Coupling Of Magnetic Moments Research Articles

Spintronics: Electrical Control of Perpendicular Magnetic Anisotropy and Spin‐Orbit Torque‐Induced Magnetization Switching (Adv. Electron. Mater. 3/2020)

In article number 1900782, Yufeng Tian and co-workers demonstrate that exchange coupling of magnetic moments across the Co/CoO interface provides an extra source of stabilization for the perpendicular magnetic anisotropy (PMA) in heterojunctions with a core structure of Pt/Co/CoO/TiO2 (TaOx). The inside cover image shows a representation of this system, with the background showing a Pt/Co/CoO/oxide/Pt cell crossbar array on a chip and the center showing an enlarged view.

Magnetic hysteresis and strong ferromagnetic coupling of sulfur-bridged Dy ions in clusterfullerene Dy2S@C82.

Two isomers of metallofullerene Dy2S@C82 with sulfur-bridged Dy ions exhibit broad magnetic hysteresis with sharp steps at sub-Kelvin temperature. Analysis of the level crossing events for different orientations of a magnetic field showed that even in powder samples, the hysteresis steps caused by quantum tunneling of magnetization can provide precise information on the strength of intramolecular Dy⋯Dy inter-actions. A comparison of different methods to determine the energy difference between ferromagnetic and antiferromagnetic states showed that sub-Kelvin hysteresis gives the most robust and reliable values. The ground state in Dy2S@C82 has ferromagnetic coupling of Dy magnetic moments, whereas the state with antiferromagnetic coupling in C s and C 3v cage isomers is 10.7 and 5.1 cm−1 higher, respectively. The value for the C s isomer is among the highest found in metallofullerenes and is considerably larger than that reported in non-fullerene dinuclear molecular magnets. Magnetization relaxation times measured in zero magnetic field at sub-Kelvin temperatures tend to level off near 900 and 3200 s in C s and C 3v isomers. These times correspond to the quantum tunneling relaxation mechanism, in which the whole magnetic moment of the Dy2S@C82 molecule flips at once as a single entity.

Electrical Control of Perpendicular Magnetic Anisotropy and Spin‐Orbit Torque‐Induced Magnetization Switching

AbstractVoltage‐driven oxygen ion migration in ferromagnetic metal/oxide heterostructures offers a highly effective means to tailor emergent interfacial functionalities. In heterojunctions with a core structure of Pt/Co/CoO/TiO2 (TaOx), it is demonstrated that exchange coupling of magnetic moments across the Co/CoO interface provides an extra source to stabilize the perpendicular magnetic anisotropy (PMA). Moreover, the strength of this interfacial coupling can be reversibly controlled through voltage‐driven oxygen ion migration at the Co/CoO interface, resulting in electrical‐field‐controllable PMA. In combination with the spin current generated from Pt, it is revealed that the spin‐orbit torque (SOT) switching of the perpendicular magnetization of Co can be turned ON/OFF by electrical field. Tunable PMA and SOT switching makes heavy metal/ferromagnetic metal/antiferromagnetic oxide heterojunctions a promising candidate to future voltage‐controlled, ultralow‐power, and high‐density spintronics devices.

Cavity-mediated dissipative coupling of distant magnetic moments: Theory and experiment

We investigate long-range coherent and dissipative coupling between two spatially separated magnets while both are coupled to a microwave cavity. A careful examination of the system shows that the indirect interaction between two magnon modes is dependent on their individual mechanisms of direct coupling to the cavity. If both magnon modes share the same form of coupling to the cavity (either coherent or dissipative), then the indirect coupling between them will produce level repulsion. Conversely, if the magnon modes have different forms of coupling to the cavity (one coherent and one dissipative), then their indirect coupling will produce level attraction. We further demonstrate the cavity-mediate nature of the indirect interaction through investigating the dependence of the indirect coupling strength on the frequency detuning between the magnon and cavity modes. Our work theoretically and experimentally explores indirect cavity mediate interactions in systems exhibiting both coherent and dissipative coupling, which opens a new avenue for controlling and utilizing light-matter interactions.

Non-Equilibrium Quantum Transport of Chiral Fluids from Kinetic Theory

We introduce the quantum-field-theory (QFT) derivation of chiral kinetic theory (CKT) from the Wigner-function approach, which manifests side jumps and non-scalar distribution functions associated with Lorentz covariance and incorporates both background fields and collisions. The formalism is utilized to investigate second-order responses of chiral fluids near local equilibrium. Such non-equilibrium anomalous transport is dissipative and affected by interactions. Contributions from both quantum corrections in anomalous hydrodynamic equations (EOM) of motion and those from the CKT and Wigner functions (WF) are considered in a relaxation-time approximation (RTA). Anomalous charged Hall currents engendered by background electric fields and temperature/chemical-potential gradients are obtained. Furthermore, chiral magnetic/vortical effects (CME/CVE) receive viscous corrections as non-equilibrium modifications stemming from the interplay between side jumps, magnetic-moment coupling, and chiral anomaly.

Exchange bias and magnetoresistance effects in La2-xSrxCoMnO6 (0.05 ≤ x ≤ 0.5)

Double perovskites with Sr2+-doping at the A-site in La2-xSrxCoMnO6 (0.05 ≤ x ≤ 0.5) are synthesized by the standard solid-state reaction method. X-ray diffraction indicates that the initial monoclinic crystal structure transforms to rhombohedra with the increasing Sr-doping from x = 0.05 to 0.5. The Sr-doped samples (0.1 ≤ x ≤ 0.3) possess mixed crystallographic phases with monoclinic and rhombohedra structure. The appropriate amount of Sr-doping (x = 0.3) can effectively increase exchange bias effect to reach a maximum value of 6.5 kOe (at 5 K) under the 10 kOe cooled magnetic field. The exchange coupling of uncompensated magnetic moment at the interface between ferromagnetic and antiferromagnetic phases is responsible for the variation trend of exchange bias effect. A large enhanced negative magnetoresistance effect (approximately 67.1%) of the x = 0.3 sample is observed at 85 K and 70 kOe. The reduction of the density of spin polarization states at Fermi level caused by disorder of Co/Mn ions is the reason for the decreasing magnetoresistance effect in the multiple Sr-doping (x > 0.3).

Vortex ferroelectric domains, large-loop weak ferromagnetic domains, and their decoupling in hexagonal (Lu, Sc)FeO3

The direct domain coupling of spontaneous ferroelectric polarization and net magnetic moment can result in giant magnetoelectric (ME) coupling, which is essential to achieve mutual control and practical applications of multiferroics. Recently, the possible bulk domain coupling, the mutual control of ferroelectricity (FE) and weak ferromagnetism (WFM) have been theoretically predicted in hexagonal LuFeO3. Here, we report the first successful growth of highly-cleavable Sc-stabilized hexagonal Lu0.6Sc0.4FeO3 (h-LSFO) single crystals, as well as the first visualization of their intrinsic cloverleaf pattern of vortex FE domains and large-loop WFM domains. The vortex FE domains are on the order of 0.1–1 μm in size. On the other hand, the loop WFM domains are ~100 μm in size, and there exists no interlocking of FE and WFM domain walls. These strongly manifest the decoupling between FE and WFM in h-LSFO. The domain decoupling can be explained as the consequence of the structure-mediated coupling between polarization and dominant in-plane antiferromagnetic spins according to the theoretical prediction, which reveals intriguing interplays between FE, WFM, and antiferromagnetic orders in h-LSFO. Our results also indicate that the magnetic topological charge tends to be identical to the structural topological charge. This could provide new insights into the induction of direct coupling between magnetism and ferroelectricity mediated by structural distortions, which will be useful for the future applications of multiferroics.

Dependence of Properties and Exchange Coupling Constants on the Charge in the Mn2O n and Fe2O n Series.

The geometrical structure and properties of the neutral and singly charged Mn2O nq and Fe2O nq clusters ( q = 0, ±1) are computed using density functional theory with the generalized gradient approximation in the range 1 ≤ n ≤ 7. The geometrical structures and spin multiplicities of the corresponding species in all six series are similar except for a few exceptions. Antiferromagnetic coupling of total spin magnetic moments of the metal atoms in the lowest total energy states is observed for the majority of species in all six series when n = 1-5; correspondingly, the computed magnetic exchange coupling constants are mostly negative. The states of Mn2O nq and Fe2O nq are nonmagnetic or weakly ferromagnetic when n > 5 except for Mn2O7+ where the ground state is antiferromagnetic. The computed adiabatic electron affinities and ionization energies of the neutral species in both series are quite close to one another and increase as n increases. However, the binding energies of a single oxygen atom and of an O2 dimer decrease as n increases and the Mn2O7+ and Fe2O7+ cations are barely stable with respect to the O2 abstraction. The most stable and least stable species at a given n are the anions and the cations, respectively. The electric dipole polarizability per atom decreases sharply when n moves from 1 to 4 and then remains nearly constant for larger n values in the anion series, whereas it is close to the asymptotic value already at n = 2 in the neutral series.

Electronic structure, stability, and magnetism of rutile-type ultrathin TiO2 nanotubes

In this study, based on density functional theory, we predicted the electronic structure, stability, and magnetism of ultrathin rutile-type TiO2 nanotubes (u-TiO2NTs) with different curvatures. The curvature energy calculations showed that the (m, 0) u-TiO2NTs follow the classical elasticity theory. As the diameter of the NTs increases, the wall thickness becomes thinner, and the binding energy and band gap increase until saturation. For the (10, 0) u-TiO2NT, the neutral Ti vacancies (VTi0) could induce a large magnetic moment and long-range ferromagnetic coupling, whereas the neutral O vacancies (VO0) cannot. In addition to calculations of the vacancy formation energy, we investigated the magnetism and magnetic coupling of the (10, 0) u-TiO2NT with VTi− or VO+. We found that the introduction of VTi− vacancies led to unstable ferromagnetic coupling in the NTs, whereas the VO+ vacancies preferred to couple in a stable ferromagnetic manner. Our results suggest a possible route for obtaining high Curie temperature ferromagnetism in TiO2 materials.

Role of atomic spin-mechanical coupling in the problem of a magnetic biocompass.

It is a well established notion that animals can detect the Earth's magnetic field, while the biophysical origin of such magnetoreception is still elusive. Recently, a magnetic receptor Drosophila CG8198 (MagR) with a rodlike protein complex is reported [S. Qin et al., Nat. Mater. 15, 217 (2016)10.1038/nmat4484] to act like a compass needle to guide the magnetic orientation of animals. This view, however, is challenged [M. Meister, Elife 5, e17210 (2016)10.7554/eLife.17210] by arguing that thermal fluctuations beat the Zeeman coupling of the proteins's magnetic moment with the rather weak geomagnetic field (∼25-65 μT). In this work, we show that the spin-mechanical interaction at the atomic scale gives rise to a high blocking temperature which allows a good alignment of the protein's magnetic moment with the Earth's magnetic field at room temperature. Our results provide a promising route to resolve the debate on the thermal behaviors of MagR, and may stimulate a broad interest in spin-mechanical couplings down to atomistic levels.

Thermal effects on ρ meson properties in an external magnetic field

A detailed study of the analytic structure of 1-loop self energy graphs for neutral and charged $\rho$ mesons is presented at finite temperature and arbitrary magnetic field using the real time formalism of thermal field theory. The imaginary part of the self energy is obtained from the discontinuities of these graphs across the Unitary and Landau cuts, which is seen to be different for $\rho^0$ and $\rho^\pm$. The magnetic field dependent vacuum contribution to the real part of the self energy, which is usually ignored, is found to be appreciable. A significant effect of temperature and magnetic field is seen in the self energy, spectral function, effective mass and dispersion relation of $\rho^0$ as well as of $\rho^\pm$ relative to its trivial Landau shift. However, for charged $\rho$ mesons, on account of the dominance of the Landau term, the effective mass appears to be independent of temperature. The trivial coupling of magnetic moment of $\rho^\pm$ with external magnetic field, when incorporated in the calculation, makes the $\rho^\pm$ to condense at high magnetic field.

Electrical resistivity of 5f -electron systems affected by static and dynamic spin disorder

Metallic $5f$ materials have very strong coupling of magnetic moments and electrons mediating electrical conduction. It is caused by strong spin-orbit interaction, coming with high atomic number $Z$, together with involvement of the $5f$ states in metallic bonding. We have used the recently discovered class of uranium (ultra)nanocrystalline hydrides, which are ferromagnets with high ordering temperature, to disentangle the origin of negative temperature coefficient of electrical resistivity. In general, the phenomenon of electrical resistivity decreasing with increasing temperature in metals can have several reasons. The magnetoresistivity study of these hydrides reveals that quantum effects related to spin-disorder scattering can explain the resistivity behavior of a broad class of actinide compounds.

Scale generation via dynamically induced multiple seesaw mechanisms

We propose a model which accounts for the dynamical origin of the electroweak symmetry breaking (EWSB), directly linking to the mass generation of dark matter (DM) candidate and active neutrinos. The standard model (SM) is weakly charged under the $U(1)_{\rm B-L}$ gauge symmetry, in conjunction with newly introduced three right-handed Majorana neutrinos and the $U(1)_{\rm B-L}$ Higgs. The model is built on the classical scale invariance, that is dynamically broken by a new strongly coupled sector, what is called the hypercolor (HC) sector, which is also weakly coupled to the $U(1)_{\rm B-L}$ gauge. At the HC strong scale, the simultaneous breaking of the EW and $U(1)_{\rm B-L}$ gauge symmetries is triggered by dynamically induced multiple seesaw mechanisms, namely bosonic seesaw mechanisms. Thus, all the origins of masses are provided singly by the HC dynamics: that is what we call the dynamical scalegenesis. We also find that an HC baryon, with mass on the order of a few TeV, can be stabilized by the HC baryon number and the $U(1)_{\rm B-L}$ charge, so identified as a DM candidate. The relic abundance of the HC-baryon DM can be produced dominantly via the bosonic-seesaw portal process, and the HC-baryon DM can be measured through the large magnetic moment coupling generated from the HC dynamics, or the $U(1)_{\rm B-L}$-gauge boson portal in direct detection experiments.

Magnetism in Yttrium Intermetallics: Ab Initio Study

This work is stimulated by the recent developments in the search of novel magnetic materials such as the simple cubic laves phase compounds of yttrium and 3d transition metals with ferromagnetic or exchange enhanced Pauli paramagnetic properties. The ground state electronic structure of the intermetallics was calculated by the full potential linearized augmented plane wave method (FP-LAPW) based on the density functional theory. The suitability of the various approximations LDA and GGA with exchange-correlations (XCs) potentials such as GGA-PBEsol, GGA-WC, etc. on the electronic structure was also investigated. From the calculated band structure and angular-momentum projected density of states (DOS), strong d–d hybridization of electrons at the Fermi level was observed. The Fe-Y and Co-Y coupling is stronger for minority component of Fe/Co d states which leads to formation of negative magnetic moment on Y sites. The magnetic moments were found to be considerably larger than the experimental moments although there is good agreement with methods based on DFT. The Y-TM compounds have mixed chemical bonding character which we have characterized by calculating the Bader charge distributions, electron localization function, as well as the magnetization spin density. High magnetization density is localized on the Fe atoms which imparts ferromagnetism to the more ionic YFe2 system, whereas the shift of electron density and magnetization density towards inter atomic regions imparts exchange enhanced paramagnetic properties in more covalent YCo2 system. In order to study the influence of d-electrons, the temperature-dependent electrical resistivity and thermopower were obtained from the band energies. Magnetism was investigated in terms of enhanced magnetic susceptibility and Sommerfeld term γ obtained from low temperature-specific heats. The dynamical stability of YX2 was investigated by calculating the vibrational phonon modes and phonon density of states. The lowermost frequency modes arise from vibrations of Y–Fe bonds which exhibit structural instability which have been interpreted in terms of electron-phonon coupling. The coupling of magnetic moments with the lattice in YFe2 verifies the magneto-elastic interactions found experimentally.

Hydrogenated Fe90M10 (M: Zr and Sc) amorphous alloys with enhanced room-temperature magnetocaloric effect

Introduction of hydrogen to Fe90M10 (M: Zr and Sc) amorphous materials is found to be effective in significantly enhancing peak magnetic entropy change (|ΔSmpk|) and simultaneously tuning Curie temperature to room temperature. The refrigerant capacity (RC) of the hydrogenated Fe90Sc10 alloy exhibits a large value of 216.2 J/kg, which is comparable to those observed in the materials exhibiting giant magnetocaloric effect. The effect of hydrogenation depends greatly on hydrogen absorption time. The underlying mechanism is revealed to be correlated with an increase in magnetic moment and exchange coupling for the Fe-based amorphous systems caused by extensions of the Fe-Fe spacing owing to the hydrogenation. It is further obtained that field dependence of the magnetic entropy change (|ΔSm|) follows the potential law as expressed by |ΔSm| = αHn. The coefficient (α) is shown to increase with the hydrogenation, which is particularly favorable from the perspective of energy-saving, as the magnetic field required for obtaining a large magnetic entropy change can be consequently decreased.

Self-organized growth and magnetic properties of epitaxial silicide nanoislands

Self-organized transition-metal (Ni and Fe) and rare-earth (Er) silicide nanostructures were grown on Si(111) and Si(001) surfaces under low coverage conditions, in a “solid phase” and “reactive deposition” epitaxial regimes. Island evolution was continuously monitored in-situ, using real-time scanning tunneling microscopy and surface electron diffraction. After anneal of a Ni/Si(111) surface at 700°C, we observed small hemispherical Ni-silicide nanoislands ∼10nm in diameter decorating surface steps in a self-ordered fashion and pinning them. Fe-silicide nanoislands formed after a 550°C anneal of a Fe-covered surface, were also self-ordered along the surface step-bunches, however were significantly larger (∼70×10nm) and exhibited well-developed three-dimensional polyhedral shapes. Ni-silicide islands were sparsely distributed, separated by about ∼100nm from one another, on average, whereas Fe-silicide islands were more densely packed, with only ∼50nm mean separation distance. In spite of the above differences between both types of island in size, shape, and number density, the self-ordering in both cases was close to ideal, with practically no islands nucleated on terraces. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, in particular in Fe-silicide islands with ∼1.9μB/Fe atom, indicating stronger ferromagnetic coupling of individual magnetic moments, contrary to Ni-silicide islands with the calculated moments of only∼0.5μB/Ni atom. To elucidate the effects of the island size, shape, and lateral ordering on the measured magnetic response, we have controllably changed the island morphology by varying deposition methods and conditions and even using differently oriented Si substrates. We have also begun experimenting with rare-earth silicide islands. In the forthcoming experiments we intend to compare the magnetic response of these variously built and composed islands and correlate between their composition, crystal and morphological characteristics, and the resultant magnetic properties.

Majorana neutrino magnetic moment and neutrino decoupling in big bang nucleosynthesis

We examine the physics of the early universe when Majorana neutrinos (electron neutrino, muon neutrino, tau neutrino) possess transition magnetic moments. These extra couplings beyond the usual weak interaction couplings alter the way neutrinos decouple from the plasma of electrons/positrons and photons. We calculate how transition magnetic moment couplings modify neutrino decoupling temperatures, and then use a full weak, strong, and electromagnetic reaction network to compute corresponding changes in Big Bang Nucleosynthesis abundance yields. We find that light element abundances and other cosmological parameters are sensitive to magnetic couplings on the order of 10^{-10} Bohr magnetons. Given the recent analysis of sub-MeV Borexino data which constrains Majorana moments to the order of 10^{-11} Bohr magnetons or less, we find that changes in cosmological parameters from magnetic contributions to neutrino decoupling temperatures are below the level of upcoming precision observations.

ChemInform Abstract: Basics and Prospective of Magnetic Heusler Compounds

AbstractReview: 61 refs.

Magnetic moment, vorticity-spin coupling and parity-odd conductivity of chiral fermions in 4-dimensional Wigner functions

We demonstrate the emergence of the magnetic moment and spin-vorticity coupling of chiral fermions in 4-dimensional Wigner functions. In linear response theory with space–time varying electromagnetic fields, the parity-odd part of the electric conductivity can also be derived which reproduces results of the one-loop and the hard-thermal or hard-dense loop. All these properties show that the 4-dimensional Wigner functions capture comprehensive aspects of physics for chiral fermions in electromagnetic fields.

Long-range antiferromagnetic order of formally nonmagneticEu3+Van Vleck ions observed in multiferroicEu1−xYxMnO3

We report on resonant magnetic x-ray scattering and absorption spectroscopy studies of exchange-coupled antiferromagnetic ordering of Eu$^{3+}$ magnetic moments in multiferroic Eu$_{1-x}$Y$_{x}$MnO$_3$ in the absence of an external magnetic field. The observed resonant spectrum is characteristic of a magnetically ordered $^7F_1$ state that mirrors the Mn magnetic ordering, due to exchange coupling between the Eu $4f$ and Mn $3d$ spins. This is the first observation of long range magnetic order generated by exchange coupling of magnetic moments of formally non-magnetic Van Vleck ions, which is a step further towards the realization of exotic phases induced by exchange coupling in systems entirely composed of non-magnetic ions.