Abstract
The magnetic spiral structure of a Y/Dy sample has been investigated for temperatures from 30 to 190 K by the method of the small-angle scattering of polarized neutrons. The sample is a sequence of layers Y50nm [Dy4.3nm /Y2.8nm ]350/Y234nm /Nd200nm Al2O3 (substrate) that is grown as a single crystal with the [001] axis of the hexagonal lattice, which is perpendicular to the layer plane. The experiments demonstrate the appearance of the magnetic peak below T N = 165.4 K, which is associated with the helicoidal phase, and the helicoid coherence length is larger than the layer thickness of the Y/Dy layer. The use of polarized neutrons allows the separation of the polarization-independent and polarization-independent components of magnetic scattering. The polarization-independent component of the magnetic neutron cross section is proportional to magnetization squared 〈S Z 〉2, whereas the polarization-dependent component is proportional to the average chirality of the system 〈C〉 = 〈[·S 1 × S 2]〉. The critical exponents βC = 1.02(1) and β = 0.39(1) have been determined for the average chirality and magnetization, respectively. The magnetization critical exponent β for Dy/Y coincides with the exponent obtained for Dy bulk samples. The difference βC − 2β = 0.24(2) shows that the chirality can be a component of the order parameter that is independent of magnetization.
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