Coherent Exchange Scattering of Low-Energy Electrons by Antiferromagnetic Crystals

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Abstract
Coherent exchange scattering has been observed by low‐energy electron diffraction from antiferromagnetic NiO. Below the Néel temperature, low‐energy electron diffraction from the (100) surface, exposed by cleavage in ultrahigh vacuum, has revealed one‐half order beams that correspond to the magnetic periodicity; these diffraction beams disappear at the Néel temperature. The intensity of the additional half‐order ``magnetic'' beams relative to that of the integer order beams is 2%–6%. Because direct dipole‐dipole and spin‐orbit interactions are much too weak to produce beams of the observed intensity, the occurrence of half‐order beams is attributed entirely to exchange scattering. The interpretation of the experimental results is discussed with the aid of a Heisenberg model Hamiltonian. Theoretical expressions for the elastic exchange cross section are in accord with the experimental data. The temperature dependence of the intensity of the exchange scattering is qualitatively explained by a molecular‐field model which includes surface corrections.