Kazuyuki Matsuhira

Observation of Magnetic Monopoles in Spin Ice

Excitations from a strongly frustrated system, the kagome ice state of the spin ice Dy 2 Ti 2 O 7 under magnetic fields along a [111] direction, have been studied. They are theoretically proposed to be regarded as magnetic monopoles. Neutron scattering measurements of spin correlations show that close to the critical point the monopoles are fluctuating between high- and low-density states, supporting that the magnetic Coulomb force acts between them. Specific heat measurements show that monopole-pair creation obeys an Arrhenius law, indicating that the density of monopoles can be controlled by temperature and magnetic field.

Metal--Insulator Transitions in Pyrochlore Oxides Ln2Ir2O7

We report the physical properties of Ln 2 Ir 2 O 7 ( Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metal–insulator transitions (MITs) at different temperatures. The transition temperature T MI increases with a reduction in the ionic radius of Ln . The ionic radius boundary for MITs in Ln 2 Ir 2 O 7 lies between Ln = Pr and Nd. MITs in Ln 2 Ir 2 O 7 have some common features. They are second-order transitions. Under the field cool condition, a weak ferromagnetic component (∼10 -3 µ B /f.u.) caused by Ir 5 d electrons is observed below T MI . The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/(K·mole), respectively. The change in entropy is much smaller than 2 R ln 2 [11.5 J/(K·mole)] expected in a magnetic transition due to localized moments of S = 1/2. The feature of continuous MITs in Ln 2 Ir 2 O 7 is discussed.

Specific Heat Study on Sm-based Filled Skutterudite Phosphides SmT4P12(T=Fe, Ru and Os)

Sm-based filled skutterudite phosphides SmFe 4 P 12 , SmRu 4 P 12 and SmOs 4 P 12 synthesized at high temperatures and high pressures have been studied by specific heat measurements in magnetic fields. SmFe 4 P 12 shows an anomaly at 1.6 K attributed to a ferromagnetic transition. SmRu 4 P 12 shows a metal–insulator transition at 16 K. This transition consists of two successive transitions, which suggests orbital and antiferromagnetic orderings. SmOs 4 P 12 shows a sharp anomaly at 4.5 K due to an antiferromagnetic ordering. We discuss the characteristic behavior of those phase transitions and the crystalline electric field state in Sm T 4 P 12 ( T =Fe, Ru and Os). Interestingly, the magnetic entropy of SmFe 4 P 12 have only 0.16 R ln 2 below the ferromagnetic ordering temperature. This suggests a strong spin fluctuation effect. On the other hand, the variation of magnetic entropy in SmRu 4 P 12 and SmOs 4 P 12 reaches to nearly R ln 4 below the ordering temperature.

Metal-insulator transition in pyrochlore iridates Ln2Ir2O7 (Ln = Nd, Sm, and Eu)

We report that Ln 2 Ir 2 O 7 for Ln = Nd, Sm, and Eu have metal–insulator transitions (MITs) at 36, 117, and 120 K, respectively. Their electrical resistivities and thermoelectric powers are not discontinuous and exhibit no thermal hysteresis at their transition temperatures T MI , indicating second-order phase transitions. In this letter, we focus on the MIT of Sm 2 Ir 2 O 7 . The specific heat and the magnetic susceptibility show a clear anomaly at T MI . These results indicate that this MIT involves a magnetic ordering produced by 5 d electrons in Ir.

A new macroscopically degenerate ground state in the spin ice compound Dy2Ti2O7 under a magnetic field

The low-temperature magnetic properties of the pyrochlore compound Dy2Ti2O7 in magnetic fields applied along the [111] direction are reported. Below 1 K, a clear plateau has been observed in the magnetization process in th efi eld range 2–9 kOe, followed by a sharp moment jump at around 10 kOe that corresponds to a breaking of the spin ice state. We found that the plateau state is disordered, with a residual entropy of nearly half the value for the zerofield state, whose macroscopic degeneracy comes from a frustration of the spins on the Kagom´ el ayers perpendicular to the magnetic field.

Specific Heat of Kagomé Ice in the Pyrochlore Oxide Dy2Ti2O7

A novel macroscopically degenerate state called kagome ice, which was recently found in a spin ice compound Dy 2 Ti 2 O 7 in a magnetic field applied along the [111] direction of the cubic unit cell, is studied by specific heat measurements. The residual entropy of the kagome ice is estimated to be 0.65 J/K mol Dy, which is nearly 40% of that for the tetrahedral spin ice obtained in a zero field (1.68 J/K mol Dy) and is in good agreement with a theoretical prediction. It is also reported that the kagome ice state, which is stabilized at a range of magnetic field of 0.3–0.6 T, is a “gas” phase and condenses into a “liquid” phase with nearly zero entropy at a critical field of 1 T.

Emergence of Magnetic Long-range Order in Frustrated Pyrochlore Nd2Ir2O7 with Metal–Insulator Transition

In this study, we performed powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd 2 Ir 2 O 7 , which exhibits a metal–insulator transition at a temperature T MI of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector q 0 of (0,0,0) and that it grows with decreasing temperature below 15 K. This structure was analyzed to be of the all-in all-out type, consisting of highly anisotropic Nd 3+ magnetic moments of magnitude 2.3±0.4µ B , where µ B is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd 3+ splits below T MI . This suggests the appearance of a static internal magnetic field at the Nd sites, which probably originates from a magnetic order consisting of Ir 4+ magnetic moments. Here, we discuss a magnetic structure model for the Ir order and the relation of the order to the metal–insulator transition in terms of frustration.

Systematic Study of Lattice Specific Heat of Filled Skutterudites

The lattice specific heat C lat of La-based filled skutterudites La T 4 X 12 ( T = Fe, Ru and Os; X = P, As, and Sb) has been systematically studied, and both the Debye temperature Θ D and the Einstein temperature Θ E of La T 4 X 12 were carefully estimated. We confirmed that a correlation exists between Θ D and the reciprocal of the square root of average atomic mass for La T 4 P 12 , La T 4 As 12 , and La T 4 Sb 12 . The Θ D of filled skutterudites was found to depend mainly on the nature of the species X forming the cage. The temperature dependence of C lat / T 3 for La T 4 X 12 exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat. Since no such broad maximum exists for the unfilled skutterudite RhP 3 , the low-energy optical modes are associated with vibration involving La ions in the X 12 cage (the so-called “guest ion modes”). The Θ E of filled skutterudites was found to roughly correspon...

Spin Dynamics at Very Low Temperature in Spin Ice Dy2Ti2O7

We have performed AC susceptibility and DC magnetic relaxation measurements on the spin ice system Dy 2 Ti 2 O 7 down to 0.08 K. The relaxation time of the magnetization has been estimated below 2 K down to 0.08 K. The spin dynamics of Dy 2 Ti 2 O 7 is well described by using two relaxation times [τ S (short time) and τ L (long time)]. Both τ S and τ L increase on cooling. Assuming the Arrhenius law in the temperature range 0.5–1 K, we obtained an energy barrier of 9 K. Below 0.5 K, both τ S and τ L show a clear deviation from the thermal activated dynamics toward temperature independent relaxation, suggesting a quantum dynamics.

Neutron scattering study of dipolar spin ice Ho2Sn2O7: Frustrated pyrochlore magnet

By means of neutron scattering techniques we have investigated the frustrated pyrochlore magnet Ho 2 Sn 2 O 7 , which was found to show ferromagnetic spin-ice behavior below T≃ 1.4 K by susceptibility measurements. High-resolution powder neutron diffraction shows no detectable disorder of the lattice, which implies the appearance of a random magnetic state solely by frustrated geometry, i.e., the corner sharing tetrahedra. Magnetic inelastic scattering spectra show that Ho magnetic moments behave as an Ising spin system at low temperatures and that the spin fluctuation has static character. The system remains in a short-range-ordered state down to at least T = 0.4 K. By analyzing the wave-vector dependence of the magnetic scattering using a mean-field theory, it is shown that the Ising spins interact via the dipolar interaction. Therefore we conclude that Ho 2 Sn 2 O 7 belongs to the dipolar-spin-ice family. Slow spin dynamics is exhibited as thermal hysteresis and time dependence of the magnetic scattering.