3.1. Structure and Composition
Figure 1 shows a backscattered electron SEM picture of two exemplary crystals of up to about 800 µm length. The powder diffraction pattern of the product (black, up), and a simulated diffractogram of unsubstituted strontium hexaferrite SrFe
12O
19 (SrM, red, down [
14]) as references, are shown in
Figure 2. A comparison of both patterns proves that the described synthesis yields single phase material. It can also be seen that minor Al substitution hardly affects reflex positions and intensities compared to unsubstituted SrM.
Both the powder pattern and the single crystal data refinement show that an aluminum containing product, like unsubstituted SrM, crystallizes in the magnetoplumbite structure [
4,
14], space group
P6
3/
mmc with
a ≈ 587 pm and
c ≈ 2300 pm. The exact lengths of the unit cell axes vary depending on Al content and annealing time. Crystallographic data, refinement parameters and measurement conditions for three investigated single crystals with different thermal history are summarized in
Table 1. Crystal 1 was investigated directly after the synthesis. Crystals 2 and 3 were annealed for 20 h at 900 °C in platinum or corundum crucibles, respectively.
The crystal structure of the magnetoplumbite SrFe
12O
19 is built from close packed oxygen or oxygen-strontium layers which are stacked in both cubic (S-block) and hexagonal sequence (R-block) and can be described as [BAB’ABCAC’AC]
n. One fourth of oxygen anions are replaced by strontium cations in B’ and C.’ Iron occupies five crystallographically unique sites in the lattice, with three of them being octahedrally, and the remaining two being tetrahedrally and trigonal-bipyramidally coordinated, respectively [
4,
14]. The extended unit cell of the magnetoplumbite structure is depicted in
Figure 3 (left), as well as a section of the structure with labelled mixed sites and coordination polyhedra around the cations, where
M = Fe, Al (right).
The literature states different favored cation sites for Al in the magnetoplumbite structure. Both Albanese and Choi et al. investigated BaM via
Mößbauer spectroscopy in 1995 and 2004 respectively. Examining the quadrupole splitting of Al substituted barium hexaferrite they found that Al preferably occupies
M(5) as well as
M(1) and
M(3) to a smaller degree [
25,
26]. In an earlier work, Albanese et al. described that occupation of
M(2) by Al in SrM is even less likely [
27]. Contrary to this, Awawdeh et al. stated in 2014 that
M(4) is preferred, using the same method [
28]. Vinnik et al. investigated Al substituted BaM using scXRD. According to the refinement, Al prefers
M(1) and then
M(5) with smaller amounts on
M(2) and
M(4). The tetrahedrally coordinated
M(3) is not occupied at small Al contents around
x = 1 [
16]. In our scXRD refinements on crystals obtained after the initial synthesis, we could confirm the general results of Vinnik.
M(3) has little to no Al but we found that the majority of Al is located on
M(5). To investigate these reported differences in the occupation, annealing experiments of single crystals were performed. During the investigation via WDX and refinement of scXRD data, it became evident that the Al distribution on the trivalent metal cation sites and the homogeneity of the received crystals are dependent on the synthesis parameters like annealing time. The asymmetric unit for every unique ion site in the structure as well as the distribution of Al on the iron sites and the isotropic displacement factors
Uiso for a crystal obtained in the initial synthesis are given in
Table 2. The same is shown in
Table 3 for a crystal annealed in a Pt crucible and in
Table 4, for a crystal annealed in a corundum crucible.
After annealing in a platinum or a corundum crucible, redistribution was indeed observed in occupation of the mixed sites. As can be taken from
Table 2,
Table 3 and
Table 4, the coordinates of the ions did not change significantly and the
Uiso are similar before and after the annealing experiments. The aluminum distribution however did change. Upon annealing, its content is doubled on the octahedrally coordinated site
M(1) whereas it is significantly reduced on
M(3), while the refined substitution levels are similar. The contents on
M(2) and
M(4) in the R-block are affected to only a small degree. Noteworthy, upon annealing, the unit cell parameters shrinks by about 0.2% leading to a density increase of 0.5% due to atomic redistribution.
According to the refinement, annealing in a corundum crucible slightly increases the Al content in the structure. WDX experiments yielded additional information about the homogeneity and Al content of the material. Crystals with edge lengths up to 1 mm, taken from the initial synthesis, after annealing in a platinum crucible or in a corundum crucible, were examined at six to ten locations on the crystal surface. The average Al contents in at.-% for the three samples are shown in
Table 5. The errors represent the highest discrepancy of a single value to the average. The high errors of the former two samples indicate that, although the Al distribution on the different metal ion sites changes, annealing alone will not suffice to homogenize the material. However, it is possible that additional Al ions can diffuse into the hexaferrite when heated while in contact with the corundum crucible and a homogeneous distribution can be achieved in this way. This can be derived from the lower error of the average content after annealing in the corundum crucible. It can also be seen that the relative content of Al increases in comparison to that obtained directly after the initial synthesis and annealing, which both took place in a platinum crucible. This strengthens the presumption that Al diffuses into the crystal from the crucible material. This is very probable as corundum and hematite crystallize isotypic [
29].
3.2. Magnetism
Magnetic measurements were performed on both powdered samples and larger crystals. The magnetic field was applied along the
c axis for the latter. In
Figure 4 and
Figure 5, the magnetic moment is plotted against the magnetic field strength to investigate the magnetic hysteresis. The former figure shows a curve for a powder (red) and a crystal (black). It can be seen that the hysteresis of the crystal is much steeper than the hysteresis of the powder, which is desired in a hard magnetic material but widens considerably in a powdered sample.
The magnetic hysteresis loops for three crystals after different degrees of annealing, as described in
Section 2, are shown in
Figure 5. The magnetic data derived from the curves, the saturation magnetization
Ms, remanence
Mr and coercivity
Hc, are summarized in
Table 6. It is obvious that the hysteresis shape and
Ms do not vary much, whereas
Mr and
Hc decrease significantly after annealing.
According to the literature, both the saturation magnetization and remanence decrease significantly and the coercivity increases with higher substitution rates [
26,
28,
30,
31]. El-Sayed et al. report a drop of
Ms from 61.2 emu/g to 49.7 emu/g,
Mr from 31.5 emu/g to 25.8 emu/g and an increase in
Hc from 1.66 kOe to 1.77 kOe [
30]. However, those values highly depend on the synthesis temperature and particle size [
30,
31]. The relatively low values presented here are to be expected as plates in millimeter scale were investigated. The critical domain size is exceeded, which results in lower overall values [
31]. A drop of
Mr was indeed observed after annealing concomitant to a decreasing
HC (see
Table 6).