M. Ziese

Extrinsic magnetotransport phenomena in ferromagnetic oxides

Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

Room-temperature magneto-optics of ferromagnetic transition-metal-doped ZnO thin films

Magneto-optic studies of ZnO doped with transition metals Co, Mn, V, and Ti indicate a significant magnetic circular dichroism (MCD) at the ZnO band edge at room temperature, together with an associated dispersive Faraday rotation. Similar spectra occur for each dopant, which implies that the ferromagnetism is an intrinsic property of the bulk ZnO lattice. At 10 K, additional paramagnetic contributions to the MCD are observed, but above about 150 K, the magnitude of the MCD signal is dominated by the ferromagnetism and is almost temperature independent. The MCD at the ZnO band edge shows room temperature hysteretic behavior.

Critical scaling and percolation in manganite films

In this work the critical behaviour of films of La0.7Ba0.3MnO3, La0.7Sr0.3MnO3, La0.7Ca0.3MnO3 and (LaMn)1-δO3 is studied. Apart from the La0.7Ca0.3MnO3 film, each of these manganite films shows a continuous phase transition with mean-field critical exponents. The analysis of the critical amplitudes indicates the formation of spin polarons. The zero-field resistivity of the La0.7Ca0.3MnO3 film is consistent with a model of percolative phase separation.

Thickness dependent magnetic and magnetotransport properties of strain-relaxed La0.7Ca0.3MnO3 films

La0.7Ca0.3MnO3 films with thicknesses between 2 and 300 nm were fabricated on LaAlO3, SrTiO3, and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates using pulsed laser deposition. After annealing at 950 °C in flowing oxygen, on LaAlO3 and LSAT, strain-relaxed epitaxial films of high quality were obtained. The magnetization, resistivity, and magnetoresistance of the films was studied as a function of thickness. Down to a thickness of about 4 nm no decrease of the saturation magnetization could be detected; the Curie temperature decreases slightly with thickness in agreement with finite size scaling theory. The thickness dependence of the conductance can be understood as a size-effect in thin films; there is no indication of an electrically dead interfacial layer.

Microstructure and Properties of Well-Ordered Multiferroic Pb(Zr,Ti)O3/CoFe2O4 Nanocomposites

A nanofabrication technique combining pulsed laser deposition and a nanoporous anodic aluminum oxide membrane mask is being proposed to prepare various types of multiferroic nanocomposites, viz. periodically ordered CoFe(2)O(4) dots covered by a continuous Pb(Zr,Ti)O(3) layer, Pb(Zr,Ti)O(3) dots covered with CoFe(2)O(4), and Pb(Zr,Ti)O(3)/CoFe(2)O(4) bilayer heterostructure dots. By properly tuning the processing parameters, epitaxial nanodot-matrix composites can be obtained. For the composite consisting of CoFe(2)O(4) nanostructures covered by a Pb(Zr,Ti)O(3) film, an unexpected out-of-plane magnetic easy axis induced by the top Pb(Zr,Ti)O(3) layer and a uniform microdomain structure can be observed. The nanocomposites tested by piezoresponse force microscopy (PFM) exhibit strong piezoelectric signals, and they also display magnetoelectric coupling revealed by magnetic-field dependent capacitance measurement.

Inverted hysteresis and giant exchange bias in La0.7Sr0.3MnO3/SrRuO3 superlattices

The magnetization reversal mechanisms in a La0.7Sr0.3MnO3/SrRuO3 superlattice with ultrathin individual layers were studied. Due to the strong exchange bias between La0.7Sr0.3MnO3 and SrRuO3 layers inverted hysteresis loops were observed at temperatures below 62 K; at higher temperatures the superlattice showed an unconventional reversal mechanism with the magnetically hard SrRuO3 layers switching first on reducing the magnetic field from saturation. These observations were corroborated by micromagnetic simulations and were interpreted as arising from interfacial Bloch walls.

Multiferroic BaTiO3–BiFeO3 composite thin films and multilayers: strain engineering and magnetoelectric coupling

BiFeO3 and BaTiO3 were used to grow homogeneous composite thin films and multilayer heterostructures with 15 double layers by pulsed laser deposition. The perpendicular strain of the films was tuned by employing different substrate materials, i.e. SrTiO3(0 0 1), MgO(0 0 1) and MgAl2O4(0 0 1). Multiferroic properties have been measured in a temperature range from room temperature down to 2 K. The composite films show a high ferroelectric saturation polarization of more than 70 µ Cc m −2 . The multilayers show the highest magnetization of 2.3 emu cm −3 , due to interface magnetic moments and exchange coupling of the included weak ferromagnetic phases. The magnetoelectric coupling of the BaTiO3–BiFeO3 films was investigated by two methods. While the ferroelectric hysteresis loops in magnetic fields up to 8 T show only minor changes, a direct longitudinal AC method yields a magnetoelectric coefficient αME = ∂E/∂H of 20.75 V cm −1 Oe −1 with a low µ0HDC of 0.25 T for the 67% BaTiO3–33% BiFeO3 composite film at 300 K. This value is close to the highest reported in the literature.

Effect of rare-earth ion doping on the multiferroic properties of BiFeO3 thin films grown epitaxially on SrTiO3(1 0 0)

High-quality epitaxial Bi1?xRExFeO3 (RE=La, Nd, Gd; x?=?0, 0.05, 0.15) thin films were prepared on SrTiO3(1?0?0) substrates using pulsed laser deposition. X-ray diffraction and RBS-channelling spectroscopy showed that the films are single-phase perovskite, free of additional phases and textured with preferential orientation along the [1?0?0] direction. The dependences of magnetization on temperature and field showed that the films exhibit weak ferromagnetic properties. Among the studied rare-earth doping ions, Bi3+ substitution by Gd3+ most considerably enhanced the ferromagnetic properties. Substitution by La3+ smoothened out the surface morphology, which is important for different potential applications. Both undoped and doped films showed clear ferroelectric response in piezoresponse force microscopy, thus confirming their multiferroic nature. The doping was found to promote a preferential ferroelectric poling of the domains.

Interfacial strain effects in epitaxial multiferroic heterostructures of PbZrxTi1−xO3∕La0.7Sr0.3MnO3 grown by pulsed-laser deposition

Ferroelectric PbZrxTi1−xO3 and ferromagnetic La0.7Sr0.3MnO3 films were grown on SrTiO3(100) substrates in order to fabricate multiferroic epitaxial heterostructures. Multilayers of PbZr0.2Ti0.8O3∕La0.7Sr0.3MnO3 with 5nm thin individual layers preserve good magnetic properties and have a transition temperature of ∼320K. The variation of the magnetic coercive field of thin La0.7Sr0.3MnO3 films, sandwiched between PbZrxTi1−xO3 films of increasing Zr content in the same epitaxial heterostructure, demonstrates the influence of the interfacial biaxial strain.

Indications for intrinsic superconductivity in highly oriented pyrolytic graphite

High-resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest nonhomogenous superconductivity with critical temperature