Atomically sharp oxide heterostructures exhibit a range of novel physical phenomena that do not occur in the parent bulk compounds including conducting, superconducting, and magnetic states. We present a new emergent phenomenon at the LaMnO3/SrTiO3 interface in which an antiferromagnetic insulator abruptly transforms into a superparamagnetic state. Above a critical thickness of LaMnO3 of five unit cells, our scanning nanoSQUID-on-tip microscopy [1] shows spontaneous formation of isolated magnetic islands with in-plane moment of 10^4 to 10^5 μ_B with characteristic diameter of 10 to 50 nm [2]. The nanoscale islands display superparamagnetic dynamics of random moment reversals by thermal activation or in response to an in-plane magnetic field. We propose a charge reconstruction model of the polar LaMnO3/SrTiO3 heterostructure which describes a sharp emergence of thermodynamic phase separation leading to nucleation of metallic ferromagnetic islands in an insulating antiferromagnetic matrix. The model suggests that a gate tunable superparamagnetic-ferromagnetic transition can be induced, holding potential for applications in magnetic storage and spintronics.
[1] D. Vasyukov et al., Nature Nanotechnology 8, 639 (2013).
[2] Y. Anahory, L. Embon, C. J. Li, S. Banerjee, A. Meltzer, H. R. Naren, A. Yakovenko, J. Cuppens, Y. Myasoedov, M. L. Rappaport, M. E. Huber, K. Michaeli, T. Venkatesan, Ariando, and E. Zeldov,
Nat. Commun. 7, 12566 (2016).
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