Margaret Kane1,Arturas Vailionis1,Megan Holtz2,Lauren Riddiford1,Purnima Balakrishnan3,Alexander Grutter3,Apurva Mehta4,Yuri Suzuki1
Stanford University1,Colorado School of Mines2,National Institute of Standards and Technology3,SLAC National Accelerator Laboratory4
Margaret Kane1,Arturas Vailionis1,Megan Holtz2,Lauren Riddiford1,Purnima Balakrishnan3,Alexander Grutter3,Apurva Mehta4,Yuri Suzuki1
Stanford University1,Colorado School of Mines2,National Institute of Standards and Technology3,SLAC National Accelerator Laboratory4
Thin films of LaNiO<sub>3</sub> (LNO) have attracted great interest because while it is the only rare-earth nickelate that does not exhibit either temperature dependent metal-insulator or magnetic transitions in the bulk, these electronic and magnetic transitions can be observed in ultra-thin films. Notably, in (111)-oriented films grown on LaAlO<sub>3</sub> (LAO), we observe evidence of a magnetic transition not seen in (001)-oriented films via the anomalous Hall effect and negative hysteretic magnetoresistance in 8-26 unit cell thick LNO films. Using gating experiments, we further explore the topological characteristics of this system.<br/>The signatures of ferromagnetism are confirmed by a polarized neutron reflectivity (PNR) study which indicates a thin region of net magnetization at the LNO/LAO interface. We propose that this interfacial region has a strain state unique to (111)-oriented films due to increased connectivity and the absence of tilts and rotations expected to relax strain in a perovskite system. Supporting this picture, PNR measurements performed on a control sample with a significantly thicker LNO film show negligible net magnetization. Interfacial relaxation in the thicker film is proposed to disrupt the unique interfacial state and suppress the magnetic transition.<br/>We directly observe the unique structural reconstruction in ultrathin (111)-oriented LNO. Synchrotron x-ray diffraction (XRD) data and dynamical XRD analysis show an elongation of the out-of-plane lattice parameter to 2.4 Å. This observation is further confirmed by 4D-scanning transmission electron microscopy measurements that indicate a region of high strain in the out-of-plane direction with no observed rotation or shear strain. Together these results indicate that growth direction can greatly enhance film functionality and that the metallicity and magnetism in LNO films is governed by a balance between the charge, lattice, and orbital degrees of freedom.<br/>Funded by Department of Energy, Director, Office of Science, Office of Basic Energy Sciences, Div. of Mat. Sci. and Eng., under Contract No. DESC0008505 and NSF GRFP.