Coupling of the Structure and Magnetism to Spin Splitting in Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites are famous for the diversity of their chemical compositions, phases, phase transitions, and associated physical properties. We use a combination of experimental and computational techniques to reveal a strong coupling between structure, magnetism, and spin splitting in a representative of the largest family of hybrid organic-inorganic perovskites: the formates. With the help of first-principles simulations, we find spin splitting in both conduction and valence bands of [NH2NH3]Co(HCOO)3 induced by spin-orbit interactions, which can reach up to 14 meV. Our magnetic measurements reveal that this material exhibits canted antiferromagnetism below 15.5 K. The direction of the associated antiferromagnetic order parameter is strongly coupled with spin splitting in the centrosymmetric phase, allowing for the creation and annihilation of spin splitting through the application of a magnetic field. Furthermore, the structural phase transition to the experimentally observed polar Pna21 phase completely changes the aforementioned spin splitting and its coupling to magnetic degrees of freedom. This reveals that in [NH2NH3]Co(HCOO)3, the structure and magnetism are strongly coupled to spin splitting and can be manipulated through electric and magnetic fields. We believe that our findings offer an important step toward a fundamental understanding and practical applications of materials with coupled properties.

Large Thermo-Spin Effects in Heusler Alloy-Based Spin Gapless Semiconductor Thin Films.

Recently, Heusler alloy-based spin gapless semiconductors (SGSs) with high Curie temperature (TC) and sizable spin polarization have emerged as potential candidates for tunable spintronic applications. We report comprehensive investigation of the temperature-dependent ANE and intrinsic longitudinal spin Seebeck effect (LSSE) in CoFeCrGa thin films grown on MgO substrates. Our findings show that the anomalous Nernst coefficient for the MgO/CoFeCrGa (95 nm) film is ≈1.86 µV K-1 at room temperature, which is nearly 2 orders of magnitude higher than that of the bulk polycrystalline sample of CoFeCrGa (≈0.018 µV K-1) and almost 3 orders of magnitude higher than that of the half-metallic ferromagnet La1-xNaxMnO3 (≈0.005 µV K-1) but comparable to that of the magnetic Weyl semimetal Co2MnGa thin film (≈2-3 µV K-1). Furthermore, the LSSE coefficient for our MgO/CoFeCrGa (95 nm)/Pt (5 nm) heterostructure is ≈20.5 nV K-1 Ω-1 at room temperature, which is twice larger than that of the half-metallic ferromagnetic La0.7Sr0.3MnO3 thin films (≈9 nV K-1 Ω-1). We show that both ANE and LSSE coefficients follow identical temperature dependences and exhibit a maximum at ≈225 K, which is understood as the combined effects of inelastic magnon scatterings and reduced magnon population at low temperatures. Our analyses not only indicate that the extrinsic skew scattering is the dominating mechanism for ANE in these films but also provide critical insights into the functional form of the observed temperature-dependent LSSE at low temperatures. Furthermore, by employing radio frequency transverse susceptibility and broad-band ferromagnetic resonance in combination with the LSSE measurements, we establish a correlation among the observed LSSE signal, magnetic anisotropy, and Gilbert damping of the CoFeCrGa thin films, which will be beneficial for fabricating tunable and highly efficient Heusler alloy-based spin caloritronic nanodevices.

Intergranular Spin Dependent Tunneling Dominated Magnetoresistance in Helimagnetic Manganese Phosphide Thin Films.

Helical magnets are emerging as a novel class of materials for spintronics and sensor applications; however, research on their charge- and spin-transport properties in a thin film form is less explored. Herein, we report the temperature and magnetic field-dependent charge transport properties of a highly crystalline MnP nanorod thin film over a wide temperature range (2 K < T < 350 K). The MnP nanorod films of ~100 nm thickness were grown on Si substrates at 500 °C using molecular beam epitaxy. The temperature-dependent resistivity ρ(T) data exhibit a metallic behavior (dρ/dT > 0) over the entire measured temperature range. However, large negative magnetoresistance (Δρ/ρ) of up to 12% is observed below ~50 K at which the system enters a stable helical (screw) magnetic state. In this temperature regime, the Δρ(H)/ρ(0) dependence also shows a magnetic field-manipulated CONE + FAN phase coexistence. The observed magnetoresistance is dominantly governed by the intergranular spin dependent tunneling mechanism. These findings pinpoint a correlation between the transport and magnetism in this helimagnetic system.

Enhanced Magnetism and Anomalous Hall Transport through Two-Dimensional Tungsten Disulfide Interfaces.

The magnetic proximity effect (MPE) has recently been explored to manipulate interfacial properties of two-dimensional (2D) transition metal dichalcogenide (TMD)/ferromagnet heterostructures for use in spintronics and valleytronics. However, a full understanding of the MPE and its temperature and magnetic field evolution in these systems is lacking. In this study, the MPE has been probed in Pt/WS2/BPIO (biphase iron oxide, Fe3O4 and α-Fe2O3) heterostructures through a comprehensive investigation of their magnetic and transport properties using magnetometry, four-probe resistivity, and anomalous Hall effect (AHE) measurements. Density functional theory (DFT) calculations are performed to complement the experimental findings. We found that the presence of monolayer WS2 flakes reduces the magnetization of BPIO and hence the total magnetization of Pt/WS2/BPIO at T > ~120 K-the Verwey transition temperature of Fe3O4 (TV). However, an enhanced magnetization is achieved at T < TV. In the latter case, a comparative analysis of the transport properties of Pt/WS2/BPIO and Pt/BPIO from AHE measurements reveals ferromagnetic coupling at the WS2/BPIO interface. Our study forms the foundation for understanding MPE-mediated interfacial properties and paves a new pathway for designing 2D TMD/magnet heterostructures for applications in spintronics, opto-spincaloritronics, and valleytronics.

Spin Seebeck Effect in Iron Oxide Thin Films: Effects of Phase Transition, Phase Coexistence, And Surface Magnetism.

Understanding the effects of phase transition, phase coexistence, and surface magnetism on the longitudinal spin Seebeck effect (LSSE) in a magnetic system is essential to manipulate the spin to charge current conversion efficiency for spincaloritronic applications. We aim to elucidate these effects by performing a comprehensive study of the temperature dependence of the LSSE in biphase iron oxide (BPIO = α-Fe2O3 + Fe3O4) thin films grown on Si (100) and Al2O3 (111) substrates. A combination of a temperature-dependent anomalous Nernst effect (ANE) and electrical resistivity measurements show that the contribution of the ANE from the BPIO layer is negligible in comparison to the intrinsic LSSE in the Si/BPIO/Pt heterostructure, even at room temperature. Below the Verwey transition of the Fe3O4 phase, the total signal across BPIO/Pt is dominated by the LSSE. Noticeable changes in the intrinsic LSSE signal for both Si/BPIO/Pt and Al2O3/BPIO/Pt heterostructures around the Verwey transition of the Fe3O4 phase and the antiferromagnetic (AFM) Morin transition of the α-Fe2O3 phase are observed. The LSSE signal for Si/BPIO/Pt is found to be almost 2 times greater than that for Al2O3/BPIO/Pt; however, an opposite trend is observed for the saturation magnetization. Magnetic force microscopy reveals the higher density of surface magnetic moments of the Si/BPIO film in comparison to the Al2O3/BPIO film, which underscores the dominant role of interfacial magnetism on the LSSE signal and thereby explains the larger LSSE for Si/BPIO/Pt.

Photopolymerization-based synthesis of iron oxide nanoparticle embedded PNIPAM nanogels for biomedical applications.

Conventional therapeutic techniques treat patients by delivering biotherapeutics to the entire body. With targeted delivery, biotherapeutics are transported to the afflicted tissue reducing exposure to healthy tissue. Targeted delivery devices are minimally composed of a stimuli responsive polymer allowing triggered release and magnetic nanoparticles enabling targeting as well as alternating magnetic field (AMF) heating. Although more traditional methods, like emulsion polymerization, have been used to realize such devices, the synthesis is problematic. For example, surfactants preventing agglomeration must be removed from the product increasing time and cost. Ultraviolet (UV) photopolymerization is more efficient and ensures safety by using biocompatible substances. Reactants selected for nanogel fabrication were N-isopropylacrylamide (monomer), methylene bis-acrylamide (crosslinker), and Irgacure 2959 (photoinitiator). The 10 nm superparamagnetic nanoparticles for encapsulation were composed of iron oxide. Herein, a low-cost, scalable, and rapid, custom-built UV photoreactor with in situ, spectroscopic monitoring system is used to observe synthesis. This method also allows in situ encapsulation of the magnetic nanoparticles simplifying the process. Nanogel characterization, performed by transmission electron microscopy, reveals size-tunable nanogel spheres between 40 and 800 nm in diameter. Samples of nanogels encapsulating magnetic nanoparticles were subjected to an AMF and temperature increase was observed indicating triggered release is possible. Results presented here will have a wide range of applications in medical sciences like oncology, gene delivery, cardiology, and endocrinology.

Evidence of superior ferroelectricity in structurally welded ZnSnO3 nanowire arrays.

Highly packed LN-type ZnSnO3 NW arrays are grown on ZnO:Al/Si substrates using a hybrid pulsed laser deposition and solvothermal process. Unique "welding" mechanism structurally joins adjacent ZnSnO3 NWs to form a nearly impervious 20 µm thick nanostructured film that shows high P r of 30 µC/cm(2) at a low E c of 25 kV/cm for the first time.

Nanocolumnar interfaces and enhanced magnetic coercivity in preferentially oriented cobalt ferrite thin films grown using oblique-angle pulsed laser deposition.

Highly textured cobalt ferrite (CFO) thin films were grown on Si (100) substrates using oblique-angle pulsed laser deposition (α-PLD). X-ray diffraction and in-depth strain analysis showed that the obliquely deposited CFO films had both enhanced orientation in the (111) crystal direction as well as tunable compressive strains as a function of the film thicknesses, in contrast to the almost strain-free polycrystalline CFO films grown using normal-incidence PLD under the same conditions. Using in situ optical plume diagnostics the growth parameters in the α-PLD process were optimized to achieve smoother film surfaces with roughness values as low as 1-2 nm as compared to the typical values of 10-12 nm in the normal-incidence PLD grown films. Cross-sectional high resolution transmission electron microscope images revealed nanocolumnar growth of single-crystals of CFO along the (111) crystallographic plane at the film-substrate interface. Magnetic measurements showed larger coercive fields (∼10 times) with similar saturation magnetization in the α-PLD-grown CFO thin films as compared to those deposited using normal-incidence PLD. Such significantly enhanced magnetic coercivity observed in CFO thin films make them ideally suited for magnetic data storage applications. A growth mechanism based on the atomic shadowing effect and strain compression-relaxation mechanism was proposed for the obliquely grown CFO thin films.

Controlled Ti seed layer assisted growth and field emission properties of Pb(Zr0.52Ti0.48)O3 nanowire arrays.

We report on the directed upright growth of ferroelectric (FE) Pb(Zr0.52Ti0.48)O3 (PZT) nanowire (NW) arrays with large aspect ratios of >60 using a Ti seed layer assisted hydrothermal process over large surface areas on ITO/glass substrates. In a two-step growth process, Ti seed layer of low surface roughness with a thickness of ~500 nm and grain size of ~100 nm was first deposited by radio frequency (RF) sputtering which was subsequently used as substrates for the growth of highly dense, single crystalline PZT NWs by controlled nucleation. The electron emission properties of the PZT NWs were investigated using the as-grown NWs as FE cathodes. A low turn-on field of ~3.4 V/µm was obtained from the NW arrays, which is impressively lower than that from other reported values. The results reported in this work give direction to the development of a facile growth technique for PZT NWs over large surfaces and also are of interest to the generation of high current electron beam from FE NW based cathodes for field emitter applications.