Makoto Yokoyama

Effects of pressure on tiny antiferromagnetic moments in the heavy-electron compound URu2Si2

We have performed elastic neutron-scattering experiments on the heavy-electron compound URu_2Si_2 for pressure P up to 2.8 GPa. We have found that the antiferrmagnetic (100) Bragg reflection below T_m ~ 17.5 K is strongly enhanced by applying pressure. For P < 1.1 GPa, the staggered moment mu_o at 1.4 K increases linearly from ~ 0.017(3) mu_B to ~ 0.25(2) mu_B, while T_m increases slightly at a rate ~ 1 K/GPa, roughly following the transition temperature T_o determined from macroscopic anomalies. We have also observed a sharp phase transition at P_c ~ 1.5 GPa, above which a 3D-Ising type of antiferromagnetic phase (mu_o ~ 0.4 mu_B) appears with a slightly reduced lattice constant.

Antiferro-Quadrupolar Ordering and Multipole Interactions in PrPb3

Antiferro-quadrupolar (AFQ) ordering in the cubic Γ 3 ground state system PrPb 3 has been studied by means of magnetization and specific heat measurements under magnetic fields H . Field variation of the AFQ transition temperature T Q (0.4 K at H =0) has been determined for the three crystallographic directions , and . The obtained T Q ( H ) is strongly anisotropic, with T Q ( H ) the largest (∼0.7 K at 6 T). The AFQ phase diagram and the magnetization behavior observed for H ∥ are well explained by a model that incorporates both antiferromagnetic and antiferroquadrupolar interactions. The interactions between field-induced staggered magnetic moments stabilize the AFQ order. This simple model, however, seriously fails to explain the observed anisotropy: T Q ( H )> T Q ( H )> T Q ( H ). Reconsidering the field-induced multipole moments in the AFQ phase, we propose that inclusion of a weak ferro-octupolar interaction between Γ 5 type octupole moments is importa...

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.

Neutron Scattering Study on Competition between Hidden Order and Antiferromagnetism in U(Ru1-xRhx)2Si2 (x ≤0.05)

We have performed elastic and inelastic neutron scattering experiments on solid solutions of U(Ru 1- x Rh x ) 2 Si 2 with Ru-rich concentrations: x =0, 0.01, 0.02, 0.025, 0.03, 0.04 and 0.05. Hidden order is suppressed with increasing x , and correspondingly, the onset temperature T m (∼17.5 K at x =0) of weak antiferromagnetic (AF) Bragg reflection decreases. For x =0.04 and 0.05, no magnetic order is detected in the investigated temperature range down to 1.4 K. In the intermediate range, 0.02 ≤ x ≤0.03, we found that the AF Bragg reflection is strongly enhanced. At x =0.02, this enhancement takes place at ∼7.7 K (≡ T M ), which is significantly lower than T m (∼ 13.7 K). T M increases with increasing x , and seems to merge with T m at x =0.03. If the AF state is assumed to be homogeneous, the staggered moment µ o estimated at 1.4 K increases from 0.02(2) µ B /U ( x =0) to 0.24(1) µ B /U ( x =0.02). The behavior is similar to that observed under hydrostatic pressure (µ o increases to ∼0.25 µ B /U at 1.0 ...

Inhomogeneous magnetism in URu2Si2 studied by muon spin relaxation under high pressure

Abstract We have studied the low-temperature phase below 17.5 K in the heavy-fermion superconductor URu 2 Si 2 , using the zero-field μSR technique under hydrostatic pressures P up to 0.85 GPa . The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic ‘hidden order’. The AF volume fraction, however, develops abruptly at around a pressure of 0.6 GPa , in contrast to the P -linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17.5 K , whereas the nominal Neel temperature seems to be about 20 K . These experimental results suggest that the AF phase exists as a metastable state.

No evidence for small-moment antiferromagnetism under ambient pressure in URu2Si2 : Single-crystal 29Si NMR study

The still unidentified hidden order (HO) phase below T 0 = 17.5 K in the heavy-electron superconductor URu 2 Si 2 has been investigated by 29 Si NMR on a single crystal under external magnetic field H ext ∥ c -axis and ambient pressure P . For a previously proposed model of the small-moment antiferromagnetism (SMAF), in which tiny-moment antiferromagnetism (AF) under ambient P is ascribed to spatially homogeneous AF with an ordered moment µ SMAF = 0.02–0.04 µ B , splitting of the line would be expected on the basis of a scaling from the corresponding splitting in the P -induced AF. However, we observed at T = 4.5 K no splitting of the line. This clearly indicates that the SMAF model cannot describe the ambient- P properties. Below T 0 down to ∼14 K, the 29 Si NMR line shows additional broadening over the paramagnetic width. The additional contribution, however, disappears below ∼14 K, which indicates that the HO itself produces almost zero internal field at the 29 Si site along the c -axis at low temperat...

Hidden order and weak antiferromagnetism in URu2Si2

Abstract The origin and nature of the low-temperature phase appearing below 17.5 K (≡T 0 ) in URu2Si2 is still not fully understood. Since the weak antiferromagnetic state (WAF; μord∼0.02–0.04μB) developing below around T0 is not simply reconciled with a large reduction in the magnetic entropy (∼0.2R ln 2) , the existence of some hidden order parameter has been argued, where WAF is considered to be of a secondary nature. The paper reviews recent development of microscopic characterization of WAF based on neutron scattering and 29 Si NMR under high pressure. It is demonstrated that, at least, for pressures ranging from ∼0.3 to ∼0.83 GPa , there are spatially competing two different ordered states in the crystal: a normal-moment (∼0.25μB/U) antiferromagnetic state and some unknown, non-magnetically ordered state. Simple extrapolation to ambient pressure suggests that WAF originates from one percent volume fraction of this antiferromagnetic phase. We also discuss the unusual magnetic properties at ambient pressure, presenting our latest results of zero-field and longitudinal-field μSR experiments.

Symmetry of the Hidden Order in URu2Si2 from Nuclear Magnetic Resonance Studies

The hidden order (HO) in URu 2 Si 2 has been investigated by both 29 Si and 99 Ru NMR on a single crystal. Tiny, but finite additional broadening of the spectrum in the HO phase in both 29 Si and 99 Ru NMR with external magnetic field applied along the crystallographic [100] or [110] direction, and its persistence as H ext →0 in 29 Si NMR clearly indicate the existence of internal hyperfine field, and the breaking of the time-reversal symmetry in the HO phase can therefore be concluded. We discuss which symmetry of the D 4 h symmetry group of the URu 2 Si 2 crystal structure is compatible with the observed dependence on the crystal-axis direction of the existence or nonexistence of the internal hyperfine field. Then, by imposing a strong constraint that the wave-vector of the HO is also Q 0 =[0,0,1], the wave-vector of the pressure-induced antiferromagnetism, we show that the two-dimensional representation E - remains as the only possible candidate for the HO symmetry. Since dipoles are obviously excluded...

Antiferromagnetic Order in Disorder-Induced Insulating Phase of SrRu1-xMnxO3 (0.4 ≤x ≤0.6)

We have performed powder neutron diffraction measurements on the solid solutions of SrRu 1- x Mn x O 3 , and found that the itinerant ferromagnetic order observed in pure SrRuO 3 changes into the C-type antiferromagnetic (AF) order with nearly localized d electrons in the intermediate Mn concentration range between x =0.4 and 0.6. With increasing x , the AF moment is strongly enhanced from 1.1 µ B ( x =0.4) to 2.6 µ B ( x =0.6), which is accompanied by the elongation of the tetragonal c / a ratio. These results suggest that the substitution of Mn for Ru suppresses the itinerant characteristic of the d electrons, and induces the superexchange interaction through the compression in the c plane. We have also found that the magnetic and transport properties observed in our tetragonal samples are similar to those of recently reported orthorhombic ones.

Magnetic properties of Sm-based filled skutterudite phosphides

Abstract Filled skutterudites SmFe4P12 and SmOs4P12 have been prepared at high temperature and high pressure. The temperature dependence of electrical resistivity in both compounds shows metallic behavior. The magnetic susceptibility and specific heat measurements indicate that SmFe4P12 shows a ferromagnetic ordering at 1.5 K , whereas SmOs4P12 is an antiferromagnet with a TN of 4.6 K .