Novel phase of solid oxygen induced by ultrahigh magnetic fields.

Magnetization measurements and magnetotransmission spectroscopy of the solid oxygen α phase were performed in ultrahigh magnetic fields of up to 193 T. An abrupt increase in magnetization with large hysteresis was observed when pulsed magnetic fields greater than 120 T were applied. Moreover, the transmission of light significantly increased in the visible range. These experimental findings indicate that a first-order phase transition occurs in solid oxygen in ultrahigh magnetic fields, and that it is not just a magnetic transition. Considering the molecular rearrangement mechanism found in the O(2)-O(2) dimer system, we conclude that the observed field-induced transition is caused by the antiferromagnetic phase collapsing and a change in the crystal structure.

High-field magnetization of a two-dimensional spin frustration system, Ni(5)(TeO(3))(4)X(2) (X = Br, Cl).

The high-field magnetization, M(H), of Ni(5)(TeO(3))(4)X(2) (X =  Br, Cl) was measured by using a pulse magnet. These compounds have a two-dimensional crystal structure and a distorted Kagome spin frustrated system which is built from the Ni(2+) ions (S = 1). The Néel transition temperatures are T(N)∼28 and 23 K for X =  Br and Cl, respectively. When TT(N), the field-dependent magnetization curves behave like a monotonically increasing straight line up to 55 T. The H(c) value is close to those obtained in previous spin resonance studies in which a model of a spin-flop scenario was proposed to explain the field-dependent resonance spectra. With the earlier model a further transition at around 23 T was predicted; however, our observations did not show any plateau behaviors, saturation or other anomalies up to 55 T, suggesting that the further transition possibly exists in a much higher field region.

Strain-induced effects on antiferromagnetic ordering and magnetocapacitance in orthorhombic HoMnO(3) thin films.

We investigated the magnetic and ferroelectric properties of c-axis oriented orthorhombic phase HoMnO(3) (o-HMO in Pbnm symmetry setting) thin films grown on Nb-doped SrTiO(3)(001) substrates. The o-HMO films exhibit an antiferromagnetic ordering near 42 K, irrespective of the orientation of the applied field. However, an additional magnetic ordering occurring around 35 K was observed when the field was applied along the c-axis of o-HMO, which was absent when the field was applied in the ab-plane. The magnetocapacitance measured along the c-axis showed that although there is evidence of dielectric constant enhancement when the temperature is below 35 K the expected abrupt change in dielectric constant appears at a much lower temperature and reaches maximum around 13.5 K, indicating that the low-temperature c-axis polarization might be related to the ordering of the Ho(3+) moment. The lattice constant analyses using x-ray diffraction and the observation of a slight magnetization hysteresis suggest that the weak second magnetic transition along the c-axis at 35 K might be more relevant to the strain-induced effect on antiferromagnetism.