Magnetic properties of selected 3d- and 4f-systems on alpha-Al 2 O 3 substrates
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Abstract
In this work the growth and the magnetic properties of the
transition metals molybdenum, niobium, and iron and of the
highly-magnetostrictive C15 Laves phases of the RFe2 compounds
(R: Rare earth metals: here Tb, Dy, and Tb{0.3}Dy{0.7}
deposited on alpha-Al2O3 (sapphire) substrates are
analyzed. Next to (11-20) (a-plane) oriented sapphire
substrates mainly (10-10) (m-plane) oriented substrates
were used. These show a pronounced facetting after high
temperature annealing in air. Atomic force microscopy (AFM)
measurements reveal a dependence of the height, width, and angle
of the facets with the annealing temperature. The observed
deviations of the facet angles with respect to the theoretical
values of the sapphire (10-1-2) and (10-11)
surfaces are explained by cross section high resolution
transmission electron microscopy (HR-TEM) measurements. These show
the plain formation of the (10-11) surface while the
second, energy reduced (10-1-2) facet has a curved
r
shape given by atomic steps of (10-1-2) layers and is
formed completely solely at the facet ridges and valleys. Thin
films of Mo and Nb, respectively, deposited by means of molecular
beam epitaxy (MBE) reveal a non-twinned, (211)-oriented epitaxial
growth as well on non-faceted as on faceted sapphire m-plane, as
was shown by X-Ray and TEM evaluations. In the case of faceted
sapphire the two bcc crystals overgrow the facets homogeneously.
Here, the bcc (111) surface is nearly parallel to the sapphire
(10-11) facet and the Mo/Nb (100) surface is nearly
parallel to the sapphire (10-1-2) surface.
(211)-oriented Nb templates on sapphire m-plane can be used for
the non-twinned, (211)-oriented growth of RFe2 films by means
of MBE. Again, the quality of the RFe2 films grown on faceted
sapphire is almost equal to films on the non-faceted substrate.
For comparison thin RFe2 films of the established (110) and
(111) orientation were prepared. Magnetic and magnetoelastic
nmeasurements performed in a self designed setup reveal a high
quality of the samples. No difference between samples with
undulated and flat morphology can be observed.
In addition to the preparation of covering, undulating thin films
on faceted sapphire m-plane nanoscopic structures of Nb and Fe
were prepared by shallow incidence MBE. The formation of the
nanostructures can be explained by a shadowing of the atomic beam
due to the facets in addition to de-wetting effects of the metals
on the heated sapphire surface. Accordingly, the nanostructures
form at the facet ridges and overgrow them. The morphology of the
structures can be varied by deposition conditions as was shown for
Fe. The shape of the structures vary from pearl-necklet strung
spherical nanodots with a diameter of a few 10 nm to oval nanodots
of a few 100 nm length to continuous nanowires. Magnetization
measurements reveal uniaxial magnetic anisotropy with the easy
axis of magnetization parallel
to the facet ridges. The shape of
the hysteresis is depending on the morphology of the structures.
The magnetization reversal processes of the spherical and oval
nanodots were simulated by micromagnetic modelling and can be
explained by the formation of magnetic vortices.