ИСТИНА

For antiferromagnet/ferromagnet [F/AF] multilayers a lot of intriguing features have been observed experimentally and predicted theoretically, in particular a rather peculiar behavior at the applied field. We report here the results for [57Fe/Cr]n samples obtained by nuclear resonance reflectivity (NRR) measurements at SPring-8. The pronounced superstructure maxima (1/2 and 3/2) are observed on the delayed NRS reflectivity curves. That confirms the doubling of the magnetic period due to an antiferromagnetic alignment (AF) between 57Fe layers. The measured time spectra of reflectivity are quite different at different Bragg angles. The fit gives the depth profiles of the hyperfine field values and its orientation Bhf. We prove that the partially coherent averaging over surface domains can explain the results instead of the noncollinear interlayer coupling. At each step of the gradually increasing external field the measurements of the delayed reflectivity curves and time spectra of reflectivity have been performed. At the smallest external field ~50 Oe no change of the intensity of the 1/2 peak is observed. However, the time spectra at the critical angle in longitudinal L- and transverse T- geometry show some changes whereas the spectra at higher angles do not change. This effect is the direct evidence of the surface spin flop (SSF) transition. At the external field about 600 Oe the delayed reflectivity curves show the decrease of the 1/2 peak intensity in the L-geometry, but the essential increase in the T- geometry. So at such field the AF connected Bhf rotate perpendicularly to the external filed. At the highest field 1500 Oe the data gives evidence that the magnetization in all 57Fe layers aligns almost parallel to the direction of the external field.