K. Katsumata

Field-Induced Magnetic Long-Range Order in the Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chain System (CH3)2CHNH3CuCl3 Observed by Specific Heat Measurements.

The compound (CH 3 ) 2 CHNH 3 CuCl 3 has recently been found to be an ideal ferromagnetic-antiferromagnetic alternating Heisenberg chain with the energy gap Δ/ k =17∼18 K [H. Manaka et al. : J. Phys. Soc. Jpn. 66 (1997) 564]. When an external field H is applied to this compound, the energy gap should vanish at the critical field H c1 = Δ/ g µ B . Just above H c1 , the three-dimensional magnetic long-range order is expected, if the interchain coupling exists. Motivated by this expectation, we performed specific heat measurements over a temperature range from 0.5 to 5 K applying H up to 12 T. As a result, we found a λ-type anomaly of specific heat over 10∼12 T, which indicates a transition to the three-dimensional long range order. The temperature at which the λ-type anomaly appears increases with increasing H . The critical field obtained by extrapolating the curve of the λ peak position versus H is 9.0 T, which is close to that obtained from a magnetization measurement.

An Incommensurate Magnetic Diffuse Scattering in Superconducting La1.92Sr0.08CuO4-δ

By neutron quasielastic scattering experiments, we observed a two-dimensional antiferromagnetic diffuse scattering in a superconducting La 1.92 Sr 0.08 CuO 4-δ sample at room temperature as well as in a superconducting phase. The line shape of the magnetic diffuse scattering at 295 K is a conventional Lorentzian, whereas it changes to a non-Lorentzian shape at low temperatures, presumably being a double peak structure with the peaks shifted by 0.05 a * from the antiferromagnetic zone center.

Phase diagram of Fe(1−x)CoxCl2; a random mixture of two antiferromagnets with competing spin anisotropies

Abstract Concentration vs transition temperature phase diagram of a random mixture of two anisotropic antiferromagnets, FeCl 2 and CoCl 2 , is obtained from the measurement of the susceptibility on the single crystals. Two distinct critical points, one along the c -axis and the other in the c -plane, are observed for the respective concentrations between x = 0.264 and x = 0.481. Three kinds of ordered phases, namely, Fe-rich and Co-rich antiferromagnetic phases and a new phase are found. The phase diagram shows a tetracritical point as well. A hump is observed in the temperature dependence of the susceptibility along the c -axis ( c -plane for x ≲ 0.3) near the ordering temperature occuring in the c -plane ( c -axis for x ≲ 0.3).

Neutron Diffraction Experiment on a Randomly Mixed Antiferromagnet with Competing Spin Anisotropies

Neutron diffraction experiment has been done on a randomly mixed antiferromagnet with competing spin anisotropies, Fe (1- x ) Co x Cl 2 . Front the experiment the spin structures in the three ordered phases which appear in the concentration versus transition temperature phase diagram, namely, the Fe-rich and Co-rich antiferromagnetic phases and the new phase are determined. The spin structure in the Fe-rich (Co-rich) phase is essentially identical with that of pure FeCl 2 (CoCl 2 ). In the new phase, although overall spin structure is the same as those of the pure materials, both of the two spin components are observed which are associated with competing anisotropies. The new phase is an OAF phase in the sense that an inclination of the sublattice magnetizations both from the easy axes of the pure substances means the simultaneous ordering of the spin components. The temperature dependences, of the spin components are discussed in favour of the result of the renormalization group theory.

X-ray diffractometer combining synchrotron radiation and pulsed magnetic fields up to 40 T

A synchrotron X-ray diffractometer incorporating a pulsed field magnet for high fields up to 40 T has been developed and a detailed description of this instrument is reported. The pulsed field magnet is composed of two coaxial coils with a gap of 3 mm at the mid-plane for passage of the X-rays. The pixel detector PILATUS 100K is used to store the diffracted X-rays. As a test of this instrument, X-ray diffraction by a powder sample of the antiferromagnet CoO is measured below the Néel temperature. A field-dependent lattice distortion of CoO due to magnetostriction is observed up to 38 T.

Magnetization process of haldane materials TMNIN and NINAZ

Magnetization measurements have been performed on the Haldane gap materials (CH 3 ) 4 NNi(NO 2 ) 3 (TMNIN) and Ni(C 3 H 10 N 2 ) 2 N 3 (ClO 4 ) (NINAZ) in pulsed high magnetic fields. TMNIN shows a typical magnetization profile of the Haldane material with the gap energy of 4.1 K. Quenching of the gap appears around 2.7 Tesla(T) and magnetic saturation is obtained above 30 T. In NINAZ, however, the quenching appears at around 30 T reflecting a large Haldane gap energy with the magnitude of 44.3 K.

Magnetic excitations from the S=1/2 two-leg ladders in La6Ca8Cu24O41

We studied the magnetic excitations from the S=1/2 two-leg ladders in La6Ca8Cu24O41. In order to understand the full dispersion of the magnon branch for the lowest triplet excitation, effects of the four-spin and the next-nearest-neighbor (diagonal) exchange interactions are analyzed. We also compare the parameters obtained from the fit to the full dispersion curve with those obtained in the related compounds.

Experiments on randomly mixed magnets with competing interactions

Abstract After a brief review of the experimental and theoretical studies on randomly mixed magnets with competing interactions, we report on our experimental study of two kinds of randomly mixed magnets: (i) random mixture of two antiferromagnets with competing spin anisotropies and (ii) random mixture of an insulating ferromagnet and an insulating antiferromagnet.

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-frequency electron spin resonance in magnetic systems

We review the recent progress in the study of magnetic materials using a high-frequency electron spin-resonance (ESR) technique. First, we show how useful high-frequency ESR is for studying antiferromagnetic materials, where the ESR frequency and magnetic field depend greatly on the exchange interaction and anisotropy energy of the materials. Next, we review the recent high-frequency ESR experiments made on spin S = 1 quasi-one-dimensional Heisenberg antiferromagnets (Q1D HAFs) and the spin-Peierls system CuGeO3. Then, we review the ESR studies performed on more complex systems, such as an S = 1 Q1D HAF with bond alternation, spin-ladder compounds and quasi-two-dimensional magnets. Each of these systems has a singlet ground state of quantum origin and an energy gap to the lowest excited state. On applying an external magnetic field, these systems show a transition from the non-magnetic to a magnetized state, and in some cases, long-range magnetic ordering occurs. Efforts are made to explain the underlying physics intuitively at the expense of rigour.