Shin-ichiro Kobayashi

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 .

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...

Kondo effect in the long-range antiferromagnetic ordered state: CexNd1-xB6

We have studied the coexistence and the competition between the Kondo singlet state and the long-range antiferro(AF)magnetic ordering in the Ce x Nd 1- x B 6 system. At high temperatures, the single impurity Kondo effect is observed in all the samples as in the Ce x La 1- x B 6 system. In the AF magnetic state formed mainly by Nd ions, Ce ions contribute to the AF magnetic ordering but at the same time exhibit the Kondo singlet character with a small effect from the magnetic ordering. The ground state of Ce x Nd 1- x B 6 changes with x ; in a small x region, the Kondo singlet state is destroyed by the exchange field from surrounding Nd moments. Above x ∼0.3, the long-range magnetic order takes place because the Ce–Ce interaction overcomes the Kondo effect. In an intermediate x value between the above two regions, the Kondo singlet state may continue to exist down to a very low temperature.

Appearance of the Phase IV in CexLa1-xB6 at x∼0.8

We have studied the transport, magnetic and elastic properties of Ce 0.8 La 0.2 B 6 single crystal. The phase IV which existed for x ≤0.75 in Ce x La 1- x B 6 was found to exist in this sample but ...

Nd Ion Doping Effects on the Multipolar Interactions in CeB6

The Nd ion doping effects on the multipolar interactions in CeB 6 were studied. At high magnetic fields, the antiferro-quadrupolar (AFQ) phase II is not significantly affected by Nd doping. This may be because the Nd ion is in the paramagnetic state. However, with decreasing magnetic field, the Nd doping effect becomes apparent. This may be because Nd–Nd interaction dominates the long-range ordering at low magnetic fields. At low magnetic fields, just after T Q and T N coincide with each other when Nd ion is doped, new phases, namely, phases IIIA and V appear. Phase IIIA is the AF magnetic state where all types of quadrupolar interaction are small. The nature of phase V seems to change largely at x ∼0.5. For x 0.5, the AF magnetic state where both Ce and Nd ions contribute may be realized.

Rapid suppression of the metamagnetic transition for H || (111) in NdB6 by La doping

We have studied the specific heat, magnetization and magnetoresistance of Nd x La 1- x B 6 ( x =0.9, 0.8, 0.7) single crystals and obtained their magnetic phase diagrams. The antiferro (AF) magnetic phase for H ∥ is divided into two phases of AFI below H c1 and AFII * (or AFII) above H c1 . In the AFI phase, there exists the component of the parallel magnetic susceptibility up to H c1 . The intermediate metamagnetic AFII phase in NdB 6 disappears already for x =0.9 and in place, new AFII * phase appears. The AFII * phase is considered to be the spin-canted phase characterized by the perpendicular magnetic susceptibility. However, the magnetization in the AFII * phase for x =0.9 and 0.8 exhibits the unusual behavior that the extrapolation of the magnetization curve down to H =0 takes a large negative magnetization value. This unusual behavior disappears at x =0.7. These results indicate that the interaction forming the interamediate metamagnetic AFII phase is rapidly suppressed by the La doping and di...

Ce Site Substitution Effects on the Multipolar Interactions in CeB6

Ce-site substitution effects on the quadrupolar and octupolar interactions in CeB 6 were investigated for Ce x R 1- x B 6 (R = Nd, Gd) single crystals. In Ce x Nd 1- x B 6 ( x =0.8,0.7), new phase V appears between phase III and I. Phase V may be the antiferro (AF) magnetic state judging from the existence of the magnetic anisotropy clearly seen for x =0.7. From the x dependence of T N and T Q , the Nd doping suppresses the AFQ and AFO interactions. In the case of Gd doping, quite unusual succesive transition III–II–V–I is observed for small Gd concentration. For larger Gd concentration, phase II is pushed up to higher fields and the AF magnetic state appears at low fields, which is not dominated by the AFQ ordering.

Anisotropic Magnetic Phase Diagram of PrB6Dominated by theOxyAntiferro-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...