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We report the first high-field x-ray diffraction experiment using synchrotron x-rays and pulsed magnetic fields exceeding 30 T. Lattice deformation due to a magnetic-field-induced valence transition in YbInCu4 is studied. It has been found that the Bragg reflection profile at 32 K changes significantly at around 27 T due to the structural transition. In the vicinity of the transition field the low-field and the high-field phases are observed simultaneously as the two distinct Bragg reflection peaks: This is a direct evidence of the fact that the field-induced valence state transition is the first order phase transition. The field-dependence of the low-field-phase Bragg peak intensity is found to be scaled with the magnetization.

The magnetic field-induced valence transition in YbInCu 4 has been studied by X-ray absorption spectroscopy at the Yb L III -edge up to 41 T. The field-induced valence transition is clearly observed as a significant change in the absorption spectra. Magnetic field dependences of Yb valence at low temperatures are directly determined for the first time. It is found that the valence above the metamagnetic transition recovers from Yb 2:84þ to Yb 2:96þ which is identical to that in the hightemperature phase. Variation of the valence is also obtained in the B-T (magnetic field-temperature) plane at 0 < B < 41 T and 5 < T < 50 K.