Coherent spin precession via photoinduced antiferromagnetic interactions in La_0.67Ca_0.33MnO₃

Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic La_0.67Ca _0.33MnO₃ 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.