High spatial resolution magnetic imaging has driven important developments in fields ranging from materials science to biology. However, to uncover finer details approaching the nanoscale with greater sensitivity requires the development of a radically new sensor technology. The nitrogenvacancy (NV) defect in diamond has emerged as a promising candidate for such a sensor based on its atomic size and quantum-limited sensing capabilities afforded by long spin coherence times. Although the NV center has been successfully implemented as a nanoscale scanning magnetic probe at room temperature, it has remained an outstanding challenge to extend this capability to cryogenic temperatures, where many solid-state systems exhibit non-trivial magnetic order. Here we present NV magnetic imaging down to 6 K with 6 nm spatial resolution and 3 μT/√Hz field sensitivity, first benchmarking the technique with a magnetic hard disk sample, then utilizing the technique to image vortices in the iron pnictide superconductor BaFe2(As0.7P0.3)2 with Tc = 30 K. The expansion of NVbased magnetic imaging to cryogenic temperatures represents an important advance in state-of-theart magnetometry, which will enable future studies of heretofore inaccessible nanoscale magnetism in condensed matter systems. and Lorentz transmission electron microscopy (TEM) 7 , and reciprocal space techniques including neutron scattering 8 have been successfully utilized to study magnetism in these systems. However, each of these techniques has limitations that must be considered. In MFM, a ferromagnetic tip must be placed in close proximity to a sample, which can perturb the magnetic order that is being probed.Scanning SQUIDs typically require a probe temperature of 10 K or lower, and generally offer micron-size spatial resolution, although recent studies have enhanced the resolution to submicron scales 9 . Lorentz TEM can provide images with high spatial resolution and magnetic contrast, but requires very thin samples, typically less than 100 nm thick. Neutron scattering requires the growth of large, high purity single-crystal samples, and is an ensemble-averaged measurement. There is therefore a significant opportunity to develop a real-space, non-invasive magnetic sensor capable of studying magnetic order at sub-10 nm spatial resolution and sub-T/Hz DC field sensitivities.The nitrogen vacancy (NV) defect center in diamond is an exceptionally versatile single spin system with unique quantum properties that have driven its application in diverse areas ranging from quantum information and photonics to quantum metrology [10][11][12][13][14][15][16][17][18][19] . Cryogenic scanning magnetometry stands out as potentially the most impactful application of NV centers, taking advantage of the exquisite magnetic field sensitivity and intrinsic atomic scale of the NV center for high resolution imaging 20 . The operation of an NV-based magnetic probe is dependent on a fundamentally different sensing principle than other imaging methods, namely the spin-dependent photolum...

Magnetic skyrmions as swirling spin textures with a nontrivial topology have potential applications as magnetic memory and storage devices. Since the initial discovery of skyrmions in non-centrosymmetric B20 materials, the recent effort has focused on exploring room-temperature skyrmions in heavy metal and ferromagnetic heterostructures, a material platform compatible with existing spintronic manufacturing technology. Here, we report the surprising observation that a room-temperature skyrmion phase can be stabilized in an entirely different class of systems based on antiferromagnetic (AFM) metal and ferromagnetic (FM) metal IrMn/CoFeB heterostructures. There are a number of distinct advantages of exploring skyrmions in such heterostructures including zero-field stabilization, tunable antiferromagnetic order, and sizable spin-orbit torque (SOT) for energy-efficient current manipulation. Through direct spatial imaging of individual skyrmions, quantitative evaluation of the interfacial Dzyaloshinskii-Moriya interaction, and demonstration of current-driven skyrmion motion, our findings firmly establish the AFM/FM heterostructures as a promising material platform for exploring skyrmion physics and device applications.

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Ohm's law describes the proportionality of current density and electric field. In solid-state conductors, Ohm's law emerges due to electron scattering processes that relax the electrical current.Here, we use nitrogen-vacancy center magnetometry to directly image the local breakdown of Ohm's law in a narrow constriction fabricated in a high mobility graphene monolayer. Ohmic flow is visible at room temperature as current concentration on the constriction edges, with flow profiles entirely determined by sample geometry. However, as the temperature is lowered below 200 K, the current concentrates near the constriction center. The change in the flow pattern is consistent with a crossover from diffusive to viscous electron transport dominated by electron-electron scattering processes that do not relax current.

Skyrmions are nanoscale magnetic structures with features promising for future low-power memory or logic devices. In this work, we demonstrate novel scanning techniques based on nitrogen vacancy center magnetometry that simultaneously probe both the magnetic dynamics and structure of room temperature skyrmion bubbles in a thin film system Ta/CoFeB/MgO. We confirm the handedness of the Dzyaloshinskii-Moriya interaction in this material and extract the helicity angle of the skyrmion bubbles. Our measurements also show that the skyrmion bubbles in this material change size in discrete steps, dependent on the local pinning environment, with their average size determined dynamically as their domain walls hop between pinning sites. In addition, an increase in magnetic field noise is observed near all skyrmion bubble domain walls. These measurements highlight the importance of interactions between internal degrees of freedom of skyrmion bubble domain walls and pinning sites in thin film systems. Our observations have relevance for future devices based on skyrmion bubbles where pinning interactions will determine important aspects of current-driven motion.

We report on the application of the local spin-density approximation (LSDA) and the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) within the linear muffintin orbital method in both the atomic sphere approximation (LMTO-ASA) and in the full-potential (FP-LMTO) method to the description of the magnetic coupling within bulk Gd. Using the LMTO-ASA approach to the band-structure problem it is found that, at the experimental lattice parameters, the PBE approximation predicts the experimentally observed ferromagnetic (FM) ground state whereas the LSDA does not. Moreover the nature of the magnetic coupling between successive layers is found to be dependent on the interlayer separation-in particular a reduction of the interlayer spacing will lead to an increased tendency towards FM coupling between successive layers and, conversely, increase of the interlayer spacing will lead to antiferromagnetic (AFM) coupling between layers being energetically favourable. A similar interdependence between the interlayer spacing and the magnetic coupling is also observed from calculations using the FP-LMTO method. These observations are used to analyse the nature of the magnetic coupling of the Gd(0001) surface to the underlying FM bulk.

Relativistic spin-polarized scattering theory is discussed and the relevant radial Dirac equations for an electron in a potential with a magnetic field component are derived. The full solution to the coupled Dirac equations treating spin-orbit coupling and spin polarization on an equal footing is found and, by matching these wavefunctions at the muffin-tin radius, the scattering amplitudes and phase shifts are calculated. The interpretation of these results gives a pictorial view of the interplay between spin-orbit coupling and spin polarization. New coupling between states is observed due to the removal of previously used approximations. The magnitude of this coupling throws doubt on some earlier calculations of magnetocrystalline anisotropy energies. A scattering analogue to the generalized Zeeman effect is also described.

The difference in absorption rate of right or left circularly polarized x-rays by magnetic materials is known as magnetic dichroism and is a well established method of investigating the magnetic properties of materials on a microscopic level. In this paper we illustrate this effect with a study of magnetic dichroism in the one-electron atom. The standard relativistic one-electron atom is solved. First-order perturbation theory is used to calculate eigenfunctions in an applied constant magnetic field. These eigenfunctions are used in a golden rule calculation of the absorption rate for right and left circularly polarized light as a function of applied field. All calculations have been done analytically. The difference in absorption rates for right and left circularly polarized light for 1s to 2p1/2 is of opposite sign to the difference for 1s to 2p3/2. We comment on the significance of these results in the interpretation of dichroism experiments on magnetic materials.