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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

The spin-partitioned total position-spread tensor: An application to Heisenberg spin chains

The spin partition of the Total Position-Spread (TPS) tensor has been performed for one-dimensional Heisenberg chains with open boundary conditions. Both the cases of a ferromagnetic (high-spin) and an anti-ferromagnetic (low-spin) ground-state have been considered. In the case of a low-spin ground-state, the use of alternating magnetic couplings allowed to investigate the effect of spin-pairing. The behavior of the spin-partitioned TPS (SP-TPS) tensor as a function of the number of sites turned to be closely related to the presence of an energy gap between the ground-state and the first excited-state at the thermodynamic limit. Indeed, a gapped energy spectrum is associated to a linear growth of the SP-TPS tensor with the number of sites. On the other hand, in gapless situations, the spread presents a faster-than-linear growth, resulting in the divergence of its per-site value. Finally, for the case of a high-spin wave function, an analytical expression of the dependence of the SP-TPS on the number of sites n and the total spin-projection S{sub z} has been derived.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Chaotic parametric soliton-like pulses in ferromagnetic-film active ring resonators

The generation of quasi-periodic sequences of parametric soliton-like pulses in an active ring resonator with a ferromagnetic film via the three-wave parametric instability of a magnetostatic surface wave is studied theoretically and experimentally. These dissipative structures form in time due to the competition between the cubic nonlinearity caused by parametric coupling between spin waves and the time dispersion caused by the resonant cavity that is present in a self-oscillatory system. The development of dynamic chaos due to the parametric instability of a magnetostatic surface wave results in irregular behavior of a phase. However, this behavior does not break a quasi-periodic pulse sequence when the gain changes over a wide range. The generated soliton-like pulses have a chaotic nature, which is supported by the maximum Lyapunov exponent estimated from experimental time series.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Anomalous magnetic behavior of Ba 2 CoO 4 with isolated CoO 4 tetrahedra

The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we discover unusual magnetic behavior in three-dimensional (3D) Ba 2 CoO 4 . In spite of isolated CoO 4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to T N =26 K. More unexpectedly, spin dynamics displays quasi-two-dimensional spin-wave dispersion with an unusually large spin gap, and one-dimensional magnetoelastic coupling. Our results postulate that Ba 2 CoO 4 is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and spin states in oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Slow-Wave Hybrid Magnonics

Cavity magnonics is an emerging research area focusing on the coupling between magnons and photons. Despite its great potential for coherent information processing, it has been long restricted by the narrow interaction bandwidth. Here, in this Letter, we theoretically propose and experimentally demonstrate a novel approach to achieve broadband photon-magnon coupling by adopting slow waves on engineered microwave waveguides. To the best of our knowledge, this is the first time that slow wave is combined with hybrid magnonics. Its unique properties promise great potentials for both fundamental research and practical applications, for instance, by deepening our understanding of the light -matter interaction in the slow wave regime and providing high -efficiency spin wave transducers. The device concept can be extended to other systems such as optomagnonics and magnomechanics, opening up new directions for hybrid magnonics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Magnetic excitations in infinite-layer nickelates

The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L 3 -edge. Undoped NdNiO 2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetic excitations and interactions in the Kitaev hyperhoneycomb iridate β –Li 2 ⁢IrO 3

Here, we present a thorough experimental study of the three-dimensional hyperhoneycomb Kitaev magnet β –Li 2 ⁢IrO 3 , using a combination of inelastic neutron scattering (INS), time-domain terahertz spectroscopy (TDTS), and heat capacity measurements. The main results include a massive low-temperature reorganization of the INS spectral weight that evolves into a broad peak centered around 12 meV, and a distinctive peak in the terahertz data at 2.8(1) meV. A detailed comparison to powder-averaged spin-wave theory calculations reveals that the positions of these two features are controlled by the anisotropic Γ coupling and the Heisenberg exchange J, respectively. The refined microscopic spin model places β –Li 2 ⁢IrO 3 in close proximity to the Kitaev spin liquid phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Perspective: Ultrafast magnetism and THz spintronics

This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time, this milestone work by Bigot and coworkers gave insight directly into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than providing an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first applications are developed: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Spontaneous magnon decays from nonrelativistic time-reversal symmetry breaking in altermagnets

Quasiparticles are central to condensed matter physics, but their stability can be undermined by quantum many-body interactions. Magnons, i.e., quasiparticles in quantum magnets, are particularly intriguing because their properties are governed by both real and spin space. While crystal symmetries may be low, spin interactions often remain approximately isotropic, limiting spontaneous magnon decay. Textbook wisdom holds that collinear Heisenberg magnets follow a dichotomy: ferromagnets host stable magnons, while antiferromagnetic magnons may decay depending on dispersion curvature. Up to now, relativistic spin-orbit coupling and noncollinear order that connect spin space to real space were shown to introduce more complex magnon instability mechanisms. Here, we show that even in nonrelativistic isotropic collinear systems, this conventional dichotomy is disrupted in altermagnets. Altermagnets, a newly identified class of collinear magnets, exhibit compensated spin order with nonrelativistic time-reversal symmetry breaking and even-parity band splitting. Using kinematic analysis, nonlinear spin-wave theory, and quantum simulations, we reveal that even weak band splitting opens a decay phase space, driving quasiparticle breakdown. Additionally, 𝑑-wave altermagnets form a rare “island of stability” at the Brillouin-zone center. Furthermore, our findings establish a quasiparticle stability trichotomy in collinear Heisenberg magnets and position altermagnets as a promising platform for unconventional spin dynamics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Systematic study of the spin stiffness dependence on phosphorus alloying in the ferromagnetic semiconductor (Ga,Mn)As

We study the dependence of the spin stiffness constant on the phosphorus concentration in the ferromagnetic semiconductor (Ga,Mn)(As,P) with the aim of determining whether alloying with phosphorus is detrimental, neutral, or advantageous to the spin stiffness. Time-resolved magneto-optical experiments are carried out in thin epilayers. Laser pulses excite two perpendicular standing spin wave modes, which are exchange related. We show that the first mode is spatially uniform across the layer corresponding to a k≈0 wavevector. From the two frequencies and k-vector spacings we obtain the spin stiffness constant for different phosphorus concentrations using weak surface pinning conditions. The mode assessment is checked by comparison to the spin stiffness obtained from domain pattern analysis for samples with out-of-plane magnetization. The spin stiffness is found to exhibit little variation with phosphorus concentration in contradiction with ab-initio predictions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electrodynamic boundary conditions for planar arrays of thin magnetic elements

Approximate electrodynamic boundary conditions are derived for an array of dipolarly coupled magnetic elements. It is assumed that the elements' thickness is small compared to the wavelength of an electromagnetic wave in a free space. The boundary conditions relate electric and magnetic fields existing at the top and bottom sides of the array through the averaged uniform dynamic magnetization of the array. This dynamic magnetization is determined by the collective dynamic eigen-excitations (spin wave modes) of the array and is found using the external magnetic susceptibility tensor. The problem of oblique scattering of a plane electromagnetic wave on the array is considered to illustrate the use of the derived boundary conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

3⁢𝑄 magnetic order with spatially alternating spin scalar chirality in overdoped C⁢o 0.336⁢ Ta⁢S 2

C⁢o 𝑥 ⁢Ta⁢S 2 (𝑥≈1/3) exhibits a spontaneous Hall effect from spin texture in antiferromagnets, with a tetrahedral triple-Q (3Q) order and uniform spin scalar chirality. Upon Co overdoping (𝑥>1/3), it undergoes a shift in magnetic ordering vectors from 𝐐 m =(1/2, 0, 0) to (1/3, 0, 0). Interestingly, the spontaneous Hall effect disappeared in the overdoped regime, which was originally attributed to the loss of 3Q order. However, a question remains whether a new type of 3Q order can exist with alternating chirality in the overdoped regime. To address this, we investigated C⁢o 0.336⁢ Ta⁢S 2 using inelastic neutron scattering (INS), neutron diffraction, and optical dichroism, and found that INS data and spin-wave simulations support a 3Q order with alternating chirality. Moreover, neutron diffraction data show field-independent Bragg peaks, while linear dichroism detects no in-plane anisotropy, consistent with threefold rotation symmetry. Our data support the scenario of an alternating-chirality 3Q order in C⁢o 0.336 ⁢Ta⁢S 2 , canceling the spontaneous Hall effect. Furthermore, this study highlights a combined neutron-optical approach to identify complex spin textures.

Landau-Lifschitz-Gilbert equation↗
DOE OSTI2015

Spin-current injection and detection in κ-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br

Spin-current injection into an organic semiconductor κ-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br film induced by the spin pumping from an yttrium iron garnet (YIG) film. When magnetization dynamics in the YIG film is excited by ferromagnetic or spin-wave resonance, a voltage signal was found to appear in the κ-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br film. Magnetic-field-angle dependence measurements indicate that the voltage signal is governed by the inverse spin Hall effect in κ-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br. We found that the voltage signal in the κ-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br/YIG system is critically suppressed around 80 K, around which magnetic and/or glass transitions occur, implying that the efficiency of the spin-current injection is suppressed by fluctuations which critically enhanced near the transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electric field controlled spin interference in a system with Rashba spin-orbit coupling

There have been intense research efforts over the last years focused on understanding the Rashba spin-orbit coupling effect from the perspective of possible spintronics applications. An important component of this line of research is aimed at control and manipulation of electron’s spin degrees of freedom in semiconductor quantum dot devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction in a two-dimensional electron system embedded in a host semiconducting material that lacks inversion-symmetry. This way, the Rashba spin-orbit coupling effect may potentially lead to fabrication of a new generation of spintronic devices where control of spin, thus magnetic properties, is achieved via an electric field and not a magnetic field. In this work we investigate theoretically the electron’s spin interference and accumulation process in a Rashba spin-orbit coupled system consisting of a pair of two-dimensional semiconductor quantum dots connected to each other via two conducting semi-circular channels. The strength of the confinement energy on the quantum dots is tuned by gate potentials that allow “leakage” of electrons from one dot to another. While going through the conducting channels, the electrons are spin-orbit coupled to a microscopically generated electric field applied perpendicular to the two-dimensional system. We show that interference of spin wave functions of electrons travelling through the two channels gives rise to interference/conductance patterns that lead to the observation of the geometric Berry’s phase. Achieving a predictable and measurable observation of Berry’s phase allows one to control the spin dynamics of the electrons. It is demonstrated that this system allows use of a microscopically generated electric field to control Berry’s phase, thus, enables one to tune the spin-dependent interference pattern and spintronic properties with no need for injection of spin-polarized electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Unexpected magnetic behavior in amorphous Co{sub 90}Sc{sub 10} alloy

An amorphous alloy Co{sub 90}Sc{sub 10} has been prepared by rapid quenching from the melt. The results of magnetization measurements show that this alloy has the highest Curie temperature reported for any amorphous transition metal based alloys. Furthermore, for a Co based amorphous alloy, the magnetic moment is remarkably high. Moreover, the alloy exhibits soft magnetic properties. Based on the findings, amorphous Co{sub 90}Sc{sub 10} appears to be an attractive candidate for applications as a soft magnetic material. The temperature dependence of the reduced magnetization can be described by the Bloch power law. The results show that the B coefficient of the amorphous Co{sub 90}Sc{sub 10} alloy, which is a measure of the rigidity of spin waves, exhibits the lowest value observed until now for any amorphous alloy and is comparable to crystalline alloys. It is found that the Sc atoms in the Co{sub 90}Sc{sub 10} alloy lead to an increase of the itinerant spin moment of Co atoms, and, in contrast to this behaviour, to a decrease of the local 3d-electrons of Co.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Dynamics and thermodynamics of the 𝑆 = $\frac{5}{2}$ almost Heisenberg triangular lattice antiferromagnet K 2⁢ Mn⁢(SeO 3 ) 2

Here, we report calorimetric, magnetic, and neutron scattering studies on an 𝑆 = 5/2, nearly Heisenberg triangular-lattice antiferromagnet K 2 ⁢Mn⁢(SeO 3 ) 2 with weak XXZ easy-axis anisotropy. Multiple magnetic phases are identified, including a noncollinear Y phase in zero field, a field-induced collinear 𝑚 = 1/3 magnetization plateau, and a high-field V phase. In the Y phase, the magnetic excitation spectrum exhibits both single-magnon excitations and an extended high-energy continuum. Both features are well described by nonlinear spin wave theory. In the field-induced phases, complex effects of the spectrum renormalization even for large 𝑆 = 5/2 material are clearly detectable. These results underscore the essential role of magnon-magnon interactions in the dynamics of large-𝑆 Heisenberg spin systems on a triangular lattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Orbital Ordering Effect on Magnetic Resonance Spectra in Charge-Ordered and Frustrated Manganites

A theoretical study of the interrelation of the crystal structure, charge, orbital, and magnetic subsystems in R{sub 1 – x}A{sub x}MnO{sub 3} charge-ordered manganites has been carried out (where R{sup 3+} is the rare earth ion, A{sup 2+} is the alkaline earth metal ion, x = 0.5, 2/3). The model of orbital-dependent exchange interactions and single-ion anisotropy is used. The presence of quasi-low-dimensional magnetic structures is exhibited. The spin waves spectra and antiferromagnetic resonance spectra are calculated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Collective dynamics of polarized spin-half fermions in relativistic heavy-ion collisions

Standard relativistic hydrodynamics has been successful in describing the properties of the strongly interacting matter produced in the heavy-ion collision experiments. Recently, there has been a significant theoretical advancement in this field to explain spin polarization of hadrons emitted in these processes. Although current models have successfully explained some of the experimental data based on the coupling between spin polarization and vorticity of the medium, they still lack a clear understanding of the differential measurements. This is commonly interpreted as an indication that the spin needs to be treated as an independent degree of freedom whose dynamics is not entirely bound to flow circulation. In particular, if the spin is a macroscopic property of the system, in equilibrium its dynamics should follow hydrodynamic laws. Here, we develop a framework of relativistic hydrodynamics which includes spin degrees of freedom from the quantum kinetic theory for Dirac fermions and use it for modeling the dynamics of matter. Following experimental observations, we assume that the polarization effects are small and derive conservation laws for the net baryon current, the energy–momentum tensor and the spin tensor based on the de Groot–van Leeuwen–van Weert definitions of these currents. We present various properties of the spin polarization tensor and its components, analyze the propagation properties of the spin polarization components, and derive the spin-wave velocity for arbitrary statistics. We find that only the transverse spin components propagate, analogously to the electromagnetic waves. Finally, using our framework, we study the space–time evolution of the spin polarization for the systems respecting certain space–time symmetries and calculate the mean spin polarization per particle, which can be compared to the experimental data. We find that, for some observables, our spin polarization results agree qualitatively with the experimental findings and other model calculations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Selective excitation of spin resonance in orthoferrite PrFeO{sub 3} with impulsive polarized terahertz radiation

Single cycle terahertz (THz) pulses were employed to excite coherent spin waves in (110)-oriented PrFeO{sub 3} single crystal. The free induction decay radiations at frequency of 0.34 THz (quasi-ferromagnetic mode, FM mode) and 0.41 THz (quasi-antiferromagnetic mode, AFM mode) were observed arising from the coupling of magnetic moment with the impulsive magnetic field of polarized terahertz radiation. These two spin modes in PrFeO{sub 3} can be excited and modulated by the magnetic field of THz pulse with a specific polarization with respect to the crystal axis. The extracted complex dielectric permittivity and magnetic permeability dispersion in THz range suggest a higher efficiency of energy transfer from the impulsive THz pulse into the AFM than the FM spin system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗