Masaki Mito

Development of Miniature Diamond Anvil Cell for the Superconducting Quantum Interference Device Magnetometer

We have developed a novel miniature diamond anvil cell (mDAC) with a diameter of 8.5 mm and a length of 35 mm for a commercial superconducting quantum interference device (SQUID) magnetometer. In this mDAC, both X–Y and tilt adjustments of diamond anvil are possible. The SQUID magnetometer with this mDAC works under pressures higher than 1 GPa in a wide range of temperature and high magnetic field region. As for the test operation of this system, we first investigated the standard pressure effects of metallic superconductor Pb up to the pressure P = 3.8 GPa at the magnetic field H = 100 G and after that we investigated the pressure effect of an f-electron ferromagnetic compound GdZn2 up to P = 4.9 GPa for H ≤20000 G. The sensitivity of the present system is about 10-6 emu.

Pressure-induced enhancement of the transition temperature of a genuine organic weak-ferromagnet up to 65 K

Abstract We investigated the magnetization of the organic weak-ferromagnet p -NC.C 6 F 4 .CNSSN in the pressure P region up to 16xa0kbar. The transition temperature T c at ambient pressure is 35.5xa0K. T c is enhanced with increasing pressure, and reaches 64.5xa0K at P =16xa0kbar, which is the highest T c for a genuine organic weak-ferromagnet. On the other hand, the net moment is suppressed by the pressure, and this can be explained with the Dzyaloshinsky–Moriya model.

Magnetism of a two-dimensional weak-ferromagnetic organic radical crystal, 1,3,5-triphenyl-6-oxoverdazyl

A genuine organic radical crystal 1,3,5-triphenyl-6-oxoverdazyl (TOV) has been revealed to be an S =1/2 two-dimensional (2D) weak-ferromagnetic Heisenberg antiferromagnet with the exchange interaction 2 J / k B =-9.0 K by the experiments of magnetic susceptibility, heat capacity, magnetization and electron paramagnetic resonance. The anomalous temperature dependence of the weak ferromagnetism at low temperatures is qualitatively explained by a four-sublattice model with two kinds of Dzyaloshinsky-Moriya (D-M) vectors of opposite sign on the a c -plane. The staggered susceptibility, which is a fictitious but intrinsic quantity for antiferromagnets, is estimated in TOV by making use of the canted weak ferromagnetic moments which dominate below 6 K. Especially the rapid growth of the magnetic susceptibility below 6 K is explained by the staggered susceptibility which makes the crossover effect from 2D Heisenberg to 2D Ising system, triggered by above the D-M anisotropy.

Pressure effects of genuine organic crystalline ferromagnet possessing intermolecular contacts between nitroxide oxygen and methyl hydrogen atoms

Abstract We have found the pressure-induced (1) periodical down–up variation of transition temperature ( T c ), (2) ferro- to antiferro-magnetic transition and (3) reduction of the magnetic lattice dimensionality in a genuine organic crystalline bulk-ferromagnet p -Cl–C 6 H 4 –CHue605N-TEMPO ( T c =0.28 K) with spin polarizations on intermolecular contacts between the nitroxide oxygen and methyl hydrogen atoms (C–H⋯O–N contacts). These phenomena can be understood by the pressure-induced rotation of the methyl moiety relevant to the interaction-mechanism through C–H⋯O–N contacts.

Exchange interactions in the genuine organic ferromagnet accompanying pressure-induced ferro- to antiferromagnetic transition

Abstract The continuous variation of magnetic intermolecular interactions has been studied in the pressurized genuine organic ferromagnet, β -phase para -nitrophenyl nitronyl nitroxide ( T c = 0.61 K), in which a pressure-induced ferro- to antiferromagnetic transition was observed under pressure around 6.5 kbar at 0.45 K. The rotation of the radical group C 6 H 4 NO 2 around its symmetrical axis in crystallized molecules is pointed out to be essential to change magnitude and a sign of the dominant exchange interactions, rather than the shrinkage of the lattice parameters under pressure.

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...

Heat Capacity Study of the Doping Effect of Paramagnetic Radicals on the Organic Spin–Peierls System (p-CyDOV Radical Crystal)

The doping effect of paramagnetic organic radical impurities ( p -CyDTV) on the organic spin–Peiels (SP) system of p -CyDOV (3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl; the SP transition temperature T SP =15.0 K) was studied by the heat capacity measurement down to 0.1 K. At low temperatures below 1 K, quite a sharp heat capacity peak was detected for each system with small impurity concentration ( x ) at the temperature T N ( x ); T N (0)=0.135 K (±0.002 K), T N (0.01)=0.290 K and T N (0.07)=0.164 K. The appearance of the peak for the system with x =0 is considered to be due to minute and inevitable impurities in the pure p -CyDOV. From the dependence of T N ( x ), T N (0)< T N (0.01), and the heat capacity behavior on x , the peaks for x =0 and 0.01 are explained to be accompanied with the three-dimensional (3D) antiferromagnetic (AFM) transition in the SP phase. While the decrease of T N ( x ) for higher impurities x =0.07 than that for x =0.01 may be interpreted as merely a 3D AFM transition from th...

Magnetic properties of 2,5-difluorophenyl-α-nitronyl nitroxide and related position isomers

Abstract 2,5-Difluorophenyl-α-nitronyl nitroxide (2,5-DFPNN) radical crystal 4 was found to possess an unusual crystal structure with co-existence of two rotational isomers in a single crystal showing bulk magnetic transition to quasi-one-dimensional ferromagnetic behavior with T c at ∼0.48xa0K in ac susceptibility as well as heat capacity measurement. On the other hand, remarkable difference on their magnetic properties was observed in related position isomers depending upon the position of the second fluoro-substituent to the o -fluorine atom.

Magnetic properties of bond-alternating quantum spin chain system: (CH3)2NH2CuX3 (X=Cl,Br)

Abstract Magnetic properties of ferromagnetic (F) and antiferromagnetic (AF) bond alternating quantum spin chain system, DMACuX3 where X=Cl, Br and DMA=(CH3)2NH2, are reported. The most prominent result is that for Cl-compound we observed the unusual mixed state in which S=1 and 0 dimers coexist evenly due to frustrated situation in the presence of comparable F and AF couplings. For Br-compound, it is shown from the susceptibility measurements that an AF coupling is dominant and it provides an example of simple dimer-limit.

Pressure-induced crossover from alternating to uniform interaction in a S=1/2 one-dimensional Heisenberg antiferromagnet

Abstract We have confirmed the pressure-induced crossover from the alternating to the uniform interaction in a one-dimensional (1D) organic antiferromagnet by the measurement of heat capacity. The pentafluorophenyl nitroxide radical (F5PNN), which is a S=1/2 1D Heisenberg antiferromagnet with the alternating interaction ratio α=|J 2 /J 1 |≃0.4(J 1 / k B =−3.1 K ) at ambient pressure, has transformed into the uniform one (α=1; J 1 / k B =−4.5 K ) under the pressure of 6.5xa0kbar. This pressure-induced crossover was made clear by observing the change of low-temperature gradient of heat capacity, which gradually transformed from exponential to linear dependence on temperature, by the pressurization.