Nonlinear optics is applied to two types of centrosymmetric magnetic materials and shown to be a powerful complement to linear optics. (i) For the investigation of centrosymmetric bulk compounds resonance-enhanced sum-frequency generation is introduced and found to be particularly useful for the investigation of antiferromagnets. The technique is demonstrated on antiferromagnetic NiO. (ii) The interfaces of centrosymmetric thin films are investigated on manganite heterostructures by interface-sensitive second-harmonic generation. Maker fringes reveal a valence difference of Mn ions at the heterointerface. Comparison with a model calculation shows good agreement.
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