Field Induced Valence Transition of Eu-based Compounds

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Some Eu compounds are known to show intermediate valence between Eu^<2+> and Eu^<3+> configurations. One of the important features of Eu systems is that they exhibit strong temperature dependence of the mean valence. Considering a large difference of magnetic moment between the divalent (7muB) and trivalcnt (0) states, the Eu valence must be affected by high magnetic fields. We have investigated the magnetic field effects on the Eu valence in this research. The following results were obtained.1) The compound EuPd_2Si_2 undergoes a precipitous, yet continuous, valence change at around 170K.We have found that this compound shows a metamagnetic transition under ultra-high filed of 93 T at 10 K.The saturation magnetization is about 7 mu B,which is in agreement with the theoretical value to the divalent state of Eu. This indicates that the metaganetic transition is a valence transition induced by high magnetic fields. This is the first example of this kind of transition found in the Eu-base … More d systems. The metamagnetic transition is accompanied with a large hysteresis, indicating a first order phase transition. This is in contrast to the continuous change of the valence in the temperature dependence.2) The substitution of Pt for Pd decreased the valence transition field (Hv) without broading the transition. It was found that the valence transition temperature (Tv) is proportional to Hv.3) Similar valence transition was observed in EuNi_2 (Sil_<1-x>Ge_x) _2 with 0.75*x*0.82. Especially, Hv of the compond with x=0.75 is lower than 45T,which can be generated by a long pulse magnet. The temperature dependence of the mean valence was studied. The linear relation between Hv and Tv is also observed in this system.4) We have discussed the field-induced valence transition in terms of the interconfigurational fluctuation (ICF) model by taking into account the Zeeman energy of the excited state. A satisfying description has been provied. Especially, we have demonstrated that a first order valcnce transition could be induced by magnetic field, even if the system shows a continuous valence transition against temperature. Less