Magnetization reversal in the anisotropy-dominated regime using time-dependent magnetic fields

We study magnetization reversal using various r.f. magnetic pulses. We show numerically that switching is possible with simple sinusoidal pulses; however the optimum approach is to use a frequency-swept (chirped) r.f. magnetic pulse, the shape of which can be derived analytically. Switching times of the order of nanoseconds can be achieved with relatively small r.f. fields, independent of the anisotropy's strength

Resonant switching using spin valves

Using micromagnetics we demonstrate that the r.f. field produced by a spin valve can be used to reverse the magnetization in a magnetic nanoparticle. The r.f. field is generated using a current that specifically excites a uniform spin wave in the spin valve. This current is swept such that the chirped-frequency generated by the valve matches the angular dependent resonant frequency of the anisotropy-dominated magnetic nanoparticle, as a result of which the magnetization reversal occurs. The switching is fast, requires currents similar to those used in recent experiments with spin valves, and is stable with respect to small perturbations. This phenomenon can potentially be employed in magnetic information storage devices or recently discussed magnetic computing schemes

Bragg scattering of Cooper pairs in an ultra-cold Fermi gas

We present a theoretical treatment of Bragg scattering of a degenerate Fermi gas in the weakly interacting BCS regime. Our numerical calculations predict correlated scattering of Cooper pairs into a spherical shell in momentum space. The scattered shell of correlated atoms is centered at half the usual Bragg momentum transfer, and can be clearly distinguished from atoms scattered by the usual single-particle Bragg mechanism. We develop an analytic model that explains key features of the correlated-pair Bragg scattering, and determine the dependence of this scattering on the initial pair correlations in the gas.Comment: Manuscript substantially revised. Version 2 contains a more detailed discussion of the collisional interaction used in our theory, and is based on three-dimensional solution

Switching spin valves using r.f. currents

We show that magnetization reversal in spin-injection devices can be significantly faster when using a chirped r.f. rather than d.c current pulse. Alternatively one can use a simple sinusoidal r.f. pulse or an optimized series of alternating, equal-amplitude, square pulses of varying width (a digitized approximation to a chirped r.f. pulse) to produce switching using much smaller currents than with a d.c. pulse.Comment: please disregard the previous versio

Natural orbits of atomic Cooper pairs in a nonuniform Fermi gas

We examine the basic mode structure of atomic Cooper pairs in an inhomogeneous Fermi gas. Based on the properties of Bogoliubov quasi-particle vacuum, the single particle density matrix and the anomalous density matrix share the same set of eigenfunctions. These eigenfunctions correspond to natural pairing orbits associated with the BCS ground state. We investigate these orbits for a Fermi gas in a spherical harmonic trap, and construct the wave function of a Cooper pair in the form of Schmidt decomposition. The issue of spatial quantum entanglement between constituent atoms in a pair is addressed.Comment: 14 pages, 4 figures, submitted to Phys. Rev.

Spin-Orbit Coupling and Symmetry of the Order Parameter in Strontium Ruthenate

Determination of the orbital symmetry of a state in spin triplet Sr$_2$RuO$_4$ superconductor is a challenge of considerable importance. Most of the experiments show that the chiral state of the $\hat{z} (k_x \pm ik_y)$ type is realized and remains stable on lowering the temperature. Here we have studied the stability of various superconducting states of Sr$_2$RuO$_4$ in the presence of spin-orbit coupling. Numerically we found that the chiral state is never the minimum energy. Alone among the five states studied it has $=0$ and is therefore not affected to linear order in the coupling parameter $\lambda$. We found that stability of the chiral state requires spin dependent pairing interactions. This imposes strong constraint on the pairing mechanism.Comment: 4 pages, 4 figure

Magnetism and superconductivity in McM_{c}Ta2_{2}S2_{2}C (M = Fe, Co, Ni, and Cu)

Magnetic properties of $M_{c}$Ta$_{2}$S$_{2}$C ($M$ = Fe, Co, Ni, Cu) have been studied using SQUID DC and AC magnetic susceptibility. In these systems magnetic $M^{2+}$ ions are intercalated into van der Waals gaps between adjacent S layers of host superconductor Ta$_{2}$S$_{2}$C. Fe$_{0.33}$Ta$_{2}$S$_{2}$C is a quasi 2D $XY$-like antiferromagnet on the triangular lattice. It undergoes an antiferromagnetic phase transition at $T_{N}$ (= 117 K). The irreversible effect of magnetization occurs below $T_{N}$, reflecting the frustrated nature of the system. The AF phase coexists with two superconducting phases with the transition temperatures $T_{cu} = 8.8$ K and $T_{cl} = 4.6$ K. Co$_{0.33}$Ta$_{2}$S$_{2}$C is a quasi 2D Ising-like antiferromagnet on the triangular lattice. The antiferromagnetic phase below $T_{N} = 18.6$ K coexists with a superconducting phase below $T_{cu} = 9.1$ K. Both Ni$_{0.25}$Ta$_{2}$S$_{2}$C and Cu$_{0.60}$Ta$_{2}$S$_{2}$C are superconductors with $T_{cu}$ ($= 8.7$ K for Ni and 6.4 K for Cu) and $T_{cl}$ (= 4.6 K common to $M_{c}$Ta$_{2}$S$_{2}$C). Very small effective magnetic moments suggest that Ni$^{2+}$ and Cu$^{2+}$ spins are partially delocalized.Comment: 15 pages, 17 figures, and 3 table