Magnon Gap Research Articles

Magnon gap excitations in van der Waals antiferromagnet MnPSe3

Magneto-spectroscopy methods have been employed to study the zero-wavevector magnon excitations in MnPSe3. Experiments carried out as a function of temperature and the applied magnetic field show that two low-energy magnon branches of MnPSe3 in its antiferromagnetic phase are gapped. The observation of two low-energy magnon gaps (at 1.70 ± 0.05 meV and 0.09 ± 0.01 meV) implies that MnPSe3 is a biaxial antiferromagnet. A relatively strong out-of-plane anisotropy imposes the spin alignment to be in-plane whereas the spin directionality within the plane is governed by a factor of 2.5 × 10−3 weaker in-plane anisotropy.

Spin and lattice dynamics of the two-dimensional van der Waals ferromagnet CrI3

Chromium tri-iodide (CrI3) is a prototypical ferromagnetic van der Waals insulator with its genuinely two-dimensional (2D) long-range magnetic order below 45 K demonstrated recently. The underlying magnetic anisotropy has not been completely understood while both the Dzyaloshinskii-Moriya (DM) interaction and the Kitaev−Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Gamma$$\\end{document} type interaction have been considered as the relevant magnetic Hamiltonian. In addition, the relation between the crystal structure and the magnetic order needs to be further elucidated concerning their possible coupling in few-layer samples and in the topmost surface layers of bulk samples. Here, we investigate these issues via temperature- and magnetic field-dependent terahertz spectroscopy on bulk CrI3 single crystals, focusing on the dynamics of ferromagnetic resonances (FMRs) and optical phonons in the terahertz (THz) region from 4 to 120 cm−1 (from 0.5 to 15 meV). We narrow down the possible ranges of the interaction parameters such as the off-diagonal symmetric terms and the single-ion anisotropy. The accurate values of these parameters significantly constrain the magnitude of possible Kitaev−Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Gamma$$\\end{document} exchange interaction and the topological magnon gap. Moreover, the structure-magnetism relationship was critically analyzed based on the temperature- and field-dependences of two Eu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\rm{E}}}_{{\\rm{u}}}$$\\end{document} in-plane optical phonon modes, which shows that the commonly believed structural phase diagram of CrI3, derived from surface-preferential data, has to be seriously modified.

Magnon gap tuning in lithium-doped MnTe

Magnon gap tuning in lithium-doped MnTe

Sizable suppression of magnon Hall effect by magnon damping in Cr2Ge2Te6

Two-dimensional (2D) Heisenberg honeycomb ferromagnets are expected to have interesting topological magnon effects as their magnon dispersion can have Dirac points. The Dirac points are gapped with finite second nearest neighbor Dzyaloshinskii-Moriya interaction, providing nontrivial Berry curvature with finite magnon Hall effect. Yet, it is unknown how the topological properties are affected by magnon damping. We report the thermal Hall effect in Cr$_2$Ge$_2$Te$_6$, an insulating 2D honeycomb ferromagnet with a large Dirac magnon gap and significant magnon damping. Interestingly, the thermal Hall conductivity in Cr$_2$Ge$_2$Te$_6$ shows the coexisting phonon and magnon contributions. Using an empirical two-component model, we successfully estimate the magnon contribution separate from the phonon part, revealing that the magnon Hall conductivity was 20 times smaller than the theoretical calculation. Finally, we suggest that such considerable suppression in the magnon Hall conductivity is due to the magnon damping effect in Cr$_2$Ge$_2$Te$_6$.

The importance of the interface for picosecond spin pumping in antiferromagnet-heavy metal heterostructures

Interfaces in heavy metal (HM) - antiferromagnetic insulator (AFI) heterostructures have recently become highly investigated and debated systems in the effort to create spintronic devices that function at terahertz frequencies. Such heterostructures have great technological potential because AFIs can generate sub-picosecond spin currents which the HMs can convert into charge signals. In this work we demonstrate an optically induced picosecond spin transfer at the interface between AFIs and Pt using time-domain THz emission spectroscopy. We select two antiferromagnets in the same family of fluoride cubic perovskites, KCoF3 and KNiF3, whose magnon frequencies at the centre of the Brillouin zone differ by an order of magnitude. By studying their behaviour with temperature, we correlate changes in the spin transfer efficiency across the interface to the opening of a gap in the magnon density of states below the Néel temperature. Our observations are reproduced in a model based on the spin exchange between the localized electrons in the antiferromagnet and the free electrons in Pt. Through this comparative study of selected materials, we are able to shine light on the microscopy of spin transfer at picosecond timescales between antiferromagnets and heavy metals and identify a key figure of merit for its efficiency: the magnon gap. Our results are important for progressing in the fundamental understanding of the highly discussed physics of the HM/AFI interfaces, which is the necessary cornerstone for the designing of femtosecond antiferromagnetic spintronics devices with optimized characteristics.

Gaps in topological magnon spectra: Intrinsic versus extrinsic effects

For topological magnon spectra, determining and explaining the presence of a gap at a magnon crossing point is a key to characterize the topological properties of the system. An inelastic neutron scattering study of a single crystal is a powerful experimental technique that is widely employed to probe the magnetic excitation spectra of topological materials. Here, we show that when the scattering intensity rapidly disperses in the vicinity of a crossing point, such as a Dirac point, the apparent topological gap size is extremely sensitive to experimental conditions including sample mosaic, resolution, and momentum integration range. We demonstrate these effects using comprehensive neutron-scattering measurements of CrCl$_3$. Our measurements confirm the gapless nature of the Dirac magnon in CrCl$_3$, but also reveal an artificial, i.e. extrinsic, magnon gap unless the momentum integration range is carefully controlled. Our study provides an explanation of the apparent discrepancies between spectroscopic and first-principles estimates of Dirac magnon gap sizes, and provides guidelines for accurate spectroscopic measurement of topological magnon gaps.

Magnon spectrum of the amorphous ferromagnet Co4P from atomistic spin dynamics

An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P, often referred to as a `roton-like' excitation, attracted much attention half a century ago. With the current interest in heat and spin currents in amorphous magnets, we apply modern simulation methods, combining reverse Monte Carlo to build the atomic structure and the stochastic Landau-Lifshitz equation for spin dynamics, to re-investigate the magnetic excitation spectrum. We find two magnon valleys, one at the origin and another at a finite wavenumber close to the observations, but without a magnon gap. We conclude that the second dip is due to Umklapp scattering caused by residual long-range order, which may be an alternative explanation of the putative roton excitation. Our study paves the way to study magnon transport in amorphous magnets and related spintronic applications.

Quasi-Two-Dimensional Anomalous Hall Mott Insulator of Topologically Engineered Jeff=1/2 Electrons

We investigate an experimental toy-model system of a pseudospin-half square-lattice Hubbard Hamiltonian in [(SrIrO3)1/(CaTiO3)1] to include both nontrivial complex hopping and moderate electronic correlation. While the former induces electronic Berry phases as anticipated from the weak-coupling limit, the later stabilizes an antiferromagnetic (AFM) Mott insulator ground state in analogous to the strong-coupling limit. Their combined results in the real system are found to be an anomalous Hall effect with a non-monotonic temperature dependence due to the self-competition of the electron-hole pairing in the Mott state, and an exceptionally large Ising anisotropy that is captured as a giant magnon gap beyond the superexchange approach. The unusual phenomena highlight the rich interplay of electronic topology and electronic correlation in the intermediate-coupling regime that is largely unexplored and challenging in theoretical modelling.

Field-angle dependence of thermal transport in Kitaev-Γ model

We investigate the magnetic field effect on the Kitaev model with the Γ-type interaction, which plays an essential role in understanding the magnetic properties of real materials. We examine the mean-field phase diagram and thermal Hall conductivity using the linear spin-wave theory. We find that the Γ interaction substantially changes the field-angle dependence of the thermal Hall conductivity and suppresses its magnitude. The suppression is caused by the enhancement of the low-energy magnon gap while increasing the Γ interaction.

MoP3SiO11 : A 4d3 honeycomb antiferromagnet with disconnected octahedra

We report the crystal structure and magnetic behavior of the $4d^3$ spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group ($R\bar 3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$ ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic order with the propagation vector $\mathbf k=0$ observed below $T_N=6.8$ K is a combined effect of the nearest-neighbor in-plane exchange coupling $J\simeq 2.6$ K, easy-plane single-ion anisotropy $D\simeq 2.2 $ K, and a weak interlayer coupling $J_c\simeq 0.8$ K. The 12% reduction in the ordered magnetic moment of the Mo$^{3+}$ ions and the magnon gap of $\Delta\simeq 7$ K induced by the single-ion anisotropy further illustrate the impact of spin-orbit coupling on the magnetism. Our analysis puts forward single-ion anisotropy as an important ingredient of $4d^3$ honeycomb antiferromagnets despite their nominally quenched orbital moment.

Field-angle dependence of thermal Hall conductivity in a magnetically ordered Kitaev-Heisenberg system

We study magnetic excitations and thermal Hall effect on the Kitaev-Heisenberg model under magnetic fields. By employing the spin-wave theory for the magnetic orders realized in this model, we examine the topological nature of the spin-wave dispersions and calculate the thermal Hall conductivity. The comprehensive investigations on the field-angle dependence clarify that the thermal Hall conductivity is sensitive to the spin ordered pattern and excitation spectra of magnons; this quantity is enhanced by the noncoplanar spin configurations and small magnon gap in the excitation spectrum. On the other hand, we also find a common feature in the field-angle dependence of the thermal Hall conductivity. It vanishes when the magnetic field is on the planes spanned by the spin axes. We reveal that the behavior is intrinsic to the Kitaev -Heisenberg model in an applied field and demonstrate that the introduction of the off-diagonal spin interaction causes the disappearance of the feature in the thermal Hall conductivity.

Magnon dispersion in bilayers of two-dimensional ferromagnets

We determine magnon spectra of an atomic bilayer magnet with ferromagnetic intra- and both ferro- and anti- ferromagnetic interlayer coupling. Analytic expressions for the full magnon band of the latter case reveal that both exchange interactions govern the fundamental magnon gap. The inter and intralayer magnetic ordering are not independent: the intralayer ferromagnetism stabilizes antiferromagnetic inter-layer order. The topology of these exchange-anisotropy spin models without spin-orbit interaction turns out to be trivial.

High-field thermal transport properties of the Kitaev quantum magnet α−RuCl3 : Evidence for low-energy excitations beyond the critical field

We investigate the phononic in-plane longitudinal low-temperature thermal conductivity kappa_ab of the Kitaev quantum magnet alpha-RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal for fields larger than the critical field Bc ~ 8 T, at which the magnetic order is suppressed, a dramatic increase of kappa_ab at all temperatures investigated. The analysis of our data shows that the phonons are not only strongly scattered by a magnetic mode at relatively large energy which scales roughly linearly with the magnetic field, but also by a small-energy mode which emerges near Bc with a square-root-like field dependence. While the former is in striking agreement with recent spin wave theory (SWT) results of the magnetic excitation spectrum at the Gamma point, the energy of the latter is too small to be compatible with the SWT-expected magnon gap at the M point, despite the matching field dependence. Therefore, an alternative scenario based on phonon scattering off the thermal excitation of random-singlet states is proposed.

Pseudo-spin rotation symmetry breaking by Coulomb interaction terms in spin–orbit coupled systems

By transforming from the pure-spin-orbital (t 2g) basis to the spin-orbital entangled pseudo-spin-orbital basis, the pseudo-spin rotation symmetry of the different Coulomb interaction terms is investigated under SU(2) transformation in pseudo-spin space. While the Hubbard and density interaction terms are invariant, the Hund’s coupling and pair-hopping interaction terms explicitly break pseudo-spin rotation symmetry systematically. The form of the symmetry-breaking terms obtained from the transformation of the Coulomb interaction terms accounts for the easy x–y plane anisotropy and magnon gap for the out-of-plane mode, highlighting the importance of mixing with the nominally non-magnetic J = 3/2 sector, and providing a physically transparent approach for investigating magnetic ordering and anisotropy effects in perovskite (Sr2IrO4) and other d 5 pseudo-spin compounds.

Charge-magnon conversion at the topological insulator/antiferromagnetic insulator interface

Efficient generation of magnons is highly desirable for magnonics. Here we theoretically investigate the charge-magnon conversion at the interface of topological insulator/antiferromagnetic insulator (AFMI), where the Dirac electrons and the antiferromagnetic magnons are coupled by the fluctuation of s-d exchange field. Our model reveals that the charge-magnon conversion efficiency for uncompensated interface is 2--3 orders higher than that for compensated interface. The conversion is more efficient with smaller magnon gap and larger s-d exchange coupling. We find a significant efficiency enhancement when the thickness of the AFMI is in the two-dimensional regime, where the magnons experience a quantum confinement effect. $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}/\ensuremath{\alpha}\ensuremath{-}\mathrm{MnTe}$ bilayers are then proposed to be a promising system to realize strong charge-magnon conversion. Our work provides the principles of manipulating charge-magnon conversion and designing efficient charge-magnon conversion structure, which would help to improve the performance of antiferromagnetic magnonics, such as magnon transistor and magnon torque memories.

Stable Higgs mode in anisotropic quantum magnets

Low-energy excitations associated with the amplitude fluctuation of an order parameter in condensed matter systems can mimic the Higgs boson, an elementary particle in the standard model, and are dubbed as Higgs modes. Identifying the condensed-matter Higgs mode is challenging because it is known in many cases to decay rapidly into other low-energy bosonic modes, which renders the Higgs mode invisible. Therefore, it is desirable to find a way to stabilize the Higgs mode, which can offer an insight into the stabilization mechanism of the Higgs mode in condensed matter physics. In quantum magnets, magnetic order caused by spontaneous symmetry breaking supports transverse (magnons) and longitudinal (Higgs modes) fluctuations. When a continuous symmetry is broken, the Goldstone magnon mode generally has a lower excitation energy than the Higgs mode, causing a rapid decay of the latter. In this work, we show that a stable Higgs mode exists in anisotropic quantum magnets near the quantum critical point between the dimerized and magnetically ordered phases. We find that an easy axis anisotropy increases the magnon gap such that the magnon mode is above the Higgs mode near the quantum critical point, and the decay of the Higgs mode into the magnon mode is forbidden kinematically. Our results suggest that the anisotropic quantum magnets provide ideal platforms to explore the Higgs physics in condensed matter systems.

Magnetic Two-Dimensional Chromium Trihalides: A Theoretical Perspective.

The discovery of ferromagnetic order in monolayer two-dimensional (2D) crystals has opened a new venue in the field of 2D materials. Two-dimensional magnets are not only interesting on their own, but their integration in van der Waals heterostructures allows for the observation of new and exotic effects in the ultrathin limit. The family of chromium trihalides, CrI3, CrBr3, and CrCl3, is so far the most studied among magnetic 2D crystals. In this Mini Review, we provide a perspective of the state of the art of the theoretical understanding of magnetic 2D trihalides, most of which will also be relevant for other 2D magnets, such as vanadium trihalides. We discuss both the well-established facts, such as the origin of the magnetic moment and magnetic anisotropy, and address as well open issues such as the nature of the anisotropic spin couplings and the magnitude of the magnon gap. Recent theoretical predictions on Moiré magnets and magnetic skyrmions are also discussed. Finally, we give some prospects about the future interest of these materials and possible device applications.

Spin dynamics and a nearly continuous magnetic phase transition in an entropy-stabilized oxide antiferromagnet

Studying magnetism in entropy-stabilized oxides (ESOs) is an exciting new research avenue following the recent discovery of antiferromagnetic order in (Mg${}_{0.2}$Co${}_{0.2}$Ni${}_{0.2}$Cu${}_{0.2}$Zn${}_{0.2}$)O (MgO-ESO). Muon spin relaxation and neutron scattering experiments reveal a continuous transition that occurs across a broad distribution of antiferromagnetic ordering temperatures throughout the MgO-ESO sample. The ground state exhibits a magnon gap and significant quasielastic neutron scattering, suggesting unusual intrinsic magnetic fluctuations. These results resolve several open questions about magnetic order in MgO-ESO and pave the way for future investigations of magnetic ESOs.

Chiral magnonic edge states in ferromagnetic skyrmion crystals controlled by magnetic fields

Achieving control over magnon spin currents in insulating magnets - where dissipation due to Joule heating is highly suppressed - is an active area of research that could lead to energy-efficient spintronics applications. However, magnon spin currents supported by conventional systems with uniform magnetic order have proven hard to control. An alternative approach that relies on topologically protected magnonic edge states of spatially periodic magnetic textures has recently emerged. A prime example of such textures is the ferromagnetic skyrmion crystal which hosts chiral edge states providing a platform for magnon spin currents. Here, we show, for the first time, an external magnetic field can drive a topological phase transition in the spin wave spectrum of a ferromagnetic skyrmion crystal. The topological phase transition is signaled by the closing of a low-energy bulk magnon gap at a critical field. In the topological phase, below the critical field, two topologically protected chiral magnonic edge states lie within this gap, but they unravel in the trivial phase, above the critical field. Remarkably, the topological phase transition involves an inversion of two magnon bands that at the $\Gamma$ point correspond to the breathing and anticlockwise modes of the skyrmions in the crystal. Our findings suggest that an external magnetic field could be used as a knob to switch on and off magnon spin currents carried by topologically protected chiral magnonic edge states.

Magnon profile on SrRuO3 films studied by inelastic neutron scattering.

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic SrRuO3 single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the SrRuO3 films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadratic relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.