The topological Hall effect in magnetic materials is considered the ultimate trademark of the skyrmion phase. The phenomenon is identified by distinct nonmonotonic features in the Hall effect signal presumed to be evidence of the topological origin. It is demonstrated here that similar features, unrelated to the skyrmion physics, arise in heterogeneous ferromagnets when components of the material exhibit the extraordinary Hall effect with opposite polarities. The relevance of this mechanism to the published data is discussed.

A characteristic dependence of voltage control of perpendicular magnetic anisotropy (VCMA) on oxygen migration at Fe/MgO interfaces was revealed by performing systematic ab initio study of the energetics of the oxygen path around the interface. We find that the surface anisotropy energy exhibits a Boltzmann sigmoidal behavior as a function of the migrated O-atoms concentration. The obtained variation of the VCMA efficiency factor $\beta$ reveals a saturation limit beyond a critical concentration of migrated O, about $54\%$, at which the anisotropy switches from perpendicular to in plane. Furthermore, depending on the range of variation of the applied voltage, two regimes associated with reversible or irreversible ions displacement are predicted to occur, yielding different VCMA response. According to our findings, one can distinguish from the order of magnitude of $\beta$ the VCMA driving mechanism: An effect of several tens of fJ/(V.m) is likely associated to charge-mediated effect combined with slight reversible oxygen displacements whereas an effect of the order of thousands of fJ/(V.m) is more likely associated with irreversible oxygen ionic migration.

We investigate the fundamental mechanism of spin-phonon coupling in the negatively charged nitrogen-vacancy center $\left({\mathrm{NV}}^{-}\right)$ in diamond in order to calculate the spin lattice relaxation time $T_1$ and its temperature dependence from first principles. Starting from the dipolar spin-spin interaction between two electrons, we couple the spins of the electrons to the movements of the ions and end up with an effective spin-phonon interaction potential $V_{\mathrm{s}\text{−}\mathrm{ph}}$. Taking this time-dependent potential as a perturbation of the system, a Fermi's golden rule expression for transition rates is obtained which allows us to calculate the spin lattice relaxation time $T_1$. We find that the temperature dependence of $T_1$ is determined by the the zero-temperature transition rate ${\mathrm{\Gamma }}_0$. We simulate the color center ab initio to extract the figures necessary to quantify ${\mathrm{\Gamma }}_0$. We calculate the local phonon modes of the color center within the harmonic approximation using the small displacement method and extract the phononic density of states and band structure by diagonalizing the dynamical matrix. We show that our model allows us to calculate $T_1$ in good agreement with experimental observations.

There has been significant interest in the generation of very short-lived superconducting states in solid-state films. Here, we consider the role of nonequilibrium superconducting fluctuations in such systems, generated by an interaction quench, considering the limit of large static disorder. In particular, we argue that because of critical slowing down, the regime of the fluctuation dominated normal state is more important than might be naively thought. We show how such a state might appear in the optical conductivity, and give the appropriate nonequilibrium generalization of the Azlamazov-Larkin and Maki-Thompson fluctuation corrections to the optical conductivity. For a quench to the superconducting critical point, we show that the fluctuation corrections lead to power-law aging behavior in the optical conductivity. The power-law aging from Azlamazov-Larkin and Maki-Thompson terms are qualitatively different, despite the effect of the two being similar in dc measurements in thermal equilibrium. These signatures provide a road map for experiments to identify the role of superconducting fluctuations in a transient state.

Motivated by recent experiments reporting superconductivity only at very low temperature in a class of heavy fermion compounds, we study the impact of energy fluctuations with small momentum transfer on the pairing instability near an antiferromagnetic quantum critical point. While these fluctuations, formed by composite spin fluctuations, were proposed to explain the thermodynamic and transport properties near the quantum critical point of compounds such as ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ and ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ at $x\approx 0.1$, here they are found to strongly suppress $T_c$ of the $d$-wave pairing of the hot quasiparticles promoted by the spin fluctuations. Interestingly, if energy fluctuations are strong enough, they can induce triplet pairing involving the quasiparticles of the cold regions of the Fermi surface. Overall, the opposing effects of energy and spin fluctuations lead to a suppression of $T_c$.

The halon is a special critical state of an impurity in a quantum-critical environment. The hallmark of the halon physics is that a well-defined integer charge gets fractionalized into two parts: a microscopic core with half-integer charge and a critically large halo carrying a complementary charge of $\pm 1/2$. The halon phenomenon emerges when the impurity-environment interaction is fine-tuned to the vicinity of a boundary quantum critical point (BQCP), at which the energies of two quasiparticle states with adjacent integer charges approach each other. The universality class of such BQCP is captured by a model of pseudospin-$1/2$ impurity coupled to the quantum-critical environment, in such a way that the rotational symmetry in the pseudospin $xy$ plane is respected, with a small local “magnetic” field along the pseudospin $z$ axis playing the role of control parameter driving the system away from the BQCP. On the approach to BQCP, the half-integer projection of the pseudospin on its $z$ axis gets delocalized into a halo of critically divergent radius, capturing the essence of the phenomenon of charge fractionalization. With large-scale Monte Carlo simulations, we confirm the existence of halons—and quantify their universal features—in O(2) and O(3) quantum critical systems.

Superconductivity in the pseudobinary pnictides ${\mathrm{Ru}}_{0.55}{\mathrm{Rh}}_{0.45}\mathrm{P}$ and ${\mathrm{Ru}}_{0.75}{\mathrm{Rh}}_{0.25}\mathrm{As}$ is probed by muon spin relaxation and rotation ($\mu \mathrm{SR}$) measurements in conjunction with magnetic susceptibility, heat capacity, and electrical resistivity measurements. Powder x-ray diffraction confirmed the MnP-type orthorhombic structure (space group $Pnma$) and showed a nearly single phase nature with small impurity phase(s) of about 5% for both the samples. The occurrence of bulk superconductivity is confirmed with $T_{\mathrm{c}}=3.7$ K for ${\mathrm{Ru}}_{0.55}{\mathrm{Rh}}_{0.45}\mathrm{P}$ and $T_{\mathrm{c}}=1.6$ K for ${\mathrm{Ru}}_{0.75}{\mathrm{Rh}}_{0.25}\mathrm{As}$. The superconducting state electronic heat capacity data reveal weak-coupling single-band isotropic $s$-wave gap BCS superconductivity. Various normal and superconducting state parameters are determined which reveal a weak-coupling electron-phonon driven type-II dirty-limit superconductivity for both the compounds. The upper critical field shows a linear temperature dependence down to the lowest measured temperatures which is quite unusual for a single-band superconductor. The $\mu \mathrm{SR}$ data confirm the conventional type-II behavior and show evidence for a single-band $s$-wave singlet pairing superconductivity with a preserved time reversal symmetry for both the compounds.

Recent STM experiments and theoretical considerations have highlighted the role of interaction-driven orbital selectivity in FeSe, and its role in generating the extremely anisotropic superconducting gap structure in this material. We study the magnetic excitation spectrum resulting from the coherent quasiparticles within the same renormalized random phase approximation approach used to explain the STM experiments, and show that it agrees well with the low-energy momentum and energy dependent response measured by inelastic neutron scattering experiments. We find a correlation-induced suppression of $(\pi ,\pi )$ scattering due to a small quasiparticle weight of states of $d_{xy}$ character. We compare predictions for twinned and untwinned crystals, and predict in particular a strongly $(\pi ,0)$ dominated response at low energies in untwinned systems, in contrast to previous itinerant theories.

Recent measurements in current-driven spin valves demonstrate magnetization fluctuations that deviate from semiclassical predictions. We posit that the origin of this deviation is spin shot noise. On this basis, our theory predicts that magnetization fluctuations asymmetrically increase in biased junctions irrespective of the current direction. At low temperatures, the fluctuations are proportional to the bias, but at different rates for opposite current directions. Quantum effects control fluctuations even at higher temperatures. Our results are in semiquantitative agreement with recent experiments and are in contradiction to semiclassical theories of spin-transfer torque.

Platelike high-quality ${\mathrm{NaYbS}}_2$ rhombohedral single crystals with lateral dimensions of a few mm have been grown and investigated in great detail by bulk methods such as magnetization and specific heat, but also by local probes such as nuclear magnetic resonance (NMR), electron-spin resonance (ESR), muon-spin relaxation ($\mu \mathrm{SR}$), and inelastic neutron scattering over a wide field and temperature range. Our single-crystal studies clearly evidence a strongly anisotropic quasi-two-dimensional magnetism and an emerging spin-orbit entangled $J_{\mathrm{eff}}=\frac{1}{2}$ state of Yb towards low temperatures together with an absence of long-range magnetic order down to 260 mK. In particular, the clear and narrow Yb ESR lines together with narrow ${}^{23}\mathrm{Na}$ NMR lines evidence an absence of inherent structural distortions in the system, which is in strong contrast to the related spin-liquid candidate ${\mathrm{YbMgGaO}}_4$ falling within the same space group $R\overline{3}m$. This identifies ${\mathrm{NaYbS}}_2$ as a rather pure spin-$\frac{1}{2}$ triangular-lattice magnet and a putative quantum spin liquid.

We report on a first-principles study of antiferromagnetic resonance (AFMR) phenomena in metallic systems $[\mathrm{Mn}X$ ($X=\text{Ir}$, Pt, Pd, Rh) and FeRh] under an external electric field. We demonstrate that the AFMR linewidth can be separated into a relativistic component originating from the angular momentum transfer between the collinear AFM subsystem and the crystal through spin-orbit coupling, and an exchange component that originates from the spin exchange between the two sublattices. The calculations reveal that the latter component becomes significant in the low-temperature regime. Furthermore, we present results for the current-induced intersublattice torque which can be separated into fieldlike and dampinglike components, affecting the intersublattice exchange coupling and AFMR linewidth, respectively.

Collective dynamics of topological magnetic textures can be thought of as a massive particle moving in a magnetic pinning potential. We demonstrate that inside a cavity resonator this effective mechanical system can feel the electromagnetic radiation pressure from cavity photons through the magneto-optical inverse Faraday and Cotton-Mouton effects. We estimate values for the effective parameters of the optomechanical coupling for two spin textures: a Bloch domain wall and a chiral magnetic soliton lattice. The soliton lattice has magnetic chirality, so that in circularly polarized light it behaves like a chiral particle with the sign of the optomechanical coupling determined by the helicity of the light and chirality of the lattice. Most interestingly, we find a level attraction regime for the soliton lattice, which is tunable through an applied magnetic field.

We consider two-body bound states in a flat band of a multiband system. The existence of pair dispersion predicts the possibility of breaking the degeneracy of the band and creating order, such as superconductivity. Within a separable interaction potential approximation, we find that the finiteness of the effective mass of a bound pair is determined by a band-structure invariant, which in the uniform case becomes the quantum metric. The results offer a simple foundation to understand and predict flat-band superconductivity. We propose an experiment to test the interaction-induced pair motion.

The composition and temperature dependence of the phase stability and elastic properties of ${\mathrm{Co}}_2\mathrm{Cr}({\mathrm{Ga}}_{1-x}{\mathrm{Si}}_x$) ($0.0\le x\le 1.0$) alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. The calculated equilibrium properties and Curie temperature of the ground FM $L{2}_1$ phase are in good agreement with the available experimental and theoretical results. Separating the thermal effects into volume expansion, phonon-smearing, and magnetic terms, we find that dominated by the magnetic excitations in each FM alloy upon cooling, the free-energy difference ($\mathrm{\Delta }F$) between the $D{0}_{22}$ and $L{2}_1$ phases increases from negative to become positive, and the tetragonal shear elastic constant ($C^{\prime }$) of the $L{2}_1$ phase stiffens, whereas that ($C_s$) of the $D{0}_{22}$ phase softens even to become negative, resulting in the reentrant martensitic transformation (RMT) from $D{0}_{22}$ to $L{2}_1$. Nevertheless, with contributions from all three thermal effects, the contrary trends of $\mathrm{\Delta }F\sim T$, $C^{\prime }\sim T$, and $C_s\sim T$ are found in each PM alloy, promoting the normal martensitic transformation (MT) from $L{2}_1$ to $D{0}_{22}$ upon cooling. From both the energetic and mechanical points of view, the evaluated critical temperature ($T_M$) of the RMT decreases whereas that of the MT is supposed to increase with increasing $x$. The MT and RMT are further attributed to the Jahn-Teller instability of the PM $L{2}_1$ and FM $D{0}_{22}$ phases, respectively, caused by the relatively weak covalent bond between Co and Ga (or Si) in them.

In this paper, we develop a self-consistent Hartree-Fock approach to theoretically study the far-from-equilibrium quantum dynamics of interacting fermions, and apply this approach to explore the onset of many-body localization (MBL) in these systems. We investigate the dynamics of a state with a nonequilibrium density profile; we find that at weak disorder the density profile equilibrates rapidly, whereas for strong disorder it remains frozen on the accessible timescales. We analyze this behavior in terms of the Hartree-Fock self-energy. At weak disorder, the self-energy fluctuates strongly and can be interpreted as a self-consistent noise process. By contrast, at strong disorder the self-energy evolves with a few coherent oscillations which cannot delocalize the system. Accordingly, the nonequilibrium site-resolved spectral function shows a broad spectrum at weak disorder and sharp spikes at strong disorder. Our Hartree-Fock theory incorporates spatial fluctuations and rare-region effects. As a consequence, we find subdiffusive relaxation in random systems; but, when the system is subjected to weak quasiperiodic potentials, the subdiffusive response ceases to exist, as rare region effects are absent in this case. This self-consistent Hartree-Fock approach can be regarded as a relatively simple theory that captures much of the MBL phenomenology.

We present a theoretical study on interfacial magnetocrystalline anisotropy for $\mathrm{Fe}/{\mathrm{MgAl}}_2{\mathrm{O}}_4$. This system has a very small lattice mismatch at the interface and therefore is suitable for realizing a fully coherent ferromagnet/oxide interface for magnetic tunnel junctions. On the basis of density functional theory, we calculate the interfacial anisotropy constant $K_{\mathrm{i}}$ and show that this system has interfacial perpendicular magnetic anisotropy (PMA) with $K_{\mathrm{i}}\approx 1.2\mathrm{mJ}/{\mathrm{m}}^2$, which is a little bit smaller than that of Fe/MgO ($K_{\mathrm{i}}\approx 1.5–1.7$ $\mathrm{mJ}/{\mathrm{m}}^2$). Second-order perturbation analysis with respect to the spin-orbit interaction clarifies that the difference in $K_{\mathrm{i}}$ between $\mathrm{Fe}/{\mathrm{MgAl}}_2{\mathrm{O}}_4$ and Fe/MgO originates from the difference in contributions from spin-flip scattering terms at the interface. We propose that the insertion of tungsten layers into the interface of $\mathrm{Fe}/{\mathrm{MgAl}}_2{\mathrm{O}}_4$ is a promising way to obtain huge interfacial PMA with $K_{\mathrm{i}}\gtrsim 3\mathrm{mJ}/{\mathrm{m}}^2$.

We present a comprehensive investigation of propagating spin waves in nanometer-thick yttrium iron garnet (YIG) films. We use broadband spin-wave spectroscopy with integrated coplanar waveguides (CPWs) and antennas on top of continuous and patterned YIG films to characterize spin waves with wave vectors up to 10 $\mathrm{rad}/\mu \mathrm{m}$. All films are grown by pulsed laser deposition. From spin-wave transmission spectra, parameters such as the Gilbert damping constant, spin-wave dispersion relation, group velocity, relaxation time, and decay length are derived, and their dependence on magnetic bias field strength and angle is systematically gauged. For a 40-nm-thick YIG film, we obtain a damping constant of $3.5\times {10}^{-4}$ and a maximum decay length of 1.2 mm. We show a strong variation of spin-wave parameters with wave vector, magnetic field strength, and field angle. The properties of spin waves with small wave vectors change considerably with in-plane magnetic bias field up to 30 mT and magnetic field angle beyond ${20}^{\circ }$. We also compare broadband spin-wave spectroscopy measurements on 35-nm-thick YIG films with integrated CPWs and antennas and demonstrate that both methods provide similar spin-wave parameters.

We report on the coexistence of magnetic order and disorder in the atomically disordered double perovskites $\mathrm{C}{\mathrm{a}}_2\mathrm{FeRu}{\mathrm{O}}_6$ and $\mathrm{CaSrFeRu}{\mathrm{O}}_6$. Powder x-ray and neutron diffraction were used to investigate the crystal structure and magnetic ordering of these oxides. Both compounds are described by the orthorhombic space group Pbnm down to 3 K, where the $B$ site is found to be statistically occupied by $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{R}{\mathrm{u}}^{5+}$ ions. The compound $\mathrm{C}{\mathrm{a}}_2\mathrm{FeRu}{\mathrm{O}}_6$ shows a $G$-type antiferromagnetic ordering at $T_N\approx 220\mathrm{K}$, where the moments are aligned parallel to the $c$ axis. The exchange of Ca by Sr suppresses long-range ordering in this system with the consequence that $\mathrm{CaSrFeRu}{\mathrm{O}}_6$ shows a diffuse scattering pattern, indicating only the presence of a short-range order of the magnetic moments. Mössbauer measurements additionally reveal the coexistence of a long-range ordered and paramagnetic phase in $\mathrm{C}{\mathrm{a}}_2\mathrm{FeRu}{\mathrm{O}}_6$, and spin-glass behavior in $\mathrm{CaSrFeRu}{\mathrm{O}}_6$. The random occupancy of iron and ruthenium atoms at the $B$ site gives rise to locally varying competing magnetic exchange interactions, which favors the emergence of reentrant magnetism with a spin-glass-like transition at $T_f\approx 87\mathrm{K}$ for $\mathrm{C}{\mathrm{a}}_2\mathrm{FeRu}{\mathrm{O}}_6$ and a spin-glass transition at $\sim 65\mathrm{K}$ for $\mathrm{CaSrFeRu}{\mathrm{O}}_6$, as evidenced by frequency dependent ac susceptibility measurements. Our results are an interesting example for crossing the borderline between antiferromagnetism and spin-glass behavior in a $3d\text{−}4d$ hybrid perovskite system by modifying the structural distortion associated to the tolerance factor of the perovskite structure rather than changing the concentration of magnetic ions.