ELSEVIER Journal of Alloys and Compounds 258 (1997) 17-19 Jouraal of ALLOY5 AN~ COMPOUNDS Hydrogen-induced anisotropy in DYo.73Tbo.27Fel.sCoo. 5 S. Kishore, G. Markandeyulu, K.V.S. Rama Rao* Magnetism and Magnetic Materials Laboratory, Department of Physics Indian Institute of Technology, Madras 600 036, India Received 5 November 1996 Abstract Large anisotropy in Dyo.73Tbo.27Fe~.sCOo.sHo. 5 when compared with DYo.73Tbo.zTFe~.sCoo. 5 has been attributed to the orientational ordering of hydrogen atoms in the lattice. Pinning effects and discontinuous jumps in magnetization have been observed at 4.2 K, up to a magnetic field of 2 kOe. The former is due to domain wall pinning and the latter is due to spin-flip metamagnetism in a field-induced transition from a modulated phase to an equal-moment phase at low temperatures. © 1997 Elsevier Science S.A. Keywords: Laves phase compounds; Anisotropy; Crystalline electric field; Spin-flip metamagnetism 1. Introduction RE-TM 2 (RE=rare earth and TM=transition metal) intermetallic compounds with C15-type cubic Laves phase structure are known to absorb large quantities of hydrogen and consequently, their magnetic properties are affected due to charge transfer effects [1]. Recent investigations on the magnetic properties of the hydrides of (Sm,Tb) (Fe,Co)2 compounds have revealed large increases in anisotropy at low temperatures [2]. In an earlier paper, we have reported the enhancement of anisotropy of DYo.73Tbo.zvFez_xCOx, on hydrogen absorption [3]. Magnetization measurements carried out on DY0.73Tb0.27Fel.sCoo. 5 at 4.2 K have revealed, in addition to the increase in anisotropy on hydrogenation, domain wall pinning effects and field-induced spin-flip transitions and these are reported in this paper. 2. Experimental details The preparation, characterization and hydrogenation of the alloys are as described elsewhere [3]. Magnetization measurements were carried out on Dy0.73Tb0.z7Fe1.sCo0.5 and Dyo.y3Tbo.27Fel.5Co0.sHo.5 at temperatures 4.2, 24 and 300 K using a PAR vibrating sample magnetometer. The *Corresponding author: Fax: 91 44 2352545; e-mail: phy2@iitm.ernet.in 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved PII S0925-8388(97)00077-7 measurements at 24 and 300 K were carded out up to a field of 10 kOe and those at 4.2 K, up to a field of 14 kOe. 3. Results and discussion Fig. la and b show the magnetization curves of DY0.73Tb0.27Fe15Coo.5 and Dy0.73Tb0.27Fel.sCo0.sHo.5 re- spectively, at temperatures 4.2, 24 and 300 K. At low temperatures, there is a substantial increase in the aniso- tropy of Dy0.y3Tb0.z7Fel.sCo0.5 on hydrogen absorption. In cubic Laves phase compounds, a rhombohedral distortion of the cubic lattice due to magnetoelastic interactions along the (111} direction has been reported [4]. In such a periodically distorted lattice, there is a spatial modulation of the 5d conduction electron polarization which, in turn, modifies the crystalline electric fields acting on the RE ions. As a result, the tetrahedral interstitial sites constituted by RE and TM atoms are rendered inequivalent with regard to their potential energy for hydrogen occupancy. It has been reported that, in cubic Laves phase compounds, for hydrogen concentrations up to one per formula unit, hydrogen occupies the interstitial sites formed by two RE and two TM atoms [5]. These rhombohedral distortions stimulate the ordering of hydrogen atoms leading to hydrogen occupancy in sites with minimum potential energy [6]. This localization, in turn, further enhances the rhombohedral distortions, resulting in an induced local uniaxial anisotropy. In the present case, the easy directions of magnetization of both Dyo.TsTbo.27Fe1.sCoo.s and