Transport and Magnetism in Thin Films of the Superconducting Candidate La₂NiO₃F

Since the discovery of superconductivity in infinite-layer nickelates, there have been continuous efforts to identify related materials with improved properties. Theoretical and experimental studies suggest that increasing the dimensional confinement across nickel oxide layers will lead to more ‘cuprate-like’ physics and higher superconducting critical temperatures. Here, we target thin films of the <i>n</i> = 1 Ruddlesden-Popper phase, La₂NiO₄, which boasts single layers of electronically isolated nickel oxide. In the parent structure, nickel adopts a 2+ oxidation state with ground state antiferromagnetic ordering. Notably, the ground state magnetic structure of the high-<i>T</i>_C cuprates is proximate to an antiferromagnetically ordered phase. We therefore use two complementary methods to tune the nickel valence and ground-state magnetic properties. First, we use molecular-beam epitaxy (MBE) to synthesize thin films of the general formula La_2−<i>x</i><i>A_x</i>NiO₄ (<i>A</i> = Sr, Ba) where nickel adopts an oxidation state of 2+<i>x</i>. After MBE synthesis, we leverage topochemical transformations including fluorine substitution and reductive de-intercalation [1,2] to synthesize thin films of the general formula La_2−<i>x</i><i>A_x</i>NiO₃F_2−<i>y</i> where nickel adopts an oxidation state of 2+<i>x</i>−<i>y</i>. Through these two methods, we can both electron- and hole-dope the nickel oxide layer, systemically probing the effect of valence on magnetic and electronic properties.<br/><br/>1.<i> Inorg. Chem.</i> <b>2018</b>, <i>57</i>, 6549−6560<br/>2.<i> Chem. Mater.</i> <b>2020</b>, <i>32</i>, 3160−3179