Spin Seebeck Effect Studies of Epitaxially Grown Cr₂O₃ Antiferromagnetic Thin Films

The source of the spin current in which antiferromagnetic spin Seebeck effect (SSE) measures have been discussed, with mechanisms for both surface and bulk generation of spin current being proposed. Recent evidence suggests that antiferromagnetic thin films produce comparable signals to those of their bulk counterparts. This has been demonstrated by epitaxially grown Cr₂O₃ films with atomically smooth surfaces on polished Cr₂O₃ substrates utilizing pulsed laser deposition (PLD). Though the homoepitaxial Cr₂O₃ thin films follow the same crystalline structure of the bulk substrate, they exhibited a distinct SSE signal with comparable magnitude to that of the bulk owing to slight lattice imperfections in the films. For thin films, both the bulk and the film signals are observed. As the grown thin film exceeds a critical thickness the bulk signal disappears, not only does the spin flop transition (SFT) broaden, but it is also shifted to a higher field, due to the local anisotropies from the defects in the film. These findings show that above a certain thickness, the film becomes opaque to the bulk spin current and acts as a source of its own spin current.