Superparamagnetism at oxide interfaces revealed by scanning SQUID-on-tip microscopy

Our novel scanning SQUID-on-tip technique[1] is used to study nanoscale magnetism present in systems such as atomically sharp oxide heterostructures. Here we report a new emergent phenomenon at the LaMnO$_{3}$/SrTiO$_{3}$ interface in which an antiferromagnetic insulator abruptly transforms into a magnetic state that exhibits unexpected nanoscale superparamagnetic dynamics. Upon increasing the thickness of LaMnO$_{3}$ above five unit cells, our scanning nanoSQUID-on-tip microscopy shows spontaneous formation of isolated magnetic islands of 10 to 50 nm diameter, which display random moment reversals by thermal activation or in response to an in-plane magnetic field[2]. Our charge reconstruction model of the polar LaMnO$_{3}$/SrTiO$_{3}$ heterostructure describes the sharp emergence of thermodynamic phase separation leading to nucleation of metallic ferromagnetic islands in an insulating antiferromagnetic matrix. The model further suggests that the nearby superparamagnetic-ferromagnetic transition can be gate tuned, holding potential for applications in magnetic storage and spintronics. [1] D. Vasyukov \textit{et al}, Nature Nanotech. 8, 639 (2013) [2] Y. Anahory \textit{et al}, arXiv:1509.01895