Near the Curie temperature T C = 30.3 К, the temperature dependences of the magnetization and heat capacity of a single-crystal ferromagnet PbMnBO 4 in the magnetic fields of 1, 3, 10 and 30 kOe are studied. In the strong magnetic fields, both the magnetic contribution to the specific heat and the nonlinearity of the field dependences of the magnetization are maintained up to the temperatures exceeding T C more than twice. It is assumed that in PbMnBO 4 the difference between T C , the paramagnetic Curie temperature  = 49 K and the broad temperature region above T C where the magnetic contribution to the specific heat is significant is due to the quasi-one-dimensional character of the magnetic structure of this ferromagnet. Using both the estimation of T C from the Ginzburg-Landau field theory and the  value, the total exchange interaction parameters 2J  40.4 K (intrachain) and z'J'  8.8 K (interchain) are determined, with z' = 4 being the number of neighboring chains. The estimation shows that the Ginzburg-Landau field theory describing the quasi-one-dimensional behavior of PbMnBO 4 is well applicable in the temperature range from to T = S 2 J  80 K. Above this temperature, the mean field approximation with the exchange parameter  θ based on the paramagnetic Curie temperature  describes well the experimental temperature dependences of the magnetization in the strong magnetic field and the specific heat is determined by the lattice contribution. 1. Introduction. The search for and synthesis of new magnetic materials, as well as the investigation of their physical properties, are the priority tasks in solid-state physics. Such studies allow one to find promising materials for technical applications, and also provide experimental data for studying new physical effects arising at the intersection of magnetic, electrical, elastic, and other properties of solid state physics. The crystal classes which admit the existence of a family of isomorphous magnets with different magnetic ions in one of the crystallographic positions attract particular interest. This property can lead to a variety of physical properties, in particular, to different magnetic structures in the family of isomorphous crystals. Orthoborate crystals with the general formula PbABO 4 are representatives of such an interesting family. Their structure for A = Ga, Al was first studied by H. Park et al. [1] where the orthorhombic structure with the space group Pnma was established. Later the same group of authors investigated the magnetic properties of polycrystalline compounds with A = Fe, Cr, and Mn [2] and found that the type of magnetic order depends on the paramagnetic ion A. The antiferromagnetic order was established for Fe and Cr with the Néel temperatures of 125 K and 8.3 K, respectively. The compound with A = Mn showed the ferromagnetic order with the Curie temperature T C = 31 K.

We have investigated the spin-wave spectrum of copper metaborate, CuB 2 O 4 , by means of inelastic neutron scattering in the commensurate magnetic phase. We have found two branches of spin-wave excitations associated with the two magnetic sublattices Cu(A) and Cu(B), respectively. In the temperature regime 10K ≤ T ≤ 21K, where only the Cu(A) magnetic moments are ordered, the interaction between the two sublattices is found to be negligible. With this approximation we have determined the 'easy plane' exchange parameters of the Cu(A) subsystem within standard spin-wave theory.

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