Yuichi Okuda

Static Spin Correlations in a Diluted Two-Dimensional Ferromagnet K2CuxZn(1-x)F4 Including the Percolation Vicinity

The static spin correlations in a diluted two-dimensional Heisenberg ferromagnet K 2 Cu x Zn (1- x ) F 4 have been studied. At low temperatures T <3.5 K, the magnetic heat capacity C m ( T ) per mole for Cu 2+ ions in the pure system ( x =1.0) has been revealed to obey the spin wave theory for a two-dimensional Heisenberg ferromagnet with the nearest neighbour exchange constant J / k B =11.4 ±0.4 K. Near the percolation limit ( x p =0.59), the absolute value of C m ( T ) in the same temperature range has been found to agree with the exact solution for the one-dimensional Heisenberg ferromagnet with the same interaction term as in the pure system. The quantities of C m ( T ) for x =0.36 and x =0.26 are discussed in comparison with the theoretical results for the finite size of open chains and closed rings, by taking account of the possible shape of the magnetic clusters presented by the computer generated random system in two dimensions.

Critical Temperature of Randomly Diluted Two-Dimensional Heisenberg Ferromagnet, K2CuxZn(1-x)F4

The susceptibility of randomly diluted two-dimensional Heisenberg-like ferromagnet K 2 Cu x Zn (1- x ) F 4 was measured down to 50 mK, using the 3 He– 4 He dilution refrigerator and a SQUID magnetometer. The ferromagnetic critical temperature T c ( x ) was obtained for x =0.98, 0.94, 0.85, 0.82, 0.68, 0.60, 0.54, 0.50 and 0.42. The value of [1/ T c (1)][(d/d x ) T c ( x )] x =1 was approximately 3.0. The critical temperature versus x curve exhibits a noticeable tail near the critical concentration, which may stem from the second nearest-neighbor interaction. The critical concentration x c , below which concentration there is no long range order down to T =0 K, was estimated to be 0.45∼0.50. The susceptibility of sample with x =0.42 behaves as if it obeys the Curie law down to 50 mK.

One-Dimensional Heisenberg Ferromagnetic Susceptibility Just at the Percolation Threshold for K2CuxZn(1-x)F4

The measurement of susceptibility in the randomly diluted two-dimensional Heisenberg-like ferromagnet K 2 Cu x Zn (1- x ) F 4 is performed for the concentrations ( x =0.68, 0.60, 0.50 and 0.40) near the percolation threshold, which is calculated as 0.59 for the square lattice with the nearest-neighbor interaction. The susceptibility for x =0.60 is excellently described by the exact solution of the one-dimensional S =1/2 Heisenberg ferromagnet with the same g-factor and the same exchange interaction as the original values of K 2 CuF 4 . The susceptibility for x =0.40 exhibits the Curie law with a slight deviation in the low temperature region.

High Field Magnetization of a Heisenberg Ferrimagnet Mn (CH3COO)2·4H2O

Magnetization measurement on Mn(CH 3 COO) 2 ·4H 2 O in an external field H 0 up to 400 kOe at 1.1 K gives the first clear example of a full magnetization process in a Heisenberg ferrimagnet, Two critical fields H c1 and H c2 are found. The system is ferrimagnetic below H c1 , ferromagnetic above H c2 and a spin canted state appears at H c1 < H 0 < H c2 . H c1 and H c2 are 125 kOe and 288 kOe, respectively for H 0 // a -axis. The biquadratic exchange interaction is taken into account to explain the magnetization process.

Sound Velocity and Attenuation in YBa2Cu3Oy

The sound velocity and attenuation in a sintered single phase YBa2Cu3O6.9 (denoted as orthor-I), which has a superconducting transition temperature, Tc of about 90 K, were measured and compared with those of the corresponding semiconductor YBa2Cu3O6.2 (denoted as tetra-II). The sound velocity in ortho-I has an additional increase below Tc, in contrast to that of tetra-II. This anomaly may imply a strong coupling between the lattice and electrons in this system. This cannot be explained by a simple thermodynamic analysis. The sound attenuation does not show any anomaly around Tc.

Temperature Dependence of Sound Velocity in High-Strength Fiber-Reinforced Plastics

Longitudinal sound velocity in unidirectional hybrid composites or high-strength fiber-reinforced plastics (FRPs) was measured along the fiber axis over a wide temperature range (from 77 K to 420 K). We investigated two kinds of high-strength crystalline polymer fibers, polyethylene (Dyneema) and polybenzobisoxazole (Zylon), which are known to have negative thermal expansion coefficients and high thermal conductivities along the fiber axis. Both FRPs had very high sound velocities of about 9000 m/s at low temperatures and their temperature dependences were very strong. Sound velocity monotonically decreased with increasing temperature. The temperature dependence of sound velocity was much stronger in Dyneema-FRP than in Zylon-FRP.

Facet Growth of 4He Crystal Induced by Acoustic Waves

Very fast growth of the c-facet of a 4 He crystal was induced by acoustic waves. The growth velocity was larger at lower temperatures and saturated below about 400 mK. The velocity was proportional to the acoustic wave power. This fast growth cannot be explained by the spiral growth mechanism for the known value of the step mobility. We developed a step multiplication model for high-power acoustic waves and found reasonable agreement with the observed temperature and power dependence of the growth velocity.

Osmolality dependence of erythrocyte sedimentation and aggregation in a strong magnetic field

In order to specify the major determinant of the magnetic enhancement of erythrocyte sedimentation observed previously, the dependence of erythrocyte sedimentation rate (ESR) on osmolality was measured under a strong magnetic field. Even at hypotonic osmolality, an increase in ESR due to aggregation was observed in plasma solution as compared with that without aggregation in saline solution. However, the magnetic field did not enhance ESR at hypotonic osmolality, when the cell shape was an isotropic sphere (spherocyte). Thus, we narrowed our search to a mechanism that would explain the enhanced ESR found specifically in anisotropic erythrocytes. It was concluded that the major determinant can only work for anisotropic erythrocytes and is a magnetic field-induced increase in an intermembrane adhesive area due to magnetic orientation of anisotropic erythrocytes.

Supercool and Superheat in bcc-hcp Transition of Solid 4He

The nucleation phenomenon in the structural transition between bcc and hcp phase of solid 4He is investigated by an optical method together with pressure and temperature. At Tc1(1.46 K), the single new phase seed was nucleated at some spot on the wall in contact with the superfluid and grown into the superfluid region. The nucleation is nothing to do with the original solid. The situation is almost the same for both cases of bcc to hcp (cooling) and hcp to bcc (warming). Whereas at Tc2(1.77 K), the new phase seeds were nucleated at many sites inside the original crystal, which is the typical Martensitic transition. Since the transition is of first order, supercool and superheat were observed. Though the transition event occurred at random, accumulated events were pretty well represented by a standard nucleation probability model. The temperature width of supercool is larger than that of superheat at Tc1. Whereas it is larger for superheat at Tc2. In other words, supercool (and superheat) is more likely in the transition from bcc to hcp regardless of warming or cooling. It is qualitatively understood by the fact that the surface tension of hcp crystal against liquid is much larger than that of bcc crystal.

Spin Dynamics near the Critical Point in (CH3NH3)2CuCl4

A dip due to the critical slowing down was observed in the real part of the high frequency susceptibility just at T c in a quasi-two dimensional ferromagnet (CH 3 NH 3 ) 2 CuCl 4 . The critical exponent, Δ , of the relaxation time for the magnetization along the easy axis, which is defined as τ∝ e - Δ ( e =( T - T c )/ T c ) was obtained as 1.05 ±0.05 for \(0.003 \lesssim \varepsilon \lesssim 0.1\). The Cole-Cole plots revealed that the Cole-Cole semi-circle became slightly but definitely depressed as the transition point was deadly approached (\( \varepsilon \lesssim 0.008\)).