Farkhad G. Aliev

Gap at the Fermi level in the intermetallic vacancy system RBiSn(R=Ti,Zr,Hf)

The new class of intermetallic compounds RNiSn(R=Ti,Zr,Hf) may be characterised by the presence of an ordered sublattice of Ni atom vacancies in comparison with normal metals RNi2Sn with no Ni vacancies. We report unusual transport and optical properties of the RNiSn system. The electrical resistivity of RNiSn is very high (3<p<100) mOhm*cm; the temperature coefficient of resistivity (TCR) is negative and strongly dependent on the annealing conditions. For some samples ZrNiSn and for a single crystal of TiNiSn the resistivity can be described by the Motts law at temperatures 0.1<T<20 K. A phase transition nearT=100 K without change of crystal structure was deduced from Hall effect data and the temperature dependence of the lattice constant. Preliminary data on transport phenomena in RPtSn and RPdSn(R=Ti,Zr,Hf) compounds are also reported. The unusual properties of RNiSn system might be related to a gap of the electron spectrum near the Fermi energy.

Layer-By-Layer Assembly of Core-Shell Magnetite Nanoparticles: Effect of Silica Coating on Interparticle Interactions and Magnetic Properties

Monodisperse PS spheres were synthesized and packed into colloidal crystals by centrifugation as described previously [6]. Sphere diameters in various batches ranged from 420 ‐ 50 nm to 660 ‐ 30 nm. To prepare macroporous nickel oxide, nickel(II) acetate (2 g) was dissolved in either acetic acid (5 mL) or a mixture of H2O (5 mL)and ethanol (5 mL) at 60 C. After cooling to room temperature, any undissolved solid was removed by filtration. Centimeter-scale, close-packed, colloidal polystyrene crystals (1 g) were soaked in this solution for 3‐5 min. Excess solution was removed from the impregnated colloidal crystals by vacuum filtration. The samples were dried at 60 C for 2 h. The dried composites were soaked in 10 mL of a saturated solution of oxalic acid in ethanol (ca. 25‐30 g in 100 mL) for 3‐5 min. After additional vacuum filtration and drying, the PS spheres were removed by calcination in flowing air at 360‐575 C for 7‐10 h (heating rate: 1 C/ min). Macroporous Ni was prepared by heating macroporous NiO in flowing H2 at 300 C for 2‐5 h (heating rate: 1 C/min) or by heating the nickel oxalate/PS composite in flowing nitrogen at 450‐500 C for 10 h. Preparation of partially reduced samples containing both NiO and Ni was possible by decreasing the reduction time or temperature, or by carrying out a fast calcination, for example at 360 C for 1 h in fast flowing air. Electrochemical tests were performed in two electrode cells using 1 M aqueous KOH as the electrolyte and nickel pellets as both electrodes. Pellets were made by pressing a mixture of 10 wt.-% poly(vinylidene fluoride) binder with 90 wt.-% nickel to 1500 psi for 30 s. An SEM of the material indicated that a porous structure was still maintained. No carbon powder was needed to enhance the pellet conductivity. Impedance analysis was performed using a Solartron 1260 Impedance/Gain-Phase Analyzer along with a Solartron 1287 Electrochemical Interface. Other product characterization was carried out as described previously [6].

Spin waves in circular soft magnetic dots at the crossover between vortex and single domain state

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions.

Spin excitation frequencies in magnetostatically coupled arrays of vortex state circular Permalloy dots

Broadband ferromagnetic resonance in square arrays of Permalloy circular dots with different interdot separations was measured in the vortex ground state. The detected spin excitations show a complicated dependence of their frequencies on the interdot coupling strength. A considerable influence of the interdot separation on the gyrotropic vortex frequency and splitting of the azimuthal spin wave frequencies was detected. The gyrotropic frequency and the first azimuthal doublet frequency splitting depend nonmonotonously on the interdot spacing, whereas the dependence of the second doublet frequency splitting on this parameter is monotonous. The observed effects are explained by the influence of both the dipolar and quadrupolar contributions to the dynamic magnetostatic interactions.

Shot noise in magnetic tunnel junctions: evidence for sequential tunneling.

We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations.

High bias voltage effect on spin-dependent conductivity and shot noise in carbon-doped Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions

Low temperature (10K) high voltage bias dynamic conductivity (up to 2.7V) and shot noise (up to 1V) were studied in epitaxial Fe(100)∕Fe–C∕MgO(100)∕Fe(100) magnetic tunnel junctions as a function of the magnetic state. The junctions show large tunnel magnetoresistance (185% at 300K and 330% at 4K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe–C interface. The shot noise shows a Poissonian character. This demonstrates a pure spin-dependent direct tunneling mechanism and validates the high structural quality of the MgO barrier.

Very low 1/f noise at room temperature in fully epitaxial Fe/MgO/Fe magnetic tunnel junctions

We report on room temperature 1∕f noise in fully epitaxial Fe (45nm)∕MgO (2.6nm)∕Fe (10nm) magnetic tunnel junctions (MTJs) with and without carbon doping of the Fe∕MgO bottom interface. We have found that the normalized noise (Hooge factor) asymmetry between parallel and antiparallel states may strongly depend on the applied bias and its polarity. Both types of MTJs exhibit record of low Hooge factors being at least one order of magnitude smaller than previously reported.

Low-frequency noise and tunneling magnetoresistance in Fe(110)/MgO(111)/Fe(110) epitaxial magnetic tunnel junctions

We report on tunneling magnetoresistance (TMR), current–voltage (I–V) characteristics and low-frequency noise in epitaxially grown Fe(110)∕MgO(111)∕Fe(110) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20μm2. The evaluated MgO energy barrier (0.50±0.08eV), the barrier width (13.1±0.5A) as well as the resistance times area product (7±1MΩμm2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. The noise power, though exhibiting large variation, was observed to be roughly anticorrelated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to an interplay between the magnetic state and the local barrier defects structure of the epitaxial MTJs.

Transient lateral photovoltaic effect in patterned metal-oxide-semiconductor films

The time dependent transient lateral photovoltaic effect has been studied with μs time resolution and with chopping frequencies in the kHz range, in lithographically patterned 21 nm thick, 5, 10, and 20 μm wide, and 1500 μm long Co lines grown over naturally passivated p-type Si (100). We have observed a nearly linear dependence of the transitorial response with the laser spot position. A transitorial response with a sign change in the laser-off stage has been corroborated by numerical simulations. A qualitative explanation suggests a modification of the drift-diffusion model by including the influence of a local inductance. Our findings indicate that the microstructuring of position sensitive detectors could improve their space-time resolution.

Precise probing spin wave mode frequencies in the vortex state of circular magnetic dots

We report on detailed broadband ferromagnetic resonance measurements of azimuthal and radial spin wave excitations in circular Permalloy dots in the vortex ground state. Dots with aspect ratio (β=height over radius) varied from 0.03 to 0.1 were explored. The frequency splitting of two lowest azimuthal modes was observed. The experimentally observed dependence of the frequency splitting on β was reasonably well described by dynamic splitting model accounting the spin waves and vortex gyrotropic mode interaction.