Here we demonstrate the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P63/mmc structure upon the isovalent larger-for-smaller A-site cation substitution in the B-site ordered double-perovskite system (Sr1-xBax)2FeSbO6. This is the same transformation sequence previously observed up to Fm-3m upon heating the parent Sr2FeSbO6 phase to high temperatures. High-pressure treatment, on the other hand, transforms the hexagonal P63/mmc structure of the other end member Ba2FeSbO6 back to the cubic Fm-3m structure. Hence we may conclude that chemical pressure, physical pressure and decreasing temperature all work towards the same direction in the (Sr1-xBax)2FeSbO6 system. Also shown is that with increasing Ba-for-Sr substitution level, i.e. with decreasing chemical pressure effect, the degree-of-order among the B-site cations, Fe and Sb, decreases.
We have fabricated a high quality magnetic Ni0.5Zn0.5Fe2O4 ferrite powder/polymer composite sheet consisting of common and environmentally friendly elements only. The sheet was then tested for its dynamic permeability by irradiating with electromagnetic waves with frequencies up to 50 GHz. Two different originally developed methods were used for the high-frequency permeability measurements, a short-circuited microstrip line method and a microstrip line-probe method. It is challenging to measure the dynamic permeability of magnetic thin films/sheets beyond 10 GHz because of the low response signal from these materials. However, the two methods produced essentially equivalent results. In the frequency dependent permeability profile, the maximum position of the profile, $$\mu ^{\prime \prime }_{max}$$
μ
max
″
, shifted towards higher frequencies upon increasing an applied (strong) static external magnetic field, $$H_{dc}$$
H
dc
. A linear relationship between $$\mu^ {\prime \prime }_{max}$$
μ
max
″
and $$H_{dc}$$
H
dc
for the entire range of $$H_{dc}$$
H
dc
was observed even at small $$H_{dc}$$
H
dc
. In general, the spinel-structured Ni-based ferrites exhibit low magnetic anisotropy, but the present sample showed a uniaxial-anisotropic behavior in the parallel direction of the sheet. Our Ni0.5Zn0.5Fe2O4 powder/polymer composite sheet thus exhibits high performance at GHz frequencies, and should be applicable e.g. as an anisotropic electromagnetic wave-interference material.
Several sample series of the (Sr0.5Ba0.5)2FeSbO6 double-perovskite compound are synthesized under various heat-treatment conditions starting from a sol-gel prepared precursor to investigate the kinetics and thermodynamics of the B-site cation ordering. The synthesis yields samples with different levels of Fe/Sb ordering ranging from essentially disordered up to fully ordered depending on the synthesis temperature and duration. In the temperature range 1050-1200 o C the degree of order is kinetically controlled following Johnson-Mehl-Avrami-Kolmogorov type kinetics, while at 1300 o C the thermodynamic equilibrium is quickly reached such that higher than 95% ordering degrees are obtained in heat-treatment periods as short as couple of hours. Also importantly, for the differently synthesized single-phase samples a linear relationship between the degree of order and the cubic subcell dimension is seen.
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