Moon-Ho Jo

Inhomogeneous transport in heteroepitaxial La0.7Ca0.3MnO3/SrTiO3 multilayers

We have investigated the electrical transport properties of heteroepitaxial La0.7Ca0.3MnO3(LCMO)/SrTiO3 multilayers; as the LCMO layer thickness was varied from 25 to 2.5 nm, the metallic transition was suppressed and enhanced magnetoresistance extended over low temperatures. The results of transport and magnetic measurements imply a vertically inhomogeneous magnetic structure in the LCMO layers, with magnetically disordered interfaces. Although strain is clearly implicated in this surface disorder, we show that intrinsic magnetic disorder must also be associated with the interfaces.

Growth of strongly biaxially aligned MgB2 thin films on sapphire by postannealing of amorphous precursors

MgB2 thin films were cold grown on sapphire substrates by pulsed laser deposition followed by postannealing in mixed, reducing gas, Mg rich, Zr gettered, environments (pO2∼10−24 atm) at 750 and 950 °C. The films had Tc in the range 29–34 K, Jc (20 K, H=0) in the range 3×104–3×105 A cm−2, and irreversibility fields H* at 20 K of 4–6.2 T. An inverse correlation was found between Tc and H*. The films had grain sizes of ∼0.1–1 μm and a strong biaxial alignment was observed in the 950 °C annealed film. (111) oriented MgO was also observed. Mg coating of films during crystallization appeared to improve film Tc.

Magnetic anisotropy of thin film La0.7Ca0.3MnO3 on untwinned paramagnetic NdGaO3 (001)

We describe in detail a method by which to establish the magnetic anisotropy of thin ferromagnetic films on strongly paramagnetic substrates that are slightly anisotropic. The film that we consider is composed of the much studied manganite La0.7Ca0.3MnO3 and the substrate is NdGaO3, a good lattice match. Below a Curie temperature Tc of 260 K it was found, using a vibrating sample magnetometer, that 72±3 nm La0.7Ca0.3MnO3 films grown epitaxially by pulsed laser deposition on untwinned orthorhombic NdGaO3 (001) substrates exhibit uniaxial anisotropy with K=(3.6±0.1)×105 erg cm−3. The easy direction is along [110] of the pseudocubic unit cell, i.e., diagonal to the O–Mn–O bond directions and parallel to the side of the actual unit cell which is orthorhombic. We attribute an 11±4% loss of the low temperature moment to the proximity of the paramagnetic substrate rather than to stress. It is argued that stress is minimal such that the observed anisotropy must be magnetocrystalline. Both the reduction in moment ...

The microwave surface impedance of MgB2 thin films

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB2. The powder sample has a Tc = 39 K and the films have Tc = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with Tc = 38 K degraded to a Tc of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.

Coherent magnetic reversal in half-metallic manganite tunnel junctions

We report coherent magnetization reversal in the La0.7Ca0.3MnO3/NdGaO3/La0.7Ca0.3MnO3 tunnel junctions probed by spin-polarized tunneling. The junction resistance shows an extremely sharp switching with distinct binary resistances and its two-dimensional angular dependence reveals two different magnetic anisotropies in play in the top and bottom La0.7Ca0.3MnO3 electrodes. The large bottom electrode (50×2000 μm) shows the effective twofold crystalline anisotropy and the top electrode (6×6 μm) shows an additional fourfold anisotropy. The geometrical expression of the switching fields in two-dimensional polar coordinates provides evidence that the coherent magnetization reversal of the junctions, particularly the top electrode, closely resembles the ideal coherent rotation model.

Microwave surface resistance of MgB2

The microwave power and frequency dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by the breakdown in the ω2 law for poor-quality samples at all temperatures. The performance of MgB2 at 10 GHz and 21 K was compared directly with that of high-quality YBCO films. The surface resistance of MgB2 was found to be approximately three times higher at low microwave power and showed an onset of nonlinearity at microwave surface fields ten times lower than the YBCO film. It is clear that MgB2 films are not yet optimized for microwave applications.

Spin- and charge-modulated trilayer magnetic junctions: La0.7Ca0.3MnO3/La0.45Ca0.55MnO3/La0.7Ca0.3MnO3

We report on magnetic tunnel junctions entirely made up of mixed-valence manganites, La0.7Ca0.3MnO3/La0.45Ca0.55MnO3/La0.7Ca0.3MnO3. In heteroepitaxial junctions, the different Mn3+/Mn4+ mixed-valence ratios can modulate the ground states throughout the trilayer, i.e. ferromagnetic metal/antiferromagnetic insulator/ferromagnetic metal. Interestingly, the tunnel magnetoresistance (TMR) of the device persists up to a higher temperature (T/TC ≤ 0.75, where TC is the Curie temperature) as compared to the case for equivalent non-manganite barrier junctions. The enhanced TMR at high temperatures in the present junction is discussed in relation to the properties of the unique interface between the metallic ferromagnet and the antiferromagnetic tunnel barrier, such as the interfacial bonding coherence and a magnetic interlayer coupling.

Magnetization reversal probed by spin-polarized tunneling

We report magnetic reversal processes in the magnetic tunnel junction (MTJ) based on a half metallic manganite, La0.7Ca0.3MnO3 by comprehensive spin-polarized tunneling (SPT) measurements. The large tunnel magnetoresistance up to 77% of (Rap−Rp)/Rap in the present MTJ is highly sensitive to the local magnetization fluctuation in the ferromagnetic electrodes and thus enables us to establish an instantaneous one-to-one correlation between the magnetization reversal and the SPT with the two-dimensional SPT measurements. We have found the systematic angular variations of the SPT features in the resistance-field curves, and based on the observed angular dependences, we propose a crucial role of the edge-domain pinning and the resultant multi-domain breakup for the magnetization reversal.

Assessment of colossal magnetoresistive manganite thin films for infrared detector applications

Recently there has been great interest in doped manganite thin films exhibiting colossal magnetoresistance (CMR), in particular for magnetic sensing applications. These perovskite oxides exhibit a metal-insulator transition at the Curie temperature which is accompanied by a large change of resistance as the material loses its ferromagnetic properties. Since this resistance change occurs over a relatively narrow temperature range it is accompanied by very large temperature coefficients of resistance in the region of the phase transition, making these materials ideal candidates for infrared detectors utilizing resistance bolometers. This paper reports measured physical and electrical properties, the latter including 1/f noise, of doped manganite thin film CMR material deposited by pulsed- laser deposition. The potential performance of CMR based resistance bolometer devices is reported.