Magnetism and Nonlinear Charge Transport in NiFe ₂ O ₄ /γ‐Al ₂ O ₃ /SrTiO ₃ Heterostructure: Toward Spintronic Applications

ABSTRACT We present the synthesis and study of the magnetic and electronic properties of NiFe ₂ O ₄ /γ‐Al ₂ O ₃ /SrTiO ₃ heterostructure. The γ‐Al ₂ O ₃ /SrTiO ₃ interface hosts a high‐mobility 2D electron gas (2DEG) with large spin‐orbit coupling, making it promising for spintronic and nano‐electronic device applications if it can be coupled to a suitable source of spin currents. For efficient spin‐charge conversion, a ferromagnetic insulator as a spin current source is more advantageous than a metallic/semiconducting ferromagnet. Here, we synthesize a ferrimagnetic insulating NiFe ₂ O ₄ (001) layer on γ‐Al ₂ O ₃ (001)/SrTiO ₃ (001) using low‐temperature reactive sputtering at 150 °C without compromising the mobility and charge carrier density of the 2DEG at the γ‐Al ₂ O ₃ (001)/SrTiO ₃ (001) interface. While γ‐Al ₂ O ₃ (8 nm)/SrTiO ₃ is nonmagnetic, NiFe ₂ O ₄ (52 nm)/γ‐Al ₂ O ₃ (8 nm)/SrTiO ₃ shows a well‐defined magnetic hysteresis loop at room temperature as well as at low temperatures. The sheet resistance of both γ‐Al ₂ O ₃ (8 nm)/SrTiO ₃ and NiFe ₂ O ₄ (52 nm)/γ‐Al ₂ O ₃ (8 nm)/SrTiO ₃ exhibits metallic behavior down to cryogenic temperatures, with a low temperature upturn driven by the combined effect of Kondo‐like scattering and weak antilocalization. Most importantly, NiFe ₂ O ₄ (52 nm)/γ‐Al ₂ O ₃ (8 nm)/SrTiO ₃ behaves as a magnetic diode at low temperatures, and its rectification performance increases significantly with increasing magnetic field strength, giving rise to a robust magneto‐electronic rectification effect at low temperatures, which provides a first step toward the development of nanoscale all‐oxide heterostructures capable of efficient spin‐charge conversion.