Glasses with mol% composition 16 Na2O∙10 CaO∙54 SiO2∙20 Fe2O3 were prepared by the melt-quenching technique. Heat treatment at 580 °C for 1 and 3 h resulted in the crystallization solely of magnetite and additionally, of hematite for longer times. Transmission electron microscopy shows growth mainly of cubic magnetite with crystallite sizes ≤ 16 nm. X-ray photoelectron spectroscopy reveals the presence of Fe3+ and Fe2+ ions in both the glass and the glass-ceramics. Mössbauer spectroscopy of the glass detects octahedrally and tetrahedrally coordinated Fe3+, while Fe2+ solely occurs in octahedral coordination. In the glass-ceramics, magnetite was detected as well as Fe3+ in both tetrahedral and octahedral coordination. Magnetization curves of the heat-treated samples recorded at room temperature and below show ferrimagnetic behaviour. From the thermomagnetic curves recorded in the temperature range from 300 to 900 K, the Curie and superparamagnetic blocking temperatures are determined. Thermomagnetic measurements in the range from 5 to 300 K reveal the nanoparticle size dependence of the observed Verwey transition.

We demonstrate a geometrical effect on the depinning line (DL) of the flux line lattice of the Bi2Sr2CaCu2O8+δ high-Tc superconductor (HTSC) micrometer ring. The DL shifts to notably lower temperatures in comparison with bulk crystals and thin flakes of the same sample. The shift is attributed to a decrease in the overall pinning potential due to a double size effect, namely (a) the ring thickness $\sim 1~\mu $m being smaller than the pinning correlation length and (b) the increase in the effective London penetration depth of the vortices (Pearl vortices). The large shift of the DL to lower temperatures may influence the suitability of this HTSC for applications in microstrip antennas and THz emitters.

The electrical, in-plane resistance as a function of temperature $R(T)$ of bulk and mesoscopic thin graphite flakes obtained from the same batch was investigated. Samples thicker than $\sim 30$ nm show metalliclike contribution in a temperature range that increases with the sample thickness, whereas a semiconductinglike behavior was observed for thinner samples. The temperature dependence of the in-plane resistance of all measured samples and several others from literature can be very well explained between 2 K and 1100 K assuming three contributions in parallel: a metalliclike conducting path at the interfaces between crystalline regions, composed of two semiconducting phases, i.e. Bernal and rhombohedral stacking. From the fits of $R(T)$ we obtain a semiconducting energy gap of $110 \pm 20$meV for the rhombohedral and $38\pm 8 $meV for the Bernal phase. The presence of these crystalline phases was confirmed by x-ray diffraction measurements. We review similar experimental data from literature of the last 33 years and two more theoretical models used to fit $R(T)$.

Control of multi-martensite phase transformations and physical properties constitute greatly unresolved challenges in Fe7Pd3-based ferromagnetic shape memory alloys. Single crystalline Fe7Pd3 thin films reveal an austenite to martensite phase transformation, continuously ranging from the face-centered cubic (fcc) to the face-centered tetragonal (fct) and body-centered cubic (bcc) phases upon irradiation with 1.8 MeV Kr+ ions. Within the present contribution, we explore this scenario within a comprehensive experimental study: employing atomic force microscopy (AFM) and high resolution transmission electron microscopy (HR-TEM), we first clarify the crystallography of the ion-irradiation- induced austenite double right arrow martensite and inter-martensite transitions, explore the multivariant martensite structures with c-a twinning and unravel a very gradual transition between variants at twin boundaries. Accompanying magnetic properties, addressed locally and globally, are characterized by an increasing saturation magnetization from fcc to bcc, while coercivity and remanence are demonstrated to be governed by magnetocrystalline anisotropy and ion-irradiation-induced defect density, respectively. Based on reversibility of ion-irradiation-induced materials changes due to annealing treatment and a conversion electron Mossbauer spectroscopy (CEMS) study to address changes in order, a quantitative defect-based physical picture of ion-irradiation-induced austenite double left right arrow martensite transformation in Fe7Pd3 is developed. The presented concepts thus pave the way for ion-irradiation-assisted optimization strategies for tailored functional alloys.

The effect of an external magnetic field B on the martensite phase transformation temperature To was studied for single crystalline Fe72Pd28 thin films. From in-plane and out-of-plane aligned magnetic field dependent resistance measurements under various fields up to 9 T, T-0 was determined. The relation between To and B is explained quantitatively by the Clausius-Clapeyron equation. The calculated value of 0.62 K/T is close to the values of the linear fits of 0.79 K/T and 0.76 K/T obtained from measurements with B aligned in-plane and out-of-plane. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In this study, the impurity concentration and magnetic response of nine highly oriented pyrolytic graphite (HOPG) samples with different grades and from different providers were determined using ion beam microscopy and SQUID magnetometry. Apart from sideface contaminations in the as-received state, bulk contamination of the samples in most cases consists of disk-shaped micron-sized particles made of Ti and V with an additional Fe contamination around the grain perimeter. The saturation magnetization typically increases with Fe concentration, however, there is no simple correlation between Fe content and magnetic moment. The saturation magnetization of one, respectively six, out of nine samples clearly exceeds the maximum contribution from pure Fe or Fe3C. For most samples the temperature dependence of the remanence decreases linearly with T - a dependence found previously for defect-induced magnetism (DIM) in HOPG. We conclude that apart from magnetic impurities, additional contribution to the ferromagnetic magnetization exists in pristine HOPG in agreement with previous studies. A comparative study between the results of ion beam microscopy and the commonly used EDX analysis shows clearly that EDX is not a reliable method for quantitative trace elemental analysis in graphite, clarifying weaknesses and discrepancies in the element concentrations given in the recent literature.

SrRuO3 films and SrRuO3/SrTiO3 superlattices grown on SrTiO3(001) were studied by structural, magnetic, magnetoresistance and Hall effect measurements. The superlattices showed heteroepitaxial growth with coherent interfaces and a Ru/Ti diffusion region of 1–1.5 unit cells. The resistivity had metallic character above a critical thickness of 3–4 unit cells, becoming insulating below. There was no hint of conduction processes along the interfaces. Both magnetization and magnetoresistance measurements showed an increase of the magnetic anisotropy, consistent with magnetostriction effects. The magnetostriction coefficient was estimated as λ100 ∼ 1.4 × 10−4. Three unit cell thick SrRuO3 layers in SrRuO3/SrTiO3 superlattices were found to have tetragonal crystal symmetry, as deduced from the sign change of the anomalous Hall constant.

We have studied the magnetic response of two bulk highly oriented pyrolytic graphite (HOPG) samples with different internal microstructure. For the sample with well defined interfaces, parallel to the graphene layers, the temperature and magnetic field hysteresis are similar to those found recently in water-treated graphite powders. The observed behavior suggests the existence of granular superconductivity above room temperature in agreement with previous reports in other graphite samples. The granular superconductivity behavior is observed only for fields normal to the embedded interfaces, whereas no relevant hysteresis in temperature or field is observed for fields applied parallel to them. Increasing the temperature above ∼400 K changes irreversibly the hysteretic response of the sample.

Ferromagnetic ordering is found for both undoped and Mn-doped zirconia (ZrO2 : Mn) thin films with 0 at% <= Mn <= 50 at% grown homoepitaxially on ZrO2 : Y2O3(0 0 1) substrates. Highly crystalline films show ferromagnetic saturation magnetization and coercive field at room temperature up to 1 emu cm(-3) and 50 mT, respectively. The Curie temperature is in all ferromagnetic samples above 300 K. Comparing optimum films with different Mn content, cubic ZrO2 : Mn films with about 25 at% Mn show reproducibly the highest magnetization, in relation to monoclinic and tetragonal films. In contrast, less crystalline films grown heteroepitaxially on LaAlO3(0 0 1) or under non-ideal conditions show negligible magnetic effects.The fraction of paramagnetically active Mn atoms in a ZrO2 film with 27% Mn at 5 K is only about 1/5 of the incorporated Mn atoms, corresponding well to the share of 20% Mn4+ in XPS. Magnetic trace impurities in the 100 ppm range cannot account for the observed effects. Our results indicate that the observed defect-induced magnetic ordering in nominally non-magnetic zirconia thin films requires a certain balance of overall crystallinity, dislocation density and film mosaicity.

HoMnO3 films were grown on pure and Nb-doped SrTiO3 (001) substrates by pulsed laser deposition. The films grew epitaxially with the c-axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn3O4 precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c-axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.

Granular superconductivity in powders of small graphite grains (several tens of micrometers) is demonstrated after treatment with pure water. The temperature, magnetic field and time dependence of the magnetic moment of the treated graphite powder provides evidence for the existence of superconducting vortices with some similarities to high-temperature granular superconducting oxides but even at temperatures above 300 K. Room temperature superconductivity in doped graphite or at its interfaces appears to be possible.

Epitaxial orthorhombic HoMnO3 films were grown on Nb-doped SrTiO3 (001) single-crystal substrates. X-ray diffractometry showed a uniform crystallographic orientation with the c axis along the substrate normal and an anisotropic compressive stress along the b axis of the Pbnm structure. The magnetization of the films was dominated by the paramagnetism of the Ho3+ ions; the latter showed a strong anisotropy with respect to the in-plane and perpendicular-to-plane magnetic field direction. The rotational anisotropy of the magnetoelectric effect was measured for magnetic field rotation in the (110)(o), (1 (1) over bar0)(o), and (001) o planes. Whereas magnetic field rotation in the (110)(o) and (1 (1) over bar0)(o) planes showed a twofold pattern with the smallest magnetoelectric effect observed in magnetic fields along the c axis, in-plane (001)(o) magnetic field rotations revealed an intricate rotational symmetry. A magnetic-field-induced crossover was observed from a low-field region with fourfold rotation patterns to a high-field region with rotation patterns up to the 12th order. This complex rotational symmetry arises from spin-orbit coupling of the Ho3+ moments that induces a modulation of the magnetoconductance as well as a magnetoelectric effect through the Maxwell-Wagner mechanism.

Low-energy muon spin rotation and SQUID magnetization measurements were performed on proton-irradiated and non-irradiated highly oriented pyrolytic graphite samples. The samples were found to be ferromagnetic above and below room temperature and to include a substantial temperature-dependent surface contribution. Assuming uniformity, the thickness of the magnetic surface layer was estimated to be 13(2) nm. The discovered surface magnetism is intrinsic and not due to irradiation.

We have measured the magnetization of bulk samples of highly oriented pyrolytic graphite (HOPG) at magnetic fields applied parallel and perpendicular to the graphene layers. Within experimental error the intrinsic ferromagnetic signals of the samples show similar magnetic moments at saturation for the two magnetic field directions, in contrast to recently published data (J. Červenka et al., Nat. Phys. 5 (2009) 840). To check that the SQUID device provides correctly the small ferromagnetic signals obtained after subtracting the 100 times larger diamagnetic background, we have prepared a sample with a superconducting Pb-film deposited on one of the HOPG surfaces. We show that the field dependence of the measured magnetic moment and after the background subtraction is highly reliable even in the sub-μ emu range providing the real magnetic properties of the embedded small ferromagnetic and superconducting signals.

We discuss recently obtained data using different experimental methods including magnetoresistance measurements that indicate the existence of metal-free high-temperature magnetic order in graphite. Intrinsic as well as extrinsic difficulties to trigger magnetic order by irradiation of graphite are discussed in view of recently published theoretical work.

The magnetic properties of MgO, MgAl2O4, SrTiO3, LaAlO3, LSAT, and ZnO single crystals were investigated. These crystals show three contributions to the magnetization, namely, an intrinsic diamagnetic contribution, a paramagnetic contribution, due to various transition-metal impurities, as well as a ferromagnetic contribution. The latter shows coercive field values that are rather independent of the actual crystal material. The ferromagnetic hysteresis loops of the magnetization per volume suggest a surface contribution. The origin of the ferromagnetic contribution as arising from either defect-induced ferromagnetism or ferromagnetic impurities is discussed.

We present an x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon pi-states, hydrogen-mediated electronic states also exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top approximate to 10 nm of the irradiated sample where the average magnetization reaches similar or equal to 15 emu g(-1) at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin to that found in proton-bombarded graphite. Our findings also show that the magnetic properties of graphite surfaces, thin films or two-dimensional graphene samples can be reliably studied using soft x-ray dichroism. Fundamental new insights into the magnetic properties of carbon-based systems can thus be obtained.

We have investigated the magnetic properties of pure ZnO thin films grown under N-2 pressure on a-, c-, and r-plane Al2O3 substrates by pulsed-laser deposition. The substrate temperature and the N-2 pressure were varied from room temperature to 570 degrees C and from 0.007 to 1.0 mbar, respectively. The magnetic properties of bare substrates and ZnO films were investigated by SQUID magnetometry. ZnO films grown on c- and a-plane Al2O3 substrates did not show significant ferromagnetism. However, ZnO films grown on r-plane Al2O3 showed reproducible ferromagnetism at 300 K when grown at 300-400 degrees C and 0.1-1.0 mbar N-2 pressure. Positron annihilation spectroscopy measurements as well as density-functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.

The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated. The structural transition of the SrTiO3 substrate induces a magnetic transition in the manganite films due to magnetoelastic coupling. Below the temperature of the structural transition additional steps in the magnetization reversal characteristics appear characterized by clearly defined coercive fields. These additional coercive fields depend on the cooling history of the sample and are related to the formation of structural domains in the La0.7Sr0.3MnO3 films induced by the substrate.

In this work we have investigated the changes of the magnetic properties of highly oriented pyrolytic graphite samples after irradiation either with ~3×1014 protons or 3.5×1013 ... 3.5×1014 iron ions with energies in the MeV range. Our results show that iron and proton irradiations can produce similar paramagnetic contributions depending on the implantation temperature. However, only protons induce a ferromagnetic effect.