Wpływ kontrolowanej modyfikacji struktury i morfologii planarnych i liniowych nanostruktur metalicznych na ich własności magnetyczne na przykładzie wielowarstw Fe/Cr i nanodrutów Co

Abstract

In this monography, we have demonstrated the results of an investigation of the influence of controlled modification of the morphology and structure on magnetic and magnetotransport properties of Fe/Cr multilayers representing planar nanostructures. Moreover, the structural and magnetic properties of cobalt nanowires being an example of linear objects were studied. The aim of the multilayer studies was to learn about the interface structure between Fe and Cr layers and then analyse the impact of this parameter on the giant magnetoresistance effect. Samples were prepared by vapor deposition in ultrahigh vacuum conditions. The layer morphology and interface structure were analyzed using the techniques of high resolution transmission electron microscopy on a cross section of the sample (HRTEM) also operated in the scanning mode (STEM), low energy electron diffraction (LEED), energy dispersive spectroscopy (EDS), Auger electron spectroscopy (AES), X-ray reflectometry (XRR) and conversion electron Mossbauer spectroscopy (CEMS). The results confirmed the slight interface roughness in the area of up to 2 - 3 atomic layers and layer continuity in all samples. An analysis of the magnetic properties carried out based on measurements of magnetooptical Kerr effect or SQUID magnetometer, indicated the antiferromagnetic coupling of Fe layers. In order to analyse the impact of interface roughness on the giant magnetoresistance value, the multilayer systems have been subjected to the controlled modification by the introduction of Bi, Pb or In surfactants as well by gold or iodine swift heavy ion irradiation. The used modification methods, depending on the type and intensity, caused the interface smoothing, which is preferably influenced by the presence of the buffer layer, the introduction of bismuth atoms as surfactant, as well as Au and I ion irradiation of polycrystalline trilayers with low fluences. In addition, we have observed changes in the magnetic arrangement of Fe layers, manifested by a decrease of antiferromagnetic coupling fraction in multilayers modified by In surfactant and in the majority of irradiated samples. The increase of the antiferromagnetic coupling fraction was observed for trilayers irradiated with low fluences, for which interface smoothening was reported. It has been demonstrated that the positive effect of the buffer layer caused the stabilisation of the antiparallel arrangement of magnetization vector in iron layers. These changes affected the giant magnetoresistance effect, which increased with increasing interface roughness caused by interface corrugation and less by interdiffusion, with kept antiferromagnetic coupling of Fe layers. An increase of interface roughness resulted mainly from interdiffusion, and it was responsible for magnetoresistance effect decrease amplified by decrease of antiferromagnetic coupling fraction. Modification of interfaces with swift heavy ions showed that irradiation with small ion fluences gives the best chance of interface smoothing and keeping antiferromagnetic ordering of the systems. The aim of investigation of the structural and magnetic properties of cobalt nanowires was to optimise their morphology, as well as electrodeposition parameters leading to the achievement of nanoobjects showing a magnetic anisotropy with an easy axis of magnetization along the nanowire and characterised by the high values of coercivity and squareness of the hysteresis loop. Cobalt nanowires were electrodeposited in the polycarbonate membranes with a diameter in the range of 30 nm to 200 nm. The nanowire length varied from 1.5 μm to 6 μm. The impact of electrodeposition parameters, such as temperature and pH of the electrolyte and the cathodic voltage on the properties of the nanowires, was also investigated. It was observed that the optimal to achieve magnetic arrangement along nanowire axis and the greatest values of the coercive field, are: small diameters (30 nm ÷ 50 nm) and small length of the nanowires (1.5 μm), low electrolyte temperature (20 °C - 25 °C), low cathodic voltage (-0.9 ÷ V - 1.0 V) and moderate pH value (3.2) with a tendency to increase in order to reduce hydrogen evolution (3.8). These observations provide the basis for ongoing studies of composed nanowires made from alternating electrodeposited layers of Co and Cu, Fe and Cu as well as Fe and Cr layers.