Yoshitami Ajiro

Spin-Singlet Ground State in Two-Dimensional S=1/2 Frustrated Square Lattice: (CuCl)LaNb2O7

We report on the magnetic properties of double-layered perovskite (CuCl)LaNb 2 O 7 with a square lattice of S =1/2 Cu 2+ ions, prepared by topotactic ion-exchange reactions. Magnetic susceptibility exhibits a thermal-activated behavior, and the analysis of the data suggests competing ferro- and antiferromagnetic exchanges for first- and second-nearest-neighbor bonds, respectively. Neutron scattering experiments provide explicit evidence for a spin-singlet ground state with an energy gap of 2.3 meV. The well-defined low-lying triplet mode is almost independent of Q despite densely packed magnetic ions in the CuCl plane. In addition, on the basis of the isolated dimer model, we found that the intradimer distance is as long as 8.8 A, which corresponds to the fourth-nearest-neighbor bond. These results indicate a sizable geometrical frustration that should impose severe constraints on the propagation of the triplet excitation, as in the case of SrCu 2 (BO 3 ) 2 . This argument is further reinforced by the obs...

New Critical Exponent β of the XY Antiferromagnet on Stacked Triangular Lattice, CsMnBr3

The phase transition of the XY antiferromagnet on a stacked triangular lattice, CsMnBr 3 is studied by neutron diffraction. The critical exponent β is found to be β=0.25±0.01. The value is in excellent agreement with the predicted value by Kawamura for a new universality class, giving convincing experimental support to the phase transition characterized by the Z 2 ×S 1 or O(2) symmetry.

New universality class of antiferromagnetic phase transition in CsMnBr3

The critical exponents γ and ν of an XY antiferromagnet on a layered-triangular lattice, CsMnBr 3 , have been measured by neutron scattering. They are γ=1.10±0.05 and ν=0.57±0.03 in the reduced temperature range 0.015< e < 0.1 above the Neel temperature. These and the previously measured β show that this phase transition belongs to the universality class characterized by the O(2) symmetry order-parameter in three dimensions, as calculated by Kawamura.

Temperature Dependence of the ESR Line Width in One-Dimensional S=1/2 Antiferromagnet CuCl2·2NC5H5

The ESR line width of a typical 1d H S=1/2 AF CuCl 2 ·2NC 5 H 5 has been measured between 1.3 and 300 K. As temperature is lowered the width anomalously decreases down to 5 K with an approximated relation Δ H ∝χ T and it turns to increase with a relation Δ H ∝ T -1.2 . It is suggested that the unusual behaviors can be understood by considering the dominant role of the q =0 fluctuations at higher temperatures and also of the q = K 0 fluctuations at lower temperatures in the spin dynamics for S=1/2 AF chains. The critical broadening for 3d ordering will appear at temperatures much nearer T N .

Magnetic Phase Transition of Antiferromagnet with Weak Canting Interaction

The magnetic critical phenomena of canted antiferromagnet in two dimensional lattice; Mn(HCOO) 2 ·2H 2 O were studied by the measurement of the dependences of AC magnetic susceptibility on amplitude and frequency of measuring AC field. The induced uniform susceptibility χ i u in the immediate neighborhood of T N could be successfully derived by analyzing the results. The critical divergence of χ i u at T N found to be well expressed as | T - T N | -γ and γ=1.7±0.1. The value of γ is discussed in the view point of magnetic ordering in two dimensional lattice. The magnetic dispersion of χ i u was observed in very low frequency region. It is probably the first observation of critical dispersion of magnetic susceptibility near T N . The result is discussed from the phenomenological point of view for canted antiferromagnet and it is suggested that it directly reflects the critical slowing down of antiferromagnetic staggered mode through weak canting interaction of this salt.

Z2 Vortex-Induced Broadening of the EPR Linewidth in the Two-Dimensional Triangular Lattice Antiferromagnets, HCrO2 and LiCrO2

The title compounds are the family of the AMO 2 -type (R\bar3m) crystal which has a pronounced layer structure and are promising candidates for the two-dimensional Heisenberg triangular lattice antiferromagnet (2DH TAL-AF). The interesting observation is that a singular point of the EPR signal is located around T KM =0.66 J S 2 for the critical temperature of phase transition associated with the pairing-dissociation of the Z 2 vortices, suggesting the present system behaves as 2DH TAL-AF down to very close to T KM . The EPR linewidth exhibits Z 2 vortex-induced broadening above the critical temperature which is inversely proportional to the thermally excited vortex density, n v ∝exp (- E v / k B T ) with the activation energy \(E_{\text{v}}{\cong}4k_{\text{B}}T_{\text{KM}}\).

Crystallographic Two Sublattice System –Staggered Susceptibility and Induced Staggered Magnetization in Paramagnetic State–

Magnetic basic properties of the crystallographic two sublattice (C 2 ) system in which magnetic ions at these lattice points have inequivalent g -tensors, are rigorously studied on the basis of the linear response theory. Such a C, system under a uniform field is quite equivalent to a fictitious single lattice (C 1 ) system under both a uniform and a staggered fields. So in such a C 2 system, the staggered magnetization as well as the uniform magnetization is induced in the paramagnetic state by applying a uniform field. On these facts, a possibility to derive the staggered susceptibility from the experimentally observed uniform and staggered magnetizations was examined based on the symmetry of zero field Hamiltonian of the system. Application of the result to the existing one and two dimensional Heisenberg antiferromagnets is discussed with some promising examples.

High Field Magnetization of Quasi One Dimensional Ising Antiferromagnet CsCoCl3

Magnetization of quasi one dimensional Ising antiferromagnet CsCoCl 3 is measured at liquid 4 He temperatures in pulsed high magnetic fields. In the case of an applied field H parallel to the Ising c -axis, the magnetization M at liq. 4 He temperatures shows a metamagnetic transition which is characterized by the sharp double peaks of the field derivative of the magnetization d M /d H at 33.0 T and 43.9 T, respectively. The results are qualitatively explained by the Ising-like linear chain model with small antiferromagnetic inter-chain exchange interaction. The role of the inter-chain interaction is discussed within the framework of molecular field approximation. The temperature dependence and the transverse case of the magnetization process are also studied.

Magnetization Process of the Triangular Lattice Antiferromagnets, RbFe(MoO4)2 and CsFe(SO4)2

High-field magnetization of two hexagonal layered compounds RbFe(MoO 4 ) 2 and CsFe(SO 4 ) 2 is measured. In the magnetization curves of both compounds, a plateau-like structure with one-third of the saturated moment is observed. This anomaly is analyzed in terms of the magnetization process of a two-dimensional triangular lattice Heisenberg antiferromagnet with weak Ising-type anisotropy.

High-field magnetization process in the triangular lattice antiferromagnet CuFeO2 up to 100 T

Abstract High-field magnetization process in a rhombohedrally stacked triangular lattice antiferromagnet, CuFeO 2 , is studied on powder and field-oriented samples using pulse magnets up to 100 T. Magnetization anomalies are successively observed at 8.0, 13.0, 22.0, 26.0, 42 and 70 T. The complicated magnetization process originates from a subtle balance of interactions in the frustrated situation. The results are discussed from the energy considerations of classical Ising triangular lattice antiferromagnets with the interactions up to the third neighbor to estimate the relevant exchange parameters.