Journal of the Korean Physical Society, Vol. 63, No. 3, August 2013, pp. 372∼376 Effect of Partial Magnetic Order on Resistivity and Thermopower of Ho(Co 1-x Al x ) 2 Alloys T. Nakama, ∗ C. Zukeran, A. Nakamura, A. Teruya, S. Hirakawa, S. Watanabe, Y. Hiranaka, M. Takeda, M. Hedo and K. Yagasaki Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan Y. Takaesu and K. Uchima General Education, Okinawa Christian Junior College, Nishihara, Okinawa 903-0207, Japan A. T. Burkov A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia (Received 24 May 2012, in final form 30 October 2012) The electrical resistivity ρ and the thermopower S of Ho(Co1-xAlx)2 alloys (x = 0 to 0.2) were measured at temperatures from 2 K to 300 K in magnetic fields up to 10 T. While ρ(T ) and S(T ) of pure HoCo2 reveal abrupt changes at the magnetic ordering temperature TC, indicating a first- order transition, the temperature variations of both properties across the ordering temperature in Ho(Co1-xAlx)2 alloys clearly show that the type of magnetic transition changes under substitution of Al for Co from first order to second order around x ≈ 0.06. ρ(T ) of the Al-substituted samples (x = 0.15 and 0.2) has a very unusual variation at temperatures below TC: in contrast to the expected and usually-observed decrease of ρ(T ), the resistivity of Ho(Co1-xAlx)2 alloys (x = 0.15 and 0.2) increases at temperatures below TC. Moreover, the magnetoresistance of these alloys is positive around TC; this is also in a sharp contrast to the usually observed negative magnetoresistance. We show that the anomalous resistivity and magnetoresistance in these alloys are related to the metamagnetic instability of the Co 3d subsystem and to the fluctuating local magnetic susceptibility due to atomic substitution. Due to this fluctuating local magnetic susceptibility, the uniform exchange field of 4f moments induces a non-uniform polarization of the 3d system, with static spatial fluctuations of the local magnetization. This static magnetic disorder gives an additional contribution (ρm) to the electrical resistance in the alloys at temperatures below TC. The degree of this disorder changes with decreasing temperature, resulting in specific variation of ρm with temperature. PACS numbers: 71.27.+a, 72.15.Gd, 72.15.Jf, 75.10.Lp Keywords: Thermopower, Electrical resistivity, Magnetoresistance DOI: 10.3938/jkps.63.372 I. INTRODUCTION HoCo 2 belongs to the family of intermetallic RCo 2 compounds with a cubic Laves phase structure (R stands for rare-earth elements) which are known for mag- netic instability of the itinerant Co 3d-electron system. When R is nonmagnetic (R=Y, Lu), the compounds are exchange-enhanced paramagnets exhibiting a metamag- netic transition in an external magnetic field B c from a state with a non-polarized 3d band to a polarized state with a 3d magnetic moment of 1 μ B /f.u. [1, 2]. This critical field was found to be about 70 T for YCo 2 [3] and about 77 T for LuCo 2 [4]. In the case of magnetic ∗ E-mail: nakama@sci.u-ryukyu.ac.jp RCo 2 compounds, the metamagnetic transition of the 3d-electron subsystem is induced by an exchange field B ex = n fd M f of the ordered 4f localized magnetic mo- ments of the rare earths. The interplay of localized and itinerant magnetism brings a wealth of new interesting phenomena. The existence of a series of compounds with different magnitudes of the 4f moment and alloys on their base on one side and alloys with substitution at cobalt sites on the other side allows a rich variety of experi- mental systems with variable magnitude of the 4f-3d ex- change field and with different degrees of enhancement of the itinerant 3d system. In the alloys, structural disor- der due to random occupation of lattice sites by the alloy components represents an additional parameter affecting the physical properties. Particularly, we have found in -372-