Magnetic behaviour of europium epitaxial thin films
Philippe Mangin, University of Nancy and NCNR
We present the magnetic behavior of thin films of a very atypical rare earth, europium, which has only recently been grown by molecular beam epitaxy along the [110] direction to form a bcc single-domain crystal. In these (110) Eu films, the [001] axis lies in the growth plane, whereas the two others cubic axes ([100] and [010]) are out of the plane at 45° relative to the growth direction. At low temperature, the sample becomes clamped to the sapphire + Niobium substrate, which gives rise to shear and tensile strains e(T). The variation of the strains has been determined as a function of both temperature and the thickness of the films. The magnetic structures have been determined using Resonant X-ray Magnetic Scattering at ESRF and neutron scattering at LLB and the NIST Center for Neutron Research.
As in bulk europium the thin films exhibit a helical magnetic ordering at TN=90K with magnetic propagation vectors along the cubic axes [100], [010] (out of the plane of the sample) and [001] (in the plane of the sample), which gives rise to three kinds of magnetic domains D1, D2 and D3 respectively. However, as the temperature is lowered, two phenomena are observed:
- The wave vectors of the D1 and D2 domains leave the cubic directions and move closer to the growth direction. They rotate through an angle beta(T).
- The D3 domains vanish at a temperature T1 and are only restored if a critical field Hc(T) is applied along [001]. Beta(T), T1 and Hc(T) are shown to be correlated to the strain since they are also dependent upon the thicknesses of the samples. Shear deformations are shown to be of prime importance.
Finally we will present new results regarding the clamping state and the order of the transition. In bulk, the magnetic transition has been shown to be of first order. In epitaxial thin films, a remanent of the first order transition is present, but only in layers that are not yet clamped at the Neel temperature.
To explain these phenomena, we will discuss several mechanisms including magnetoelastic contributions and exchange anisotropy induced by the deformations.
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