Coexistence of glassy antiferromagnetism and giant magnetoresistance (GMR) in Fe/Cr multilayer structures

Using temperature-dependent magnetoresistance and magnetization measurements on Fe/Cr multilayers that exhibit pronounced giant magnetoresistance (GMR), we have found evidence for the presence of a glassy antiferromagnetic (GAF) phase. This phase reflects the influence of interlayer exchange coupling (IEC) at low temperature (T < 140K) and is characterized by a field-independent glassy transition temperature, Tg, together with irreversible behavior having logarithmic time dependence below a "de Almeida and Thouless" (AT) critical field line. At room temperature, where the GMR effect is still robust, IEC plays only a minor role, and it is the random potential variations acting on the magnetic domains that are responsible for the antiparallel interlayer domain alignment.Comment: 5 pages, 4 figure

Grazing Incidence X-ray Scattering from Magnetic Thin Films and Nanostructures

Grazing incidence scattering of synchrotron x-rays has been used to characterize the structure of magnetic thin films and periodic nanostructures. The combined metal and metal oxide films have been chosen to clarify the effects of growth processing techniques in technologically important magnetic and magnetoresistive thin film materials, and have particular relevance to the magnetic tunnel junction (MTJ) class of magnetic sensor. Co/Al2O3 thin films and Co/MgO multilayer thin films have been characterized using x-ray reflectivity and diffuse scatter analysis to explain how preparatory oxidation of the lower ferromagnet in an MTJ can reduce Néel interlayer coupling and improve the consistency of magnetoresistance. Measurements reveal differing effects for Al2O3 and MgO tunnel barrier materials. In Co/Al2O3 systems, preoxidation is found to reduce significantly chemical interdiffusion at the interface between the two layers, implying a more uniform oxidation of the barrier layer. In Co/MgO multilayers, an increased in-plane correlation length of the inherited interface roughness is seen after preoxidation. This implies that preoxidation suppresses the short wavelength undulations on both sides of the tunnel barrier that cause Néel coupling. Grazing incidence in-plane diffraction measurements on epitaxial Fe/MgO/Fe [001] and Fe/Au/MgO/Fe [001] films during annealing to 600 K have shown that, in both cases, the MgO lattice is initially strained towards being commensurate with the iron and gold layers, but relaxes after annealing towards a typical bulk MgO lattice. The iron and gold layers display linear thermal expansion at rates consistent with the bulk material. These in-plane lattice measurements demonstrate how the strain and strain dispersion in an epitaxial MgO barrier layer can be relieved under controlled annealing conditions. Finally, patterned thin film surfaces with submicron periodic symmetries have been studied by grazing incidence x-ray scattering. A novel semi-kinematical theory has been developed into a numerical algorithm capable of simulating the scatter from a wide range of arbitrary and disordered nanoscale arrays. This has allowed key structural parameters including array periodicity, symmetry and array coherence lengths to be extracted from experimental data

Fluoride Thin Films: from Exchange Bias to Multferroicity

This dissertation concerns research into the growth and characterization fluoride thin films by molecular beam epitaxy. After a discussion of relevant background material and experimental procedures in the first two chapters, we study exchange bias in magnetic multilayers incorporating the uniaxial antiferromagnet FeF2, grown to varying thicknesses, sandwiched between ferromagnetic Co layers with fixed thicknesses of 5 and 20 nm. Several bilayers with only the 20 nm thick Co layer were grown for comparative study. The samples were grown on Al2O3 (112¯0) substrates at room temperature. In-situ RHEED and x-ray diffraction indicated the films were polycrystalline. The films were determined to have low surface and interlayer roughness, as determined by AFM and x-ray reflectivity. After field-cooling to below the Neel temperature of FeF2 in a magnetic field of 1 kOe, magnetic hysteresis loops were measured as a function of temperature. We found that both layers had a negative exchange bias, with the exchange bias of the thinner layer larger than that of the thicker layer. In addition, the coercivity below the blocking temperature TB of the thinner layer was significantly larger than that of the thick layer, even though the coercivity of the two layers was the same for T \u3e TB. The exchange bias effect, manifested by a shift in these hysteresis loops, showed a strong dependence on the thickness of the antiferromagnet. Anisotropic magnetoresistance measurements provided additional insight into the magnetization reversal mechanism within the ferromagnets. The thickness dependent exchange anisotropy of trilayer and bilayer samples is explained by adapting a random field model to the antiferromagnet/ferromagnet interface.;Finally, We investigate the temperature dependent growth, as well as the magnetic and ferroelectric properties of thin films of the multiferroic compounds BaMF4, where M = Fe, Co, Ni. The films were grown to thicknesses of 50 or 100 nm on single crystal Al2O3 (0001) substrates. X-ray diffraction showed that this family of films grew epitaxially in the (010) orientation, but were twinned in the plane, with three domain orientations rotated by 120 degrees relative to one another. Measurements of the remanent hysteresis via interdigitated electrodes showed that the compounds M = Co, and Ni were ferroelectric, but no switching behavior was observed in the Fe system at electric fields up to 400 kV/cm. Measurements of the field-cooled and zero-field-cooled magnetic moment confirmed low temperature antiferromagnetic behavior, and found new weak ferromagnetic phases induced by strain