Correlating the Dipolar Interactions Induced Magneto-Viscoelasticity and Thermal Conductivity Enhancements in Nanomagnetic Fluids.

The effective thermal management of electronic system holds the key to maximize their performance. The recent miniaturization trends require a cooling system with high heat flux capacity, localized cooling, and active control. Nanomagnetic fluids (NMFs) based cooling systems have the ability to meet the current demand of the cooling system for the miniaturized electronic system. However, the thermal characteristics of NMFs have a long way to go before the internal mechanisms are well understood. This review mainly focuses on the three aspects to establish a correlation between the thermal and rheological properties of the NMFs. First, the background, stability, and factors affecting the properties of the NMFs are discussed. Second, the ferrohydrodynamic equations are introduced for the NMFs to explain the rheological behavior and relaxation mechanism. Finally, different theoretical and experimental models are summarized that explain the thermal characteristics of the NMFs. Thermal characteristics of the NMFs are significantly affected by the morphology and composition of the magnetic nanoparticles (MNPs) in NMFs as well as the type of carrier liquids and surface functionalization that also influences the rheological properties. Thus, understanding the correlation between the thermal characteristics of the NMFs and rheological properties helps develop cooling systems with improved performance.

Charge-transfer-induced 2D ferromagnetism and realization of thermo-remnant memory effect in ultrathinß-NiOOH-encapsulated graphene.

For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagnetic metal leads to many intriguing phenomena. However, because of the unidirectional spin orientation in ferromagnetic transition metals, interface interaction plays a detrimental role in diminishing the magnetic parameters on 2D surfaces. To overcome this issue, we have synthesized ultrathin 2D weak antiferromagneticß-NiOOH layers on a graphene surface. By exploiting the charge transfer effect and tuning the thickness of the thinß-NiOOH layers, conversion of ferromagnetism along with giant coercivity and the thermo-remnant magnetic memory effect were observed. As antiferromagnets have two spin orientations, transfer of charge at the interface breaks the nullifying effect of zero magnetization in antiferromagnets and the combined system behaves like a 2D ferrimagnet. Whenever, the sandwich structure ofß-NiOOH/graphene/ß-NiOOH is formed, it also shows interlayer exchange coupling those results in huge exchange bias and anomalous temperature dependence of coercivity. Due to the strong exchange interaction between the layers, the combined system also shows a robust temperature-based memory effect. Spin-polarized density functional theory was also calculated to confirm the interface interaction and its quantitative evaluation by means of Bader charge analysis and charge-density mapping.

Anomalous coherent and dissipative coupling in dual photon-magnon hybrid resonators.

We explored the distinctive behavior of coherent and dissipative photon-magnon coupling (PMC) in dual hybrid resonators, each incorporating an Inverted Split-Ring Resonator (ISRR) paired with a Yttrium Iron Garnet (YIG) film, positioned in close proximity but with varying relative split-gap orientations. These orientations led to notable shifts in the dispersion spectra, characterized by level repulsion and attraction, signaling coherent and dissipative coupling, respectively, in single ISRR/YIG hybrids at certain orientations. Through analytical modeling, we determined that the observed shifts in coupling types are primarily due to the effect of photon-photon (ISRR-ISRR) interactions altering the phase difference between the coupled ISRR and magnon modes. Our findings highlight that precise manipulation of the relative split-gap orientations in the ISRR resonators enables controlled coherent and dissipative coupling within planar PMC systems. This capability opens new avenues for applications in quantum information technologies and quantum materials.

Magnetic-field controlled on-off switchable non-reciprocal negative refractive index in non-Hermitian photon-magnon hybrid systems.

Photon-magnon coupling, where electromagnetic waves interact with spin waves, and negative refraction, which bends the direction of electromagnetic waves unnaturally, constitute critical foundations and advancements in the realms of optics, spintronics, and quantum information technology. Here, we explore a magnetic-field-controlled, on-off switchable, non-reciprocal negative refractive index within a non-Hermitian photon-magnon hybrid system. By integrating an yttrium iron garnet film with an inverted split-ring resonator, we discover pronounced negative refractive index driven by the system's non-Hermitian properties. This phenomenon exhibits unique non-reciprocal behavior dependent on the signal's propagation direction. Our analytical model sheds light on the crucial interplay between coherent and dissipative coupling, significantly altering permittivity and permeability's imaginary components, crucial for negative refractive index's emergence. This work pioneers new avenues for employing negative refractive index in photon-magnon hybrid systems, signaling substantial advancements in quantum hybrid systems.

Magnetically Actuated Trigger Transient Soft Actuators Comprising On-Demand Photo-Initiated and Thermo-Degradable Polypropylene Carbonate-Photo-Acid Generator.

Lifetime-reconfigurable soft robots have emerged as a new class of robots, emphasizing the unmet needs of futuristic sustainability and security. Trigger-transient materials that can both actuate and degrade on-demand are crucial for achieving life-reconfigurable soft robots. Here, we propose the use of transient and magnetically actuating materials that can decompose under ultraviolet light and heat, achieved by adding photo-acid generator (PAG) and magnetic particles (Sr-ferrite) to poly(propylene carbonate) (PPC). Chemical and thermal analyses reveal that the mechanism of PPC-PAG decomposition occurs through PPC backbone cleavage by the photo-induced acid. The self-assembled monolayer (SAM) encapsulation of Sr-ferrite preventing the interaction with the PAG allowed the transience of magnetic soft actuators. We demonstrate remotely controllable and degradable magnetic soft kirigami actuators using blocks with various magnetized directions. This study proposes novel approaches for fabricating lifetime-configurable magnetic soft actuators applicable to diverse environments and applications, such as enclosed/sealed spaces and security/military devices.

Magnetization reversals in core-shell sphere clusters: finite-element micromagnetic simulation and machine learning analysis.

Recently developed permanent magnets, featuring specially engineered microstructures of inhomogeneous magnetic phases, are being considered as cost-effective alternatives to homogeneous single-main-phase hard magnets composed of Nd2Fe14B, without compromising performance. In this study, we conducted a comprehensive examination of a core-shell sphere cluster model of Ce-substituted inhomogeneous Nd2-δCeδFe14B phases versus homogeneous magnetic phases, utilizing finite-element micromagnetic simulation and machine learning methods. This involved a meticulous, sphere-by-sphere analysis of individual demagnetization curves calculated from the cluster model. The grain-by-grain analyses unveiled that these individual demagnetization curves can elucidate the overall magnetization reversal in terms of the nucleation and coercive fields for each sphere. Furthermore, it was observed that Nd-rich spheres exhibited much broader ranges of nucleation and coercive field distributions, while Nd-lean spheres showed relatively narrower ranges. To identify the key parameter responsible for the notable differences in the nucleation fields, we constructed a machine learning regression model. The model utilized numerous hyperparameter sets, optimized through the very fast simulated annealing algorithm, to ensure reliable training. Using the kernel SHapley Additive eXplanation (SHAP) technique, we inferred that stray fields among the 11 parameters were closely related to coercivity. We further substantiated the machine learning models' inference by establishing an analytical model based on the eigenvalue problem in classical micromagnetic theory. Our grain-by-grain interpretation can guide the optimal design of granular hard magnets from Nd2Fe14B and other abundant rare earth transition elements, focusing on extraordinary performance through the careful adjustment of microstructures and elemental compositions.

Highly efficient heat-dissipation power driven by ferromagnetic resonance in MFe2O4 (M = Fe, Mn, Ni) ferrite nanoparticles.

We experimentally demonstrated that heat-dissipation power driven by ferromagnetic resonance (FMR) in superparamagnetic nanoparticles of ferrimagnetic MFe2O4 (M = Fe, Mn, Ni) gives rise to highly localized incrementation of targeted temperatures. The power generated thereby is extremely high: two orders of magnitude higher than that of the conventional Néel-Brownian model. From micromagnetic simulation and analytical derivation, we found robust correlations between the temperature increment and the intrinsic material parameters of the damping constant as well as the saturation magnetizations of the nanoparticles' constituent materials. Furthermore, the magnetization-dissipation-driven temperature increments were reliably manipulated by extremely low strengths of applied AC magnetic fields under resonance field conditions. Our experimental results and theoretical formulations provide for a better understanding of the effect of FMR on the efficiency of heat generation as well as straightforward guidance for the design of advanced materials for control of highly localized incrementation of targeted temperatures using magnetic particles in, for example, magnetic hyperthermia bio-applications.

Edge-soliton-mediated vortex-core reversal dynamics.

We report an additional reversal mechanism of magnetic vortex cores in nanodot elements driven by currents flowing perpendicular to the sample plane, occurring via dynamic transformations between two coupled edge solitons and bulk vortex solitons. This mechanism differs completely from the well-known switching process mediated by the creation and annihilation of vortex-antivortex pairs in terms of the associated topological solitons, energies, and spin-wave emissions. Strongly localized out-of-plane gyrotropic fields induced by the fast motion of the coupled edge solitons enable a magnetization dip that plays a crucial role in the formation of the reversed core magnetization. This work provides a deeper physical insight into the dynamic transformations of magnetic topological solitons in nanoelements.

Recursive evolution of spin-wave multiplets in magnonic crystals of antidot-lattice fractals.

We explored spin-wave multiplets excited in a different type of magnonic crystal composed of ferromagnetic antidot-lattice fractals, by means of micromagnetic simulations with a periodic boundary condition. The modeling of antidot-lattice fractals was designed with a series of self-similar antidot-lattices in an integer Hausdorff dimension. As the iteration level increased, multiple splits of the edge and center modes of quantized spin-waves in the antidot-lattices were excited due to the fractals' inhomogeneous and asymmetric internal magnetic fields. It was found that a recursive development (Fn = Fn-1 + Gn-1) of geometrical fractals gives rise to the same recursive evolution of spin-wave multiplets.

Ultra-high rate of temperature increment from superparamagnetic nanoparticles for highly efficient hyperthermia.

The magneto-thermal effect, which represents the conversion of magnetostatic energy to heat from magnetic materials, has been spotlighted for potential therapeutic usage in hyperthermia treatments. However, the realization of its potential has been challenged owing to the limited heating from the magnetic nanoparticles. Here, we explored a new-concept of magneto-thermal modality marked by low-power-driven, fast resonant spin-excitation followed by consequent energy dissipation, which concept has yet to be realized for current hyperthermia applications. We investigated the effect of spin resonance-mediated heat dissipation using superparamagnetic Fe3O4 nanoparticles and achieved an extraordinary initial temperature increment rate of more than 150 K/s, which is a significant increase in comparison to that for the conventional magnetic heat induction of nanoparticles. This work would offer highly efficient heat generation and precision wireless controllability for realization of magnetic-hyperthermia-based medical treatment.

Optimizing machine learning models for granular NdFeB magnets by very fast simulated annealing.

The macroscopic properties of permanent magnets and the resultant performance required for real implementations are determined by the magnets' microscopic features. However, earlier micromagnetic simulations and experimental studies required relatively a lot of work to gain any complete and comprehensive understanding of the relationships between magnets' macroscopic properties and their microstructures. Here, by means of supervised learning, we predict reliable values of coercivity (µ0Hc) and maximum magnetic energy product (BHmax) of granular NdFeB magnets according to their microstructural attributes (e.g. inter-grain decoupling, average grain size, and misalignment of easy axes) based on numerical datasets obtained from micromagnetic simulations. We conducted several tests of a variety of supervised machine learning (ML) models including kernel ridge regression (KRR), support vector regression (SVR), and artificial neural network (ANN) regression. The hyper-parameters of these models were optimized by a very fast simulated annealing (VFSA) algorithm with an adaptive cooling schedule. In our datasets of randomly generated 1,000 polycrystalline NdFeB cuboids with different microstructural attributes, all of the models yielded similar results in predicting both µ0Hc and BHmax. Furthermore, some outliers, which deteriorated the normality of residuals in the prediction of BHmax, were detected and further analyzed. Based on all of our results, we can conclude that our ML approach combined with micromagnetic simulations provides a robust framework for optimal design of microstructures for high-performance NdFeB magnets.

Intrinsic DMI-free skyrmion formation and robust dynamic behaviors in magnetic hemispherical shells.

We performed finite-element micromagnetic simulations to examine the formation of skyrmions without intrinsic Dzyaloshinskii-Moriya interaction (DMI) in magnetic hemispherical shells. We found that curvature-induced DM-like interaction allows for further stabilization of skyrmions without the DMI in curved-geometry hemispherical shells for a specific range of uniaxial perpendicular magnetic anisotropy (PMA) constant Ku. The larger the curvature of the shell, the higher the Ku value required for the formation of the skyrmions. With well-stabilized skyrmions, we also found in-plane gyration modes and azimuthal spin-wave modes as well as an out-of-plane breathing mode, similarly to previously found modes for planar geometries. Furthermore, additional higher-frequency hybrid modes were observed due to coupling between the gyration and azimuthal modes. This work provides further physical insight into the static and dynamic properties of intrinsic DMI-free skyrmions formed in curved-geometry systems.

Wireless Control of Two- and Three-Dimensional Actuations of Kirigami Patterns Composed of Magnetic-Particles-Polymer Composites.

We demonstrate wireless remote control of two-dimensional (2D) and three-dimensional (3D) shape transformations of specially designed kirigami patterns by application of static magnetic fields. The kirigami patterns consist of hinge-linked periodic unit blocks composed of magnetic-particle-elastomer composites. By designing the axis of magnetic anisotropy in each unit block and determining the placement of the hinges that link the individual unit blocks, 2D and 3D transformations of the patterns were demonstrated under application of uniform magnetic fields with specific field directions. Magnetic nanoparticles in an elastomer matrix within unit blocks were aligned in-plane or out-of-plane with respect to the frame of the individual unit blocks by application of magnetic fields. Such 2D and 3D actuations of kirigami patterns might offer a first step toward the development of spatiotemporal actuation and transformation of more complex 3D shapes using magnetic-particle-elastomer composites.

Spin-wave duplexer studied by finite-element micromagnetic simulation.

We conceptually designed a robust nano-scale waveguide structure suitable for potential use as a spin-wave duplexer that allows signal propagation only of selected narrow-band frequencies and duplex transmission in a three-port device comprising a receiver, a transmitter, and their common antenna. The waveguide structure combines three different arms and a circular ring, both made of nanostrip waveguides and a single magnetic material for reliably controllable propagations of spin waves. We attribute the observed duplex transmission of spin waves of narrow pass bands to scattering of spin waves by edge solitons placed at contact areas between the arms and the circular ring. This work proposes the first concept of nano-scale magnonic duplexers operating beyond GHz-frequency ranges.

Robust magnon-photon coupling in a planar-geometry hybrid of inverted split-ring resonator and YIG film.

We experimentally demonstrate strongly enhanced coupling between excited magnons in an Yttrium Iron Garnet (YIG) film and microwave photons in an inverted pattern of split-ring resonator (noted as ISRR). The anti-crossing effects of the ISRR's photon mode and the YIG's magnon modes were found from |S21|-versus-frequency measurements for different strengths and directions of externally applied magnetic fields. The spin-number-normalized coupling strength (i.e. single spin-photon coupling) [Formula: see text] was determined to 0.194 Hz ([Formula: see text] = 90 MHz) at 3.7 GHz frequency. Furthermore, we found that additional fine features in the anti-crossing region originate from the excitation of different spin-wave modes (such as the magnetostatic surface and the backward-volume magnetostatic spin-waves) rather than the Kittel-type mode. These spin-wave modes, as coupled with the ISRR mode, modify the anti-crossing effect as well as their coupling strength. An equivalent circuit model very accurately reproduced the observed anti-crossing effect and its coupling strength variation with the magnetic field direction in the planar-geometry ISRR/YIG hybrid system. This work paves the way for the design of new types of high-gain magnon-photon coupling systems in planar geometry.

Coupled gyration modes in one-dimensional skyrmion arrays in thin-film nanostrips as new type of information carrier.

We report on a micromagnetic numerical simulation study of dynamic coupling between neighboring skyrmions periodically arranged in narrow-width nanostrips. We explored the coupled gyration modes and their characteristic dispersions in terms of the interdistance between the neighboring skyrmions. The application of perpendicular magnetic fields allows for the control and modification of the dispersion of the coupled gyration modes. The coupled gyration modes of individual skyrmions might provide a new type of information carrier in narrow-width straight and curved nanostrips, as driven by magnetic interactions in such continuous thin films.

Resonant vortex-core reversal in magnetic nano-spheres as robust mechanism of efficient energy absorption and emission.

We report on novel vortex-core reversal dynamics in nano-spheres of single-vortex spin configuration as revealed by micromagnetic simulations combined with analytical derivations. When the frequency of an AC magnetic field is tuned to the frequency of the vortex-core precession around the direction of a given static field, oscillatory vortex-core reversals occur, and additionally, the frequency is found to change with both the strength of the applied AC field and the particle size. Such resonant vortex-core reversals in nano-spheres may provide a new and efficient means of energy absorption by, and emission from, magnetic nanoparticles, which system can be effectively implemented in bio-applications such as magnetic hyperthermia.

Resonantly excited precession motion of three-dimensional vortex core in magnetic nanospheres [corrected].

We found resonantly excited precession motions of a three-dimensional vortex core in soft magnetic nanospheres and controllable precession frequency with the sphere diameter 2R, as studied by micromagnetic numerical and analytical calculations. The precession angular frequency for an applied static field HDC is given as ωMV = γeffHDC, where γeff = γ〈mΓ〉 is the effective gyromagnetic ratio in collective vortex dynamics, with the gyromagnetic ratio γ and the average magnetization component 〈mΓ〉 of the ground-state vortex in the core direction. Fitting to the micromagnetic simulation data for 〈mΓ〉 yields a simple explicit form of 〈mΓ〉 ≈ (73.6 ± 3.4)(lex/2R)(2.20±0.14), where lex is the exchange length of a given material. This dynamic behavior might serve as a foundation for potential bio-applications of size-specific resonant excitation of magnetic vortex-state nanoparticles, for example, magnetic particle resonance imaging.

Magnetic microstructure of rolled-up single-layer ferromagnetic nanomembranes.

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.