Etching of Cr tips for scanning tunneling microscopy of cleavable oxides

We report a detailed three-step roadmap for the fabrication and characterization of bulk Cr tips for spin-polarized scanning tunneling microscopy. Our strategy uniquely circumvents the need for ultra-high vacuum preparation of clean surfaces or films. First, we demonstrate the role of $ex$-$situ$ electrochemical etch parameters on Cr tip apex geometry, using scanning electron micrographs of over 70 etched tips. Second, we describe the suitability of the $in$-$situ$ cleaved surface of the layered antiferromagnet La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$ to evaluate the spin characteristics of the Cr tip, replacing the UHV-prepared test samples that have been used in prior studies. Third, we outline a statistical algorithm that can effectively delineate closely-spaced or irregular cleaved step edges, to maximize the accuracy of step height and spin-polarization measurements.