Quasi-antiferromagnetic Modes Research Articles

Ultrafast Spin Dynamics in the Iron Borate Easy-Plane Weak Ferromagnet

Ultrafast processes of the spin dynamics in iron borate FeBO3 are considered theoretically; the mechanisms responsible for excitation of quasi-ferromagnetic as well as quasi-antiferromagnetic spin resonance modes by a one-period terahertz pulse are indicated. In full agreement with experimental observations [27], the excitation of the high-frequency quasi-antiferromagnetic mode is resonant by nature, and its amplitude is a linear function of the electric field of the terahertz pulse. The amplitude of the low-frequency quasi-ferromagnetic mode is a quadratic function of the electric field of the pulse, and the excitation of this mode is governed by the mechanism of the inverse Cotton–Mouton effect.

THz generation and frequency manipulation in AFM/HM interfaces

In this paper, we propose an approximate nonlinear theory of a spintronic terahertz-frequency oscillator based on antiferromagnet-heavy metal interfaces. We present a model of excitation of nonlinear oscillations of Neel vector in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. We determine that, with increasing pumping pulse amplitude, the spin system response increases nonlinearly in the fundamental quasi-antiferromagnetic mode. Our results theoretically show that a spin-current flowing from a heavy metal due to the spin-Hall effect vary the frequencies of the output EM oscillations in a wide range, which could be detected by a standard pump-and-probe spectroscopy. Our study paves the way to laser-induced, electrically tunable, low-power, ultrafast AFM-based oscillator that operates without external magnetic fields at room temperature for telecommunication systems, bio-inspired networks and optical networks on chip. The nonlinear dynamics of the antiferromagnet-based emitters discussed here is of importance in terahertz-frequency spintronic technologies.

Terahertz spectroscopy study of doping and magnetic field induced effects on spin reorientation in Ho<sub>1–<i>x</i></sub>Y<sub><i>x</i></sub>FeO<sub>3</sub> single crystals

In this paper, the effects of magnetic field and nonmagnetic Y<sup>3+</sup> doping on spin state and spin reorientation in HoFeO<sub>3</sub> single crystal are systematically studied by the self-developed terahertz time-domain spectroscopy (THz-TDS) under magnetic field. By doping nonmagnetic Y<sup>3+</sup>, we find that the spin reorientation temperature range decreases. Meanwhile, we also find the type of spin reorientation of HoFeO<sub>3</sub> does not change with Y<sup>3+</sup> doping, indicating that the Y<sup>3+</sup> doping can exchange the interaction energy of Ho<sup>3+</sup>-Fe<sup>3+</sup> without introducing any new magnetic structure. Moreover, the resonance frequency of quasi-ferromagnetic mode (q-FM) decreases with temperature increasing in the low temperature range, while the resonance frequency of quasi-antiferromagnetic mode (q-AFM) increases with temperature increasing in the high temperature range in Ho<sub>1–<i>x</i></sub>Y<sub><i>x</i></sub>FeO<sub>3</sub> single crystals. With the external magnetic field (<i> <b>H</b> </i><sub>DC</sub>) applied along the (110) axis, on the one hand the magnetic field can not only tune the resonant frequency of q-FM but also induce the spin reorientation in Ho<sub>1–<i>x</i></sub>Y<sub><i>x</i></sub>FeO<sub>3</sub> single crystals, and on the other hand this magnetic field induced spin reorientation phenomenon can happen more easily if the temperature approaches to the intrinsic spin reorientation temperature range of the single crystals. Besides, the critical magnetic field induced spin reorientation increases with the doping of Y<sup>3+</sup> increasing. Our research shows that THz spectroscopy data can be used to detect the doping concentration of Y<sup>3+</sup> ions in HoFeO<sub>3</sub>; in addition, Y<sup>3+</sup> doping can make the spin state in HoFeO<sub>3</sub> crystal more stable and not easily affected by external magnetic fields. We anticipate that the role of doping and magnetic field in spin reorientation transition will trigger great interest in understanding the mechanism of the spin exchange interaction and the mechanism of external field tuning effect in the vast family of rare earth orthoferrites.

Terahertz Optomagnetism: Nonlinear THz Excitation of GHz Spin Waves in Antiferromagnetic FeBO_{3}.

A nearly single cycle intense terahertz (THz) pulse with peak electric and magnetic fields of 0.5 MV/cm and 0.16T, respectively, excites both modes of spin resonances in the weak antiferromagnet FeBO_{3}. The high frequency quasiantiferromagnetic mode is excited resonantly and its amplitude scales linearly with the strength of the THz magnetic field, whereas the low frequency quasiferromagnetic mode is excited via a nonlinear mechanism that scales quadratically with the strength of the THz electric field and can be regarded as a THz inverse Cotton-Mouton effect.THz optomagnetism is shown to be more energy efficient than similar effects reported previously for the near-infrared spectral range.

Spintronic terahertz-frequency nonlinear emitter based on the canted antiferromagnet-platinum bilayers

In this paper, we propose an approximate nonlinear theory of a spintronic terahertz-frequency emitter based on canted antiferromagnet-platinum bilayers. We present a model accounting for the excitation of nonlinear oscillations of the Néel vector in an antiferromagnet using terahertz pulses of an electromagnetic field. We determine that, with increasing amplitude of the pumping pulse, the spin system’s response increases nonlinearly in the fundamental quasiantiferromagnetic mode. We demonstrate control of the Néel vector trajectory by changing the terahertz pulse peak amplitude and frequency and determine the bands of nonlinear excitation using Fourier spectra. Finally, we develop an averaging method which gives the envelope function of an oscillating output electromagnetic field. The nonlinear dynamics of the antiferromagnet-based emitters discussed here is of importance in terahertz-frequency spintronic technologies.

Research progress of coherent control of terahertz spin waves and strong coupling in rare-earth orthoferrites

Antiferromagnets (AFM) are promising for future spintronic applications due to their advantageous properties. Antiferromagnets produce no stray fields and are insensitive to external magnetic field perturbations. Furthermore, antiferromagnets show intrinsic high terahertz (THz) frequency dynamics. The THz pulses are a direct and general probe of ultrafast spin dynamics in insulating antiferromagnets. In this review article, we discuss the excitation and control of the antiferromagnetic spin resonances in rare-earth orthoferrites (<i>R</i>FeO<sub>3</sub>, <i>R</i> indicates Y and rare-earth element) with the THz electromagnetic pulsetime-domain spectroscopy. We believe that this approach is general and can be applied to a broad range of materials with different AFM spin alignments, giving a novel non-contact approach to probing AFM order with ps temporal resolution. We summarize different quasi-ferromagnetic modes (qFM) and quasi-antiferromagnetic modes (qAFM), as well as the spin reorientation transition temperatures of <i>R</i>FeO<sub>3</sub>. Coherent control of spin waves at THz frequency promises fruitful applications in ultrafast magnetization control and has received increasing attention. It is demonstrated that not only the delay time between the excitation and control THz pulses arriving DyFeO<sub>3</sub>, but also the intrinsic dielectric anisotropy of YFeO<sub>3</sub> in the THz range allow the coherent control of both the amplitude and the phase of the excited spin waves. Moreover, we outline the current observation of Dicke cooperativity in magnetic interaction of Er<i><sub>x</sub></i>Y<sub>1－</sub><i><sub>x</sub></i>FeO<sub>3</sub>, which presents a route to understanding, controlling, and predicting novel phases of condensed matter by using the concepts and tools available in quantum optics. Finally, magnon-polaritonsare demonstrated to play a key role in preparing the THz waves through TmFeO<sub>3</sub>.

Ultrafast Photoinduced Multimode Antiferromagnetic Spin Dynamics in Exchange-Coupled Fe/RFeO3 (R = Er or Dy) Heterostructures.

Antiferromagnetic spin dynamics is important for both fundamental and applied antiferromagnetic spintronic devices; however, it is rarely explored by external fields because of the strong exchange interaction in antiferromagnetic materials. Here, the photoinduced excitation of ultrafast antiferromagnetic spin dynamics is achieved by capping antiferromagnetic RFeO3 (R = Er or Dy) with an exchange-coupled ferromagnetic Fe film. Compared with antiferromagnetic spin dynamics of bare RFeO3 orthoferrite single crystals, which can be triggered effectively by ultrafast laser heating just below the phase transition temperature, the ultrafast photoinduced multimode antiferromagnetic spin dynamic modes, for exchange-coupled Fe/RFeO3 heterostructures, including quasiferromagnetic resonance, impurity, coherent phonon, and quasiantiferromagnetic modes, are observed in a temperature range of 10-300 K. These experimental results not only offer an effective means to trigger ultrafast antiferromagnetic spin dynamics of rare-earth orthoferrites, but also shed light on the ultrafast manipulation of antiferromagnetic magnetization in Fe/RFeO3 heterostructures.

Far-IR magnetospectroscopy of magnons and electromagnons in TbFeO3 single crystals at low temperatures

Transmittance spectra of $\mathrm{TbFe}{\mathrm{O}}_{3}$ single crystals have been studied in the far-infrared spectral range $15\ensuremath{-}120\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in the external magnetic fields up to 9 T and at low temperatures down to 1.5 K. The temperature and magnetic-field dependencies of the antiferromagnetic resonance (AFMR) modes have been measured below and above the magnetic ordering temperature of Tb moments ${T}_{N}^{\mathrm{Tb}}=3.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at $Tl{T}_{N}^{\mathrm{Tb}}$. The quasiantiferromagnetic mode gains electric-dipole activity along the $c$ axis below ${T}_{N}^{\mathrm{Tb}}$ and thus behaves as a hybrid, i.e., both electric- and magnetic-dipole active, mode. In addition to AFMR modes, below ${T}_{N}^{\mathrm{Tb}}$ we observed the appearance of electromagnon excitation at about $27\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which is electric-dipole active along the $c$ axis. The electromagnon is optically active only in a narrow temperature range, $2.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl{T}_{N}^{\mathrm{Tb}}$ at $H=0$, and in a narrow range of magnetic fields 2.4--2.7 T applied along the $b$ axis at $T=1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We argue that this electromagnon appears in magnetic phases, which are compatible with a spontaneous electric polarization along the $b$ axis.

Ultralow temperature terahertz magnetic thermodynamics of perovskite-like SmFeO3 ceramic.

The terahertz magnetic properties of perovskite-like SmFeO3 ceramic are investigated over a broad temperature range, especially at ultralow temperatures, using terahertz time-domain spectroscopy. It is shown that both resonant frequencies of quasi-ferromagnetic and quasi-antiferromagnetic modes have blue shifts with the decreasing temperature due to the enhancement of effective magnetic field. The temperature-dependent magnetic anisotropy constants are further estimated using the resonant frequencies, under the approximation of omitting the contribution of Sm3+ magnetic moments to the effective field. Specially, the effective anisotropy constants in the ca and cb planes at 3 K are 6.63 × 105 erg/g and 8.48 × 105 erg/g, respectively. This thoroughly reveals the terahertz magnetic thermodynamics of orthoferrites and will be beneficial to the application in terahertz magnetism.

Magnetization-free measurements of spin orientations in orthoferrites using terahertz time domain spectroscopy

The spin orientation during spin reorientation phase transition in a weak ferromagnetic orthoferrite DyxEr1−xFeO3 (x = 0.7) has been studied by using terahertz time domain spectroscopy under zero and weak external magnetic fields. The spin orientation angle was deduced from the ratio of the absorption intensity of quasi-ferromagnetic and quasi-antiferromagnetic modes, which appear around 0.25 and 0.4 THz, respectively. Between 11.4 and 17.9 K, this material showed a rotation-type reorientation transition rather than Morin-type transition. The temperature and magnetic field dependence of the orientation angle of the magnetic moment was found to follow a simple model assuming a linear temperature dependence of the anisotropy parameter. It has been also shown that this method is insensitive to the domain structure with opposite polarity and that it allows measurement without macroscopic magnetization.

Spin reorientation transition process in single crystal NdFeO3

The spin reorientation transition in single crystal NdFeO3 is studied using AC magnetic susceptibility, hysteresis loops, and polarized terahertz (THz) time domain spectroscopy measurements. Different frequency dependence behaviors of AC susceptibility reflect that the dynamic response of magnetization inside the spin reorientation region differs from the phase outside the transition region. The magnetization hysteresis loops at different temperatures reveal that domains formed during the spin reorientation process, which coincides with the abrupt increase of AC magnetic susceptibility during the transition. In addition, temperature dependent THz wave excitation of quasi-antiferromagnetic mode indicates the process of spin reorientation as a continuous rotation of Fe3+ spins rather than a mixed phase of Γ4 and Γ2.

Terahertz probes of magnetic field induced spin reorientation in YFeO3 single crystal

Using the terahertz time-domain spectroscopy, we demonstrate the spin reorientation of a canted antiferromagnetic YFeO3 single crystal, by evaluating the temperature and magnetic field dependence of resonant frequency and amplitude for the quasi-ferromagnetic (FM) and quasi-antiferromagnetic modes (AFM), a deeper insight into the dynamics of spin reorientation in rare-earth orthoferrites is established. Due to the absence of 4f-electrons in Y ion, the spin reorientation of Fe sublattices can only be induced by the applied magnetic field, rather than temperature. In agreement with the theoretical predication, the frequency of FM mode decreases with magnetic field. In addition, an obvious step of spin reorientation phase transition occurs with a relatively large applied magnetic field of 4 T. By comparison with the family members of RFeO3 (R = Y3+ or rare-earth ions), our results suggest that the chosen of R would tailor the dynamical rotation properties of Fe ions, leading to the designable spin switching in the orthoferrite antiferromagnetic systems.

Terahertz emission spectroscopy of laser-induced spin dynamics inTmFeO3andErFeO3orthoferrites

Using the examples of laser-induced spin-reorientation phase transitions in ${\mathrm{TmFeO}}_{3}$ and ${\mathrm{ErFeO}}_{3}$ orthoferrites, we demonstrate that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of \ensuremath{\sim}0.3--0.35 THz. By recording how the amplitude and phase of the excited oscillations depend on temperature and applied magnetic field, we show that the unexpected mode has all the features of a spin resonance of the ${\mathrm{Fe}}^{3+}$ ions. We suggest that it can be assigned to transitions between the multiplet sublevels of the ${}^{6}{A}_{1}$ ground state of the ${\mathrm{Fe}}^{+3}$ ions occupying rare-earth positions.

Terahertz spectroscopic observation of spin reorientation induced antiferromagnetic mode softening in DyFeO3 ceramics

Dysprosium orthoferrite has been found to undergo the Γ4(Gx,Fz)→Γ1(Gy) spin reorientation transition (Morin transition) at about 37K. In this study, the Morin transition in DyFeO3 ceramic is investigated using a temperature-tunable terahertz spectroscopy system. It is experimentally demonstrated that the DyFeO3 ceramics exhibit obvious terahertz absorption and emission at the antiferromagnetic resonant frequency. It is also shown that the lifetime of magnetic dipole emission remarkably shortens and the quasi-antiferromagnetic mode softens below 198K, which can be ascribed to the decreased terahertz absorption and reduced internal magnetic field induced by the spin reorientation, respectively. This contributes to the research of rare earth orthoferrites׳ terahertz electromagnetic properties.

The polarization trajectory of terahertz magnetic dipole radiation in (110)-oriented PrFeO3 single crystal

By using the polarized terahertz (THz) time-domain spectroscopy, the macro-magnetization motion in (110)-oriented PrFeO3 single crystal was constructed. We emphasize that the trajectory of the emitted THz waveforms relies on not only the motion of macroscopic magnetization vector, but also the spin configuration in the ground state and the propagation of THz pulse. The azimuthal angle (the incident THz pulse polarization with respect to the crystal axes) enables us to control the polarization trajectories of the quasiferromagnetic and quasiantiferromagnetic mode radiations that can lead to further applications on multiple information storing and quantum processing.

Selective excitation of spin resonance in orthoferrite PrFeO3 with impulsive polarized terahertz radiation

Single cycle terahertz (THz) pulses were employed to excite coherent spin waves in (110)-oriented PrFeO3 single crystal. The free induction decay radiations at frequency of 0.34 THz (quasi-ferromagnetic mode, FM mode) and 0.41 THz (quasi-antiferromagnetic mode, AFM mode) were observed arising from the coupling of magnetic moment with the impulsive magnetic field of polarized terahertz radiation. These two spin modes in PrFeO3 can be excited and modulated by the magnetic field of THz pulse with a specific polarization with respect to the crystal axis. The extracted complex dielectric permittivity and magnetic permeability dispersion in THz range suggest a higher efficiency of energy transfer from the impulsive THz pulse into the AFM than the FM spin system.

Terahertz Magnetic Response in ReFeO<sub>3</sub>-Type Antiferromagnetism

ReFeO3 type orthoferrites with the perovskite structure are important antiferromagnetic materials, which show antiferromagnetic resonance and magnetic dipole radiation in the terahertz frequency region. In this paper, the interaction between GdFeO3 ceramic and terahertz magnetic field were investigated with a terahertz spectroscopy. It is shown that the GdFeO3 ceramic has remarkable absorption and emission at 0.61 THz and the permeability exhibits a resonant phenomenon, which can be attributed to the quasi-antiferromagnetic mode. This is of significance in the developments of novel terahertz devices.