N. V. Vorob’eva

Giant magnetoresistance in a ferromagnet-polymer system

Giant magnetoresistance in a ferromagnet-electroactive polymer-nonmagnetic metal structure is studied. The insulator-to-metal transition in the conductivity of the system is attributed to the change in the shape of the current-voltage characteristics of the sample in a magnetic field. It is concluded that the giant magnetoresistance observed in the experiment may be of injection nature, and this is why the effect is realized at the ferromagnet-electroactive polymer interface alone.

On the role of spin polarization of electrons in the effect of giant injection magnetoresistance in the Ni-polymer-Cu system

The effect of spin polarization of electrons injected from a ferromagnet on the giant injection magnetoresistance was investigated for a ferromagnet-polymer-nonmagnetic metal experimental structure. The degree of spin polarization was varied by introducing a depolarizing nonmagnetic metal (Cu) layer between the ferromagnet and the organic transport layer. It was established that the coefficient of giant injection magnetoresistance depends significantly on the thickness of the depolarizing layer. In particular, the effect was not observed at a thickness larger than 12 nm and decreased exponentially at a smaller thickness as the thickness increased. The conclusion was drawn that the spin polarization of electrons plays a decisive role in the effect of giant injection magnetoresistance.

Static and dynamic photoinduced magnetic effects in yttrium-iron garnet lightly doped with barium ions

In yttrium-iron garnet lightly doped with barium, direct measurements of the photoinduced changes in magnetostrictive strains disagree with those in magnetostriction constants at 78–100 K. This is attributed to a considerable photoinduced modification of the initial state in this sample due to a redistribution of the charge (during illumination) between cations of the ferromagnetic octahedral sublattice. In the same sample, the temperature dependence of the photoinduced disaccomodation of magnetic permeability characterizing the initial demagnetized state is measured and calculated. A change in the electron mechanism of the phenomenon during the transition to room temperature is shown. The conclusion about the promising prospects for using such samples for remagnetization by light is advanced.

The nature of photoinduced changes in the magnetostriction of yttrium-iron garnet single crystals

A model of the occurrence of photoinduced changes in linear magnetostriction is proposed based on a complex experimental study of magnetostrictive strains in yttrium-iron garnets Y3Fe5O12 with low contents of different impurities. Analytical expressions for calculating the magnetostriction in yttrium-iron garnet single crystals with different types of doping are presented. The correlation of the photoinduced change in the magnetostriction with the crystallographic features of the samples is demonstrated. The changes in the magnetostriction constants are analyzed quantitatively for samples prepared in different ways.

On the nature of giant magnetoresistance in a ferromagnet-polymer-nonmagnetic metal system

The role of magnetostriction in manifestations of the giant magnetoresistance (GMR) in a nickel-poly(arylenephthalide)-copper system has been studied. It is established that the appearance (disappearance) of GMR at the nickel-polymer interface is not correlated with a significant change in magnetostrictive deformations, since the transverse and longitudinal magnetostriction in the substrate plane remains almost unchanged in the entire interval of magnetic fields inducing the transition. The role of the longitudinal magnetostriction perpendicular to the sample plane cannot be considered as determining, since the same sample exhibits a change in the sign of the conductivity switching depending on the initial conditions for the same sign of the longitudinal magnetostriction in the substrate. The possibility of controlling the GMR threshold field by changing the applied bias voltage is also not indicative of the influence of magnetostriction on this phenomenon.

The role of the wide-band-gap polymer layer for the existence of conductivity switching in the valve structure

The effect of electronic switching of the conductivity by a magnetic field lower than 500 mT has been implemented in the spin-valve structure in the “current perpendicular to plane” geometry with a transport layer of a wide-band-gap polymer to 1200 nm thick. The problem of spin transfer through the polymer layer has been discussed in the model of ballistic charge transport over nanoscale conducting channels.

Influence of a metallic substrate’s magnetic state on the huge magnetoresistance of a ferromagnet-polymer structure

The association between the threshold value of conductance switching in an external magnetic field and the initial magnetic state of a ferromagnetic plate substrate is shown for a ferromagnet/polymer/nonmagnetic metal system. The threshold magnetic field change is explained by appearance of residual magnetization after being held in an external magnetic field and, correspondingly, by the change in the initial state upon further remagnetization.

Effect of doping with Pb, Ba, and Si on the properties of Y3Fe5O12 single crystals

In doped single-crystal yttrium iron garnet, Y3Fe5O12, photoinduced changes in the cubic magnetostriction constant λ111 and coefficient (λσ) that characterizes extra stress in the material are determined by the sublattice the dopant is incorporated into. The incorporation of dopant cations into a particular crystallographic site can be controlled by adjusting the crystal growth process. Varying the process for the growth of doped Y3Fe5O12 single crystals with semiconducting properties, one can tune the type and magnitude of their photoinduced response. Since an ideal sample cannot be created in practice, and even the lowest densities of growth defects in a single crystal lead to photomagnetism, the present results suggest that, at sufficiently low temperatures, photomagnetic changes occur in almost all Y3Fe5O12 single crystals. The behavior of their photomagnetic response depends on the type of defect.

Huge magnetoresistance of structures based on nonferromagnetic wideband polymers

The phenomenon of huge magnetoresistance in structures of the ferromagnetic metal-polymer insulator (FM-PI) type is considered in comparison to other types of magnetoresistance. Similarities and differences between the effects of huge magnetoresistance at an FM-PI interface and on an array of paramagnetic nanoparticles are shown. Conclusions are drawn as to the decisive role of change in the magnetic field of tunneling conditions of a charge through a potential barrier.

Photoinduced changes in magnetostriction of Y3Fe5O12 single crystals

A decrease in the value of the magnetostriction constant λ111 is observed at 78 K in YIG(Ba) and YIG(Si) single crystals exposed to light in the spectral range 0.65– 2 μm. Exposure to infrared light leads to a decrease in the value of λ111 for YIG(Si), but causes no change for YIG(Ba).