Tuning ferromagnetic properties of LaMnO3 films by oxygen vacancies and strain

We systematically investigated the ferromagnetic properties of LaMnO3 (LMO) films grown on SrTiO3 (STO) substrates with varying deposition oxygen pressure P-o2 (0.2 <= P-o2 <= 200 mTorr). It was found that the films only deposited with P-o2 <= 20 mTorr showed exchange bias behavior and meantime the coercive field H-c of LMO films deposited below 20 mTorr is about one order of magnitude larger than that of films deposited at 200 mTorr. When the LMO films were deposited at 20 mTorr on different substrates [LaAlO3 (LAO), (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) (LAST), SrTiO3 (STO) and KTaO3], LMO/STO sample showed the maximum H-c, even though all of them exhibit exchange bias behavior. Thickness dependence of magnetic properties in LMO/STO films deposited at 20 mTorr indicated a maximum coercive field and the existence of exchange bias for 20 nm-thick LMO sample. Based on our results, the enhancement of coercive field and exchange bias was attributed to the competition between different magnetic phases induced by oxygen vacancies and Jahn-Teller effect from strain-induced orbital ordering.