S. Koleśnik

Carrier-induced ferromagnetism in p − Zn 1 − x Mn x Te

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped

Carrier-induced ferromagnetic interactions in p-doped Zn(1−x)MnxTe epilayers

{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}

Indication of ferromagnetic ordering in p-Zn1−xMnxTe

epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance, and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY) model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.

Neutron irradiation damage effect on superconducting and normal state properties of the YBa2Cu3O7 system

Abstract p-type doping of molecular-beam-epitaxy grown layers of the diluted magnetic semiconductor Zn(1−x)MnxTe is achieved by using an active nitrogen cell. The strong interaction between the localized Mn spins and the holes deeply modifies the transport properties (metal–insulator transition, spin-dependent Hall effect). In spite of the weak localization of the carriers at low temperature, the holes clearly induce a ferromagnetic interaction between the localized spins, which is discussed as a function of Mn content and hole concentration.

Carrier induced ferromagnetic interactions and transport properties of p-Zn(1−x)MnxTe epilayers

Abstract P-type doped Zn 1− x Mn x Te has been grown by molecular beam epitaxy and by the Bridgman method. Our SQUID measurements indicate ferromagnetic ordering above 1 K, for p⩾10 19 cm −3 . It is argued that weakly localized holes are responsible for the ferromagnetic correlation in the insulating phase.

Colossal magnetoresistance and superconductivity in REAMnO/YBCO heterostructures

Abstract Effect of irradiation by fast neutrons on superconducting and normal state properties of the YBa2Cu3O7 samples is presented. Transport measurements showed a degradation of all superconducting parameters, especially the transport critical current density. Critical current densities obtained from magnetization data exhibited a substantial increase / about 15 times at 77 K and H = 10 kOe/ after the irradiation with the fluence in the range 2.4·1017 ÷ 8.7 · 1017 n/cm2. Such an anomalous bahavior is explained in terms of neutron created defects at intergrain regions and improved intragrain pinning.

Flux pinning by anisotropic arrays of antidots in superconducting thin films

The conductivity and Hall effect of p-type doped Zn(1−x)MnxTe layers are described and analyzed in terms of a two-fluid model where part of the carriers are weakly localized as bound magnetic polarons. At low temperature, the Hall effect is dominated by the spin-dependent component (extraordinary Hall effect), in good agreement with a crude estimate taking into account the properties of the valence band. The long range ferromagnetic interaction appears to be induced by the weakly localized holes.

Ferromagnetism and superconductivity in La0.7Sr0.3MnO3/YB2Cu3O7 superlattices

Abstract We report on the fabrication of RE1−xSrxMnO3/YBa2Cu3O7 ( RE = Nd , La ) multilayers grown by high pressure DC sputtering on LaAlO3 substrates. We show negative magnetoresistance in the multilayer structures above the superconducting transition temperature Tc. Magnetic moment measurements suggest a superconducting coupling through the ferromagnetic spacer as well as a ferromagnetic coupling through the superconducting spacer in this heterostructures.

Angular Dependence of Flux Pinning in High-T c Superconductor Thin Films and Superconductor-Ferromagnetic Heterostructures

Anisotropic arrays of antidots have been prepared on Y-Ba-Cu-O and Nb thin films by electron-beam lithography. Fast-Fourier-Transform analysis of the magnetization curves for Y-Ba-Cu-O thin film reveals their periodical character which corresponds to the matching effect of the vortex lattice. For niobium thin film, matching peaks can be observed on the decreasing branches of the magnetization curves. Magneto-optical investigations at lower temperatures show unidirectional flux entry and exit along the shorter antidot lattice parameter. In higher magnetic fields, an anisotropic critical state is observed. Our observations strongly suggest formation of anisotropic vortex lattices in the patterned thin films.

Magnetic properties of oxygen-deficient GdBa2Cu3Oy single crystals

Abstract We report on the fabrication of La 0.7 Sr 0.3 MnO 3 / YBa 2 Cu 3 O 7 , (LSMO/YBCO) multilayers grown by high pressure dc sputtering on LaAlO 3 substrates. X-ray diffraction analysis of the single and bilayer films confirms c-axis epitaxial growth; superlattice structure shows second order satellite peaks. Magnetic moment measurements show both superconducting and ferromagnetic order below the superconducting transitions T c 0 . Superconducting transitions for a series of multilayers with fixed YBCO thickness d YBCO = 5.8 nm and increasing thickness of LSMO layers d LSMO from 1.2 to 3.6 nm show monotonic decrease of T c 0 .