G. Q. Gong

Low-field magnetoresistive properties of polycrystalline and epitaxial perovskite manganite films

The low-field magnetoresistance (MR) properties of polycrystalline La0.67Sr0.33MnO3 and La0.67CaO33MnO3 thin films with different grain sizes have been investigated and compared with epitaxial films. MR as high as 15% has been observed in the polycrystalline films at a field of 1500 Oe at low temperatures, whereas the MR of the epitaxial films is less than 0.3% in the same field range. Based on the magnetization dependence of the MR, the current-voltage characteristics, and the temperature dependence of the resistivity, we attribute the low-field MR to spin-dependent scattering of polarized electrons at the grain boundaries which serve as pinning centers for the magnetic domain walls.

Colossal magnetoresistance of 1 000 000‐fold magnitude achieved in the antiferromagnetic phase of La1−xCaxMnO3

Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3 system. At 125 K, the CMR of the La0.5Ca0.5MnO3 reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated.

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.

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.

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.

Magnetic field induced properties of manganite perovskites with colossal magnetoresistance (invited)

We present a systematic study of the magnetotransport and magnetic properties of the half-doped La0.5Ca0.5MnO3+δ system. The solid is a metamagnet which undergoes a first-order antiferromagnet (AFM) to ferromagnet (FM) phase transition under a field or by changing temperature. Associated with the AFM–FM transition is an insulator to metal transition. A maximum 109-fold magnetoresistance ratio has been observed at 4.2 K between the least and the most conductive states. At low T (⩽50 K), we have also observed two additional metastable electronic states in the canted AFM state at certain fields. The resistivity of each state differs from one another by at least one order of magnitude. The existence of these multiple states may be related to the unique charge- and spin-ordered state of the half-doped manganite.

Colossal magnetoresistance in the antiferromagnetic La0.5Ca0.5MnO3 system

We have explored the colossal magnetoresistance (CMR) effect in the antiferromagnetic La0.5Ca0.5MnO3 compound. In the absence of a magnetic field (H), the solid is a canted antiferromagnetic (AFM) insulator. An applied H in the Tesla scale induces a first order AFM to FM phase transition, and correspondingly, an insulator to metal transition. The observed CMR is attributed to the H‐induced charge localization‐delocalization behavior associated with the AFM–FM transition. At low temperatures (T<100 K), the solid remains in the AFM phase, where we have observed a phenomenal one millionfold change in resistivity between 0 and 8 Tesla. The origin of CMR in low T‐region is a thermal activation energy gap which is strongly dependent on H.

Emission studies of the gas‐phase oxidation of Mn during pulsed laser deposition of manganates in O2 and N2O atmospheres

The plasma produced during pulsed laser deposition of manganate films has been probed using optical emission spectroscopy. The studies have been carried out using Mn, Mn2O3, and La0.67Sr0.33MnO3 (LSMO) as target materials in the presence of two different oxidizing gases: nitrous oxide (N2O) and oxygen (O2). Emission from excited MnO (MnO*) has been observed in all cases resulting primarily from reaction of the ablated Mn atoms with the background gas. Consistent with the oxidation reaction energetics, the emission intensity from MnO* is found to be about an order of magnitude stronger with N2O than with O2. Magnetization measurements of LSMO films show improved magnetic properties of films prepared in N2O compared to O2 at low pressures. The improvement in film quality can be attributed, at least in part, to the increased oxidation of Mn in the plasma plume.

LARGE MAGNETIC HALL EFFECT IN FERROMAGNETIC FEXPT100-X THIN FILMS

We have observed a very large extraordinary Hall effect (EHE) in a series of Fe–Pt thin films with various Fe contents. The origin of this remarkable EHE is the large spin–orbit interaction in the Fe–Pt alloys. At certain Fe content, the Hall resistivity can be saturated with a magnetic field less than 2 kG. The large EHE persists to room temperature with little change in magnitude. The EHE, which to our knowledge is the largest among magnetic transition metals, may find potential applications in magnetic sensors and nonvolatile magnetic random access memories. We will present structural analysis of the Fe–Pt films.