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[en] Absolute spin-wave band gaps can be substantially opened and tuned by rotating noncircular rods in two-dimensional magnonic crystals. Spin-wave band structures of two-dimensional magnonic crystals composed of Fe (EuO) square rods squarely arranged in a EuO (Fe) matrix are numerically calculated using the plane-wave method. The results show that it is possible to increase the width of the band gaps or to create band gaps by rotating the noncircular rods. For the system of EuO rods in Fe matrix, the largest absolute spin-wave gap in the structure of square rods is 227% of the size of that in the corresponding structure of circular rods. Such an approach may open up a new scope for engineering band gaps of two-dimensional magnonic crystals.
[en] The pressure-volume relationship for EuO was investigated to 630 kilobars at room temperature with a diamond-anvil, high-pressure cell. Volumes were determined by x-ray diffraction; pressures were determined by the ruby R1 fluorescence method. The preferred interpretation involves normal compression behavior for EuO, initially in the B1 (NaCl-type) structure, to about 280 kilobars. Between approx. =280 and approx. =350 kilobars a region of anomalous compressibility in which the volume drops continuously by approximately 2% is observed. A second-order electronic transition is proposed with the 6s band overlapping with the 4f levels, thereby reducing the volume of EuO without changing the structure. This is not a semiconductor-to-metal transition. In reflected light, this transition is correlated with a subtle and continuous change in color from brown-black to a light brown. The collapsed B1 phase (postelectronic transition) is stable between approx. =350 and approx. =400 kilobars. At about 400 kilobars the collapsed B1 structure transforms to the B2 (CsCl-type) structure, with a zero pressure-volume change of approximately 12 +/- 1.5%