Y. H. Jeong

Magnetism in Mn-doped ZnO bulk samples prepared by solid state reaction

Contrasting magnetic properties were obtained from bulk Mn-doped ZnO synthesized under different processing conditions. While a ferrimagnetic phase transition was observable in a Zn0.95Mn0.05O sample processed at 1170 K, no such transition was found for a sample with the same composition processed at 1370 K. The detailed magnetic, structural, and spectroscopic studies of these two samples have revealed that the ferrimagnetic transition in the former sample is attributable to the secondary phase, (Mn,Zn)Mn2O4, in the system. For the latter sample processed at higher temperature, no secondary phase was detected and the major feature of the system remained paramagnetic down to 4 K. The implication of the present results for Mn-doped ZnO thin films is discussed.

Large magnetocaloric effect in a La0.7Ca0.3MnO3 single crystal

We report the results of a thorough study of the magnetocaloric effect (MCE) in a La0.7Ca0.3MnO3 single crystal, which undergoes a first-order magnetic phase transition at ∼227 K. The magnetic entropy change ΔSM and the adiabatic temperature change ΔTad reach, respectively, ∼6.42 J/kg K and 4.76 K for ΔB=5 T and even if both are smaller than those exhibited by gadolinium (∼9.8 J/kg K and ∼11.7 K), the ΔSM distribution here is much more uniform than that of gadolinium and polycrystalline manganites. This is desirable for an Ericson-cycle magnetic refrigerator. The MCE is larger in the single-crystalline manganite compared to the polycrystalline one. The manganite single crystal has large ΔSM induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. All these make the lanthanum manganite single crystal an attractive candidate as a working substance for AMR. The molecular field model provides a fairly good description of the magnet...

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.

Universal Ti-rich termination of atomically flat SrTiO3 (001), (110), and (111) surfaces

We have studied the surface termination of atomically flat SrTiO3 surfaces treated by chemical etching and subsequent thermal annealing, for all commercially available orientations (001), (110), and (111). Atomic force microscopy confirms that our treatment processes produce unit cell steps with flat terrace structures. We have also determined the topmost atomic layer of SrTiO3 surfaces through time-of-flight mass spectroscopy. We found that all three orientations exhibit a Ti-rich surface. Our observation opens doors for interface engineering along the [110] and [111] directions in addition to a well known [100] case, which widens the range of functional heterostructures and interfaces.

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.

Multiphase segregation and metal-insulator transition in single crystal La 5 / 8 − y Pr y Ca 3 / 8 MnO 3

The insulator-metal transition in single crystal La{sub 5/8-y}Pr{sub y}Ca{sub 3/8}MnO{sub 3} with y{approx}0.35 was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature T{sub MI}, the charge-ordering (CO) peaks exhibit no anomaly at this temperature and continue to grow below T{sub MI}. Our data suggest then, that in addition to the CO phase, another insulating phase is present below T{sub CO}. In this picture, the insulator-metal transition is due to the changes that occur within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolationlike insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.

Progress in experimental techniques for dynamic calorimetry

Abstract The dynamic heat capacity of a substance is a fundamental quantity related to the dynamics of the material and its measurement provides invaluable information on the system dynamics. We developed dynamic calorimeters based on the temperature modulation method and the adiabatic method. These dynamic calorimetric techniques were applied to the study of slow relaxation of a supercooled liquid [Ca(No 3 ) 2 ] 0.4 (KNO 3 ) 0.6 near the glass transition. After defining dynamic heat capacity from the linear response theory, we describe theoretical as well as technical aspects of modulation calorimeters and adiabatic calorimeters developed in this laboratory in detail. Based on the dynamic heat capacity data of calcium-potassium nitrate obtained with these calorimeters, merits and demerits of the various methods are pointed out. Future prospects are given.

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.

Structural and magnetic properties of CoFe alloy films

The investigation of the structural and magnetic properties of bcc Co1−xFex (0.1<x<0.4) alloy films prepared on MgO(001) substrates using e-beam evaporation and pulsed laser ablation deposition (PLAD) methods is reported. Both structural and magnetic properties seem to be strongly correlated to the deposition conditions, i.e., substrate temperatures (Ts) and deposition rates (DR). To identify the stability limits of the bcc structure in the phase diagram, our study is limited to the Co-rich compounds. For x⩾0.3 at high Ts (500 °C), alloy films deposited by e-beam evaporation exhibit a bcc (001) growth, showing the magnetic anisotropy with easy axis parallel to the MgO [100] crystal axis. In the PLAD technique, the excellent bcc (001) film growth is observed at Ts=300 °C for x=0.25 which is close to the concentration limit of the bcc regime in the equilibrium bulk phase diagram, whereas the same compound films deposited at high Ts reveal the mixed phase of fcc (001) and bcc (001) structures. Magnetic hyste...

Enthalpy and dielectric relaxation in the glass transition region of polypropylene glycol

Abstract We investigated enthalpy and dielectric relaxation behaviors in the glass transition region of polypropylene glycol of molecular weight 1000 and 4000 by measuring both dynamic heat capacity and dielectric susceptibility as a function of frequency and temperature. The data at each temperature were fitted to the Kohlrausch–Williams–Watt (KWW) relaxation function, and the relaxation time, the shape parameter β , and the relaxation strength were extracted. The relaxation times follow the same Vogel–Tammann–Fulcher (VTF) equation for both enthaply and dielectric responses, while β of the KWW relaxation function showed different temperature dependences. β of the enthalpy relaxation shows a linear temperature dependence, whereas it stays constant in the dielectric relaxation. Significant molecular weight dependence occurs only in the relaxation strength. We discuss the temperature dependence of the shape parameter in terms of broadening of the relaxation time distribution associated with cooperative molecular rearrangements.