Journal of Magnetism and Magnetic Materials 83 (1990) 267-269 267 N orth-Holland EFFECT OF SPIN-REORIENTATION ON THE DOMAIN STRUCTURE IN HoCo3Ni2 R. SZYMCZAK, J. ZAWADZKI Institute of Physics, Polish Academy of Sciences, Warsaw, Poland and Z. DRZAZGA Institute of Physics, Silesian University, Katowice, Poland The domain structure in polycrystalline HoCo3Ni 2 was studied above (room temperature) and below (5 K) the reorientation range, using the Kerr effect, the powder technique and the oxygen cryocodensation method. The domain wall energy was determined for both temperature regions. 1. Introduction Unusual magnetic properties of heavy rare-earth (RE)-cobalt compounds result from antiferromagnetic coupling between RE and Co sublattices as well as from the extremely large anisotropy characteristic of both RE and Co ions. A particularly complicated situation arises for systems exhibiting competition between the uniaxial Co anisotropy and the easy plane RE anisotropy. An example of such a compound is HoCo3Ni 2 which dem- onstrates a compensation point, as well as magnetiza- tion reorientation from the hexagonal c-axis to the basal plane in the temperature range of 143-160 K [1]. The torque curves measured below the reorientation temperature show the six easy directions in the basal plane. The decrease of magnetization upon approaching the low temperatures observed in this compound [1] may be explained by the presence of narrow domain walls, responsible for a large intrinsic coercivity. The main purpose of this work is to study the domain structure in HoCo3Ni 2 above and below the spin-reorientation range, in order to compare the char- acter of the domain structure and domain wall energy in both easy axis and easy plane regions. anisotropy magnets. Characteristic "rosette"-like pat- terns are seen (figs. la, b) on the basal plane perpendic- ular to the easy axis (c-axis). Whenever the c-axis is oriented nearly parallel to the grain surface, the usual stripe domains with reverse spikes appear (fig. lc). Below the reorientation range, the character of the domain structure does not appear to be changed. There is, however, a principal difference. At low temperature "rosette"- and "star"-like domains are now observed not on the basal plane, but on the axial plane, while laminar domains appear on the basal plane, which is, after reorientation, the easy plane (compare figs. 2a-d, 2b-e and 2c-f). After reorientation, some large grains consist of several blocks with .laminar domains (fig. 2f). Observations of temperature changes of the domain structure were performed and gave direct evidence of spin-reorientation. 3. Discussion In order to determine the density of the domain wall energy y, the domain patterns obtained on the grains with the easy axis normal (or almost normal) to the grain surface, have been analysed. The Bodenberger- 2. Experiment The polycrystalline samples of HoCo3Ni 2 investi- gated were prepared by melting the high-purity ele- ments in an arc argon furnace. Homogeneity of the samples was confirmed by an X-ray microanalysis. Ob- servations of magnetic domains were performed on relatively large grains, by means of the magneto-optical Kerr effect (fig. la), or the usual Bitter powder tech- nique (figs. lb, c and 2a, b, c) - at room temperature, and using the cryocondensation method [2] at 5 K (figs. 2d, e, f). At room temperature, the domain structure patterns are similar to those observed earlier on uniaxial high 0304-8853/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland) a b ~i~ii~ ¸ !ili!~! !d~i~ ll~ii~? _ . , // • 1 • ! C Fig. 1. Magnetic domains on the basal (a, b) and axial planes (c) at room temperature (100 x ).