Journal of Magnetism and Magnetic Materials 80 (1989) 281-284 281 North-Holland, Amsterdam AC SUSCEPTIBILITY MEASUREMENTS IN SOME RTiFe,I_ xCox (R = Dy, Ho, Er) COMPOUNDS: SPIN-REORIENTATION BEHAVIOR V.K. SINHA, S.K. MALIK a, D.T. ADROJA a, j. ELBICKI, S.G. SANKAR and W.E. WALLACE MEMS Department and Mellon Institute, Carnegie Mellon Unioersity, Pittsburgh, PA 15213, USA Tata Institute of Fundamental Research, Bombay, India Received 14 February 1989 The DyTiFell_xCo x (x = 0, 1, 3), HoTiFe n_xCox (x = 0, 3) and ErTiFell compounds, all exhibiting the ThMn~2-type structure, were studied for their ac susceptibility in the temperature range 20-300 K. The Dy- and Ho-containing compounds exhibited spin re, orientation from axial to cone to planar, or from axial to cone in the temperature range 250-50 K, as expected. In this structure Dy and Ho with negative second-order Stevens' coefficients (av < 0) favor planar rare earth sublattice anisotropy, while Fe and Co sublattices favor uniaxial and planar anisotropies, respectively and would be in competition as a function of temperature. A spin-reorientation type of behavior was also observed in the ErTiFe H compound near 50 K. This observation appears to imply that, in this compound, the higher-order crystal-field terms are important. The magnetization data revealed that in these systems the rare earth and transition metal sublattices coupled antiparallel, as is normally the case in the heavy rare-earth-transition-metal compounds. 1. Introduction Intermetallic compounds of rare earth (R) and 3d transition metals (TM) having a uniaxial mag- netocrystalline anisotropy are important candi- dates as permanent magnet materials. High uni- axial anisotropies in these compounds arise as a consequence of the electrostatic interactions of the asymmetric charge cloud of the 4f electrons with the charges of the surrounding metal atoms. Prom- ising candidates in this respect have been SmC% and its structural derivatives, such as 5m2Co17 [1], and the more recent R2FelaB-based compounds [2-4]. However, while the former materials lack very high energy products, the latter ones possess considerably lower magnetic ordering temperature and large temperature coefficients of remanence and coercivity, limiting their range of applications. The quest for better permanent magnet materials has recently focused on compounds of the type RTiFell (R = rare earth), which crystallize in the body-centered tetragonal ThMnl2 structure [5-10]. In this particular crystal structure, those rare earths with a positive second-order Stevens' factor (aj > 0), e.g. Sm, Er, Tm and Yb, prefer magnetization along the c-axis, whereas those rare earths with a s < 0, e.g. Tb, Dy and Ho, prefer planar magneti- zation. In the R-TM intermetallics, the tempera- ture dependence of R and TM sublattice anisotro- pies is generally quite different and, as a conse- quence, in systems where the R and TM anisotro- pies are in opposition the direction of easy magne- tization is likely to be temperature dependent, leading to spin reorientations. In the ternary RTiFell compounds (R = Dy, Tb and Ho), spin reorientation of this type has recently been ob- served by both MSssbauer and bulk magnetization measurements [11-14]. It is of considerable inter- est to study the spin-reorientation behavior of these systems by the ac susceptibility method, and to investigate the effect of Co substitution. A series of studies on the RTiFelm_xCo x systems has been undertaken at this laboratory to understand their fundamental magnetic properties, and thus to assess the potential of these systems as perma- nent magnet materials. In the present paper we report the spin-reorien- tation behavior of the RTiFell_xCox compounds 0304-8853/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)