The magnetic susceptibility and magnetization measured along the b axis of a KTb(WO 4 ) 2 single crystal was investigated experimentally in the temperature range 70 mK-6 K and in magnetic fields up to 8 T. The results allow us to make conclusion that singlet magnet KTb(WO 4 ) 2 undergoes an antiferromagnetic phase transition T c = 0.65 K. Alkali-rare-earth double molybdates and tungstates have been actively studied for a long time. Many compounds of this class are characterized by a strong magnetic anisotropy of the rare-earth ions, a low local symmetry, and a pronounced chain structure. The electronic spectrum of rare-earth ions in the crystalline field usually has lowlying excited levels, and that leads to the possibility of reorganization of the corresponding electronic states both by an external magnetic field and by displacements of the ions. This is responsible, in particular, for the magneticfield-induced structural phase transitions observed in various compounds of this class [1][2][3][4][5]. In the case of nonKramers rare-earth ions the lowest electronic state often form a quasi-doublet with a gap Δ of the order of few K, well separated from the rest of the spectrum. The magnetic dipole and exchange interactions in these compounds are of the same order of magnitude. Therefore, singlet and excitonic types of magnets can be realized in these compounds (in the second case the interactions are insufficient to induce magnetic order suppressed by the gap Δ [6,7]. In the non-Kramers doublet case the contribution to the magnetic properties of the crystal from the higher-lying excitations of the rare-earth ions are also unusual [6]. All of these circumstances cause great interest to the study of such systems. Here it is possible to study a number of topical questions in solid state physics in a comparatively simple situation. Among such topics are the interaction of electronic excitations with lattice vibrations (the JahnTeller effect, polaron effects, etc.), structural phase transitions taking place by unusual scenarios (incommensurability, strong fluctuations), and nonlinear regimes of microwave energy absorption, which are comparatively easy to achieve here because of the long relaxation times of the elementary excitations [8,9].
PACSOne member of this family of compounds that has practically escaped study is KTb(WO 4 ) 2 , which contains the rare-earth ion Tb 3+ ( 7 F 6 ) with an odd number of electrons.X-ray studies of the magnet KTb(WO 4 ) 2 have shown that this compound belongs to the monoclinic class C 2 /c with a chain structure [10]. There are 4 formulae units per elementary cell with cell parameters: a = 10.653 Å, b = = 10.402 Å, c = 7.573 Å, β = 130.76°. Crystal structure is represented in Fig. 1. The Tb ions form chains along [101] direction, distance between nearest Tb 3+ is 4.071 Å.The angular dependence of the magnetic susceptibility [10] shows that at low temperatures a purely Ising aniso-