Antiferromagnetic magnonic charge current generation via ultrafast optical excitation

Néel spin-orbit torque allows a charge current pulse to efficiently manipulate the Néel vector in antiferromagnets, which offers a unique opportunity for ultrahigh density information storage with high speed. However, the reciprocal process of Néel spin-orbit torque, the generation of ultrafast charge current in antiferromagnets has not been demonstrated. Here, we show the experimental observation of charge current generation in antiferromagnetic metallic Mn2Au thin films using ultrafast optical excitation. The ultrafast laser pulse excites antiferromagnetic magnons, resulting in instantaneous non-equilibrium spin polarization at the antiferromagnetic spin sublattices with broken spatial symmetry. Then the charge current is generated directly via spin-orbit fields at the two sublattices, which is termed as the reciprocal phenomenon of Néel spin-orbit torque, and the associated THz emission can be detected at room temperature. Besides the fundamental significance on the Onsager reciprocity, the observed magnonic charge current generation in antiferromagnet would advance the development of antiferromagnetic THz emitter.


Onsager reciprocity, the observed magnonic charge current generation in antiferromagnet would advance the development of antiferromagnetic THz emitter.
Antiferromagnets (AFM) with the intrinsic frequency of terahertz (THz) and high stability against disturbing external fields, are prime candidates to develop new types of memory and logic devices [1][2][3] .In addition to the effects associated with spin-orbit torques (SOT) from the heavy metal (HM) layer [4][5][6][7] , the thermo-magnetoelastic effect 7,8 is also one of the mechanisms for Né el vector switching in the AFM/HM heterostructure.In turn, the high intrinsic frequency of AFM offers an opportunity to pump ultrafast spin current and generate the ultrafast charge current.For instance, in the AFM/HM bilayer, sub-terahertz spin pumping was observed by detecting the voltage signal converted via inverse spin Hall effect (ISHE) 9,10 .Based on this spin-pumping effect, an antiferromagnetic THz emitter has been achieved, and an ultrafast charge current can be generated via ISHE 11,12 .
However, whether the charge current can be directly generated in AFM single film is still unclear 13 .
A direct conversion from magnons to charge current via spin-orbit coupling has been observed in ferromagnetic (Ga, Mn) As thin films.In this case, the magnetization pression can be directly converted into a charge current without additional HM layers 14 .In AFMs with local inversion asymmetry, such as CuMnAs and Mn 2 Au 15 , magnetization switching can be induced without the assistance of HM and achieved only in single AFM film 13,16,17 .Due to the local inversion symmetry breaking, a current induces non-equilibrium spin polarization.Opposite inversion symmetry breaking in the two sublattices results in staggered spin polarization, leading to the coherent switching of the moments at two antiferromagnetic sublattices.Such current-induced torque for Né el vector switching is known as the Né el spin-orbit torque (NSOT) 13,[16][17][18] .Accordingly, in the reciprocal process of NSOT, magnonic charge current is expected to occur in AFM with local inversion symmetry breaking, when staggered non-equilibrium spin polarization is excited by antiferromagnetic magnetization dynamic, i.e., a fluctuation of the Né el vector n.
Since the frequency of antiferromagnetic magnetization dynamic is in THz region, it is extremely difficult to excite AFM magnons using on-chip waveguides as in the ferromagnetic structures.In this regard, ultrafast optical excitation is suggested as the most promising tool to explore the antiferromagnetic magnetization dynamic 19- 24 , and is able to excite a fluctuation of the Né el vector n in picosecond time scale 21 .
The charge current generated from the fluctuation of n is accompanied by the emission of THz wave.In this work, we experimentally demonstrate the THz waveforms generated by the magnonic charge current in AFM metallic Mn 2 Au thin films, which are excited by ultrafast optical pulse.
The crystalline structure of Mn 2 Au is shown in Fig. 1a, whose magnetic sublattices exhibit a local inversion symmetry breaking.The femtosecond laser pulse triggers the orientation fluctuation 12,25,26 and magnitude reduction 27 of the local magnetic moments, resulting in locally non-equilibrium spin polarization (σ A = -σ B ) with opposite signs on the Mn sublattices (See Supporting Information S2).This staggered spin polarization of the electrons is directly converted into a charge current by spin-orbit coupling due to the local inversion symmetry breaking.The magnonic charge current can be expressed as  ∝  A,B ×  ̂, where the charge current J and spin polarization σ A,B are orthogonal (Fig. 1b and c).The observed AFM magnonic charge current generated from the magnetic moment fluctuation can be a favorable building block for antiferromagnetic THz emitter and provides a promising platform to deepen the understanding of NSOT from the view of Onsager reciprocity.

THz signal from Mn 2 Au single layer
We demonstrate the AFM magnonic charge current generation in 15 nm Mn 2 Au thin films utilizing the THz emission spectroscopy technique.The schematic of THz emission spectroscopy setup is shown in Fig. 2a, and the coordination system (xyz) is defined for the laboratory frame.Ultrafast laser pulses with y-axis polarization are used as a pump that propagates along the z-axis, and the sample is arranged perpendicular to the z-axis with an in-plane rotation angle referred to as θ.The y-component of the THz electric field is measured in the time domain through electro-optical sampling.The Mn 2 Au films are (103)-oriented (Supplementary Section S1) and the uniaxial magnetic anisotropy (UMA) of Mn 2 Au films is ascribed to the uniaxial strain from the Pb(Mg 1/3 Nb 2/3 ) 0.7 Ti 0.3 O 3 (PMN-PT) (110) substrate 13,28 .
A typical waveform of the THz emission is observed in PMN-PT/Mn 2 Au thin film (Fig. 2b), which is linearly polarized along y-axis and is pumped with the laser fluence of 12 uJ mm -2 .The corresponding Fourier spectrum of the THz waveform is plotted in Fig. 2c, where the effective spectral width is about 2.8 THz.The linear dependence of the THz amplitude on the laser fluence is shown in Fig. 2d and the THz amplitude does not reach saturation within the energy range.

THz emission mechanism for single Mn 2 Au layers
A comparison of THz emission between Mn 2 Au and other AFM-based structures is shown in Fig. 3a.The results indicate that there is no spin-to-charge current conversion in the heavy metal layer Pt to enhance the THz signal of Mn 2 Au thin film, and the behavior is significantly different from that of NiO and NiO/Pt 11 .It shows that the amplitude of THz signal from the Mn 2 Au single layer is much higher than that of the Mn 2 Au/Pt bilayer because the THz absorption of heavy metal layer decreases the terahertz signal (See Supporting Information S6).Moreover, FeRh is a particularly interesting system that displays a collinear antiferromagnetic phase without inversion symmetry breaking.Thus, a 15 nm FeRh thin film 29 is used as a control sample for Mn 2 Au, and a very weak THz signal from FeRh is observed, which is 50 times weaker than that of the Mn 2 Au single layer (inset of Fig. 3a).The charge current generated in the opposite magnetic sublattice should be cancel out due to the antisymmetric structure of FeRh, so that the magnetic dipole radiation in FeRh is negligible 30,31 .While, a non-vanished charge is generated in Mn 2 Au due to the local broken inversion symmetry, in which the amplitude of the THz signal is much higher than that of the FeRh.It suggests that the mechanism of the magnetic dipole radiation 32 as THz wave emission from Mn 2 Au thin films can be excluded.AFM Né el vector fluctuation triggered by femtosecond laser pulse results in a non-equilibrium spin polarization at two magnetic sublattices, where the staggered polarized spins convert into a charge current via the reciprocal phenomenon of NSOT.There is no obvious influence on the THz emitted signal when a magnetic field (around 4 kOe) is applied to Mn 2 Au (Fig. 3b), indicating that the THz emission caused by antiferromagnetic order cannot be changed by the external magnetic field.This feature also reflects the advantage of antiferromagnetic THz emitter.

Sample azimuth dependence after Né el vector switching
To clarify the relationship between the polarization direction of emitted THz signal and antiferromagnetic Né el vector n, we employ ferroelastic strain from the ferroelectric material PMN-PT to switch the Mn 2 Au Né el vector 13,33 .The experimental results of emitted THz signals are summarized in Fig. 4, in which the amplitude and symmetry of the charge signal can be determined by the Né el vector n.Fig. 4a and b show the schematics of ferroelastic switching of uniaxial magnetic anisotropy in PMN-PT/Mn 2 Au.When a positive electric field E 1 (+ 4 kV cm -1 ), which is larger than the ferroelectric coercive field on PMN-PT (011), is applied (Fig. 4a), n would be switched from [100] to [01 ̅ 1] (parallel to the y-axis) 13 .In contrast, when a negative E 2 (-2 kV cm -1 ) with the opposite polarity up to the ferroelectric coercive field of PMN-PT is applied (Fig. 4b), the strain state and n would be aligned back to [100] (parallel to the x-axis).Note that such a switching is non-volatile, and all of the THz waveforms were measured after removing E.
The amplitude of THz waveforms from the Mn 2 Au films as a function of the sample azimuth θ for the original state, E 1 state and E 2 state are summarized in Fig. 4c,   4d, and 4e, respectively (where θ is the sample in-plane rotation angle.When the Né el vector n is along [100] (Fig. 4c and 4e), the angular dependence of the amplitude is clearly consistent with the uniaxial magnetic anisotropy of Mn 2 Au Né el vector n with a period of 180º (cosθ) with the maximum of the THz at θ = 0º and 360º and the minimum at θ = 180º .The amplitude of the THz waveform after applying electric field is almost 3 times larger than that of the original state 34 , revealing that the electric field clearly modulates the antiferromagnetic domain distribution of Mn 2 Au, guaranteeing the Né el-type domains along the [100] direction more ordered compared to the original state, and the amplitude of the emitted THz signal is enhanced.In contrast, a different symmetry is found after applying a strain electric field E 1 (Fig. 4d).The Né el vector is rotated from [100] to [01 ̅ 1] (90º rotation).The most eminent feature is that the THz waveform data can be fitted by the cosine function with a 30º phase shift.The maximum THz amplitude appears at θ = 330º , and the minimum appears at θ = 150º .This phenomenon arises because the size of the laser spot used in our work is ~3 mm, which is much larger than the size of the Né el-type domains of Mn 2 Au 35,36 .Especially, XMLD-PEEM directly exhibits that the domain switching is almost 30% with NSOT switching 36 .The results indicate that the proportion of the Né el vector reorientation in this sample is approximately 30% in the macroscopic region of the sample.
Similarly, the magnitude of the charge current in the original state is expected to be sample-dependent (See Supporting Information S5), its symmetry is determined by both the magnetocrystalline anisotropy 37 and Né el vector of the sample.The resulting THz signal is emitted by the y-component of total charge current generation, and the charge current is perpendicular to the Né el vector n, which verifies that the mechanism of charge current on satisfies the reciprocal relationship of NSOT.

Conclusions
In conclusion, we demonstrate the AFM magnonic charge current generation in collinear AFM metallic Mn 2 Au at room temperature via the reciprocal relationship of NSOT.In the THz emission experiments, the laser pump pulse triggers the AFM magnon fluctuation in thin films.Instantaneous spin polarization in the staggered magnetic sublattice is achieved, which pumps AFM magnonic charge that is converted into charge current directly.In addition to Mn 2 Au, the AFM magnonic charge current generation is expected to exist in collinear AFM metallic systems with broken space-reversal symmetry, such as CuMnAs and RuO 2 38 .The generation of AFM magnonic charge current generation opens up new prospects for THz antiferromagnetic magnonics and sets the stage for future exploring of magnonic pumping in the emerging field of AFM spintronics.

Fig. 1
Fig. 1 Principle of magnonic charge current generation in Mn 2 Au.a, The

Fig. 2
Fig. 2 Experimental setup and THz spectrum.a, The schematic of THz emission

Fig. 3
Fig. 3 THz emission for AFM vs. AFM/HM layers.a, A comparison of THz

Fig. 4
Fig. 4 Symmetric and amplitude of angular dependence of charge current

3
Nb 2/3 ) 0.7 Ti 0.3 O 3 (PMN-PT) (011) substrates were deposited by magnetron sputtering at 573 K.The base pressure is 2 × 10 −5 Pa, and the growth rate is 0.07 nm/s using a Mn 2 Au alloy target (atomic ratio of 2:1).X-ray diffraction XRD of the Mn 2 Au films were measured using Cu Kα 1 radiation with λ = 1.5406Å.The surface roughness was characterized by an atomic force microscope (AFM).Magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometry with a field of up to 5 Tesla.THz emission spectroscopyA commercial Ti:sapphire laser (central wavelength of 800 nm, pulse duration of 100 fs, repetition rate of 1 kHz) was used for THz emission measurements.The pumping laser beam was split into two parts (at a 9:1 ratio of intensities) for photogenerating and electro-optic sampling of the THz spin currents.The pumping laser pulses were focused onto the emission samples with a pot diameter of around 3 mm.The laser fluence was 12 μJ mm −2 for most of the measurements.Our measurements were obtained with linearly polarized laser pulses, and the emitted THz wave was collected and refocused by two parabolic mirrors with a reflected focal length of 5 cm.The THz electric field was temporally probed by measuring the ellipticity modulation of the probe beam in a 1-mm-thick (110)-oriented ZnTe crystal.All of the measurements were conducted at room temperature with dry air.