Alireza Qaiumzadeh

Staggered dynamics in antiferromagnets by collective coordinates.

Antiferromagnets can be used to store and manipulate spin information, but the coupled dynamics of the staggered field and the magnetization are very complex. We present a theory which is conceptually much simpler and which uses collective coordinates to describe staggered field dynamics in antiferromagnetic textures. The theory includes effects from dissipation, external magnetic fields, as well as reactive and dissipative current-induced torques. We conclude that, at low frequencies and amplitudes, currents induce collective motion by means of dissipative rather than reactive torques. The dynamics of a one-dimensional domain wall, pinned at 90° at its ends, are described as a driven harmonic oscillator with a natural frequency inversely proportional to the length of the texture.

Antiferromagnetic domain wall motion induced by spin waves.

Spin waves in antiferromagnets are linearly or circularly polarized. Depending on the polarization, traversing spin waves alter the staggered field in a qualitatively different way. We calculate the drift velocity of a moving domain wall as a result of spin wave-mediated forces and show that the domain wall moves in opposite directions for linearly and circularly polarized waves. The analytical results agree with micromagnetic simulations of an antiferromagnetic domain wall driven by a localized, alternating magnetic field.

Spin-Superfluidity in Biaxial Antiferromagnetic Insulators

Antiferromagnets may exhibit spin superfluidity since the dipole interaction is weak. We seek to establish that this phenomenon occurs in insulators such as NiO, which is a good spin conductor according to previous studies. We investigate nonlocal spin transport in a planar antiferromagnetic insulator with a weak uniaxial anisotropy. The anisotropy hinders spin superfluidity by creating a substantial threshold that the current must overcome. Nevertheless, we show that applying a high magnetic field removes this obstacle near the spin-flop transition of the antiferromagnet. Importantly, the spin superfluidity can then persist across many micrometers, even in dirty samples.

Spin-orbit torques in two-dimensional Rashba ferromagnets

Magnetization dynamics in single-domain ferromagnets can be triggered by a charge current if the spin-orbit coupling is sufficiently strong. We apply functional Keldysh theory to investigate spin-orbit torques in metallic two-dimensional Rashba ferromagnets in the presence of spin-dependent disorders. A reactive, antidamping-like spin-orbit torque as well as a dissipative, field-like torque is calculated microscopically, to leading order in the spin-orbit interaction strength. By calculating the first vertex correction we show that the intrinsic antidamping-like torque vanishes unless the scattering rates are spin dependent.

Controlling chiral domain walls in antiferromagnets using spin-wave helicity

In antiferromagnets, the Dzyaloshinskii-Moriya interaction lifts the degeneracy of left- and right-circularly polarized spin waves. This relativistic coupling increases the efficiency of spin-wave-induced domain wall motion and leads to higher drift velocities. We show that in biaxial antiferromagnets, the spin-wave helicity controls both the direction and magnitude of the magnonic force on chiral domain walls. By contrast, in uniaxial antiferromagnets, the magnonic force is propulsive with a helicity dependent strength.

Generation of single skyrmions by picosecond magnetic field pulses

We numerically demonstrate an ultrafast method to create single skyrmions in a collinear ferromagnetic sample by applying a picosecond (effective) magnetic field pulse in the presence of Dzyaloshinskii-Moriya interaction. For small samples the applied magnetic field pulse could be either spatially uniform or nonuniform while for large samples a nonuniform and localized field is more effective. We examine the phase diagram of pulse width and amplitude for the nucleation. Our finding could ultimately be used to design future skyrmion-based devices.

Manipulation of ferromagnets via the spin-selective optical Stark effect

We investigate the nonresonant all-optical switching of magnetization. We treat the inverse Faraday effect (IFE) theoretically in terms of the spin-selective optical Stark effect for linearly or circularly polarized light. In the dilute magnetic semiconductors (Ga,Mn)As, strong laser pulses below the band gap induce effective magnetic fields of several teslas in a direction which depends on the magnetization direction as well as the light polarization and direction. Our theory demonstrates that the polarized light catalyzes the angular momentum transfer between the lattice and the magnetization.

Spin polarization dependence of quasiparticle properties in graphene

We address spin polarization dependence of graphenes Fermi liquid properties quantitatively using a microscopic random phase approximation theory in an interacting spin-polarized Dirac electron system. We show an enhancement of the minority-spin many-body velocity renormalization at fully spin polarization due to reduction in the electron density and consequently increase in the interaction between electrons near the Fermi surface. We also show that the spin dependence of the Fermi velocity in the chiral Fermi systems is different than that in a conventional two-dimensional electron liquid. In addition, we show that the ratio of the majority-to-minority-spin lifetime is smaller than unity and related directly to the polarization and electron energy. The spin-polarization dependence of the carrier Fermi velocity is of significance in various spintronic applications.

Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide

1 Institute for Physics, Johannes Gutenberg-University Mainz, 55099 Mainz, Germany 2 Graduate School of Excellence Materials Science in Mainz, Staudingerweg 9, 55128, Mainz, Germany 3 Utrecht University, Princetonplein 5, 3584 CC Utrecht, Netherlands 4 Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology, NO7491 Trondheim, Norway 5 Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands * Both authors contributed equally to this work.

Intrinsic Spin-Orbit Torque Arising from the Berry Curvature in a Metallic-Magnet/Cu-Oxide Interface

We report the observation of the intrinsic dampinglike spin-orbit torque (SOT) arising from the Berry curvature in metallic-magnet/CuO_{x} heterostructures. We show that a robust dampinglike SOT, an order of magnitude larger than a fieldlike SOT, is generated in the heterostructure despite the absence of the bulk spin-orbit effect in the CuO_{x} layer. Furthermore, by tuning the interfacial oxidation level, we demonstrate that the fieldlike SOT changes drastically and even switches its sign, which originates from oxygen-modulated spin-dependent disorder. These results provide important information for a fundamental understanding of the physics of the SOTs.