Double-spiral magnetic structure of the Fe/Cr multilayer revealed by nuclear resonance reflectivity

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We have studied the magnetization depth profiles in a [⁵⁷Fe(dFe)/Cr(dCr)]₃₀ multilayer with ultrathin Fe layers and nominal thickness of the chromium spacers dCr≈2.00.16em0exnm using nuclear resonance scattering of synchrotron radiation. The presence of a broad pure-magnetic half-order (1/2) Bragg reflection has been detected at zero external field. The joint fit of the reflectivity curves and M\"ossbauer spectra of reflectivity measured near the critical angle and at the ``magnetic'' peak reveals that the magnetic structure of the multilayer is formed by two spirals, one in the odd and another one in the even iron layers, with the opposite signs of rotation. The double-spiral structure starts from the surface with the almost-antiferromagnetic alignment of the adjacent Fe layers. The rotation of the two spirals leads to nearly ferromagnetic alignment of the two magnetic subsystems at some depth, where the sudden turn of the magnetic vectors by ~180^∘ (spin flop) appears, and both spirals start to rotate in opposite directions. The observation of this unusual double-spiral magnetic structure suggests that the unique properties of giant magnetoresistance devices can be further tailored using ultrathin magnetic layers.