I. Harada

Field-induced long-range ordering in an S = 1 quasi-one-dimensional Heisenberg antiferromagnet

We have measured the heat capacity and magnetization of the spin one one-dimensional Heisenberg antiferromagnet NDMAP and constructed a magnetic field versus temperature phase diagram. We found a field induced long-range magnetic ordering. We have been successful in explaining the phase diagram theoretically.

High-magnetic-field X-ray absorption spectroscopy of field-induced valence transition in EuNi2(Si1-xGex)2

The magnetic-field-induced valence transition in EuNi 2 (Si 1- x Ge x ) 2 ( x =0.82, 0.85) has been studied by X-ray absorption spectroscopy at low temperatures. The field dependence of the Eu valence is directly observed for the first time. We find a significant decrease in the valences at high magnetic fields above 40 T. The valence of Eu changes from v * =2.75 at a zero field to v * =2.33 at 40 T for x =0.82. The results are analyzed using a theoretical model. The hybridization parameter V between the Eu 2+ and Eu 3+ states and the energy separation Δ E are determined, which are crucial for quantum mechanical mixing.

Magnetic circular X-ray dichroism at Fe K-edge in ferrimagnetic Fe-oxides☆

Abstract Magnetic circular X-ray dichroism at the Fe K-edge has been recorded from ferrimagnetic Fe-oxides. The characteristic pre-peak structure is specified to the 1s → 3d transitions relating to the Fe 3+ ion at tetrahedral sites. Main absorption depends on local symmetry around the Fe ion lying at octahedral sites.

Theory of magnetic circular dichroism in Ce L2,3-edges of mixed valence compound CeFe2

Abstract Theoretical analysis of the magnetic circular dichroism at Ce L 2,3 edges in X-ray absorption spectra (XAS) of mixed valence compound CeFe 2 is presented. Impurity Anderson model is used to describe the valence mixing between Ce 4f orbital and Fe 3d band with LCAO approach to describe the band nature of the Fe 3d and Ce 5d states. Our results reproduce well the two-peak structure observed both in XAS and MCD experiments, especially their energy positions and spectral widths. Anomalously small magnetic moment is consistent with our model.

Rigorous determination of the ground-state phases for a generalized ANNNI chain with alternating spins

Abstract We determine exactly the possible ground-state configurations for a generalized axial-next-nearest-neighbor Ising (ANNNI) chain with alternating spins s and S in an external field, by extending the Morita-Horiguchi technique [Phys. Lett. A 38 (1972) 223]. The existence of noncompensating magnetic moments brings forth a ground-state phase diagram with full variety.

Magnetic Phases and Long-Range Ordering of the Haldane System in High Magnetic Fields

Critical properties of field-induced magnetic phases are investigated in the one-dimensional S = 1 Haldane system with planar anisotropy, using the finite-temperature density-matrix renormalization group method. We point out that the underlying physics of the properties differs significantly between cases of different field directions: The magnetic phase is characterized as the gapless Tomonaga-Luttinger regime in one case and as the gapful soliton regime in the other. In connection with the information obtained for the one-dimensional spin chain, possible implications for the field-induced magnetic long-range ordering are discussed.

Static spin structure factor of CuGeO3 above spin-Peierls transition temperature

Abstract We study, by means of the quantum transfer matrix method, the finite-temperature behavior of the S =1/2 antiferromagnetic chain with nearest neighbor and competing next nearest neighbor interactions, which is a model for the spin-Peierls material, CuGeO 3 . In particular, the static spin structure factor is found to display an interesting phenomenon as a consequence of the competition: a characteristic wave number for the maximum value of the structure factor takes a commensurate or an incommensurate value, depending on temperature. The boundary, called a Lifshitz temperature, can be found in this material. We discuss such behavior in connection with the experimental result of the neutron scattering in CuGeO 3 above the spin-Peierls transition temperature.

Multi-frequency ESR studies on a Haldane magnet in a field-induced phase at ultra-low temperatures

We report the results of multi-frequency electron spin resonance (ESR) measurements on single crystals of Ni(C5H14N2)2N3(PF6) which is regarded as the one-dimensional Heisenberg antiferromagnet with spin one, namely the Haldane magnet, at very low temperatures down to about 100 mK. We observed the lowest resonance branch below about 500 mK for the field along the chain direction (H||c), which was observed previously only in an inelastic neutron scattering experiment at 30 mK. We compare the resonance branch with that calculated by a phenomenological field theory, and discuss the field dependence and the temperature sensitivity of this ESR branch.

Field-induced long-range ordering in a quasi-one-dimensional Haldane system

Abstract Quantum S =1 spin chain with single-ion anisotropy in the presence of an external magnetic field is investigated by means of the finite-temperature density-matrix renormalization group. Emphasis is laid on the point that the underlying physics differs significantly with respect to magnetic-field directions. There appear various phases, which are absent in the classical counterpart studied extensively in the past. We argue possible implications of our numerical results in the field-induced magnetic long-range ordering observed experimentally in NDMAP.

Dynamic properties of one-dimensional S = 1/2 Heisenberg antiferromagnets with bond-alternation and competition

Abstract We have studied the dynamic spin structure factor as well as the static one in the ground state of the one-dimensional S=1/2 Heisenberg antiferromagnet with alternating nearest neighbor and competing next nearest neighbor couplings, under a uniform magnetic field. They show an anomalous behavior at a wavevector characterized by the magnetization. We have discussed further the effect of the spin–phonon couplings on the dynamic spin structure factor, in order to demonstrate the difference from what are obtained for the simple dimerized Hamiltonian.