LAUSR

We propose a new scheme for modifying conventional spin waves so as to precisely describe low-dimensional Heisenberg ferrimagnets at finite temperatures. What is called the modified spin-wave theory was initiated by Takahashi, who intended to calculate the low-temperature thermodynamics of low-dimensional Heisenberg ferromagnets, where Holstein–Primakoff bosons are constrained to keep the total uniform magnetization zero in a straightforward manner. If the concept of an ideal Bose gas with a fixed density is applied to antiferromagnets and ferrimagnets, the formulation is no longer trivial, having rich variety in the way how the conventional spin waves, especially those in ferrimagnets, are constrained and brought into interaction. Which magnetization should be kept zero, uniform, staggered, or both? One or more chemical potentials can be introduced so as to satisfy the relevant constraint condition either in diagonalizing the Hamiltonian or in minimizing the free energy, making the Bogoliubov transformat...