Strain engineering to control the magnetic and magnetotransport properties of La0.67Sr0.33MnO3 thin films

TLDR: Yang et al. as discussed by the authors studied the control of the magnetic and magnetotransport properties of La 0.67 Sr 0.33 MnO 3 (LSMO) thin films through strain engineering.

Abstract: Strain engineering to control the magnetic and magnetotransport properties of La 0.67 Sr 0.33 MnO 3 thin films F. Yang 1 , N. Kemik 1 , M.D. Biegalski 2 , H.M. Christen 2 , E. Arenholz 3 , Y. Takamura 1 Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616, USA Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA This work studies the control of the magnetic and magnetotransport properties of La 0.67 Sr 0.33 MnO 3 thin films through strain engineering. The strain state is characterized by the tetragonal distortion (c/a ratio), which can be varied continuously between a compressive strain of 1.005 to a tensile strain of 0.952 by changing the type of substrate, the growth rate, and the presence of an underlying La 0.67 Sr 0.33 FeO 3 buffer layer. Increasing tensile tetragonal distortion of the La 0.67 Sr 0.33 MnO 3 thin film decreases the saturation magnetization, changes the temperature dependence of the resistivity and magnetoresistance, and increases the resistivity by several orders of magnitude. The perovskite oxides have been widely investigated in recent years since they possess various important physical properties such as ferromagnetism, superconductivity, and ferroelectricity. 1 In particular, La 0.67 Sr 0.33 MnO 3 (LSMO) is an attractive candidate for spintronic devices 2,3 because it displays colossal magnetoresistance (CMR) and half-metallicity, and possesses a Curie temperature, T C, above room temperature (~ 360 K). 4,5 In this material, the T C marks the transition between the ferromagnetic (FM)/ metallic and the paramagnetic (PM)/ insulating states, as well as the peak in the CMR. This correlation between the electrical and magnetic properties is explained by the double-exchange mechanism 6,7 which involves the hopping of electrons between Mn 3+ and Mn 4+ ions with parallel spin through a bridging O 2- ion. Due to the strong interactions between the charge and orbital degrees of freedom, these properties can be manipulated by a number of different parameters, including external pressure 8 , oxygen stoichiometry 9 , and the doping level. 10,11 With thin films, the epitaxial strain imposed from the underlying substrate provides an additional tuning parameter for the functional properties. It has been shown that coherently strained LSMO thin films can be grown on a wide range of different single crystal oxide substrates and that the resulting strain dramatically impacts the magnetic and magnetotransport properties of the thin films. 12-16 The strain state can be characterized by the tetragonal distortion, defined as the c/a ratio, where the in-plane lattice parameter of the film, a, is dictated by the lattice parameter of the substrate, and the out-of-plane lattice parameter, c, is allowed to respond accordingly. For example, Kwon et al. reported that an in-plane easy magnetization direction is observed in tensile-strained films (c/a ratio 1) grown on (001)-oriented LaAlO 3 (LAO) substrates exhibit an out-of-plane easy axis. 12 Furthermore, it has been shown that the magnitude of this tetragonal distortion depends on the crystallographic orientation of the film and the substrate. 13,14,17 Fully strained LSMO films grown on (110)-oriented substrates show enhanced electrical and magnetic properties due to the reduced tetragonal distortion relative to the films grown on (001)-oriented substrates. Previously, Takamura et al. 15 showed that a dramatic change in the properties of LSMO films occurred as the c/a ratio decreased from 0.984 to 0.962. However, the strain states were limited to discrete values corresponding to the lattice parameters of the commercially available single crystal oxide substrates, STO and DyScO 3 , respectively, therefore intermediate strain states were not available. One alternative involves using piezoelectric substrates such as Pb(Mg 1/3 Nb 2/3 ) 0.72 Ti 0.28 O 3 , which allow for the dynamic modulation of the strain by varying the applied electric field. 16 However, the crystalline quality of the LSMO films grown on these piezoelectric substrates suffers due to the large lattice mismatch and the rhombohedral structure of the substrate. Furthermore, the brittleness of the substrates limits the practical range of strain that can be attained. In this work, we demonstrate the ability to control the epitaxial strain of LSMO thin films continuously over a large range through the choice of substrate, the growth rate, and the presence of an underlying La 0.7 Sr 0.3 FeO 3 (LSFO) buffer layer. Bulk LSMO has a rhombohedral perovskite structure which can be approximated as