Magnetic and magnetoelastic phenomena in rare earth dysprosium/lutetium superlattices and thin films

Abstract

A new regime in the study of rare-earth magnetism began with the discovery that three dimensional long-range order can occur in epitaxial rare-earth/yttrium superlattice structures. In the Dy/Y system, heli-magnetic order propagates along the c-axis growth direction through non-magnetic Y layers as thick as 100 A. This phase persists even at 4.2 Kelvins, the lowest temperature measured, despite the fact that Dy is spontaneously ferromagnetic at T$\sb{\rm c}$ = 85 K in bulk form. This thesis focuses on a new epitaxial system, dysprosium/lutetium, and demonstrates that, in this case, the ferromagnetic transition temperature of Dy is enhanced above the bulk value by nearly 100%. In addition, microscopic domains of magnetoelastic distortion appear at T$\sb{\rm c}$, reducing the local symmetry of the structure from hexagonal to orthorhombic. Similar phenomena are found to occur in Lu/Dy/Lu trilayer films, which suggests that epitaxially induced stresses and strains are responsible for this novel effect. Further, we find in the Dy/Lu superlattices that the helical phase is coherent over regions encompassing several superlattice bilayers. The low temperature magnetic structures consist of either aligned or anti-aligned ferromagnetic Dy blocks. The anti-aligned phase, found in samples with Lu layers thicker than 22 A, Lu is more coherent than the helix. The aligned structure occurs only for samples with the thinnest Lu layers, and exhibits a shorter coherence length. We discuss the construction of the samples by molecular beam epitaxy, and the experimental methods by which their magnetic properties were determined. We address possible mechanisms underlying the observed physical phenomena, both the interlayer magnetic coupling and the enhancement of T$\sb{\rm c}$