N. M. Kreines

Photoinduced superconductivity in YBaCuO films

Abstract The phenomenon of persistent photoconductivity in YBa2Cu3O6+x (x ≈ 0.4) films near the semiconductor-metal transition has been studied. Transport and magnetic measurements of the illuminated films were performed. The transition to the metallic state and photoinduced superconductivity have been observed. The mechanism of persistent photoconductivity in reduced YBa2Cu3O6+x films is also discussed.

Brillouin-Mandelstam scattering from thermal and excited magnons

Abstract Brillouin-Mandelstam scattering (BMS) is the scattering of light from acoustical quasiparticles (phonons, magnons and others). The frequency shift under BMS is 10–100 GHz. The observation of BMS from magnons became possible only after J. Sandercock had designed a multi-pass Fabry-Perot interferometer with a high contrast (1971). BMS from magnons has, by now, been observed in CrBr 3 (Sandercock), YIG, FeBO 3 (Jantz, Sandercock, Wettling), CoCO 3 (Borovik-Romanov, Jotikov, Kreines), EuO, EuS (Grunberg, Metawe), Ni, Fe (Sandercock, Wettling), metglasses (Chang, Malozemoff, Grimsditch, Senn, Winterling). In this review the main results of the above works are presented. The dispersion laws of magnons were studied by BMS in the energy range (inaccessible for neutron diffraction) where the contributions due to three types of interaction: magnetic, dipole-dipole, and exchange can be separated. Investigation of BMS in EuO and metals has led to the discovery of surface magnons. BMS from standing spin waves has been observed in thin films of metglasses. By observing BMS, it is possible to study quasiparticles pumped by microwave power. It was found that under ferro- or antiferromagnetic resonance an excess of quasiparticles arises, these quasiparticles being magnons with the frequency equal to that of microwave power and phonons with half the microwave frequency. Scattering of light from parametrically excited magnons has also been observed. This opens new possibilities for studying relaxation processes in magnetic materials.

Control of Interlayer Exchange Coupling in Fe=Cr=Fe Trilayers by Ion Beam Irradiation

The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.

Interaction of the moving domain wall with phonons

Abstract The interaction between the moving domain wall (DW) and acoustic phonons in the weak ferromagnet YFeO 3 has been investigated by means of Brillouin-Mandelstam spectroscopy method for the first time. The light scattering by the moving DW with the frequency shift due to the Doppler effect has been observed. The DW velocity and the intensity of the scattered light were determined from the spectra as a function of pulsed magnetic field at different temperatures. It was determined that as the DW velocity approaches that of transverse of longitudinal sound extra phonons, or sound soliton, are generated. The light scattering from the excited phonons was observed directly. The space and time evolution of this sound soliton was investigated at T =2 K. Nonstationary supersound DW motion has been observed. Nonlinear excitation of longitudinal sound was discovered. The temperature dependence of the DW mobility was also measured. The general picture of the DW motion at v ≈ s was discussed.

Spin-glass state of Fe/Cr multilayer structures with ultrathin iron layers

The evolution of the magnetic properties of Fe/Cr superlattices with a decrease in the nominal thickness of the iron layers down to atomic dimensions at which these layers are not continuous has been analyzed. Investigations have been carried out with multilayer samples with Fe-layer thicknesses in a range of 2–6 Å and Cr-layer thicknesses of 10 and 20 Å. It has been found that the system with various Fe-layer thicknesses and at various temperatures exhibits various magnetic phases—superparamagnetic, magnetically ordered, and nonergodic—characterized by the dependence of the magnetization of the sample on its magnetic prehistory. It has been shown that the observed nonergodic phase has the properties of a spin glass. A qualitative phase diagram of the magnetic states of the system has been obtained.

Interlayer interaction in a Fe/Cr/Fe system: Dependence on the thickness of the chrome interlayer and on temperature

A three-layer sample of Fe(100 Å)/Cr(0–20 Å)/Fe(100 Å) is used to study the dependence of inter-layer exchange on the thickness of the chrome interlayer and on temperature. The method of Kerr magnetometry in the temperature range from 77 to 473 K and the method of Brillouin scattering of light by spin waves at room temperature are used. The data for magnetization curves and spin wave spectra are treated in the model of biquadratic exchange. The range of validity of this model is established, which is apparently determined by interlayer exchange. The resultant dependence of the constant of bilinear interaction on the interlayer thickness demonstrates an oscillating behavior with two oscillation periods of about 3 and 18 Å. Within the experimental error, the magnitude of this constant, the amplitude, and the period and phase of its oscillation are independent of temperature. It is found that the constant of biquadratic exchange decreases in inverse proportion to the chrome thickness, the proportionality factor decreasing linearly as the temperature rises. In order to interpret the observed singularities in the behavior of the biquadratic exchange constant, a theoretical model is suggested which includes the nonideality of the interface and the presence of magnetic hardness in the chrome interlayer. This rigidity exceeds in magnitude the interaction on the interface between iron and chrome. The suggested model gives an adequate qualitative description of the experimental results.

Metastable photoinduced superconductivity in YBaCuO films close to metal-semiconductor transition

The paper gives a brief overview of experiments concerned with the metastable photoinduced superconductivity phenomenon that have been recently discovered. As evidenced, illuminating the YBa2Cu3O6.4 films by visible light to enhancement of metal properties and occurrence of a superconducting transition at low temperatures. Both the temperature of the onset of a superconducting transition and the superconducting phase space in the films rise with the increase of exposure dose. The phenomena discovered are due to photoexcitation of extra mobile holes in the CuO2 plane allowing a new metastable superconducting phase to be initiated (photodoping). Also discussed are microscopic mechanisms of photoinduced superconductivity.

Dependence of photoinduced superconductivity on illumination dose in YBa2Cu3O6.4 films

Abstract Photoinduced superconductivity has been investigated in YBa 2 Cu 3 O 6.4 films near to the semiconductor-metal transition. Light-induced changes in the resistance and growth of the diamagnetic moment were observed. The data revealed a dependence between the photoinduced reduction of the normal state resistance and the increase of the onset temperature of superconductivity in the films. Prolonged irradiation led to a seemingly complete loss of resistivity below 5 K. These phenomena are attributed to photogeneration of additional mobile holes in CuO 2 layers resulting in superconductivity.

Interlayer coupling in Fe/Cr/Gd multilayer structures

The effect of the chromium layer thickness on the magnetic state of an [Fe/Cr/Gd/Cr]n multilayer structure is studied. A series of Fe/Cr/Gd structures with Cr spacer thicknesses of 4–30 Å is studied by SQUID magnetometry and ferromagnetic resonance in the temperature range 4.2–300 K. The obtained experimental results are described in terms of an effective field model, which takes into account a biquadratic contribution to the interlayer coupling energy and a nonuniform magnetization distribution inside the gadolinium layer (which was detected earlier). Depending on the magnetic field and temperature, the following types of magnetic ordering are identified at various chromium layer thicknesses: ferromagnetic, antiferromagnetic, and canted ordering. A comparison of the experimental and calculated curves allowed us to determine the dependence of the bilinear (J1) and biquadratic (J2) exchange constants on chromium layer thickness tCr. Weak oscillations at a period of about 18 Å are detected in the J1(tCr) dependence in the range 8–30 Å. The interlayer coupling oscillations in the system under study are assumed to be related to the RKKY exchange interaction mechanism via the conduction electrons of Cr.

Inhomogeneous ferromagnetic resonance modes in [Fe/Cr]n superlattices with a high biquadratic exchange constant

In a set of [Fe/Cr]n superlattices, magnetization curves and spectra of ferromagnetic resonance under an in-plane magnetic field have been studied at room temperature. Along with the acoustic branch, several additional branches have been observed in resonance spectra. Resonance spectra have been calculated analytically for a structure with an infinite number of layers and numerically for finite numbers of layers in real samples using a model of biquadratic exchange taking account of the fourth-order magnetic anisotropy. A possibility of describing both static and resonance properties of the system in terms of this model has been demonstrated.