This study presents a model to elucidate the impact of defect creation on the electrical resistance of nanometer-thick, micrometer-wide graphite samples induced by low-energy ion implantation. Electrically contacted samples of highly oriented pyrolytic graphite were irradiated with N2, Ne, Ar, and Kr ions at 5 keV and the resulting changes in electrical resistance were measured in situ as a function of the ion fluence. Building upon the Johnson–Mehl–Avrami formalism, originally developed for nucleation and growth of defects in solids, this works adapts the framework to describe the influence of ion-induced damage cascades on the electrical resistance of graphite. Moreover, the model allows for an estimate of a lower bound for the electron mean-free path in graphite.

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.

Magnetic order at room temperature induced by atomic lattice defects, like vacancies, interstitials, or their pairs, has been observed in a large number of different non-magnetic hosts, such as pure graphite, oxides, and silicon-based materials. High Curie temperatures and time-independent magnetic response at room temperature indicate the extraordinary robustness of this new phenomenon in solid-state magnetism. In this work, we review experimental and theoretical results of pure TiO2 (anatase), whose magnetic order can be triggered by low-energy ion irradiation. In particular, we discuss the systematic observation of an ultrathin magnetic layer with perpendicular magnetic anisotropy at the surface of this oxide.

A broad effort is underway to understand and harness the interaction between superconductors and spin-active color centers with an eye on hybrid quantum devices and novel imaging modalities of superconducting materials. Most work, however, overlooks the interplay between either system and the environment created by the color center host. Here we use a diamond scanning probe to investigate the spin dynamics of a single nitrogen-vacancy (NV) center proximal to a superconducting film. We find that the presence of the superconductor increases the NV spin coherence lifetime, a phenomenon we tentatively rationalize as a change in the electric noise due to a superconductor-induced redistribution of charge carriers near induced redistribution of charge carriers near the NV. We then build on these findings to demonstrate transverse-relaxation-time-weighted imaging of the superconductor film. These results shed light on the dynamics governing the spin coherence of shallow NVs, and promise opportunities for new forms of noise spectroscopy and imaging of superconductors.

In this work, we demonstrate the feasibility of detecting low energy single ions using nanometer-thick and few-micrometer-wide graphite flakes at room temperature. We used electrically contacted samples of highly oriented pyrolytic graphite and measured in situ the change in electrical resistance upon ion irradiation using a self-made nanovoltmeter. Eight samples of different dimensions were prepared and subjected to ion irradiation from various species. A fluence as low as 5×108cm−2 corresponding to a few hundreds of ions irradiated was sufficient to produce a signal significantly above the noise level. Furthermore, we used a model to describe the dependence of the change in electrical resistance on the defect concentration induced by ion irradiation. By extrapolating this model to the level of single ions, our findings support the feasibility of detecting single ions at room temperature using thin graphite as a detector material.

Herein, magnetotransport properties of microstructured c‐ and a‐plane ZnO thin films grown on a‐ and r‐plane sapphire substrates are investigated. The grain and grain boundary contributions to the electrical transport are verified using impedance spectroscopy. Photoluminescence measurements show maxima related to oxygen and zinc vacancies (VZn) present in both kinds of samples, which, especially VZn, can contribute to the ferromagnetic behavior. The temperature dependence of the resistance indicates the existence of two different regimes, a variable‐range hopping mechanism at temperatures T ≤ 30 K, whereas thermally activated transport dominates at higher temperatures. The magnetoresistance between 2 and 250 K is negative for all samples, indicating the existence of spin‐scattering processes. Hall‐effect measurements reveal that the samples are n‐type but have a small anomalous‐like contribution related to different types of charge carriers.

Manipulation and control of defects triggered by an electron beam allow us to conduct defect engineering on layered materials. We investigate topologically stable helices within a [Dy(10 nm)/Tb(10 nm)](30) multilayer subjected to MeV electron(e)-irradiation up to a maximum fluence of 9.58 x 10(18) e/cm(2). As electrons can go through the sample homogeneously and with high penetration depth, they produce defects without doping. Our e-irradiation results indicate defect induced magnetic manipulation, which increases the blocking/freezing temperature of spin-frustrated interfaces by 4%. This increase implies an increase in the spin-cluster volume. Consequently, the reduced uncompensated pinning centres decrease the interfacial exchange bias coupling by 45%. Direct manipulation of pinning centres would thereby allow us to tailor spintronic devices in a clean way.

This study describes electrical transport measurements as a function of magnetic field and temperature that provide hints of the existence of superconductivity with a critical temperature of T c ∼ 350 K in a natural graphite sample. The measurements were done in a restricted temperature 300 K ⩽ T ⩽ 450 K and magnetic field B ⩽ 400 mT range. Electrical resistance measurements at remanence (zero field), after the application of a magnetic field, indicate the existence of trapped flux, which remains nearly unchanged within ∼ 30 min but it vanishes at a temperature of ∼ 330 K. The apparent transition is accompanied by a clear enhancement of the magnetoresistance at T < T c . Raman measurements on the bulk sample reveal the existence of the rhombohedral stacking order, which interfaces with the usual Bernal phase were predicted to lead to high temperature superconductivity due to the formation of robust flat bands in the electron dispersion relation.

In spite of 40 years of experimental studies and several theoretical proposals, an overall interpretation of the complex behavior of the magnetoresistance (MR) of multilayer graphene, i.e. graphite, at high fields ($B \lesssim 70~$T) and in a broad temperature range is still lacking. Part of the complexity is due to the contribution of stacking faults (SFs), which most of thick enough multilayer graphene samples have. We propose a procedure that allows us to extract the SF contribution to the MR we have measured at 0.48~K $\leq T \leq$ 250~K and 0~T$\leq B \lesssim$ 65~T. We found that the MR behavior of part of the SFs is similar to that of granular superconductors with a superconducting critical temperature $T_c \sim $ 350~K, in agreement with recent publications. The measurements were done on a multilayer graphene TEM lamella, contacting the edges of the two-dimensional SFs.

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs.

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) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were 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.

Although ferromagnets are found in all kinds of technological applications, their natural occurrence is rather unusual because only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? In this study, a universal scheme for the computational discovery of new artificial functional magnetic materials is proposed, which is guided by experimental constraints and based on first principles. The obtained predictions explain very well the experimental data found in the literature. The potential of the method is further demonstrated by the experimental realization of a truly 2D ferromagnetic phase at room temperature, created in nominally non-magnetic TiO2 films by ion irradiation, which follows a characteristic 2D magnetic percolation transition and exhibits a tunable magnetic anisotropy.

Regina Ariskina ,1 Michael Schnedler ,2,* Pablo D. Esquinazi ,1,† Ana Champi ,3 Markus Stiller,1 Wolfram Hergert ,4 R. E. Dunin-Borkowski,2 Philipp Ebert,2 Tom Venus,5 and Irina Estrela-Lopis5 1Division of Superconductivity and Magnetism, Felix-Bloch Institute for Solid State Physics, University of Leipzig, D-04103 Leipzig, Germany 2Peter Gruenberg Institut, Forschungszentrum Juelich, D-52425 Juelich, Germany 3Centro de Ciencias Naturais e Humanas, Universidade Federal do ABS, Santo Andre, 09210-580 SP, Brazil 4Institute of Physics, Martin Luther University Halle-Wittenberg. D-06120 Halle, Germany 5Institute of Medical Physics and Biophysics, University of Leipzig, D-04107 Leipzig, Germany

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so‐called C or P1 centers, are 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 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers 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 centers, stressing the extraordinary role of the C centers 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.

Herein, a ferrimagnetic manganese zinc ferrite (Mn0.5Zn0.5Fe2O4) film with a thickness of 200 nm is prepared without a buffer layer on strontium titanate (001) (SrTiO3) using pulsed laser deposition. Its magnetic properties are investigated using superconducting quantum interference device (SQUID), X‐ray absorption spectroscopy with subsequent X‐ray magnetic circular dichroism (XMCD) and magneto‐optic Kerr effect (MOKE). Hysteresis loops derived from SQUID exhibits bulk‐like properties. This can further be confirmed by bulk‐like XMCD spectra. In remanent magnetization, an in‐plane magnetization with basically no out‐of‐plane component is found. The magnetic moments derived by the sum rule formalism from the XMCD data are in good agreement to the magnetization observed by SQUID and MOKE. XMCD as well as MOKE reveal an in‐plane angular fourfold magnetic anisotropy with the easy direction along [110] for (Mn0.5Zn0.5)Fe2O4 on SrTiO3. The element‐specific magnetic moments from XMCD show a stronger contribution of Fe to the anisotropy than of Mn and distinct contributions of the orbital moments.

Cryogenic vector magnet systems make it possible to study the anisotropic magnetic properties of materials without mechanically rotating the sample but by electrically tilting and turning the magnetic field. Vector magnetic fields generated inside superconducting vector magnets are generally measured with three Hall sensors. These three probes must be calibrated over a range of temperatures, and the temperature-dependent calibrations cannot be easily carried out inside an already magnetized superconducting magnet because of remaining magnetic fields. A single magnetometer based on an ensemble of nitrogen vacancy (NV) centers in diamond is proposed to overcome these limitations. The quenching of the photoluminescence intensity emitted by NV centers can determine the field in the remanent state of the solenoids and allows an easy and fast canceling of the residual magnetic field. Once the field is reset to zero, the calibration of this magnetometer can be performed in situ by a single measurement of an optically detected magnetic resonance spectrum. Thereby, these magnetometers do not require any additional temperature-dependent calibrations outside the magnet and offer the possibility to measure vector magnetic fields in three dimensions with a single sensor. Its axial alignment is given by the crystal structure of the diamond host, which increases the accuracy of the field orientation measured with this sensor, compared to the classical arrangement of three Hall sensors. It is foreseeable that the magnetometer described here has the potential to be applied in various fields in the future, such as the characterization of ferromagnetic core solenoids or other magnetic arrangements.

High anisotropy and the existence of two-dimensional highly conducting interfaces at stacking faults parallel to the graphene planes of the graphite structure influence, in a non-simple way, the transport properties of highly oriented graphite. We report two related effects on the electrical resistance of highly oriented pyrolytic as well as of natural graphite bulk samples, measured with the four points method in the temperature range 300 K ≤T≤ 410 K. A qualitative and quantitative change in the temperature dependence of the resistance was obtained by simply enlarging the electrodes and contacting the edges of the internal interfaces on the same sample. Additionally, at temperatures T≳350 K the resistance can change with time. We show that this temperature-dependent annealing effect is related to the stacking faults and can irreversibly change the absolute value of the resistance and its temperature dependence. A partial recovery is obtained after leaving the sample at normal conditions for several days. The overall results stress the importance of the electrodes location on a bulk graphite sample, the contribution of the stacking faults in the interpretation of the measured transport properties and the need of systematic studies on the influence of high temperature annealing on the interfaces properties.

Noncollinear spin textures such as skyrmions lead to a novel Hall effect contribution called the topological Hall effect. In recent years, research has focused on the Weyl metal SrRuO3 with topological signatures reported for SrRuO3 layers adjacent to layers with strong spin–orbit coupling. However, SrRuO3 films are known to be prone to structural modifications when interfaced with different materials. In this work the Hall effect of SrRuO3/La0.7Sr0.3MnO3 superlattices (SLs) is correlated with structural and magnetic information to study whether a topological Hall effect is induced by interfacing to ferromagnetic layers. High‐quality SLs with ultrathin layers are fabricated by pulsed laser deposition on SrTiO3 substrates. The symmetry of the SrRuO3 layers is studied by angular magnetoresistance measurements and the magnetocrystalline anisotropy by magnetization measurements. Two SLs are compared in detail, both with 8 unit cell thick SrRuO3 layers, but with La0.7Sr0.3MnO3 layers 2 and 4 unit cell thick. Only the sample with 2 unit cell thick manganite layers shows features resembling a topological Hall effect. Depending on the La0.7Sr0.3MnO3 layer thickness the SrRuO3 crystalline symmetry is orthorhombic or tetragonal. The results indicate that the topological Hall effect features arise from an intricate interplay between magnetocrystalline anisotropy and antiferromagnetic interlayer coupling.

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.

This work focuses on the generation of ferromagnetism at the surface of anatase TiO2 films by low-energy ion irradiation. Controlled Ar+-ion irradiation resulted in a thin (similar to 10) nm ferromagnetic surface layer. The intrinsic origin and robustness of the magnetic order has been characterized by x-ray magnetic circular dichroism at room temperature revealing that a Ti band is spin-polarized. These results, together with density functional theory calculations, indicate that Ti vacancy-interstitial pairs are responsible for the magnetic order. Superconducting quantum interference device measurements show the existence of a perpendicular magnetic anisotropy and a low remanent magnetization. Magnetic force microscopy reveals that this low remanence is due to oppositely aligned magnetic domains with magnetization vectors normal to the main surface. The weak domain-wall pinning, the magnetic anisotropy, together with the simplicity of the preparation method, open up interesting possibilities for future applications. As an example, single domain patterns of similar to 1 mu m width and several pan length can be easily prepared.