Fyzikální ústav Akademie věd ČR

The rare earth (4f) intermetallic compounds with Fe or Co are famous as extremely powerful permanent magnets with exceptionally high uniaxial magneto-crystalline anisotropy. In R₂Co₁₇ compounds (R is a rare-earth metal), the strongest interaction is the Co-Co exchange interaction which primarily determines their Curie temperature, T_C. The R-Co interaction, although much weaker than the Co-Co one, is of special importance since by this interaction the strongly anisotropic R-sublattice magnetization is coupled to the much less anisotropic Co-sublattice magnetization. The exchange interaction between the 4f and 3d electrons is usually represented by the molecular field by which the R- and Co-sublattice moments are coupled. For the Co-rich R-Co compounds, the values for the molecular field are typically of the order of 100 T, so that large magnetic fields are needed to determine molecular field by induction of changes in the magnetic configurations of the two magnetic sublattices. Due to large magnetic anisotropy the study should be done on single crystals.

Er₂Co₁₇ and Tm₂Co₁₇ (hexagonal crystal structure of the Th₂Ni₁₇ type) are ferrimagnets with rather similar magnetic properties [1, 2]. They have T_C = 1170 K and spontaneous magnetic moment M_s = 10.1 and 13.4 µ_B/f.u. for the Tm and Er compound, respectively, directed along the Co sublattice. In Er₂Co₁₇, the metamagnetic transition at 40 T has been found in magnetic field applied along the c axis. The transition is clearly of the first order and corresponds to sharp rotation of the Er sublattice by approx. 50 degrees with its further continuous alignment towards the collinear ferromagnet.

We performed the high-field magnetization measurements of the Tm₂Co₁₇ single crystal as well. Our most interesting finding is a large rise of magnetization in a field directed along the c axis at µ₀H_cr = 39 T. It is attributed to a direct ferri-to-ferromagnetic transformation by way of paramagnetic remagnetization of the Tm sublattice. This is a continuous process – no intermediate canted-spin phases occur and the sublattice moments remain collinear with the applied field. Thus, despite the similarity of magnetic properties in fields below 20 T, the field-induced transitions in Er₂Co₁₇ and Tm₂Co₁₇ have completely different nature. Process of remagnetization of the R sublattice without rotation of the magnetic moments is discovered for the first time. Then we confirmed this process in related compound Tm₂Fe₁₇ [3].

Fig. 1: High-field magnetization curves of the Er₂Co₁₇ and Tm₂Co₁₇ compounds with the pulsed magnetic field applied along the principal crystal axes. The unit cell of the R₂Co₁₇ and R₂Co₁₇ demonstrates packing of the 4f and 3d elements in the crystal structure. The insets show a typical crystal growth by Czochralski method and the resulting ingot.

References:
[1] A.V. Andreev, Y. Skourski, M.D. Kuzmin, S. Yasin, S. Zherlitsyn, R. Daou, J. Wosnitza, A. Iwasa, A. Kondo, A. Matsuo, K. Kindo, Magnetic and magnetoelastic anomalies of an Er₂Co₁₇ single crystal in high magnetic fields Phys. Rev. B 83 (2011) 184422.
[2] A.V. Andreev, M.D. Kuzmin, Y. Narumi, Y. Skourski, N.V. Kudrevatykh, K. Kindo, F.R. de Boer, J. Wosnitza, High-field magnetization study of a Tm₂Co₁₇ single crystal Phys. Rev. B 81 (2010) 134429.
[3] O. Isnard, A.V. Andreev, M.D. Kuzmin, Y. Skourski, D.I. Gorbunov, J. Wosnitza, N.V. Kudrevatykh, A. Iwasa, A. Kondo, A. Matsuo, K. Kindo, High magnetic field study of the Tm₂Fe₁₇ and Tm₂Fe₁₇D_3.2 compounds Phys. Rev. B 88 (2013) 174406.