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In this work, we demonstrate that cutting diamond crystals with a laser (532~nm wavelength, 0.5~mJ energy, 200~ns pulse duration at 15~kHz) produces a $\lesssim 20~$nm thick surface layer with magnetic order at room temperature. We have measured the magnetic moment of five natural and six CVD diamond crystals of different size, nitrogen content and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300~K is observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals are much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface., 8 pages with 8 figures

We have studied the magnetization of a recently synthesized Cu9S5 compound and found two phase transitions around room temperature. The phase transitions in the crystalline structure, chara...

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of $\lesssim 80~$ppm, the so-called C- or P1-centres, were irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below $\sim 30$~K, after a decrease of $\lesssim 40~$ppm in the concentration of C-centres produced by the electron irradiation. This result indicates that magnetic C-centres are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C-centres, stressing the extraordinary role of the C-centres in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered., 15 pages, 10 figures

In the last 43 years several hints were reported suggesting the existence of granular superconductivity above room temperature in different graphite-based systems. In this paper, some of the results are reviewed, giving special attention to those obtained in water and n-heptane treated graphite powders, commercial and natural bulk graphite samples with different characteristics as well as transmission electron microscope lamellae. The overall results indicate that superconducting regions exist and are localized at certain internal interfaces of the graphite structure. The existence of the rhombohedral graphite phase in all samples with superconducting-like properties suggests its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical calculations predict. High precision electrical resistance and magnetization measurements were used to identify a transition at $$T_\mathrm{c} \gtrsim 350~$$ K. To check for the existence of true zero resistance paths in the samples we used local magnetic measurements, which results support the existence of superconducting regions at such high temperatures.

The effect of an external magnetic field B on the martensite phase transformation temperature T 0 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 T 0 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.

Fe layers with thicknesses between 5 and 100 nm were sputtered on mesoporous nanostructured anatase TiO2templates. The morphology of these hybrid films was probed with grazing-incidence small-angle X-ray scattering and X-ray reflectivity, complemented with magnetic measurements. Three different stages of growth were found, which are characterized by different correlation lengths for each stage. The magnetic behavior correlates with the different growth regimes. At very small thicknesses the TiO2template is coated and a porous Fe film results, with in-plane and out-of-plane magnetization components. With increasing thickness, agglomeration of Fe occurs and the magnetization gradually turns mostly in plane. At large thicknesses, the iron grows independently of the template and the magnetization is predominantly in plane with a bulk-like characteristic.

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 $R(T)$., 11 pages, 10 figures

Trying to dope graphite flakes we found that the magnetization of pure, several tens of micrometers grain size graphite powder and after a simple treatment with pure water shows clear and reproducible granular superconducting behavior with a critical temperature above 300K. The observed magnetic characteristics as a function of temperature, magnetic field and time, provide evidence for weakly coupled grains through Josephson interaction, revealing the existence of superconducting vortices., Comment: 19 pages, 5 figures; http://onlinelibrary.wiley.com/doi/10.1002/adma.201202219/abstract (2012)

Substrate-attached and freestanding single crystalline Fe70Pd30 ferromagnetic shape memory alloy membranes, which were synthesized by molecular beam epitaxy on MgO (001) and later released from their substrates, are characterized with respect to their structural, thermal and magnetic properties. Residing in the two-phase region of austenite and the correct martensite phase with face centered tetragonal (fct) structure at room temperature, they reveal martensite transition with little hysteresis at 326 K and 320 K, respectively. Comparing substrate-attached with freestanding films, which show fundamentally different magnetic fingerprints, it is proposed that domain structure is capable of posing a bias on the austenite fct-martensite phase transition by favoring martensite variants with their easy axis aligned along the field – just as the substrate constitutes a mechanical constraint on the transition. If confirmed, this would suggest thermo-magnetic actuation as an alternative where only moderate magnetic fields are feasible, but moderate temperature changes are possible.

HoMnO 3 films were grown on pure and Nb-doped SrTiO 3 (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 Mn 3 O 4 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.

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. Cervenka 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.

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.

Zn 90 Mn 7.5 Cu 2.5 O 100 films have been prepared on a-plane sapphire substrates by pulsed laser deposition. Paramagnetism mainly caused by Mn 2+ ions was observed in the films from room temperature down to 2 K. Magnetotransport properties (magnetoresistance (MR) and Hall effect) were studied from 5 K to 290 K up to a field of 6 T. Negative MR was observed at temperature above 100 K. Low field positive MR and high field negative MR was observed at 5 K. Clear anomalous Hall effect with a kink at low field was observed below 20 K, indicating that there exist two different scattering mechanisms for the Mn 2+ and Cu 2+ ions.

In this work we have investigated the changes of the magnetic properties of highly oriented pyrolytic graphite samples after irradiation either with $\sim 3 \times 10^{14}$ protons or $ 3.5 \times 10^{13} ... 3.5 \times 10^{14}$ 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., Comment: 4 pages with three figures. To be published in EPJ B

We have studied the field and temperature dependence of the magnetic moment of single crystalline sapphire substrates with different surface orientations. All the substrates show a ferromagnetic behavior that partially changes after surface cleaning. The amount of magnetic impurities in the substrates was determined by particle induced X-ray emission. The overall analysis of the data indicates that the magnetic impurities very likely contribute to the measured ferromagnetic behavior but does not rule out completely intrinsic contributions. Our work stresses the necessity to use other than bulk characterization methods for the study of weak ferromagnetic signals of thin films grown on oxide substrates.

Ferromagnetic, semi-insulating Mn-alloyed ZnO films with a Curie temperature above 375 K have been grown by pulsed laser deposition on c-plane sapphire substrates. Antiferromagnetic coupling is revealed by temperature-dependent magnetization measurements. The antiferromagnetic coupling would be compatible with the observed weak ferromagnetism by assuming that the magnetic moments order antiferromagnetically but nonparallel (canted). We find a clear correlation between coercivity and mosaicity of the ferromagnetic Mn-alloyed ZnO films and explain it on the basis of a coercivity mechanism known from soft magnetic materials.

In this study, ferromagnetic microstructures in highly oriented pyrolytic graphite and superparamagnetic spots in polyimide foils were created by 2.25 MeV proton microbeam irradiation and characterized using atomic and magnetic force microscopy. For this purpose, graphite samples were irradiated with cross-like patterns of 15 μm × 15 μm size using ion fluences in the range of (0.003–2.5) × 1018 cm−2. The irradiated crosses showed strong magnetic signals and a complex domain structure in the magnetic images depending on the geometrical dimensions of the crosses. Furthermore, polyimide foils were irradiated with microspots and fluences in the range of (0.016–3.1) × 1019 cm−2. Magnetic force microscopy shows very strong phase shifts in these irradiated areas.

Recently, magnetic order in highly oriented pyrolytic graphite (HOPG) induced by proton broad- and microbeam irradiation was discovered. Theoretical models propose that hydrogen could play a major role in the magnetism mechanism. We analysed the hydrogen distribution of pristine as well as irradiated HOPG samples, which were implanted to micrometer-sized spots as well as extended areas with various doses of 2.25 MeV protons at the Leipzig microprobe LIPSION. For this we used the sensitive 3D hydrogen microscopy system at the Munich microprobe SNAKE. The background hydrogen level in pristine HOPG is determined to be less than 0.3 at-ppm. About 4.8e15 H-atoms/cm^2 are observed in the near-surface region (4 um depth resolution). The depth profiles of the implants show hydrogen located within a confined peak at the end of range, in agreement with SRIM Monte Carlo simulations, and no evidence of diffusion broadening along the c-axis. At sample with microspots, up to 40 at-% of the implanted hydrogen is not detected, providing support for lateral hydrogen diffusion., accepted for publication in Nucl. Instr. and Meth

Room-temperature ferromagnetic Zn1−x(TM)xO semiconductor thin films have been grown by pulsed laser deposition (PLD) on c -plane sapphire substrates and the effect of the substitution of Zn2+ ions by transition metal ions (Mn2+ or Ti2+) has been investigated. In contrast to ZnTiO thin films, the grain size of the ZnMnO thin films can be controlled in a wide range by the PLD growth conditions. The magnetic properties of the Zn1−x(TM)xO films have been investigated by a superconducting quantum interference device (SQUID). For semi-insulating ZnMnO films with an optimized grain size, we observed a weak ferromagnetism for temperatures up to 400 K [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.-W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117–121], while the ZnTiO films were only paramagnetic or superparamagnetic. This is expected with respect to the micromagnetic model.

The incorporation of 3d transition metals into ZnO can produce ferromagnetism above room temperature. For Mn-doped ZnO films grown at low temperatures around 500 °C by pulsed laser deposition, the ferromagnetic behavior is correlated to their textured structure. For optimized films showing homogeneous magnetic domain formation in magnetic force microscopy, the saturation magnetization and coercive field strength amount to 0.013 emu/g and 234 Oe at 300 K, respectively. Compared to undoped ZnO films, the luminescence of Mn-doped ZnO films is strongly quenched. However, luminescence can be observed around antiferromagnetic MnO or MnO2 clusters that crystallize during the growth process on the film surface.

In this study ferromagnetic microstructures in highly oriented pyrolytic graphite (HOPG) were created by 2.25 MeV proton microbeam irradiation and characterized using AFM/MFM and SQUID measurements. For this purpose, spots of 1.8 × 1.8 μm2 size as well as cross-like patterns of 15 μm × 15 μm size were irradiated with different ion doses. Furthermore, arrays of 600 μm × 600 μm size containing 100 × 100 micro-spots were produced with ion doses of ∼0.3 nC/μm2. As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 × 1018 cm−3 and 1 × 1021 cm−3 for the range of ion doses used. The irradiated crosses showed strong magnetic signals and a complex domain structure in the MFM images. SQUID measurements on a HOPG sample successively irradiated with micro-spots arrays showed that the irradiation increased the magnetic moment of the sample up to ∼0.6 × 10−5 emu and the remanent magnetization Mr(B = 0) up to 4.5 × 10−4 emu/g. However, annealing effects during irradiation have to be carefully taken into account as they can result in a complete loss of the magnetic ordering in the irradiated areas.

The pulsed laser deposition technique has been used to produce magnetite (Fe 3 O 4 ) films with thickness ranging from 26 to 320 nm grown on MgAl 2 O 4 (1 0 0) substrates. Microstructural characterisation of the films has revealed enlarged grains with increasing thickness, i.e. deposition time. Magnetisation hysteresis loops have been measured at different temperatures in the range 25–300 K for three samples: two films with thickness of 26 and 320 nm deposited on MgAl 2 O 4 (1 0 0) and, for comparison, a 30 nm thick film deposited on MgO (1 0 0). The evolution of the remanence and the coercivity with temperature shows significant differences between the thinner films and the thicker one which are explained in terms of the effect of the interfacial strain as well as the influence of the deposition time on the grain sizes and the variation in the density of antiphase boundaries. All the studied films exhibit a minimum of the coercivity at about 130 K, the temperature at which the magnetocrystalline anisotropy constant K 1 vanishes for the bulk material.

In this article several aspects concerning the induction of magnetic ordering in graphite via proton bombardment using nuclear nanoprobes are addressed such as proton range and lateral straggling, defect densities, heat load, and simultaneous impurity analysis via particle induced X-ray emission.

The recently found ferromagnetic signals in pure graphite and in polymerised fullerenes are received mainly with scepticism by most of the scientific community. Actually, before those results were published, there were already several reports claiming an unusually large magnetization in pyrolytic carbon and oxidised fullerenes, without attracting the attention of the community. This scepticism may be well founded since not always a careful and systematic impurity study was provided to quantify the influence of ferromagnetic impurities. In this article and after a brief review of the existing data on magnetic carbon, we present and discuss recently obtained results on the ferromagnetism (or ferrimagnetism) induced by proton irradiation on pure pyrolytic graphite, which strengthen the importance of hydrogen in the formation of the magnetic ordering. The overall results indicate that room-temperature ferromagnetism in carbon-based structures containing only p- and s-electrons is a reality.

A C60 polymer has been characterized for the first time with respect to impurity content and ferromagnetic properties by laterally resolved particle induced X-ray emission (PIXE), superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM) in order to detect intrinsic ferromagnetic domains. In parts of the pure regions (concentration of magnetic impurities

Magnetization hysteresis loop $M(H)$ measurements performed on a single crystalline Ba$_{0.63}$K$_{0.37}$BiO$_3$ superconducting thick film reveal pronounced sample geometry dependence of the "second magnetization peak" (SMP), i.e. a maximum in the width of $M(H)$ occurring at the field $H_{\rm SMP}(T)$. In particular, it is found that the SMP vanishes decreasing the film dimension. We argue that the observed sample geometry dependence of the SMP cannot be accounted for by models which assume a vortex pinning enhancement as the origin of the SMP. Our results can be understood considering the thermomagnetic instability effect and/or non-uniform current distribution at $H < H_{\rm SMP}$ in large enough samples.

We have performed superconducting quantum interference device and \ensuremath{\mu}-Hall local magnetization measurements on Nb thin films of different thicknesses $(120\mathrm{nm}l~dl~1200\mathrm{nm})$ and widths $(95\ensuremath{\mu}\mathrm{m}l~wl~900\ensuremath{\mu}\mathrm{m})$ in order to characterize the effects due to thermomagnetic instabilities (TMI's) and their geometry dependence. We show that the second magnetization peak which occurs at fields ${H}_{\mathrm{SMP}}(T)\ensuremath{\ll}{H}_{c2}(T)$ depends on the sample dimensions and discuss it in terms of the classical theory of TMI's. Local magnetization measurements reveal a domelike flux profile at low fields which is not due to edge barriers. The Bean-like flux distribution is recovered at applied fields $Hg~{H}_{\mathrm{SMP}}(T).$

In recent years, the number of nominally nonmagnetic solids showing magnetic order induced by some kind of defect has increased continuously. From the single element material graphite to several covalently bonded nonmagnetic compounds, the influence of defects like vacancies and/or nonmagnetic ad-atoms on triggering magnetic order has attracted the interest of experimentalists and theoreticians. We review and discuss the main theoretical approach as well as recently obtained experimental evidence based on different experimental methods that support the existence of defect-induced magnetism in nonmagnetic as well as in magnetic materials.

The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form new states of matter. Here La0.7Sr0.3MnO3/SrRuO3 superlattices with coherently grown interfaces and layer thicknesses down to one unit cell were fabricated by pulsed laser deposition. The superlattices were studied by X-ray, HRTEM, magnetization and magnetotransport measurements. For such small thicknesses La0.7Sr0.3MnO3 films are antiferromagnetic and insulating. Despite the small layer thickness, the LaSrMnO layers in the superlattices were ferromagnetic with Curie temperatures close to room temperature. Whereas the resistivity of the superlattices showed metallic behaviour and was dominated by the conducting SrRuO3 layers, the off-diagonal resistivity showed an anomalous Hall effect with ferromagnetic loop shape even far above the Curie temperature of the SrRuO3 layers as well as a positive high field slope. This indicates the presence of a highly conducting, ferromagnetically ordered hole gas at the interfaces that might be formed by a charge-transfer process. This result opens up an alternative route for the fabrication of quasi-two-dimensional systems.

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 indicates 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 $\sim 400$ K changes irreversibly the hysteretic response of the sample., 36 pages with 13 figures

In the last years the number of nominally non-magnetic solids showing magnetic order induced by some kind of defects has increased continuously. From the single element material graphite to several covalently bonded non-magnetic compounds, the influence of defects like vacancies and/or non-magnetic ad-atoms on triggering magnetic order has attracted the interest of experimentalists and theoreticians. We review and discuss the main theoretical approach as well as recently obtained experimental evidence based on different experimental methods that supports the existence of defect-induced magnetism (DIM) in non-magnetic as well as in magnetic materials., Comment: 2 Figures, 9 pages, invited contribution at LAW3M (Buenos Aires) to be published in IEEE Transaction on Magnetics

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., Comment: submitted to Carbon

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, (110)o, and (001)o planes. Whereas magnetic field rotation in the (110)o and (110)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 Fil: Ziese, M.. Universität Leipzig D.superconductivity And Magnetism; Alemania Fil: Setzer, A.. Universität Leipzig D.superconductivity And Magnetism; Alemania Fil: Wunderlich, R.. Universität Leipzig D.superconductivity And Magnetism; Alemania Fil: Zandalazini, Carlos Ivan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina. Universität Leipzig D.superconductivity And Magnetism; Alemania Fil: P. Esquinazi. Universität Leipzig D.superconductivity And Magnetism; Alemania

A doping-independent ferromagnetic hysteresis at 300 and 5 K was found for both undoped and Mn-doped zirconia thin films with up to $20\text{ }\text{at}\text{.}\text{ }\mathrm{%}$ Mn. The lack of Mn-induced magnetic ordering is a strong indication that the observed ferromagnetic effects are defect related. Electron paramagnetic resonance analysis supports the conclusion that Mn ions are in a magnetically ``silent'' state. Cubic ${\text{ZrO}}_{2}:\text{Mn}$ films show a noticeable layer ferromagnetic saturation magnetization at 5 K, as compared to the monoclinic and tetragonal films, which clearly correlates with strain and a higher density of dislocations as the dominating microstructural defect.

LSAT, and ZnO single crystals were inves-tigated. These crystals show three contributions to the magnetization, namely, an intrinsic diamagnetic contri-bution, a paramagnetic contribution, due to various transition-metal impurities, as well as a ferromagneticcontribution. 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 originof the ferromagnetic contribution as arising from either defect-induced ferromagnetism or ferromagnetic im-purities is discussed.DOI: 10.1103/PhysRevB.81.214414 PACS number s : 77.84.Bw, 75.50.Pp, 75.20.Ck, 75.70.Rf

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.

We have investigated the magnetic properties of pure ZnO thin films grown under ${\text{N}}_{2}$ pressure on $a$-, $c$-, and $r$-plane ${\text{Al}}_{2}{\text{O}}_{3}$ substrates by pulsed-laser deposition. The substrate temperature and the ${\text{N}}_{2}$ pressure were varied from room temperature to $570\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ 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 ${\text{Al}}_{2}{\text{O}}_{3}$ substrates did not show significant ferromagnetism. However, ZnO films grown on $r$-plane ${\text{Al}}_{2}{\text{O}}_{3}$ showed reproducible ferromagnetism at 300 K when grown at $300--400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and 0.1--1.0 mbar ${\text{N}}_{2}$ pressure. Positron annihilation spectroscopy measurements as well as density-functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.

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., 4 figures, 6 pages. Invited contribution at JEMS08 to be published in JMMM

Iron, fluorine and boron ions were implanted into highly oriented pyrolytic graphite (HOPG). The samples were characterized before and after ion implantation as well as after heat treatments in vacuum by measurements of the magnetic moment and element analysis. Whereas the main magnetic contribution remains diamagnetic the paramagnetic one clearly increases with implantations and correlates with the amount of implanted ions. It is shown that a large part of the paramagnetic contribution is caused by the structural disorder created by particle bombardment using iron, fluorine or boron. All implanted HOPG samples show practically no change of the small ferromagnetic signal observed in their virgin state. No particular influence of iron on the ferromagnetic properties of HOPG is observed, up to ∼4000 μg/g Fe-concentration in the implanted region. For comparison, ferrous sulphates were added to ultra-clean graphite powder. This iron addition increases the number of paramagnetic spins proportional to the iron content in the untreated samples. In heat-treated samples however, a clear ferromagnetic behaviour is observed due to the formation of a ferromagnetic iron compound.

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 martensite and inter-martensite transitions, explore the multi-variant 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 M??bauer spectroscopy (CEMS) study to address changes in order, a quantitative defect-based physical picture of ion-irradiation-induced austenite ? martensite transformation in Fe7Pd3 is developed. The presented concepts thus pave the way for ion-irradiation-assisted optimization strategies for tailored functional alloys.

This chapter studies induced magnetic order by ion irradiation of carbon-based structures. Proton irradiation can trigger magnetic ordering in graphite. The results after proton irradiation show that magnetic ordering exists in a carbon structure without the influence of magnetic ions. There is a negligible influence of the topography onto the magnetic signal, and the magnetic signal depends on the implanted charge, proton current, and probably also on the carbon structure. Hydrogenated nanographite can have a spontaneous magnetization due to different numbers of mono- and dihydrogenated carbon atoms. Theoretical simulations indicate that a magnetic ordered state should be easier to achieve with a mixture of carbon–hydrogen bonds than fluorinated nanographite. Muon spin rotation/relaxation experiments indicate that a positive muon in graphite triggers a local magnetic moment around it. Hydrogen may also play an important role in the magnetic ordering found in fullerenes.

We have studied the magnetization of various, well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite to investigate the recently reported ferromagnetic-like signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers - to minimize the diamagnetic background - show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal., Comment: 11 figures

A magnetic-field-driven transition from metallic- to semiconducting-type behavior in the basal-plane resistance takes place in highly oriented pyrolytic graphite at a field $H_c \sim 1~$kOe applied along the hexagonal c-axis. The analysis of the data reveals a striking similarity between this transition and that measured in thin-film superconductors and Si MOSFET's. However, in contrast to those materials, the transition in graphite is observable at almost two orders of magnitude higher temperatures., Comment: 4 Figures

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.

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.

We have studied the magnetization of disordered, pure carbon films in order to search for signs of para-, ferro-or antiferromagnetism. Carbon films on silicon substrates were produced by pulsed laser deposition in high vacuum and at room temperature using pressed ultra-clean graphite powder as target. The magnetization shows a Curie–Weiss behaviour added to a temperature-independent term. Annealing the films at 1000°C and in high-vacuum eliminates completely the Curie–Weiss contribution. We compare our results with those recently obtained in activated carbon fibers.