Tracking the ultrafast motion of an antiferromagnetic order parameter

Abstract

The unique functionalities of antiferromagnets offer promising routes to advance information technology. Their compensated magnetic order leads to spin resonances in the THz-regime, which suggest the possibility to coherently control antiferromagnetic (AFM) devices orders of magnitude faster than traditional electronics. However, the required time resolution, complex sublattice interactions and the relative inaccessibility of the AFM order parameter pose serious challenges to studying AFM spin dynamics. Here, we reveal the temporal evolution of an AFM order parameter directly in the time domain. We modulate the AFM order in hexagonal YMnO 3 by coherent magnon excitation and track the ensuing motion of the AFM order parameter using time-resolved optical second-harmonic generation. The dynamic symmetry reduction by the moving order parameter allows us to separate electron dynamics from spin dynamics. As transient symmetry reductions are common to coherent excitations, we have a general tool for tracking the ultrafast motion of an AFM order parameter.

Efficient spin excitation via ultrafast damping-like torques in antiferromagnets

“…With respect to this, we note that any ultrafast mechanism that induces a net magnetisation in an AFM will benefit from the observed exchange enhancement. While field-like excitations via the IFE or using THz pulses typically induce spin-cantings of the order of 1 ∘ 15 , 38 , there are various possibilities beyond increasing the pump fluence that may allow us to exceed a 90 ∘ spin canting and thus induce antiferromagnetic switching. Exploiting different excitation mechanisms such as thermally and optically induced ultrafast spin-transfer torques 35 , 39 , 40 may drastically increase the impulsively induced net magnetisation.…”

“…In AFMs, however, the competition between magnetic anisotropy and the ≳100 times stronger exchange interaction results in strongly elliptical trajectories. An oscillating net magnetisation occurs along the minor axis of the ellipse, whereas the antiferromagnetic order parameter is modulated due to a collective in-phase spin canting along the major axis 15 , 17 , 18 . Therefore, the efficiency of impulsive spin excitations in antiferromagnets depends strongly on the direction of the initial spin canting.…”

“…The inverse Faraday effect (IFE), i.e. the optical generation of effective sub-picosecond magnetic field pulses with circularly polarised light, allows to impulsively create a spin canting in AFMs [8][9][10][11][12][13][14][15] .…”

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

“…More recently, SHG has been used to identify the antiferromagnetic contribution of the SHG dependence on incident light polarization and to probe sub-micron sized antiferromagnetic domains in BFO thin films (Figure 7e) [141]. Furthermore, this optical tool can be used for ultrafast dynamics investigations such as the recent example of tracking motion of antiferromagnetic order parameter in YMnO 3 crystals [142].…”

Spin Dynamics in Antiferromagnets with Domain Walls and Disclinations

“…The application of AFMs in the terahertz technique is based on various physical effects. The application of femtosecond lasers (with a pulse duration time shorter than 100 fs) allows the non-thermal excitation of spin oscillations in transparent AFMs [19][20][21][22][23][24][25][26][27], which can be used to create generators of electromagnetic waves in the terahertz interval with optical pumping and a control over the radiation parameters [28][29][30]. The capability of such excitation is based on the effects that are inverse to the well-known magneto-optical Faraday and Cotton-Mouton effects [7,8].…”