Joonghoe Dho

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents. Doping of 2at.% Ti4+ ions increased the dc resistivity by more than three orders of magnitude. In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude. Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant.

Strain-induced magnetic stripe domains in La0.7Sr0.3MnO3 thin films

We have investigated magnetic microstructures of magnetoresistive La0.7Sr0.3MnO3 (LSMO) thin films. Magnetic images are strongly dependent on structural strain induced by the substrates. The LSMO film on SrTiO3 dominated by tensile stress effect displays a feather-like pattern, whereas LSMO films on LaAlO3 and NdGaO3 substrates under compressive stress show stripe domains. In particular, the magnetic image of the film on NdGaO3 reveals distinctive straight stripe domain patterns on the order of about 120 nm, suggesting the presence of a sizable out-of-plane magnetization. The ordering of the stripe domains is also sensitive to the field direction.

High-resolution x-ray diffraction and transmission electron microscopy of multiferroic BiFeO3 films

High-resolution x-ray diffraction and transmission electron microscopy (TEM) have been used to study BiFeO3 thin films grown on the bare and SrRuO3 buffered (001) SrTiO3 substrates. Reciprocal space mapping (RSM) around (002) and (103) reflections revealed that BFO films with a thickness of about 200 nm were almost fully relaxed and had a rhombohedral structure. Cross-sectional, high-resolution TEM showed that the films started to relax at a very early stage of growth, which was consistent with the RSM results. A thin intermediate layer of about 2 nm was observed at the interface, which had a smaller lattice than the overgrown film. Twist distortions about the c axis to release the shear strain introduced by the growth of rhombic (001) BiFeO3 on cubic (001) SrTiO3 were also observed. The results indicate that a strained, coherent BiFeO3 film on (001) SrTiO3 is very difficult to maintain and (111) STO substrates are preferable.

Oxygen pressure and thickness dependent lattice strain in La0.7Sr0.3MnO3 films

We report magnetic and electronic properties of La0.7Sr0.3MnO3 (LSMO) thin films epitaxially grown on perovskite substrates by pulsed laser deposition, which are varied with oxygen background pressure and film thickness. The strains of the LSMO films are tuned by the two parameters but their resulting effects are somewhat different. The lattice strain induced by the oxygen pressure suppresses the ferromagnetic transition (TC) and metal–insulator transition (TMI) temperatures. With decreasing film thickness from 110 to 11 nm, however, small changes in both TC and TMI were observed. These results suggest that the physical properties of the LSMO films are strongly dependent on the oxygen content but less sensitive to the film thickness.

Competing functionality in multiferroic YMnO3

We have investigated the dual functionality of multiferroic YMnO3, which simultaneously possesses antiferromagnetic and ferroelectric properties. When the YMnO3 was used as a pinning layer, it gave rise to an exchange bias and enhanced coercivity in a soft ferromagnetic film which depended on the YMnO3 crystal orientation. Significantly, the exchange bias and coercivity were minimal for the (0001) YMnO3 surface, which is optimal for ferroelectric applications. Consequently, although multiferroics, such as YMnO3, can be exploited in magnetic and ferroelectric applications independently, applications which aim to exploit their multifunctionality will be limited by the competing crystal orientations required for each application.

Controlling the exchange bias in multiferroic BiFeO3 and NiFe bilayers

We have investigated how the exchange bias and coercive field of NiFe/BiFeO3 bilayers on (001)SrTiO3 is affected by changes in substrate temperature, oxygen pressure, and film thickness for BiFeO3 deposition. Our results suggest that the exchange bias and the coercive field are mainly dependent on the roughness at the interface because (001)BiFeO3 has a compensated surface. The change in exchange bias associated with training effect and/or time relaxation, which are probably caused by a rearrangement of BiFeO3 domains under magnetic fields opposite to the initial magnetic field, may limit future spintronic applications of BiFeO3.

Thermal relaxation of field-induced irreversible ferromagnetic phase in Pr-doped manganites

We have investigated the magnetic and transport properties of

Percolative phase separation induced by nonuniformly distributed excess oxygen in low-doped La1-xCaxMnO3+delta (x <= 0.2)

{\mathrm{Pr}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{MnO}}_{3}

Anomalous optical switching and thermal hysteresis behaviors of VO2 films on glass substrate

and

Magnetic and microstructural properties on Mn- and Ru-doped hexagonal barium ferrites

{\mathrm{Pr}}_{0.82}{\mathrm{Na}}_{0.18}{\mathrm{MnO}}_{3}