Distinct skyrmion phases at room temperature in two-dimensional ferromagnet Fe3GaTe2.

Distinct skyrmion phases at room temperature hosted by one material offer additional degree of freedom for the design of topology-based compact and energetically-efficient spintronic devices. The field has been extended to low-dimensional magnets with the discovery of magnetism in two-dimensional van der Waals magnets. However, creating multiple skyrmion phases in 2D magnets, especially above room temperature, remains a major challenge. Here, we report the experimental observation of mixed-type skyrmions, exhibiting both Bloch and hybrid characteristics, in a room-temperature ferromagnet Fe3GaTe2. Analysis of the magnetic intensities under varied imaging conditions coupled with complementary simulations reveal that spontaneous Bloch skyrmions exist as the magnetic ground state with the coexistence of hybrid stripes domain, on account of the interplay between the dipolar interaction and the Dzyaloshinskii-Moriya interaction. Moreover, hybrid skyrmions are created and their coexisting phases with Bloch skyrmions exhibit considerably high thermostability, enduring up to 328 K. The findings open perspectives for 2D spintronic devices incorporating distinct skyrmion phases at room temperature.

Current-Controllable and Reversible Multi-Resistance-State Based on Domain Wall Number Transition in 2D Ferromagnet Fe3GeTe2.

Controlling the multi-state switching is significantly essential for the extensive utilization of 2D ferromagnet in magnetic racetrack memories, topological devices, and neuromorphic computing devices. The development of all-electric functional nanodevices with multi-state switching and a rapid reset remains challenging. Herein, to imitate the potentiation and depression process of biological synapses, a full-current strategy is unprecedently established by the controllable resistance-state switching originating from the spin configuration rearrangement by domain wall number modulation in Fe3GeTe2. In particular, a strong correlation is uncovered in the reduction of domain wall number with the corresponding resistance decreasing by in-situ Lorentz transmission electron microscopy. Interestingly, the magnetic state is reversed instantly to the multi-domain wall state under a single pulse current with a higher amplitude, attributed to the rapid thermal demagnetization by simulation. Based on the neuromorphic computing system with full-current-driven artificial Fe3GeTe2 synapses with multi-state switching, a high accuracy of ≈91% is achieved in the handwriting image recognition pattern. The results identify 2D ferromagnet as an intriguing candidate for future advanced neuromorphic spintronics.

Vertical nanoscale strain-induced electronic localization in epitaxial La2/3Sr1/3MnO3 films with ZrO2 nanopillar inclusions.

Unusual electrical transport properties associated with weak or strong localization are sometimes found in disordered electronic materials. Here, we report experimental observation of a crossover of electronic behavior from weak localization to enhanced weak localization due to the spatial influence of disorder induced by ZrO2 nanopillars in (La2/3Sr1/3MnO3)1-x:(ZrO2)x (x = 0, 0.2, and 0.3) nanocomposite films. The spatial strain regions, identified by scanning transmission electron microscopy and high-resolution x-ray diffraction, induce a coexistence of two-dimentional (2D) and three-dimentional (3D) localization and switches to typical 2D localization with increasing density of ZrO2 pillars due to length scale confinement, which interestingly accords with enhancing vertically interfacial strain. Based on the excellent agreement of our experimental results with one-parameter scaling theory of localization, the enhanced weak localization exists in metal range close to the fixed point. These films provide a tunable experimental model for studying localization in particular the transition regime by appropriate choice of the second epitaxial phase.

Effect of Mn doping and charge transfer on LaTi1-xMnxO3.

We report the magnetic and electronic transport properties of Mn-doped LaTi1-xMnxO3(x= 0, 0.1, 0.3, 0.5) as a function of temperature and an applied magnetic field. It was found that the Mn-doped samples show a magnetic transition which is not present in the parent LaTiO3. The Mn-doped samples showed fluctuations in magnetization at low fields below their Néel transition temperature indicating electronic phase separation in the material. Increased Mn content in the sample strengthens the ferromagnetic-like moment while maintaining G-type antiferromagnetic phase by charge transfer from Mn to Ti and influencing orbital ordering of the Ti3+t2gorbitals. The results are discussed in parallel with transport and bulk magnetization measurements detailing the electronic behavior. An additional context for the mechanism is supported by first-principles density-function theory calculations.

Enhancing Perpendicular Magnetic Anisotropy in Garnet Ferrimagnet by Interfacing with Few-Layer WTe2.

Engineering magnetic anisotropy in a ferro- or ferrimagnetic (FM) thin film is crucial in a spintronic device. One way to modify the magnetic anisotropy is through the surface of the FM thin film. Here, we report the emergence of a perpendicular magnetic anisotropy (PMA) induced by interfacial interactions in a heterostructure comprised of a garnet ferrimagnet, Y3Fe5O12 (YIG), and a low-symmetry, high spin-orbit coupling (SOC) transition metal dichalcogenide, WTe2. At the same time, we also observed an enhancement in Gilbert damping in the WTe2-covered YIG area. Both the magnitude of interface-induced PMA and the Gilbert damping enhancement have no observable WTe2 thickness dependence down to a single quadruple layer, indicating that the interfacial interaction plays a critical role. The ability of WTe2 to enhance the PMA in FM thin film, combined with its previously reported capability to generate out-of-plane damping like spin torque, makes it desirable for magnetic memory applications.

Quantum Hall Effect Measurement of Spin-Orbit Coupling Strengths in Ultraclean Bilayer Graphene/WSe2 Heterostructures.

We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe2 heterostructure devices. Contact mode atomic force microscopy (AFM) cleaning yields ultraclean interfaces and high-mobility devices. In a perpendicular magnetic field, we measure the quantum Hall effect to determine the Landau level structure in the presence of out-of-plane Ising and in-plane Rashba SOC. A distinct Landau level crossing pattern emerges when tuning the charge density and displacement field independently with dual gates, originating from a layer-selective SOC proximity effect. Analyzing the Landau level crossings and measured inter-Landau level energy gaps yields the proximity-induced SOC energy scale. The Ising SOC is ∼2.2 meV, 100 times higher than the intrinsic SOC in graphene, whereas its sign is consistent with theories predicting a dependence of SOC on interlayer twist angle. The Rashba SOC is ∼15 meV. Finally, we infer the magnetic field dependence of the inter-Landau level Coulomb interactions. These ultraclean bilayer graphene/WSe2 heterostructures provide a high mobility system with the potential to realize novel topological electronic states and manipulate spins in nanostructures.

Spin density wave instability in a ferromagnet.

Due to its cooperative nature, magnetic ordering involves a complex interplay between spin, charge, and lattice degrees of freedom, which can lead to strong competition between magnetic states. Binary Fe3Ga4 is one such material that exhibits competing orders having a ferromagnetic (FM) ground state, an antiferromagnetic (AFM) behavior at intermediate temperatures, and a conspicuous re-entrance of the FM state at high temperature. Through a combination of neutron diffraction experiments and simulations, we have discovered that the AFM state is an incommensurate spin-density wave (ISDW) ordering generated by nesting in the spin polarized Fermi surface. These two magnetic states, FM and ISDW, are seldom observed in the same material without application of a polarizing magnetic field. To date, this unusual mechanism has never been observed and its elemental origins could have far reaching implications in many other magnetic systems that contain strong competition between these types of magnetic order. Furthermore, the competition between magnetic states results in a susceptibility to external perturbations allowing the magnetic transitions in Fe3Ga4 to be controlled via temperature, magnetic field, disorder, and pressure. Thus, Fe3Ga4 has potential for application in novel magnetic memory devices, such as the magnetic components of tunneling magnetoresistance spintronics devices.

Dimensionality Effects in FeGe2 Nanowires: Enhanced Anisotropic Magnetization and Anomalous Electrical Transport.

We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.

Ferromagnetism and nonmetallic transport of thin-film α-FeSi(2): a stabilized metastable material.

A metastable phase α-FeSi_{2} was epitaxially stabilized on a silicon substrate using pulsed laser deposition. Nonmetallic and ferromagnetic behaviors are tailored on α-FeSi_{2} (111) thin films, while the bulk material of α-FeSi_{2} is metallic and nonmagnetic. The transport property of the films renders two different conducting states with a strong crossover at 50 K, which is accompanied by the onset of a ferromagnetic transition as well as a substantial magnetoresistance. These experimental results are discussed in terms of the unusual electronic structure of α-FeSi_{2} obtained within density functional calculations and Boltzmann transport calculations with and without strain. Our finding sheds light on achieving ferromagnetic semiconductors through both their structure and doping tailoring, and provides an example of a tailored material with rich functionalities for both basic research and practical applications.

Magnetic/NIR-responsive drug carrier, multicolor cell imaging, and enhanced photothermal therapy of gold capped magnetite-fluorescent carbon hybrid nanoparticles.

This paper reports a type of multifunctional hybrid nanoparticle (NP) composed of gold nanocrystals coated on and/or embedded in a magnetite-fluorescent porous carbon core-shell NP template (Fe3O4@PC-CDs-Au) for biomedical applications, including magnetic/NIR-responsive drug release, multicolor cell imaging, and enhanced photothermal therapy. The synthesis of the Fe3O4@PC-CDs-Au NPs firstly involves the preparation of core-shell template NPs with magnetite nanocrystals clustered in the cores and fluorescent carbon dots (CDs) embedded in a porous carbon shell, followed by an in situ reduction of silver ions (Ag(+)) loaded in the porous carbon shell and a subsequent replacement of Ag NPs with Au NPs through a galvanic replacement reaction using HAuCl4 as a precursor. The Fe3O4@PC-CDs-Au NPs can enter the intracellular region and light up mouse melanoma B16F10 cells in multicolor mode. The porous carbon shell, anchored with hydrophilic hydroxyl/carboxyl groups, endows the Fe3O4@PC-CDs-Au NPs with excellent stability in the aqueous phase and a high loading capacity (719 mg g(-1)) for the anti-cancer drug doxorubicin (DOX). The superparamagnetic Fe3O4@PC-CDs-Au NPs with a saturation magnetization of 23.26 emu g(-1) produce localized heat under an alternating magnetic field, which triggers the release of the loaded drug. The combined photothermal effects of the Au nanocrystals and the CDs on/in the carbon shell can not only regulate the release rate of the loaded drug, but also efficiently kill tumor cells under NIR irradiation. Benefitting from their excellent optical properties, their magnetic field and NIR light-responsive drug release capabilities and their enhanced photothermal effect, such nanostructured Fe3O4@PC-CDs-Au hybrid NPs are very promising for simultaneous imaging diagnostics and high efficacy therapy.

A Facile Solvothermal Synthesis of Octahedral Fe3 O4 Nanoparticles.

Anisotropic Fe3 O4 octahedrons are obtained via a simple solvothermal synthesis with appropriate sizes for various technological applications. A complete suite of materials characterization methods confirms the magnetite phase for these structures, which exhibit substantial saturation magnetization and intriguing morphologies for a wide range of applications.

Multifunctional 1D magnetic and fluorescent nanoparticle chains for enhanced MRI, fluorescent cell imaging, and combined photothermal/chemotherapy.

While the assembled 1D magnetic nanoparticle (NP) chains have demonstrated synergistic magnetic effects from the individual NPs, it is essential to prepare new 1D NP chains that can combine the magnetism with other important material properties for multifunctional applications. This paper reports the fabrication and multifunctional investigation of a new type of 1D NP chains that combine the magnetic properties with fluorescent properties, photothermal conversion ability, and drug carrier function. The building block NPs are composed of magnetic Fe(3)O(4) nanocrystals clustered in the core and fluorescent carbon dots embedded in the mesoporous carbon shell with hydroxyl/carboxyl groups anchored on their surface. These NPs can assemble under the induction of external magnetic field and form stable 1D NP chains of diameter ∼ 90 nm and length ∼ 3 µm via the hydrogen bonding and π-π stacking linkage of the carbon shell. The resulted 1D hybrid NP chains not only demonstrate much higher magnetic resonance imaging (MRI) contrasting ability than the dispersed building block NPs, but also enter into intracellular region and light up the B16F10 cells under a laser excitation with strong and stable fluorescence. While the mesoporous carbon shell provides high drug loading capacity, the embedded fluorescent carbon dots convert near-infrared (NIR) light to heat, and hence kill the tumor cells efficiently and enhance the drug release rate to further improve the therapeutic efficacy under NIR irradiation. Such designed 1D magnetic-fluorescent hybrid NP chains with enhanced MRI contrast, fluorescent imaging ability, and combined chemo-/photothermal therapeutic ability have great potential for various biomedical applications.

Magnetic iron oxide-fluorescent carbon dots integrated nanoparticles for dual-modal imaging, near-infrared light-responsive drug carrier and photothermal therapy.

Multifunctional hybrid nanoparticles (NPs, ∼100 nm) that combine magnetic Fe3O4 nanocrystals and fluorescent carbon dots (CDs) in porous carbon (C) were successfully synthesized using a one-pot solvothermal method by simply increasing the H2O2 concentration. The resultant Fe3O4@C-CDs hybrid NPs not only demonstrate excellent magnetic responsive properties (Ms = 32.5 emu g-1) and magnetic resonance imaging ability (r = 674.4 mM-1 s-1) from the Fe3O4 nanocrystal core, but also exhibit intriguing photoluminescent (quantum yield ∼6.8%) properties including upconversion fluorescence and excellent photostability from the CDs produced in the porous carbon. The hybrid NPs can enter the intracellular region and illuminate mouse melanoma B16F10 cells under different excitation wavelengths. Meanwhile, the mesoporous carbon shell and hydrophilic surface functional groups endow the hybrid NPs with high loading capacity (835 mg g-1) for the anti-cancer drug doxorubicin and excellent stability in aqueous solutions. More importantly, the hybrid NPs can absorb and convert near-infrared (NIR) light to heat due to the existence of CDs, and thus, can realise NIR-controlled drug release and combined photothermo/chemotherapy for high therapeutic efficacy. Such nanostructured Fe3O4@C-CDs hybrid NPs demonstrate great promise towards advanced nanoplatforms for simultaneous imaging diagnostics and high efficacy therapy.

Atomically resolved spectroscopic study of Sr2IrO4: experiment and theory.

Particularly in Sr2IrO4, the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion stabilizes a J(eff) = 1/2 spin-orbital entangled insulating state at low temperatures. Whether this insulating phase is Mott- or Slater-type, has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). STS results clearly illustrate the opening of an insulating gap (150 ~ 250 meV) below the Néel temperature (TN), in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the temperature dependence of the gap is qualitatively consistent with our DFT + dynamical mean field theory (DMFT) results, both showing a continuous transition from a gapped insulating ground state to a non-gap phase as temperatures approach TN. These results indicate a significant Slater character of gap formation, thus suggesting that Sr2IrO4 is a uniquely correlated system, where Slater and Mott-Hubbard-type behaviors coexist.

Porous carbon protected magnetite and silver hybrid nanoparticles: morphological control, recyclable catalysts, and multicolor cell imaging.

A simple and facile synthetic strategy is developed to prepare a new class of multifunctional hybrid nanoparticles (NPs) that can integrate a magnetic core with silver nanocrystals embedded in porous carbon shell. The method involves a one-step solvothermal synthesis of Fe3O4@C template NPs with Fe3O4nanocrystals in the core protected by a porous carbon shell, followed by loading and in situ reduction of silver ions in the carbon shell in water at room temperature. The core-satellite and dumbbell-like nanostructures of the resulted Fe3O4@C-Ag hybrid NPs can be readily controlled by loading amount of silver ions. The hybrid NPs can efficiently catalyze the reduction reaction of organic dyes in water. The easy magnetic separation and high stability of the catalytically active silver nanocrystals embedded in the carbon shell enable the hybrid NPs to be recycled for reuse as catalysts. The hybrid NPs can also overcome cellular barriers to enter the intracellular region and light up the mouse melanoma B16F10 cells in multicolor modal, with no cytotoxicity. Such porous carbon protected Fe3O4@C-Ag hybrid NPs with controllable nanostructures and a combination of magnetic and noble metallic components have great potential for a broad range of applications in the catalytic industry and biomedical field.

A persistent metal-insulator transition at the surface of an oxygen-deficient, epitaxial manganite film.

The oxygen stoichiometry has a large influence on the physical and chemical properties of complex oxides. Most of the functionality in e.g. catalysis and electrochemistry depends in particular on control of the oxygen stoichiometry. In order to understand the fundamental properties of intrinsic surfaces of oxygen-deficient complex oxides, we report on in situ temperature dependent scanning tunnelling spectroscopy experiments on pristine oxygen deficient, epitaxial manganite films. Although these films are insulating in subsequent ex situ in-plane electronic transport experiments at all temperatures, in situ scanning tunnelling spectroscopic data reveal that the surface of these films exhibits a metal-insulator transition (MIT) at 120 K, coincident with the onset of ferromagnetic ordering of small clusters in the bulk of the oxygen-deficient film. The surprising proximity of the surface MIT transition temperature of nonstoichiometric films with that of the fully oxygenated bulk suggests that the electronic properties in the surface region are not significantly affected by oxygen deficiency in the bulk. This carries important implications for the understanding and functional design of complex oxides and their interfaces with specific electronic properties in catalysis, oxide electronics and electrochemistry.

Multifunctional PEG encapsulated Fe3O4@silver hybrid nanoparticles: antibacterial activity, cell imaging and combined photothermo/chemo-therapy.

A class of multifunctional hybrid nanoparticles (NPs) that can integrate a magnetic core, silver (Ag) nanocrystals, and a biocompatible poly(ethylene glycol) (PEG) shell were synthesized and characterized and their applications as antibacterial agents, optical labels for cellular imaging and drug carriers were tested. The synthetic strategy involves a one-step solvothermal synthesis of Fe3O4@PEG template NPs (∼60 nm) with magnetic Fe3O4 nanocrystals in the core and porous PEG as the shell, followed by loading and in situ reduction of Ag+ ions to form Ag nanocrystals in the shell. The size and number of the Ag nanocrystals embedded in the PEG shell can be readily controlled via a simple reaction condition change, resulting in different nanostructures and properties of the hybrid NPs. Such designed Fe3O4@Ag-PEG hybrid NPs can combine the properties and functions from each component. While the Fe3O4 core provides an easy magnetic separation and targeting and magnetic resonance imaging (MRI) contrast ability, the Ag nanocrystals provide stable strong fluorescence and antibacterial activity. The porous PEG shell with excellent stability in water and non-cytotoxicity can be used as a drug carrier for combined photothermo/chemo-therapy. The small hybrid NPs can enter the intracellular region and light up the mouse melanoma B16F10 cells. This class of hybrid NPs with rational integration of functional building blocks should offer broad opportunities for external magnetic manipulation, imaging diagnostics, antibacterial applications and as drug carriers.

Anomalous in-plane electronic scattering in charge ordered Na0.41CoO2·0.6H2O.

We report electronic transport measurements on high quality floating zone grown Na(x)CoO2 and Na0.41CoO2·0.6H2O single crystals. We find an in-plane electronic scattering minimum near 11 K and a clear charge ordering at approximately 50 K. The electronic and magnetic properties in hydrated and nonhydrated Na0.41CoO2 samples are similar at higher temperature, but evolve in markedly different ways below ∼50 K, where a strong ferromagnetic tendency is observed in the hydrated sample. Model calculations show the relationship of this tendency to the structure of the Fermi surface. The results, particularly the clear differences between the hydrated and nonhydrated material show a substantially enhanced ferromagnetic tendency upon hydration. Implications for superconductivity are discussed.