Temperature Evolution of Magnon Propagation Length in Tm3Fe5O12 Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping.

The magnon propagation length, ⟨ξ⟩, of a ferro-/ferrimagnet (FM) is one of the key factors that controls the generation and propagation of thermally driven magnonic spin current in FM/heavy metal (HM) bilayer based spincaloritronic devices. For the development of a complete physical picture of thermally driven magnon transport in FM/HM bilayers over a wide temperature range, it is of utmost importance to understand the respective roles of temperature-dependent Gilbert damping (α) and effective magnetic anisotropy (Keff) in controlling the temperature evolution of ⟨ξ⟩. Here, we report a comprehensive investigation of the temperature-dependent longitudinal spin Seebeck effect (LSSE), radio frequency transverse susceptibility, and broad-band ferromagnetic resonance measurements on Tm3Fe5O12 (TmIG)/Pt bilayers grown on different substrates. We observe a significant drop in the LSSE voltage below 200 K independent of TmIG film thickness and substrate choice. This is attributed to the noticeable increases in effective magnetic anisotropy field, HKeff (∝Keff) and α that occur within the same temperature range. From the TmIG thickness dependence of the LSSE voltage, we determined the temperature dependence of ⟨ξ⟩ and highlighted its correlation with the temperature-dependent HKeff and α in TmIG/Pt bilayers, which will be beneficial for the development of rare-earth iron garnet based efficient spincaloritronic nanodevices.

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.

Competing Magnetic Interactions and Field-Induced Metamagnetic Transition in Highly Crystalline Phase-Tunable Iron Oxide Nanorods.

The inherent existence of multi phases in iron oxide nanostructures highlights the significance of them being investigated deliberately to understand and possibly control the phases. Here, the effects of annealing at 250 °C with a variable duration on the bulk magnetic and structural properties of high aspect ratio biphase iron oxide nanorods with ferrimagnetic Fe3O4 and antiferromagnetic α-Fe2O3 are explored. Increasing annealing time under a free flow of oxygen enhanced the α-Fe2O3 volume fraction and improved the crystallinity of the Fe3O4 phase, identified in changes in the magnetization as a function of annealing time. A critical annealing time of approximately 3 h maximized the presence of both phases, as observed via an enhancement in the magnetization and an interfacial pinning effect. This is attributed to disordered spins separating the magnetically distinct phases which tend to align with the application of a magnetic field at high temperatures. The increased antiferromagnetic phase can be distinguished due to the field-induced metamagnetic transitions observed in structures annealed for more than 3 h and was especially prominent in the 9 h annealed sample. Our controlled study in determining the changes in volume fractions with annealing time will enable precise control over phase tunability in iron oxide nanorods, allowing custom-made phase volume fractions in different applications ranging from spintronics to biomedical applications.

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.

From multi-segmented to core/shell nanorods: morphology evolution in Fe-Au nanorods by tuning fabrication conditions.

Aiming to obtain hybrid magneto-plasmonic nanostructures, we have developed multisegmented and core/shell structured Fe-Au nanorods using template assisted electrochemical deposition. A facile method of tuning the growth pattern of multisegmented nanorods into core/shell structured is demonstrated. With a precise control of current density and deposition time, a brick-stacked wire like growth led to the formation of hollow nanotubes that could be further tuned to multilayered hollow nanotubes and core/shell structured nanorods. TEM imaging and STEM-EELS technique were used to explore the morphology, microstructure and the distribution of Au and Fe in the nanorods. The easy magnetization direction was found to be perpendicular to the nanorods' growth direction in the segmented nanorods. On the other hand, core/shell nanorods exhibited isotropic behavior. Our findings provide deeper insights into the fabrication of hybrid nanorods and the opportunity to tune the fabrication method to vary their morphology accordingly. Such studies will benefit design of hybrid nanorods with specific morphologies and physical properties and hence their integration into sensing, spintronics and other potential biomedical and technological applications.

Emergent magnetism and exchange bias effect in iron oxide nanocubes with tunable phase and size.

We report a systematic investigation of the magnetic properties including the exchange bias (EB) effect in an iron oxide nanocube system with tunable phase and average size (10, 15, 24, 34, and 43 nm). X-ray diffraction and Raman spectroscopy reveal the presence of Fe3O4, FeO, andα-Fe2O3phases in the nanocubes, in which the volume fraction of each phase varies depending upon particle size. While the Fe3O4phase is dominant in all and tends to grow with increasing particle size, the FeO phase appears to coexist with the Fe3O4phase in 10, 15, and 24 nm nanocubes but disappears in 34 and 43 nm nanocubes. The nanocubes exposed to air resulted in anα-Fe2O3oxidized surface layer whose thickness scaled with particle size resulting in a shell made ofα-Fe2O3phase and a core containing Fe3O4or a mixture of both Fe3O4and FeO phases. Magnetometry indicates that the nanocubes undergo Morin (of theα-Fe2O3phase) and Verwey (of the Fe3O4phase) transitions at ∼250 K and ∼120 K, respectively. For smaller nanocubes (10, 15, and 24 nm), the EB effect is observed below 200 K, of which the 15 nm nanocubes showed the most prominent EB with optimal antiferromagnetic (AFM) FeO phase. No EB is reported for larger nanocubes (34 and 43 nm). The observed EB effect is ascribed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe3O4phase and AFM FeO phase, while its absence is related to the disappearance of the FeO phase. The Fe3O4/α-Fe2O3(FiM/AFM) interfaces are found to have negligible influence on the EB. Our findings shed light on the complexity of the EB effect in mixed-phase iron oxide nanosystems and pave the way to design exchange-coupled nanomaterials with desirable magnetic properties for biomedical and spintronic applications.

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.

Iron Oxide Nanorings and Nanotubes for Magnetic Hyperthermia: The Problem of Intraparticle Interactions.

Magnetic interactions can play an important role in the heating efficiency of magnetic nanoparticles. Although most of the time interparticle magnetic interactions are a dominant source, in specific cases such as multigranular nanostructures intraparticle interactions are also relevant and their effect is significant. In this work, we have prepared two different multigranular magnetic nanostructures of iron oxide, nanorings (NRs) and nanotubes (NTs), with a similar thickness but different lengths (55 nm for NRs and 470 nm for NTs). In this way, we find that the NTs present stronger intraparticle interactions than the NRs. Magnetometry and transverse susceptibility measurements show that the NTs possess a higher effective anisotropy and saturation magnetization. Despite this, the AC hysteresis loops obtained for the NRs (0-400 Oe, 300 kHz) are more squared, therefore giving rise to a higher heating efficiency (maximum specific absorption rate, SARmax = 110 W/g for the NRs and 80 W/g for the NTs at 400 Oe and 300 kHz). These results indicate that the weaker intraparticle interactions in the case of the NRs are in favor of magnetic hyperthermia in comparison with the NTs.

Hybrid magnetic nanoparticles as efficient nanoheaters in biomedical applications.

Heating at the nanoscale is the basis of several biomedical applications, including magnetic hyperthermia therapies and heat-triggered drug delivery. The combination of multiple inorganic materials in hybrid magnetic nanoparticles provides versatile platforms to achieve an efficient heat delivery upon different external stimuli or to get an optical feedback during the process. However, the successful design and application of these nanomaterials usually require intricate synthesis routes and their magnetic response is still not fully understood. In this review we give an overview of the novel systems reported in the last few years, which have been mostly obtained by organic phase-based synthesis and epitaxial growth processes. Since the heating efficiency of hybrid magnetic nanoparticles often relies on the exchange-interaction between their components, we discuss various interface-phenomena that are responsible for their magnetic properties. Finally, followed by a brief comment on future directions in the field, we outline recent advances on multifunctional nanoparticles that can boost the heating power with light and combine heating and temperature sensing in a single nanomaterial.

Shell-mediated control of surface chemistry of highly stoichiometric magnetite nanoparticles.

Magnetite (Fe3O4) nanoparticles are one of the most studied nanomaterials for different nanotechnological and biomedical applications. However, Fe3O4 nanomaterials gradually oxidize to maghemite (γ-Fe2O3) under conventional environmental conditions leading to changes in their functional properties that determine their performance in many applications. Here we propose a novel strategy to control the surface chemistry of monodisperse 12 nm magnetite nanoparticles by means of a 3 nm-thick Zn-ferrite epitaxial coating in core/shell nanostructures. We have carried out a combined Mössbauer spectroscopy, dc magnetometry, X-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy study on iron oxide and Fe3O4/Zn0.6Fe2.4O4 core/shell nanoparticles aged under ambient conditions for 6 months. Our results reveal that while the aged iron oxide nanoparticles consist of a mixture of γ-Fe2O3 and Fe3O4, the Zn-ferrite-coating preserves a highly stoichiometric Fe3O4 core. Therefore, the aged core/shell nanoparticles present a sharp Verwey transition, an increased saturation magnetization and the possibility of tuning the effective anisotropy through exchange-coupling at the core/shell interface. The inhibition of the oxidation of the Fe3O4 cores can be accounted for in terms of the chemical nature of the shell layer and an epitaxial crystal symmetry matching between the core and the shell.

Unlocking the Potential of Magnetotactic Bacteria as Magnetic Hyperthermia Agents.

Magnetotactic bacteria are aquatic microorganisms that internally biomineralize chains of magnetic nanoparticles (called magnetosomes) and use them as a compass. Here it is shown that magnetotactic bacteria of the strain Magnetospirillum gryphiswaldense present high potential as magnetic hyperthermia agents for cancer treatment. Their heating efficiency or specific absorption rate is determined using both calorimetric and AC magnetometry methods at different magnetic field amplitudes and frequencies. In addition, the effect of the alignment of the bacteria in the direction of the field during the hyperthermia experiments is also investigated. The experimental results demonstrate that the biological structure of the magnetosome chain of magnetotactic bacteria is perfect to enhance the hyperthermia efficiency. Furthermore, fluorescence and electron microscopy images show that these bacteria can be internalized by human lung carcinoma cells A549, and cytotoxicity studies reveal that they do not affect the viability or growth of the cancer cells. A preliminary in vitro hyperthermia study, working on clinical conditions, reveals that cancer cell proliferation is strongly affected by the hyperthermia treatment, making these bacteria promising candidates for biomedical applications.

Correction: An electron transfer driven magnetic switch: ferromagnetic exchange and spin delocalization in iron verdazyl complexes.

Correction for 'An electron transfer driven magnetic switch: ferromagnetic exchange and spin delocalization in iron verdazyl complexes' by David J. R. Brook et al., Dalton Trans., 2018, 47, 6351-6360.

An electron transfer driven magnetic switch: ferromagnetic exchange and spin delocalization in iron verdazyl complexes.

The verdazyl 'pincer' ligand, 1-isopropyl-3,5-dipyridyl-6-oxoverdazyl (dipyvd), coordinates iron to form a series of pseudooctahedral coordination compounds [Fe(dipyvd)2]n+ (n = 0-3). In the case where n = 2, the molecular geometry and physical and spectral properties are consistent with a low spin (S = 0) iron(ii) ion coordinated by two ferromagnetically coupled radical ligands. Upon one electron reduction, the room temperature effective magnetic moment of the complex jumps from µeff = 2.64 to µeff = 5.86 as a result of spin crossover of the iron atom combined with very strong ferromagnetic coupling of the remaining ligand centered unpaired electron with the metal center. The sign of the exchange is opposite to that observed in other high spin iron/radical ligand systems and appears to be a result of delocalization of the ligand unpaired electron across the whole molecule. The large change in magnetic properties, combined with a delocalized electronic structure and accessible redox potentials, suggests the utility of this and related systems in the development of novel molecular spintronic devices.

Roles of bulk and surface magnetic anisotropy on the longitudinal spin Seebeck effect of Pt/YIG.

A clear understanding of the temperature evolution of the longitudinal spin Seebeck effect (LSSE) in the classic Pt/yttrium iron garnet (YIG) system and its association with magnetic anisotropy is essential towards optimization of its spin-caloric functionality for spintronics applications. We report here for the first time the temperature dependences of LSSE voltage (V LSSE), magnetocrystalline anisotropy field (H K) and surface perpendicular magnetic anisotropy field (H KS) in the same Pt/YIG system. We show that on lowering temperature, the sharp drop in V LSSE and the sudden increases in H K and H KS at ~175 K are associated with the spin reorientation due to single ion anisotropy of Fe2+ ions. The V LSSE peak at ~75 K is attributed to the H KS and M S (saturation magnetization) whose peaks also occur at the same temperature. The effects of surface and bulk magnetic anisotropies are corroborated with those of thermally excited magnon number and magnon propagation length to satisfactorily explain the temperature dependence of LSSE in the Pt/YIG system. Our study also emphasizes the important roles of bulk and surface anisotropies in the LSSE in YIG and paves a new pathway for developing novel spin-caloric materials.

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.

Critical Behavior and Macroscopic Phase Diagram of the Monoaxial Chiral Helimagnet Cr1/3NbS2.

Cr1/3NbS2 is a unique example of a hexagonal chiral helimagnet with high crystalline anisotropy, and has generated growing interest for a possible magnetic field control of the incommensurate spin spiral. Here, we construct a comprehensive phase diagram based on detailed magnetization measurements of a high quality single crystal of Cr1/3NbS2 over three magnetic field regions. An analysis of the critical properties in the forced ferromagnetic region yields 3D Heisenberg exponents ß = 0.3460 ± 0.040, γ = 1.344 ± 0.002, and T C = 130.78 K ± 0.044, which are consistent with the localized nature the of Cr3+ moments and suggest short-range ferromagnetic interactions. We exploit the temperature and magnetic field dependence of magnetic entropy change (ΔS M) to accurately map the nonlinear crossover to the chiral soliton lattice regime from the chiral helimagnetic phase. Our observations in the low field region are consistent with the existence of chiral ordering in a temperature range above the Curie temperature, T C < T < T*, where a first-order transition has been previously predicted. An analysis of the universal behavior of ΔS M(T,H) experimentally demonstrates for the first time the first-order nature of the onset of chiral ordering.

Epitaxial magnetite nanorods with enhanced room temperature magnetic anisotropy.

Nanostructured magnetic materials with well-defined magnetic anisotropy are very promising as building blocks in spintronic devices that operate at room temperature. Here we demonstrate the epitaxial growth of highly oriented Fe3O4 nanorods on a SrTiO3 substrate by hydrothermal synthesis without the use of a seed layer. The epitaxial nanorods showed biaxial magnetic anisotropy with an order of magnitude difference between the anisotropy field values of the easy and hard axes. Using a combination of conventional magnetometry, transverse susceptibility, magnetic force microscopy (MFM) and magneto-optic Kerr effect (MOKE) measurements, we investigate magnetic behavior such as temperature dependent magnetization and anisotropy, along with room temperature magnetic domain formation and its switching. The interplay of epitaxy and enhanced magnetic anisotropy at room temperature, with respect to randomly oriented powder Fe3O4 nanorods, is discussed. The results obtained identify epitaxial nanorods as useful materials for magnetic data storage and spintronic devices that necessitate tunable anisotropic properties with sharp magnetic switching phenomena.

Remotely Controlled Micromanipulation by Buckling Instabilities in Fe3O4 Nanoparticle Embedded Poly(N-isopropylacrylamide) Surface Arrays.

The micromanipulation of biological samples is important for microbiology, pharmaceutical science, and related bioengineering fields. In this work, we report the fabrication and characterization of surface-attached microbeam arrays of 20 µm width and 25 µm height made of poly(N-isopropylacrylamide), a thermoresponsive polymer, with embedded spherical or octopod Fe3O4 nanoparticles. Below 32 °C, the microbeams imbibe water and buckle with an amplitude of approximately 20 µm. Turning on an AC-magnetic field induces the microbeam array to expel water due to the heating effect of the nanoparticles (magnetic hyperthermia), leading to a reversible transition from a buckled to nonbuckled state. It is observed that the octopod nanoparticles have a heating rate 30% greater (specific absorption rate, SAR) than that of the spherical nanoparticles, which shortens the time scale of the transition from the buckled and nonbuckled state. The return of the microbeams to the buckled state is accomplished by turning off the AC magnetic field, the rate of which is dictated by dissipation of heat and is independent of the type of nanoparticle. It is further demonstrated that this transition can be used to propel 50 µm spherical objects along a surface. While the motion is random, this study shows the promise of harnessing shape-shifting patterns in microfluidics for object manipulation.

Enhanced magnetic anisotropy and heating efficiency in multi-functional manganese ferrite/graphene oxide nanostructures.

A promising nanocomposite material composed of MnFe2O4 (MFO) nanoparticles of ∼17 nm diameter deposited onto graphene oxide (GO) nanosheets was successfully synthesized using a modified co-precipitation method. X-ray diffraction, transmission electron microscopy, and selected area electron diffraction confirmed the quality of the synthesized samples. Fourier transform infrared measurements and analysis evidenced that the MFO nanoparticles were attached to the GO surface. Magnetic measurements and analysis using the modified Langevin model evidenced the superparamagnetic characteristic of both the bare MFO nanoparticles and the MFO-GO nanocomposite at room temperature, and an appreciable increase of the effective anisotropy for the MFO-GO sample. Magnetic hyperthermia experiments performed by both calorimetric and ac magnetometry methods indicated that relative to the bare MFO nanoparticles, the heating efficiency of the MFO-GO nanocomposite was similar at low ac fields (0-300 Oe) but became progressively larger with increasing ac fields (>300 Oe). This has been related to the higher effective anisotropy of the MFO-GO nanocomposite. In comparison with the bare MFO nanoparticles, a smaller reduction in the heating efficiency was observed in the MFO-GO composites when embedded in agar or when their concentration was increased, indicating that the GO helped minimize the physical rotation and aggregation of the MFO nanoparticles. These findings can be of practical importance in exploiting this type of nanocomposite for advanced hyperthermia. Magnetoimpedance-based biodetection studies also indicated that the MFO-GO nanocomposite could be used as a promising magnetic biomarker in biosensing applications.