X. K. Wang

Validity of the force theorem for magnetocrystalline anisotropy

Abstract The validity of the so-called force theorem is critical for the computational/theoretical determination of the magnetocrystalline anisotropy (MCA) in the framework of local density theory. This theorem states that the spin-orbit coupling induced MCA energy is given by the difference in the fully relativistic band energies between two magnetization directions calculated with the same self-consistent scalar-relativistic potential. We show that the charge- and spin-density variations caused by spin-orbit coupling vanish to first order in the spin-orbit coupling strength. By the stationary property of the total energy functional, we establish rigorously the validity of the force theorem for surface/interface MCA. We show that our arguments also apply to a variant of the MCA force theorem and discuss problems of applying the force theorem for MCA in bulk systems with cubic crystalline symmetry.

A miniature x‐ray compatible sputtering system for studying in situ high Tc thin film growth

We describe a miniature sputtering system incorporating an unconventional magnetron gun and a right angle sputtering geometry. The design essentially eliminates the negative ion resputtering effect encountered in the sputtering of high Tc oxide films. The geometry is compatible with the in‐plane and conventional θ–2θ diffraction geometries and a chamber with two appropriate sector x‐ray windows has been constructed and operated at a synchrotron. Detailed data on the performance of the sputtering gun are presented.

Spin polarons in the double-exchange model

Abstract We consider the possible formation of spin polarons in the double exchange model at finite temperatures. We show that the average size of a spin polaron in this model increases as the temperature decreases. Based on this, we argue that the transition to the ferromagnetic phase in the double-exchange model happens when the spin polarons start overlapping. The effect of the external field is shown to reduce the effective temperature and thus to increase the size of the polarons. The theory predicts a linear increase of T c versus the applied external magnetic field, a temperature dependence of the magnetic susceptibility above T c , that deviates from Curie—Weiss behavior, and an increase of T c as the electron/hole density increases.

Microstructure studies of epitaxial YBa2Cu3O7–δ films

Thin films of YBaCuO were prepared as a superlattice of three constituents on SrTiO3 (100) substrates from three electron guns using a dedicated, computer‐controlled evaporator. The results indicate that the as‐deposited thin films grow as a superlattice of the three constituents, with a mixture of amorphous and polycrystalline regions. After annealing, however, the multilayer films are converted to the homogeneous superconducting phase; the morphology, studied with transmission electron microscopy, consisted of an array of orthogonal interconnecting rectangular bars. The better films had Tc(R=0)=90 K and Jc=2.9×106 A/cm2 at 4.2 K. The selected area electron diffraction pattern, and the high‐resolution electron microscopy images show that the interconnecting bars are single crystals of the 1‐2‐3 phase. During annealing, the growth rate along the a and b directions is faster than along the c direction. For the films with the a axis oriented perpendicular to the substrate, the grains nucleated directly on t...

In situ x‐ray diffraction studies of YBa2Cu3Ox

Using a specially designed off‐axis faced magnetron sputtering chamber we have performed in situ x‐ray diffraction studies of the growth of YBa2Cu3Ox films using a synchrotron light source. The orientation and rocking curve width were studied as a function of substrate temperature, O2/Ar partial pressures, and deposition rate. Growth rate was studied on SrTiO3, LaAlO3, and MgO.

In situ x-ray diffraction studies of YBa sub 2 Cu sub 3 O sub x

Using a specially designed off‐axis faced magnetron sputtering chamber we have performed in situ x‐ray diffraction studies of the growth of YBa2Cu3Ox films using a synchrotron light source. The orientation and rocking curve width were studied as a function of substrate temperature, O2/Ar partial pressures, and deposition rate. Growth rate was studied on SrTiO3, LaAlO3, and MgO.

Superconducting YBa2Cu3O7−δ thin films with three different orientations

Epitaxial thin films of YBaCuO have been prepared with (1) the a axis perpendicular to (100) SrTiO3 ; (2) the c axis perpendicular to (100) SrTiO3 ; and (3) the [110] axis perpendicular to (110) SrTiO3. Films were fabricated using a multilayer deposition technique involving three electron guns containing Y, BaF2, and Cu under a pressure of 5×10−5 Torr of O2. As deposited films, which contained polycrystalline and amorphous regions, were later annealed in a furnace under a flowing O2‐H2 O atmosphere. X‐ray diffraction patterns as well as scanning electron microscopy and high‐resolution electron microscopy images confirm that the films are highly oriented, essentially epitaxial. The a‐axis oriented film exhibits zero resistance at 90 K and a critical current density of 2.9×106 A/cm2 at 4.2 K while the c‐axis oriented film exhibits a Tc of 88 K and a Jc of 0.9×107 A/cm2 at 4.2K; the Jc values were determined magnetically. The [110]‐orientation film shows the sharpest transition with a transition width of 1 K...

Orientation and structure dependence of interface magnetocrystalline anisotropy of Co/Cu overlayers and superlattices

The full potential linearized augmented plane wave method and atomic force approach are employed for the theoretical determination of interface magnetocrystalline anisotropy (MCA) for superlattice systems of Co/Cu in (001), (110), and (111) orientations, and overlayer systems of the monolayer Co on Cu (111) substrate adsorbed by different further coverage of Cu. It is found in superlattices that the interface MCA is sensitive to the geometry arising from different orientations. In good agreement with experiment, the interface MCA with Cu overlayers is found to peak at 1 monolayer of Cu‐coated Co/Cu(111) and then to decrease with further Cu deposition. In addition to the hybridization of electronic states at the Co/Cu interface, the interaction between the interface layers and the next‐to‐interface layers in superlattices and structure relaxation in overlayers may have a significant influence on the MCA of the Co layer.

PROPERTIES OF BUCKYTUBES AND DERIVATIVES

The structural, magnetic, and transport properties of bundles of buckytubes (buckybundles) have been studied. High-resolution electron microscopy (HREM) images have revealed the detailed structural properties and the growth pattern of buckytubes and their derivatives. The magnetic susceptibility of a bulk sample of buckybundles is −10.75 × 10−6 emu/g for the magnetic field parallel to the bundle axes, which is approximately 1.1 times the perpendicular value and 30 times larger than that of C60. The magnetoresistance (MR) and Hall coefficient measurements on the buckybundles show a negative MR at low temperature and a positive MR at a temperature above 60 K and a nearly linear increase in conductivity with temperature. The results show that a buckybundle may best be described as a semimetal. Using a stable glow discharge, buckybundles with remarkably large diameters (up to 200 μm) have been synthesized. These bundles are evenly spaced, parallel, and occupy the entire central region of a deposited rod. HREM images revealed higher yield and improved quality buckytubes produced by this technique compared to those produced by a conventional arc discharge.

In situ x‐ray diffraction studies of YBa2Cu3Ox/LaAlO3 interfaces

We report on the oriented growth by sputtering of YBCO/LaAlO3/YBCO S/I/S trilayers and YBCO/LaAlO3 multilayers using a new synchrotron compatible multisource deposition chamber.