Anisotropic Magnetization in DyCo2Single Crystal

Abstract

Intermetallic Laves phase compounds RCo2 (R=rare earth element) exhibit various interesting magnetic properties. YCo2 and LuCo2 are exchange enhanced paramagnets, which show an itinerant metamagnetic transition at 70T. When R is one of the magnetic elements, Co 3d-moment is induced by a large internal field from the rare earth 4f -moment. In light rare earth compounds, PrCo2 and NdCo2 are ferromagnets, 2) where the coupling between rare earthand Co-moments is parallel. On the other hand, in heavy rare earth compounds, GdCo2, TbCo2, DyCo2, HoCo2, ErCo2 and TmCo2 are ferrimagnets, where the coupling between rare earthand Co-moments is antiparallel. CeCo2 is an exceptional one due to the tetravalence of Ce-ion, and is a paramagnet down to 1.5K below which it exhibits superconductivity. The magnetocrystalline anisotropy of this system, especially in heavy rare earth RCo2 series, was investigated intensely by mean of magnetization,5-8) neutron scattering,9-11) and Mossbauer spectroscopy measurements. 13) However, for DyCo2, there is no magnetic anisotropy data except Mossbauer study with polycrystalline sample, in which only the easy of magnetization is inferred as the 〈100〉. Until now, a single crystal growth of DyCo2 has not been succeeded presumably because of the difficulty due to its incongruent phase. In this note, we report the first successful growth of the single crystalline DyCo2, on which magnetic anisotropy has been directly investigated by magnetization measurements. For comparison with the experimental results, we calculated Dy-moment based on the crystalline electric field (CEF) effect. A single crystal of DyCo2 was prepared by Czochralski pulling method in an induction furnace. The raw materials were 3N (99.9% pure)-Dy and 4N-Co. In order to reduce impurity phase, e.g. DyCo3, Dy-rich composition was selected, namely the starting composition was DyCo1.8. No other peak except DyCo2 was observed in X-ray powder diffraction measurement. The residual resistivity ratio (RRR) of the present single crystal was 20. We also prepared a polycrystalline sample whose RRR was 128 for comparison. The single crystal ingot was cut into a cube by a spark cutter. The weight of the sample was 9.2mg. Magnetization measurements were carried out up to 5.5T using a SQUID magnetometer axis (Quantum Design MPMS). The temperature dependence of magnetic susceptibility χ under 0.01T is shown in Fig. 1. With decreasing temperature, χ increases rapidly below Curie temperature Tc. Tc of the present single crystal is found to be 136K, which agrees with the previous report and our data on a polycrystalline sample. χ of a polycrystalline sample follows a Curie-Weiss law above Tc. However, for the single crystalline sample, χ shows deviation from the Curie-Weiss law above Tc, which is considered to be due to magnetic impurities (Co and/or DyCo3). The estimated amount of the impurities is of the order of ∼ 1%, whose influence on the high field magnetization measurement is negligible. The field dependence of magnetization for fields along the each principle direction at 4.2K is shown in Fig. 2. The correction by subtracting the demagnetizing field is made in Fig. 2. The easy axis is found to be the 〈100〉. The absolute value of magnetization for each direction at 1T and 5T is listed in Table I. The ratio of these absolute values among the 〈100〉-, the 〈110〉and the 〈111〉directions is close to 1 : cos 45◦(= 0.71) : cos 54◦(= 0.59); the ratio of magnetizations projected to the field directions assuming the magnetic moment of Dy aligns to the