arXiv subjects

M. Nishino

Publications and source records attributed to M. Nishino.

3 recordsLinked to original sources

arXiv2016

Equilibrium, Metastability, and Hysteresis in a Model Spin-crossover Material with Nearest-neighbor Antiferromagnetic-like and Long-range Ferromagnetic-like Interactions

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical endpoints and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic Properties and Metastable States in Spin-Crossover Transition of Co-Fe Prussian Blue Analogues

The combination of spin transitions and magnetic ordering provides an interesting structure of phase transitions in Prussian blue analogues (PBAs). To understand the structure of stable and metastable states of Co-Fe PBA, it is necessary to clarify free energy as a function of magnetization and the fraction of the high-temperature component. Including the magnetic interaction between high-temperature states, we study the magnetic phase transition of Co-Fe PBA in addition to spin transitions. Here, we take into account the degeneracy changes due to charge transfer between Co and Fe atoms accompanying the spin transition. In this study, the charge transfer between Co and Fe atoms is explicitly taken into account and also the ferrimagnetic structure of Co-Fe PBAs is expressed in the proper way. First, we found systematic changes in the structures of stable and metastable states as functions of system parameters using mean field theory. In particular, the existence of a metastable magnetic-ordered high-temperature state is confirmed at temperatures lower than that of the hysteresis region of spin transitions. Second, we found that the magnetic interaction causes complex ordering processes of a spin transition and a magnetic phase transition. The effect of a magnetic field on the phase structure is also investigated and we found metamagnetic magnetization processes. Finally, the dynamical properties of this metastable state are studied by Monte Carlo method.

cond-mat.mtrl-sci↗

arXiv2000

Local magnetic structures induced by inhomogeneities of the lattice in S=1/2 bond-alternating chains and response to time-dependent magnetic field with a random noise

We study effect of inhomogeneities of the lattice in the S=1/2 bond-alternating chain by using a quantum Monte Carlo method and an exact diagonalization method. We adopt a defect in the alternating order as the inhomogeneity and we call it `bond impurity'. Local magnetic structures induced by the bond impurities are investigated both in the ground state and at very low temperatures. The local magnetic structure can be looked on as an effective S=1/2 spin and the weakness of the interaction between the local structures causes the quasi-degenerate states in the low energy. We also investigate the force acting between bond impurities and find that the force is generally attractive. We also study the dynamical property of the local magnetic structure. While the local magnetic structure behaves as an isolated S=1/2 spin in the response to a time-dependent uniform field, it is found to be robust against the effect of a random noise applied at each site individually in the sweeping filed.

cond-mat.mtrl-sci↗