Fig. 1. (a) Sketch of the system under consideration. A circularly polarized magnetic field illuminates a magnetic sample whose dimensions are smaller than the region for which the E-field can be considered negligible. This field can trigger ultrafast magnetization dynamics. (b) Two crossed azimuthally polarized beams of 30 THz and peak intensity 2.1 W/cm² define a spatial region of radius nm in which the E-field is lower than 100 MV/m, as depicted in the panel. In such a region, a constant circularly polarized B-field of amplitude 10.5 T and central frequency 30 THz is found.

Fig. 2. Micromagnetic simulation results of the temporal evolution (color code) of the magnetization components (, ) of CoFeB excited by B-fields with different polarization states. (a) RCP (yellowish color scale) and LCP (greenish color scale) B-fields (, THz, ps). The RCP (LCP) B-field induces a measurable negative (positive) component. In both cases the anisotropy field induces a precession of around the equilibrium configuration. The bottom part sketches the mechanism during a B-field period of constant amplitude. The B-field (red), magnetization (black) and torque (green) vector representations at four different times reveal the magnetization dynamics mechanism over one period. (b) Linear polarization along x (yellowish trace) or y (greenish trace). (c) Circular polarization perpendicular to the equilibrium magnetization with RCP (yellowish trace) and LCP (greenish trace) helicities.

Fig. 3. Analysis of the nonlinear effect dependencies. Total magnetization rotation as a function of (a) the polarization state of the B-field (characterized by , and using ) and (b) the inverse of the frequency of a circularly polarized B-field. In both (a) and (b), three different B-field amplitudes (60 T blue, 100 T red and 140 T black) oscillating at are represented. (c) Total magnetization rotation as a function of the circularly polarized B-field amplitude, with three different central frequencies ( blue, red and black). In (a), (b) and (c), the B-field pulse duration is . (d) Total magnetization rotation as a function of the circularly polarized B-field pulse duration, , with three different B-field amplitudes ( blue, red and black) and a central frequency of . In all panels, symbols indicate results from micromagnetic simulations while lines correspond to Equation (10).

Fig. 4. Micromagnetic simulation results of the temporal evolution of the magnetization components ( blue, yellow, black) of CoFeB excited by an RCP B-field. The normalized B-field envelope is shown in dashed red. While a B-field of , THz and ps shows switching at the ps timescale, a B-field of , THz and  ps achieves it at the femtosecond timescale.