Makoto Isoda

Coexistent States of Ferro- and Antiferromagnetism in Uniaxially Anisotropic Itinerant Electron Systems

From the general considerations, the coexistence of ferro- and antiferromagnetism is discussed in uniaxially anisotropic itinerant electron systems. Especially in an easy-axis type anisotropic system, it is pointed out that the coexistent states and the phase transitions are significantly different from those in an isotropic system: (i) The following two canted states realize in addition to a ferrimagnetic state. The magnetization vectors cant symmetrically with respect to the easy axis in one state and to the hard plane in another state. (ii) Both ferrimagnetic and canted states can be realized successively in an anisotropic system. The conditions for the coexistence and the phase diagrams in χ Q -1 -χ 0 -1 and H - T planes are discussed.

Pressure-Induced Antiferromagnetism of Fe2P

The magnetization of Fe 2 P single crystal was measured as a function of magnetic field H , temperature T and hydrostatic pressure p , for 0≤ H ≤20 kOe, 4.2K≤ T ≤260 K and 0≤ p ≤14 kbar. It was found that the pressure-induced new magnetic phase below T =170 K and above p =5 kbar is the antiferromagnetic with one and unusual two steps metamagnetic behaviours. H - T magnetic phase diagrams were obtained at various pressures and they contains ferromagnetic, antiferromagnetic, intermediate and paramagnetic phases. In contrast with the drastic change in the Curie temperature with pressure, the magnetization at 4.2 K was pressure-insensitive. The pressure-induced magnetic transition and the antiferromagnetism of Fe 2 P are qualitatively discussed on the basis of the competition between the two different interactions and of the itinerant electron theory recently developed.

Magnetic Correlation and Breakdown of Fermi Liquid Picture in Frustrated Itinerant Electron Magnet.

Three-dimensional geometrically frustrated itinerant magnet is investigated based on the single band Hubbard model on the pyrochlore lattice. The magnetic correlation and the quasiparticle properties are discussed by the weak correlation approach for half-filled case, based on the single electron spectrum in consideration of the lattice geometry. The spectrum reveals the existence of doubly degenerate flat band over the whole Brillouin zone. The spin fluctuation spectrum in RPA is anisotropic and shows the independence on frequency. The result is globally consistent with the observed in Y(Sc)Mn 2 . It is also shown that the frequency spectrum is clearly different from the conventional Fermi liquid one, which brings the breakdown of the Fermi liquid picture owing to the divergence of quasiparticle mass.

Magnetization Process of the S = 1/2 Heisenberg Antiferromagnet on the Cairo Pentagon Lattice

We study the S = 1/2 Heisenberg antiferromagnet on the Cairo pentagon lattice by the numerical-diagonalization method. We tune the ratio of two antiferromagnetic interactions coming from two kinds of inequivalent sites in this lattice, examining the magnetization process of the antiferromagnet; particular attention is given to one-third of the height of the saturation. We find that quantum phase transition occurs at a specific ratio and that a magnetization plateau appears in the vicinity of the transition. The plateau is accompanied by a magnetization jump on one side among the edges due to the spin-flop phenomenon. Which side the jump appears depends on the ratio.

Valence-Bond Crystal and Anisotropic Excitation Spectrum on 3-Dimensionally Frustrated Pyrochlore

The valence-bond crystal is proposed as a ground state of spin-1/2 Heisenberg antiferromagnet on 3-dimensional fully frustrated pyrochlore by the bond-operator theory. This crystal, formed as a set of RVB-like tetrahedral singlet states, has lower energy than the state consisting of two dimers in a tetrahedron. The spectra of triplet excitation have a finite gap and a symmetry lower than that of the lattice. The comparison with Y(Sc)Mn 2 is discussed.

Magnetic Properties of Half-Metallic f-Electron Systems.

A microscopic model is proposed to describe magnetic properties of half-metallic f -electron systems. We explicitly take into account the energy gap in the conduction band and the intra-atomic exchange interaction between localized f -electrons and conduction eleictrons. Based on the model Hamiltonian, the temperature dependence of the magnetic properties are studied assuming the ferromagnetic ground state. The relative stability of ferromagnetic state against antiferromagnetic state is also discussed by perturbation expansion in terms of the hybridization between the conduction and the localized f -electron states. Some implications are given for UNiSn, which shows the transition from paramagnetic semiconductor to antiferromagnetic metal by lowering temperature.

Specific Heat and Magnetic Susceptibility of Ising-Like Anisotropic Heisenberg Model on Kagome Lattice

The specific heat and magnetic susceptibility of a spin-1/2 Ising-like anisotropic Heisenberg model (IAHM) on a kagome lattice are investigated by an exact diagonalization method for small clusters up to 18 spins with periodic boundary conditions. The enhancement of magnetic susceptibility from the Curie–Weiss law and the peak structure of specific heat at intermediate temperatures are attributed to a longitudinal (classical) spin fluctuation, and the steep decrease and the second peak at lower temperatures in those respective quantities to a transverse (quantum) spin fluctuation. It is verified that such anomalies may be produced by interruption by a semiclassical liquidlike state consisting of doubletlike triangular trimmers between a higher-temperature classical spin gaslike state and a lower-temperature quantum spin singlet state. The IAHM may give a general viewpoint for interpreting the experimentally observed thermodynamic properties of spin systems with various triangle-based lattices.

A Consistent Description of Magnetic Properties of the Triangulated-Kagome System Cu9X2(cpa)6·nH2O

A consistent description of the behaviors of both the magnetic susceptibility and of the 1/3-plateau like magnetization under a magnetic field in the triangulated-kagome compound Cu 9 X 2 (cpa) 6 · n H 2 O (X = F, Cl, Br; cpa = the anion of 2-carboxypentonic acid) is given by the Heisenberg model with the antiferromagnetic next-nearest-neighbor interaction, in addition to the two types of antiferromagnetic nearest-neighbor interactions in previous studies. This description is provided by the method of numerical diagonalization. The newly introduced interaction derives the quantum phase transition from the quantum-disordered-state to the ferrimagnetic one. The stability of the magnetization plateaus is complemented using the perturbation theory.

THERMODYNAMIC PROPERTIES OF SPIN-1/2 ISING-LIKE HEISENBERG MODEL ON TRIANGLE-BASED LATTICES

Thermodynamic properties, specific heat and magnetic susceptibility, of spin-1/2 Ising-like Heisenberg model are investigated by an exact diagonalization method for small finite size kagome- and triangular-lattices up to 18-spins. The enhancement of magnetic susceptibility from the Curie–Weiss law and the peaks-structure of specific heat, rather generally detected experimentally in triangle-based spin systems including herbertsmithite, are interpreted as an intrinsic property of triangle-based frustrated spin systems with some extent of exchange anisotropy and are inferred as owing to a quantum-classical crossover.

Numerical exact diagonalization study of triangulated kagome Heisenberg spin system

The thermodynamic properties and the magnetization under magnetic field at zero temperature of the spin-1/2 Heisenberg model on triangulated kagome lattice, which composed of two sublattices, are investigated by numerical exact diagonalization method for 18 and up to 27 spin clusters, respectively. The temperature dependence of magnetic susceptibility may be reproduced qualitatively with the weak ferromagnetic intersublattice nearest neighbor exchange coupling constant JAB as proposed by previous studies, although it is quantitatively insufficient in comparison with the experimental result. The magnetization under magnetic field shows the 5/9 plateau for every examined values of JAB, but 1/3 plateau, visible in the experiment, may be reproduced only for not so weak antiferromagnetic case of JAB. This discrepancy for JAB in those two qualities has also been recognized in previous studies on classical spin models and is not still resolved even for the present quantum spin system.