Low-temperature measurements of magnetic susceptibility, magnetization, and specific heat have been made on single-crystal and polycrystal samples of Tb(OH), . The experimental techniques and methods of analysis were similar to those used previously to study isostructural Gd(OH), . The results are consistent with earlier reports that Tb(OH)3 is a highly anisotropic ferromagnet resembling a three-dimensional Ising model, but a detailed analysis revealed an interesting range dependence of the efFective spin-spin interactions. Estimates of the interactions between first-, secondand third-nearest neighbors were made from measurements at temperatures high and low compared with the Curie temperature (T~= 3.72 + 0.01 K), using asymptotically exact series expansions correct to second order, and for both the firstand second-nearest neighbors an unusual cancellation between the magnetic dipole and "exchange" interactions was found. As a result of this cancellation, the net interaction between second-nearest neighbors becomes comparable to that between the nearest neighbors, but both are reduced in absolute magnitude so that they become comparable to the long-range dipole interactions between more distant neighbors. Some consequences of this competition on the cooperative properties are discussed.
The rare-earth hydroxides, R(OH)3 with R–La to Yb and Y form a series of simple magnetic crystals isostructural with the hexagonal rare-earth trichlorides. Compared with most ionic rare-earth crystals they have relatively small lattice parameters, and one may therefore expect magnetic cooperative effects at readily accessible temperatures. In this paper we report the first series of magnetic measurements on small, hydrothermally grown single crystals (1 to 10 mg) which show that Tb(OH)3, Dy(OH)3, and Ho(OH)3 order ferromagnetically at 3.72°, 3.50°, and 2.55°K, respectively, while Nd(OH)3 and Gd(OH)3 undergo more complex antiferromagnetic transitions near 1.7° and 2.0°K. Er(OH)3 remains paramagnetic down to 1.2°K. Magnetization measurements on Tb(OH)3, Dy(OH)3, and Ho(OH)3 in fields up to 14 kG give saturation moments (corrected for Van Vleck temperature-independent paramagnetism) of 1350 emu/cc (9.0 μB/ion), 1418 emu/cc (9.6 μB/ion), and 1121 emu/cc (7.6 μB/ion) parallel to the c axis and almost zero perpendicular to the c axis. The correspondingly large and anisotropic magnetic dipole interactions account approximately for the observed Curie temperatures, suggesting that Tb(OH)3, Dy(OH)3, and Ho(OH)3 may be among the simplest examples of Ising-like systems studied so far, with dominant interactions along linear chains of nearest neighbors. Single crystals are found to be chemically stable with apparently good optical properties, suggesting useful applications to magneto-optic devices. These are discussed briefly.
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