K. Takeda

Bulk ferro- and antiferro-magnetic behavior in 1,5-dimethyl verdazyl radical crystals with similar molecular structure

Abstract The organic radical crystal, 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV), has been revealed to be a new and bulk ferromagnet with the second highest Curie temperature T C = (0.67 ± 0.02) K of genuine organic ferromagnets by the simultaneous measurements of heat capacity and magnetic susceptibility. On the other hand, 3-(4-bromophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-BDTV) behaves as a one-dimensional antiferromagnet with an intermolecular interaction of 2J k B = −41.2 K .

Magnetism of one-dimensional copper oxides related to HTSC

Abstract Magnetic and electronic properties of two kinds of copper oxides with the linear chains linked by planar CuO 4 , Ca 2 CuO 3 and Li 2 CuO 2 , were investigated by magnetic susceptibility (χ), specific heat ( C p ) and XPS measurements. Ca 2 CuO 3 has scarcely any magnetic contribution to χ and C p in striking contrast with Li 2 CuO 2 which shows 3D-AF magnetic ordering at 9.3 K. They are discussed on the linear chain model.

Thermal and magnetic properties of M(HCOO)2·2urea: M=Mn, Fe, Co, Ni

Abstract Properties of newly synthesized quadratic AF are summarized. T N =3.77 K (Mn), 7.9 K (Fe), 6.34 K (Co), 15.0 K (Ni). Their usefulness comes from their moderate magnitude of exchange and is exemplified with heat capacity and magnetization measurements.

Pressure induced enhancement of ordering temperature of a ferromagnetic organic radical crystal: p-CDTV

The magnetic susceptibility of 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) has been measured down to 0.2 K under hydrostatic pressures. The ordering temperature ofTc=0.65 K at the ambient pressure is found to increase under pressure up toTc=0.80 K at 6.0 kbar, giving the demonstrative temperature independent susceptibility of a ferromagnetic state at low temperatures. This pressure dependence is the opposite direction to that observered in the other organic ferromagnet β-phase p-NPNN, and is interpreted in terms of tne virtual charge transfer mechanism rather than pressure effects on the dipolar interactions.

Pressure-induced reduction of the Curie temperature of the organic ferromagnet p-NPNN

Abstract Under hydrostatic pressure, a drastic reduction of the Curie temperature T C ( p ) of β-phase p-NPNN, a genuine organic ferromagnet, has been found; from T C ( p 0 ) = 0.61 K ( p = p 0 = 0.001 kbar) to T C ( p ) = 0.35 K ( p = 7.2 kbar), for example. The results are discussed in comparison with the positive effect of pressure on the Neel temperatures of organic antiferromagnets.

Prototype one-dimensional Heisenberg ferromagnetic interaction in the organic radical crystal

Abstract An experimental check of quantum statistics for the most isotropic Heisenberg ferromagnet in one dimension has been carried out with a genuine organic 1,5 dimethylverdazyl radical crystal. A prototype Heisenberg ferromagnetic behavior, including magnetic susceptibility with the critical index γ = 2.0 and field dependence of heat capacity, is observed.

Simultaneous observation of heat capacity and magnetic susceptibility of a genuine organic ferromagnet under high pressures

The magnetic interactions in β-phase p-NPNN (p-nitrophenyl nitronyl nitroxide), a prototype genuine organic ferromagnet, have been studied by simultaneous measurements of magnetic heat capacity and susceptibility under hydrostatic pressures. A drastic reduction of the Curie temperature from 0.6 K at ambient pressure to 0.35 K at 7.2 kbar has been observed. This makes a significant contrast to the pressure-induced enhancement of the Neel temperature of organic antiferromagnets. A lowering of the magnetic lattice dimensionality is also observed at temperatures slightly above the Curie temperature under pressure. These observations are interpreted in terms of the virtual charge transfer mechanism.

Thermodynamical study of the magnetic transition in commensurate phases in PrCo2Si2

Abstract In the magnetically commensurate phases in the axially anisotropic compounds PrCo 2 Si 2 , the paramagnetic ions with individual entropy k B ln 2 has been detected to be immersed regularly by the heat capacity measurement and the adiabatic magnetization experiment. The possibility of the phase-slipping of the sinusoidal exchange field has been suggested to be driven by the variation of temperature in zero field.

Heat capacities of helium in one- and three-dimensional channels at low temperatures

Heat capacities of Ne adsorbed in the one- and three-dimensional channels in the high-silica zeolites ZSM-23 and ZSM-5 have been observed in the temperature range from 2 K to 23 K for various concentrations of Ne. For lower concentrations of Ne, the low-temperature heat capacities for both the one-dimensional and the three-dimensional channels, both about 5.5 AA in diameter, are described by the Einstein model, which implies that Ne atoms are localized independently or construct clusters of minute size in the channels. However, for the higher concentration where channels are nearly filled with Ne atoms, the heat capacities of Ne reflect the difference between the topological channel structures of the two zeolites. In the case of three-dimensional channels, the heat capacity is revealed to be well reproduced by the Debye model for a continuous solid with the characteristic temperature Theta D=29 K, which is unexpectedly smaller than the values Theta 3D approximately=67 K for the bulk solid Ne and Theta 2D approximately=50 K for the two-dimensional solid Ne. This suggests the possibility of an unusual softening effect of phonons in solid of Ne in the three-dimensional network of channels. While the heat capacity of Ne in the one-dimensional channels cannot be understood in terms of the one-dimensional Debye model, it can be explained using the Einstein model. These results for the classical Ne particle are significant when compared with the thermal behaviour of He in the same zeolites, which reflects the characteristic quantum effect at low temperatures.

Thermodynamic properties of the Haldane and non-Haldane systems with the iso-structural compounds (CH3)4NM(NO2)3, M=Ni, Co

Abstract In a one-dimensional compound (CH 3 ) 4 NCo(NO 2 ) 3 , which has the same crystal structure as (CH 3 ) 4 NNi(NO 2 ) 3 , we have found that next-nearest neighbor exchange interactions exist to frustrate the nearest neighbor interactions. A little larger Haldane gap energy obtained in the Ni 2+ compound is suggested to be due to this kind of frustration.