Significant exchange bias (EB) is induced across the spin reorientation (Morin) transition (TM) in epitaxial alpha - Fe2O3(001)/Ni81Fe19 (hematite/permalloy) heterostructures, far below the antiferromagnetic transition temperature. In contrast to typical exchange coupled systems, these heterostructures exhibit two distinct features: (i) exchange bias is established at the Morin transition, rather than the N & eacute;el ordering, and (ii) a qualitatively different temperature dependence where EB is confined to a narrow temperature window near TM instead of increasing monotonically upon cooling as in conventional systems. The temperature- and angular-dependent measurements imply that this thermal behavior is caused by separate transitions of the interfacial and bulk hematite spins. As a consequence, interdiffusion drastically alters the thermal dependence of EB, which highlights the role of the interfacial spins. Our study reveals that the spin reorientation transition breaks the magnetic isotropy at the antiferromagnetic hematite (001) surface and controls the interfacial exchange interactions. These results demonstrate that the spin reorientation transition in hematite, including its widely available doped variants, can be exploited to realize thermally programmable and temperature-selective exchange bias. With the growing interest in antiferromagnetic spintronics, they further show that spin reorientation transitions in antiferromagnets provide a tunable window independent of the N & eacute;el temperature for manipulating AFM spins and may assist N & eacute;el vector switching in spin torque devices.

In this work, we have controlled locally the magnetic anisotropy of PdCo multilayers using ion beam modification. First, we have used techniques with different probing depths to confirm that cobalt is partially oxidized in the Pd/Co multilayers exhibiting PMA fabricated by sputtering, presumably due to the diffusion of atmospheric oxygen migrated through the Pd capping layer. Then, low energy ion bombardment has been employed to induce a structural disorder in the upper layers of the multilayer, gradually reducing the PMA at local scale with increasing ion doses. Medium ion doses lead to a marked decline in PMA, resulting in equivalent magnetic anisotropy directions and the loss of characteristic labyrinthine magnetic domain morphology. High ion doses completely suppress PMA, yielding a typical in-plane shape anisotropy. By combining controlled PMA reduction via lithographic techniques, we create elongated structures displaying distinct magnetic responses; bombarded regions exhibit reduced PMA, whereas adjacent regions maintain their PMA. Notably, regions with reduced PMA do not disrupt the distribution of perpendicular magnetic domains in neighboring PMA regions, even at widths as narrow as 500 nm, demonstrating the robustness of the PMA domain structure.

Vanadium dioxide (VO2) is a correlated oxide that undergoes a sharp, reversible metal-insulator transition (MIT) near 340 K, making it promising for energy-efficient electronics. However, integrating VO2 into flexible substrates with low power consumption via a straightforward approach remains a challenge. Here, we demonstrate the direct deposition of flexible VO2 thin films on Kapton via reactive magnetron sputtering. While applying a predeposited Al2O3 layer improves adhesion and strain control for flexible uses, the direct deposition also offers a straightforward procedure to produce freestanding VO2 films. The resulting flexible devices exhibit millimeter-scale resistive switching at submilliampere currents, showing two orders of reduction in power consumption compared to typical sapphire-based films. In operando midwave infrared imaging reveals filamentary conduction driven by Joule heating and local dissipation. Furthermore, we show that mechanical deformation modulates the resistance and can trigger the MIT under a preload voltage bias, enabling deformation-induced resistive switching. These results introduce a versatile and low-power consumption VO2 platform for flexible electronics, tactile sensors, and neuromorphic systems, offering functionalities through the synergy of correlated electron physics, thermal confinement, and substrate compliance.

The metal-insulator transition (MIT) in vanadium dioxide (VO2) thin films is strongly affected by grain size, thickness, and interfacial properties. Typically, a minimum thickness around 50 nm is required for VO2 to exhibit a significant MIT when functional substrates like sapphire and silicon are used. Several works have shown that thin films below 20 nm, with up to 2-3 decades of change in the resistance across the MIT, can be achieved but require complex pre- or postprocessing of the samples. We show that predeposition substrate condition control facilitates the direct growth of VO2 ultrathin 15 nm films, exhibiting a resistance change between 3 and 4 decades across the MIT. Our findings indicate that the interface between the film and the substrate is crucial in determining the initial growth layers and the structural evolution. With appropriate substrate surface treatment, the desired VO2 MIT can be enhanced regardless of the substrate crystallographic orientation. Moreover, we propose a novel approach to obtain large resistance changes across the MIT in ultrathin VO2 films by incorporating a predeposited 1.5 nm vanadium oxide buffer layer, thereby eliminating the need to use different materials or complex pre- or postprocessing of the samples. We also demonstrate that this method improves the transition of 25-50 nm VO2 thin films on silicon substrates. Our study reveals a simple approach for direct growth of ultrathin VO2 films exhibiting a significant MIT, which is commonly accepted unattainable over substrates of technological importance, such as sapphire and silicon.

The spin Seebeck effect is useful for probing the spin correlations and magnetic order in magnetic insulators. Here, we report a strong longitudinal spin Seebeck effect (LSSE) in antiferromagnetic V2O3 thin films. The LSSE response at cryogenic temperatures increases as a function of the external magnetic field until it approaches saturation. The response at given power and field exhibits a non-monotonic temperature dependence, with a pronounced peak that shifts toward higher temperatures as the field increases. Furthermore, the magnitude of the LSSE signal decreases consistently with increasing thickness, implying that the bulk SSE dominates any interfacial contribution. This negative correlation between the SSE and the thickness implies that the magnon energy relaxation length in V2O3 is shorter than the thickness of our thinnest film, 50 nm, consistent with the strong spin-lattice coupling in this material.

Volatile resistive switching in neuromorphic computing can be tuned by external stimuli such as temperature or electric-field. However, this type of switching is generally coupled to structural changes, resulting in slower reaction speed and higher energy consumption when incorporated into an electronic device. The vanadium dioxide (VO2), which has near room temperature metal-insulator transition (MIT), is an archetypical volatile resistive switching system. Here, we demonstrate an isostructural MIT in an ultrathin VO2 film capped with a photoconductive cadmium sulfide (CdS) layer. Transmission electron microscopy, resistivity experiments, and first-principles calculations show that the hole carriers induced by CdS photovoltaic effect are driving the MIT in rutile VO2. The insulating-rutile VO2 phase has been proved and can remain stable for hours. Our finding provides a new approach to produce purely electronically driven MIT in VO2, and widens its applications in fast-response, low-energy neuromorphic devices.

The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic excitations in magnetic insulators. Vanadium dioxide in its insulating low temperature phase is expected to lack magnetic degrees of freedom, as vanadium atoms are thought to form singlets upon dimerization of the vanadium chains. Instead, we find a paramagnetic SSE response in VO2 films that grows as the temperature decreases below 50 K. The field and temperature dependent SSE voltage is qualitatively consistent with a general model of paramagnetic SSE response and inconsistent with triplet spin transport. The microscopic nature of the magnetic excitations in VO2 requires further examination.

Interfacial effects between antiferromagnetic (AFM) and ferromagnetic (FM) materials have long been a center of magnetism studies. Aside from the exchange bias occurring at the AFM/FM interface, controlling the coercivity is another significant topic in magnetic recordings. The coercivity of FM materials is often determined through varying grain size, alloy composition, density of defects, etc., which is set during material growth and offers limited room for modification after growth. Hematite (α-Fe2O3) is an AFM material that undergoes a temperature-controlled spin-flip transition, the so-called Morin transition. This transition gives an extra degree of freedom making hematite an intriguing component to study the exchange coupling when interfaced with an FM material. In this work, changes in the magnetic properties of soft magnetic permalloy (Ni81Fe19, or Py) thin films grown on hematite were studied across the Morin transition. Surprisingly, these samples showed a remarkable change in coercivity during the Morin transition. We attribute this effect to the magnetic domain mixture of hematite during the Morin transition. Our findings present a novel method of controlling the coercivity of plain ferromagnetic thin films.

Vanadium sesquioxide (V2O3) is a strongly correlated electronic material that famously undergoes a triple coupled first-order transition where it transitions from a paramagnetic metal with a rhombohedral structure at high temperature to an antiferromagnetic insulator with a monoclinic structure. While several studies have used one of both electronic and structural transitions to control the properties of a heterostructure, evidence of magnetic coupling has notoriously yet to be found. In this paper, we report on a robust magnetic coupling between the antiferromagnetic (AFM) V2O3 and ferromagnetic (FM) Permalloy (Py) layers that results in a significant exchange bias and strain-induced coercivity enhancement. We provide a temperature and angle-dependent study of magnetic properties, which clearly indicates exchange bias at the AFM/FM interface that appears at the onset of the metal-insulator transition. The magnitude of the exchange bias is strongest when the field is applied along the [001] V2O3 crystallographic orientation which corresponds to an AFM spin configuration on the [110] surface. This opens the door to designing and implementing novel functionalities in transition metal oxide-based computing using the connection between magnetism and the metal-insulator transition.

While the connection between colossal magnetoresistance and phase separation in praseodymium-doped rareearth manganites has been well established, the underlying mechanisms enabling this connection still need to be fully understood. This paper thoroughly examines the phase separation in La5/8-xPrxCa3/8MnO3 (x = 0.40) using an integrated approach including magnetotransport, electron spin resonance, and magnetic susceptibility. We observed that the initially fluidlike magnetic phase separation at intermediate temperatures transforms into the solidlike phase separation via a glass transition at low temperatures. Magnetization dynamics measurements revealed that this transition is concurrent with the reentrance of the paramagnetic phase. Our findings offer a perspective on the freezing of phase separation fluidlike behavior resulting in the advent of the low-temperature solidlike state, contributing to a deeper understanding of the temperature-dependent behaviors of praseodymiumdoped rare-earth manganites.

Fast and sensitive phase transition detection is one of the most important requirements for new material synthesis and characterization. For solid-state samples, microwave absorption techniques can be employed for detecting phase transitions because it simultaneously monitors changes in electronic and magnetic properties. However, microwave absorption techniques require expensive high-frequency microwave equipment and bulky hollow cavities. Due to size limitations in conventional instruments, it is challenging to implement these cavities inside a laboratory cryostat. In this work, we designed and built a susceptometer that consists of a small helical cavity embedded into a custom insert of a commercial cryostat. This cavity resonator operated at sub-GHz frequencies is extremely sensitive to changes in material parameters, such as electrical conductivity, magnetization, and electric and magnetic susceptibilities. To demonstrate its operation, we detected superconducting phase transition in Nb and YBa2Cu3O7-δ, metal-insulator transitions in V2O3, ferromagnetic transition in Gd, and magnetic field induced transformation in meta magnetic NiCoMnIn single crystals. This high sensitivity apparatus allows the detection of trace amounts of materials (10-9-cc) undergoing an electromagnetic transition in a very broad temperature (2-400 K) and magnetic field (up to 90 kOe) ranges.

Machine learning has experienced unprecedented growth in recent years, often referred to as an "artificial intelligence revolution." Biological systems inspire the fundamental approach for this new computing paradigm: using neural networks to classify large amounts of data into sorting categories. Current machine-learning schemes implement simulated neurons and synapses on standard computers based on a von Neumann architecture. This approach is inefficient in energy consumption, and thermal management, motivating the search for hardware-based systems that imitate the brain. Here, the present state of thermal management of neuromorphic computing technology and the challenges and opportunities of the energy-efficient implementation of neuromorphic devices are considered. The main features of brain-inspired computing and quantum materials for implementing neuromorphic devices are briefly described, the brain criticality and resistive switching-based neuromorphic devices are discussed, the energy and electrical considerations for spiking-based computation are presented, the fundamental features of the brain's thermal regulation are addressed, the physical mechanisms for thermal management and thermoelectric control of materials and neuromorphic devices are analyzed, and challenges and new avenues for implementing energy-efficient computing are described.

The strongly correlated material La0.7Sr0.3MnO3 (LSMO) exhibits metal-to-insulator and magnetic transition near room temperature. Although the physical properties of LSMO can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. To include additional flexibility and tunability, we developed a hybrid optoelectronic heterostructure that uses photocarrier injection from cadmium sulfide (CdS) to an LSMO layer to change its electrical conductivity. LSMO exhibits no significant optical response, however, the CdS/LSMO heterostructures show an enhanced conductivity, with ~ 37 % resistance drop, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect of a 9 T magnetic field in pure LSMO. Surprisingly, the optical and magnetic responses of CdS/LSMO heterostructures are decoupled and exhibit different effects when both stimuli are applied. This unexpected behavior shows that heterostructuring strongly correlated oxides may require a new understanding of the coupling of physical properties across the transitions and provide the means to implement new functionalities.

Manipulation of antiferromagnetic (AFM) materials as active elements provides a crucial combination of electrical, thermal, and magnetic properties for spintronics. This study shows how the spin current generated in heavy metal is induced by spin-orbit torque into an adjacent AFM insulator. The bulk unpinned spins of the AFM layer drive a spin current that is transmitted to the top ferromagnet. This mechanism allows the electrical control of the exchange bias, coercive field, and blocking temperature of the system. Further support is provided by a model calculation that quantitatively describes the effect of the spin current injection into the AFM.

Recent discoveries of high-Tc superconducting hydrides at high pressure have opened up new possibilities for improving the superconducting transition temperature (Tc) using hydrogenation. Here, a unique thermodynamic approach is developed based on the concept of rapid thermal annealing and is adopted to study the hydrogenation effect on the superconducting properties of Nb/Pd bilayer films. Below 300 degrees C annealing temperatures, the Tc is enhanced from 8.77 to 9.06 K and is correlated with the compression of the Nb unit cell. A weak lattice expansion occurs at higher annealing temperatures, and the Tc is gradually suppressed. Furthermore, the rapid thermal hydrogenation affects the Tc differently depending on the substrate on which the Nb/Pd bilayers are grown. For the c-cut Al2O3 substrate, the Tc reduction starts at 250 degrees C, while for the r-cut Al2O3, this occurs at 350 degrees C. We associate these features with the elastic behavior of Nb film upon hydrogenation. A proposed model shows that the increase of Tc could be caused by the compressive stress related to the rapid nucleation of hydrides or the removal of impurities. Our discoveries provide insights into how superconductivity can be manipulated by rapid thermal hydrogenation.

Low-field microwave absorption techniques are ultrasensitive, nondestructive methods for probing electric and magnetic properties of solids. Nonresonant low-field microwave absorption techniques such as magnetic field modulated microwave spectroscopy (MFMMS) can easily detect electromagnetic phase transitions in minute and inhomogeneous samples. While this technique can easily and almost selectively identify superconducting transitions, magnetic phase transitions produce more varied responses. Here, we present a technique to investigate the electric and magnetic properties of a sample with complex electromagnetic responses. This technique involves taking a series of magnetic hysteresis loops and magnetoresistance measurements. These can be compared to MFMMS data to identify features having electric or magnetic origin. This approach is applied to magnetite (Fe3O4), which possesses an electric, magnetic, and structural phase transition across its Verwey transition. By measuring high-quality Fe3O4 thin films in MFMMS and complementary techniques, the previously inscrutable MFMMS signal is analyzed. Furthermore, a model of the MFMMS signal can be calculated from the magnetic and electric data, which reproduces most of the features of the experimentally obtained MFMMS signal. This technique broadens the capabilities of MFMMS beyond the detection of superconductors.

We report on a temperature-driven reversible change of the in-plane magnetic anisotropy of V$_2$O$_3$/Ni bilayers. This is caused by the rhombohedral to monoclinic structural phase transition of V$_2$O$_3$ at $T_C$ = 160 K. The in-plane magnetic anisotropy is uniaxial above $T_C$, but as the bilayer is cooled through the structural phase transition, a secondary magnetic easy axis emerges. Ferromagnetic resonance measurements show that this change in magnetic anisotropy is reversible with temperature. We identify two structural properties of the V$_2$O$_3$/Ni bilayers affecting the in-plane magnetic anisotropy: (1) a growth-induced uniaxial magnetic anisotropy associated with step-like terraces in the bilayer microstructure and (2) a low-temperature strain-induced biaxial anisotropy associated with the V$_2$O$_3$ structural phase transition. Magnetoresistance measurements corroborate the change in magnetic anisotropy across the structural transition and suggest that the negative magnetostriction of Ni leads to the emergence of a strain-induced easy-axis. This shows that a temperature-dependent structural transition in V$_2$O$_3$ may be used to tune the magnetic anisotropy in an adjacent ferromagnetic thin film.

We have studied the influence of controlled defects on the magnetic properties of La0.67Sr0.33MnO3 (LSMO) thin films. We used 100 nm thick epitaxial LSMO films, which were grown on SrTiO3 substrates and introduced stoichiometry changes and defects using oxygen ion irradiation through nanoporous alumina masks. Oxygen irradiation through the mask creates cascades of defects in spatially-modulated regions in the plane of the LSMO film. The magnetic properties of the samples were compared before and after the irradiation. We have found that irradiation reduces the ferromagnetic ordering temperature, decreases the total magnetization, enhances the coercivity, and induces exchange bias below 50 K. The coercivity enhancement is dramatically higher below 50 K and can be associated with exchange bias. These results can be explained by the formation of Mn-rich antiferromagnetic or ferrimagnetic phases within the bombarded regions that are exchange coupled to the non irradiated ferromagnetic host.

Controlling the magnetic ground states at the nanoscale is a long-standing basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shaped nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (tRu) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling dominates and forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at which AFM coupling dominates, the magnetic state in remanence is a double pole structure. Our results showed that the interlayer exchange coupling interaction remains finite even at 4 nm Ru thickness. The magnetic states in remanence, observed by magnetic force microscopy (MFM), are in good agreement with corresponding hysteresis loops obtained by the magneto-optic Kerr effect (MOKE) and micromagnetic simulations.

Effect of growth-induced uniaxial anisotropy and thickness of non-magnetic Cu spacer between the ferromagnetic/antiferromagnetic Co/CoO multilayers on the exchange bias, coercive field, and magnetic anisotropy have been studied in 10?300 K temperature range. Co(8 nm)/Cu(tCu)/CoO(4 nm) (tCu = 0.6?2.6 nm) multilayers on Si (100) substrates were prepared using the oblique-angle magnetron sputtering deposition to induce growth induced magnetic anisotropy. The top Co layer was allowed to be naturally oxidized to obtain antiferromagnetic CoO. The films show a strong uniaxial in-plane anisotropy determined by ferromagnetic resonance measurements. Magnetization measurements for both easy and hard magnetization directions revealed that the exchange bias and coercive fields decrease by increasing Cu thickness. The existence of strong exchange bias across the thickest Cu layer shows the long-range character of exchange and dipolar interactions at 10 K. Dependence of Cu spacer thickness shows an oscillatory behavior that can be ascribed to the competitions between Ruderman?Kittel?Kasuya?Yosida (RKKY) and dipolar interactions.