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Register a new userElectromagnon resonance in a collinear spin state of a polar antiferromagnet Fe2Mo3O8
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Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by magnetic field or Zn-doping. We propose that the observed electromagnon mode involves the collective precession of the spins with oscillating in-plane electric polarization through the mechanism of the linear magnetoelectric effect.
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Terahertz time-domain spectroscopy was performed to directly probe the low-energy (1-5 meV) electrodynamics of triangular lattice antiferromagnets CuFe1-xGaxO2 (x = 0.00, 0.01, and 0.035). We discovered an electromagnon (electric-field-active magnon) excitation at 2.3 meV in the paraelectric up-up-down-down collinear magnetic phase, while this electromagnon vanishes in the ferroelectric helimagnetic phase. Anti-correlation with noncollinear magnetism excludes the exchange-striction mechanism as the origin of dynamical magnetoelectric coupling, and hence evidences the observation of spin-orbit coupling mediated electromagnon in the present compound.
Strong coupling between magnon and electromagnetic wave can lead to the formation of a coupled spinphoton quasiparticle named as magnon-polariton. The phenomenon is well studied for ferromagnetic systems inside microwave cavities in recent years. However, formation of magnon-polariton is rarely seen for an antiferromagnet (AFM) because the strong coupling condition is not easily fulfilled. Here we present time-domain terahertz measurement on a multiferroic polar antiferromagnet Fe2Mo3O8. We find clearly beating between two modes at frequencies above and below the electric-active magnon frequency below TN, which we assign to the formation of AFM magnon-polariton. An ultra-strong spin-photon coupling effect is derived based on the energy level splitting. However, the AFM magnon-polariton is absent in the frequency domain measurement. Our work reveals that the coherent magnon formation driven by the ultrashort THz pulse provides a new way to detect polariton mode splitting.
The notion of a simple ordered state implies homogeneity. If the order is established by a broken symmetry, elementary Landau theory of phase transitions shows that only one symmetry mode describes this state. Precisely at points of phase coexistence domain states formed of large regions of different phases can be stabilized by long range interactions. In uniaxial antiferromagnets the so-called metamagnetism is an example of such a behavior, when an antiferromagnetic and field-induced spin-polarized paramagnetic/ferromagnetic state co-exist at a jump-like transition in the magnetic phase diagram. Here, combining experiment with theoretical analysis, we show that a different type of mixed state between antiferromagnetism and ferromagnetism can be created in certain acentric materials. In the small-angle neutron scattering experiments we observe a field-driven spin-state in the layered antiferromagnet Ca3Ru2O7, which is modulated on a scale between 8 and 20 nm and has both antiferromagnetic and ferromagnetic parts. We call this state a metamagnetic texture and explain its appearance by the chiral twisting effects of the asymmetric Dzyaloshinskii-Moriya (DM) exchange. The observation can be understood as an extraordinary coexistence, in one thermodynamic state, of spin orders belonging to different symmetries. Experimentally, the complex nature of this metamagnetic state is demonstrated by measurements of anomalies in electronic transport which reflect the spin-polarization in the metamagnetic texture, determination of the magnetic orbital moments, which supports the existence of strong spin-orbit effects, a pre-requisite for the mechanism of twisted magnetic states in this material.
Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO3 (001) films on SrTiO3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.
Ruthenium compounds play prominent roles in materials research ranging from oxide electronics to catalysis, and serve as a platform for fundamental concepts such as spin-triplet superconductivity, Kitaev spin-liquids, and solid-state analogues of the Higgs mode in particle physics. However, basic questions about the electronic structure of ruthenates remain unanswered, because several key parameters (including the Hunds-rule, spin-orbit, and exchange interactions) are comparable in magnitude, and their interplay is poorly understood - partly due to difficulties in synthesizing sizable single crystals for spectroscopic experiments. Here we introduce a resonant inelastic x-ray scattering (RIXS) technique capable of probing collective modes in microcrystals of $4d$-electron materials. We present a comprehensive set of data on spin waves and spin-state transitions in the honeycomb antiferromagnet SrRu$_{2}$O$_{6}$, which possesses an unusually high Neel temperature. The new RIXS method provides fresh insight into the unconventional magnetism of SrRu$_{2}$O$_{6}$, and enables momentum-resolved spectroscopy of a large class of $4d$ transition-metal compounds.
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