V. Leiner and R. Siebrecht (ILL), D. Labergerie, Ch. Sutter, and H. Zabel (Univ. Bochum).
for the magnetic moments from one
layer to the next is observed, implying a shorter period for the spin helix.
Due to new advances in thin-film deposition techniques it is
possible today to grow ultra-thin epitaxial magnetic films with very high structural
quality. This allows to raise the question of how the bulk magnetic properties are
modified in thin films either due to dimensionality effects or due to proximity with
other material layers. Information on nanofabricated magnetic films is of high importance
for the design and functionality of magnetic nanostructures integrated in new electronic
devices. The most complete information on such ultra-thin structures comes from neutron
scattering. For revealing antiferromagnetic or incommensurate spin structures, neutron
scattering is unique. However, known as a bulk probe, its applicability to thin films
has to be proven. In this context, we have studied the magnetic phase-transition
in single ultra-thin Holmium films. The strong magnetic moment of Ho and its long-range
magnetic order yield intense magnetic peaks at small Q-values, thus, making Ho a
perfect model system for scattering experiments in the ultra-thin regime.
In the bulk, Holmium exhibits an incommensurate spin helix below the Néel
temperature, TN, of ~ 132 K. Between 20 K and 132 K the magnetic moments order ferromagnetically
in the hexagonal basal plane. From one plane to the next, the average orientation
of the magnetic moments encloses a turn angle , setting up a magnetic spiral along the direction
normal to the planes. The turn angle is about 30° at 20 K and increases continuously
to 50° with rising temperature up to the Néel temperature. Below 20 K
the magnetic moments lock into a commensurate magnetic cone structure with a turn
angle of 30° in the basal plane and a ferromagnetic component normal to the planes.
In Fig. 1, the phase diagram is schematically reproduced, which is the result of
magnetic neutron scattering [1-3] and resonant magnetic x-ray scattering experiments
at the Ho – LII and LIII absorption edge [4-6].
|
|
Figure 1: Schematic phase diagram of bulk Holmium. |
In magnetic scattering experiments the helical magnetic structure gives rise to magnetic
satellite reflections at distance ± 
* around the allowed Bragg peak along the c*
direction.*
in units of c* is related to the turn angle via = 180°. *.
Using resonant magnetic x-ray scattering at the Ho – LIII absorption edge, the minimum
thickness which can be studied is around 200 Å. Below this value the charge
background from the Laue-oscillations of the nearby Bragg-reflection becomes too
strong and buries the magnetic (00.2 ± *) reflections [7]. Using neutron scattering, we have now
succeeded to study the magnetic phase-transition in a single Ho(00.1) film which
has a thickness of only 46 Å. The epitaxial Ho film was grown on a sapphire
substrate with the use of a Nb and Y buffer layer. The Ho film was capped with an
Y and Nb layer to prevent it from oxidation. The embedding of Ho between Y layers
ensures symmetric structural and magnetic boundary conditions. The film thickness
was determined from x-ray reflectivity measurements; the out-of-plane roughness was
found to be ~ 10 Å. For the magnetic scattering experiments we use the ADAM
reflectometer at the ILL because of its high flux, low background and high Q-resolution.
These features are essential for studying weak satellite reflections close to the
(00.0) Bragg peak. Throughout the experiments we used a wavelength of 4.41 Å.
Figure 2 shows the magnetic (00.0+*) peak from the Ho film with 46 Å thickness. With
increasing temperature the peak shifts to higher Q values due to an increasing turn
angle. At the same time the intensity drops due to a decreasing order parameter.
For this thin film we measure a Néel temperature of only 105 K which is considerably
lower than in the bulk. The inset shows the order parameter as a function of temperature.
Below 20 K, we could not observe a transition to a commensurate magnetic structure.
The magnetic order in the ultra-thin Ho film remains incommensurate, in agreement
with results obtained previously from Ho/Y superlattices [8]. Looking at the length of the magnetic
helix at = 20 K, we find * = 0.251c*, i.e. an increase of the turn angle to 46° (as compared
to 30° found for bulk Ho).
We have demonstrated for the first time that it is possible to study the magnetism
in such ultra-thin films by investigating the magnetic ordering in Ho films as thin
as 46 Å using a high-resolution neutron reflectometer. The neutron results
indicate that even thinner samples can be investigated with reasonable counting statistics
and underline impressively the importance of neutron scattering for the investigation
of nanostructured magnetic systems.
|
|
| Figure 2: Radial neutron scans of the (00.0+*) magnetic satellite peak for the
46 Å thick single epitaxial Ho(00.1) film for different temperatures. The peak
shifts to higher scattering vectors due to an increasing turn angle of the Ho spin
helix with increasing temperature. Simultaneously the intensity decreases due to
a loss of long-range magnetic-order with increasing temperature. In the inset the
magnetic-order parameter is plotted as a function of temperature. |
[1] W.C. Koehler, et al., Rev. 158 (1967) 450 n [2] G.P. Felcher, et al., Spedding, Phys. Rev. B 13 (1976) 3034 . [3] M.J. Pechan, C. Stassis, J. Appl. Phys. 55 (1984) 1900. [4] D. Gibbs, et al., Phys. Rev. Lett. 55 (1985) 234. [5] J. Bohr, et al., Physica 140A (1986) 349. [6] D. Gibbs, et al., Phys. Rev. B 43 (1991) 5 663. [7] C. Sutter, E. Weschke, R. Meier, C. Schûssler-Langeheine, G. Grübel, D. Abernathy, to be published. [8] D.A. Jehan, et al., Phys. Rev. B 48 (1993) 5594.
