Tatsuya Kawae

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.

Non-Fermi Liquid Behavior in Dilute Quadrupolar System PrxLa1-xPb3 with x ≤0.05

We have studied the low-temperature properties of Pr x La 1- x Pb 3 with non-Kramers Γ 3 quadrupolar moments of the crystal-electric-field ground state, for a wide concentration range of Pr ions. For x ≤0.05, the specific heat C / T increases monotonically below T =1.5 K, which can be scaled with a characteristic temperature T * defined at each concentration x . The electrical resistivity ρ( T ) in the corresponding temperature region shows a marked decrease deviating from a Fermi-liquid behavior ρ( T ) ∝ T 2 . The Kondo effect arising from the correlation between the dilute Γ 3 moments and the conduction electrons may give rise to such anomalous behavior.

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.

Specific heat study of an s = 1/2 alternating heisenberg chain system : F5PNN in a magnetic field

We have measured the specific heat of an S = 1/2 antiferromagnetic alternating Heisenberg chain pentafulorophenyl nitronyl nitroxide in magnetic fields up to H > H(C2). This compound has a field-induced magnetic ordered (FIMO) phase between H(C1) and H(C2). Characteristic behaviors are observed depending on the magnetic field up to above H(C2) outside of the H-T boundary for the FIMO. The temperature and field dependence of the specific heat are qualitatively in good agreement with the theoretical calculation on an S = 1/2 two-leg ladder [Wang et al., Phys. Rev. Lett. 84, 5399 (2000)]]. This agreement suggests that the observed behaviors are related with the low-energy excitation in the Tomonaga-Luttinger liquid.

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.

Antiferromagnetism and geometric frustration in tetrahedral lattice hydroxyhalides M2(OH)3X

A systematic study of magnetic susceptibility measurements is carried out on distorted tetrahedral lattice hydroxyhalides Fe2(OH)3Cl, Mn2(OH)3Cl, Mn2(OH)3Br, and Co2(OH)3Br, following our recent finding of coexisting antiferromagnetic order and disorder in Cu2(OH)3Cl (clinoatacamite). These transition metal hydroxyhalides are found to undergo antiferromagnetic transitions, at TN = 14 K for Fe2(OH)3Cl, at two successive transitions of TN1 = 3.4 K and TN2 = 2.7 K for Mn2(OH)3Cl, at TN1 = 3.3 K and TN2 = 2.4 K for Mn2(OH)3Br, and at TN = 5 K for Co2(OH)3Br, respectively. All hydroxychlorides show coexisting glassiness below the magnetic transitions; meanwhile, the glassiness is not found in the hydroxybromides. The distorted tetrahedral lattice for the magnetic ions consists of stacked layers of triangular lattice planes and Kagome lattice planes, with the magnetic ions on the Kagome lattice planes being bonded by the halogen ions. The contrasting behaviours in Mn2(OH)3Cl and Mn2(OH)3Br suggest that the glassiness arises from competing magnetic interactions in the tetrahedron, specifically on the Kagome lattice planes.

Low-Temperature Properties in Dilute Quadrupolar Compound PrxLa1-xInAg2

The low-temperature properties of Pr x La 1- x InAg 2 have been studied for a wide concentration range of Pr ions between 0≤ x ≤0.6. The susceptibility above T =15 K is well reproduced by the crystal-electric-field level scheme with a non-Kramers Γ 3 doublet in the ground state for each concentration, while that below T =15 K shows a non-Fermi-liquid behavior with a logarithmic temperature dependence. The temperature dependence of the specific heat C / T shows a monotonic enhancement below T =10 K, which can be scaled by a characteristic temperature T * defined at each concentration. These results suggest that the anomalous low-temperature properties are understood as the single-ion nature of Γ 3 quadrupolar moments. The origin of the anomalous properties is discussed in connection with the quadrupolar Kondo effect arising from the correlation between the Γ 3 moments and the conduction electrons.

Field-induced magnetic ordering in an alternating Heisenberg chain F5PNN

Abstract We have found the field-induced magnetic ordering of an S= 1 2 alternating linear Heisenberg antiferromagnet F5PNN by measuring magnetic field dependence of the specific heat. A cusp-like anomaly is observed below 0.2 K between 3.5 and 6 T . From the extrapolation of H vs. T phase diagram to 0 K , the lower and the upper critical fields are expected to be about 3 and 7 T , respectively. The phase boundary is almost symmetric with respect to a horizontal line of H=5 T . This behavior is consistent with the previously observed magnetization measurement in which magnetization began to increase at about 3 T and saturated at about 7 T .

Specific Heat Study of Novel Spin-Gapped System: (CH3)2NH2CuCl3

Specific heat measurements in a magnetic field up to 8 T have been performed on an S =1/2 weakly coupled alternating ferromagnetic (F) and antiferromagnetic (AF) dimer chain system, (CH 3 ) 2 NH 2 CuCl 3 . We have observed a spontaneous AF ordering and a field-induced one below and above the 1/2 magnetization plateau field range from 2 to 3.5 T, respectively. The H – T phase boundaries for the orderings are completely different from the general reentrant ones of the other quantum spin systems investigated thus far. In the plateau field range, a double-peaked structure is observed in the temperature dependence of specific heat, reflecting the coexistence of F and AF excitations.