Terahertz spin dynamics in rare-earth orthoferrites

Abstract

Recent interest in developing fast spintronic devices and laser-controllable magnetic solids has sparked tremendous experimental and theoretical efforts to understand and manipulate ultrafast dynamics in materials. Studies of spin dynamics in the terahertz (THz) frequency range are particularly important for elucidating microscopic pathways toward novel device functionalities. Here, we review THz phenomena related to spin dynamics in rare-earth orthoferrites, a class of materials promising for antiferromagnetic spintronics. We expand this topic into a description of four key elements. (1) We start by describing THz spectroscopy of spin excitations for probing magnetic phase transitions in thermal equilibrium. While acoustic magnons are useful indicators of spin reorientation transitions, electromagnons that arise from dynamic magnetoelectric couplings serve as a signature of inversion-symmetry-breaking phases at low temperatures. (2) We then review the strong laser driving scenario, where the system is excited far from equilibrium and thereby subject to modifications to the free-energy landscape. Microscopic pathways for ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore, we review a variety of protocols to manipulate coherent THz magnons in time and space, which are useful capabilities for antiferromagnetic spintronic applications. (4) Finally, new insights into the connection between dynamic magnetic coupling in condensed matter and the Dicke superradiant phase transition in quantum optics are provided. By presenting a review on an array of THz spin phenomena occurring in a single class of materials, we hope to trigger interdisciplinary efforts that actively seek connections between subfields of spintronics, which will facilitate the invention of new protocols of active spin control and quantum phase engineering.

Resonant-dissipative model of ultrafast spin dynamics

“…It follows from this that, without taking into account the spin-orbit interaction (or coupling), i.e. at 𝜆 = 0, the last equation reduces to the resonant-dissipative model of the Faraday effect in non-magnetic dielectrics proposed in [7][8][9].…”

Precession of magnetic moments with change of their orientations

“…Recently, one of the intensively developed areas of femtomagnetism is the control of the magnetization of nano-sized magnets, and also the speed of ultrafast laser-induced spin precession with spin-orbit interaction [1][2][3][4][5][6][7][8].…”

Overcoming losses in superlenses with synthetic waves of complex frequency

“…To overcome this limitation, a negative refractive index lens has been proposed to enhance the evanescent waves to recover the deep-subwavelength resolution of imaging (1,2). Subsequently, superlenses, made of either natural materials with negative permittivities (3)(4)(5)(6)(7) or hyperbolic materials with mixed signs of dielectric constants along different directions (8)(9)(10)(11)(12)(13), have been proposed to attain subdiffractional limited imaging. Nevertheless, losses are non-negligible in materials with negative parameters (14)(15)(16), which reduces the deep-subwavelength information of the superlenses and seriously affects the resolution of imaging (17)(18)(19).…”