Andriy Ya. Vovk

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.

Room temperature tunneling magnetoresistance of electron beam deposited (Co50Fe50)x(Al2O3)1−x cermet granular films

A series of (Co50Fe50)x-(Al2O3)1−x cermet granular thin films deposited on glass substrates by dual electron beam evaporation was studied for their structural, magnetotransport, and magnetic properties. Upon varying the magnetic volume fraction, x, from 0.07 to 0.52 the percolation threshold (xc) was determined from resistivity measurements to be ∼0.17. This value agrees well with the theoretical prediction for a three-dimensional system of spherical particles. Values of the isotropic tunneling magnetoresistance (TMR) as high as 10% at room temperature were found for films with x<0.16. The relationship between magnetotransport (maximum TMR) and the granular film topology (the percolation threshold) is discussed.

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.

Magnetic and structural properties of nonstoichiometric Ni2MnGa alloys with Ni and Ga excess

The influence of nickel and gallium excess on local structure and magnetic properties of Ni–Mn–Ga alloys has been studied. Nuclear magnetic resonance experiments showed that excessive Ni occupies Mn and Ga positions. This leads to the appearance of low-frequency line from the nearest Mn55 nuclei and generation of fractional nuclear echo signals due to the increase of electrical field gradients on these nuclei. Magnetic measurements revealed the difference of Curie temperature determined from ac susceptibility and extrapolated from high-temperature magnetization behavior. The most probable explanation of this fact is the reduction of manganese–manganese indirect exchange via the faults in Mn–Ga layers interchange caused by excessive Ga.

Magnetotransport in NiMnGa thin films

The influence of substrate temperature and annealing conditions on structure, magnetic, and magnetotransport properties of NiMnGa films was investigated, and the crucial effect of substrate temperature was confirmed. It was found that amorphous disordered films are formed during deposition on a substrate held at room temperature. Postdeposition annealing leads to partial recrystallization and recovery of magnetic properties. Annealing at 773 K does not allow the formation of crystal structure, which is created during deposition on substrates held at the same temperature. The highest values of magnetoresistance were observed in the films with a well-defined crystal structure.

Magnetic and transport properties of NiMnAl thin films

The magnetic and transport properties of Ni2MnAl thin films prepared using pulse laser deposition were investigated. It was shown that the films are granular and multiphase. Contrary to the data reported earlier we observe nonmonotonic temperature dependence of resistance with minimum in the vicinity of 100 K for the films deposited on substrates held at 773 K and negative magnetoresistance (MR) values of about 2.5% at 5 K and 1.6% at RT in magnetic field of 50 kOe. These values of MR are the highest reported up to date for Ni2MnAl films. Large negative MR in our films arises from a nonhomogeneous structure and its origin is the same as for granular films.