Journal of Materials Research 2022 www.mrs.org/jmr Vol.:(0123456789) DOI:10.1557/s43578-022-00822-9 Effect of annealing in the formation of well‑crystallized and textured SrFe 12 O 19 films grown by RF magnetron sputtering G. D. Soria 1,4 , A. Serrano 2 , J. E. Prieto 1 , A. Quesada 2 , G. Gorni 3 , J. de la Figuera 1 , J. F. Marco 1,a) 1 Instituto de Química Física “Rocasolano”, CSIC, 28006 Madrid, Spain 2 Instituto de Cerámica y Vidrio, CSIC, 28049 Cantoblanco, Madrid, Spain 3 Alba Synchrotron Light Facility, CELLS, 08290 Barcelona, Spain 4 Present Address: Laboratorio Nacional de Fusión, CIEMAT, 28040 Madrid, Spain a) Address all correspondence to this author. e-mail: jfmarco@iqfr.csic.es Received: 2 August 2022; accepted: 7 November 2022 We have studied the influence of the annealing treatment on the crystalline growth of SrFe 12 O 19 previously deposited on Si (100) substrates using radio frequency (RF) magnetron sputtering. For this goal, two grown films, with and without ex situ heating step, have been analyzed and compared to determine the differences in their structural, compositional, and magnetic properties. The results obtained by the different analysis techniques, in particular Mössbauer spectroscopy together with EXAFS and XANES data, suggest that the as‑grown film is composed of nanocrystalline maghemite nanoparticles and amorphous strontium oxide. Specifically, Mössbauer spectroscopy results pointed out the presence of Fe 3+ cations occupying octahedral and tetrahedral sites with hyperfine magnetic fields 49.3 T and 44.2 T, respectively, characteristic of a spinel‑related structure. A strontium hexaferrite canonical structure with a c‑axis orientation in the sample plane was found for the annealed film. Introduction M-type ferrites are among the most popular permanent mag- netic materials. is is mainly due to their high magnetocrystal- line anisotropy which leads to elevated coercive fields. In addi- tion, these hard ferrites exhibit excellent thermal and chemical stability, high Curie temperatures, and good wear resistance, as well as being environmentally friendly and competitively priced products [1, 2]. ese materials are widely applied for different uses, such as motors, magneto-mechanical devices and actua- tors, among others [3–5]. e M-hexaferrite family is composed by strontium hexaferrite SrFe 12 O 19 (SFO), barium hexaferrite BaFe 12 O 19 , and lead hexaferrite PbFe 12 O 19 [6, 7]. Specifically, the hard magnetic material selected in this study has been SFO. e SFO crystal structure (Fig. 1) consists of alternately arranged blocks of spinel and rock salt units. e structure shows a hex- agonal closed-packing of oxygen ions, where 2 of the 40 sites are occupied by divalent strontium ions, while the trivalent iron ions are located in interstitial sites [2, 6, 8, 9]. Such configuration confers to this material a markedly ferrimagnetic character where iron can be found in five distinct cationic environments: three octahedral sites (12 k, 4f2, 2a), one tetrahedral site (4f1), and one bipyramidal site (2b). In SFO, the easy magnetization axis is the c crystallographic axis. Such characteristic of SFO is very promising for both longi- tudinal and perpendicular recording media applications where magnetic thin films are oſten required. Strontium hexaferrite films have been grown by pulsed laser deposition [11], chemical solution deposition [12], metal–organic chemical vapor deposi- tion [13], spin coating sol–gel process [14], and RF magnetron sputtering [15]. For the latter deposition method, it has been reported that the growth of the SFO crystalline phase needs two steps: deposition and post-annealing treatment at a high temperature (800–900 °C) [15–17]. Multiple previous works have established the requirement for the annealing step either ex situ or in situ the deposition chamber [18–20]. Such stud- ies pointed out by X-ray diffraction (XRD) analysis that the  FOCUS ISSUE © The Author(s) 2022 Article MÖSSBAUER SPECTROSCOPY FROM ARTIFICIAL NANO ARCHITECTURES TO ENVIRONMENTAL APPLICATIONS