Munehisa Matsumoto

Magnetic properties of Ni 2.18 Mn 0.82 Ga Heusler alloys with a coupled magnetostructural transition

Polycrystalline Ni 2 . 1 8 Mn 0 . 8 2 Ga Heusler alloys with a coupled magnetostructural transition are studied by differential scanning calorimetry, magnetic and resistivity measurements. Coupling of the magnetic and structural subsystems results in unusual magnetic features of the alloy. These uncommon magnetic properties of Ni 2 . 1 8 Mn 0 . 8 2 Ga are attributed to the first-order structural transition from a tetragonal ferromagnetic to a cubic paramagnetic phase.

Ground-state phase diagram of quantum Heisenberg antiferromagnets on the anisotropic dimerized square lattice

The S = ½ and S= I two-dimensional quantum Heisenberg antiferromagnets on the anisotropic dimerized square lattice are investigated by the quantum Monte Carlo method. By finite-size-scaling analyses on the correlation lengths, the ground-state phase diagram parametrized by strengths of the dimerization and of the spatial anisotropy is determined much more accurately than the previous works. It is confirmed that the quantum critical phenomena on the phase boundaries belong to the same universality class as that of the classical three-dimensional Heisenberg model. Furthermore, for S= 1, we show that all the spin-gapped phases, such as the Haldane and dimer phases, are adiabatically connected in the extended-parameter space, though they are classified into different classes in terms of the string order parameter in the one-dimensional, i.e., the zero-interchain-coupling, case.

Bond-dilution effects on two-dimensional spin-gapped Heisenberg antiferromagnets

Abstract Bond-dilution effects on spin-1/2 spin-gapped Heisenberg antiferromagnets of coupled alternating chains on a square lattice are investigated by means of the quantum Monte Carlo method. It is found that, in contrast with the site-diluted system having an infinitesimal critical concentration, the bond-diluted system has a finite critical concentration of diluted bonds, xc, above which the system is in an antiferromagnetic (AF) long-range ordered phase. In the disordered phase below xc, plausibly in the concentration region significantly less than xc, the system has a spin-gap due to singlet pairs of induced magnetic moments reformed by the AF interactions through the two-dimensional shortest paths.

Site-dilution-induced antiferromagnetic long-range order in a two-dimensional spin-gapped Heisenberg antiferromagnet

Effects of the site dilution on spin-gapped Heisenberg antiferromagnets with

Plaquette-singlet solid state and topological hidden order in a spin-1 antiferromagnetic Heisenberg ladder

S=1/2

Influence of three-dimensional transition elements on magnetic and structural phase transitions of Ni-Mn-Ga alloys

and S=1 on a square lattice are investigated by means of the quantum Monte Carlo method. It is found that effective magnetic moments induced around the diluted sites exhibit the antiferromagnetic long-range order in the medium of spin-singlet pairs. Their microscopic structure is examined in detail and important roles of the higher dimensionality than one on the phenomenon are discussed.

Dilution Effects on Two-Dimensional Heisenberg Antiferromagnets with Non-Magnetic Spin-Gapped Ground State

Ground-state properties of the spin-1 two-leg antiferromagnetic ladder are investigated precisely by means of the quantum Monte Carlo method. It is found that the correlation length along the chains and the spin gap both remain finite regardless of the strength of interchain coupling, i.e., the Haldane state and the spin-1 dimer state are connected smoothly without any quantum phase transitions between them. We propose a plaquette-singlet solid state, which qualitatively describes the ground state of the spin-1 ladder quite well, and also a corresponding topological hidden order parameter. It is shown numerically that the new hidden order parameter remains finite up to the dimer limit, though the conventional string order defined on each chain vanishes immediately when infinitesimal interchain coupling is introduced.

Improvement of magnetic hardness at finite temperatures : Ab initio disordered local-moment approach for YCo5

Differential scanning calorimetry and magnetic measurements were performed to study the influence of the ferromagnetic three-dimensional transition elements Fe and Co on the structural and magnetic properties of the ferromagnetic shape memory alloys Ni-Mn-Ga. Substitution of Fe or Co for Ni decreases the temperature of martensitic phase transition Tm, whereas substitution of Co for Mn results in a considerable increase of Tm. Magnetic measurement revealed that Curie temperature TC increased upon substitution of Fe or Co for Ni. This may be useful for design of high-temperature ferromagnetic shape memory alloys.

Quantum phase transitions of spin chiral nanotubes

Dilution effects on spin-½ quantum Heisenberg antiferromagnets with a non-magnetic spin-gapped ground state are studied by means of the quantum Monte Carlo simulation. In the site-diluted system, an antiferromagnetic long-range order (AF-LRO) is induced at an infinitesimal concentration of dilution due to an effective coupling J m n between induced magnetic moments. In the bond-diluted case, on the other hand, the AF-LRO is not induced up to a certain concentration of dilution due to singlet pairs reformed by an AF coupling J a f between the edge spins of the diluted bond through the two-dimensional shortest paths. The competition between J m n and J a f yields peculiar phenomena in the bond-diluted system.

Investigation of phase transformations in Ni2MnGa using high magnetic field low-temperature X-ray diffraction system

Temperature dependence of the magnetocrystalline anisotropy energy and magnetization of the prototypical rare-earth magnet YCo5 is calculated from first principles, utilizing the relativistic disordered local-moment approach. We discuss a strategy to enhance the finite-temperature anisotropy field by hole doping, paving the way for an improvement of the coercivity near room temperature or higher.