T. Y. Koo

Anisotropic strains and magnetoresistance of La0.7Ca0.3MnO3

Thin films of perovskite manganite La0.7Ca0.3MnO3 were grown epitaxially on SrTiO3(100), MgO(100) and LaAlO3(100) substrates by the pulsed laser deposition method. Microscopic structures of these thin film samples as well as a bulk sample were fully determined by x-ray diffraction measurements. The unit cells of the three films have different shapes, i.e., contracted tetragonal, cubic, and elongated tetragonal for SrTiO3, MgO, and LaAlO3, respectively, while the unit cell of the bulk is cubic. It is found that the samples with a cubic unit cell show smaller peak magnetoresistance at low fields (≲1 T) than the noncubic ones do. The present result demonstrates that the magnetoresistance of La0.7Ca0.3MnO3 at low fields can be controlled by lattice distortion via externally imposed strains.

Strain-induced magnetoelectric coupling in BaTiO3/Fe3O4 core/shell nanoparticles

We report a strain-induced magnetoelectric coupling in BaTiO3/Fe3O4 ferroelectric core/ferrimagnetic shell nanoparticles. The temperature-dependent magnetoresistance and magnetization clearly show several jumps near the structural phase transition temperatures of BaTiO3. Below the Verwey transition temperature of Fe3O4, i.e., at 20 K, the dielectric constant of BaTiO3/Fe3O4 decreases continuously upon application of an external magnetic field, and the observed magnetodielectric curve does not follow the square of the magnetization. We discuss the effect of strain on the electric field-dependent magnetic anisotropy near the core/shell interface.

Magnetodielectric coupling in core/shell BaTiO3∕γ-Fe2O3 nanoparticles

We report an intriguing magnetodielectric coupling in BaTiO3∕γ-Fe2O3 dielectric core/ferrimagnetic shell nanoparticles. The dielectric constant steeply increases with magnetic field, and the frequency dependent magnetodielectric curve shows a resonancelike peak at high temperatures, while it decreases smoothly with field and no peak appears in the frequency dependent magnetodielectric curve at low temperatures. We attribute the observed magnetodielectric coupling to the Maxwell-Wagner effect combined with magnetoresistance at high temperatures and to possible spin-lattice coupling and its modification near interfaces at low temperatures.

Orbital ordering and enhanced magnetic frustration of strained BiMnO3 thin films

Epitaxial thin films of multiferroic perovskite BiMnO3 were synthesized on SrTiO3 substrates, and orbital ordering and magnetic properties of the thin films were investigated. The ordering of the Mn3+ eg orbitals at a wave vector (¼¼¼) was detected by Mn K-edge resonant X-ray scattering. This peculiar orbital order inherently contains magnetic frustration. While bulk BiMnO3 is known to exhibit simple ferromagnetism, the frustration enhanced by in-plane compressive strains in the films brings about cluster-glass-like properties.

Synthesis of nanoscale composites of exchange biased MnFe2O4 and Mn-doped BiFeO3

Exchange bias may be used to couple a normal ferromagnet to a ferroelectric antiferromagnet and thus create a multiferroic system with nonzero magnetization. In implementing this idea the authors developed a synthesis method for composite films of nanoscale clusters of ferromagnetic MnFe2O4 embedded in multiferroic Mn-doped BiFeO3. The method utilizes the Bi volatility to obtain the composite films via thermal annealing of multilayers composed of BiFeO3 and BiMnO3. The cluster size varies from 20to80nm depending on the film thickness. The composite films possess both ferroelectric and ferromagnetic properties; the magnetic hysteresis curves, in particular, manifest exchange bias.

Local strain-induced 90° domain switching in a barium titanate single crystal

Local strain analysis for a BaTiO3 single crystal under electric loads was conducted using x-ray microdiffraction technique. During the polarization reversal, local 90° domain switchings were observed at certain positions instead of uniform 180° domain switching. Field-dependent strain analysis demonstrated that inhomogeneous domain switching was strongly related to local strain distributions. The results can be understood in terms of competition between elastic and electrostatic energies and provide a crystallographic evidence for the role of local strains on 90° domain switching.

Evidence of a consolute critical point in the phase separation regime of La 5 ∕ 8 − y Pr y Ca 3 ∕ 8 MnO 3 ( y ∼ 0.4 ) single crystals

We report on DC and pulsed electric field sensitivity of the resistance of mixed valent Mn oxide based La(5/8-y)Pr(y)Ca(3/8)MnO(3) (y = 0.4) single crystals as a function of temperature. The low temperature regime of the resistivity is highly current and voltage dependent. An irreversible transition from high (HR) to a low resistivity (LR) is obtained upon the increase of the electric field up to a temperature dependent critical value (V_c). The current-voltage characteristics in the LR regime as well as the lack of a variation in the magnetization response when V_c is reached indicate the formation of a non-single connected filamentary conducting path. The temperature dependence of V_c indicates the existence of a consolute point where the conducting and insulating phases produce a critical behavior as a consequence of their separation.

aMagnetic control of spin reorientation and magnetodielectric effect below the spin compensation temperature in TmFeO3

The onset of antiferromagnetic transition, spin reorientation, and spin compensation of TmFeO3 single crystals were investigated by the magnetic and heat capacity measurements. Control of spin reorientation by magnetic field and anomalous hysteretic behavior in domain switching were clarified. No appreciable magnetodielectric effect was observed in spin reorientation temperature region. On the other hand, below the spin compensation temperature both a dielectric anomaly along the c axis and a concomitant magnetodielectric effect up to ∼4% at 80kOe were observed. This suggests that rare-earth orthoferrites can be another candidate for magnetodielectric system through the mediation of spin compensation phenomena.

Fisher–Langer relation and scaling in the specific heat and resistivity of La0.7Ca0.3MnO3

Abstract We have measured the specific heat and resistivity of mixed valence manganite La 0.7 Ca 0.3 MnO 3 as a function of temperature and magnetic field. We demonstrate that the magnetic contribution to the specific heat in this system is proportional to the temperature coefficient of the resistivity in the magnetic phase transition region, following the Fisher–Langer relation. We also show that the specific heat and the temperature coefficient of resistivity measured as a function of temperature at various magnetic fields can be superposed to a single curve if they are properly scaled.

Dynamic specific heat of KH2PO4 near the Curie point

Abstract We have developed the technique, called 3ω method, capable of measuring the dynamic specific heat of a solid in the frequency range from 0.01 Hz to 2 kHz and used this technique to measure the specific heat of KH2PO4 as a function of frequency. The specific heat becomes frequency-dependent near the Curie point.