N. I. Polushkin

Magnetic nanodot arrays produced by direct laser interference lithography

Periodic magnetic nanodot arrays have been produced on an area as large as 1 cm×1 cm by direct nanolithography using interferometric laser radiation. The dots are formed by the local annealing of sputtered amorphous Co–C films in regions where the laser intensity is highest. At room temperature the dots exhibit ferromagnetic order and are embedded in a paramagnetic matrix. The onset of room-temperature ferromagnetism is caused by nanoscale chemical and morphological changes during dot formation and reflects the phase separation of magnetic Co-rich clusters. The present single-step nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays.

Femtosecond laser-induced nanofabrication in the near-field of atomic force microscope tip

Femtosecond laser-induced formation of nanostructures in the near field of an atomic force microscope tip is demonstrated. Mechanism of nanofabrication will be discussed.

Ferromagnetic Co-C nanodot arrays produced by direct interferometric laser annealing

Magnetic properties of Co-C nanodot arrays produced by direct interferometric laser annealing are investigated by magnetic force microscopy (MFM) and magnetization measurements. The dots are formed by locally annealing sputtered amorphous Co-C films in regions where the laser intensity is highest. As-sputtered Co-C films do not exhibit ferromagnetic order at room temperature, but MFM shows that the dots become magnetic upon annealing, possibly due to the agglomeration or phase separation of Co-rich clusters. The dots are embedded in either a paramagnetic or weakly magnetic matrix. The magnetic properties of the generated pattern can be changed by varying the laser power. The present results show that direct interferometric lithography may become a useful tool for fabricating future patterned magnetic nanostructures.

Arrays of magnetic wires created in phase-separating Fe-containing alloys by interference laser irradiation

Using the interference of laser beams, we have fabricated the periodic arrays of submicron wires in Fe/C and Fe/Cr thin-film alloys. The array formation is found to arise from iron agglomeration in interference maxima. The dramatic decay of the in-plane magnetic anisotropy was observed in the fabricated arrays with decrease in the interference periodicity down to 170 nm. This decay can rather be explained by transforming the wires into separate granules than a smallness of the temperature modulation at small periodicities. Such an explanation is supported by the electronmicroscopic and ferromagnetic resonance data.

Observation of Laser-Induced Local Modification of Magnetic Order in Transition Metal Layers

Laser-induced local modifications of magnetic order in thin Fe-Cr layers were investigated. Local modification in the layers were induced by interfering laser beams. The results of the study give evidence for the formation of submicron-sized anisotropically shaped ferromagnetic regions with a well-defined direction of the easy magnetic axis in the interference maxima at the modification threshold. It was also found that the magnetic anisotropy of a medium is drastically reduced with changing the shapes of these local regions and distances between them. This may be due to the strengthening of the interaction between the regions through the paramagnetic matrix.

Direct patterning of magnetic media via focusing light by microsphere arrays

An approach to nanoscale patterning using focused laser beams by two-dimensional lattices of microspheres is presented. The arrays were positioned in a controllable way near the sample surface with submicron accuracy to find the focal plane. This technique has been applied for direct patterning of Fe–Cr layers which exhibit laser-induced modifications in magnetic properties. The obtained data indicate the occurrence of some irregularities in the patterned features. To understand: (1) The reasons for the observed irregularities and (2) how small patterned features can be obtainable, the Mie solution for the intensity of scattered light has been used.

Local magnetostrictive response of small magnetic entities in artificial Fe–Cr composites

Nanoscale ferromagnetic entities are directly patterned in superparamagnetic Fe–Cr layers by interfering laser beams. To characterize the formed entities, in addition to the conventional methods, we used a technique for magnetic imaging based on the atomic force microscopy (AFM) with nonmagnetic tips and an ac magnetic field applied in situ. The observed AFM dynamic response is interpreted in terms of magnetostriction and a related quantity, the ac susceptibility.

Magnetic force microscopy observations of the magnetic behavior in Co–C nanodot arrays

The nanomagnetic behavior of Co–C nanodot arrays was investigated by magnetic force microscopy (MFM) and an alternative gradient force magnetometer. The direction of the easy axis can be observed directly with MFM by comparing the saturated magnetization state and the remanent magnetization state. Interaction of the domain wall with local defects was observed by field dependent MFM measurements. Some types of defects that can pin domain wall movement were identified.

Observation of remanent states of small magnetic particles: Micromagnetic simulation and experiment

Micromagnetic properties of submicron ferromagnetic elements ellipsoidal in shape are studied theoretically and experimentally. By numerically solving the equations of magnetodynamics, it is found that different remanent magnetization states can be obtained, depending on the manner of magnetization reversal in such elements: one-vortex states, two-vortex states, and vortex-free states with skew-symmetric spin pinning. The magnetization configurations predicted in the calculations have been observed experimentally using magnetic force microscopy in regular lattices of microstructures formed in thin-film samples of Fe-Cr alloys under irradiation by interfering laser beams.

Formation of magnetic nanosize gratings in the illumination of thin-film Fe-Cr mixtures by interfering laser beams

The conditions for formation of nanosize gratings (∼100 nm) of ferromagnetic stripes in the illumination of thin-film (10–15 nm) paramagnetic Fe-Cr mixtures by interfering beams from an excimer laser are investigated. The ferromagnetic ordering arises as a result of the thermally stimulated clustering of Fe atoms. The gratings are formed in a certain energy interval of the laser radiation. The width of this interval depends substantially on both the interference period and the illumination time τi. For τi=10 ns there exists an energy interval in which gratings with periods as small as 300 nm are formed.