Low-dimensional hybrid structures of heterogeneous constituents usually exhibit abnormal properties, a fact that makes such hybrids attractive for various cryogenic and room-temperature applications. Here, we studied Co/(1 - x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3)/Co (Co/PMN-xPT/Co) with x = 0.29 and 0.30, specifically focusing on the evolution of the remanent ferromagnetic state, m(rem) of the Co outer layers in the whole temperature range from 300K down to 10 K, upon application of an external electric field, E-ex. We observed that m(rem) was vulnerable to degradation through the occurrence of electric field-induced magnetic instabilities (EMIs) that appeared only when E-ex not equal 0 kV/cm and were facilitated as E-ex increases. However, EMIs completely ceased below a characteristic temperature T-ces = 170 K even for the maximum vertical bar E-ex vertical bar = 5 kV/cm applied in this work. A direct comparison of the magnetization data of the Co/PMN-xPT/Co hybrids reported here with the electromechanical properties of the parent PMN-xPT crystals plausibly indicates that EMIs are motivated by the coupling of the ferromagnetic domains of the Co outer layers with the ferroelectric domains of the PMN-xPT crystal. These results highlight the drawback of EMIs in relevant hybrids and delimit the temperature regime for the reliable operation of the Co/PMN-xPT/Co ones studied here. (c) 2014 AIP Publishing LLC.

The properties of multiferroic and specifically piezoelectric (PE) materials are, nowadays, intensively investigated by means of well established, however relatively complicate methods. In this work we present a method for the direct visual demonstration of the underlying electro-mechanical processes occurring in PE materials and the estimation of the respective coefficients d(ij). The method is based on the utilization of optical microscopy for the local observation of the deformation of a PE specimen upon application of an electric field. The direct comparison of the snapshots obtained before and after application of the electric field and simple algebraic calculations enables the estimation of the d(ij) coefficients. The method was evaluated in unpoled single crystals of 0.71Pb(Mg1/3Nb2/3)O-3-0.29PbTiO(3), at room temperature. Various locations of each crystal surface were surveyed. Non-homogeneous electro-mechanical response was observed. Accordingly, the estimated d(ij) coefficients depended on the specific location of the crystal surface. Specifically, the d(zx) coefficient ranged within 500-1000 pm/V over the investigated locations (for electric fields E<1kV/mm). The present method directly unveils non-homogeneous electro-mechanical processes occurring at the surface of PE crystals and clarifies how these observations can be quantified through the respective d(ij) coefficients.

The Exchange Bias (EB) effect is observed at the interface of Antiferromagnet/Ferromagnet (AF/FM) structures and depends on the interface roughness (IR). Until today, only low IR values, usually below 10 nm, have been investigated. Here we investigate an extended range of IR through controlling the surface roughness (SR) of the employed substrates. We employ CoO/Co bilayers (thickness within 10-60 nm), a classic AF/FM structure that exhibits intense EB. ZnO was employed as the substrate in both film and bulk forms, enabling us to vary the SR up to 840 nm. Our data reveal a strong relative decrease, ranging within 20-65%, of both the shift H-shift(EB) and coercive H-c(EB) fields upon increase of SR (IR), for both parallel and normal magnetic field-sample configurations. For the explanation of these findings we propose that in thin AF/FM structures deposited on rough substrates the local magnetization, M-f of the FM is 'locked' mainly in-layer due to shape anisotropy, thus it is forced to follow the morphologically rough landscape of the substrate. This imposes misalignment between M-f, that is 'directionally random', and H-ex, that is 'directionally oriented'. This weakens the biasing potential of H-ex on M-f and reduces the relative macroscopic parameters H-shift(EB) and HcEB

In recent years the microstructure, mechanical and magnetic properties of Eurofer-97 steel are studied intensively due to its application in nuclear fusion power plants. Its microstructure is usually accessed by means of electron microscopy. Here we present an alternative approach utilizing Atomic Force Microscopy (AFM) to study as-received Eurofer-97 steel. We recorded both the Height Signal (HS) and Phase Signal (PS) that provided information on the morphologic and inelastic topography, respectively. With the HS we detected spherical particles (SPs) of size 50-2000 nm. Interestingly, micrometer SPs (0.1-2.0 mu m) are randomly distributed, while nanometer SPs (50-100 nm) are sometimes arranged in correlation to grain boundaries. The PS clearly revealed that the micrometer SPs exhibit inelastic properties. Though we cannot identify the elemental composition of the SPs with AFM, based on relevant electron microscopy data we ascribe the nanometer ones to the TaC, TiN and VN and the coarse micrometer ones to M23C6 (M=Cr, Fe). The latter class of SPs can probably be active sites that influence the mechanical properties of Eurofer-97 steel upon annealing as observed in relevant electron microscopy based studies.

Using a vector network analyzer equipped with a calibrated rectangular wave guide the electric permittivity and the element of the magnetic permeability tensor for Y3Fe5O12, ZnFe2O4 and NiFe2O4 are measured. The electric permittivity can be estimated from the body resonances (d = n lambda/2) if a sufficient long sample is used. The estimation of the magnetic permeability tensors' parameters can be estimated comparing the experimental results with computer simulations using the magnetic properties of the materials as derived from the magnetic measurements.

In ferromagnetic/superconducting (FM/SC) planar hybrids, a reentrance of the upper-critical field line H-c2(T) is observed close to T-c. The effect is generally ascribed to the so-called domain-wall superconductivity and has been theoretically explored in great detail. Experimental investigations are limited mostly to FM/SC bilayers in which the FM layers host out-of-plane magnetic domains of strong anisotropy and large width (>300 nm), the SC layer is quite thick (>30 nm), and the external magnetic field was applied normal. To expand our knowledge to until now unexplored conditions, we study a series of Co(d(Co))/Nb(d(Nb))/Co(d(Co)) trilayers under a parallel magnetic field; Co outer layers exhibit out-of-plane magnetic domains of weak anisotropy and small width (<150 nm) and the Nb interlayer is very thin (<20 nm). We demonstrate a strong reentrance of H-c2(T) that can be tuned through the irreversible magnetization processes of the FM outer layers, the two-dimensional character of the SC interlayer, and the matching between the interfering SC and FM length scales. These results refine the nature of the reentrance of H-c2(T) in FM/SC hybrids and can motivate new theoretical and experimental investigations. (C) 2014 AIP Publishing LLC.

Ferroelectric materials have attracted much interest due to their wide and important technological applications. Regarding their piezoelectric properties, these materials are evaluated by means of relatively complicate global methods. In this work a comparatively simple and efficient local method for the direct estimation of the d(ij) coefficients is presented. The method is based on conventional optical microscopy (OM) and advanced Atomic Force Microscopy (AFM) employed to image the local deformation of a specimen upon variation of a dc electric field. The feasibility and reliability of the method is demonstrated at room temperature in single crystals of (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3). Non-homogeneous electromechanical processes are detected. Accordingly, the estimated d(ij) coefficients exhibit a spatial variation over the crystal surface. Except for electro-mechanical systems, the introduced local method could find wide application for the investigation of spatially non-homogeneous properties that possibly exist in relevant magneto-mechanical and thermo-mechanical complex systems. (C) 2014 Elsevier B.V. All rights reserved.

Artificial (CoO-)Co/Nb/Co trilayers (TLs) are studied by means of magnetization, transport, and magnetic force microscopy measurements. By using these TLs as a model system, we provide definite means to distinguish the exchange-fields and stray-fields based magnetoresistance effects observed in relevant Ferromagnetic/Superconducting/Ferromagnetic (FM/SC/FM) structures. Accordingly, we isolate and exclusively focus on the stray-fields version, to report a complete supercurrent switch, Delta R/R-nor approximate to 100%, achieved at operating temperature T > 4.2 K. Detailed series of Co(60 nm)/Nb(d(Nb))/Co(60 nm) TLs are studied, in which all crucial parameters that relate to the outer Co layers and to the Nb interlayer were systematically varied to unveil the factors that motivate and/or promote the observed effect. We propose a compact model that fairly captures the underlying physics of the Co(60 nm)/Nb(d(Nb))/Co(60 nm) TLs studied here and can be generalized to assist understanding of the relevant processes in every kind of FM/SC/FM TLs. The presented results and accompanying model are both provocative for basic research and attractive for the design of cryogenic devices. (C) 2014 AIP Publishing LLC.

Nowadays, Ferromagnetic/Superconducting/Ferromagnetic trilayers (FM/SC/FM TLs) are intensively studied. Here, based on (CoO-) Co/Nb/Co TLs of thin Nb interlayer (below 30 nm) we introduce two classes of low-T-c SC-based cryogenic devices, depending on the thickness of the Co outer layers and the presence of a CoO underlayer. An extended range of Co thickness (from 10 to 80 nm) was investigated and an underlayer of CoO was selectively employed, practically aiming to control in-plane and out-of-plane magnetization processes through utilization of shape anisotropy and exchange bias. To this effect magnetic force microscopy, magnetization and magnetoresistance data are presented. Ancillary atomic force microscopy and Rutherford back scattering data are presented, as well. CoO-Co/Nb/Co TLs of the first class have thin Co outer layers (10-30 nm) and are further assisted by the presence of a CoO underlayer to behave as efficient spin valves (Delta R/R-nor = 1.5% and Delta R/R-min = 2.4%) under the action of the FMs in-plane exchange fields; the effect is termed superconducting Spin-Valve Effect (sSVE). Co/Nb/Co TLs of the second class have thick Co outer layers (50-80 nm) and without the need of a CoO underlayer act as almost absolute supercurrent switches (Delta R/R-nor = 97.7% and Delta R/R-min = 28000%) under the action of FMs out-of-plane stray fields; the effect is termed superconducting Magneto-Resistance Effect (sMRE). The properties of these (CoO-) Co/Nb/Co TLs resemble the behavior of standard FM/normal-metal/FM and FM/insulator/FM TLs that exhibit the effects giant (GMR) and tunnel (TMR) magnetoresistance, respectively. Aiming to utilize the FM/SC/FM TLs studied here into cryogenic applications we thoroughly surveyed their operational H-T phase diagram and discuss how can be used to realize binary ('0'-'1') elemental devices for information management in both read heads and memory units. The underlying physical mechanisms responsible for the sSVE and sMRE observed in the two different classes of TLs, that is CoO-Co/Nb/Co and Co/Nb/Co, respectively and the technical requisites that the SC and FM ingredients should meet are discussed.