Peculiarities of a magnetic transition in a quasi-one-dimensional ferromagnet PbMnBO4

Near the Curie temperature T_C = 30.3 K, the temperature dependences of the magnetization and heat capacity of a single-crystal ferromagnet PbMnBO₄ 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 PbMnBO₄ the difference between T_C, the paramagnetic Curie temperature θ = 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 θ 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 PbMnBO₄ is well applicable in the temperature range from to T = S²J ≈ 80 K. Above this temperature, the mean field approximation with the exchange parameter λ_θ based on the paramagnetic Curie temperature θ describes well the experimental temperature dependences of the magnetization in the strong magnetic field and the specific heat is determined by the lattice contribution.