Scientific reports of Bukhara State University


Background. The anisotropy of the transverse magnetoresistance of single-crystal nickel films was studied in this work. The measurements were carried out on samples whose surface plane coincided with the [001] plane. Studies of the magnetoresistance in a single-crystal nickel film have shown the effect of tensile stresses acting on it from the side of magnesium oxide. The modification of the anisotropy of magnetoreflection of a film on a substrate as compared to a free sample is apparently associated with a change in the shape of the Fermi surface of carriers. In accordance with the technology the films were grown at a substrate temperature of 1000°C. The thermal expansion coefficient of nickel, αNi, exceeds the corresponding coefficient αMgO for magnesium oxide. For example, at room temperature αNi = 12,5·10-6, αMgO = 11,2·10-6 deg-1. Consequently, upon cooling, the film, being bonded to the substrate, is subjected to in-plane tensile stress, which leads to tetragonal deformation of the crystal lattice. This conclusion was confirmed by comparing the crystal lattice parameters measured by X-ray diffraction at room temperature Tk and T = 77 K. Methods. The magnetoresistance was measured by an unbalanced double Thomson bridge with an accuracy of the order of 10-6 Ohm. The magnetoresistance measurements were carried out in the longitudinal and transverse directions of the external magnetic field, as well as depending on the angle φ (between the [100] axis and the direction of the external field). In all measurements the current passed through the samples remained constant 5 x 10-4 A. When measuring the longitudinal and transverse magnetoresistance the external magnetic field varied from 0 to 21000 Oe. The effect measurements in our experiments were carried out with an accuracy of 2-4% on average. The measurement of the magnetoresistance as a function of the angle φ was carried out at [100] ǁ i ǁ r ^ H = φ. Findings. Oxidation of nickel films occurs through the transfer of electrons, nickel cations and oxygen anions through the film. Annealing controls the oxidation process and film morphology. According to the results of the analysis of this study we find that Ni films 50 nm thick are almost completely oxidized at an annealing temperature of 700 ° C. It is assumed that tensile density stresses transform the Ni/MgO film into a “strong” ferromagnet, characterized by a high magnitude of magnetoresistance (up to 0.25% at low temperatures). In this work the anisotropy of the transverse magnetoresistance of single-crystal nickel films was studied. The measurements were carried out on samples whose surface plane coincided with the [001] plane. We used samples with a thickness of d = 5.0 ± 0.5 μm with a ratio ƞ = 103. Conclusions. The modification of the anisotropy of the magnetoresistance of a film on a substrate as compared to a free sample is apparently associated with a change in the shape of the Fermi surface of carriers. In this case, the volume occupied by carriers in momentum space does not change that is indicated by the existence of a general universal dependence for the magnetoresistance of films on a substrate and films separated from it. The magnetoresistance in both longitudinal and transverse magnetic fields for all investigated film thicknesses has a different course of curves: in the first case- with a positive value, in the second-with a negative value of the effect magnitude. The magnitude of the magnetoresistance at a saturation field of 5800 Oe depending on the angle φ between the [100] axis and the direction of the magnetic field H shifts towards a negative effect with decreasing film thickness and reaches zero at φ equal to 1450 and 1350 for films with a thickness of 600 Ȧ. On films with a thickness of 500 Å the magnitude of the magnetoresistance in the entire range of angle variation has a negative sign of the magnitude of the effect. With a decrease in temperature following the transition of the Ni/MgO film to the “strongly magnetic” state a structural phase transition occurs in it.

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