Nanoscale Magnetic Arrays through Block Copolymer Templating of Polyoxometalates.

Magnetic nanoarrays promise to enable new energy-efficient computations based on spintronics or magnonics. In this work, we present a block copolymer-assisted strategy for fabricating ordered magnetic nanostructures on silicon and permalloy substrates. Block copolymer micelle-like structures were used as a template in which polyoxometalate (POM) clusters could assemble in an opal-like structure. A combination of microscopy and scattering techniques was used to confirm the structural and organizational features of the fabricated materials. The magnetic properties of these materials were investigated by polarized neutron reflectometry, nuclear magnetic resonance, and magnetometry measurements. The data show that a magnetic structural design was achieved and that a thin layer of patterned POMs strongly influenced an underlying permalloy layer. This work demonstrates that the bottom-up pathway is a potentially viable method for patterning magnetic substrates on a sub-100 nm scale, toward the magnetic nanostructures needed for spintronic or magnonic crystal devices.

Significantly Enhanced Room-Temperature Ferromagnetism in Multiferroic EuFeO3-δ Thin Films.

Regulating the magnetic properties of multiferroics lays the foundation for their prospective application in spintronic devices. Single-phase multiferroics, such as rare-earth ferrites, are promising candidates; however, they typically exhibit weak magnetism at room temperature (RT). Here, we significantly boosted the RT ferromagnetism of a representative ferrite, EuFeO3, by oxygen defect engineering. Polarized neutron reflectometry and magnetometry measurements reveal that saturation magnetization reaches 0.04 µB/Fe, which is approximately 5 times higher than its bulk phase. Combining the annular bright-field images with theoretical assessment, we unravel the underlying mechanism for magnetic enhancement, in which the decrease in Fe-O-Fe bond angles caused by oxygen vacancies (VO) strengthens magnetic interactions and tilts Fe spins. Furthermore, the internal relationship between magnetism and VO was established by illustrating how the magnetic structure and magnitude change with VO configuration and concentration. Our strategy for regulating magnetic properties can be applied to numerous functional oxide materials.

Topological Surface State Annihilation and Creation in SnTe/Crx(BiSb)2-xTe3 Heterostructures.

Topological surface states are a new class of electronic states with novel properties, including the potential for annihilation between surface states from two topological insulators at a common interface. Here, we report the annihilation and creation of topological surface states in the SnTe/Crx(BiSb)2-xTe3 (CBST) heterostructures as evidenced by magneto-transport, polarized neutron reflectometry, and first-principles calculations. Our results show that topological surface states are induced in the otherwise topologically trivial two-quintuple-layers thick CBST when interfaced with SnTe, as a result of the surface state annihilation at the SnTe/CBST interface. Moreover, we unveiled systematic changes in the transport behaviors of the heterostructures with respect to changing Fermi level and thickness. Our observation of surface state creation and annihilation demonstrates a promising way of designing and engineering topological surface states for dissipationless electronics.

Dimensional Control of Octahedral Tilt in SrRuO3 via Infinite-Layered Oxides.

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO3 (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO2 (SCO) layers. The infinite-layered CuO2 exhibits a structural transformation from "planar-type" to "chain-type" with reduced film thickness. Two orientations dramatically modify the polyhedral connectivity at the interface, thus altering the octahedral distortion of SRO. The local structural variation changes the spin state of Ru and orbital hybridization strength, leading to a significant change in the magnetoresistance and anomalous Hall resistivity. These findings could launch investigations into adaptive control of functionalities in quantum oxide heterostructures using oxygen coordination.

Migration Kinetics of Surface Ions in Oxygen-Deficient Perovskite During Topotactic Transitions.

Oxygen diffusivity and surface exchange kinetics underpin the ionic, electronic, and catalytic functionalities of complex multivalent oxides. Towards understanding and controlling the kinetics of oxygen transport in emerging technologies, it is highly desirable to reveal the underlying lattice dynamics and ionic activities related to oxygen variation. In this study, the evolution of oxygen content is identified in real-time during the progress of a topotactic phase transition in La0.7 Sr0.3 MnO3-δ epitaxial thin films, both at the surface and throughout the bulk. Using polarized neutron reflectometry, a quantitative depth profile of the oxygen content gradient is achieved, which, alongside atomic-resolution scanning transmission electron microscopy, uniquely reveals the formation of a novel structural phase near the surface. Surface-sensitive X-ray spectroscopies further confirm a significant change of the electronic structure accompanying the transition. The anisotropic features of this novel phase enable a distinct oxygen diffusion pathway in contrast to conventional observation of oxygen motion at moderate temperatures. The results provide insights furthering the design of solid oxygen ion conductors within the framework of topotactic phase transitions.

Reversible Control of Interfacial Magnetism through Ionic-Liquid-Assisted Polarization Switching.

The ability to control magnetism of materials via electric field enables a myriad of technological innovations in information storage, sensing, and computing. We use ionic-liquid-assisted ferroelectric switching to demonstrate reversible modulation of interfacial magnetism in a multiferroic heterostructure composed of ferromagnetic (FM) La0.8Sr0.2MnO3 and ferroelectric (FE) PbZr0.2Ti0.8O3. It is shown that ionic liquids can be used to persistently and reversibly switch a large area of a FE film. This is a prerequisite for polarized neutron reflectometry (PNR) studies that are conducted to directly probe magnetoelectric coupling of the FE polarization to the interfacial magnetization.

Syntropic spin alignment at the interface between ferromagnetic and superconducting nitrides.

The magnetic correlations at the superconductor/ferromagnet (S/F) interfaces play a crucial role in realizing dissipation-less spin-based logic and memory technologies, such as triplet-supercurrent spin-valves and 'π' Josephson junctions. Here we report the observation of an induced large magnetic moment at high-quality nitride S/F interfaces. Using polarized neutron reflectometry and DC SQUID measurements, we quantitatively determined the magnetization profile of the S/F bilayer and confirmed that the induced magnetic moment in the adjacent superconductor only exists below T C. Interestingly, the direction of the induced moment in the superconductors was unexpectedly parallel to that in the ferromagnet, which contrasts with earlier findings in S/F heterostructures based on metals or oxides. First-principles calculations verified that the unusual interfacial spin texture observed in our study was caused by the Heisenberg direct exchange coupling with constant J∼4.28 meV through d-orbital overlapping and severe charge transfer across the interfaces. Our work establishes an incisive experimental probe for understanding the magnetic proximity behavior at S/F interfaces and provides a prototype epitaxial 'building block' for superconducting spintronics.

Anchor Group Bottlebrush Polymers as Oil Additive Friction Modifiers.

Surface-tethered polymers have been shown to be an efficient lubrication strategy for boundary and mixed lubrication by providing a solvated film between solid surfaces. We have assessed the performance of various graft copolymers as friction modifier additives in oil and revealed important structure-property relationships for this application. The polymers consisted of an oil-soluble, grafted poly(lauryl acrylate) segment and a polar, linear poly(4-acryloylmorpholine) anchor group. Reversible addition-fragmentation chain transfer polymerization was used to access various architectures with control of the grafting density and position of the anchor group. Macrotribological studies displayed promising results with ≈50% reduction in friction coefficient at low polymer treatment rates. QCM-D experiments, neutron reflectometry, small-angle neutron scattering, and atomic force microscopy were used to gather detailed information on these polymers' surface adsorption characteristics, film structure, and solution behavior.

Atomic-Scale Modulation of Synthetic Magnetic Order in Oxide Superlattices.

Atomic-scale precision control of magnetic interactions facilitates a synthetic spin order useful for spintronics, including advanced memory and quantum logic devices. Conventional modulation of synthetic spin order has been limited to metallic heterostructures that exploit Ruderman-Kittel-Kasuya-Yosida interaction through a nonmagnetic metallic spacer; however, they face issues arising from Joule heating and/or electric breakdown. The practical realization and observation of a synthetic spin order across a nonmagnetic insulating spacer will lead to the development of spin-related devices with a completely different concept. Herein, the atomic-scale modulation of the synthetic spiral spin order in oxide superlattices composed of ferromagnetic metal and nonmagnetic insulator layers is reported. The atomically controlled superlattice exhibits an oscillatory magnetic behavior, representing the existence of a spiral spin structure. Depth-sensitive polarized neutron reflectometry evidences modulated spiral spin structures as a function of the nonmagnetic insulator layer thickness. Atomic-scale customization of the spin state can move the field one step further to actual spintronic applications.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites.

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO3 (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO2 (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO6 octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 µB /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

Optically Induced Static Magnetization in Metal Halide Perovskite for Spin-Related Optoelectronics.

Understanding the feasibility to couple semiconducting and magnetic properties in metal halide perovskites through interface design opens new opportunities for creating the next generation spin-related optoelectronics. In this work, a fundamentally new phenomenon of optically induced magnetization achieved by coupling photoexcited orbital magnetic dipoles with magnetic spins at perovskite/ferromagnetic interface is discovered. The depth-sensitive polarized neutron reflectometry combined with in situ photoexcitation setup, constitutes key evidence of this novel effect. It is demonstrated that a circularly polarized photoexcitation induces a stable magnetization signal within the depth up to 7.5 nm into the surface of high-quality perovskite (MAPbBr3) film underneath a ferromagnetic cobalt layer at room temperature. In contrast, a linearly polarized light does not induce any detectable magnetization in the MAPbBr3. The observation reveals that photoexcited orbital magnetic dipoles at the surface of perovskite are coupled with the spins of the ferromagnetic atoms at the interface, leading to an optically induced magnetization within the perovskite's surface. The finding demonstrates that perovskite semiconductor can be bridged with magnetism through optically controllable method at room temperature in this heterojunction design. This provides the new concept of utilizing spin and orbital degrees of freedom in new-generation spin-related optoelectronic devices.

Electrically Enhanced Exchange Bias via Solid-State Magneto-ionics.

Electrically induced ionic motion offers a new way to realize voltage-controlled magnetism, opening the door to a new generation of logic, sensor, and data storage technologies. Here, we demonstrate an effective approach to magneto-ionically and electrically tune the exchange bias in Gd/Ni1-xCoxO thin films (x = 0.50 and 0.67), where neither of the layers alone is ferromagnetic at room temperature. The Gd capping layer deposited onto antiferromagnetic Ni1-xCoxO initiates a solid-state redox reaction that reduces an interfacial region of the oxide to ferromagnetic NiCo. An exchange bias is established after field cooling (FC), which can be enhanced by up to 35% after a voltage conditioning and subsequently reset with a second FC. These effects are caused by the presence of an interfacial ferromagnetic NiCo layer, which further alloys with the Gd layer upon FC and voltage application, as confirmed by electron microscopy and polarized neutron reflectometry studies. These results highlight the viability of the solid-state magneto-ionic approach to achieve electric control of exchange bias, with potential for energy-efficient magneto-ionic devices.

Controlling Magnetization Vector Depth Profiles of La0.7Sr0.3CoO3/La0.7Sr0.3MnO3 Exchange Spring Bilayers via Interface Reconstruction.

The La0.7Sr0.3CoO3-δ/La0.7Sr0.3MnO3-δ (LSCO/LSMO) bilayer system is an ideal perovskite oxide platform for investigating interface reconstruction and its effect on their magnetic properties. Previous studies have shown that LSCO can separate into magnetic sublayers, which possess distinct trends as the total LSCO thickness increases. In this study, we used polarized neutron reflectometry to quantify changes in the magnetic and chemical depth profiles, and it confirms the formation of ∼12 Å-thick interfacial LSCO and LSMO layers, characterized by a decreased nuclear scattering length density compared to the bulk of the layers. This decrease is attributed to the combined effects of oxygen vacancy formation and interfacial charge transfer, which lead to magnetically active Co2+ ions with ionic radii larger than the Co3+/Co4+ ions typically found in bulk LSCO or single-layer films. The interfacial magnetization values, as well as Co2+ ion and oxygen vacancy concentrations, depend strongly on the LSCO layer thickness. These results highlight the sensitive interplay of the cation valence states, oxygen vacancy concentration, and magnetization at interfaces in perovskite oxide multilayers, demonstrating the potential to tune their functional properties via careful design of their structure.

Magnetic Structure of Ion-Beam Imprinted Stripe Domains Determined by Neutron Scattering.

We present a detailed analysis of the in-plane magnetic vector configuration in head-to-head/tail-to-tail stripe domain patterns of nominal 5 µm width. The patterns have been created by He-ion bombardment induced magnetic patterning of a CoFe/IrMn3 exchange bias thin-film system. Quantitative information about the chemical and magnetic structure is obtained from polarized neutron reflectometry (PNR) and off-specular scattering (OSS). The technique provides information on the magnetic vector orientation and magnitude along the lateral coordinate of the sample, as well as the chemical and magnetic layer structure as a function of depth. Additional sensitivity to magnetic features is obtained through a neutron wave field resonance, which is fully accounted for in the presented analysis. The scattering reveals a domain width imbalance of 5.3 to 3.7 µm of virgin and bombarded stripes, respectively. Further, we report that the magnetization in the bombarded stripe significantly deviates from the head-to-head arrangement. A domain wall of 0.6 µm with homogeneous magnetization direction is found to separate the two neighboring domains. The results contain detailed information on length scales and magnetization vectors provided by PNR and OSS in absolute units. We illustrate the complementarity of the technique to microscopy techniques for obtaining a quantitative description of imprinted magnetic domain patterns and illustrate its applicability to different sample systems.

Reversible Control of Physical Properties via an Oxygen-Vacancy-Driven Topotactic Transition in Epitaxial La0.7 Sr0.3 MnO3- δ Thin Films.

The vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure-property relationship in such systems. Here, an oxygen-vacancy-induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La0.7 Sr0.3 MnO3- δ thin films is identified by real-time X-ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen-deficient interfacial layer with drastically reduced nuclear scattering length density, further enabling a quantitative determination of the oxygen stoichiometry (La0.7 Sr0.3 MnO2.65 ) for the intermediate state. Associated physical properties of distinct topotactic phases (i.e., ferromagnetic metal and antiferromagnetic insulator) can be reversibly switched by an oxygen desorption/absorption cycling process. Importantly, a significant lowering of necessary conditions (temperatures below 100 °C and conversion time less than 30 min) for the oxygen reloading process is found. These results demonstrate the potential applications of defect engineering in the design of perovskite-based functional materials.

Direct Measurement of the Intrinsic Sharpness of Magnetic Interfaces Formed by Chemical Disorder Using a He+ Beam.

Using ion beams to locally modify material properties and subsequently drive magnetic phase transitions is rapidly gaining momentum as the technique of choice for the fabrication of magnetic nanoelements. This is because the method provides the capability to engineer in three dimensions on the nanometer length scale. This will be an important consideration for several emerging magnetic technologies (e.g., spintronic devices and racetrack and random-access memories) where device functionality will hinge on the spatial definition of the incorporated magnetic nanoelements. In this work, the fundamental sharpness of a magnetic interface formed by nanomachining FePt3 films using He+ irradiation is investigated. Through careful selection of the irradiating ion energy and fluence, room-temperature ferromagnetism is locally induced into a fractional volume of a paramagnetic (PM) FePt3 film by modifying the chemical order parameter. A combination of transmission electron microscopy, magnetometry, and polarized neutron reflectometry measurements demonstrates that the interface over which the PM-to-ferromagnetic modulation occurs in this model system is confined to a few atomic monolayers only, while the structural boundary transition is less well-defined. Using complementary density functional theory, the mechanism for the ion-beam-induced magnetic transition is elucidated and shown to be caused by an intermixing of Fe and Pt atoms in antisite defects above a threshold density.

The high-intensity reflectometer of the Jülich Centre for Neutron Science: MARIA.

MARIA (magnetism reflectometer with high incident angle) is a world class vertical sample reflectometer dedicated to the investigation of thin films in the fields of magnetism, soft matter and biology. The elliptical vertically focusing guide allows one to measure small samples with a typical size of 1 × 1 cm very efficiently. The double-bounce polarizer and the in situ pumped 3He SEOP (spin-exchange optical pumping) neutron spin filter cell for analysing the polarization of the reflected neutron beam can be moved into the beam in seconds. The polarized flux of MARIA amounts to 5 × 107 n (s cm2)-1 at the sample position with a horizontally collimated beam of 3 mrad, a wavelength of λ = 4.5 Šand a wavelength resolution of Δλ/λ = 10%. In the non-polarized mode a flux of 1.2 × 108 n (s cm2)-1 is achieved in this configuration. MARIA is also capable of grazing-incidence small-angle neutron scattering measurements, using a pinhole collimation with two four-segment slits and an absorber that prevents the focusing of the elliptical guide in the vertical direction.

Self-Assembled Layering of Magnetic Nanoparticles in a Ferrofluid on Silicon Surfaces.

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.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures.

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

Orientation Control of Interfacial Magnetism at La0.67Sr0.33MnO3/SrTiO3 Interfaces.

Understanding the magnetism at the interface between a ferromagnet and an insulator is essential because the commonly posited magnetic "dead" layer close to an interface can be problematic in magnetic tunnel junctions. Previously, degradation of the magnetic interface was attributed to charge discontinuity across the interface. Here, the interfacial magnetism was investigated using three identically prepared La0.67Sr0.33MnO3 (LSMO) thin films grown on different oriented SrTiO3 (STO) substrates by polarized neutron reflectometry. In all cases the magnetization at the LSMO/STO interface is larger than the film bulk. We show that the interfacial magnetization is largest across the LSMO/STO interfaces with (001) and (111) orientations, which have the largest net charge discontinuities across the interfaces. In contrast, the magnetization of LSMO/STO across the (110) interface, the orientation with no net charge discontinuity, is the smallest of the three orientations. We show that a magnetically degraded interface is not intrinsic to LSMO/STO heterostructures. The approach to use different crystallographic orientations provides a means to investigate the influence of charge discontinuity on the interfacial magnetization.