A. Stupakiewicz

Ultrafast nonthermal photo-magnetic recording in a transparent medium

Discovering ways to control the magnetic state of media with the lowest possible production of heat and at the fastest possible speeds is important in the study of fundamental magnetism, with clear practical potential. In metals, it is possible to switch the magnetization between two stable states (and thus to record magnetic bits) using femtosecond circularly polarized laser pulses. However, the switching mechanisms in these materials are directly related to laser-induced heating close to the Curie temperature. Although several possible routes for achieving all-optical switching in magnetic dielectrics have been discussed, no recording has hitherto been demonstrated. Here we describe ultrafast all-optical photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet. A single linearly polarized femtosecond laser pulse resonantly pumps specific d−d transitions in the cobalt ions, breaking the degeneracy between metastable magnetic states. By changing the polarization of the laser pulse, we deterministically steer the net magnetization in the garnet, thus writing ‘0’ and ‘1’ magnetic bits at will. This mechanism outperforms existing alternatives in terms of the speed of the write–read magnetic recording event (less than 20 picoseconds) and the unprecedentedly low heat load (less than 6 joules per cubic centimetre).

Direct imaging of the magnetization reversal in microwires using all-MOKE microscopy

We report a method of imaging of the magnetization reversal process using analysis of real-time images of magnetic domain structures in cylindrically shaped microwires. This method uses wide-field polarizing optical microscopy and is based on the magneto-optical Kerr effect (MOKE). The aperture diaphragm in MOKE microscope was used to control the incident angles of the light rays that reached the non-planar surface of the microwire and also determined the MOKE geometries. The movement of the non-central position of the hole in this diaphragm leads to a change in the orientation of the plane of incidence of the light along the perpendicular or the parallel direction to the axial direction of the wire. The visualization of the surface magnetic domain structures is obtained using polar and longitudinal MOKE geometries. The hysteresis loops were obtained by plotting the averaged image contrast as a function of the external magnetic field. The separation of the all-magnetization components is performed using different MOKE geometries in a microscope. We demonstrate the use of vector magnetometry to analyze the orientation of the magnetization in a cylindrically shaped microwire under the influence of an external magnetic field.

Direct observation of giant Barkhausen jumps in magnetic microwires

Magnetization reversal induced by a circular magnetic field has been studied using the magneto-optical Kerr effect in magnetic microwires. The visualization of the classical effect of a giant Barkhausen jump is reported in a magnetic microwire. It was directly confirmed that the surface giant Barkhausen jump consists of the nucleation of a single circular domain followed by the long distance quick motion of the solitary circular domain walls.

Irreversible modification of magnetic properties of Pt/Co/Pt ultrathin films by femtosecond laser pulses

Annealing ultrathin Pt/Co/Pt films with single femtosecond laser pulses leads to irreversible spin-reorientation transitions and an amplification of the magneto-optical Kerr rotation. The effect was studied as a function of the Co thickness and the pulse fluence, revealing two-dimensional diagrams of magnetic properties. While increasing the fluence, the creation of two branches of the out-of-plane magnetization state was found.

Phase-controllable spin wave generation in iron garnet by linearly polarized light pulses

A phase-controlled spin wave was non-thermally generated in bismuth-doped rare-earth iron garnet by linearly polarized light pulses. We controlled the initial phase of the spin wave continuously within a range of 180° by changing the polarization azimuth of the excitation light. The azimuth dependences of the initial phase and amplitude of the spin wave were attributed to a combination of the inverse Cotton-Mouton effect and photoinduced magnetic anisotropy. Temporally and spatially resolved spin wave propagation was observed with a CCD camera, and the waveform was in good agreement with calculations. A nonlinear effect of the spin excitation was observed for excitation fluences higher than 100 mJ/cm2.

Metal-rich Au-silicide nanoparticles for use in nanotechnology

We present a route to functionalize chemically and magnetically silicon surfaces by a local passivation, taking advantage of Stranski–Krastanov growth mode of the Au–Si(111) system. Metal-rich Au-silicide nanoparticles, supported on a Si-rich two-dimensional Au-silicide layer, are obtained. Subsequently deposited Co is used to form magnetic nanostructures. The two Au silicides display a different chemical reactivity with Co enabling the fabrication of localized magnetic Co nanodots. These magnetic nanostructures can be aligned along step bunches of a vicinal Si(111) surface. By varying the growth parameters, the particle density can be tuned from 109 to the low 1012 dots/in.2.

Wide frequencies range of spin excitations in a rare-earth Bi-doped iron garnet with a giant Faraday rotation

Ultrafast magnetization dynamics of a rare-earth Bi-doped garnet were studied using an optical pump–probe technique via the inverse Faraday effect. We observed a wide range of frequency modes of the magnetization precession, covering two orders of magnitude. The excitation efficiency of low-frequency precessions in the GHz range, together with a significant beating effect, strongly depended on the amplitude of the external magnetic field. On the contrary, high-frequency precession was independent of the external magnetic field. The obtained results may be exploited in the development of wide class of microwave and magneto-optical devices.

Giant magnetoimpedance effect and domain wall dynamics in Co-rich amorphous microwires

The giant magnetoimpedance (GMI) effect, as well as the magnetic and magneto-optical properties, of Co69.2Fe4.1B11.8Si13.8C1.1 and Fe73.8Cu1Nb3.1B9.1Si13 amorphous microwires is studied. It is observed that the magnetic properties and the GMI effect of the Co-rich microwire can be tuned by heat treatment. A high GMI effect has been observed in as-prepared Co-rich microwires. After appropriate annealing of the Co-rich microwires, fast domain wall propagation and a GMI effect can be simultaneously observed in the same sample, and annealing is seen to affect both the diagonal and off-diagonal GMI components. Using complementary studies of the bulk and surface magnetic properties, an attempt is made to explain the great difference observed in the GMI properties of Co- and Fe-based microwires exhibiting very similar bulk hysteresis loops.

Circular domains nucleation in magnetic microwires

We report on the nucleation of circular domains in the outer shell of magnetic, Co-rich, amorphous microwires over a wide range of magnetic field pulse frequencies and amplitudes. Nucleation in the low barrier regime of different numbers of single domains was detected using the magneto-optical Kerr effect. The amplitude and frequency dependence of the domain nucleation probability is described in the framework of a model which agrees well with the experimental data. We have shown that to efficiently drive magnetic domains in microwires with alternating electric current excitation, precise tuning of the frequency and amplitude of the pulse is necessary.

Tunable magnetic properties in ultrathin co/garnet heterostructures

We demonstrate how the magnetic properties of metal/dielectric Co/yttrium iron garnet heterostructures can be engineered by both changing the garnet thickness and adding an ultrathin Co cover layer. The observed magnetization reversal process in the heterostructures is explained by both cubic and perpendicular growth-induced magnetic anisotropy of the garnet films. In particular, the perpendicular magnetic anisotropy can be strongly increased for reduced thickness. A strong influence of a 2 nm Co layer on the domain structure geometry and magnetization processes has been found for 1.8 µm garnet films.