Coherent Spin Precession via Photoinduced Antiferromagnetic Interactions in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$

Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ thin films using the time-resolved magneto-optical Kerr effect. The precession amplitude monotonically decreases with increasing field, indicating that the coherent spin rotation may be triggered by a transient exchange field and not by demagnetization and/or anisotropy field modulation. We attribute the transient exchange field to emergent antiferromagnetic interactions due to charge transfer and modification of the kinetic energy of $e_g$ electrons under optical excitation.

Figure 1

(a) Transient reflectivity $\Delta R$, and transient Kerr rotation $\Delta \theta$ of LCMO film with magnetic field $H(0.5\mathrm{T})$ applied perpendicular to the sample plane  (b), with $H$ applied along the in-plane easy axis (c), and with zero field (d) at $\sim 20\mathrm{K}$; (e) Fourier transform of (d).

Figure 2

Transient Kerr rotations of Fe film with magnetic field applied along the in-plane hard axis (a) and easy axis (b) at RT.

Figure 3

Amplitude of precession of (a) 60 nm and 100-nm LCMO films with field applied at 45° to the easy axis and along the easy axis at 20 K, and (b) Fe film with field applied at 55° to the easy axis and of 10° to the easy axis at RT.

Figure 4

(a) Transient exchange field and (b) transient anisotropy field in a state of canted magnetization (magnetic field at 5° to the easy axis) of 60-nm thick LCMO film. The dashed line is a guide to the eye of the data.

Figure 5

Transient Kerr rotations of LCMO film with applied field $H=0.5\mathrm{T}$ at various temperatures. The inset shows the uniform precession frequency as a function of the temperature.