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][13][14][15][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 SrTiO 3 (STO) substrates, while compressively 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 propertie...

Soft x-ray magnetic dichroism, magnetization, and magnetotransport measurements demonstrate that the competition between different magnetic interactions (exchange coupling, electronic reconstruction, and long-range interactions) in La0.7Sr0.3FeO3 (LSFO) / La0.7Sr0.3MnO3 (LSMO) perovskite oxide superlattices leads to unexpected functional properties. The antiferromagnetic order parameter in LSFO and ferromagnetic order parameter in LSMO show a dissimilar dependence on sublayer thickness and temperature, illustrating the high degree of tunability in these artificially-layered materials.

Using x-ray magnetic dichroism we characterize the magnetic order in La 0.7 Sr 0.3 MnO 3 (LSMO) / La 0.7 Sr 0.3 FeO 3 (LSFO) superlattices with 6 unit cell thick sublayers. The LSMO layers exhibit a reduced Curie temperature compared to the bulk while antiferromagnetic order in the LSFO layers persists up to the bulk Neél temperature. Moreover, we find that aligning the LSMO magnetization by a magnetic field within the (001) surface plane leads to a reorientation of the Fe moments as well maintaining a perpendicular orientation of Fe and Mn moments. This perpendicular alignment is due to the frustrated exchange coupling at the LSMO/LSFO interface.

Resonant x-ray reflectivity (XRR) is utilized to characterize the structural properties of La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 superlattices. XRR spectra at the Mn and Fe absorption edges provide additional structural information compared to spectra obtained with a conventional Cu x-ray source. We demonstrate that this technique provides individual layer thicknesses and intermixing behavior of perovskite superlattices with small density contrast in a non-destructive manner. These results are consistent with scanning transmission electron microscopy and electron energy loss spectroscopy.

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