Zahra Haghshenasfard

Spin-wave instabilities of ferromagnetic nanowire stripes under parallel pumping

A microscopic (or Hamiltonian-based) theory is presented for the spin-wave instability thresholds in ferromagnetic nanowires under conditions of parallel pumping with a microwave field. A dipole-exchange Hamiltonian is employed in which the external magnetic field is either longitudinal or transverse to the length of the nanowire. This contrasts with most previous work on nonlinear spin waves which concentrate on situations where a macroscopic (or continuum) theory is appropriate. In nanowire stripes with lateral dimensions of order 100 nm or less, the quantization of the eigenmodes becomes modified due to strong spatial confinement and edge effects, making it appropriate to employ a microscopic dipole-exchange approach analogous to that used recently for ultrathin films. Numerical applications are presented for the dependence of the threshold microwave field amplitude for instability on the static applied field (the analog of the butterfly curves). The cases of longitudinal and transverse applied fields are found to lead to distinctly different behaviors, which are illustrated using EuS and permalloy as materials with different ratios of dipolar to exchange interactions. Structural effects due to the discrete spin-wave branches lead to structural aspects of the butterfly curves that are significantly modified compared with those for ultrathin films due to the edge modes in the nanowires.

Quantum statistics and squeezing for a microwave-driven interacting magnon system

Theoretical studies are reported for the statistical properties of a microwave-driven interacting magnon system. Both the magnetic dipole-dipole and the exchange interactions are included and the theory is developed for the case of parallel pumping allowing for the inclusion of the nonlinear processes due to the four-magnon interactions. The method of second quantization is used to transform the total Hamiltonian from spin operators to boson creation and annihilation operators. By using the coherent magnon state representation we have studied the magnon occupation number and the statistical behavior of the system. In particular, it is shown that the nonlinearities introduced by the parallel pumping field and the four-magnon interactions lead to non-classical quantum statistical properties of the system, such as magnon squeezing. Also control of the collapse-and-revival phenomena for the time evolution of the average magnon number is demonstrated by varying the parallel pumping amplitude and the four-magnon coupling.

Controlling the repetition rate of a mode-locked laser using an f-deformed Bose–Einstein condensate

Investigations are made for the possibility of using an f-deformed Bose–Einstein condensate (BEC) inside the resonator of a mode-locked laser to achieve tunable control of the repetition rate. Slow light propagation of the probe field through the f-deformed BEC may lead to a large observable increase in the cavity round-trip delay and, therefore, to a reduction in the effective repetition rate of the laser. Conversely, negative propagation velocity through the f-deformed BEC can lead to a decrease in the cavity round-trip and to an increase in the effective repetition rate. Simulations are presented using an f-deformed BEC of Λ-type three-level atoms beyond the rotating wave approximation. Collisions between the atoms are considered as a special kind of f-deformation where the collision rate κ is regarded as the deformation parameter. In particular, it is found that the repetition rate can be controlled effectively by changing the collision rate κ, the total number of atoms N and the parameter λ for the counter-rotating terms.

Quantum statistics for a two-mode magnon system with microwave pumping: application to coupled ferromagnetic nanowires

A microscopic (Hamiltonian-based) method for the quantum statistics of bosonic excitations in a two-mode magnon system is developed. Both the exchange and the dipole-dipole interactions, as well as the Zeeman term for an external applied field, are included in the spin Hamiltonian, and the model also contains the nonlinear effects due to parallel pumping and four-magnon interactions. The quantization of spin operators is achieved through the Holstein-Primakoff formalism, and then a coherent magnon state representation is used to study the occupation magnon number and the quantum statistical behaviour of the system. Particular attention is given to the cross correlation between the two coupled magnon modes in a ferromagnetic nanowire geometry formed by two lines of spins. Manipulation of the collapse-and-revival phenomena for the temporal evolution of the magnon number as well as the control of the cross correlation between the two magnon modes is demonstrated by tuning the parallel pumping field amplitude. The role of the four-magnon interactions is particularly interesting and leads to anti-correlation in some cases with coherent states.

Parallel Pumping of Spin Waves for Ferromagnetic Nanowires and Nanotubes With Circular Cross Sections

Calculations are reported for the spin-wave instabilities in a ferromagnetic nanowire with circular cross section, as well as nanotubes, under conditions of parallel microwave pumping. The quantized spin waves are characterized by a one-dimensional wave vector along the nanowire and the external applied magnetic field is taken to be parallel to the longitudinal axis. In nanowires with lateral dimensions less than about 200 nm, the spatial quantization of the eigenmodes become predominant and our theory employs a microscopic (or Hamiltonian-based) dipole-exchange method analogous to that used recently for ultrathin films and nanowire stripes with rectangular cross section. Numerical applications are made to wires with different radii and comparisons are presented with nanowires having square cross sections. When our nanowire results are compared with those for macroscopic samples or for ultrathin films, it is found that the “butterfly curves” (threshold field vs. applied field) are significantly modified and show more structural features.