Zugl.: Regensburg, Univ., Diss., 2009

This paper discusses the search of an FeNi25-based alloy with a face-centered cubic crystal structure exhibiting a Curie point around room temperature, for magnetocaloric applications. Fe was substituted in various amounts with FCC-stabilising elements Mn and Co as these elements respectively decrease and increase the Curie Temperature, thus enabling to tune the Curie point. Three characterization methods were carried out on the samples: Magneto- thermo-gravimetry (MTG), X-ray diffraction (XRD) and finally, vibrating sample magnetometer (VSM) measurements were performed. All samples displayed several Curie points, each corresponding to various FCC phases. Also, the last sample, FeNi25Mn6Co2, had an FCC phase fraction of almost 99% and presented two Curie points in the continuity of one another one at -35°C and another at 91°C. Hence, at room temperature, the sample underwent a magnetic phase transition passing from its ferromagnetic state to a paramagnetic one. / Detta arbete består i att utröna möjligheterna att med utgångspunkt från den binära sammansättningen FeNi25 erhålla en ytcentrerad kubisk fas (fcc) med en Curie punkt vid rumstemperatur. Syftet är att använda dessa legeringar i magnetokaloriska tillämpningar. Strategin är att både Mn och Co är fcc stabliliserande grundämnen, och att Mn sänker och Co ökar Curie temperaturen. Tre olika karakteriseringsmetoder användes; röntgendiffraktometri (struktur), Magneto-Termo-Gravimetri (magnetisering vs temperatur) och konventionell magnetometri vid rumstemperatur (magnetisering vs magnetiskt fält, Vibrating Sample Magnetometry VSM). Resultaten visar att även om kristallstrukturen i det närmaste är fullständigt fcc, så ger de magnetiska mätningarna vid handen att flera olika faser är vid handen med avesvärt olika Curie temperaturer. Som en illustration av detta förhållande kan nämnas att sammansättningen FeNi25Mn6Co2 uppvisar en fcc-fraktion på i det närmaste 99%, men har vid en M(T) mätning ett förlopp som enklast förklaras med en Curie punkt vid ca -35C och en ytterligare vid ca 90°C. Denna observation signalerar att de magnetiska egenskaperna torde vara mer beroende av exakt distribution av de ingående atomslagen i fcc strukturen än vad de röntgendiffraktometriska undersökningarna kan detektera.

The increasing demand for nearline storage capacity in data centers calls for a continued enhancement in hard disk drive recording density far beyond one terabit per square inch. The thermal stability limit forces the drive manufacturers to develop new concepts in order to achieve this in the long term. Potential solutions are microwave-assisted magnetic recording (MAMR), heat-assisted magnetic recording (HAMR) and bit-patterned media (BPM). A simple example of BPM based on sputter-deposited Co/Pd multilayers and prepatterned substrates at hypothetical recording densities up to one terabit per square inch was studied by magnetic force microscopy (MFM). This system achieved promising results at lower densities, but an actual application for data storage, especially at one terabit per square inch and higher densities, requires elaborate optimizations. For some time now, FePt thin films have attracted much attention as prospective recording layers for high-density magnetic data storage due to their high magnetic anisotropy. The use of FePt films in HAMR is especially promising. This application has been tested successfully by Seagate and its key customers in recent years and is about to be introduced into the nearline hard disk drive market. It requires a tuning of the magnetic properties of FePt, especially of its Curie temperature. The addition of Cu proved to be effective in this regard and can also facilitate the formation of the crucial L10 structure and (001) texture during rapid thermal annealing of sputter-deposited thin films. Such films were prepared as bilayers of Cu and FePt on Si substrates, annealed for 30 s, and analyzed by X-ray diffraction (XRD) and SQUID vibrating sample magnetometry (SQUID-VSM). The influence of large Cu additions on important properties like lattice parameters, mosaicity, magnetic anisotropy and Curie temperature is discussed. The chemical long-range order was calculated from the XRD data, and a dedicated chapter of this thesis covers the most important factors to be considered in such calculations for textured thin films and other samples. The feasibility of creating patterned Fe-Cu-Pt films with perpendicular magnetic anisotropy, as needed for a combination of HAMR and BPM, by deposition through a PMMA mask, a lift-off process and subsequent annealing was investigated as well. The results indicate that the chosen approach might not lead to the required (001) texture when the nanostructures are small enough to compete with today's recording densities, so that either a continuous film might need to be etched after annealing or a seed layer might be required to induce the texture.:1. Motivation: Magnetic Data Storage 2. Experimental Techniques 3. Co/Pd Multilayers on Prepatterned Substrates 4. Fe-Pt and Fe-Cu-Pt Alloys 5. Rapid Thermal Annealing of FePt and FePt/Cu Films 6. Order Parameter Calculation 7. Summary