F. G. Aliev

Plain superconducting films as magnetic field tunable two-dimensional rectifiers

Longitudinal and transversal permanent electric fields generated by an ac current through superconducting Pb and Nb thin strips have been studied as the function of the drive frequency (103 105Hz) the rectification becomes two dimensional. The rectification strongly depends on the magnetic field, temperature and ac drive. The unusual dc electric field topology generated by the ac current in the superconducting strips can be explained by a local rectification due the oppositely directed asymmetric edge (Bean-Livingston type) barriers.

Unusual dc electric fields induced by a high frequency alternating current in superconducting Nb films under a perpendicular magnetic field

We report a systematic study of dc electric fields produced by sinusoidal high frequency ac currents in Nb superconducting films subject to a constant magnetic field perpendicular to the film plane. At frequencies in the 100kHz to MHz range appears a new rectification effect which has not been previously observed at lower frequencies. We have observed the dc electric field generated in this regime in films without intentionally created anisotropic pinning centres, i.e. plain films, both in strip geometry as in cross- shape geometry, and also in films with symmetric periodic pinning centres. The electric field appears in both directions along and transverse to the alternating current and is essentially different at opposite film sides. It depends strongly on the intensity of the magnetic field and may exceed by nearly an order of magnitude the rectified electric fields recently reported at lower frequencies (few kHz) in systems with artificially induced anisotropic vortex pinning. The effect has a non-monotonic dependence on the drive current frequency, being maximum around a few 100kHz to MHz, and shows a complicated temperature dependence. It is found to be different in long strips and cross shape samples. In the case of films with symmetric periodic pinning centres the rectified voltage

Conductance in Co|Al2O3|Si|Al2O3 Permalloy with asymmetrically doped barrier

We report on dependence of conductance and tunnelling magnetoresistance on bias voltage at dierent temperatures down to 2K in Co |Al2O3(10A)|Si(�)|Al2O3(2A)|Permalloy magnetic tunnel junctions. Complementary low frequency noise measurements are used to understand the conduc- tance results. The obtained data indicate the breakdown of the Coulomb blockade for thickness of the asymmetric silicon layer exceeding 1.2A. The crossover in the conductance, the dependence of the tunnelling magnetoresistance with the bias voltage and the noise below 80K correspond to 1 monolayer coverage. Interestingly, the zero bias magnetoresistance remains nearly unaected by the presence of the silicon layer. The proposed model uses Larkin-Matveev approximation of tunnelling through a single impurity layer generalized to 3D case and takes into account the variation of the barrier shape with the bias voltage. The main dierence is th e localization of all the impurity levels within a single atomic layer. In the high thickness case, up to 1.8A, we have introduced a phe- nomenological parameter, which reects the number of singl e levels on the total density of silicon atoms. PACS numbers:

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

Abstract We report on tunnelling magnetoresistance (TMR), current–voltage ( IV ) characteristics and low-frequency noise in epitaxially grown Fe(1xa01xa00)/MgO(1xa01xa01)/Fe(1xa01xa00) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20xa0μm 2 . The evaluated MgO energy barrier (0.50±0.08xa0eV), the barrier width (13.1±0.5xa0A) as well as the resistance times area product (7±1xa0MΩxa0μm 2 ) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures ( T

Electron interaction with domain walls in Fe/Cr multilayers at low temperatures

Abstract For antiferromagnetically coupled [Fe/Cr] 10 multilayers with Fe(Cr) thickness 30(13.5) Axa0and 12(12) A, we found that at low fields the contribution of the domain walls to the resistivity, ρ DW , is small at room temperature and strongly enhanced at low temperatures. Magnetic force microscopy images clearly reveal the presence of domain walls. In a wide interval above 1.9xa0K ρ DW varies as ρ DW ( T )= ρ DW (0)− AT α with the exponent α ≃0.7–1, indicating the possible presence of quantum interference effects in the quasiballistic electron transport through domain walls.

Nonlinear electron transport in magnetic multilayers

We report on measurements of the second harmonics in the magnetovoltage generated in [Fe(12u200aA)/Cr(12u200aA)]10 epitaxial multilayers. It is shown that the variation of the amplitude of the second-harmonic signal with magnetic field is up to three times larger compared to the first harmonic. The enhanced “magnetovoltage” second-harmonic effect may be of practical use in systems based on spin electronic phenomena.

Evolution of calorimetric, magnetic and transport properties of UxTh1−xBe13 (0.64 ≤ x ≤ 1) solid solutions

Abstract We have recently found that U 0.9 Th 0.1 Be 13 displays a non-Fermi liquid ground state which is consistent with predictions of the quadrupolar Kondo effect (QKE). We report the evolution of transport, calorimetric and magnetic properties of U x Th 1− x Be 13 (0.64 ≤ x ≤ 1) solid solutions. The magnetoresistance of U 0.9 Th 0.1 Be 13 is compared with those of UBe 13 and U 0.64 Th 0.36 Be 13 . We present the first experimental study of the Hall effect in the heavy fermion compound with a non-Fermi liquid ground state, with a drastic reduction of the small field Hall resistivity in the QKE state. The field dependence is opposite to those found previously for Kondo lattices.

Complex magnetic response in magnetic tunnel junctions determined via magnetic and transport measurements

Abstract : We have studied the low frequency complex magnetization dynamics in Co/Al2O3/Ni80Fe20 magnetic tunnel junctions (MTJs) at temperatures between 4.2K and 300K. The measurements were carried out by using two different experimental techniques. The first method probes directly magnetic properties via DC magnetization and AC susceptibility while the second one measures AC magnetization dynamics of the ferromagnetic electrodes near the cross area which is related to the tunnelling resistance.

Domain Wall Magnetoresistance and Complex magnetic Response in Antiferromagnetically Coupled Fe/Cr Multilayers

Abstract : For antiferromagnetically coupled Fe/Cr(100) multilayers the low field contribution to the resistivity, which is caused by the domain walls (DWs), is strongly enhanced at low temperatures. The low temperature resistivity increases approximately according to a power law with the exponent 0.7-1. This behaviour can be explained by the suppression of anti-localization effects by the nonuniform gauge fields caused by the domain walls. Analyses of complex low frequency magnetic susceptibility shows an enhancement of the magnetic losses at low magnetic fields, which may be related to the AC field induced DWs movement. At low temperatures (T is less than 100K) DWs become pinned. For frequencies (10(2) is less than f is less than 10(3)) Hz at temperatures below 10K, this hysteretic low field peak in the magnetic losses transforms to a non-hysteretic dip for the absolute value of H less than 20 Oe, indicating a possible qualitative change in the dynamics of the DWs. The frequency dependence of the dissipation at 2K, may be reasonably well fitted by the expression that describes the losses of a damped oscillator with a single relaxation time of about 10(-4) sec.

Low-frequency response in antiferromagnetically coupled Fe/Cr multilayers

Abstract We have studied the magnetic field dependences of the real (χ) and imaginary (χ′) contributions to the low-frequency magnetic susceptibility in epitaxial antiferromagnetically coupled [Fe(Cr(1xa00xa00)]n (n=10–50) multilayers. For the magnetic field directed along (1xa01xa00), the magnetic susceptibility shows on orientation phase transition. For the magnetic field either along the easy or the hard axes we observe a strong enhancement of the χ′(H) (i.e. magnetic losses) at low magnetic fields (H