Influence of finite size and image charge screening on the antiferromagnetic ordering of CoO ultrathin films

• Published in: Journal of magnetism and magnetic materials Vol. 515; p. 167292
• Main Authors: Barman, Sukanta, Kundu, Asish K., Menon, Krishnakumar S.R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.12.2020; Elsevier BV
• Subjects: Antiferromagnetism; Coherent scattering; Critical exponent; Critical temperature; Crossovers; Electron beams; Electronic structure; Energy gap; Exchange scattering; Half-order; Image charge; Low energy electron diffraction; Oxide thin films; Periodic variations; Photoelectric emission; Substrates; Surface antiferromagnetism; Thin films; Unit cell; Valence band; Oxide thin films; Surface antiferromagnetism; Half-order; Critical exponent; Exchange scattering
• ISSN: 0304-8853; 1873-4766
• DOI: 10.1016/j.jmmm.2020.167292

•Coherent exchange scattering of low energy electrons from the CoO(001) surface.•Half-order spots in LEED reflect the antiferromagnetic ordering of the CoO film.•Evolution of surface Néel temperature with CoO film coverage.•Significant enhancement of surface Néel temperature beyond its bulk value.•Dimensional crossovers in the magnetic critical behaviour of CoO film. The surface antiferromagnetic (AFM) ordering of ultrathin CoO(001) films grown on Ag(001) has been investigated using low energy electron diffraction (LEED) and valence band photoemission spectroscopy. We have observed coherent exchange scattered half-order spots in LEED with a periodicity of the magnetic unit cell of the CoO(001) surface. These spots show very low intensities (2–3% of the integer-order spots) and are visible only below a critical temperature for low electron beam energies (<70 eV), revealing its AFM origin. Significant enhancement of surface Nèel temperature (TN) of CoO(001) for different film coverages has been observed and it is exceeding the bulk TN even in the ultrathin limit. The enhancement of surface TN has been explained by the combined effects of finite size, surface strain, and polarizability of the underlying substrate. The critical behaviour near TN shows a coverage dependent evolution as the magnetic critical exponent (β) varies from 0.16 to 0.48 upon increasing film coverage from 0.5 ML to 10 ML, suggesting crossovers in the critical behaviour. Furthermore, upon transition from paramagnetic (PM) to AFM phase, minor modifications of the valence band electronic structure have been observed such as enhancement of spectral weight in the Co 3d band as well as widening of an energy gap relative to EF by ~150 meV, in agreement with the theoretical predictions.