Lance Horng

Effect of the substitutions of Ni3+, Co3+, and Fe3+ for Mn3+ on the ferromagnetic states of the La0.7Pb0.3MnO3 manganite

The substitution at the Mn site of various transitional metal elements dramatically modifies the magnetic and electronic properties of the La0.7Pb0.3Mn0.8Me0.2O3 (Me=Ni, Co, and Fe) manganites. The powder x-ray diffraction pattern shows a single-phase rhombohedral distorted perovskite structure with space group R3c (No. 167) for all samples. The present article is focused on the effects induced by the partial substitution of Ni, Co, and Fe for the Mn site. The field cooled–zero field cooled curves indicate that the magnetic order is modified from a ferromagnetic long-range spin order for La0.7Pb0.3MnO3 to a spin cluster induced short-range spin order for all the doped compositions. The saturation magnetization, transition temperature, and resistance peak temperature decrease significantly from Ni-doped, Co-doped, to Fe-doped composition. In conclusion, the variation of the magnetic and electronic behaviors is due to the dissimilar 3d electron configuration of the substitutions.

Effect of Au cap layer on the magnetic properties and the microstructure for FePt thin films

We have investigated the effect of interfacial diffusion on the magnetic properties of Au∕Fe51Pt49 bilayer thin films. The samples were prepared in two stages. First, an ordered Fe51Pt49 continuous thin film was sputtered on a quartz substrate. Then an aurum cap layer was deposited onto the Fe51Pt49 film at room temperature, followed by a postannealing at 300–800°C to promote the interfacial diffusion. A high coercivity of 23.5kOe was achieved in the isotropic polycrystalline FePt-based film. Transmission electron micrographs indicate that the bilayer sample exhibits Ll0 ordered FePt grains with a diameter of about 20nm, which were partially isolated by Au phase. We consider that the isolation of FePtLl0 grains and the grain refining effect can be two contributory factors for the enhanced coercivity.

Anisotropic pinning effect on a nb thin film with triangular arrays of pinning sites

A superconducting Nb thin film with triangular arrays of pinning sites has been fabricated to investigate the commensurate vortex lattices. Vortex-vortex interactions in this pattern film make the observed anisotropy in the pinning properties. Periodic minima appear in the magnetoresistance curves at equal field intervals that can be associated with matching effect. The transport properties at near critical temperatures show anisotropic pinning properties related to the configuration of pinning centers. The critical current is depressed when the driving force is applied along the short diagonal in the rhombic unit cell of pinning array. It implies that the pinning potential along the short diagonal creates a moving channel of vortices in triangular arrays of defects. The pinned and interstitial vortices can be movable in an incommensurate vortex row and a straight unhindered one-dimensional path.

Influence of asymmetry on vortex nucleation and annihilation in submicroscaled permalloy disk array

Magnetic properties of vortex nucleation, annihilation, and switching field distribution (SFD) in NiFe disk arrays, where the elements are with 300nm diameter and different degrees of asymmetry, were investigated through measurements and simulations of hysteresis loop. The nucleation and annihilation of vortex state show strong dependences on the asymmetry. More interestingly, the width of SFD, the crucial factor for high-density storage application, oscillating with the degree of asymmetry is observed. The simulation results agree well with the experimental data.

Influence of Y-doped induced defects on the optical and magnetic properties of ZnO nanorod arrays prepared by low-temperature hydrothermal process

One-dimensional pure zinc oxide (ZnO) and Y-doped ZnO nanorod arrays have been successfully fabricated on the silicon substrate for comparison by a simple hydrothermal process at the low temperature of 90°C. The Y-doped nanorods exhibit the same c-axis-oriented wurtzite hexagonal structure as pure ZnO nanorods. Based on the results of photoluminescence, an enhancement of defect-induced green-yellow visible emission is observed for the Y-doped ZnO nanorods. The decrease of E2(H) mode intensity and increase of E1(LO) mode intensity examined by the Raman spectrum also indicate the increase of defects for the Y-doped ZnO nanorods. As compared to pure ZnO nanorods, Y-doped ZnO nanorods show a remarked increase of saturation magnetization. The combination of visible photoluminescence and ferromagnetism measurement results indicates the increase of oxygen defects due to the Y doping which plays a crucial role in the optical and magnetic performances of the ZnO nanorods.

Effect of annealing temperature on exchange coupling in NiFe∕FeMn and FeMn∕NiFe systems

Most studies on exchange bias in spin valves are for antiferromagnetic/ferromagnetic structures, and Fe[50]Mn[50]∕Ni[79]Fe[21] is widely used. The results of the exchange-bias field (Hex) and coercivity (Hc) as a function of the annealing temperature in NiFe∕FeMn and FeMn∕NiFe systems are given in the study. We prepare two types of films, type I: substrate/∕Ta∕NiFe∕FeMn∕Ta and type II: substrate/Ta∕FeMn∕NiFe∕Ta, respectively. Annealing was performed at 150–450°C under 720Oe for 2h, Hex and Hc of type I samples increase slightly with the increase of the annealing temperature. But Hex and Hc of type II samples increase rapidly after annealing over 300°C, then decrease after annealing at 375°C. The strong exchange-bias field and low coercivity were exhibited for type I samples, where NiFe is the buffer layer and is also the pinned layer. In type II samples, the increase in the exchange-bias field is attributed to the altered interface of FeMn∕NiFe due to interdiffusion. The interpretation of the fluctuation ...

Fabrication and Characterization of Microstructured Magnetic Tunnel Junction Rings

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

Critical persistent current for a loop formed by a Bi-2223 Ag-sheathed superconducting tape

A persistent current loop formed by a Bi-2223 Ag-sheathed superconducting tape has been fabricated by tape joint technique. The current decay measurement was used to determine the persistent current of the loop at 77 K. A dc current exceeding the critical current I/sub c/ of the loop defined by 1 /spl mu/V/cm criterion was transmitted into the loop. The persistent current is found to about 25% of its I/sub c/ value of the tape. Based on detailed measurements of the voltage-current curves, the low level resistances down to very low voltage (<1 nanovolt) at the jointed and unjointed parts were tested, respectively. The jointed section has a larger I/sub c/ value of 1 /spl mu/V/cm criterion than the unjointed one. However, it is found that the persistent current of this test loop is limited by the jointed section, because the residual resistance of the jointed section is larger. These results illustrate that in the jointed section the flux motion is more serious and the weaker grain boundaries or jointing interfaces limits the persistent transport currents.

Temperature dependent pinning phenomenon in superconducting Nb films with triangular and honeycomb pinning arrays

The pinning phenomena in superconducting Nb films with triangular and honeycomb pinning arrays were explored. Special temperature dependent phenomena were found for both films. For the film with a triangular pinning array, the pronounced matching peaks in the critical currents as a function of magnetic field reduce from six to three within a narrow temperature range. This temperature dependent matching effect is explained by considering the dramatic change of coherent length with temperature when the temperature is close to T(c). In order to compare with the film with a triangular pinning array, we fabricated a film with a honeycomb pinning array with similar pinning site spacing and pinning size. Special prominent matching peaks at H = 3.5H(1) were found for this film. Molecular dynamic simulations were made to study this phenomenon. The ground state distribution of vortices obtained from simulations reasonably explains the prominent matching peaks. Pronounced temperature dependent matching effects were also found for the film with a honeycomb pinning array.

Evolution of Magnetotransport Properties and Spin-Glass Behavior of the La0.7-xNdxPb0.3MnO3 System

The effect of neodymium substitution on the lanthanum site in La0.7-xNdxPb0.3MnO3 perovskite polycrystalline has been systematically studied. The substitution of neodymium for lanthanum causes structure change from rhombohedral to orthorhombic. Ferromagnetism is suppressed as neodymium content increases. The magnetic order changes from a ferromagnetic long-range order with x=0.0 to a spin-glass nature with x=0.7. Besides, the ferromagnetic-paramagnetic transition temperature values are lower for the neodymium phases than for the lanthanum ones. Therefore, the introduction of neodymium degrades symmetry of the structure, diminishes the spin-coupling exchange interaction, and results in changes of magnetic properties. This fact is in good agreement with increasing the B value in Blochs T3/2 law and decreasing the spin-wave stiffness parameter D in quadratic dispersion relation.