Masako Hitaka

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.

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 –CHN-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.

Thermal Analysis of Freedom of Spin in Partially Disordered State of the Antiferromagnetic Triangular Lattice in CuFeO2

The partially disordered spins in the triangular antiferromagnetic lattice in CuFeO 2 have been studied by heat capacity measurements. A cusp and a sharp peak in the heat capacity curve were observed at T N1 =14.0±0.5 K and T N2 =11.0±0.5 K, respectively. The analysis of heat capacity revealed that the magnetic entropy associated with the anomaly in the heat capacity around T N2 is attributable to the disordering of one-fifth of Fe 3+ spins. The present result is discussed in comparison with partially disordered states of CeSb and PrCo 2 Si 2 , anisotropic spin systems with frustration. The heat capacity cusp around T N1 is compared with the characteristic thermal behavior for frustrated spin systems.

Magnetism of an Organic Radical Crystal with Weak Ferromagnetic Moment Below 5.4 K

Abstract From the measurements of magnetic susceptibility, heat capacity, magnetization and EPR, the magnetism of the organic radical crystal of 1,3,5-triphenyl-6-oxoverdazyl is revealed to be descrived by a two-dimensional quantum Heisenberg antiferromagnet with the effective intra-plane exchange interaction J/Kb= − 4.3 K. The appearance of weak ferromagnetic moment is observed below 5.4 K. The magnetic susceptibility, however, shows its broad maximum around 2 K and decrease toward the low temperatures. These results are shown to be described by a four-sublattice model with two Dzyaloshinsky-Moriya vectors of opposite sign on the ac-plane. A proposal is made for crystal symmetry in which weak ferromagnetic moment is stabilized without direct ferromagnetic interactions.

Site Dilution Study of a Square Lattice Heisenberg Antiferromagnet with S= 5/ 2 Covering the Percolation Threshold

The site dilution problems on a square lattice antiferromagnet with spin S =5/2, Mn(HCOO) 2 2(NH 2 ) 2 CO ( T N =3.77 K), have been studied for a wide range of magnetic concentration x , covering the percolation threshold x p =0.59 for the square lattice, by the measurements of magnetic heat capacity and susceptibility. The N\({\rm \acute{e}}\)el temperature T N ( x ) has been found to decrease as d/ d x { T N ( x )/ T N (1)}= R =2.7 just below x =1 for the three kinds of non-magnetic impurities, Mg 2+ (2p 6 ), Zn 2+ (3d 10 ) and Cd 2+ (4d 10 ), at almost the same reduction rate as in the cases of the antiferromagnet K 2 MnF 4 with S =5/2 and the ferromagnet K 2 CuF 4 with S =1/2. The present value R =2.7, however, makes a contrast to R =3.1 for the diluted La 2 CuO 4 , an S =1/2 Heisenberg antiferromagnet. The extrapolation of T N ( x ), with R =2.7(3.1) down to T N ( x )=0 gives a pseudo-critical concentration x = x c =0.63(0.68) deviating from x p =0.59. This indicates that T N ( x ) must draw concavel...

Doping effect of non-magnetic and magnetic impurities on magnetic susceptibility of the Haldane system (CH3)4NNi(NO2)3

Magnetic susceptibility measurement has been carried out on the Haldane system (CH 3 ) 4 NNi(NO 2 ) 3 containing non-magnetic Zn 2+ and magnetic Cu 2+ impurity ions. For each impure system, magnetic susceptibility is described by the sum of two contributions; one is from the Haldane state and the other is a paramagnetic contribution. The latter contribution increases proportionally with impurity concentration. For 1D system shortened by Zn 2+ impurity ions, the paramagnetic contribution obeys the Curie law, and its absolute value of the susceptibility is found to amount nearly 40% of the theoretical estimation for the VBS model where the spin with its value S =1/2 appears at the both edges of 1D segments. In the case for the system with Cu 2+ impurity ions, the paramagnetic contribution is much smaller than the results for the Zn 2+ impure systems. This reduction of the magnetic moment of Cu 2+ ion for S =1/2 is qualitatively different from the case for the enhanced moment of Cu 2+ ion in the anisotropic ...

Pressure-induced enhancement of Tc in a genuine organic bulk-ferromagnet

Abstract Few genuine organic bulk-ferromagnets have been synthesized until now, but their transition temperatures (Tc) do not exceed 1.5 K. The representative organic bulk-ferromagnets such as β-phase p-NPNN (T c =0.61 K ) and p-Cl-C6H5CH=N=TEMPO (T c =0.28 K ) show a ferro-to-antiferromagnetic transition under pressure. In 2,5-DFPNN (T c =0.48 K ) , however, we find that ferromagnetism is stable even under pressure, and Tc increases with pressure. This result gives us a clue for realizing the high-temperature organic bulk-ferromagnet.

Pressure Effects on Intermolecular Interactions in Genuine Organic Radical Crystals

Abstract Pressure induced enhancement of magnetic ordering temperature has been found in a quasi-one-dimensional ferromagnetic organic radical crystal, 3-(4-chlorophenyl)-l,5-dimethyl-6-thioxovcrdazyl by the measurements of magnetic susceptibility and heat capacity under hydrostatic pressures up to 13 kbar (1.3 GPa). This pressure dependence is the opposite direction to the results in the ferromagnetic β-phase p-NPNN observed recently, and in the same way as in ordinary antiferromagnets. The effects of pressurization suggest to bring about frustration in kinetic exchange interactions via various charge transfer paths between molecular orbitals.

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.