The magnetization process of the one-dimensional antiferromagnetic Heisenberg model with the Ising-like anisotropic exchange interaction is studied by the exact diagonalization technique. It results in the evidence of the first-order spin-flop transition with a finite magnetization jump in the N\'eel ordered phase for
S> 1. It implies that the
S=21 chain is an exceptional case where the metamagnetic transition becomes second order due to large quantum fluctuations.
We generalize a class of Heisenberg antiferromagnets in one, two, and three dimensions, which have been shown to exhibit magnetization plateaus for spin-
21. In a certain parameter range of the general model, which is formally defined in
D dimensions, we obtain the exact ground state(s) in the presence of an external magnetic field for arbitrary values of spin. In this range, the magnetization remains a constant as a function of the external field, except at some special values of the field where there is a jump from one plateau to the next. The plateaus are formed at certain specific fractions of the full magnetization which are determined by the spin and the lattice. Our general spin-
S result reproduces the known cases for spin-
21 in various lattices. Furthermore, we argue that outside the exact regime, the mechanism for the plateau formation is different. This results in first order phase transitions along some of the plateaus as the coupling constant is varied. We rigorously show the existence of such transitions for some particular cases. Finally, we numerically analyze a spin-
1 model in one dimension using exact diagonalization to obtain its complete phase diagram. It agrees with our analytic results.
The ground state phase diagram of the alternating spin-1/2 chains with anisotropic ferromagnetic coupling under the influence of a symmetry breaking transverse magnetic field is studied. We have used the exact diagonalization technique. In the limit where the antiferromagnetic coupling is dominant, we have identified two Ising-type quantum phase transitions. We have calculated two critical fields
hc1 and
hc2, corresponding to the transition between different magnetic phases of the system. It is found that the intermediate state ($h_{c_{1}}
The spin-1/2 Heisenberg antiferromagnet on the frustrated diamond-decorated square lattice is known to feature various zero-field ground-state phases, consisting of extended monomer-dimer and dimer-tetramer ground states as well as a ferrimagnetic regime. Using a combination of analytical arguments, density matrix renormalization group (DMRG), exact diagonalization as well as sign-problem-free quantum Monte Carlo (QMC) calculations, we investigate the properties of this system and the related Lieb lattice in the presence of a finite magnetic field, addressing both the ground-state phase diagram as well as several thermodynamic properties. In addition to the zero-field ground states, we find at high magnetic field a spin-canted phase with a continuously rising magnetization for increasing magnetic field strength as well as the fully polarized paramagnetic phase. At intermediate field strength, we identify a first-order quantum phase transition line between the ferrimagnetic and the monomer-dimer regime. This first-order line extends to finite temperatures, terminating in a line of critical points that belong to the universality class of the two-dimensional Ising model.
The magnetization process of the S = 1/2 spin ladder system with ferromagnetic rung exchange interaction is investigated by the numerical exact diagonalization of finite-size clusters. It is found that in the presence of an easy-axis anisotropy the system exhibits a two-step field induced phase transition corresponding to the spin flop. Due to large quantum fluctuation, each step is a second-order transition and the second step occurs between two different Tomonaga-Luttinger liquid phases. Some phase diagrams are also presented.
The field-induced long-range antiferromagnetic order in the quasi-one-dimensional S=1 antiferromagnet is investigated with the exact diagonalization of finite chains and the mean field approximation for the interchain interaction. In the presence of the single-ion anisotropy D, we present the phase diagrams in the plane of the magnetic field H and the temperature T for H∥D and H⊥D. It is found that the two boundary curves of the ordered phase for H∥D and H⊥D have two intersections in the case of D>0 (XY like), while no intersection in the case of D<0 (Ising like).