Antiferromagnet spintronics is an emerging frontier for magnetic memories, envisaging a new generation of devices with augmented packing density, due to the absence of stray fields with respect to their ferromagnetic counterparts, and improved robustness versus external magnetic fields . Chromium has recently emerged as a promising candidate because of its Néel temperature above room temperature and the possibility of growing fully epitaxial heterostructures, but the antiferromagnetic functionality at the thin films level of both metal and oxides has not been demonstrated up to now.
In this contribution, we report on the growth of ultrathin Cr films (about 2 nm thick) on a BaTiO3/Nb SrTiO3(001) template, with the idea, mimicking the experience on the Fe/BaTiO3 bilayer by the authors , to tune the Cr magnetic properties by an electric field, thus envisaging the electrical control of the spin configuration. Cr is grown by Molecular Beam Epitaxy at room temperature on a 40 nm thick BaTiO3 (BTO) film with ferroelectric character, and followed by post annealing in ultra-high vacuum conditions.
While Cr is metallic for a single post annealing up to 800 K, larger annealing temperatures and/or times lead to the coexistence of Cr and Cr oxide (CrOx) phases, and then to a full Cr oxidation.
Both the phases are crystalline with a cubic structure, with Cr//BTO and CrOx//BTO, as shown by XPD and LEED. Note that this is true even when the two phases coexist: by deconvoluting the Cr2p peak measured by XPS into the metallic and the oxidized components, the angular dependence of the two shows a diffraction pattern compatible with a cubic structure in both the cases, but with different crystal orientations (Cr// CrOx//BTO).
In-situ XPS and ex-situ XAS point towards a Cr3+ oxidation state of the CrOx phase, as in Cr2O3 (the most stable chromium oxide, with antiferromagnetic character). However, as discussed above, XPD and LEED suggest a cubic structure of CrOx, that is different from its known phases, corundum α-Cr2O3 and defective spinel γ-Cr2O3, and suggest a defective rocksalt structure.
Further investigation are in progress to demonstrate the antiferromagnetic character of both Cr and CrOx films capped by a Pt overlayer, by measuring the anisotropic magneto-resistance and the magnetic-field-invariant-magnetization , in order to discern on their potential application to antiferromagnet spintronics.
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