Nonlinear Magnetization Dynamics Driven by Strong Terahertz Fields

Abstract

We present a comprehensive experimental and numerical study of magnetization dynamics in a thin metallic film triggered by single-cycle terahertz pulses of ∼ 20 MV/m electric field amplitude and ∼ 1 ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect, and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing table-top terahertz sources.

Inertial spin dynamics in ferromagnets

“…In Ref. [47] it is shown that the response of a similar magnetic film, when either the externally applied magnetic field or the terahertz magnetic field polarity are reversed, has the symmetry expected of a magnetic effect. The polar MOKE configuration is the optimal geometry since the torque acts out of the film plane when both H THz and M are in the plane.…”

Inertial spin dynamics in epitaxial cobalt films

“…2 shows the time-resolved MOKE measurements of the terahertz-field induced dynamics in all three samples. The plotted traces represent the difference of the data recorded with magnetic fields of equal magnitude but opposite polarity, ensuring the magnetic character of the signal [49]. For all samples, the MOKE response is dominated by the coherent precession of the magnetization around the applied THz magnetic field H THz , which in fcc and bcc films is larger in amplitude than in the hcp one.…”

Terahertz charge and spin transport in metallic ferromagnets: the role of crystalline and magnetic order

“…In recent years, however, the use of intense terahertz (THz) radiation to study ultrafast magnetization dynamics in various materials [11][12][13][14][15][16][17][18][19][20][21][22][23] has gathered much attention, owing to the lower photon energy (in the meV range), much closer to the characteristic lattice energies. Indeed, early indications that the lattice structure plays a role in terahertz-driven demagnetization were already discussed in Ref.…”

“…This magnetic field causes the easy plane magnetization vector M to tilt outof plane, while preserving a sizable in-plane component. An intense THz magnetic field H THz with polarization normal to the direction of M excites the system which maximizes the M × H THz torque on the magnetization 14,18,19 . The dynamical response of the magnetization is measured in the polar magneto-optical Kerr effect (MOKE) geometry using a nominally 40 fs near-infrared probe pulse and a balanced detection scheme 18 .…”

“…From these plots, we first notice the two distinct dynamics already observed in Refs. 14,18 : a coherent response of the magnetization, which is associated with the torque exerted by the magnetic component single-cycle THz field and which depends on its sign, and an incoherent response that is independent on the field polarity. The latter is understood in terms of an ultrafast demagnetization which is recovered within a few picoseconds.…”