Search

The present study grows a series of Fe3-xMgxO4 (0≤x≤1.5) films and systematically measure both structure and magnetization of these films. These films are grown on MgO and SrTiO3 (STO), which have small (∼-0.3%) and large (∼7.5%) lattice mismatch in order to have either strained or relaxed films, by plasma-oxygen-assisted molecular beam epitaxy, respectively. X-ray diffraction (XRD) is carried out to analyze the crystalline structure. Saturation magnetization (Ms) of pure Fe3O4 (x=0) on both substrates is ∼500 emu/cm3, which is consistent with the bulk value. However, Ms has a fast decrease with increasing x for the films grown on MgO(001), from 340 to ∼100 emu/cm3 in the region of 0.33 for x>1.35. On the other hand, Ms remains unchanged with x increasing from 0.3 to 1 for the film grown on STO. With x>1, Ms drops abruptly to ∼100 emu/cm3, which is comparable to Ms of the film grown on MgO. The discrepancy in Ms of Fe3-xMgxO4 film grown on MgO and STO may imply that the cation distribution of these films may be fundamentally different. Possible cation distribution and the substrate strain effect will be discussed. [ABSTRACT FROM AUTHOR]

We fabricated a series of Fe3-{x}MnxO4 (0\leqx\leq 1.5) films by plasma–oxygen-assisted molecular beam epitaxy and did magnetic and electrical characterizations of these films. The magnetization measurement shows that the saturation magnetization (Ms) is consistent with the curve of minimum possible Ms. Accordingly, the present result suggests that Mn^3+ replace Fe^3+ mainly in the B-site, which is basically different from the bulk distribution mainly in the A-site. Resistance as a function of temperature in the range of 80–300 K is carried out for all films. The resistivity presents a typical Arrhenius temperature dependence with \rho=\rho0\exp(Ep/{\rm k}B{\rm T}) indicating that the transport is due to a hopping mechanism. The prefactor \rho0 increases with x in Fe3-{x}MnxO4 at smaller x but tends to level out at x>0.6, suggesting that the structure varies from a inverse spinel to a normal spinel in that the Fe^2+/Fe^3+ ratio remains constant at x>0.6. The activation energy Ep of electrical hopping remains a constant value \sim50 meV at x<0.9 but abruptly rises up at x>0.9 and reaches 300 meV at x=1.5, suggesting a semiconductor—insulator transformation. This increase in the activation energy may indicate a percolation limit which is different from the previous transformation observed in other ferrites such as Fe3-{x}CoxO4. [ABSTRACT FROM AUTHOR]

Microstructured magnetic tunnel junction ring with outer/inner diameter of 2/1 μm has been fabricated to investigate the angular dependence of magnetoresistance during magnetization reversal process. The minor loop of magnetoresistive curve reveals four distinct resistance levels associated with four magnetization configurations within the free layer throughout the magnetization reversal. The magnetoresistance decreases with increasing angle between applied external field and biasing direction, which is resulted from the relative alignment of total magnetization of pinned and free layer. An extra feature appeared in the minor loop when the external field is transverse to the biasing direction; this can be attributed to a vortex-pair formation/annihilation in the free layer. Furthermore, a series of schemes of magnetic configurations of pinned and free layers are illustrated to explain the routes of minor loops. [ABSTRACT FROM AUTHOR]

Microstructured magnetic tunnel junction rings have been fabricated by a top-down technique combining electron beam lithography and ion milling process. Four-terminal magnetoresistance measurements and magnetic force microscopy were used to successfully explore a four-transition process within the free layer throughout the magnetization reversal. Various magnetization configurations were identified to be the onion state, vortex-pair state, vortex state, vortex-core state, and reverse onion state. In addition, the various durations of each magnetic state observed in the magnetoresistance curve can be utilized for the study of a coupling effect between the pinned layer and the free layer. [ABSTRACT FROM AUTHOR]

Fe1-xCoxOy (0 ≤ x ≤ 1,1 ≤ y ≤ 4/3) mixture oxide films are grown, with epitaxial quality, on MgO(001) substrates by molecular beam epitaxy. The X-ray diffraction result along (001) plane is similar to Fe3O4 for the films with x < 0.5 (Fe rich)and is simllar to CoO for the films with x > 0.5 (Co rich). An abrupt change, over 2%, on the lattice spacing is observed around Fe/Co ∼ (x = 0.5) suggesting a phase transformation from spinel to rocksalt. A fast suppression of the saturation magnetization is found markedly different from the bulk materials as the Co ratio increases. Crystalline anisotropy and anomalous magnetic properties are affected substantially by Co ions. These unique magnetic behaviors of Fe-Co-O films are believed to be related to cation distribution in spinel structure originated by the low-temperature process of thin-film fabrication. [ABSTRACT FROM AUTHOR]

In this report, we grow MnCr2O4 (124) films, and Mn2CrO4 (214) films, for comparison, on MgO and SrTiO3 (STO) substrates by oxygen-plasma-assisted molecular beam epitaxy and also carry out the measurements of the magnetization as a function of temperature (5-200 K) and external field (0-5 T) of these films. The results of magnetization versus temperature further show that ferrimagnetic transition (Tc) occurs at 44 K for 214 films grown on both MgO and STO. The Tc of 124 films raises to 45 K but is only observed for the film grown on STO. No or very weak magnetization is observed for the 124 films grown on MgO(001). The vanishing of the magnetization of the 124 films grown on MgO is attributed to the instability of spinel structure for Cr-rich films. Magnetic hysteresis curves of the films further show the magnetization saturation (Ms) of 124 is <1/2 bohr magneton, which is noticeably smaller than the bulk Ms 1 bohr magneton. The reduction of Ms in these chromites films may result from the cation distribution in that Cr cations distribute in both A and B sites, which is different from the bulk materials. The distribution of Cr cations in both A and B is attributed to the low temperature growth and similar results have been found in previous growth of various ferrite films. [ABSTRACT FROM AUTHOR]

Abstract: Magneto-impedance (MI) of a double-barrier MgO magnetic tunnel junction (DMTJ) is measured as a function of frequency (20 Hz–3 MHz) and DC bias (−1 to 1 V) under magnetic parallel and antiparallel states. A strong nonlinear effect is observed at all frequencies and the MI changes from positive to negative at the relaxation frequency ∼1/RC. A frequency–dc bias () “phase diagram” is mapped out to describe the combination effects of the magneto-impedance response. The decay of the tunnel magneto-resistance (TMR) and tunnel magneto-capacitance (TMC) at high voltage is sufficiently reduced by the double-MgO barrier. The V1 / 2 (the voltage that magnetic response drops to 1/2 of its maximum) of TMC reaches an average value of ∼1.35 V, which is larger than the V1 / 2 of TMR ∼0.95 V, suggesting that the response of TMC is more stable than TMR for high frequency application. [ABSTRACT FROM AUTHOR]

Fe3O4 (magnetite) has been suggested as a half-metal material. On the other hand, Mn3O4 has an extremely strong anisotropy effect. Using plasma–oxygen-assisted molecular beam epitaxy, we are able to grow Fe3O4, Mn3O4 and a series of Fe3-x MnxO4 (0⩽x⩽3) films on MgO (0 0 1) and (0 1 1), with epitaxial quality. A cubic-to-tetragonal structural transformation is observed first time in Fe3-x MnxO4 films. The magnetic response also shows an abrupt change of magnetic anisotropy and high coercivity, which is consistent with structural transition. [Copyright &y& Elsevier]

Abstract: The size dependence of magnetization reversal of magnetic tunnel junction (MTJ) rings has been investigated. The MTJ rings, with outer diameter of 4, 2 and 1μm and inner diameter of 1.5, 1 and 0.5μm were fabricated by a top–down technique. The magnetoresistance curves manifest all of the magnetic domain configurations during magnetization reversal in different sized rings. Various transition processes were observed, such as four transition, three transition and two transition in the largest, middle and smallest MTJ ring, respectively. Furthermore, the biasing fields observed from major loops decrease with decreasing size, which may result from edge roughness produced in the ion-milling process. [Copyright &y& Elsevier]