H.D. Yang

Antisite disorder driven spontaneous exchange bias effect in La2-xSrxCoMnO6 (0<x<1)

Doping at the rare-earth site by divalent alkaline-earth ions in perovskite lattice has witnessed a variety of magnetic and electronic orders with spatially correlated charge, spin and orbital degrees of freedom. Here, we report an antisite disorder driven spontaneous exchange bias effect as a result of hole carrier (Sr(2+)) doping in La(2-x)Sr(x)CoMnO6 (0u2009u2009<u2009u2009xu2009u2009<u2009u20091) double perovskites. X-ray diffraction and Raman spectroscopy have evidenced an increase in disorder with the increase of Sr content up to xu2009u2009=u2009u20090.5 and thereby a decrease from xu2009u2009=u2009u20090.5 to 1. X-ray absorption spectroscopy has revealed that only Co is present in the mixed valence of Co(2+) and Co(3+) states with Sr doping to compensate the charge neutrality. Magnetotransport is strongly correlated with the increase of antisite disorder. The antisite disorder at the B-site interrupts the long-range ferromagnetic order by introducing various magnetic interactions and instigates reentrant glassy dynamics, phase separation and canted type antiferromagnetic behavior with the decrease of temperature. This leads to a novel magnetic microstructure with unidirectional anisotropy that causes a spontaneous exchange bias effect that can be tuned with the amount of antisite disorder.

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 42xa0K, irrespective of the orientation of the applied field. However, an additional magnetic ordering occurring around 35xa0K 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 35xa0K the expected abrupt change in dielectric constant appears at a much lower temperature and reaches maximum around 13.5xa0K, 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 35xa0K might be more relevant to the strain-induced effect on antiferromagnetism.

Metamagnetic behaviour and effect of field cooling on sharp magnetization jumps in multiferroic Y2CoMnO6

We present sharp magnetization jumps and field-induced irreversibility in magneti- zation in multiferroic Y2CoMnO6. The appearance of magnetic relaxation and field sweep rate dependence of magnetization jumps resembles the martensite-like scenario and suggests the coexis- tence of E*-type antiferromagnetic and ferromagnetic phases at low temperatures. In Y2CoMnO6, the critical field required for the sharp jump can be increased or decreased depending on the mag- nitude and direction of the cooling field; this is remarkably different from manganites or other metamagnetic materials where the critical field increases irrespective of the direction of the applied field cooling. The cooling field dependence on the sharp magnetization jumps has been described by considering exchange pinning mechanism at the interface, like in the exchange bias model. Copyright c EPLA, 2014

Anomalous dielectric behavior in nanoparticle Eu2O3 : SiO2 glass composite system

Eu2O3 (0.5 mol%) nanoparticles have been synthesized in a silica glass matrix by the sol-gel method at calcination temperatures of 700 °C and above. Compared with the parent material SiO2, this nano-glass composite system shows enhancement of dielectric constant and diffuse phase transition along with magnetodielectric effect around room temperature (~270 K). The observed conduction mechanism is found to be closely related to the thermally activated oxygen vacancies. Magnetodielectric behavior is strongly associated with magnetoresistance changes, depending on the nanoparticle size and separation. Such a material might be treated as a potential candidate for device miniaturization.

A low-temperature specific heat study of giant dielectric constant materials

A low-temperature specific heat study has been performed on the insulating giant dielectric constant material CaCu3Ti4O12 and two related compounds, Bi2/3Cu3Ti4O12 and La0.5Na0.5Cu3Ti4O12, from 0.6 to 10 K. From analyzing the specific heat data in a very low-temperature range, 0.6–1.5 K, and moderately low-temperature range, 1.5–5 K, in addition to the expected Debye terms, we observed significant contributions originating from the linear and Einstein terms, which we attributed as the manifestation of low-lying elementary excitations due to lattice vibrations occurring at the grain boundaries and induced by local defects. Together with the findings on electronic and mechanical properties, a phenomenological model is proposed to explain the high dielectric constant behavior in both low and high frequency regions.

Effects of Jahn–Teller distortion on the skyrmion stability of (Cu1−xNix)2OSeO3

We explore the Jahn–Teller distortion of Ni2+ ions on the skyrmion phase of Cu2OSeO3. Polycrystalline (Cu1−xNix)2OSeO3 (0 ≤ x ≤ 0.06) samples were synthesized by solid-state reaction. X-ray diffraction, X-ray absorption near-edge spectroscopy, extended X-ray absorption fine structure spectroscopy, magnetic and magnetodielectric measurements were performed and comprehensively analyzed. These revealed that the Ni2+ ion occupies both the Cu sites almost equally, and the Jahn–Teller distortion of the Ni2+ ion invokes a change in the local trigonal bipyramidal structure, which has a significant influence on the H–T magnetic phase diagram and electrical polarization of (Cu1−xNix)2OSeO3. A large enhancement of the skyrmion area upon Ni-doping can be ascribed to the magnetic anisotropy of the Jahn–Teller active Ni2+ ion in the Cu(I) site. The observed results indicate that chemical doping is a unique way to control and manipulate skyrmion boundaries in Cu2OSeO3.