Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure

Abstract

In the bulk, LaCoO3 (LCO) is a paramagnet, yet in tensile-strained thin films at low temperature ferromagnetism (FM) is observed, and its origin remains unresolved. Polarized neutron reflectometry (PNR) is a powerful tool to determine the depth profiles of the structure and magnetization simultaneously and, thus, the evolution of the interfacial FM with strain can be accurately revealed. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by PNR and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments.The magnetization can be directly controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO6 octahedra in combination with the ferroelastic order parameter.

Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO3

“…These models commonly considered epitaxial LCOs as ideally stretched homogeneous thin films, supported by HAADF STEM images without any dark stripes. , However, it should be noted that the dark stripes would not be observed in HAADF STEM images if the beam direction is not parallel to the dark stripe planes (see Figure 2S in the Supporting Information) . Recently, the internal structure suggested by the dark-striped HAADF STEM images was further verified by polarized neutron reflectometry . Therefore, the ferromagnetism of epitaxial LCOs is believed to be explained within the dark-striped atomic structure.…”

“…27 Recently, the internal structure suggested by the dark-striped HAADF STEM images was further verified by polarized neutron reflectometry. 34 Therefore, the ferromagnetism of epitaxial LCOs is believed to be explained within the dark-striped atomic structure.…”

Dynamics of Anisotropic Oxygen-Ion Migration in Strained Cobaltites

“…First of all, cobaltites have relatively low E Vö (∼1.5 eV) and oxygen migration barrier (∼0.8 eV), compared to other 3 d transition metal oxides . Second, earlier work shows that the stoichiometric LCO 3 epitaxial films exhibit an unconventional strain relaxation behavior, resulting in the stripe-like domain patterns due to the formation of OVCs. − The pseudocubic ( pc ) lattice constant of bulk LCO 3 is a pc = 3.81 Å (Figure a). Single-crystalline SrTiO 3 ( a = 3.905 Å) and LaAlO 3 ( a pc = 3.79 Å) substrates may introduce −0.5% (compressive) and +2.5% (tensile) strain to the as-grown LCO 3 films, respectively.…”

“…The LCO 2.67 phases contain alternatively stacked one CoO 4 tetrahedral and two CoO 6 octahedral layers . Tensile strain would favor the creation of ordered V ö in the films along the out-of-plane direction because the V ö -related chemical expansion can reduce the lattice misfit strain. − On the contrary, compressive strain leads to in-plane V ö orderings, which are parallel to the interface due to the concomitant lattice elongation along the out-of-plane direction.…”

Controlling Strain Relaxation by Interface Design in Highly Lattice-Mismatched Heterostructure

“…Tailoring the strain state in materials is a critical building block for engineering the functions of materials, since it allows for effectively tuning the multiple degrees of freedom in systems, like lattice distortion, charge distribution, spin reorientation, and orbital ordering. − This approach is referred to as strain engineering and has been widely exerted to construct enormous functional heterostructures, which provide a versatile platform for researching many intriguing phenomena, like flexoelectricity and multiferroicity. − Also, with such a wide range of functions generated by strain engineering it broadens potential applications of thin films, such as solar cells, information storage, signal processing, and so forth. − …”