V. V. Ustinov

Hysteresis of microwave absorption of hard superconductors in magnetic field

Abstract Field- and temperature-dependent peculiarities of the microwave power loss in hard superconductors (Nb, NbSe2, V3Si, La1.8Sr0.2CuO4, YBa2Cu3O7 etc.) were discovered and investigated. The qualitative interpretation suggested allows to connect the low field mw-absorption to the peculiarity appearing near Hc1 and hysteresis of mw-absorption — to the pinning of the Abrikosovs flux-lines.

High frequency properties of magnetic multilayers

The review of the experimental methods of the microwave giant magnetoresistive (GMR) effect measurement is presented. The absorption of the electromagnetic power in the cavity or any other resonant circuit and the direct penetration through the multilayer are the most usable methods. It was found that magnetic field dependence of microwave absorption in the metallic multilayers is governed by GMR effect. The direct correlation between the GMR and the transmission of electromagnetic waves through the metallic magnetic superlattices was established. The results of microwave transmission through the Fe/Cr superlattices are presented in the frequency range from 26 to 38 GHz.

Conduction and disorder in -based materials

We propose a simple theory of colossal magnetoresistance in which the metal-insulator transition in heavily doped lanthanum manganites is assumed to be caused by a shift of the mobility edge. The theory involves a small set of parameters which have clear physical meaning and can be easily found experimentally. The detailed comparison with experimental data available is made.

Penetration of the electromagnetic waves through doped lanthanum manganites

Penetration of the electromagnetic field through plate-shaped bulk polycrystalline sintered Pb and Y–Ba doped manganite samples is studied in the frequency range from 20 kHz to 30 MHz. Strong magnetic-field dependence of the transmission coefficient is observed. The penetration data are compared with the other magnetic-field dependent parameters: the magnetic permeability, magnetoresistance, and magnetoimpedance. Unusual frequency dependence of the penetration depth is observed.

Correlation between microwave transmission and giant magnetoresistance in Fe/Cr superlattices

Magnetoresistance and microwave transmission of Fe/Cr superlattices grown by MBE method are studied in magnetic fields at wave frequencies of GHz range. It is shown that a well-defined correlation exists between these effects.

High-sensitive hysteresisless spin valve with a composite free layer

Method of dc magnetron deposition has been used to prepare metallic nanostructures of the (Si, Al2O3, glass)/Ta/[NiFe/CoFe]/Cu/CoFe/(FeMn, MnIr)/Ta spin-valve type with a composite “free layer” Ni80Fe20/Co90Fe10. A three-stage method of decreasing hysteresis of the free layer has been suggested. The dependence of the magnitude of the giant magnetoresistance and hysteresis has been investigated depending on the angle between the direction of the easy magnetization axis of the free layer and the direction of the applied magnetic field in the plane of layers. It is shown that in the spin valves studied the hysteresis can be reduced to a few tenths of an oersted at the magnitude of the magnetoresistance more than 8%. In the case of a spin valve with an antiferromagnet Fe50Mn50 the magnetoresistive sensitivity in the range of the hysteresisless variation of the magnetoresistance was equal to 1%/Oe, whereas the maximum sensitivity in the presence of a hysteresis exceeded 6%/Oe. A greater sensitivity (2.5%/Oe) at the magnitude of the free-layer hysteresis of 0.6 Oe was obtained for spin valves on the basis of the antiferromagnetic Mn75Ir25 alloy. For them, the maximum magnetoresistance was equal to 11.6%.

The effect of annealing on the magnetoresistance of polycrystalline (La - Y - Ca) perovskites

The magnetization, resistivity and magnetoresistance of yttrium-doped polycrystalline have been studied as a function of annealing conditions. The magnetoresistance amplitude depends upon the annealing time, with an optimum corresponding to 12 h annealing at under oxygen. Different contributions to the magnetoresistance are separated; the first is related to the technical magnetization, the second, dominant one, to the influence of the total magnetic induction on the band splitting and the spin correlation function.

Magnetic neutron off-specular scattering for the direct determination of the coupling angle in exchange-coupled multilayers

Abstract A new method is presented for the direct determination of the coupling angle in magnetic exchange multilayers. Strong anomalies in the off-specular neutron spin–flip scattering intensity at critical angles give a new tool, which is tested with Fe/Cr multilayers. In addition, a complete two-dimensional data analysis of specular reflection and off-specular scattering has been employed to verify atomic spin correlation in Fe/Cr multilayers, a typical system showing the GMR effect.

Semiconductor-metal transition and magnetoresistance in (La,Ca)MnO3 : Experiments and simple model

The effect of Ca-doping on bulk samples of La1−xCaxMnO3 has been investigated, as well as the effect of sample annealing under air and oxygen. The maximum magnetoresistance is observed for ferromagnetic samples, with La0.67Ca0.33MnO3 composition, annealed at an optimum temperature of 1200°C. A simple phenomenological model has been used which describes the magnetic and transport properties as resulting from an exchange-induced band-crossing SC-metal transition.

Giant changes in magnetic and magnetoresistive properties of CoFe/Cu multilayers at subnanosized variations in the thickness of the chromium buffer layer

The article discusses the magnetic and magnetoresistive properties of the [Co90Fe10/Cu]n multilayers prepared by magnetron sputtering as a function of the type of the substrate and the material and thickness of a buffer layer (Co90Fe10, Cu, Cr, Fe). The multilayers with a chromium buffer layer exhibit a sharp transition from low-coercive to high-coercive state at low variations in the thickness of the chromium layer (from 15 to 20 Å). It has been demonstrated that the revealed sudden change in the hysteresis properties of the multilayers with a Cr buffer layer is accompanied by structural changes in the Cu and Co90Fe10 layers, characterized by the disappearance of the 〈111〉 texture at tCr > 20 Å. It has been found that the most efficient buffer layer, which at room temperature ensures a threefold increase in the magnetoresistance at a minimum increase in hysteresis, is the Co90Fe10 (15 Å) layer. It has been established that the substrate type ((100)Si,