Magnetic defects in KTaO₃ and KTaO₃:Fe nanopowders

Abstract

Magnetic defects in non‐doped and iron‐doped nanopowders of KTaO3 were studied by resonance and static magnetic measurements. The resonance spectra and hysteresis loops of KTaO3:Fe nanocrystalline powders were observed and investigated for the first time. Unlike bulk crystals, both non‐doped and doped nanopowders exhibit two types of resonance lines, paramagnetic ones from the isolated Fe3+ ions and a ferromagnetic signal from ions involved in clusters. Theoretical description and full identification of the electron paramagnetic resonance (EPR) spectra were made. It was established that low doping (up to 0.4 mol%) increases the number of paramagnetic Fe3+ centers with axial and rhombic symmetries, and has no effect on the centers with cubic symmetry and magnetic clusters. Analysis of the temperature dependence of the spectra showed that the cluster's resonance line follows the Bloch law “T3/2”, which indicates a ferromagnetic nature of the cluster. A partial collapse of the ferromagnetic subsystem in the temperature range of 100–400 K was also found. Magnetization and other characteristics of ferromagnetic and paramagnetic subsystems were determined. An average cluster size was estimated to be equal to a length of 6.2 nm and the fractional volume of the ferromagnetic cluster system was found.

Semiconductor Physics, Quantum Electronics & Optoelectronics. 2016. V. 19, N 4. P. 343-351. DOI: https://doi.org/10.15407/spqeo19.04.343 SrTiO3:Eu3+ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study

“…The former include, for example Fe 3+ -ions, randomly included in the material during the synthesis and forming the paramagnetic centers, in the structure of which the oxygen vacancy V(O) is present. This signal from paramagnetic centers Fe 3+ -V(O) was observed in KTaO 3 [34,35] and SrTiO 3 [36]. Defects of internal origin in the investigated powders are, for example, centers of О − and О 2− .…”

Percolation Magnetism in Ferroelectric Nanoparticles

“…With some probability, one oxygen vacancy V(O) or two vacancies 2 V(O) may occur. The probability is 50% in our case that corresponds to the experimentally determined ratio of the axial (Fe-V(O)) and rhombic (Fe-2 V(O)) centers in KTaO 3 and KNbO 3 [9, 10, 15]. The defects were modeled by spheres in our calculations, at that the radiuses of iron ion and oxygen vacancy are r (Fe 3+ ) = 0.064 nm and r (V(O)) = 0.132 nm, respectively.…”

“…Among ferroelectrics, recently, in nanocrystals (average particle size is 80 nm) of potassium tantalate and potassium niobate, produced by the novel technology of the metal (Ta or Nb) oxygenation in molten potassium nitrate [ 14 ], magnetic resonance and static magnetization methods established experimentally the appearance of ferromagnetic subsystem along with a paramagnetic subsystem [ 10 , 15 , 16 ]. This effect is absent in larger crystals (with sizes >200 nm) of the compounds obtained by the same technology.…”

“…The compounds are nonmagnetic in the bulk. The intentional doping of potassium tantalate (KNbO 3 ) and potassium niobate (KTaO 3 ) nanocrystals by iron and manganese separately resulted in the increase of the paramagnetic component, while the ferromagnetic subsystem remains unchanged [ 15 , 16 ]. It has been suggested that the reason for the appearance of the magnetic properties of pristine ferroelectric nanocrystals are magnetic defects, which can be both iron impurity atoms, forming metallic clusters on the surface of the nanoparticles, and oxygen vacancies [ 9 ].…”

“…It is also known that the oxygen vacancies always exist in oxide ferroelectrics, such as the perovskite-type (general formula ABO 3 ), resulting in some degree of non-stoichiometry in these compounds. It has been shown experimentally that two magnetic subsystems (paramagnetic and ferromagnetic) are present in nanocrystals KTaO 3 and KNbO 3 [ 9 , 10 , 15 ]. Paramagnetic subsystem consists of separate noninteracting magnetic Fe 3+ ions in KTaO 3 , and Fe 3+ and Mn 2+ ions in KNbO 3 .…”