SPIN

Magnetite is a highly utilized transition metal oxide with many interesting magnetic and transport properties. The presence of anti-phase boundaries (APBs) and charge-orbital ordering (COO) are two of the most exciting properties of epitaxial magnetite thin films. Here, epitaxial stepped Fe₃O₄ films were prepared to investigate the competition between APBs and COO via measurements of in-plane anisotropy. The anisotropy was probed for two orthogonal configurations, with magnetic field applied or electrical-contacts aligned either along or perpendicular to the steps. We reveal that the APBs dominate the magnetic and transport properties of the films above the Verwey transition temperature ($T_{\mathrm{\text{V}}})$. However, below $T_{\mathrm{\text{V}}}$ film thickness becomes a decisive factor in determining the magnetic nature of stepped magnetite films, due to its correlation with domain size. When the film is thinner than a critical thickness, the anisotropy is dominated by the APBs, and a higher anisotropy constant and MR ratio are observed when the magnetic field or contacts are oriented along the steps. Conversely, for sufficiently thick films, below $T_{\mathrm{\text{V}}}$, the magnetic and electrical transport properties are dominated by COO. Thus a higher anisotropy constant and MR ratio are observed when the magnetic field or contacts are oriented perpendicular to the steps.

We have studied the effect of the silicon concentration on the structural and hyperfine properties of nanostructured Fe${}_{100-x}$Si_x powders ($x=10$, 20, 25 and 30at.%) prepared by mechanical alloying. The X-ray diffraction (XRD) studies indicated that after 72h of milling, the solid solution bcc-$\alpha$-Fe(Si) is formed. The grain sizes, $〈$D$〉$ (nm), decreases with increasing Si concentration and reaches a minimum value of 11nm. We have found that the lattice parameter decreases with increasing Si concentration. The changes in values are attributed to the substitutional dissolution of Si in Fe matrix. From the adjustment of Mössbauer spectra, we have shown that the mean hyperfine magnetic field, $〈|$H${}_{\mathrm{\text{hyp}}}|〉$ (T), decreases with increasing Si concentration. The substitutional dependence of $〈|$H${}_{\mathrm{\text{hyp}}}|〉$ (T) can be attributed to the effect of p electrons Si influencing electrons d of Fe.

Inhomogeneous magneto-electric effect (IMEE) driven by exchange forces is studied. A phenomenological theory of the effect is proposed. The theory predicts the existence of the radial electric polarization in the systems with the magnetic structures of vortex type. Polarization does not depend on the sign of the winding number and appears even in a magnetic “hedgehog”. A microscopic theory of the exchange IMEE in a medium with the vortex magnetization distribution is developed. Electric polarization is caused by the exchange interaction of free charge carriers with the localized ones. On the basis of the microscopic theory, the electrical polarization is estimated for vortices in magnetic semiconductors and metals.

We propose and analyze a “spintronic/straintronic” reconfigurable equality bit comparator implemented with a nanowire spin valve whose two contacts are two-phase multiferroic nanomagnets and possess bistable magnetization. A reference bit is “written” into a stable magnetization state of one contact and an input bit in that of the other with electrically generated strain. The spin-valve’s resistance is lowered (raised) if the bits match (do not match). Multiple comparators can be interfaced in parallel with a magneto-tunneling junction to determine if an $N$-bit input stream matches an $N$-bit reference stream bit by bit. The system is robust against thermal noise at room temperature and a 16-bit comparator can operate at $\sim 743$MHz while dissipating $\sim$28fJ per cycle. This implementation is more energy-efficient than CMOS-based implementations and the reference bits can be stored in the comparator itself without the need for refresh cycles or the need to fetch them from a remote memory for comparison. That improves reliability, speed and security.

We present here micromagnetic study of variation of switching field distribution (SFD) in a high-density patterned media as a function of magnetic anisotropy of the system. We consider the manifold effect of magnetic anisotropy in terms of its magnitude, tilt in anisotropy axis and random arrangements of magnetic islands with random anisotropy values. Our calculation shows that reduction in anisotropy causes linear decrease in coercivity because the anisotropy energy tries to align the spins along a preferred crystallographic direction. Tilt in anisotropy axis results in decrease in squareness of the hysteresis loop and hence facilitates switching. Finally, the experimental challenges like lithographic distribution of magnetic islands, their orientation, creation of defects, etc. demanded the distribution of anisotropy to be random along with random repetitions. We have explained that the range of anisotropy values and the number of bits with different anisotropy play a key role over SFD, whereas the position of the bits and their repetitions do not show a considerable contribution.

Superconducting transition temperatures of 164 K in cuprate high temperature superconductors (HTS) and recently 200 K in H₃S under high pressure encourage us to believe that room temperature superconductivity (RTS) might be possible. In considering paths to RTS, we contrast conventional (BCS) SC, such as probably manifested by H₃S, with the unconventional superconductivity (SC) in the cuprate HTS family. Turning to SC models, we show that in the presence of one or more van Hove singularities (vHs) near the Fermi level, SC mediated by classical phonons ($k_B{T}_c>\hslash \times$phonon frequency) can occur. The phonon frequency in the standard $T_c$ formula is replaced by an electronic cutoff, enabling a much higher $T_c$ independent of phonon frequency. The resulting $T_c$ and isotope shift plot versus doping strongly resembles that seen experimentally in HTS. A more detailed theory of HTS, which involves mediation by classical phonons, satisfactorily reproduces the chief anomalous features characteristic of these materials. We propose that, while a path to RTS through an H₃S-like scenario via strongly-coupled ultra-high frequency phonons is attractive, features perhaps unavailable at ordinary pressures, a route involving SC mediated by classical phonons which can be low frequency may be found.