XRD study of strongly oriented thin films of hexagonal ferrites
with potential magnetoelectric effect
J. Buršík1, M. Soroka1, R. Kužel2, J.
Prokleška2
1Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež, Czech Republic
2Faculty of Mathematics
and Physics, Charles University, Prague, Czech Republic,
* kuzel@karlov.mff.cuni.cz
X-ray diffraction and reflectivity techniques for
analysis of thin polycrystalline thin films have been well-developed and they
are routinely used when often low-angle incidence setup is applied instead or
in addition to symmetrical q-q
scan. However, for strongly oriented films more different asymmetric scans
and/or reciprocal space mapping are required. During study of nanocrystalline ZnO films [1] we have applied a combination of different
scans to characterize both the so-called out-of-plane orientation and in-plane
orientation. Strong out-of-plane orientations were found, and only basal (00l)
reflections were available in symmetric scans. Therefore, the lattice
parameters, profile analysis (crystallite size and strains) and residual
stresses were studied by combination of several asymmetric reflections scanned
at specific suitable angles of inclinations j and y
(i.e. on the axis perpendicular to the film surface and axis perpendicular to
the goniometer axis, respectively). For comparison of measured lattice
parameters there was often a problem of either no data in the PDF-4+ database
for specific phase or multiple but different data.
The methods were also used in later extensive study of
M, Y, Z and W hexagonal ferrites with a potential of magnetoelectric effect.
The films were prepared by chemical solution deposition (CSD) method and metalorganic precursor solutions prepared using the modified Pechini method. Number of processing parameters were
tested and optimized with the aim to minimize the amount of impurities that
could spoil the magnetic properties of final material. For preparation of
highly oriented ferrite films, several substrates were used, and different
substrate/seeding layer/ferrite layer architectures were proposed. From seven M
phases with different chemical composition, magnetic character and lattice
misfit values investigated in their use as template and buffer layers for Y
ferrite growth, the best results were achieved when the misfit values between
seed layer and substrate, and between seed layer and top Y-layer are
approximately equal and when the surface of seed layers are formed by hexagons
for which the surface area formed by top surface of hexagons is much larger
than surface area formed by side walls of hexagons [2, 3].
New Y-ferrite phases were prepared with the composition BaSrZnCoFe11(Me)O22 (Me = Al, Ga, In, Sc). and it was found that for Me = Al, Ga the magnetic structure is of non-colinear ferrimagnetic type with unspecified helical magnetic structure. For Me = Ga this is a new system with potential ME effect. Moreover, these films could be prepared as well-oriented both out-of-plane and in-plane on STO - SrTiO3(111) substrates directly without any seeding layers.
ME Z-type ferrite Sr3Co2Fe24O41
and BaxSr3-xCo2Fe24O41
thin films were prepared and characterized for the first time [4]. According to
the XRD texture analysis the orientation relationship between Z ferrite and substrate
can be expressed as (00l)Z || (111)STO || and 
Z || 
STO. However, in these films the analysis was complicated by the presence
of M and S (spinel) phases that were also oriented (aligned with the substrate)
and therefore also many asymmetric reflections were overlapped and many of them
were weak. Therefore, a careful selection of reflections suitable for the analysis
had to be made.
Composition series of W-type SrCo2-xZnxFe16O27 hexaferrite thin films and powders were also successfully synthetized by CSD method. For successful growth of W hexaferrite films on SrTiO3(111) substrate, annealing temperature and time falls into a very narrow interval of 1225 to 1250°C and 60 to 120 minutes, respectively, depending on the amount of substituting Zn2+ ion. The strong anisotropy of magnetic properties was confirmed. Currently, oriented Y-Z composites are studied that is another challenge for the analysis.