K. Matsubayashi

Magnetic properties (Communication arising): Parasitic ferromagnetism in a hexaboride?

Surprisingly for a compound with no magnetic element, Young et al. have observed ferromagnetism in calcium hexaboride (CaB6) doped with lanthanum (La) — the system has a ferromagnetic Curie temperature as high as 600 K, which is comparable to that of transition-metal ferromagnets such as iron (Fe). Here we show that high-temperature ferromagnetism in this CaB6 system is not intrinsic but that it is instead due to alien phases of iron and boride.

Extrinsic origin of high-temperature ferromagnetism in CaB6

The discovery of ferromagnetism in CaB 6 doped with La was surprising, because there is no magnetic element in the compound nevertheless the system involved a ferromagnetic Curie temperature as high as 600 K. In order to resolve the origin of the ferromagnetism, we have made magnetization measurements and chemical analysis of CaB 6 and LaB 6 samples synthesized under various conditions. We have found that the ferromagnetism disappears almost completely by the acid etching, and that there is strong correlation between the magnitude of reduced magnetization and the iron concentration involved in the acid. Therefore we conclude that the ferromagnetism is of extrinsic origin and can be ascribed to iron impurity phases, probably FeB and Fe 2 B that have nearly the same Curie temperature as CaB 6 . We suggest that these iron impurities originate from the crucible used in the synthesis. Furthermore we have measured the thermoelectric power at high temperatures including the Curie temperature. There seems to be n...

Insulator–metal transition studied by heat capacity measurements on SmS

Abstract We have measured the specific heat of SmS at pressures up to ∼7.5 kbar . We have observed that the electronic specific heat coefficient γ shows a steep increase at around P c ∼3.5 kbar , which corresponds to an insulator–metal phase transition. In the metal phase above Pc, we have found that γ reaches 100 mJ/mol K 2 , indicating a strongly correlated metallic state. In the insulator phase, on the other hand, we have observed an anomaly in the temperature dependence of the specific heat around 1 K , which seems to survive in the metal phase.