H. Kikuchi

Single crystal susceptibility of the S=1 one-dimensional Heisenberg antiferromagnet AgVP2S6

Abstract A reliable single crystal susceptibility data is reported to reveal the ground state property of the S = 1 one-dimensional Heisenberg antiferromagnet AgVP2S6. The observed susceptibilities for the parallel and perpendicular direction to the chain axis fall down rapidly as the temperature is lowered down to about 50 K, revealing a non-magnetic singlet ground state nature of the system. The results are interpreted quantitatively within the framework of the Haldane prediction, giving the quantum gap of E g k B ≈ 250 K which should be compared with 320 K from neutron scattering.

Magnetic ordering in triangulated kagomélattice compound, Cu9Cl2(cpa)6 · nH2O

Abstract A triangulated kagomelattice compound Cu9Cl2(cpa)6 · nH2O was found to show an anomalous magnetic properties due to strong geometrical frustration. In spite of a strong antiferromagnetic interaction among S = ½ Cu2+ moments deduced from magnetic susceptibility at high temperatures and the g-factor measured by ESR, no long-range magnetic order was detected down to 2 K. The Curie constant decreases remarkably at low temperatures and the magnetization in an external magnetic field of 38 T amounts to only 0.3 μB. These facts suggest strongly the reduction in the number of magnetic moments at low temperatures due to the local formation of singlet state.

Submillimeter wave ESR of triangular-kagome antiferromagnet Cu9X2(cpa)6 (X=Cl, Br)

Abstract Cu9X2(cpa)6 which has two-dimensional triangles-in-triangles kagome lattice magnetic structure is considered as frustrated spin system. High field ESR of Cu9X2(cpa)6 powder have been performed in the submillimeter wave region at temperatures from 4.2 to 50xa0K. Both X=Cl and Br samples showed constant resonance field and the broadening of line width as the temperature was decreased. Observed resonances were nearly isotropic in contrast with the ESR of usual Cu2+ substances with anisotropic g-value.

High-field ESR measurements of a bond alternating Heisenberg chain system Ni2(μ-N3)3(dpt)2(ClO4)

Abstract High-field electron spin resonance has been measured on a Ni 2 (μ-N 3 ) 3 (dpt) 2 (ClO 4 ) single crystal, which is an S =1 Heisenberg antiferromagnet with bond alternation, in the frequency region from 50 to 370xa0GHz using pulsed magnetic fields up to 16xa0T. The g -values for H perpendicular and parallel to the single-crystal plane were determined to be 2.19 and 2.13, respectively. By analysing our results on frequency dependence at 20xa0K by the simple dimer model considering the singlet and triplet states, we were able to determine the single-ion anisotropy of the system as | D |=10.6xa0K and | E |=3.1xa0K for the first time. The spin gap was estimated to be 93xa0K from the analysis of the temperature dependence of the integrated intensity. These results will be discussed in connection with other S =1 Heisenberg antiferromagnets with bond alternation.

Submillimeter wave ESR of S = 1 antiferromagnetic Heisenberg chain system with bond alternation

Abstract Submillimeter wave ESR of Ni 2 (μ-C 2 O 4 )(μ-N 3 )(Medpt) 2 (ClO 4 ) has been performed. A broad resonance has been observed above 10 K at 300 GHz for H ⊥ chain. We also have measured frequency dependence of the resonance. The energy gap and the anisotropy term DS 2 z of this system is discussed from our results.

Observation of direct transition in quantum spin system Cu2(C5N12N2)2Cl4 by high-field ESR

Cu 2 (C 5 H 12 N 2 ) 2 Cl 4 (abbreviated as CHpC) is a new spin-gap system with a gap of about 10 K, which is proposed to be an S = 1/2 ladder system. We performed the high-field ESR measurements down to 0.5 K using the 3 He cryostat and the Vector Network Analyzer System in IMR. The measurements have been performed in V and W bands using the superconducting magnet up to 14 T, and we have observed the direct transition of CHpC below 0.9 K for the first time. Our results are discussed in connection with our previous high-field ESR results above 1.8 K using the pulsed magnetic field.

Submillimeter-wave ESR measurements of low-dimensional quantum spin systems

Temperature-dependent electron spin resonance (ESR) measurements of two low-dimensional quantum spin systems, theS = 1/2 spin-ladder system Cu2(1,4-diazacycloheptane)2Cl4 (Cu2(C5H12N2)2Cl4) and the gaplessS = 1 bond-alternating one-dimensional antiferromagnettrans-Ni(333-tet)(N3)(ClO4), have been performed. X-band electron spin resonance (ESR) measurements of single-crystal Cu2(C5H12N2)2Cl4 show the increase of linewidth and anisotropicg-shifts below 8 K similar to those known for one-dimensional antiferromagnet. On the other hand, Ni(333-tet)(N3)(ClO4) has broad line-width and ESR has been observed for the first time by our high-field ESR. Its linewidth increases as the temperature is decreased, while theg-shift seems to be isotropic and theg-value decreases as the temperature is decreased. Thisg-shift can be connected to the quantum fluctuation of the system.

High-field magnetization measurement of (YxNd1-x)2BaNiO5

Abstract We performed the magnetization measurements of (Y x Nd 1- x ) 2 BaNiO 5 at 4.2 K in high magnetic fields up to 30 T. The cross-over of the magnetic properties from one-dimensional to three-dimensional antiferromagnetism was observed by replacing the Y ions by Nd ions. Two magnetic phase transitions were found in Nd 2 BaNiO 5 .

Submillimeter wave ESR measurements of gapless S=1 antiferromagnetic bond alternating chain system

Abstract Submillimeter wave ESR measurements have been performed on gapless S =1 antiferromagnetic bond alternating chain system Ni(333−tet)( μ −N 3 )(ClO 4 ) using the pulsed magnetic field up to 16 T and the resonance has been observed in the temperature region from 4.2 to 260 K for the first time. The g -shift and the broadening of ESR are observed below 113 K, which corresponds to the maximum of the magnetic susceptibility. These results suggest the importance of the quantum fluctuation in the system at low temperature.

High-field ESR measurements on CsCoCl3

Abstract High-field ESR measurements on the quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3 have been performed in the frequency region from 526 to 1840xa0GHz in high pulsed magnetic fields up to 55xa0T. A number of ESR signals were observed in the antiferromagnetic phase. Moreover, for the first time new ESR signals were observed in the high-field phase above B c =33xa0T. The observed ESR signals are discussed in connection with previous high-frequency ESR, Raman-scattering and high-field magnetization measurements