FE-01: Generation of Highly Anisotropic Physical Properties in Ferromagnetic Thin Films Controlled by their Differently Oriented Nano-sheets

The generation and control of magnetic anisotropy in thin films at the nano-scale is a key question for addressing applications in many different fields, including sensors, actuators, permanent magnets, magneto-electronics, information processing or storage devices. For the first time we successfully fabricated and studied pulsed laser deposited ferromagnetic nano-crystalline thin films constituted by nano-sheets with a controlled slant. Films of Fe, Co, Co–Fe and Co-rich Co–MT (MT= V, Cr, Cu, Zn, Cd, Hf) were grown. The visualization of these nano-sheets by Scanning Tunneling Microscopy (STM) and High-Resolution Transmission Electron Microscopy (HRTEM) showed typically tilt angles ≈ 56 o with respect to the substrate plane, and nano-sheets ≈ 3.0–4.0 nm thick, ≈ 30–100 nm wide, and ≈ 200–300 nm long, with an inter-sheet distance of ≈ 0.9–1.2 nm, depending on their constitutive elements. Induced by this special nano-morphology, all these films exhibited a large uniaxial magnetic anisotropy in the plane of the films, the easy direction of magnetization being parallel to the longitudinal direction of the nano-sheets and the hard direction being perpendicular to the nano-sheets. In the as-deposited films, typical values of the anisotropy field were measured between Hk = 600 and 1400 Oe (≈ 48 and 112 kA/m) depending on composition. The deduced effective shape anisotropy constant associated to the nano-sheets was, in some cases, similar or even higher than the magnetocrystalline anisotropy constant. The magnetic domains configuration of these thin films was also studied through the observation of charged domain walls. In addition to this magnetic anisotropy, we also found large uniaxial anisotropy in other physical properties of the films, such as transport, light reflection, mechanical and magneto-mechanical properties. The discovery of the nano-sheets allowed us to elucidate the origin of all these anisotropies. The changes in the nano-morphology of these films caused by thermal treatments, and hence in their anisotropic properties, were also studied. HRTEM, including chemical analysis at the nano-scale, allowed seeing these morphological and chemical changes. The nature and the activation energy spectra of the corresponding atomic relaxation processes developed were stablished when the dependence of the anisotropic resistance on temperature was simultaneously monitored. Remarkably interesting, some films retained their nano-sheet morphology, at least up to 500 oC, and retained and even increased their anisotropies by up to three times, with anisotropy fields Hk≈ 3000-3500 Oe (≈240-280 kA/m). For example, the thermal treatments produced crystallization processes and the growth of CoV magnetic phases maintaining the nano-sheets morphology. Similar and useful behavior has recently been found in some Co-Fe films with these nano-sheets. In contrast, other annealed films, CoZn, CoCu, CoCd… lost their nano-sheet-morphology. This work opens a new path of study for these new magnetically anisotropic materials, particularly with respect to the nano-morphological and structural changes related to the increase in magnetic anisotropy. A method for generating anisotropies by controlling the nano-sheet morphology of films, including oriented crystallization processes at the nano-scale, has been established in this work.References: [1] V. Madurga, J. Vergara, C. Favieres. J. Magn. Magn. Mater. 272–276, 1681 (2004) [2] V. Madurga, C. Favieres, J. Vergara. Nanotechnology 21, 095702 (2010) [3] V. Madurga, J. Vergara, C. Favieres. Nanoscale Research Letters 6, 325 (2011) [4] J. Vergara, C. Favieres, V. Madurga. J. Phys. D: Appl. Phys. 48, 435003 (2015) [5] C. Favieres, J. Vergara, C. Magén, M. R. Ibarra, V. Madurga. J. Alloy. Compd. 664, 695 (2016) [6] J. Vergara, C. Favieres, C. Magén, J.M. De Teresa, M. R. Ibarra, V. Madurga. Materials 10, 1390 (2017) [7] C. Favieres, J. Vergara, C. Magén, M. R. Ibarra, V. Madurga. J. Alloy. Compd. 911, 164950 (2022) [8] C. Favieres, J. Vergara, V. Madurga. J. Appl. Phys. 133, 124301 (2023)