Tuning ferromagnetic properties of LaMnO3 thin films by oxy-gen vacancies and strain

The compound of LaMnO $_{3}($ LMO) is an archetypal manganite for investigating the interface coupling when adjoining with other complex oxides [1]–[3]. Although showing an A-type antiferromagnetic (AFM) insulating ground state in bulk [4], LMO thin films in heterostructures tend to exhibit ferromagnetic (FM) behavior [1]–[3]. This unexpected FM behavior has gained renewed interests recently [5]–[8]. In this work, we fabricate a series of LMO thin films on SrTiO $_{3}($ STO) substrates with different oxygen pressures $( \mathrm {P}_{O2})$ from 0.2 to 200 mTorr and systematically studied their magnetic properties. LMO/STO films deposited at low $\mathrm {P}_{O2}(20$ mTorr or below) show obvious enhancement of coercive field (HC) that can reach up to 30 times that of LMO film deposited at 200 mTorr (Fig. 1(a)). On the other hand, exchange bias effect is observed in LMO/STO samples with $\mathrm {P}_{O2} \quad \le 20$ mTorr (Fig. 1(b)). $H_{C}$ and $H_{EB}$ decrease with rising measurement temperature, and $H_{EB}$ vanishes at the blocking temperature $( T_{B}$, about 60 K). Fig. 1(c) shows dependence of $H_{C}$ and $H_{EB}$ on $\mathrm {P}_{O2}$. Obviously, HC and HEB show similar variation with PO2, such that HC and HEB firstly increase with decreasing PO2 and then remain constant. On the other hand, we deposited the LMO films on different substrates [LaAlO $_{3}($ LAO), (LaAlO $_{3})_{0.3}($ Sr
2
AlTaO $_{6})_{0.7}$, STO and KTaO $_{3}]$ at 20 mTorr, to investigate the strain effect in FM properties of LMO film grown on low oxygen pressure. X-ray diffraction measurements indicate LMO films shows epitaxial growth but with strongly tensile, weakly tensile, more weakly tensile, and strongly compressive strains, respectively (Fig. 2(a)). Fig. 2(b) exhibits the field cooling $M- T$ curves of LMO films upon cooling field, both the measured and cooled field are 1 kOe. The saturation magnetization of LMO films under strong compressive or tensile strains are much smaller below 100 K, and the magnetization of LMO/LSAT is slight smaller than that of LMO/STO. Fig. 2(c) depicts the hysteresis loops of LMO films on different substrates (hence strain states) at 3 K. All the samples were field cooled in a 2 T external field from 300 K to 3 K. It can be seen that the $M- H$ loops of highly-stressed LMO films (with LAO, LSAT and KTO) are much narrower than that deposited on STO, and only in LMO/STO do $H_{C}$ enhancement and the “kinked” behavior in $M- H$ loop prevail. The temperature dependence of magnetization for LMO films deposited at 20 mTorr is consistent with the strain state of LMO films on different substrates. When the LMO film is slightly strained it shows robust ferromagnetic behavior, and the compressive or tensile strain will result in the progressive reduction of magnetization in LMO film. Theoretical investigated on the LMO films also indicated that a large cooperative coupling of Jahn-Teller distortion to oxygen-octahedron rotations has a significant effect on its magnetic property, which can be manipulated by the epitaxial strain [9]. Our results confirm the magnetic properties of LMO thin film are closely related to the deposited oxygen pressure and substrate, and the enhanced coercive field and exchange bias are attributed to the competition between different magnetic phases induced by oxygen vacancies and strain-induced Jahn-Teller effect. This work was supported by the National Natural Science Foundation of China (Grant No. 51502129), the Hong Kong Research Grant Council (PolyU 153015/14P) and The Hong Kong Polytechnic University (1-ZE25, 1-ZVGH).