Masafumi Sera

Magnetic properties of the 4 sublattice model for the antiferro (AF) quadrupolar order dominated by the AF octupolar and AF exchange interactions -A simple model for CeB6

The inconsistency between the results of NMR and neutron scattering in antiferro-quadrupolar (AFQ) ordered phase II of CeB 6 which remained to be solved over 15 years was explained recently by Saka...

Possible Long-Range Order with Singlet Ground State in CeRu2Al10

We investigate the thermal and transport properties of Ce x La 1- x Ru 2 Al 10 to clarify the origin of the recently discovered mysterious phase below T 0 =27 K in CeRu 2 Al 10 where a large magnetic entropy is released, however, the existence of an internal magnetic field is ruled out by 27 Al-NQR measurement. We find that T 0 decreases with decreasing x and disappears at x ∼0.45. T 0 of CeRu 2 Al 10 is suppressed down to 26 K under H =14.5 T along the a -axis. These results clearly indicate that the transition has a magnetic origin and is ascribed to the interaction between Ce ions. Considering the results of specific heat, magnetic susceptibility, thermal expansion, and electrical resistivity and also 27 Al-NQR, we propose that the transition originates from the singlet pair formation between Ce ions. Although its properties in a Ce dilute region is basically understood by the impurity Kondo effect, CeRu 2 Al 10 shows a Kondo-semiconductor-like behavior. The phase transition at T 0 may be characterized...

Unusual magnetic Phase diagram of CexLa1-xB6 (x = 0.5, 0.75) studied by the magnetoresistance

We have studied the magnetoresistance of Ce x La 1- x B 6 ( x =0.25, 0.5, 0.75) single crystals under the magnetic fields up to 15 T and in a temperature range between 0.4 K and 15 K. In Phase I (paramagnetic), in all the samples the negative magnetoresistance is observed, whose magnitude becomes larger with decreasing temperature. However, in Ce 0.5 La 0.5 B 6 , the magnetoresistance below T ∼1 K becomes very small neverthless the negative magnetoresistance is observed above ∼1 K. This is completely different from the magnetoresistance in Ce 0.25 La 0.75 B 6 where the magnitude of the negative magnetoresistance becomes larger with decreasing temperature. The weak magnetic field dependence of the electrical resistivity is observed also in Phase IV in Ce 0.75 La 0.25 B 6 . These results indicate that the ground state of Ce 0.5 La 0.5 B 6 is not the Kondo singlet but the AF (antiferro) magnetic state called Phase VI ′ is realized below ∼1 K. Phase IV ′ is similar to Phase IV in Ce 0.75 La 0.25 B 6 . The ani...

Long-range order and low-energy magnetic excitations in CeRu2Al10 studied via neutron scattering

= 27 K wasinvestigated by neutron scattering. Powder diffraction patterns show clear superstructure peakscorresponding to forbidden (h+ k)-odd reflections of the Cmcm space group. Inelastic neutronscattering experiments further reveal a pronounced magnetic excitation developing in the orderedphase at an energy of 8 meV.

Detection of Neutron Scattering from Phase IV of Ce0.7La0.3B6: A Confirmation of the Octupole Order

We have performed a single crystal neutron scattering experiment on Ce0.7La0.3B6 to investigate the order parameter of phase IV microscopically. Below the phase transition temperature 1.5 K of phase IV, weak but distinct superlattice reflections at the scattering vector (h/2,h/2,l/2) (h, l = odd number) have been observed by neutron scattering for the first time. The intensity of the superlattice reflections is stronger for high scattering vectors, which is quite different from the usual magnetic form factor of magnetic dipoles. This result directly evidences that the order parameter of phase IV has a complex magnetization density, consistent with the recent experimental and theoretical prediction in which the order parameter is the magnetic octupoles Tbeta with Gamma5 symmetry of point group Oh. Neutron scattering experiments using short wavelength neutrons, as done in this study, could become a general method to study the high-rank multipoles in f electron systems.

Magnetic Properties in Phase IV of Ce0.7La0.3B6 Studied by Muon Spin Relaxation

The magnetic properties of Ce 0.7 La 0.3 B 6 single crystal have been studied by muon spin relaxation (µSR). The absence of a clear muon spin precession under zero external field in phase IV indicates that no commensurate long-range magnetic order is present in this phase. Moreover, the time spectra under a longitudinal field strongly suggests that there are two sources of local fields exerting coherently on muons in phase IV. It was inferred from the muon Knight shift under a transverse field that the direction of the effective internal magnetic field probed by muon exhibits a marked change between phase III and IV.

Drastic Change of the Magnetic Phase Diagram of Ce xLa 1- xB 6 between x = 0.75 and 0.5

We have studied the magnetoresistance and magnetic phase diagram of Ce x La 1- x B 6 ( x = 0.65, 0.7). The antiferromagnetic (AF) phase IV recently discovered in Ce 0.75 La 0.25 B 6 , where it exists only in a narrow temperature and magnetic field region, extends down to T = 0 K for x = 0.7. The magnetoresistance in phase IV is much smaller than that in the AF phase III dominated by the antiferro-quadrupolar (AFQ) ordering. A first-order phase transition takes place at a critical magnetic field from phase IV to III and that field for H // is smaller than that for H // . These can be explained by considering the difference of the free energy in magnetic fields between phases IV and III. A tetra-critical point seems to exist for x = 0.75 but it is separated into two tri-critical points by the IV-II boundary for H // in Ce 0.65 La 0.35 B 6 .

Existence of Fine Structure inside Spin Gap in CeRu2Al10

We investigate the magnetic field effect on the spin gap state in CeRu 2 Al 10 by measuring the magnetization and electrical resistivity. We found that the magnetization curve for the magnetic field H ∥ c shows a metamagnetic-like anomaly at H * ∼4 T below T 0 =27 K, but no anomaly for H ∥ a or H ∥ b . A shoulder of the electrical resistivity at T s ∼5 K for I ∥ c is suppressed by applying a longitudinal magnetic field above 5 T. Many anomalies are also found in the magnetoresistance for H ∥ c below ∼5 K. The obtained magnetic phase diagram consists of at least two or three phases below T 0 . These results strongly indicate the existence of a fine structure on a low-energy side in a spin gap state with an excitation energy of 8 meV, which has recently been observed in inelastic neutron scattering experiments.

Magnetoelastic Properties of CexLa1-xB6

The thermal expansions and magnetostrictions of Ce x La 1- x B 6 ( x =1.0, 0.5 and 0.25) single crystals in both longitudinal and transverse magnetic fields up to 80 kOe have been performed in the temperature range from 1.39 K and ∼20 K. In the paramagnetic region, the results show the systematic but drastic variations with increasing Ce concentration indicating the importance of the interaction between Ce ions. The magnetic field induced enhancement of the crystal volume shrinkage observed in Phase II indicates that the antiferro-quadrupolar ordering has a magnetic field induced character. The anisotropic expansion of the crystal volume was observed in the single domain state in Phase III.

Anisotropic Magnetic Phase Diagram of PrB6 Dominated by the Oxy Antiferro-Quadrupolar Interaction

We have studied the specific heat, magnetoresistance and magnetization of PrB 6 which shows two successive phase ransitions at T IC from the commensurate (C) to incommensurate (IC) magnetic phase and at T N from the IC to paramagnetic phase and obtained the magnetic phase diagram. The suppression of T N by the magnetic field shows the following anisotropy. Below ∼12 T, T N and above ∼12 T, T N . T IC is suppressed by the magnetic field for H ∥ and but T IC is enhanced largely with increasing magnetic field up to ∼10 T. It was found that the IC phase is divided into two phases below and above ∼7 T for the magnetic field around the H ∥ direction. The anisotropy of T N is explained by the different magnitude of the parallel and perpendicular magnetic susceptibility whose weights depend on the magnetic field direction. The anisotropic field dependence of T IC at low magnetic field is explained by the different magnitude of the i...We have studied the specific heat, magnetoresistance and magnetization of PrB 6 which shows two successive phase ransitions at T IC from the commensurate (C) to incommensurate (IC) magnetic phase and at T N from the IC to paramagnetic phase and obtained the magnetic phase diagram. The suppression of T N by the magnetic field shows the following anisotropy. Below ∼12 T, T N and above ∼12 T, T N . T IC is suppressed by the magnetic field for H ∥ and but T IC is enhanced largely with increasing magnetic field up to ∼10 T. It was found that the IC phase is divided into two phases below and above ∼7 T for the magnetic field around the H ∥ direction. The anisotropy of T N is explained by the different magnitude of the parallel and perpendicular magnetic susceptibility whose weights depend on the magnetic field direction. The anisotropic field dependence of T IC at low magnetic field is explained by the different magnitude of the i...