X. W. Li

Transport and magnetic properties of epitaxial and polycrystalline magnetite thin films

The transport and magnetic properties of magnetite (Fe3O4) thin films grown epitaxially on single crystal MgO(100) and SrTiO3(100) substrates, and with multiple grain orientations on polycrystalline SrTiO3 substrates, have been investigated. The films are grown using pulsed laser deposition and their epitaxial quality determined using ion channeling measurements. Transport and magnetic studies of Fe3O4 films as a function of thickness and morphology suggest that epitaxial strain and growth defects affect the width and temperature of the Verwey transition. In addition, these factors also significantly influence the magnetic coercivity of the films. The low-field magnetoresistance (MR) behaviors of epitaxial and polycrystalline films as a function of temperature have been compared and they were found to be quite similar, suggesting very small contribution to the MR from grain boundaries.

Magnetoresistance and Hall effect of chromium dioxide epitaxial thin films

Epitaxial CrO2 thin films have been grown on TiO2(100) and Al2O3(0001) substrates by atmospheric pressure chemical vapor deposition. The films have a Curie temperature (Tc) of around 393 K with the ones grown on TiO2 exhibiting in-plane uniaxial magnetic anisotropy. They also display metallic characteristics, with room temperature resistivity of about 285 μΩu200acm, dropping by about two orders of magnitude upon cooling down to 5 K. Magnetoresistance (MR) properties of the films have been measured with the magnetic field in the plane. For a field of 40 kOe, a positive transverse MR of about 25% at 5 K and a negative MR of about 7% at near Tc have been observed. In addition, Hall resistivity has been measured with magnetic field up to 40 kOe. A positive ordinary Hall effect is found at low temperatures, indicating the conduction carriers are holes.

Magnetic and transport properties of epitaxial and polycrystalline chromium dioxide thin films (invited)

The magnetic and transport properties of epitaxial and polycrystalline chromium dioxide (CrO2) thin films have been investigated. They are grown epitaxially on single crystal TiO2 (100) substrates, and with multiple grain orientations on polycrystalline TiO2 substrates, by chemical vapor deposition. The films have a Curie temperature (TC) of 390–395 K, with the epitaxially grown CrO2 (100) films exhibiting in-plane uniaxial magnetic anisotropy. While the epitaxial samples display metallic characteristics, the polycrystalline films are semiconducting with a dominant grain boundary contribution to the resistance at low temperatures. The magnetoresistance (MR) properties have also been measured with the magnetic field applied in the plane. For the epitaxial films, the MR is negative at temperatures near TC and is positive at low temperatures. A negative MR is also observed near TC for the polycrystalline samples. However, unlike the epitaxial films, the MR is found to be negative also at low temperatures, wi...

Bias voltage and temperature dependence of magnetotunneling effect

We have studied systematically the magnetotunneling properties of several metallic magnetictunnel-junction systems (Ni80Fe20–insulator–Ni80Fe20,Ni80Fe20–I–Co,Co–I–Co, Ni40Fe60–I–Co). The room-temperature magnetoresistance MR value at zero-bias ranges between 16% and 27%, depending on the spin polarization of the electrodes. There seems to be a general bias dependence of MR in all of these systems. In particular, it requires a bias in the range of 0.22–0.23 V to suppress the maximum MR value by half. We have also measured the bias dependence of MR as a function of barrier parameters (thickness and oxidation time). At low temperature, a sharp cusplike feature appears near zero bias. In some cases, low-temperature MR values substantially exceed expectations from established spin-polarization.

Magnetic domain structures of La0.67Sr0.33MnO3 thin films with different morphologies

Using a wide-field Kerr microscope, we have studied the magnetic domain structures of epitaxial and polycrystalline La0.67Sr0.33MnO3 thin films as well as a film having thermally induced 〈110〉 microcracks. The epitaxial film on a (001) SrTiO3 substrate has different magnetic domain behaviors for in-plane fields applied along the 〈100〉 and 〈110〉 directions. Magnetic domain orientation and contrast suggest a biaxial magnetic anisotropy with 〈110〉 easy axes. Defects such as microcracks and grain boundaries have a strong perturbing effect on the local magnetization and can lead to an enhanced and controllable spin-dependent scattering.

Extraordinary Hall effect in (111) and (100)-orientated Co/Pt superlattices

We have fabricated a series of (111)- and (100)-orientated Co/Pt superlattices of high quality. (111)-orientated superlattices acquire a perpendicular magnetic anisotropy for samples with thin Co layers whereas for the (100)-orientated superlattices the easy magnetization direction remains in plane. Analysis of the extraordinary Hall effect (EHE) verifies the applicability of simple scaling laws between the resistivity and the EHE coefficient for these layered materials. In addition, we have found a marked dependence of the skew coefficient on Co layer thickness. The contribution to EHE from quantum side-jump remains relatively constant.

Epitaxial La0.67Sr0.33MnO3 magnetic tunnel junctions

We report the observation of a large magnetoresistance (83%) at low magnetic fields of tens of Oe at 4.2 K in the epitaxial trilayer junction structure, La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3. The spin-polarization parameter of the manganite has been determined from the magnetoresistance value. The switching fields of the two magnetic layers were designed by using the magnetic shape anisotropy. By limiting the sweeping field in a low field range (∼100 Oe), we have achieved bistable resistive states at zero field, which is of potential interest for magnetoelectronic applications.

Sub-200 Oe giant magnetoresistance in manganite tunnel junctions

Metallic manganite oxides, La{sub 1{minus}x}D{sub x}MnO{sub 3} (D = Sr, Ca, etc.), display colossal magnetoresistance (CMR) near their magnetic phase transition temperatures ({Tc}) when subject to a Tesla-scale magnetic field. This phenomenal effect is the result of the strong interplay inherent in this class of materials among electronic structure, magnetic ordering, and lattice dynamics. Though fundamentally interesting, the CMR effect achieved only at large fields poses severe technological challenges to potential applications in magnetoelectronic devices, where low field sensitivity is crucial. Among the objectives of the research effort involving manganite materials is to reduce the field scale of MR by designing and fabricating tunnel junctions and other structures rich in magnetic domain walls. The junction electrodes were made of doped manganite epitaxial films, and the insulating barrier of SrTiO{sub 3}. The interfacial epitaxy has been imaged by using high-resolution transmission electron microscopy (TEM). The authors have used self-aligned lithographic process to pattern the junctions to micron scale in size. Large MR values close to 250% at low fields of a few tens of Oe have been observed. The mechanism of the spin-dependent transport is due to the spin-polarized tunneling between the half-metallic electrodes, in which the spins of the conductionmorexa0» electrons are nearly fully polarized. The authors present results of field and temperature dependence of MR in these structures and discuss the electronic structure of the manganite inferred from tunneling measurement. Results of large MR at low fields due to the grain-boundary effect is also presented.«xa0less

Bias Voltage and Temperature Dependence of Magneto-Tunneling

The large magnetoresistance in spin dependent tunneling (SDT) junctions has great device potential. One of the puzzling features of SDT is its strong dependence of MR on bias voltage. We have studied a variety of junctions and have observed a correlation between this voltage dependence and the non-linearity of the I-V characteristic. Since trap states in tunneling barriers can lead to such non-linear behavior, we have developed a model based on the assumption of traps which provide a second, spin-independent, channel for tunneling, in addition to the spin-dependent direct tunneling channel. Such traps in our case might arise from nonuniformity oxidized aluminum or impurities. The ratio of the two currents, which depends on the trap state density, determines the magnetoresistance ratio. The model predicts that the spin independent current is largely responsible for the non-linearity of the I-V curve. The relative contribution of the spin independent current increases as bias voltage increases, resulting in a MR decrease. A slower decrease is predicted for low trap state density. When the trap state density is high, the major portion of the tunneling current is spin independent, resulting in a low MR. Thus low MR, large non-linearity and strong MR-V are correlated as observed. Further, a thicker barrier tends to result in a stronger MR-V dependence. These predictions have been confirmed by experimental results. It is concluded that trap states are mainly responsible for the observed MR-V dependency. To minimize this voltage dependence and to increase the MR. a high quality barrier with large barrier height and low trap density is essential. Thin barriers, which allow low device resistance for high speed and low noise applications. also help to decrease the MR-V dependence. These results suggest that the voltage dependence of SDT magnetoresistance can be used to study the trap state distribution in the barrier.