Interface effects in magnetic metal/metal-oxide thin film systems

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Cortie, David
School of Physics
exchange bias; multiferroic; spintronics
thesis / dissertation description
With the emergence of the spintronics paradigm, there is a compelling reason to find novel methods to manipulate and measure magnetic spin in condensed matter systems at nanometre length scales. In metal-oxide materials, the combination of magnetic super-exchange, strongly correlated electrons and low symmetry crystalline environments gives rise to a plethora of useful phenomena: antiferromagnetism, ferroelectricity, semiconducting behaviour and magneto-electric coupling. The break in translational symmetry at an interface further modifies the intrinsic properties of such magnets in nanometre-thick films. This thesis focuses on the study of the magnetic structure of cubic and perovskite metal-oxide magnets, particularly when these are incorporated intometal/metal-oxide thin film systems. The goal is to investigate the varied effects that an interface exerts on neighbouring magnetic layers to understand the delicate role of nano-architecture and microstructure on the overall magnetic properties. Although three thin film systems are discussed - Ni80Fe20/α-Fe2O3, BiMn0.5Fe0.5O3 and Ni80Fe20/CoO/Co - these are united through the importance of interfacial effects. A major part of the work was the deployment of advanced experimental techniques such as polarised neutron reflectometry and high-angle neutron diffraction to reveal atomic-scale magnetic information buried at interfaces below the surface of the thin film. A versatile classical spin model was developed using a computer simulation of 2D/3D spin-lattices obeying a modified Heisenberg equation to model the experimental findings.A pronounced exchange bias effect was found experimentally for an antiferromagnetic hematite α-Fe2O3/ ferromagnetic permalloy Ni80Fe20 bilayer film below the blocking temperature of 40 K. Polarised neutron reflectometry was used to characterise the asymmetric reversal behaviour in the exchange biased state. Using complementary techniques, a detailed fitting and error analysis was performed of the polarised neutron reflectometry data to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface in the saturated state. The data is consistent with an induced magnetic region at the antiferromagnetic interface of 0.5 - 1.0 µB per Fe within a depth of 1 - 2 nanometres. By extending contemporary models of exchange bias to the nanocrystalline case, the Monte Carlo computer simulation provides a qualitative explanation for the mechanisms of formation of uncompensated magnetic spins at the interface, the magnetic reversal behaviour and the consequence of variable antiferromagnetic anisotropy.For BiMn0.5Fe0.5O3 epitaxial thin films, a temperature-dependent experiment was performed to investigate the antiferromagnetic order. High-angle neutron diffraction was used to directly reveal the atomic-scale magnetic structure of the single-crystalline thin film deposited on a SrTiO3 (001) substrate. A transition to long-range G-type antiferromagnetism was observed below 120 K with a (½½½) propagation vector. A weak ferromagnetic component was observed using SQUID magnetometry. The thin film has a reduced critical temperature compared to the related bulk material La0.2Bi0.8Fe0.5Mn0.5O3 attributed to the strained growth of the film on the substrate interface. There is no indication of the spin cycloid, known for BiFeO3, in the BiMn0.5Fe0.5O3 thin film. Monte Carlo simulations were used to elucidate the types of local and global perturbation of the molecular fields required to generate or destroy an incommensurate spin structure and lead to a weak ferromagnetism.For the Ni80Fe20/CoO/Co section, a series of ferromagnet/antiferromagnet/ferromagnet trilayer thin films were fabricated by ion-beam assisted deposition with varying CoO spacer thickness. A microstructural study was performed using electron microscopy, X-ray reflectometry and polarised neutron reflectometry to determine the relationship between the film morphology and the magnetic properties. At 200 K, after field-cooling, both perpendicular and longitudinal magnetic exchange bias was present in a double-shifted hysteresis loop. Based on layer-resolved polarised neutron reflectometry, it is shown that ion-beam modification during the deposition led to an oxygen-rich CoO/Co nanocomposite interface region.