Dynamic behavior of neodymium magnets
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Loeffler, Colin↗
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Results for “magnetization dynamics”
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Geostrophic to magnetically-damped turbulence in liquid metal rotating magnetoconvection
Understanding planetary core convection dynamics requires the study of convective flows in which the Coriolis and Lorentz forces attain a leading-order, so-called magnetostrophic balance. Experimental investigations of rotating magnetoconvection (RMC) in the magnetostrophic regime are therefore essential to broadly characterize the properties of local-scale planetary core flow. Toward this end, we present here the first thermovelocimetric measurements of magnetostrophic, liquid metal convection, which are made using liquid gallium as the working fluid, at moderate rotation rates (Ekman numbers 10 -4 ≤ Ek ≤ 10 -5 ) and in the presence of dynamically strong magnetic fields (Elsasser number Λ = 1). Complementary rotating convection (RC) experiments are performed at the same rotation rates to serve as reference cases. Our RMC velocity measurements adequately follow a geostrophic turbulent scaling for cases in which local-scale convective inertial forces exceed the Lorentz forces in the fluid bulk. In cases where Lorentz forces exceed local-scale inertia (local or convective interaction parameter N ℓ or N C ≳ 3), the root-mean-square RMC velocities are magnetically damped, yielding values below the geostrophic turbulent RC scaling prediction. An enhancement in heat transfer is observed, which we attribute to the increased coherence of vertically aligned magnetostrophic convective flow. Finally, our results and comparisons to dynamo simulations suggest that the system is in the strong-field regime, where convection-scale flows are magnetically damped with N C ≳ 3).
Convective turbulence↗
Electron Drift Kinetics in Space and in the Lab [Slides]
In a magnetized plasma, dynamics are fundamentally split between the directions parallel and perpendicular to the magnetic field. Generally, gyration about the magnetic field will occur on smaller spatial and time scales than the bulk dynamics of the system. From Hamiltonian mechanics, a periodic motion on a time scale much shorter than the other system dynamics leads to an adiabatic invariant. The adiabatic invariant for this motion is the magnetic moment $\mu=\frac{mv⟂^2}{2B}$, which will be approximately conserved.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Formation of collisionless shocks driven by strongly magnetized relativistic electrons in the laboratory
In experiments performed with the OMEGA EP laser system, proton deflectometry captured magnetic field dynamics consistent with collisionless shock formation driven by strongly magnetized relativistic electrons. During laser-foil interactions, shocks can form as relativistic electrons and strong surface magnetic fields generated by a short-pulse laser impinge on a cooler plasma produced by a longer-pulse laser. Three-dimensional particle-in-cell simulations reproduce the magnetic draping and fast formation speeds measured in the experiment and reveal that this relativistic-electron-driven shock forms at an interface that is unstable to shear and streaming instabilities. The simulation results provide insight into the microphysics that may influence high-energy shocks observed in extreme astrophysical environments. Published by the American Physical Society 2024
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Detecting axion dynamics on the surface of magnetic topological insulators
Axions, initially proposed to solve the strong CP problem, have recently gained attention in condensed matter physics, particularly in topological insulators. However, detecting axion dynamics has proven challenging, with no experimental confirmations to date. In this paper, we identify the surface of magnetic topological insulators as an ideal platform for observing axion dynamics. The vanishing bulk gap at the surface allows for order O(1) variations in the axion field, making the detection of axionlike phenomena more feasible. In contrast, these phenomena are strongly suppressed in the bulk due to the small magnetic exchange gap. We investigate two-photon decay as a signature of axion dynamics and calculate the branching ratio using a perturbative approach. Our findings reveal that the photon flux emitted from the surface is in plane and orders of magnitude larger than that from the bulk, making it detectable with modern microwave technology. We also discuss potential material platforms for detecting axion two-photon decay and strategies to enhance the signal-to-noise ratio.
Gao, Zhi-Qiang↗
Dynamic Symmetry Breaking in Chiral Magnetic Systems
The Dzyaloshinskii–Moriya interaction (DMI) in magnetic systems stabilizes spin textures with preferred chirality, applicable to next-generation memory and computing architectures. In perpendicularly magnetized heavy-metal/ferromagnet films, the interfacial DMI originating from structural inversion asymmetry and strong spin-orbit coupling favors chiral Néel-type domain walls (DWs) whose energetics and mobility remain at issue. Thus, a new effect is characterized in which domains expand unidirectionally in response to a combination of out-of-plane and in-plane magnetic fields, with the growth direction controlled by the in-plane field strength. These growth directionalities and symmetries with applied fields cannot be understood from static treatments alone. The authors theoretically demonstrate that perpendicular field torques stabilize steady-state magnetization profiles highly asymmetric in elastic energy, resulting in a dynamic symmetry breaking consistent with the experimental findings. This phenomenon sheds light on the mechanisms governing the dynamics of Néel-type DWs and expands the utility of field-driven DW motion to probe and control chiral DWs.
36 MATERIALS SCIENCE↗
Memory and rejuvenation in glassy systems
Here, the memory effect in a single crystal spin glass (Cu 0.92 Mn 0.08 ) has been measured using 1 Hz ac susceptibility measurements over a reduced temperature range of 0.4 - 0.7 T g and a model of the memory effect has been developed. A double-waiting-time protocol is carried out where the spin glass is first allowed to age at a temperature below T g , T w$_{1}$ , followed by a second aging 4 K lower, T w$_{2}$ . The 4 K separation is sufficient to ensure rejuvenation has occurred. The model is based on calculating overlaps between the growth of the correlation lengths at the two temperatures. It accounts for the absolute magnitude of the memory effect as a function of both waiting times and temperatures. The data can be explained by the memory loss being a function of the relative change in the correlated volume at the first waiting temperature due to growth in the correlations at the second waiting temperature.
36 MATERIALS SCIENCE↗
Temperature-induced hexagonal–orthorhombic phase transition in lutetium ferrite nanoparticles
The x-ray diffraction, Raman, and infrared spectroscopies and magnetic measurements were used to explore the correlated changes of the structure, lattice dynamics, and magnetic properties of the LuFeO3 nanoparticles, which appear in dependence on their sintering temperature. We revealed a gradual substitution of the hexagonal phase by the orthorhombic phase in the nanoparticles, with sintering temperature increasing from 700 to 1100 °C. The origin and stability of the hexagonal phase in the LuFeO3 nanoparticles are of the special interest, because the nanoparticles in the phase can be a room-temperature multiferroic with a weak ferromagnetic and pronounced structural and ferroelectric long-range ordering. The antiferromagnetic and nonpolar orthorhombic phase is more stable in the bulk LuFeO3. To define the ranges of the hexagonal phase stability, we determine the bulk and interface energy densities of different phases from the comparison of the Gibbs model with experimental results. Using effective parameters of the Gibbs model, we predict the influence of size effects and temperature on the structural and polar properties of the LuFeO3 nanoparticles. Analysis of the obtained results shows that the combination of the x-ray diffraction, Raman and infrared spectroscopies, magnetic measurements, and theoretical modeling of structural and polar properties allows us to establish the interplay between the phase composition, lattice dynamics, and multiferroic properties of the LuFeO3 nanoparticles prepared under different conditions.
Materials Science↗
Critical magnetic Reynolds number of the turbulent dynamo in collisionless plasmas
The intracluster medium of galaxy clusters is an extremely hot and diffuse, nearly collisionless plasma, which hosts dynamically important magnetic fields of ~μG strength. Seed magnetic fields of much weaker strength of astrophysical or primordial origin can be present in the intracluster medium. In collisional plasmas, which can be approximated in the magnetohydrodynamical (MHD) limit, the turbulent dynamo mechanism can amplify weak seed fields to strong dynamical levels efficiently by converting turbulent kinetic energy into magnetic energy. However, the viability of this mechanism in weakly collisional or completely collisionless plasma is much less understood. In this study, we explore the properties of the collisionless turbulent dynamo using three-dimensional hybrid-kinetic particle-in-cell simulations. We explore the properties of the collisionless turbulent dynamo in the kinematic regime for different values of the magnetic Reynolds number, Rm, initial magnetic-to-kinetic energy ratio, (E mag /E kin ) i , and initial Larmor ratio, (r Larmor /L box ) i , i.e. the ratio of the Larmor radius to the size of the turbulent system. We find that in the ‘un-magnetized’ regime, (r Larmor /L box ) i > 1, the critical magnetic Reynolds number for the dynamo action Rm crit ≈ 107 ± 3. In the ‘magnetized’ regime, (r Larmor /L box ) i ≲ 1, we find a marginally higher Rm crit = 124 ± 8. We find that the growth rate of the magnetic energy does not depend on the strength of the seed magnetic field when the initial magnetization is fixed. We also study the distribution and evolution of the pressure anisotropy in the collisionless plasma and compare our results with the MHD turbulent dynamo.
79 ASTRONOMY AND ASTROPHYSICS↗
Electronic Probing and Manipulation of Low-Dimensional Magnets (Final Report)
We investigate the effects of reduced dimensionality of nanomagnets on the dynamical properties at low temperatures where quantum effects such as confinement, coherence, decoherence, and environmental coupling may be important. The experimental focus is on low-temperature transport measurements of such nanomagnets and cryogenic preamplification, followed by theoretical modeling of magnetic dynamics. In addition, we explore quantum effects in magnetic dynamics for their potential as quantum sensors of spin. We are particularly interested in the regime of strong environmental coupling, where novel phases of magnetic matter such as spin-boson may develop. The key new results are improved signal-to-noise ratio of magnetic tunneling junctions through low temperature preamplification that enables us to find the spin-dependent component of the current noise; a new powerful technique to simulate magnetic dynamics in the quantum coherent regime using the master equation formalism; and, finally, experimental hint of a novel state of matter in rare-earth doped permalloy ferromagnetic samples.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Spin Dynamics of Triple-𝐐 Magnetic Orderings in a Triangular Lattice: Implications for Multi-𝐐 Orderings in General Two-Dimensional Lattices
Multi-𝐐 magnetic structures on two-dimensional (2D) lattices provide a key route to realizing topological physics in 2D magnetism. A major experimental challenge is to unambiguously confirm their formation by excluding the possibility of topologically trivial multidomain single- or double-𝐐 magnetic orders, which cannot be distinguished using conventional diffraction techniques. Here, we propose that long-wavelength spin dynamics offers a universal diagnostic for triangular lattices: Triple-𝐐 orders that preserve rotational symmetry and single- or double-𝐐 orders that break it exhibit qualitatively distinct anisotropies in their Goldstone-mode velocities, stemming from fundamental differences in their underlying spin configurations. We validate this concept using the metallic triangular-lattice antiferromagnet Co 0.325 TaS 2 , which hosts both a stripe-type single-𝐐 state and a triple-𝐐 tetrahedral ordering at different temperatures. Using inelastic neutron-scattering and spin dynamics simulations, we first refine the spin Hamiltonian by fitting the paramagnetic excitation spectra, allowing us to develop an unbiased model independent of magnetic ordering. We then show that the observed velocity profiles of the Goldstone modes agree with the high-temperature model’s predictions: markedly anisotropic for the single-𝐐 phase and near isotropic for the triple-𝐐 phase. Importantly, this contrast persists across various exchange parameters, highlighting its model-independent nature and suggesting potential applicability to other 2D lattice systems. Beyond the long-wavelength regime, we present a substantial discrepancy between the measured and simulated magnon spectra exclusively in the triple-𝐐 phase. We attribute this discrepancy to magnon energy renormalization arising from order-of-magnitude-enhanced magnon-magnon interactions in the triple-𝐐 phase, due to its noncollinear configuration. This work provides universal insight into the dynamical properties of topological multi-𝐐 magnetic orderings in 2D lattice structures, offering a broadly applicable diagnostic to distinguishing them from topologically trivial single- or double-𝐐 counterparts. The unequivocal confirmation of the triple-𝐐 structure in Co 0.325 TaS 2 further establishes it as a prominent material platform for exploring topological spin textures in the genuine 2D limit.
Park, Pyeongjae [Oak Ridge National Laboratory (OR↗
N - V –Diamond Magnetic Microscopy Using a Double Quantum 4-Ramsey Protocol
We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables wide-field magnetic imaging using nitrogen-vacancy (N-V) centers in diamond, with enhanced homogeneity of the magnetic sensitivity relative to conventional single quantum (SQ) techniques. The DQ 4-Ramsey protocol employs microwave-phase alternation across four consecutive Ramsey (4-Ramsey) measurements to isolate the desired DQ magnetic signal from any residual SQ signal induced by microwave pulse errors. In a demonstration experiment employing a 1-μm-thick N-V layer in a macroscopic diamond chip, the DQ 4-Ramsey protocol provides a volume-normalized dc magnetic sensitivity of η V = 34nTHz –1/2 μm 3/2 across a 125 μm × 125 μm field of view, with about 5× less spatial variation in sensitivity across the field of view compared to a SQ measurement. Furthermore, the improved robustness and magnetic sensitivity homogeneity of the DQ 4-Ramsey protocol enables imaging of dynamic broadband magnetic sources such as integrated circuits and electrically active cells.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Conservation of angular momentum in ultrafast spin dynamics
The total angular momentum of a closed system is a conserved quantity, which should remain constant in time for any excitation experiment once the pumping signal has been extinguished. Such conservation, however, is never satisfied in practice in any real-time first-principles description of the demagnetization process. Furthermore, there is growing experimental evidence that the same takes place in experiments. The missing angular momentum is usually associated to lattice vibrations, which are not measured experimentally and are never considered in real-time simulations. Here, in this study, we critically analyze the issue and conclude that current state-of-the-art simulations violate angular momentum conservation already at the electronic level of description. This shortcoming originates from an oversimplified description of the spin-orbit coupling, which includes atomic contributions but neglects completely that of itinerant electrons. We corroborate our findings with time-dependent simulations using model tight-binding Hamiltonians, and show that indeed such conservation can be reintroduced by an appropriate choice of spin-orbit coupling. The consequences of our findings on recent experiments are also discussed.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Dynamics of a fractal set of first-order magnetic phase transitions in frustrated Lu 2 CoMnO 6
The axial next-nearest-neighbor Ising model predicts a fractal (infinite) set of phases with incommensurate wave vectors that are separated by first-order phase boundaries. This complexity results from a simple frustration condition between nearest- and next-nearest-neighbor interactions along a chain of Ising spins. Using x-ray photon correlation spectroscopy (XPCS), we investigate the surprising antiferromagnetic dynamics that emerge from such a complex phase diagram over a wide range of temperatures. In this work, we present XPCS measurements of the frustrated magnetic chain compound Lu 2 CoMnO 6 and Monte Carlo simulations. Incommensurate magnetic Bragg peaks slide towards commensurate “up-up-down-down” spin order with decreasing temperature and increasing time. Both simulation and experiment support a counterintuitive “upside-down” temperature dependence of the magnetic dynamics: at higher temperatures in the region of first-order phase boundaries, slower dynamics are observed where the speckle maintains its coherence. At the lowest temperatures, where part of the sample adopts commensurate order, the dynamics speed up and result in fast decoherence.
36 MATERIALS SCIENCE↗
Spin-glass dynamics in the presence of a magnetic field: exploration of microscopic properties
The synergy between experiment, theory, and simulations enables a microscopic analysis of spin-glass dynamics in a magnetic field in the vicinity of and below the spin-glass transition temperature T g . The spin-glass correlation length, ξ(t, t w ; T), is analysed both in experiments and in simulations in terms of the waiting time t w after the spin glass has been cooled down to a stabilised measuring temperature T < T g and of the time t after the magnetic field is changed. This correlation length is extracted experimentally for a CuMn 6 at. % single crystal, as well as for simulations on the Janus II special-purpose supercomputer, the latter with time and length scales comparable to experiment. The non-linear magnetic susceptibility is reported from experiment and simulations, using ξ(t, t w ; T) as the scaling variable. Previous experiments are reanalysed, and disagreements about the nature of the Zeeman energy are resolved. The growth of the spin-glass magnetisation in zero-field magnetisation experiments, M ZFC (t, t w ; T), is measured from simulations, verifying the scaling relationships in the dynamical or non-equilibrium regime. Lastly, our preliminary search for the de Almeida–Thouless line in D = 3 is discussed.
36 MATERIALS SCIENCE↗
Stimulated Nucleation of Skyrmions in a Centrosymmetric Magnet
Understanding the dynamics of skyrmion nucleation and manipulation is important for applications in spintronic devices. In this contribution, the spin textures at magnetic domain-boundaries stimulated by application of in-plane magnetic fields in a centrosymmetric kagome ferromagnet, Fe 3 Sn 2 , with thickness gradient are investigated using Lorentz transmission electron microscopy. Switching of the in-plane magnetic field is shown to induce a reversible transformation from magnetic stripes to skyrmions, or vice versa, at the interface between differently oriented domains. Micromagnetic simulations combined with experiments reveal that the rotatable anisotropy and thickness dependence of the response to the external in-plane field are the critical factors for the skyrmion formation. In addition, it is shown that the helicity of skyrmions can also be controlled using this dynamic process. Finally, the results suggest that magnetic materials with rotatable anisotropy are potential skyrmionic systems and provides a different approach for nucleation and manipulation of skyrmions in spintronic devices.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Control of the Folding Dynamics of Self-Reconfiguring Magnetic Microbots Using Liquid Crystallinity
Reconfigurable microdevices are being explored in a range of contexts where their life-like abilities to move and change shape are important. While much work has been done to control the motion of these microdevices by engineering their geometry and composition, little is known about their dynamics in complex fluid environments with non-Newtonian rheology. Here in this paper, it is shown how the actuation dynamics of reconfigurable microdevices made by assembly of patchy magnetic microcubes, which are referred to as “microbots,” can be modulated by their interactions with the anisotropic viscoelastic environment of a liquid crystal (LC). The free energy arising from the elastic strain of LC and formation of topological defects around the microbots influences their folding dynamics, which can be tuned by tailoring both the far-field orientation of the LC and the local ordering of the LC at the microbot surfaces. These findings represent a first step toward establishing a general set of design rules to control the dynamics of microbots using complex anisotropic fluids.
36 MATERIALS SCIENCE↗
Nonclassical Spin Transfer Effects in an Antiferromagnet
We simulate scattering of electrons by a chain of antiferromagnetically coupled quantum Heisenberg spins, to analyze spin-transfer effects not described by the classical models of magnetism. Our simulations demonstrate efficient excitation of dynamical states that would be forbidden by the semiclassical symmetries, such as generation of multiple magnetic excitation quanta by a single electron. Furthermore, quantum interference of spin wave functions enables generation of magnetization dynamics with amplitudes exceeding the transferred magnetic moment. Here, the efficiency of excitation is almost independent of the electron spin polarization, and is governed mainly by the transfer of energy. Nonclassical spin transfer may thus enable efficient electronic control of antiferromagnets not limited by the classical constraints.
36 MATERIALS SCIENCE↗