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Exchange-coupled ferromagnetic/antiferromagnetic (FM/AF) bilayers are widely used in spin valves, magnetic tunnel junctions and spin–orbit torque devices. While most studies focused on AF layers exchange coupled with single FM layer, the exchange interaction with FM multilayer were not well understood. In this work, [Co/Ni]n/PtMn multilayers with different PtMn thickness were prepared. Polycrystalline face-centered cubic Co, Ni, and PtMn phases were evidenced by the grazing incidence x-ray diffraction measurements. At room temperature, the [Co/Ni]n/PtMn multilayers show similar soft ferromagnetism as the reference [Co/Ni]2 layers. Reducing the temperature results in broadening of the hysteresis loops, and exchange bias was not observed until 100 K. Temperature-dependent DC magnetization and AC susceptibility revealed blocking temperatures of 210–240 K and a spin freezing temperature of 140 K. This work has revealed the temperature evolution of exchange interaction in [Co/Ni]n multilayer interfacing with A1-phase PtMn. The results are important for understanding the physical process of the establishment of exchange coupling in various FM/AF interfaces.

Multifunctional, Fe-doped indium tin oxide (ITO) nanostructured films were grown by pulsed laser deposition on sapphire substrates. The films showed low resistivity and n-type conductivity with a carrier density as high as $3\times 10^{21}~{\rm cm}^{-3}$ . A magnetic moment at room temperature larger than those of any iron oxides was measured in films with Fe doping as low as 2%. We ascribe it to the formation of bound polarons. A strong green photoemission at room temperature was observed, which we ascribe to quantum confinement effect. The coexistence of ferromagnetism and luminescence in the visible range makes our films of potential for magneto-optoelectronic applications.

Extended Abstract Memristive switching in oxide semiconductors relies on the formation and disruption of conductive nano-filaments [1]. This effect is considered promising for the next generation of non-volatile memories. Yet, the switching event is a complicated electronic and ionic process, which may involve more than one mechanism. In order to elucidate the switching mechanism, advanced microscopy investigations proved challenging because of heavily dependence of the results on specimen preparation techniques. Important information can be obtained if switching leads to a macroscopic change of nonelectrical properties, for instance magnetic properties. In this respect, n-type Mn-ZnO and p-type NiO provide a unique testbeds since the magnetic properties of these oxides are strongly dependent on the distribution of oxygen vacancies [2]. We show that resistive switching in n-type ferromagnetic Mn-ZnO and p-type antiferromagnetic NiO coexists with a switching of the magnetic phase. Thin films of these oxides were sandwiched between two metallic electrodes and resistive switching was induced. Nano-devices were patterned out of the trilayers by resorting to electron beam lithography and physical etching. We found that a switching of the resistance corresponds to a switching of the magnetic phase in the film [3, 4]. By measuring the magnetic properties of the devices in the two resistive states, we can draw important conclusions on the underlying switching mechanism. For instance, in Mn-ZnO the effect is not filamentary type and occurs uniformly under the interface, whereas in NiO the effect is filamentary type. By measuring the change of magnetic properties we could exclude that switching was due to the formation of Ni-ion filaments across the device. We have demonstrated [4] that the switching is due to the formation and rapture of oxygen-vacancy filaments.

We have investigated the origin of the ferromagnetic ordering in undoped and Mn-doped ZnO thin films. A systematic comparison is presented between two sets of films grown by pulsed laser deposition from targets prepared under the same conditions. We show that ferromagnetic ordering in both compounds is directly related to the concentration of oxygen vacancies. The incorporation of Mn dramatically increases the magnetic moment in highly oxygen-deficient films. By analyzing the photoluminescence spectra and the structural properties of the films, we can exclude that magnetic ordering in our films originates from unintentional doping.

Magnetic vortex-based oscillators represent a unique opportunity to study the mutual spin-transfer torque coupling between two ferromagnetic layers in a spin-valve configuration. This will eventually lead to a complete understanding of this phenomenon and, potentially, to electronic devices of practical application. Mutual spin-transfer torque can lead to multiple-mode high-frequency dynamics in a spin-valve system. We here report our study on the emission response of an ohmic nanocontact to a spin-valve multilayer subject to perpendicularly applied magnetic field. In this configuration, three modes are accessible, two of which correspond to the precessional motion of a vortex in one of the two ferromagnetic layers with the other working as a static polarizer. At high currents, the third mode can be observed that is ascribed to the simultaneous precession of two vortices, one in each layer, with the other layer working as a dynamic polarizer. The simultaneous application of a field in the sample plane tends to suppress the mode that corresponds to the precessional motion of the vortex in the thinner ferromagnetic layer.

The studies of the effects of magnetic field on surface enhanced Raman scattering (SERS) have been so far limited to the case of ferromagnetic/noble-metal, core/shell nano-particles, where the influence was always found to be negative. In this work, we investigate the influence of magnetic field on a diluted magnetic semiconductor/metal SERS system. Guided by three dimensional finite-difference time-domain simulations, a high efficient SERS substrate was obtained by diluting Mn into Au-capped ZnO, which results in an increase of the dielectric constant and, therefore, an enhancement of Raman signals. More remarkably, an increase of intensities as well as a reduction of the relative standard deviation (RSD) of Raman signals have been observed as a function of the external magnetic strength. We ascribe these positive influences to magnetic-field induced nucleation of bound magnetic polarons in the Mn doped ZnO. The combination of diluted magnetic semiconductors and SERS may open a new avenue for future magneto-optical applications.

We investigated the role of strain on the magnetic properties of La0.7Sr0.3MnO3 epitaxial films grown on substrates with different crystallographic orientation. The magnetic characterization confirms the large magnetostriction due to the tensile stress applied by the SrTiO3 substrates. The data demonstrate that the easy direction of magnetization is aligned to the elongated, in-plane axis both in (110)- and (001)-oriented films. The in-plane anisotropy of (110) films is interpreted in the frame of a simple structural model. We provide evidence of a spatial separation between a conducting ferromagnetic and an insulating paramagnetic phase at the Curie temperature. The possible effect of such phase separation on the coercitive field and on the magnetization versus temperature is discussed. (c) 2006 Elsevier B.V. All rights reserved.

The critical Josephson current (IC) in superconducting/ferromagnetic (S/F) multilayer-based junctions can be controlled by changing the relative directions of the magnetization in the F-layers. Recent experimental works [1, 2] show that an enhancement of IC is achieved in S/F weak links when the alternating F-layers are antiparallel aligned. We present preliminary experimental results concerning the dependence of IC on the relative orientation of the ferromagnetic layers in S/F1/I/F2/S tunnel junctions where the F-layers are obtained by changing the relative composition of NiCu alloys. The multilayers were grown by electron beam deposition, and processed by Focused Ion Beam lithography. The magnetic state of the devices was directly determined by measuring the current perpendicular to plane (CPP) magnetoresistance (MR) at high bias. IC was found to be larger when the F-layers are antiparallel aligned. The maximum change of IC corresponds to the maximum change of MR. The application of a magnetic field induces a transition in the shape of the currentvoltage curve that seems to suggest Coulomb blockade effect.

The dependence of the critical Josephson current (I-C) and magnetoresistance (MR) on the relative orientation of the ferromagnetic layers is investigated in current perpendicular to plane (CPP) measurements of superconductor/ferromagnetic (S/F) multilayers. Two different CuxNi1-x alloys were chosen so that alternate F-layers reversed at different applied magnetic fields and that the ferromagnetic coherence length was of the order of several nanometres. These structures show a significant CPP high-bias MR, which enables the magnetic state of the devices to be determined directly. We found that I-C is substantially larger when the adjacent F-layers are aligned antiparallel compared to the case of parallel orientation. Since in a pseudo-spin-valve structure the relative F-layer orientation is approximately parallel for zero applied magnetic field, I-C can experience an enhancement when a magnetic field is applied in the plane of the junction. The maximum change of I-C corresponds to the maximum change of MR.

Perpendicular magnetic anisotropy (PMA) was studied at low temperature (T=30 K) in dc-magnetron sputtered Ni60Cu40/Cu multilayers. PMA has been observed in many multilayer structures for ferromagnetic layer thicknesses less than a certain thickness t⊥. In general cases t⊥ is less than a few nanometers, making such structures unsuitable for low-cost fabrication techniques. Our results show a strong perpendicular easy direction of magnetization for NiCu layer thickness between 4.2 nm and 34 nm. The thickness t⊥ at which the multilayers change the preferential orientation from perpendicular to in-plane is estimated to be 55 nm. Structural studies show that the low magnetostatic energy density is likely to be the main reason for the large t⊥ value obtained in this system.

We present novel low-field magnetoresistive devices based on the ferromagnetic manganite La0.7Sr0.3MnO3 operating in the current in plane (CIP) configuration. In these planar spin-valve devices, a focused Ga+ beam is used to create pinning centers for magnetic domain walls. The spin-dependent scattering of polarized electrons at the domain walls (DW) is responsible for the magnetoresistance observed in the patterned tracks. The magneto-transport properties of these devices are interpreted within a model for DW magnetoresistance. Applications such as magnetic data storage can be envisaged for the structures we investigated. (C) 2006 Elsevier B.V. All rights reserved.

The magnetic properties of hematite (?-Fe2O3) nanoparticles synthesized by in situ thermal decomposition of an iron-mercaptide precursor in polymeric matrices are studied. In Polystyrene, nanoparticles with average size ~20 nm can be produced that show ferromagnetic behavior with no Morin transition for temperatures as low as 10 K. Poly(9-vinylcarbazole)-based nanocomposites, produced by the same synthesis process, show a different magnetic behavior with a clear Morin transition. The effect of the polymeric matrix on the structural and magnetic properties of dispersed hematite nanoparticles is discussed. It is concluded that, in heavily viscous medium, the ability to control the particle growth and distribution is compromised.

We report our study on the origin of ferromagnetism in Ti-doped ZnO. A series of Ti doped ZnO films with increasing concentration of Ti dopant were grown and characterized in terms of structural, electrical, and magnetic properties. We found that Ti has a low solubility in the ZnO wurtzite structure. This favors stabilization of a large number of Zn vacancies, and theoretical calculations have shown that they can carry substantial magnetic moment. A carrier mediated exchange interaction between Zn vacancies is at the origin of the surprisingly high magnetic moment we measure in this compound.

The magnetic and magnetotransport properties of La0.7Sr0.3MnO3/Permalloy heterostructures are investigated. La0.7Sr0.3MnO3 films were grown on (110) SrTiO3 in order to induce an in-plane anisotropy with the easy axis along the [001] direction. An easy axis of magnetization was induced in the Permalloy in the same direction through the application of a magnetic field during the growth. The magnetic characterization of the bilayer is presented and discussed. The transport properties of devices operated in the current perpendicular to plane mode are shown. The possibility of fabricating magnetoresistive devices by simply depositing a ferromagnetic counterelectrode on a manganite film is demonstrated. (c) 2006 American Institute of Physics.

We present preliminary experimental results concerning both single and stacked tunnel junctions in which one of the electrodes was formed by a superconductor/ferromagnet (S/F) bi-layer. In particular, in the stacked configuration a Nb/NiCu bi-layer was used as the intermediate electrode, and it was probed by tunneling on both sides. Tunnel junctions have been characterized in terms of current-voltage characteristics (IVC), and differential conductance. Preliminary steady-state injection-detection measurements performed in the stacked devices at T 4: 2 K are also presented and discussed. (C) 2003 Elsevier B.V. All rights reserved.

Eight percent of Mn was successfully diluted into nonpolar ZnO films deposited by pulsed laser deposition on single crystal (100) SrTiO3 substrates. X-ray diffraction patterns and energy-dispersive X-ray spectroscopy confirmed high crystallinity of the films and excluded unintentional magnetic doping. A unique surface domain structure was observed by scanning electron microscope and atomic force microscope, which might play a vital role to strain release induced by lattice mismatch between nonpolar (11–20) ZnO film and (100) SrTiO3 substrate. In addition, the films showed strong ferromagnetism with a large coercivity HC ∼ 180 Oe at room temperature. The large magnetic moment is ascribed to carrier-mediated exchange interaction between the Mn ions, where the majority of the carriers are oxygen vacancies.

We show that the magnetic properties of a dilute semiconductor oxide can be altered in a reversible and non-volatile manner by the application of an electric field. The selected ferromagnetic oxide was manganese-substituted zinc oxide. Bipolar resistive memory switching was induced in the film sandwiched between two metallic electrodes. The bistable switching of the resistive state was accompanied by a bistable switching of the magnetic moment. The scalability of the system was investigated by fabricating devices with lateral size down to 400 nm.

We induced bipolar resistive switching in p-type nickel oxide. By probing the magnetic properties of the films, we proved that bipolar resistive switching in this antiferromagnetic oxide was due to the formation and rupture of oxygen-vacancy filaments, rather than electrochemical growth and dissolution of nickel-ion filaments. In the low resistive state, oxygen-mediated super-exchange interaction was suppressed along the conductive paths. This led to a reduction of the saturation moment but not the appearance of a ferromagnetic phase, excluding the formation of nickel filaments.

We report on the observation of a non-volatile, reversible switching of the magnetic moment in Mn-doped ZnO thin films. The system is a typical oxide memristor based on an oxygen-deficient semiconductor oxide. In the present study, the oxide semiconductor is ferromagnetic at room temperature. We found that the bistable switching of the resistive state was accompanied by a bistable switching of the magnetic moment at room temperature. Our results support the hypothesis that ferromagnetism in Mn-doped ZnO is mediated by oxygen-vacancies.

We report our study on the emission response of a magnetic nanocontact with dynamic polarizer in perpendicular magnetic field. In this configuration three modes are accessible, two of which correspond to the precessional motion of a vortex in one of the two ferromagnetic layers with the other working as a static polarizer. At high currents a third mode can be observed that is ascribed to the simultaneous precession of two vortices, one in each layer, with the other layer working as a dynamic polarizer.

We fabricated pseudo-spin-valves by using La0.7Sr0.3MnO3 and Co33Fe67 as ferromagnetic electrodes. A natural interface layer present between metal and manganite layers eliminated the need of depositing any nonmagnetic spacers. The magnetic layers were decoupled from each other, and the structure exhibited a positive magnetoresistive behavior. Direct comparison between magnetic and transport measurements concluded the occurrence of giant magnetoresistive effect in such a spacerless metal-oxide pseudo-spin-valve structure. The results have implications for a simple route to fabricate oxide-based spintronic devices.

In this work arrays of Co∕Cu multilayer nanowires were fabricated by electrodeposition to develop giant magnetoresistive sensors. Structure and morphology defects within 6μm long portions of single nanowires have been probed by electron and ion-beam microscopies. It has been discovered that the most recurrent growth defect is a tilt of the nanowire layers, which varies along the length of the nanowires, this effect being more significant for nanowires of larger diameters; nevertheless, the nanowire arrays fabricated in this work demonstrated a magnetoresistive response which is very close to the behavior of analogous systems previously reported in the literature.