shows a sharp change *S&10 lV/K (at 4.5 K) + across H . From these results, we inferred that the high field state is a 5f-band split ferromagnetic state. 1999 Elsevier + Science B.V. All rights reserved. Keywords: UCoAl; Metamagnetic transition; Transport properties
The uranium intermetallic compound UCoAl is known to have a paramagnetic ground state [1]. This compound has attracted much attention, since it exhibits metamagnetism induced by magnetic field (H) below 1 T for H#c-axis. Recently, Tran et al. have suggested that both ferromagnetic and antiferromagnetic interactions are almost equally important in UCoAl based on the magnetic susceptibility measurements on UCo Ni Al \V V [2]. The origin of the metamagnetic transition (MT) in UCoAl has not yet been elucidated. From the view point of MT from a paramagnetic ground state, it might be of interest to compare the transition with the metamagnetic-like anomaly reported for heavy fermion Ce compounds such as CeRu Si , CeCu and CeNi Ge . Recently, based on the experiment of thermal properties, we have inferred that the origin of the metamagnetic transition in UCoAl is different from that of 4f-electron systems [3]. Here, we report the effect of the MT on the
* Corresponding author. Tel.: #81-426-77-2487; fax: #81426-77-2483; e-mail: [email protected].
magnetoresistance (MR) and thermoelectric power (TEP) in UCoAl. A single crystal of UCoAl was grown by Czochralski pulling method. The raw materials were 3N5-U, 3N>Co, 6N-Al. From the same sample, we prepared several bar-shaped samples (typically 4;1.3;0.5 mm), on which we can measure the transport properties both in the transverse and longitudinal geometries. The resistivity was measured by a conventional DC four-probe method. TEP was measured by the ordinary differential method using AuFe (0.07%)-normal silver thermocouples. Fig. 1 shows the transverse MR(o ) and longitudinal , MR(o ) at 2 and 4.2 K. Across MT, MR(o ) shows an , , increase with increasing H, which agrees with the previously reported data [4] except the lower transition field and the lower resistivity owing to a higher quality of the present sample. In Ref. [4], the increase in MR(o ) is , interpreted as due to the reduction of number of carriers resulting from the 5f-band splitting. In contrast, MR(o ) , in the present experiment shows a clear decrease with increasing H. We believe that to explain these phenomena, additional contributions should be considered
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 0 7 8 - 3
T.D. Matsuda et al. / Physica B 259–261 (1999) 240—241
241
Fig. 2. The field dependence of thermoelectric power in the transverse geometry. Fig. 1. The magnetoresistance in the (a) transverse and (b) longitudinal geometries. The closed circles and open squares denote the data for increasing and decreasing field, respectively.
besides that arising from a simple mechanism such as a Fermi surface change proposed in Ref. [4]. Actually, we must consider at least three contributions to the change of MR(*o) across H . Firstly, the Fermi surface changes + across H as suggested in Ref. [4], since the mean value + of *o, which is defined as *o"(*o #2*o )/3 at , , ¹P0 K, increases across H . Secondly, the field-in+ duced band split ferromagnetic state can be inferred from the resistivity anisotropy, we must take into account another contribution to MR: namely the so-called anisotropic MR that is usually observed in ferromagnetic materials. In particular, it is well known for 3d-ferromagnetic metals and alloys that MR is usually positive for the longitudinal and negative for the transverse geometry due to the anisotropy in the spin—orbit coupling. Thirdly, there exists a magnetic scattering contribution from magnetic fluctuations to *o, which dominates at high temperature and is suppressed above H . + Fig. 2 shows the field dependence of TEP along c-axis at 4.5 K and 10 K. TEP shows a drastic decrease of *S&10 lV/K at 4.5 K and &20 lV/K at 10 K across
H with increasing H. These changes of TEP strongly + indicate the change of the Fermi surface across H . + Concluding the results on MR and TEP, we suggest the following scenario for the metamagnetic transition in UCoAl. In zero field, the 5f-band is located near Fermi level, and it is split in magnetic field. In this process, the Fermi surface change is reflected in the change of TEP, the specific heat coefficient c [3] and MR. The 5f-band split state leads to the anisotropic MR. This is consistent with the results of band structure calculations by Eriksson et al. [5].
References [1] V. Sechovsky, L. Havela, F.R. de Boer, J.J.M. Franse, P.A. Veenhuizen, J. Sebek, J. Stehno, A.V. Andreev, Physica 142B (1986) 283 and references therein. [2] V.H. Tran, R. Troc, H. Noe¨l, J. Alloys and Compounds 262—263 (1997) 471. [3] T.D. Matsuda et al., to be published. [4] A.V. Kolomiets, L. Havela, V. Sechovsky, J. Appl. Phys. 83 (1998) 6435. [5] O. Eriksson, B. Johansson, M.S.S. Brooks, J. Phys. Condens. Matter 1 (1989) 4005.