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Kondo temperature TK’s for Kondo insulator alloys Yb1-xRxB12 (R = Y, Lu, Sc and Zr) show drastic variation as every x increase. It is found that YbB12 in a virtual gap-less state would have a peak at about 25 K in magnetic susceptibility χ(T) from analysis of χ(T) of all these alloys. Up-turns shown in χ(T) below 20 K grow as development of an energy gap ΔE. Those behaviors may be related to the second-gap development within the in-gap state showing metallic behaviors.

YbB12 has been investigated energetically as a typical Kondo insulator. To study detailed formation mechanism of Kondo energy gap, we have investigated transport and magnetic properties of Yb1-xR3+xB12 (R3+ = non-magnetic trivalent ions). Recently, we have recently succeeded in substitution of Ca2+ for Yb site in YbB12 by high-pressure synthesis. In Ca- and Lu-substituted alloys both of magnetic susceptibility and specific heat C/T at low temperatures, remarkably show larger increase than those of powdered YbB12. These increases are due to giant increase of density of state in an in-gap state.

We synthesized Ca1−xSrxFeO3 containing tetravalent iron Fe4+ with high valence by low temperature heat treatment along with ozone oxidization. We investigated the magnetic properties of perovskite Ca1−xSrxFeO3. The charge disproportionation transition temperature (TCD) of Ca1−xSrxFeO3 was observed from x = 0.0 to 0.6 in the magnetic susceptibility measurements. The decrease in TCD occurs is attributed to deformation of the crystal structure. The metamagnetic transitions with hysteresis were observed in the magnetization curves of all the compositions. The metamagnetic behavior is due to helical magnetism, which means that there are intermediate stable states before magnetization is saturated. In the Sr-rich region with x = 0.8 and 1.0, the saturation magnetization shows forced ferromagnetic order with a value of ∼3.2 μB at high magnetic field, H ∼ 60 T.