Search

The strong exchange interaction between 3$d$-4$f$ magnetic sublattice in rare-earth perovskites introduces a variety of complex magnetic states hosting fascinating electronic ground states with exotic properties. Especially when it comes to rare-earth nickelate and cobaltite perovskites, tuning their rich magnetic phase diagram and spin-state transitions make them potential candidates for spintronic applications. Here, we report the observation of antiferromagnetic coupling between Pr 4$f$ and Ni/Co 3$d$ magnetic sublattices and its tunability with strain in PrCo$_{0.5}$Ni$_{0.5}$O$_{3-\delta}$ (PCNO) thin films. SQUID magnetization measurements reveal ferromagnetic (FM) ordering around 25 K, followed by a spin glass transition at low temperatures subject to spin reorientation. Competing magnetic interactions arise owing to the 3$d$-4$f$ antiferromagnetic (AFM) coupling between Pr and Co/Ni sublattice as revealed by the X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Pr $M_{4,5}$ and Co/Ni $L_{2,3}$ absorption edges. Strain dependence on these AFM coupling reveals an increase (decrease) in the AFM exchange interaction for tensile (compressive) strained films, leading to a net decrease (increase) in the magnetization of PCNO films at low temperatures. The relative increase in low-temperature negative magnetoresistance for compressively strained films also reflects the enhanced ferromagnetic ordering in the system. The angle-dependent magnetoresistance measurements reveal a two-fold anisotropic magnetoresistance (AMR) in tensile strained PCNO films. In contrast, temperature-dependent switching of AMR accompanied by a two- to four-fold symmetry crossover is observed for LaAlO$_3$-grown compressive strained films.

Thin films of polycrystalline $\beta$-Mn structure CoZn have been grown on thermally oxidized Si substrates by co-sputtering from elemental targets followed by annealing. A range of films grown with variable Co deposition power and fixed Zn deposition power were produced, so as to vary the proportions of the two elements reaching the substrate, which were annealed post-growth. Whilst all films exhibited a (211) $\beta$-Mn structure CoZn texture in X-ray diffraction, transmission electron microscopy showed that the composition with the highest integrated intensity for that Bragg peak contained large vacancies and was covered by a thick ZnO cap owing to being Co-deficient overall. CoZn films deposited at ratios tuned to give the optimal volume fraction of $\beta$-Mn were continuous, with crystallites up to 200~nm in size, with a much thinner ZnO cap layer. Magnetic measurements show that such optimal CoZn films have a Curie temperature $T_\mathrm{C} \sim 420$~K and saturation magnetization of 120~emu/cm$^3$, properties close to those reported for bulk crystals. The $\beta$-Mn structure is chiral (P4$_{1}$32/P4$_{3}$32 space group) and is known to give rise to a Dzyaloshinkii-Moriya interaction (DMI) that stabilizes room-temperature skyrmions in the bulk. Our thin films are thus a potential materials platform, compatible with planar processing technology, for magnetic skyrmions arising from a bulk DMI., Comment: v2 corrects minor typographical errors

We demonstrate nonreciprocal critical current in 65 nm thick polycrystalline and epitaxial Nb thin films patterned into tracks. The nonreciprocal behavior gives a supercurrent diode effect, where the current passed in one direction is a supercurrent and the other direction is a normal state (resistive) current. We attribute fabrication artefacts to creating the supercurrent diode effect in our tracks. We study the variation of the diode effect with temperature and magnetic field, and find a dependence with the width of the Nb tracks from 2-10 $\mu$m. For both polycrystalline and epitaxial samples, we find that tracks of width 4 $\mu$m provides the largest supercurrent diode efficiency of up to $\approx$30$\%$, with the effect reducing or disappearing in the widest tracks of 10 $\mu$m. We propose a model based on the limiting contributions to the critical current density to explain the track width dependence of the induced supercurrent diode effect. It is anticipated that the supercurrent diode will become a ubiquitous component of the superconducting computer., Comment: 21 pages, 5 figures

Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ferromagnetic multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain Co$_x$Pt$_{100-x}$ alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic Co$_{50}$Pt$_{50}$ and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the $L1_1$ and $L1_0$ ordered compounds. Primarily, interest in Co$_{50}$Pt$_{50}$ has been in the $L1_0$ crystal structure, where layers of Pt and Co are stacked alternately in the [100] direction. There has been less work on $L1_1$ crystal structure, where the stacking is in the [111] direction. For the latter $L1_1$ crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former $L1_0$ crystal structure, the magnetic anisotropy is perpendicular to the [100] plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the $L1_1$ and $L1_0$ Co$_{50}$Pt$_{50}$ to be $47\pm3\%$., Comment: 17 pages main text, 7 figures main text. 12 pages SI, 10 figures SI

We report on the electrical transport properties of Nb based Josephson junctions with Pt/Co$_{68}$B$_{32}$/Pt ferromagnetic barriers. The barriers exhibit perpendicular magnetic anisotropy, which has the main advantage for potential applications over magnetisation in-plane systems of not affecting the Fraunhofer response of the junction. In addition, we report that there is no magnetic dead layer at the Pt/Co$_{68}$B$_{32}$ interfaces, allowing us to study barriers with ultra-thin Co$_{68}$B$_{32}$. In the junctions, we observe that the magnitude of the critical current oscillates with increasing thickness of the Co$_{68}$B$_{32}$ strong ferromagnetic alloy layer. The oscillations are attributed to the ground state phase difference across the junctions being modified from zero to $\pi$. The multiple oscillations in the thickness range $0.2~\leqslant~d_\text{CoB}~\leqslant~1.4$~nm suggests that we have access to the first zero-$\pi$ and $\pi$-zero phase transitions. Our results fuel the development of low-temperature memory devices based on ferromagnetic Josephson junctions., Comment: Main text: 11 pages, 3 figures, 1 table. Supplementary information: 9 pages, 5 figures

We demonstrate a Josephson junction with a weak link containing two ferromagnets, with perpendicular magnetic anisotropy and independent switching fields in which the critical current can be set by the mutual orientation of the two layers. Such pseudospin-valve Josephson junctions are a candidate cryogenic memory in an all superconducting computational scheme. Here, we use Pt/Co/Pt/CoB/Pt as the weak link of the junction with $d_\text{Co} = 0.6$ nm, $d_\text{CoB} = 0.3$ nm, and $d_\text{Pt} = 5$ nm and obtain a $60\%$ change in the critical current for the two magnetization configurations of the pseudospin-valve. Ferromagnets with perpendicular magnetic anisotropy have advantages over magnetization in-plane systems which have been exclusively considered to this point, as in principle the magnetization and magnetic switching of layers in the junction should not affect the in-plane magnetic flux., Comment: 5 pages, 3 figures

Spin-wave resonance measurements were performed in the mixed magnetic phase regime of a Pd-doped FeRh epilayer that appears as the first-order ferromagnetic-antiferromagnetic phase transition takes place. It is seen that the measured value of the exchange stiffness is suppressed throughout the measurement range when compared to the expected value of the fully ferromagnetic regime, extracted via the independent means of a measurement of the Curie point, for only slight changes in the ferromagnetic volume fraction. This behavior is attributed to the influence of the antiferromagnetic phase: inspired by previous experiments that show ferromagnetism to be most persistent at the surfaces and interfaces of FeRh thin films, we modelled the antiferromagnetic phase as forming a thin layer in the middle of the epilayer through which the two ferromagnetic layers are coupled up to a certain critical thickness. The development of this exchange stiffness is then consistent with that expected from the development of an exchange coupling across the magnetic phase boundary, as a consequence of a thickness dependent phase transition taking place in the antiferromagnetic regions and is supported by complimentary computer simulations of atomistic spin-dynamics. The development of the Gilbert damping parameter extracted from the ferromagnetic resonance investigations is consistent with this picture., Comment: Main manuscript: 11 pages, 6 figures. Supplemental material: 2 pages, 1 figure

We have imaged N\'eel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 \mu m diameter dots, using scanning transmission x-ray microscopy. The skyrmion bubbles can be nucleated by the application of an external magnetic field and are stable at zero field with a diameter of 260 nm. Applying an out of plane field that opposes the magnetisation of the skyrmion bubble core moment applies pressure to the bubble and gradually compresses it to a diameter of approximately 100 nm. On removing the field the skyrmion bubble returns to its original diameter via a hysteretic pathway where most of the expansion occurs in a single abrupt step. This contradicts analytical models of homogeneous materials in which the skyrmion compression and expansion are reversible. Micromagnetic simulations incorporating disorder can explain this behaviour using an effective thickness modulation between 10 nm grains., Comment: 18 pages, 5 figures

Superconducting spintronics in hybrid superconductor/ferromagnet (S-F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, $T_c$, of the S-layer can be controlled by the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using field history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both $T_c$ and the shape of the resistive transition, R(T), to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the effective exchange field of the ferromagnet and link this, using conventional S-F theory, to the suppression of $T_c$. We observe stepped features in the R(T) which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description., Comment: 19 pages, 4 figures

Superconducting spintronics has emerged in the last decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom. Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by proximity of a conventional superconductor to a ferromagnetic material with inhomogeneous macroscopic magnetization. Using low-energy muon spin rotation experiments, we find an entirely unexpected novel effect: the appearance of a magnetization in a thin layer of a non-magnetic metal (gold), separated from a ferromagnetic double layer by a 50 nm thick superconducting layer of Nb. The effect can be controlled by either temperature or by using a magnetic field to control the state of the remote ferromagnetic elements and may act as a basic building block for a new generation of quantum interference devices based on the spin of a Cooper pair., Comment: 17 pages, 4 figures

The temperature dependence of the spin diffusion length typically reflects the scattering mechanism responsible for spin relaxation. Within non-magnetic metals it is reasonable to expect the Elliot-Yafet mechanism to play a role and thus the temperature dependence of the spin diffusion length might be inversely proportional to resistivity. In lateral spin valves measurements have found that at low temperatures the spin diffusion length unexpected decreases. By measuring the transport properties of lateral Py/Cu/Py spin valves fabricated with different purities of Cu, we extract a spin diffusion length which shows this suppression below 30K only in the presence of the Kondo effect. We have calculated the spin-relaxation rate and isolated the contributions from magnetic impurities. We find the spin-flip probability of a magnetic impurity to be 34%. Our semi-quantitative analysis demonstrates the dominant role of Kondo scattering in spin relaxation, even in low concentrations of order 1 p.p.m., and hence accounts for the reduction in spin diffusion length observed by ourselves and others.