Hidehiko Ishimoto

Magnetization tunneling in single-molecule magnets

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM’s). SMM’s are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)16(H2O)4] solvate. The two Mn12 complexes are isomers that both differ in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn–Teller elongation at the Mn III ions. The magnetization versus magnetic field hysteresis loop is quite different for the two isomeric Mn12 complexes. One Mn12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn12 isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn4 complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.

Superconductivity and metal-insulator transition in amorphous Si1−xAux system

Abstract Electrical conductivity of metallic amorphous Si 1− x Au x films was measured down to 10 mK. Superconducting transition was observed in a certain range of Au content. In the normal state, the temperature variation of electrical conductivity showed √ T -dependence at higher temperatures and ln T -dependence at lower temperatures. This crossover is interpreted as being due to a change of sample dimensionality.

Two-stage nuclear demagnetization refrigerator reaching 27 µK

A two-stage nuclear demagnetization refrigerator with large cooling capacity has been constructed and successfully operated. The first stage consists of 11 moles of the Van Vleck paramagnet PrNi5 in an initial field of 6 T, and the second stage consists effectively of 19 moles of copper in an initial field of 7.6 T. The minimum temperature as measured with pulsed platinum NMR is 27 µK, the lowest ever obtained in a low-field experimental region.

[Mn 18 ] 2+ and [Mn 21 ] 4+ single-molecule magnets

Abstract The synthesis and structural characterization of the two new Mn complexes [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) and [Mn21O16(O2CMe)16(hmp)6(hmpH)2(pic)2(py)(H2O)](ClO4)4 (3) are presented, together with a detailed study of their magnetic properties. Complex 1 possesses a ground-state spin of S=13, and the ground-state spin for 3 is estimated to be S=17/2 or 19/2. Both complexes 1 and 3 are new examples of single-molecule magnets (SMMs), displaying frequency-dependent out-of-phase AC signals, as well as magnetization vs. DC field hysteresis at temperatures below 1 K. Complex 1 straddles the classical/quantum interface by also displaying quantum tunneling of the magnetization (QTM).

ANTIFERROMAGNETIC LONG-RANGE ORDER IN CU1-XZNXGEO3 WITH EXTREMELY LOW ZN CONCENTRATION

We have measured the magnetic susceptibilities of single crystals of Cu_{1-x}Zn_xGeO_3 with extremely low Zn concentration (x) lower than x=5x10^{-3} at very low temperatures to investigate the spin-Peierls and antiferromagnetic transitions. The results show that the undoped CuGeO_3 has no antiferromagnetic phase down to 12 mK and there exists an antiferromagnetic long-range order with the easy axis along the

Metal-insulator transition in the amorphous Si1−xAux system with a strong spin-orbit interaction

c

Observation of Magnetic Transition in Quantum Nanomagnet Mn4Br.

axis for x down to 1.12(2)x10^{-3}. The minimum observed Neel temperature was 0.0285 K for x=1.12(2)x10^{-3} sample. From the concentration dependence of the Neel temperature it is concluded that there is no critical concentration for the occurrence of the antiferromagnetic long-range order.

New Anomalous Phenomena in NMR Absorption and Spin Lattice Relaxation of Solid Hydrogen

Abstract The metal-insulator transition in a-Si1−xAux system was studied through electrical conductivity measurements and tunnelling experiments at temperatures down to 10 mK and in magnetic fields up to 5 T. This system makes a metal-insulator transition at xc ∼ 0.14 at low temperatures. As the variable range hopping conduction has been observed in an insulating side, this transition has a character of Anderson transition. On the other hand, it has also a character of Mott transition, because the tunnelling experiments showed that a correlation gap begins to open at xc when x approaches xc from the metallic side. The existence of a strong spin-orbit interaction was revealed by the studies of temperature and magnetic field dependences of electrical conductivity σ(T, B) of metallic a-Si : Au thin films. The zero-temperature limit conductivity σ0 continuously goes to zero as the content of gold approaches xc from the metallic side. This behaviour is expressed by σ0α |x/xc − 1|s with xc ∼ 0.14 and s ∼ 1.0 both in 0 T and 5 T. The value of 1.0 is consistent with the one obtained by Altshuler and Aronov for interacting electrons in disordered electron system with a strong spin-orbit interaction.

Systematic measurement of the intrinsic losses in various kinds of bulk fused silica

The magnetization of the polycrystalline manganese cluster Mn 4 Br was measured down to 0.13 K under a magnetic field of ±15 kOe. It shows a gradual drop at around 1.5 K and the heat capacity measurement at 0 kOe reveals a sharp anomaly at 1.33 K, indicating the occurrence of a second-order antiferromagnetic (AFM) phase transition. The M – H curve well below the transition temperature T N shows a step around 2.5 kOe, which probably corresponds to the transition from the AFM phase to metamagnetic (MM) phase. The observed field agrees with that estimated from T N for the MM transition, using a simple molecular field theory. This is the first observation of a well-defined AFM and also a MM phase among the many manganese clusters with a high-spin ground state that function as molecular nanomagnets.

Search for superfluidity of3He in3He-4He solution

NMR line shape and spin lattice relaxation time of solid hydrogen are measured at 22 MHz for the fractional ortho content of 0.55> X >0.08 and temperature range of 1 K> T >0.2 K. As the sample temperature decreases below 0.35 K, the absorption line changes from singlet to doublet. The phenomenon is observed at ortho contents as small as 0.08. The critical temperature ( T * ), below which the line exhibits the doublet structure, weakly depends on X and is expressed as T * =(0.3±0.1)( X +0.5) K (0.55> X >0.08). The spin lattice relaxation time ( T 1 ) is found to show an anomalous increase by a factor of 10 2 at T ≃ T * . The increase of T 1 with decreasing temperature below T * gives Δ L =0.25±0.05 K for the lower edge of the excited state with J z =±1. Temperature dependence of the field separation of the doublet line is analyzed by using the order parameter formulated by Ueyama and Matsubara. The result gives Δ =1.0±0.2 K for the mean energy gap between the rotational substate with J z =0 and J z =±1.