Strain relaxation dynamics of multiferroic orthorhombic manganites.

Resonant ultrasound spectroscopy has been used to characterise strain coupling and relaxation behavior associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3and TbMn0.98Fe0.02O3through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic to colinear-sinusoidal incommensurate antiferromagnetic transition at ∼41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ∼150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy,Ea, of ≳0.04 eV from a loss peak at ∼80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ∼10-6s. Comparison with previously published data for Sm0.6Y0.4MnO3confirms that this pattern may be typical for multiferroic orthorhombicRMnO3perovskites (R= Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3and TbMn0.98Fe0.02O3, but not for GdMnO3, yieldedEa⩾ ∼0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ∼0.1-1‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic-orthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients.

Exchange bias in bulk layered hydroxylammonium fluorocobaltate (NH3OH)2CoF4.

The magnetic properties of layered hydroxylammonium fluorocobaltate (NH(3)OH)(2)CoF(4) were investigated by measuring its dc magnetic susceptibility in zero-field-cooled (ZFC) and field-cooled (FC) regimes, its frequency dependent ac susceptibility, its isothermal magnetization curves after ZFC and FC regimes, and its heat capacity. Effects of pressure and magnetic field on magnetic phase transitions were studied by susceptibility and heat capacity measurements, respectively. The system undergoes a magnetic phase transition from a paramagnetic state to a canted antiferromagnetic state exhibiting a weak ferromagnetic behavior at T(C) = 46.5 K and an antiferromagnetic transition at T(N) = 2.9 K. The most spectacular manifestation of the complex magnetic behavior in this system is a shift of the isothermal magnetization hysteresis loop in a temperature range below 20 K after the FC regime-an exchange bias phenomenon. We investigated the exchange bias as a function of the magnetic field during cooling and as a function of temperature. The observed exchange bias was attributed to the large exchange anisotropy which exists due to the quasi-2D structure of the layered (NH(3)OH)(2)CoF(4) material.

Effect of pressure on the magnetic properties of TM(3)[Cr(CN)(6)](2)·12H(2)O.

We present the results of magnetization and AC susceptibility measurements performed on ferrimagnetic Mn(3)(2+)[Cr(III)(CN)(6)](2)·12H(2)O and ferromagnetic Ni(3)(2+)[Cr(III)(CN)(6)](2)·12H(2)O systems under pressures up to 0.9 GPa in a commercial SQUID magnetometer. The magnetization process is affected by pressure: magnetization saturates at higher magnetic field, saturated magnetization µ(s) of Ni(3)[Cr(CN)(6)](2) is reduced and almost unaffected for Mn(3)[Cr(CN)(6)](2) at low temperatures. The Curie temperature T(C) of Mn(3)[Cr(CN)(6)](2) increases with the applied pressure, ΔT(C)/Δp = 25.5 K GPa(-1), due to a strengthened super-exchange antiferromagnetic interaction J(AF), but it is not affected significantly in the case of Ni(3)[Cr(CN)(6)](2) with a dominant ferromagnetic J(F) super-exchange interaction. The increase in the J(AF) interaction is attributed to the enhanced value of the single electron overlapping integral S and the energy gap Δ of the mixed molecular orbitals t(2g) (Mn(2+)) and t(2g) (Cr(III)) induced by pressure.