Antiferromagnetic Resonance Modes Research Articles

Coupling of Ferromagnetic and Antiferromagnetic Spin Dynamics in Mn_{2}Au/NiFe Thin Film Bilayers.

We investigate magnetization dynamics of Mn_{2}Au/Py (Ni_{80}Fe_{20}) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40GHz, far above the single-layer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn_{2}Au. The coupling strength is in the order of 1.6T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn_{2}Au/Py interface.

Phase coupling of acoustic and optical modes in antiferromagnetic materials CrCl3

In layered antiferromagnetic material CrCl3, due to the antiferromagnetic coupling between two magnetic sublattices, there are two antiferromagnetic resonance modes, called acoustic mode with in-phase precession and optical mode with out-of-phase precession. By using Landau–Lifshitz–Gilbert equation, we study the magnetization dynamics of the two sublattices in CrCl3. A coupling resonance mode appears at the coupling point when the acoustic and optical magnon modes are tuned by an applied magnetic field, which is called ‘coupling mode’. In this paper, we present an explanation for the coupling of the acoustic and the optical mode. Our calculation shows that the coupling of the acoustic and optical mode is accomplished by change of the precession phase-difference between two magnetic sublattices.

K2Ni(SeO3)2: A Perfect S = 1 Triangular-Lattice Antiferromagnet with Strong Geometric Frustration and Easy-Plane Anisotropy

We synthesized a single crystal of K2Ni(SeO3)2 (S = 1) with an equilateral triangular lattice (space group R-3m). The magnetic susceptibility χ(T) and heat capacity Cp(T) show that the compound is antiferromagnetically ordered at TN ≈ 7 K. The Curie–Weiss temperature is about θCW = −72.41 and −71.83 K when the field is applied along the ab plane (H//ab) and parallel to the c axis (H//c), respectively, indicating a strong geometric frustration (f ≈ 10). Both the χ(T) and M(H) point to the presence of easy-plane anisotropy. Density functional theory (DFT) calculations estimate the intraplane exchange (J/kB = −19.04 K) and the interplane exchange (J′/kB = −0.01 K), proving the excellent two-dimensionality. The 50 T high-field magnetization M(H) at 2 K indicates a sign of 1/3 magnetization plateau. The high-field electron spin resonance demonstrates the antiferromagnetic resonance modes with easy-plane anisotropy, yielding J highly consistent with the DFT result. Thus, K2Ni(SeO3)2 is a perfect triangular-lattice antiferromagnet with strong frustration and easy-plane anisotropy.

Electric-dipole and magnetic absorption in TbFeO3 single crystals in the THz–IR range

Various mechanisms of absorption including both electric-dipole excitations and magnetic ones in a broadband (5–5000 cm−1) polarized transmission and reflection spectra were studied in orthoferrite TbFeO3 using a coherent submillimeter, pulsed THz, and IR Fourier transform spectroscopy. The classical oscillator model and generalized four-parameter model were used to analyze the spectra obtained, and the parameters of the models were determined. We analyzed main absorption processes, which include contributions of both phonons and multi-phonon processes as well as electron transitions in Tb3+ ions and two antiferromagnetic resonance modes in the Fe subsystem. As a result, the spectra of real and imaginary parts of permeability and permittivity were obtained. A distinctive feature of the studied electrodynamic response in TbFeO3 is a significant absorption in the THz range due to multi-phonon processes, which exceeds optical phonon contribution by an order of magnitude at room temperature.

High-field magnetization and electronic spin resonance study in the twisted honeycomb lattice α-Mn2V2O7

We report the single-crystal growth of Mn2V2O7 and the results of magnetic susceptibility, high-field magnetization up to 55 T and high-frequency electric spin resonance (ESR) measurements for its low-temperature α phase. Two antiferromagnetic (AFM) ordering at 17.5 K and 3 K and obvious magnetic anisotropy are observed in α-Mn2V2O7 upon cooling. In pulsed high magnetic fields, the compound reaches the saturation magnetic moment of ∼10.5 μ B for each molecular formula at around 45 T after two undergoing AFM phase transitions at H c1 ≈ 16 T, H c2 ≈ 34.5 T for H//[] and H sf1 = 2.5 T, H sf2 = 7 T for H//[]. In these two directions, two and seven resonance modes are detected by ESR spectroscopy, respectively. The ω 1 and ω 2 modes of H//[] can be well described by two-sublattice AFM resonance mode with two zero-field gaps at 94.51 GHz and 169.28 GHz, indicating a hard-axis feature. The seven modes for H//[] are partially separated by the critical fields of H sf1 and H sf2, displaying the two signs of spin-flop transition. The fittings of ω c1 and ω c2 modes yield zero-field gaps at 69.50 GHz and 84.73 GHz for H//[], confirming the axis-type anisotropy. The saturated moment and gyromagnetic ratio indicate the Mn2+ ion in α-Mn2V2O7 is in a high spin state with orbital moment completely quenched. A quasi-one-dimensional magnetism with a zig-zag-chain spin configuration is suggested in α-Mn2V2O7, due to the special neighbor interactions caused by a distorted network structure with honeycomb layer.

High-Angular Momentum Excitations in Collinear Antiferromagnet FePS3.

We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS3. Our measurements reveal an excitation that closely resembles the antiferromagnetic resonance mode typical of easy-axis antiferromagnets; nevertheless, it displays an unusual, four-times larger Zeeman splitting in an applied magnetic field. We identify this excitation with an |Sz| = 4 multipolar magnon─a single-ion 4-magnon bound state─that corresponds to a full reversal of a single magnetic moment of the Fe2+ ion. We argue that condensation of multipolar magnons in large-spin materials with a strong magnetic anisotropy can produce new exotic states.

Role of magnetoelastic coupling and magnetic anisotropy in MnTiO3

We report the thermodynamic properties studied by thermal expansion, magnetostriction, magnetisation, and specific heat measurements as well as the low-energy magnetic excitations of \mto\ and investigate how magneto-elastic coupling and magnetic anisotropy affect the evolution of long-range order and the magnetic phase diagram. Specifically, we utilise high-resolution capacitance dilatometry and antiferromagnetic resonance (AFMR) studies by means of high-frequency electron spin resonance (HF-ESR) spectroscopy. The role of anisotropy is reflected by spin-reorientation at $B_{\rm{SF}}$~$\simeq 6$~T and a corresponding sign change in $\partial T_{\rm N}/\partial B$. Analysis of the AFMR modes enables us to establish the zero-field excitation gap $\Delta$ as well as its temperature dependence. We derive the effective anisotropy field $B_\mathrm{A} = 0.16(1)$~T which predominately originates from out-of-plane nearest-neighbour dipole-dipole interactions. Despite the nearly fully quenched orbital moment, our data show pronounced thermal expansion and magnetostriction anomalies at $T_{\rm N}$ and $B_{\rm{SF}}$ , respectively, which allows the experimental determination of sizable uniaxial pressure dependencies, i.e., $\partial B_{\rm SF}/\partial p_{\rm c} = -0.20(2)$~T/GPa, $\partial T_{\rm N}/ \partial p_b = 0.69(12)$~K/GPa, and $\partial T_{\rm N}/ \partial p_c = -2.0(4)$~K/GPa. Notably, short-range magnetic order appears up to at least 3$T_{\rm N}$ , as indicated by anisotropic lattice distortion, the violation of a constant Gr\"uneisen behavior, and the presence of local magnetic fields detected by HF-ESR.

Terahertz spectroscopy of antiferromagnetic resonances in YFe1−xMnxO3 ≤x≤0.4 across a spin reorientation transition

We have conducted a terahertz spectroscopic study of antiferromagnetic resonances in bulk orthoferrite YFe1−xMnxO3 0≤x≤0.4. Both the quasi-ferromagnetic resonance mode and the quasi-antiferromagnetic resonance mode in the weak ferromagnetic Γ4 phase disappear near the spin reorientation temperature, TSR, for the onset of the collinear antiferromagnetic Γ1 phase (x ≥ 0.1). Below TSR, an antiferromagnetic resonance mode emerges and exhibits a large blueshift with decreasing temperature. However, below 50 K, this mode softens considerably, and this tendency becomes stronger with Mn doping. We provide a deeper understanding of such behaviors of the antiferromagnetic resonance modes in terms of the influence of the Mn3+ ions on the magnetocrystalline anisotropy. Our results show that terahertz time-domain spectroscopy is a useful, complementary tool in tracking magnetic transitions and probing the interaction between disparate magnetic subsystems in antiferromagnetic materials with multiple ionic species.

Zero-field magnon–photon coupling in antiferromagnet CrCl3

We experimentally and theoretically study the coupling between magnons and microwave photons at zero static magnetic field in a system combined by a planar superconducting cavity and a van der Waals antiferromagnetic insulator CrCl3 crystal. Using high-temperature superconductor DyBaCO, we fabricate a planar superconducting cavity with a high-quality factor. The antiferromagnetic resonance of CrCl3 has a finite resonance frequency within a microwave electronics range at zero static magnetic field, and this frequency is sensitive to the temperature. Tuning the temperature to make the antiferromagnetic resonance mode of CrCl3 approaching the cavity mode, we experimentally achieve the coupling between the magnon in CrCl3 and the microwave photon in cavity. An anticrossing feature of the transmission spectra mapping indicates the coupling of magnons and photons at zero static magnetic field. The cooperativity of the hybrid system is 76, indicating a strong coherent information exchange between photons and magnons in CrCl3.

High-efficiency optical vortex generation with hybrid all-dielectric geometric-metasurface in visible frequency

We report a high-efficiency geometric-phase metasurface composed of hybrid all-dielectric nanofins, of which the conversion efficiency 93% is obtained in visible frequency. Such high efficiency originates from the stimulation of the anti-ferromagnetic resonance modes of polarization-dependent parity, i.e. even and odd number of anti-parallel magnetic dipoles would introduce π phase delay among two orthogonal components of the transmitted field. We further demonstrate the generation of the focused optical vortex (OV), Bessel OV and autofocusing Airy OV with the hybrid all-dielectric metasurfaces, which shows promising applications in flexible optical trapping and optical communications.

Antiferromagnetic Resonance in a Spin-Gap Magnet with Strong Single-Ion Anisotropy

Quasi-one-dimensional magnet NiCl$_2\cdot$4SC(NH$_2$)$_2$, usually abbreviated as DTN, does not order at zero field down to $T=0$: due to the strong single-ion anisotropy of the "easy plane" type acting on $S=1$ Ni$^{2+}$ ions, the $S_z=0$ ground state is separated from $S_z=\pm 1$ excitations by an energy gap. Once the magnetic field is applied along the main anisotropy axis, the gap closes at $B_{c1}=2.18$ T and the field-induced antiferromagnetic order arises. The low-energy excitations spectrum of this field-induced ordered state includes two branches of excitations, one of them have to be a gapless Goldstone mode. Recent studies of excitations spectrum in a field-induced ordered state of DTN (T.Soldatov et.al, Phys.Rev.B 101, 104410 (2020)) have revealed that Goldstone mode became gapped as magnetic field deviates from the main symmetry axis. This paper proposes simple description of antiferromagnetic resonance modes of quasi-one-dimensional quantum $S=1$ magnet with strong single-ion anisotropy. The approach used is based on a combination of the strong coupling model for the anisotropic spin chain with the conventional mean-field model of antiferromagnetic resonance. The resulting model fits to the known experimental results without additional tuning parameters.

Heavy Fermion Metal CeB6 in SubTHz and THz Range: The Electron Spin Resonance and Neutron Scattering Studies

Cerium hexaboride attracts attention of solid-state community for almost 50 years. Nowadays, this material is considered as a unique example of a system with orbital ordering resulting in formation of low-temperature magnetic phase with quadrupolar order, the so-called antiferroquadrupole phase. In crystal electric field 4f 1 state of Ce3+ splits into Γ7 doublet and Γ8 quartet, the latter is believed to be the ground state, as long as just Γ8 allows quadrupolar effects. An additional tool for studying rich physics of CeB6 appeared since the discovery in this material the ESR, which is specific to antiferroquadrupole (AFQ) phase and is missing in the paramagnetic phase. In the present review, we have brought together electron spin resonance and inelastic neutron scattering data, which allowed obtaining most detailed structure of the magnetic resonance modes in the AFQ phase in a wide-frequency range up to 0.6 THz. It is shown that apart paramagnetic resonance main mode with homogeneous precession of the magnetization, there are antiferromagnetic resonance modes as well, some of them existing in the AFQ phase. Analysis of the g factors demonstrates that available theories based on Γ8 ground state are facing difficulties with accounting experimental data, thus providing a challenge for developing of a theory correctly describing dynamic magnetic properties of this interesting material.

Magnetocrystalline anisotropy of the easy-plane metallic antiferromagnet Fe2As

Magnetocrystalline anisotropy is a fundamental property of magnetic materials that determines the dynamics of magnetic precession, the frequency of spin waves, the thermal stability of magnetic domains, and the efficiency of spintronic devices. We combine torque magnetometry and density functional theory calculations to determine the magnetocrystalline anisotropy of the metallic antiferromagnet Fe$_2$As. Fe$_2$As has a tetragonal crystal structure with the N\'eel vector lying in the (001) plane. We report that the four-fold magnetocrystalline anisotropy in the (001)-plane of Fe$_2$As is extremely small, ${K_{22}} = - 150~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$ at T = 4 K, much smaller than perpendicular magnetic anisotropy of ferromagnetic structure widely used in spintronics device. ${K_{22}}$ is strongly temperature dependent and close to zero at T > 150 K. The anisotropy ${K_1}$ in the (010) plane is too large to be measured by torque magnetometry and we determine ${K_1} = -830~{\rm{ kJ/}}{{\rm{m}}^{\rm{3}}}$ using first-principles density functional theory. Our simulations show that the contribution to the anisotropy from classical magnetic dipole-dipole interactions is comparable to the contribution from spin-orbit coupling. The calculated four-fold anisotropy in the (001) plane ${K_{22}}$ ranges from $- 292~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$ to $280~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$, the same order of magnitude as the measured value. We use ${K_1}$ from theory to predict the frequency and polarization of the lowest frequency antiferromagnetic resonance mode and find that the mode is linearly polarized in the (001)-plane with $f = $ 670 GHz.

Anisotropic energy gap of low-frequency AFMR mode in FexGa1-xBO3 single crystals

Diamagnetically diluted iron-gallium borate FexGa1-xBO3 single crystals have been studied by Antiferromagnetic Resonance (AFMR) at 77 K in the frequency range of 17 to 25 GHz in magnetizing fields up to 10 kOe. Anisotropic energy gap of the low-frequency AFMR mode has been determined under the rotation of the magnetizing field in the basal plane of the crystals. The results show a reduction of the anisotropic energy gap with decreasing x.

Properties of antiferromagnetic resonance in GdB6

Antiferromagnetic resonance in GdB6 exhibits a series of anomalous features such as jump of the resonance position at TN = 15.5 K and resonance evolution to four line structure with further temperature decrease. Here we report the study of temperature behaviour of antiferromagnetic resonance modes v(H0) in AFM phase. We analyzed v(H0) dependencies at several temperatures T < TN within the model of an antiferromagnet with easy anisotropy axis. In the result we have demonstrated that AFMR behaviour is determined by two following factors: (i) abrupt change of the gyromagnetic ratio γ at TN, which remains constant (corresponding to g-factor g = 1.27) in AFM phase and (ii) jump of zero field frequency v0 (from v0 ≈ 0 to v0 ≈ 20 GHz at TN with its further growth to v0 ≈ 35 GHz at T = 1.9 K.

Electron magnetic resonance of iron-gallium borate single crystals

Electron magnetic resonance (EMR) studies of iron-gallium borate, FexGa1−xBO3, single crystals have been carried out in the frequency range ca. 8–38 GHz in magnetizing fields up to 10 kOe and the temperature range of 4–310 K. With decreasing x in the range of 0.34≤x≤1, the EMR spectra show a gradual passage from a low-frequency antiferromagnetic resonance (AFMR) mode at x = 1 toward a coexistence of AFMR and cluster magnetic resonance arising, respectively, from completely and partially magnetically ordered crystal regions. Temperature and concentration dependences of magnetic characteristics of iron-gallium borates, namely, the Néel temperature, the Dzyaloshinskii–Moriya field, and the isotropic energy gap, have been determined by means of AFMR. In contrast to unmixed FeBO3, FexGa1−xBO3 crystals with 0.34≤x≤0.85 show anomalous nonmonotonic temperature dependences of the Dzyaloshinskii–Moriya field with a maximum well below the Néel temperature suggesting the occurrence of another magnetic transition in this temperature range.

The decisive role of magnetic anisotropy in honeycomb layered Li3Ni2SbO6 and Na3Ni2SbO6

The decisive role of magnetic anisotropy even in systems with small anisotropy is illustrated for the honeycomb-layered antiferromagnets The decisive role of magnetic anisotropy in honeycomb layered Li$_3$Ni$_2$SbO$_6$ and Na$_3$Ni$_2$SbO$_6$ with $A$ = Li and Na. Both systems evolve long range magnetic order below $T_{\rm N}$ = 14 and 16.5~K, respectively. The magnetic phase diagrams obtained from static magnetisation studies up to 15~T imply competing antiferromagnetic phases and a tricritical point at $T_{\rm N}$. The phase boundaries are visible in the dynamic response of the antiferromagnetic resonance modes, too, which investigation by means of high frequency/high field electron spin resonance enables precise determination of magnetic anisotropy. The anisotropy gap amounts to $\Delta = 360 \pm 2$~GHz in Na$_3$Ni$_2$SbO$_6$ while in Li$_3$Ni$_2$SbO$_6$ orthorhombicity is associated with $\Delta = 198 \pm 4$ and $218 \pm 4$~GHz. Above $T_{\rm N}$, the data imply short-range antiferromagnetic order up to at least 80~K. The data suggest a crucial role of anisotropy for selecting the actual spin structure at $B=0$~T.

Magnetic field dependence of antiferromagnetic resonance in NiO

We report on measurements of magnetic field and temperature dependence of antiferromagnetic resonances in the prototypical antiferromagnet NiO. The frequencies of the magnetic resonances in the vicinity of 1 THz have been determined in the time-domain via time-resolved Faraday measurements after selective excitation by narrow-band superradiant terahertz (THz) pulses at temperatures down to 3 K and in magnetic fields up to 10 T. The measurements reveal two antiferromagnetic resonance modes, which can be distinguished by their characteristic magnetic field dependencies. The nature of the two modes is discussed by comparison to an eight-sublattice antiferromagnetic model, which includes superexchange between the next-nearest-neighbor Ni spins, magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman interaction with the external magnetic field. Our study indicates that a two-sublattice model is insufficient for the description of spin dynamics in NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy play important roles.

Magnetism and the phase diagram of MnSb2O6

Static and dynamic magnetic properties of P\={3}1$m$-phase MnSb$_2$O$_6$ have been studied by means of muon spin relaxation ($\mu$SR), high-frequency electron spin resonance (HF-ESR), specific heat, and magnetisation studies in magnetic fields up to 25\,T. The data imply onset of long-range antiferromagnetic order at $T_N$ =~8~K and a spin-flop-like transition at $B_{SF}\approx 0.7 - 1$~T. Below $T_N$, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below $T_2$ =~5.3~K is evidenced by a step in the magnetisation and a slight kink of the relaxation rate. Above $T_N$ , both $\mu$SR and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least $12\cdot T_{\rm N}\approx 100$\,K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of $\Delta \approx 18$\,GHz which implies small but finite magnetic anisotropy.

Néel Spin-Orbit Torque Driven Antiferromagnetic Resonance in Mn_{2}Au Probed by Time-Domain THz Spectroscopy.

We observe the excitation of collective modes in the terahertz (THz) range driven by the recently discovered Néel spin-orbit torques (NSOTs) in the metallic antiferromagnet Mn_{2}Au. Temperature-dependent THz spectroscopy reveals a strong absorption mode centered near 1THz, which upon heating from 4 to 450K softens and loses intensity. A comparison with the estimated eigenmode frequencies implies that the observed mode is an in-plane antiferromagnetic resonance (AFMR). The AFMR absorption strength exceeds those found in antiferromagnetic insulators, driven by the magnetic field of the THz radiation, by 3 orders of magnitude. Based on this and the agreement with our theory modeling, we infer that the driving mechanism for the observed mode is the current-induced NSOT. Here the electric field component of the THz pulse drives an ac current in the metal, which subsequently drives the AFMR. This electric manipulation of the Néel order parameter at high frequencies makes Mn_{2}Au a prime candidate for antiferromagnetic ultrafast memory applications.