We show that linear polarized x-ray-absorption spectroscopy can be used to measure the temperature and thickness dependence of magnetic moments in NiO thin films. We demonstrate that both the long-range order and the nearest-neighbor spin-spin correlations can be revealed. NiO (100) films with thicknesses of 5, 10, and 20 monolayers epitaxially grown on MgO (100) are studied. The Neel temperature is found to be strongly reduced from the bulk value even for the 20 monolayer film

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A new effect on the X-ray absorption line shape is described which is proportional to the nearest-neighbor spin-spin correlation function. We present theory and demonstrate the use of linear polarized X-ray absorption spectroscopy to study the temperature dependence of the long-range order parameter and the nearest-neighbor spin-spin correlation function in antiferromagnetic NiO.

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