We present here the combined structural and magnetic investigation of [110] europium films, ranging between 35nm and 2000nm in thickness, that have been recently grown by molecular beam epitaxy [1] to form a bcc single-domain crystal.
We show first that these films undergo an interesting clamping effect [2] that occurs when cooling the sample below a temperature Tcl: while the Eu lattice contracts as bulk element between room temperature and Tcl, the in-plane lattice constants freeze below Tcl to follow the very small thermal expansion coefficient of the sapphire substrate. This clamping temperature increases for small Eu thicknesses. At low temperature, the films are thus submitted to tensile and shear strains, the variation of which have been determined as a function of both temperature and the thickness of the films. A significant thermal hysteretic behavior of the lattice constants has been also observed.
The magnetic arrangement has been determined using SQUID magnetization measurements, Resonant X-ray Magnetic Scattering at ESRF (Grenoble, France) and neutron scattering (under zero and under and external magnetic field) at LLB (Saclay, France) and at 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 sample plane at 45° relative to the growth direction) and [001] (in the growth plane), which gives rise to three kinds of magnetic domains D1, D2 and D3 respectively. However, as the temperature is lowered, two phenomena are observed:
i) 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 ß(T).
ii) The D3 domain vanishes at a temperature Td and is only restored if a critical field Hc(T) is applied along the magnetic propagation direction [001].
ß(T), Td and Hc(T) are shown to be correlated to the strain since they are also dependent upon the thicknesses of the samples. Besides, shear deformations are shown to be of prime importance.
In order to address these issues and to correlate structural and magnetic investigations, we discuss several mechanisms including magnetoelastic contributions and exchange anisotropy induced by the lattice deformations.
[1] S. Soriano, K. Dumesnil, C. Dufour, D. Pierre, Journal of crystal growth 265, 582 (2004)
[2] S. Soriano, C. Dufour, K. Dumesnil, J.A. Borchers, Ph. Mangin, accepted for publication in Applied Physics Letters (2004)
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