Lanczos exact diagonalization study of field-induced phase transitions forIsing and Heisenberg antiferromagnets

Using an exact diagonalization treatment of Ising and Heisenberg model Hami ltonians, we study field-induced phase transitions for two-dimensional anti ferromagnets. It is found that a first-order phase transition occurs for bo th the Ising and Heisenberg antiferromagnets. For the Ising antiferromagnet a mixture of antiferromagnetic and ferromagnetic phases exists only at a c ritical value of Zeeman energy, while for the Heisenberg antiferromagnet th e mixed phase appears over a wide range of Zeeman energy. It is shown that the mixed phase far the Heisenberg antiferromagnet occurs owing to the pres ence of a spin-flop process as an intermediate step. [S0163-1829(99)13633-6 ].