Leszek Malkinski

Anomalous magnetoresistance in NiMnGa thin films

The origin of anomalous negative magnetoresistance and its temperature dependence in polycrystalline Ni–Mn–Ga films prepared by pulse laser deposition was studied. The investigation of structural, transports, magnetic, and ferromagnetic resonance properties of the films suggests contributions of different mechanisms in magnetotransport. At low magnetic fields the main contribution to magnetoresistance is due to the transport between the areas with different orientation of magnetic moments, while at high fields it is an electron scattering of in spin-disordered areas.

Fabrication of pseudo-spin-valves and 100 nm sized periodic elements for magnetic memory application

Abstract Co–Cu–Co pseudo spin-valves (PSV’s) and Co–Cu multilayers have been fabricated and studied for the giant magnetoresistance (GMR) effect. Attention is focused on optimizing the low field response. They are deposited on Si (001) wafers without removing the natural silicon-dioxide layer, using S-research sputtering guns in high vacuum. A buffer layer of Cr–Cu was used before depositing the PSVs. Realization magnetic switching at low field for the soft layer and high field for the hard-layer makes the structure suited for MRAM application. The low and high switching fields are typically 100 and 2200 Oe, respectively. The magnetic measurement using SQUID magnetometry and VSM confirms the switching fields in the hysteresis loops of the magnetization. The GMR multilayers have a low switching field of 60 Oe at low temperatures. They are patterned into two-dimensional 100 nm sized periodic dot-array using ultraviolet interferometric lithography. The purpose is to develop processes of fabricating GMR structures over large areas with nanometer feature sizes for high-density memory devices. The coercive field for the patterned structures is enhanced by a factor of four at low temperatures and by a small fraction at higher temperatures. The magnetization remains to be in plane as in un-patterned case.

CoFe2O4 nanostructures with high coercivity

Nanometer-sized ferrite magnetic materials are the subject of intense research interest due to their potential applications in high-density magnetic information storage. One of the most explored ferrite materials is the cobalt ferrite (CoFe2O4). We have synthesized cobalt ferrite nanowires using cobalt ferrite nanoparticles in a porous anodic alumina template (AAT). The process of embedding ferrimagnetic particles into the pores was assisted by the magnetic field of a permanent magnet placed in vacuum directly under the substrate. Particles synthesized in the template were subsequently annealed at 600°C for 2h in Ar gas forming arrays of cobalt ferrite nanowires inside the AAT. The morphology of the ferrite before and after annealing was observed using a field-emission scanning electron microscope. The crystallographic structure of the nanowires was analyzed using x-ray diffraction and transmission electron microscopy. The magnetization was measured by a superconducting quantum interference device. The co...

Channel switching and magnetoresistance of a metal-SiO2-Si structure

We report a switching of conducting channel and magnetoresistance (MR) in very thin metallic films deposited on Si substrate with native SiO2 surface. The resistance of the metal-oxide-semiconductor (MOS) structure significantly increases when the temperature is lowered to a threshold value Tc of about 250 K. At room temperature the samples exhibit a high conductivity, and a positive MR of about 18%. Below Tc, the resistivity is increased by ~50% and the positive MR disappears. This effect can be explained by the conducting channel switching from the Si electron inversion layer to the upper metallic films when the temperature is decreased. It has also been found that the MR of Cu80Co20-SiO2-Si structure changes sign with the change of temperature, which is correlated to the switching of the conducting channel.

Electrodeposited nickel and gold nanoscale metal meshes with potentially interesting photonic properties

Nickel and gold meshes having three-dimensional periodicity at optical wavelengths and nanoscale structural fidelity have been prepared by electrodeposition within close-packed silica sphere arrays.

Increasing the switching speed of liquid crystal devices with magnetic nanorods

Liquid crystal (LC)/magnetic nanorods colloids were fabricated and tested using a magneto-optical setup. These thermotropic ferronematics do not show any signs of macroscopic aggregation, exhibit enhanced magnetic sensitivity, and faster time response in the simultaneous presence of crossed electric and magnetic fields. Magnetic nanorods increase an effective magnetic anisotropy of the colloid and decrease magnetic Freedericksz threshold. Applying a magnetic field along the direction perpendicular to the applied electric field leads to a decrease of the time OFF by a factor of 6 for pure liquid crystals, and by a factor of 9—for ferronematics.

Microwave absorption of patterned arrays of nanosized magnetic stripes with different aspect ratios

Arrays consisting of nanosized stripes of Permalloy with different length-to-width ratios have been fabricated using electron beam nanolithography, magnetron sputtering, and lift-off process. These stripes have a thickness of 100nm, a width of 300nm, and different lengths ranging from 300nmto100μm. The stripes are separated by a distance of 1μm. Magnetization hysteresis loops were measured using a superconducting quantum interference device susceptometer. Microwave absorption at 9.8GHz was determined by means of ferromagnetic resonance technique. The dependence of the resonant field on the angle between the nanostructure and the in-plane dc magnetic field indicates the presence of uniaxial magnetic anisotropy associated with the aspect ratio of the stripes. A maximum change of the resonant field of 1600Oe was observed in the longest stripes, yet it was only 200Oe for square shaped stripes. The linewidth of the resonant curve varied with the angle, in the range from 120to300Oe. Most of the ferromagnetic re...

Preparation, structural characterization, and dynamic properties investigation of permalloy antidot arrays

Regular nanosized structures are considered to be promising materials for magnetic information storage media with high density of information. Recently attention was paid to static and dynamic magnetic properties arising from dimensional confinement in such nanostructures. Here we present an investigation of permalloy antidot arrays of different thicknesses. Thin permalloy films of thickness ranging from 10to500nm were deposited on nanoporous Al2O3 membranes with a pore size of 100nm. It was found that additional ferromagnetic resonance peaks appear for film thicknesses below 100nm, while films with larger thicknesses show resonance properties similar to continuous films. A comparison between the films deposited onto Si wafers and porous media was done. An evolution of the domain structures observed in MFM experiments was confirmed by micromagnetic calculations.

Synthesis of mild-hard AAO templates for studying magnetic interactions between metal nanowires

The sequential application of mild and hard anodization techniques in the fabrication of porous alumina membranes allows one to decrease the number of continuous pores in anodized aluminium oxide (AAO) templates. Initially, standard mild anodization techniques were used to create porous templates with 100 nm interpore distances and 70 nm pore diameters. Hard anodization treatment on the same membrane then produced interpore distances of about 265 nm with diameters of 110 nm. At the interface between the two anodization steps, many of the mild-side pores were terminated to create a mild–hard membrane (Mi–Ha AAO) where the functional interpore distances were 200–300 nm. Growth (electrodeposition) of nickel and cobalt nanowires in the various pore structures of the mild and hard sides of the Mi–Ha AAO templates allows one to probe magnetic interactions between nanowires and compares them to wires grown in standard mild templates (Mi-AAO). The magnetic properties of nanowires in Mi–Ha AAO and Mi-AAO showed distinct differences in the squareness of hysteresis loops and coercivity both as a function of pore structure and magnetic component. In general, the squareness of the hysteresis loops increased with aspect ratio and greater interpore distance. Coercivity also showed an increase with aspect ratio, but varied differently with interpore distance for Ni and Co. These various magnetic behaviors are discussed with respect to crystalline structure, morphology, and interactions of adjacent sets of nanowires.

Size dependence of static and dynamic magnetic properties in nanoscale square Permalloy antidot arrays

Permalloy antidot arrays with different square hole sizes (1200×1200, 800×800, and 400×400nm2) have been fabricated by means of electron-beam lithography and lift-off techniques. The smaller square hole size results in enhanced remanence and reduced coercivity in the antidot array. Multiple resonance modes were clearly observed for the magnetic field applied normal to the array plane, and double uniform resonance modes occurred when the field deviated more than 30° from the normal to the plane. Two distinct dipolar field patterns with different orientations and magnitudes split the uniform resonance into double resonance modes. The double resonance modes show uniaxial in-plane anisotropy and the easy axes are orthogonal. The magnitude of the induced dipolar anisotropy remains almost constant with changes in the square hole size. The double resonance peaks move to low field with reduction of the square hole size.