All-Optical Control of Spin in a 2D van der Waals Magnet

Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. While ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin, little is known so far about how laser pulses influence the spins in 2D magnets. Stacking of 2D magnetic layers promises to deliver heterostructures with new and tuneable magnetic and magneto-optical properties. Optical control of their magnetic state would provide new opportunities in both data storage and photonics, where the non-volatility of magnetism could deliver improved energy efficiency.<br/>The search for all optical switching (AOS) of magnetic order was ignited by the observation of ultrafast demagnetization in Ni on timescales of ~100fs [1]. AOS of the magnetization was observed in ferrimagnetic GdFeCo [2] but was found to be a thermally driven toggle switching, dependent upon the exchange coupling and different demagnetization times of Gd and FeCo [3]. Helicity dependent AOS has been observed in a number of materials including [Co/Pt] multilayers [4] but usually requires multiple fs optical pulses in a process that is believed to involve full demagnetization followed by the growth of domains driven by thermally generated spin currents. More recently, toggle switching was observed in rare-earth-free synthetic ferrimagnet structures, with switching driven by the flow of optically induced spin current between the constituent ferromagnetic layers [5].<br/>The selective optical excitation of spin-polarised electrons by circularly polarized light in direct band gap semiconductors is well established. Optical orientation of spin within magnetic semiconductors such as GaMnAs has been found to induce coherent precessional dynamics of the Mn moments [6]. However, AOS was not observed and interest waned due to the stubbornly low Curie temperature (~200K). The electronic structure of transition metal dichalcogenides (TMDCs) also permits selective optical excitation of spin-polarised electrons. By tuning to the band gap, circularly polarized light can excite spin-polarised electrons within just one of the K(K’) valleys [7]. Transfer of optically excited carriers across the interface within a WSe₂/CrI₃ bilayer leads to the presence of an interfacial exchange field that influences the photoluminescence polarization dependence and intensity [8].<br/>Here we demonstrate laser-induced magnetic domain formation and all-optical switching in ferromagnetic CrI₃ [9]. While the magnetism of bare CrI₃ layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in CrI₃/WSe₂ bilayers. The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results suggest that the switching is driven by spin-dependent interfacial charge transfer between the WSe₂ and the CrI₃.<br/>[1] E. Beaurepaire et al., Phys. Rev. Lett<i>. </i><b>76, </b>4250 (1996).<br/>[2] C. D. Stanciu et al., Phys. Rev. Lett. <b>99, </b>047601 (2007).<br/>[3] J. Barker et al., Sci. Rep. <b>3</b>, 3262 (2013).<br/>[4] C-H. Lambert et al., Science <b>345</b>, 1337 (2014).<br/>[5] M. Dabrowski et al. Nano Lett. <b>21</b>, 9210 (2021)<br/>[6] H. Li et al., Appl. Sci. <b>8</b>, 1880 (2018).<br/>[7] D. Xiao et al., Phys. Rev. Lett. <b>108</b>, 196802 (2012).<br/>[8] D. Zhong et al., Nat. Nanotech. <b>15</b>, 187 (2020).<br/>[9] M. Dabrowski et al., Nat. Commun. <b>13</b>, 5976 (2022).