Hiroyuki Deguchi

Magnetic Phase Transition in the Two-Dimensional Heisenberg Antiferromagnet Mn(HCOO)2·2(NH2)2CO

The development of the short-range ordering in Mn(HCOO) 2 ·2(NH 2 ) 2 CO has been observed by the measurements of magnetic heat capacity and susceptibility. The experimental results are well reproduced by the high-temperature series expansion for the two-dimensional Heisenberg antiferromagnet with the exchange constant J / k B =-0.34±0.02 K in the temperature region T >1.2 T SK ( T SK ; the Stanley-Kaplan temperature). The critical temperature T N =3.77±0.02 K≃1.1 T SK nearly coincides with that of Mn(HCOO) 2 ·2H 2 O. It is suggested that the long-range ordering is responsible for the small Ising-like anisotropy in the exchange interaction. A two-dimensional spin structure with the canceled Weiss field from the adjacent layers, as in K 2 NiF 4 , is conjectured.

The Numerical Comparison of Magnetic Susceptibility and Heat Capacity of TMNIN with the Result of a Quantum Monte Carlo Method for the Haldane System

The measurements of magnetic heat capacity and susceptibility of one-dimensional S =1 antiferromagnet (CH 3 ) 4 NNi(NO 2 ) 3 (TMNIN) have been carried out in order to make comparison with the theoretical results of a quantum Monte Carlo method for the Haldane system. The results for the heat capacity, which show a broad maximum around 10 K, are well reproduced by the theory with the interaction J / k B =-12.0±1.0 K in the temperature range T >0.2∣ J ∣ S ( S +1)/ k B . The low temperature heat capacity exhibits an exponential decay with gap energy Δ/ k B =5.3±0.2 K, which gives Δ=0.44∣ J ∣, in contrast to the linear dependence on temperature as in the case for half integer spin. The residual magnetic entropy below 0.7 K is estimated to be 0.07% of N k B ln 3, which denies the possibility of three-dimensional ordering of the spin system at lower temperatures. The observed susceptibility also agrees with the theory with J / k B =-10.9 K and g =2.02 in the whole temperature region, when we take the effect fro...

Magnetic Behavior of Organic Free Radicals with Localized and Delocalized Electrons

Magnetic and thermal properties have been studied on two kinds of organic free radicals, TANOL(2,2,6,6-tetramethyl-4-piperidinol-1-oxyl) and TPV(1,3,5-triphenylverdazyl), where the unpaired electrons are localized on the NO bond in the former compound but delocalized on the larger molecular orbitals in the latter. TANOL has been confirmed magnetically as a typical one-dimensional Heisenberg system with localized quantum spins coupled with the exchange constant J / k B =-5.2 K. On the other hand, the experimental results on TPV can not reasonably be understood on the basis of a localized model with the possible paths of magnetic interactions such as a railroad trestle lattice. It is suggested that organic free radicals offer promising real systems for getting essential physical phenomena which can check model Hamiltonians for a delocalized and frustrated quantum system.

Magnetic Susceptibility of a Spin 1/2 Heisenberg Antiferromagnetic Chain: [3,3′-Dimethyl-2,2′-Thiazolinocyanine]-TCNQ

Magnetic susceptibility of an organic ion-radical salt, [3,3 ′ -dimethyl-2,2 ′ -thiazolinocyanine]-TCNQ, is measured in the temperature range of 1.4–300 K. Above 10 K, it is quantitatively explained in terms of a S =1/2 Heisenberg antiferromagnetic chain with J / k B =-32.5 K based on the Bonner-Fisher curve. Below 10 K, however, the measured susceptibility seems to show the characteristic slope reported by Eggert et al. down to the ordering temperature T c =1.49 K. Below T c the heat capacity of this salt displays the temperature dependence expected for a gapless spin-wave excitation, which rules out the spin-Peierls transition in this system.

Magnetic Phase Diagram of a Random Mixture with Competing Interactions: Co1-xMnxCl2·2H2O

Magnetic and thermal properties of Co 1- x Mn x Cl 2 ·2H 2 O with competing exchange interactions have been studied experimentally, For the system with x ∼0.5, the magnetic heat capacity have revealed only a broad maximum around 5 K, which is quite different from the heat capacity peak of λ-type for the pure systems and similar to that observed in metallic spin-glasses. The hysteresis in the magnetic susceptibility and thermoremanent magnetization have been observed in the mixtures with x ≤0.5, reflecting the frustration of magnetic interactions, while normal antiferromagnetic behavior has been seen in the region x >0.5. The magnetic phase diagram of this system is given, which contains four states: paramagnetic, antiferromagnetic, frustrated antiferromagnetic and anisotropic spin-glass states.

Magnetic phase transition in two-dimensional antiferromagnets M(HCOO)2 · 2(NH2)2CO (M: Mn, Co, Ni)

The magnetic phase transition in the quasi-two-dimensional system M(HCOO) 2 ·2(NH 2 ) 2 CO (M: Mn, Co, Ni) has been studied taking the crossover effect of spin dimensionality of lattice dimensionality into consideration. The magnetization of the Ni 2+ salt decreases anomalously as the temperature decreases, which suggests additional antisymmetric or biquadratic exchange interaction.

Magnetic phase diagram of DyNi2Si2 with two-dimensionally modulated spin structures

The fully mapped T - H (temperature-magnetic field) phase diagram of DyNi 2 Si 2 has been determined by heat capacity and magnetic susceptibility measurements. In zero magnetic fields, the heat capacity exhibits a small but distinct cusp at T 1 =3.7 K corresponding to the magnetic order-order phase transition, and a clear λ-type transition at T N =5.6 K. Except for the paramagnetic phase, three magnetic phases are found below T N and below the critical field H c =21.5 kOe. The results are discussed in comparison with recent experimental evidence which indicates the two-dimensionally modulated spin structures in the c -plane.

Proton NMR Study of Random Mixture Fe1-xMnxCl2 2H2O with Competing Anisotropies and Exchange Interactions.

Proton NMR measurements of Fe 1- x Mn x Cl 2 ·2H 2 O are carried out. In the Mn-type antiferromagnetic phase all spins are parallel to the b -axis which is the easy axis of MnCl 2 ·2H 2 O. In the Fe-type antiferromagnetic phase, for Fe-rich region, all spins are parallel to the α-axis which is the easy axis of FeCl 2 ·2H 2 O, and with increasing Mn concentration some Mn spins tilt from the α-axis. In the “intermediate” phase Fe- or Mn-type (oblique) antiferromagnetic spins coexist with the spin glass spins. All spins lie in the α b -plane with the angle depending on the Mn concentration.

Heat Capacities of H2 and D2 in Some Restricted Geometries

Heat capacities of H2 and D2 adsorbed in restricted geometries of zeolites and montmorillonites are measured at temperatures below 20K. The results are strongly concentration dependent and give temperature dependence characteristic of resricted geometries. Einstein type temperature dependence in zeolite, broad maxima or anomalous peak in pillar intercalated montmorillonites are observed.

Heat capacities in Haldane-gap antiferromagnet NENP and TMNIN under magnetic field

Abstract Heat capacity measurements are performed in Haldane-gap antiferromagnet NENP and TMNIN under magnetic field up to 13T. For NENP, the exponential temperature dependence of heat capacity gives strong evidence of existence of the energy gap. The field dependence of the gap energy shows characteristic increase above the critical field, which is considered to correspond to appearance of the staggered moment observed in 1 H-NMR measurement. We obtain similar results qualitatively for TMNIN. Moreover, the density of state in the lowest excited state is also discussed.