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The exchange coupling between a hard magnetic layer MnBi and a soft magnetic layer Co-Fe has been found to significantly improve the maximum energy product. In this work, the spin structure of exchange-coupled MnBi:Co-Fe bilayers is experimentally investigated by X-ray magnetic circular dichroism (XMCD) and polarized neutron reflectometry (PNR). We find that the out-of-plane magnetization reversal process of the MnBi:Co-Fe bilayer structure involves formation of a curling-type twisting of the magnetization in the film plane at low or intermediate reversal fields. Micromagnetic simulations are further performed to provide a detailed view of the spins at the curling center. Reminiscent of chiral spin structures known as spin bobbers, this curling in the exchange-coupled hard-soft magnetic bilayers is a new type of skyrmionic spin structure and worth further investigation.

Magnetic skyrmions have captivated physicists due to their topological nature and novel physical properties. In addition, skyrmions hold significant promise for future information technologies. A key barrier to realizing skyrmion-based devices has been stabilizing these spin structures under ambient conditions. In this paper, we demonstrate that the tunable magnetic properties of amorphous Fe/Gd mulitlayers enable the formation of skyrmion lattices which are stable over a large temperature and magnetic field parameter space, including room temperature and zero magnetic field. These skyrmions, having a hybrid nature displaying both Bloch-type and Néel-type characteristics, are stabilized by dipolar interactions rather than Dzyaloshinskii-Moriya interactions, typically considered a requirement for the generation of skyrmions. Small angle neutron scattering (SANS) was used in combination with soft x-ray microscopy to provide a unique, multiscale probe of the local and long-range order of these structures. The hexagonal lattice seen in SANS results from the hybrid skyrmion picture obtained with micromagnetic simulations. These results identify a pathway to engineer controllable skyrmion phases in thin film geometries which are stable at ambient conditions.

Control of materials through custom design of ionic distributions represents a powerful new approach to develop future technologies ranging from spintronic logic and memory devices to energy storage. Perovskites have shown particular promise for ionic devices due to their high ion mobility and sensitivity to chemical stoichiometry. In this work, we demonstrate a solid-state approach to control of ionic distributions in (La,Sr)CoO3 thin films. Depositing a Gd capping layer on the perovskite film, oxygen is controllably extracted from the structure, up to 0.5 O/u.c. throughout the entire 36-nm thickness. Commensurate with the oxygen extraction, the Co valence state and saturation magnetization show a smooth continuous variation. In contrast, magnetoresistance measurements show no change in the magnetic anisotropy and a rapid increase in the resistivity over the same range of oxygen stoichiometry. These results suggest significant phase separation, with metallic ferromagnetic regions and oxygen-deficient, insulating, nonferromagnetic regions, forming percolated networks. Indeed, x-ray diffraction identifies oxygen-vacancy ordering, including transformation to a brownmillerite crystal structure. The unexpected transformation to the brownmillerite phase at ambient temperature is further confirmed by high-resolution scanning transmission electron microscopy which shows significant structural - and correspondingly chemical - phase separation. This work demonstrates room-temperature ionic control of magnetism, electrical resistivity, and crystalline structure in a 36-nm-thick film, presenting opportunities for ionic devices that leverage multiple material functionalities.

We demonstrate substantial enhancement in the energy product of MnBi-based magnets by forming robust ferromagnetic exchange coupling between a MnBi layer and a thin CoFe layer in a unique perpendicular coupling configuration, which provides increased resistance to magnetization reversal. The measured nominal energy product of 172kJ/m3 at room temperature is the largest value experimentally attained for permanent magnets free of expensive raw materials. Our finding shows that exchange-coupled MnBi/CoFe magnets are a viable option for pursuing rare-earth-free magnets with energy products approaching those containing rare-earth elements.

We demonstrate reversible control of magnetization and anisotropy in La0.67Sr0.33MnO3 films through interfacial oxygen migration. Gd metal capping layers deposited onto La0.67Sr0.33MnO3 leach oxygen from the film through a solid-state redox reaction to form porous Gd2O3. X-ray absorption and polarized neutron reflectometry measurements show Mn valence alterations consistent with high oxygen vacancy concentrations, resulting in suppressed magnetization and increased coercive fields. Effects of the oxygen migration are observed both at the interface and also throughout the majority of a 40 nm thick film, suggesting extensive diffusion of oxygen vacancies. After Gd-capped La0.67Sr0.33MnO3 is exposed to atmospheric oxygen for a prolonged period of time, oxygen diffuses through the Gd2O3 layer and the magnetization of the La0.67Sr0.33MnO3 returns to the uncapped value. These findings showcase perovskite heterostructures as ideal candidates for developing functional interfaces through chemically-induced oxygen migration.

Magnetization reversal mechanisms and depth-dependent magnetic profile have been investigated in Co/Pd thin films magnetron-sputtered under continuously varying pressure with opposite deposition orders. For samples grown under increasing pressure, magnetization reversal is dominated by domain nucleation, propagation, and annihilation; an anisotropy gradient is effectively established, along with a pronounced depth-dependent magnetization profile. However, in films grown under decreasing pressure, disorders propagate vertically from the bottom high-pressure region into the top low-pressure region, impeding domain wall motion and forcing magnetization reversal via rotation; depth-dependent magnetization varies in an inverted order, but the spread is much suppressed. © 2014 AIP Publishing LLC.

The role of the capping material in stabilizing a thin ferromagnetic layer at the interface between a FeRh film and cap in the nominally antiferromagnetic phase at room temperature was studied by x-ray magnetic circular dichroism in photoemission electron microscopy and polarized neutron reflectivity. These techniques were used to determine the presence or absence of interfacial ferromagnetism (FM) in films capped with different oxides and metals. Chemically stable oxide caps do not generate any interfacial FM while the effect of metallic caps depends on the element, showing that interfacial FM is due to metallic interdiffusion and the formation of a ternary alloy with a modified antiferromagnetic to ferromagnetic transition temperature.

Magnetic skyrmions are topologically protected, nanoscale whirls of the spin configuration that tend to form hexagonally ordered arrays. As a topologically non-trivial structure, the nucleation and annihilation of the skyrmion, as well as the interaction between skyrmions, varies from conventional magnetic systems. Recent works have suggested that the ordering kinetics in these materials occur over millisecond or longer timescales, which is unusually slow for magnetic dynamics. The current work investigates the skyrmion ordering kinetics, particularly during lattice formation and destruction, using time-resolved small angle neutron scattering (TR-SANS). Evaluating the time-resolved structure and intensity of the neutron diffraction pattern reveals the evolving real-space structure of the skyrmion lattice and the timeframe of the formation. Measurements were performed on three prototypical skyrmion materials: MnSi, (Fe,Co)Si, and Cu 2 OSeO 3 . To probe lattice formation and destruction kinetics, the systems were prepared in the stable skyrmion state, and then a square-wave magnetic field modulation was applied. The measurements show that the skyrmions quickly form ordered domains, with a significant distribution in lattice parameters, which then converge to the final structure; the results confirm the slow kinetics, with formation times between 10 ms and 99 ms. Comparisons are made between the measured formation times and the fundamental material properties, suggesting the ordering temperature, saturation magnetization and magnetocrystalline anisotropy may be driving the timeframes. Micromagnetic simulations were also performed and support a scaling of the kinetics with sample volume, a behavior which is caused by the reconciling of misaligned domains.

Magnetic skyrmions exhibit unique, technologically relevant pseudo-particle behaviors which arise from their topological protection, including well-defined, 3D dynamic modes that occur at microwave frequencies. During dynamic excitation, spin waves are ejected into the interstitial regions between skyrmions, creating the magnetic equivalent of a turbulent sea. However, since the spin waves in these systems have a well-defined length scale, and the skyrmions are on an ordered lattice, ordered structures from spin-wave interference can precipitate from the chaos. This work uses small-angle neutron scattering (SANS) to capture the dynamics in hybrid skyrmions and investigate the spin-wave structure. Performing simultaneous ferromagnetic resonance and SANS, the diffraction pattern shows a large increase in low-angle scattering intensity, which is present only in the resonance condition. This scattering pattern is best fit using a mass fractal model, which suggests the spin waves form a long-range fractal network. The fractal structure is constructed of fundamental units with a size that encodes the spin-wave emissions and are constrained by the skyrmion lattice. These results offer critical insights into the nanoscale dynamics of skyrmions, identify a new dynamic spin-wave fractal structure, and demonstrate SANS as a unique tool to probe high-speed dynamics., (© 2023 Wiley-VCH GmbH.)

Large spin-orbit torques (SOTs) generated by topological materials and heavy metals interfaced with ferromagnets are promising for next-generation magnetic memory and logic devices. SOTs generated from y spin originating from spin Hall and Edelstein effects can realize field-free magnetization switching only when the magnetization and spin are collinear. Here we circumvent the above limitation by utilizing unconventional spins generated in a MnPd 3 thin film grown on an oxidized silicon substrate. We observe conventional SOT due to y spin, and out-of-plane and in-plane anti-damping-like torques originated from z spin and x spin, respectively, in MnPd 3 /CoFeB heterostructures. Notably, we have demonstrated complete field-free switching of perpendicular cobalt via out-of-plane anti-damping-like SOT. Density functional theory calculations show that the observed unconventional torques are due to the low symmetry of the (114)-oriented MnPd 3 films. Altogether our results provide a path toward realization of a practical spin channel in ultrafast magnetic memory and logic devices., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Advances in the synthesis and characterization of colloidal magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behavior, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic system, for example, achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a non-ferromagnetic dead shell indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structures of Ni NP ensembles via unpolarized, half-polarized, and fully polarized small-angle neutron scattering (SANS) measurements and analyses coupled with X-ray diffraction and magnetometry. The postulated nanometric magnetic dead shell is not only detected but conclusively identified as a non-ferromagnetic Ni phosphide derived from the trioctylphosphine commonly used in hot-injection colloidal NP syntheses. The phosphide shell thickness is tunable via synthesis temperature, falling to as little as 0.5 nm at 170 °C. Temperature- and magnetic field-dependent polarized SANS measurements additionally reveal essentially bulk-like ferromagnetism in the Ni core and negligible interparticle magnetic interactions, quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis-structure-property relationships in metallic magnetic NPs, point to a simple potential route to ligand-free stabilization, and highlight the power of the currently available suite of polarized SANS measurement and analysis capabilities for magnetic NP science and technology.

We have investigated the magnetic correlations among 7 nm iron oxide nanoparticles embedded in stretched silicone elastomers using polarized Small Angle Neutron Scattering (SANS). The magnetic nanoparticle (MNP)-elastomer composite can be stretched during experiments, and macroscopic deformations cause rearrangement of the iron oxide particles on the nanoscale. Polarized neutrons can be used to nondestructively probe the arrangement of magnetic nanoparticles before and after stretching, so that the relationship between applied stress and nanoscale magnetization can be interrogated. We find that stretching the MNP-elastomer composite past a certain threshold dramatically changes the structural and magnetic morphology of the system. The unstretched sample is modeled well by ~40 nm clusters of ~7 nm particles arranged in a hard sphere packing with a "volume fraction" parameter of 0.3. After the sample is stretched 3× its original size, however, the scattering data can be modeled by smaller, 16 nm clusters with a higher volume fraction of 0.4. We suggest that the effect can be understood by considering a stretching transformation on FCC-like crystallites of iron oxide nanoparticles embedded in an elastomeric medium., Competing Interests: Conflict of Interest The authors have no conflicts to disclose.

We study the assembly of magnetite nanoparticles in water-based ferrofluids in wetting layers close to silicon substrates with different functionalization without and with an out-of-plane magnetic field. For particles of nominal sizes 5, 15, and 25 nm, we extract density profiles from neutron reflectivity measurements. We show that self-assembly is only promoted by a magnetic field if a seed layer is formed at the silicon substrate. Such a layer can be formed by chemisorption of activated N -hydroxysuccinimide ester-coated nanoparticles at a (3-aminopropyl)triethoxysilane functionalized surface. Less dense packing is reported for physisorption of the same particles at a piranha-treated (strongly hydrophilic) silicon wafer, and no wetting layer is found for a self-assembled monolayer of octadecyltrichlorosilane (strongly hydrophobic) at the interface. We show that once the seed layer is formed and under an out-of-plane magnetic field further wetting layers assemble. These layers become denser with time, larger magnetic fields, higher particle concentrations, and larger moment of the nanoparticles.

Magnetic skyrmions have been the focus of intense research due to their unique qualities which result from their topological protections. Previous work on Cu 2 OSeO 3 , the only known insulating multiferroic skyrmion material, has shown that chemical substitution alters the skyrmion phase. We chemically substitute Zn, Ag, and S into powdered Cu 2 OSeO 3 to study the effect on the magnetic phase diagram. In both the Ag and the S substitutions, we find that the skyrmion phase is stabilized over a larger temperature range, as determined via magnetometry and small-angle neutron scattering (SANS). Meanwhile, while previous magnetometry characterization suggests two high temperature skyrmion phases in the Zn-substituted sample, SANS reveals the high temperature phase to be skyrmionic while we are unable to distinguish the other from helical order. Overall, chemical substitution weakens helical and skyrmion order as inferred from neutron scattering of the q ≈ 0.01 Å - 1 magnetic peak., Competing Interests: The authors report no conflicts of interest in this investigation.

We reveal the assembly of magnetite nanoparticles of sizes 5 nm, 15 nm and 25 nm from dilute water-based ferrofluids onto an amorphous magnetic template with out-of-plane anisotropy. From neutron reflectometry experiments we extract density profiles and show that the particles self-assemble into layers at the magnetic surface. The layers are extremely stable against cleaning and rinsing of the substrate. The density of the layers is determined by and increases with the remanent magnetic moment of the particles.

This work reports the ferromagnetism of topological insulator, (Bi,Sb) 2 Te 3 (BST), with a Curie temperature of approximately 120 K induced by magnetic proximity effect (MPE) of an antiferromagnetic CrSe. The MPE was shown to be highly dependent on the stacking order of the heterostructure, as well as the interface symmetry: Growing CrSe on top of BST results in induced ferromagnetism, while growing BST on CrSe yielded no evidence of an MPE. Cr-termination in the former case leads to double-exchange interactions between Cr 3+ surface states and Cr 2+ bulk states. This Cr 3+ -Cr 2+ exchange stabilizes the ferromagnetic order localized at the interface and magnetically polarizes the BST Sb band. In contrast, Se-termination at the CrSe/BST interface yields no detectable MPE. These results directly confirm the MPE in BST films and provide critical insights into the sensitivity of the surface state., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

In this article we review the process by which magnetite nanoparticles self-assemble onto solid surfaces. The focus is on neutron reflectometry studies providing information on the density and magnetization depth profiles of buried interfaces. Specific attention is given to the near-interface "wetting" layer and to examples of magnetite nanoparticles on a hydrophilic silicon crystal, one coated with (3-Aminopropyl)triethoxysilane, and finally, one with a magnetic film with out-of-plane magnetization.

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa 2 Cu 3 O 7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.

We report observation of a radial dependence in the magnetic anisotropy of epitaxially strained CoFe 2 O 4 nanopillars in a BaTiO 3 matrix. This archetypal example of a multiferroic heterostructure with a self-assembling three-dimensional architecture possesses significant out-of-plane uniaxial magnetic anisotropy. The anisotropy originates from the large magnetostriction of CoFe 2 O 4 and the state of stress within the nanocomposite. Magnetometry suggests the existence of two magnetic phases with different anisotropies. Micromagnetic simulations of a core-shell magnetic anisotropy qualitatively reproduce features of the magnetic hysteresis and elucidate the magnetization reversal mechanism: The magnetization initially reorients within the pillar core, followed by that of the shell. This is consistent with polarized small-angle neutron scattering which can be described by a CoFe 2 O 4 magnetization that is nonuniform on nanometer length scales. As the length scale of inhomogeneity of the magnetic anisotropy is similar to estimates of the relaxation of the displacement field from the CoFe 2 O 4 -BaTiO 3 interface, stress and its influence on structure provide an important route to new functionality of vertically aligned nanopillars for applications in low-power memory, computing, and sensing.

We report the effects of nitrogen diffusion on exchange bias in MnN/CoFeB heterostructures as a function of MnN thickness and field-annealing temperature. We find that competing effects occur in which high-temperature annealing enhances exchange bias in heterostructures with thick MnN through improved crystallinity, but in thinner samples this annealing ultimately eliminates the exchange bias due to nitrogen deficiency. Using polarized neutron reflectometry and magnetic x-ray spectroscopy, we directly observe increasing amounts of nitrogen migration from MnN into the underlying Ta seed layer with increased annealing temperature. In heterostructures with thin MnN layers, the resulting nitrogen deficiency becomes significant enough to alter the antiferromagnetic state before the Ta seed layer is nitrogen saturated. Furthermore, we observe intermixing at the MnN/CoFeB interface which is attributed to the nitrogen deficiency creating vacancies in the MnN layer after annealing in a field. This intermixing of Mn with Co and Fe is not believed to be the cause for loss of exchange bias when the MnN layer is too thin but is instead a secondary effect due to increased vacancies after nitrogen migration.

Here we report on the microstructural factors influencing the formation of the interfacial exchange bias effect in three-dimensional transition-metal-based nanocomposite systems, with relevance to permanent magnet applications. Bulk phase-separated nanocomposites consisting of the ferromagnetic α -Fe and metastable antiferromagnetic γ - Fe 70 Mn 30 phases exhibit a notable low-temperature exchange bias and substantial coercivity ( H ex = 24.6 kA / m , H C = 95.7 kA / m ) as well as a near room-temperature blocking temperature. Structural investigation by synchrotron X-ray diffraction, neutron scattering, and transmission electron microscopy confirm that the ferromagnetic α -Fe phase nucleates as small precipitates ( d ≈ 50 nm ) at the grain boundaries of the antiferromagnetic γ - Fe 70 Mn 30 grains ( d = 360 - 740 nm ) and grows anisotropically upon heat treatment, resulting in an elliptical geometry. These results indicate that optimization of the exchange bias effect in bulk nanocomposite systems may be achieved through maximizing the surface-to-volume ratio of ferromagnetic precipitates in an antiferromagnetic matrix, enhancing the magnetocrystalline anisotropy of the antiferromagnetic phase to facilitate interfacial pinning and ensuring a balanced distribution of the ferromagnetic and antiferromagnetic phases. This work further clarifies critical factors influencing the formation of an exchange bias in an inexpensive transition-metal-based bulk nanocomposite system with potential for scalable production., Competing Interests: The authors declare no competing financial interest.

Magnetic iron oxide nanoparticles (MIONs) have established a niche as a nanomedicine platform for diagnosis and therapy, but they present a challenging surface for ligand functionalization which limits their applications. On the other hand, coating MIONs with another material such as gold to enhance these attachments introduces other complications. Incomplete coating may expose portions of the iron oxide core, or the coating process may alter their magnetic properties. We describe synthesis and characterization of iron oxide/silica/gold core-shell nanoparticles to elucidate the effects of a silica-gold coating process and its impact on the resulting performance. In particular, small angle neutron scattering reveals silica intercalates between iron oxide crystallites that form the dense core, likely preserving the magnetic properties while enabling formation of a continuous gold shell. The synthesized silica-gold-coated MIONs demonstrate magnetic heating properties consistent with the original iron oxide core, with added x-ray contrast for imaging and laser heating.

We show the effects of a time-invariant magnetic field on the physical structure and magnetic properties of a colloid comprising 44 nm diameter magnetite magnetic nanoparticles, with a 24 nm dextran shell, in water. Structural ordering in this colloid parallel to the magnetic field occurs simultaneously with the onset of a colloidal uniaxial anisotropy. Further increases in the applied magnetic field cause the nanoparticles to order perpendicular to the field, producing unexpected colloidal unidirectional and trigonal anisotropies. This magnetic behavior is distinct from the cubic magnetocrystalline anisotropy of the magnetite and has its origins in the magnetic interactions among the mobile nanoparticles within the colloid. Specifically, these field-induced anisotropies and colloidal rearrangements result from the delicate balance between the magnetostatic and steric forces between magnetic nanoparticles. These magnetic and structural rearrangements are anticipated to influence applications that rely upon time-dependent relaxation of the magnetic colloids and fluid viscosity, such as magnetic hyperthermia and shock absorption.

Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe 3 O 4 /Mn x Fe 3-x O 4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.

This article describes the three-dimensional self-assembly of monodisperse colloidal magnetite nanoparticles (NPs) from a dilute water-based ferrofluid onto a silicon surface and the dependence of the resultant magnetic structure on the applied field. The NPs assemble into close-packed layers on the surface followed by more loosely packed ones. The magnetic field-dependent magnetization of the individual NP layers depends on both the rotational freedom of the layer and the magnetization of the adjacent layers. For layers in which the NPs are more free to rotate, the easy axis of the NP can readily orient along the field direction. In more dense packing, free rotation of the NPs is hampered, and the NP ensembles likely build up quasi-domain states to minimize energy, which leads to lower magnetization in those layers. Detailed analysis of polarized neutron reflectometry data together with model calculations of the arrangement of the NPs within the layers and input from small-angle scattering measurements provide full characterization of the core/shell NP dimensions, degree of chaining, arrangement of the NPs within the different layers, and magnetization depth profile.

Inelastic neutron scattering is utilized to directly measure inter-nanoparticle spin waves, or magnons, which arise from the magnetic coupling between 8.4 nm ferrite nanoparticles that are self-assembled into a close-packed lattice, yet are physically separated by oleic acid surfactant. The resulting dispersion curve yields a physically-reasonable, non-negative energy gap only when the effective Q is reduced by the inter-particle spacing. This Q renormalization strongly indicates that the dispersion is a collective excitation between the nanoparticles, rather than originating from within individual nanoparticles. Additionally, the observed magnons are dispersive, respond to an applied magnetic field, and display the expected temperature-dependent Bose population factor. The experimental results are well explained by a limited parameter model which treats the three-dimensional ordered, magnetic nanoparticles as dipolar-coupled superspins.

Pulsed laser deposition films from Ba 2 FeMoO 6 (BFMO) targets onto SrTiO 3 [001] (STO) substrates have been reported previously to have non-zero magnetism at 300 K, a majority of magnetic ordering at 240 K that is less than the 370 K ordering temperature of polycrystalline BFMO, and suppressed saturation magnetization compared to polycrystalline BFMO. To interrogate these previously reported observations of BFMO on STO, we have used a combination of x-ray diffraction, atomic force microscopy, x-ray and neutron reflectivity, and x-ray photoelectron spectroscopy that shows inhomogeneities. The present results show off-stoichiometry on the A-site by incorporation of Sr from the substrate and on the B-site to have %Fe/%Mo > 1 by evolution of BaMoO 4 . There is an enhanced ordering temperature and magnetic response nearer to the SrTiO 3 interface compared to the air interface. Depth dependent strain and microstructure are needed to explain the magnetic response. Holistic considerations and implications are also discussed.

A comprehensive three-dimensional picture of magnetic ordering in high-density arrays of segmented FeGa/Cu nanowires is experimentally realized through the application of polarized small-angle neutron scattering. The competing energetics of dipolar interactions, shape anisotropy, and Zeeman energy in concert stabilize a highly tunable spin structure that depends heavily on the applied field and sample geometry. Consequently, we observe ferromagnetic and antiferromagnetic interactions both among wires and between segments within individual wires. The resulting magnetic structure for our nanowire sample in a low field is a fan with magnetization perpendicular to the wire axis that aligns nearly antiparallel from one segment to the next along the wire axis. Additionally, while the low-field interwire coupling is ferromagnetic, application of a field tips the moments toward the nanowire axis, resulting in highly frustrated antiferromagnetic stripe patterns in the hexagonal nanowire lattice. Theoretical calculations confirm these observations, providing insight into the competing interactions and resulting stability windows for a variety of ordered magnetic structures. These results provide a roadmap for designing high-density magnetic nanowire arrays for spintronic device applications.

Magnetic topological insulators such as Cr-doped (Bi,Sb) 2 Te 3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb) 2 Te 3 , we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.

Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3-the geometric ferroelectric with the greatest known planar rumpling-we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially-from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.

The emergence of complex new ground states at interfaces has been identified as one of the most promising routes to highly tunable nanoscale materials. Despite recent progress, isolating and controlling the underlying mechanisms behind these emergent properties remains among the most challenging materials physics problems to date. In particular, generating ferromagnetism localized at the interface of two nonferromagnetic materials is of fundamental and technological interest. Moreover, the ability to turn the ferromagnetism on and off would shed light on the origin of such emergent phenomena and is promising for spintronic applications. We demonstrate that ferromagnetism confined within one unit cell at the interface of CaRuO3 and CaMnO3 can be switched on and off by changing the symmetry of the oxygen octahedra connectivity at the boundary. Interfaces that are symmetry-matched across the boundary exhibit interfacial CaMnO3 ferromagnetism while the ferromagnetism at symmetry-mismatched interfaces is suppressed. We attribute the suppression of ferromagnetic order to a reduction in charge transfer at symmetry-mismatched interfaces, where frustrated bonding weakens the orbital overlap. Thus, interfacial symmetry is a new route to control emergent ferromagnetism in materials such as CaMnO3 that exhibit antiferromagnetism in bulk form.

Electric field control of magnetism provides a promising route towards ultralow power information storage and sensor technologies. The effects of magneto-ionic motion have been prominently featured in the modification of interface characteristics. Here, we demonstrate magnetoelectric coupling moderated by voltage-driven oxygen migration beyond the interface in relatively thick AlOx/GdOx/Co(15 nm) films. Oxygen migration and Co magnetization are quantitatively mapped with polarized neutron reflectometry under electro-thermal conditioning. The depth-resolved profiles uniquely identify interfacial and bulk behaviours and a semi-reversible control of the magnetization. Magnetometry measurements suggest changes in the microstructure which disrupt long-range ferromagnetic ordering, resulting in an additional magnetically soft phase. X-ray spectroscopy confirms changes in the Co oxidation state, but not in the Gd, suggesting that the GdOx transmits oxygen but does not source or sink it. These results together provide crucial insight into controlling magnetism via magneto-ionic motion, both at interfaces and throughout the bulk of the films.

Here we present polarized neutron reflectometry measurements exploring thin film heterostructures composed of a strongly correlated Mott state, GdTiO_{3}, embedded with SrTiO_{3} quantum wells. Our results reveal that the net ferromagnetism inherent to the Mott GdTiO_{3} matrix propagates into the nominally nonmagnetic SrTiO_{3} quantum wells and tracks the magnetic order parameter of the host Mott insulating matrix. Beyond a well thickness of 5 SrO layers, the magnetic moment within the wells is dramatically suppressed, suggesting that quenched well magnetism comprises the likely origin of quantum critical magnetotransport in this thin film architecture. Our data demonstrate that the interplay between proximate exchange fields and polarity-induced carrier densities can stabilize extended magnetic states within SrTiO_{3} quantum wells.

Ionic transport in metal/oxide heterostructures offers a highly effective means to tailor material properties via modification of the interfacial characteristics. However, direct observation of ionic motion under buried interfaces and demonstration of its correlation with physical properties has been challenging. Using the strong oxygen affinity of gadolinium, we design a model system of GdxFe1-x/NiCoO bilayer films, where the oxygen migration is observed and manifested in a controlled positive exchange bias over a relatively small cooling field range. The exchange bias characteristics are shown to be the result of an interfacial layer of elemental nickel and cobalt, a few nanometres in thickness, whose moments are larger than expected from uncompensated NiCoO moments. This interface layer is attributed to a redox-driven oxygen migration from NiCoO to the gadolinium, during growth or soon after. These results demonstrate an effective path to tailoring the interfacial characteristics and interlayer exchange coupling in metal/oxide heterostructures.

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.

New mechanisms for achieving direct electric field control of ferromagnetism are highly desirable in the development of functional magnetic interfaces. To that end, we have probed the electric field dependence of the emergent ferromagnetic layer at CaRuO_{3}/CaMnO_{3} interfaces in bilayers fabricated on SrTiO_{3}. Using polarized neutron reflectometry, we are able to detect the ferromagnetic signal arising from a single atomic monolayer of CaMnO_{3}, manifested as a spin asymmetry in the reflectivity. We find that the application of an electric field of 600 kV/m across the bilayer induces a significant increase in this spin asymmetry. Modeling of the reflectivity suggests that this increase corresponds to a transition from canted antiferromagnetism to full ferromagnetic alignment of the Mn^{4+} ions at the interface. This increase from 1 μ_{B} to 2.5-3.0 μ_{B} per Mn is indicative of a strong magnetoelectric coupling effect, and such direct electric field control of the magnetization at an interface has significant potential for spintronic applications.

Neutron reflectometry was used to study the assembly of magnetite nanoparticles in a water-based ferrofluid close to a silicon surface. Under three conditions, static, under shear and with a magnetic field, the depth profile is extracted. The particles have an average diameter of 11 nm and a volume density of 5% in a D2O-H2O mixture. They are surrounded by a 4 nm thick bilayer of carboxylic acid for steric repulsion. The reflectivity data were fitted to a model using a least square routine based on the Parratt formalism. From the scattering length density depth profiles the following behavior is concluded: the fits indicate that excess carboxylic acid covers the silicon surface and almost eliminates the water in the densely packed wetting layer that forms close to the silicon surface. Under constant shear the wetting layer persists but a depletion layer forms between the wetting layer and the moving ferrofluid. Once the flow is stopped, the wetting layer becomes more pronounced with dense packing and is accompanied by a looser packed second layer. In the case of an applied magnetic field the prolate particles experience a torque and align with their long axes along the silicon surface which leads to a higher particle density.

We report on the magnetic structure and ordering of hexagonal LuFeO_{3} films of variable thickness grown by molecular-beam epitaxy on YSZ (111) and Al_{2}O_{3} (0001) substrates. These crystalline films exhibit long-range structural uniformity dominated by the polar P6_{3}cm phase, which is responsible for the paraelectric to ferroelectric transition that occurs above 1000 K. Using bulk magnetometry and neutron diffraction, we find that the system orders into a ferromagnetically canted antiferromagnetic state via a single transition below 155 K regardless of film thickness, which is substantially lower than that previously reported in hexagonal LuFeO_{3} films. The symmetry of the magnetic structure in the ferroelectric state implies that this material is a strong candidate for linear magnetoelectric coupling and control of the ferromagnetic moment directly by an electric field.

The nature of near-surface spin canting within Fe3O4 nanoparticles is highly debated. Here we develop a neutron scattering asymmetry analysis which quantifies the canting angle to between 23° and 42° at 1.2 T. Simultaneously, an energy-balance model is presented which reproduces the experimentally observed evolution of shell thickness and canting angle between 10 and 300 K. The model is based on the concept of Td site reorientation and indicates that surface canting involves competition between magnetocrystalline, dipolar, exchange, and Zeeman energies.

Here it is demonstrated that multiple-energy, anomalous small-angle X-ray scattering (ASAXS) provides significant enhancement in sensitivity to internal material boundaries of layered nanoparticles compared with the traditional modeling of a single scattering energy, even for cases in which high scattering contrast naturally exists. Specifically, the material-specific structure of monodispersed Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles is determined, and the contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles with a diameter of 8.2 ± 0.2 nm consist of a core with a composition near Fe₃O₄ surrounded by a (Mn(x)Fe(1-x))₃O₄ shell with a graded composition, ranging from x ≈ 0.40 at the inner shell toward x ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe₃O₄ cores without a coating shell. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multiple-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.

We have found ferromagnetism in epitaxially grown superlattices of CaRuO(3)/CaMnO(3) that arises in one unit cell at the interface. Scanning transmission electron microscopy and electron energy loss spectroscopy indicate that the difference in magnitude of the Mn valence states between the center of the CaMnO(3) layer and the interface region is consistent with double exchange interaction among the Mn ions at the interface. Polarized neutron reflectivity and the CaMnO(3) thickness dependence of the exchange bias field together indicate that the interfacial ferromagnetism is only limited to one unit cell of CaMnO(3) at each interface. The interfacial moment alternates between the 1 μ(B)/interface Mn ion for even CaMnO(3) layers and the 0.5 μ(B)/interface Mn ion for odd CaMnO(3) layers. This modulation, combined with the exchange bias, suggests the presence of a modulating interlayer coupling between neighboring ferromagnetic interfaces via the antiferromagnetic CaMnO(3) layers.

We have observed long-range spin-triplet supercurrents in Josephson junctions containing ferromagnetic (F) materials, which are generated by noncollinear magnetizations between a central Co/Ru/Co synthetic antiferromagnet and two outer thin F layers. Here we show that the spin-triplet supercurrent is enhanced up to 20 times after our samples are subject to a large in-plane field. This occurs because the synthetic antiferromagnet undergoes a "spin-flop" transition, whereby the two Co layer magnetizations end up nearly perpendicular to the magnetizations of the two thin F layers. We report direct experimental evidence for the spin-flop transition from scanning electron microscopy with polarization analysis and from spin-polarized neutron reflectometry. These results represent a first step toward experimental control of spin-triplet supercurrents.

We demonstrate that delta doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AFM) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Theoretical consideration of these additional carriers shows that they cause a local enhancement of ferromagnetic double exchange with respect to AFM superexchange, resulting in local canting of the AFM spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy, and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AFM state as was predicted by de Gennes [Phys. Rev. 118, 141 (1960)] but had remained elusive.

A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.

One potential cancer treatment selectively deposits heat to the tumor through activation of magnetic nanoparticles inside the tumor. This can damage or kill the cancer cells without harming the surrounding healthy tissue. The properties assumed to be most important for this heat generation (saturation magnetization, amplitude and frequency of external magnetic field) originate from theoretical models that assume non-interacting nanoparticles. Although these factors certainly contribute, the fundamental assumption of 'no interaction' is flawed and consequently fails to anticipate their interactions with biological systems and the resulting heat deposition. Experimental evidence demonstrates that for interacting magnetite nanoparticles, determined by their spacing and anisotropy, the resulting collective behavior in the kilohertz frequency regime generates significant heat, leading to nearly complete regression of aggressive mammary tumors in mice.

We report the antiferromagnetic (AFM) interlayer exchange coupling between Ga0.97Mn0.03As ferromagnetic semiconductor layers separated by Be-doped GaAs spacers. Polarized neutron reflectivity reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers is AFM. When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by doped charge carriers.