Exchange Bias in La_0.67Sr_0.33MnO₃/YFeO₃ Ferromagnet/Antiferromagnet Heterostructures

Thin film multilayers can exhibit emerging phenomena at their interfaces which are absent in their individual constituents. In particular, magnetic exchange interactions at the interfaces between ferromagnetic and antiferromagnetic-based materials provide a broad range of possibilities for fundamental studies and development of novel devices based on exchange bias. Various oxide-oxide, metal-metal or metal-oxide ferromagnet-antiferromagnet interfaces show exchange bias after field-cooling which produces a shift of the magnetic hysteresis loop. Perovskites with ABO₃ composition can exhibit ferromagnetism (e.g. La_0.67Sr_0.33MnO₃, LSMO), or antiferromagnetism (e.g. orthoferrites, AFeO₃) among other properties, and therefore provide a rich environment for exploring interfacial exchange in an all-oxide heterostructure.<br/><br/>Here, we investigate and optimize magnetic coupling of perovskite multilayers made of LSMO and YFeO₃ (YFO) as ferromagnetic and antiferromagnetic layers respectively. Films were made by pulsed laser deposition which produced heterostructures with interface widths of around one unit cell (1 u.c. = 0.4 nm), according to atomically resolved scanning transmission electron microscopy coupled with electron diffraction spectroscopy. We first fabricated single layers and bilayers of YFO and LSMO on SrTiO₃ (STO) substrates. Field cooling from 673 K in a 10 kOe field led to an exchange bias of up to 306 Oe at 50 K for 10 u.c. LSMO/49 u.c. YFO/STO with a Curie temperature around 280 K. Although exchange bias increases with decreasing ferromagnetic thickness, the LSMO layers must be thick enough, usually around 3-5 nm, to maintain their magnetic ordering and suppress magnetic dead layers. LSMO thickness also influences the Curie temperature of the LSMO layer, and the blocking temperature of the LSMO/YFO bilayer which was slightly below room temperature. On the contrary, exchange bias increases with thicker YFO layers until reaching a saturation value at 49 u.c. YFO for a fixed LSMO thickness of 10 u.c..<br/><br/>Most work on exchange bias in perovskite heterostructures has been done on bilayers, but we demonstrate here that perovskite-based multilayers exhibit enhanced exchange bias. A five-layer LSMO [10 u.c.]/YFO[49 u.c.]/LSMO[10 u.c.]/YFO[49 u.c.]/LSMO[10 u.c.]/ stack yields a 424 Oe exchange bias at 50 K after field cooling at 10 kOe. To demonstrate a heterostructure with both pinned and unpinned layers, an all-perovskite spin-valve stack was synthesized consisting of STO/LSMO(10 u.c.)/STO(8 u.c.)/LSMO(10 u.c.)/YFO(26 u.c.). After field cooling at 10 kOe and 673 K the hysteresis loops shows two steps switching from the unpinned and pinned layers, and parallel and antiparallel remanent states can be achieved at 200K. These results show the capabilities of PLD for making oxide heterostructures with sharp interfaces, and show how oxide heterostructures can exhibit spin valve behavior analogous to that of exchange-biased metallic spin valves.