The magnetic properties of the (FexNi1-x)2B family of materials are explored using DFT calculations utilizing the FPLO and SPR-KKR code packages. It is found that a uniaxial magnetocrystalline anisotropy exists at around x = 0.8 with a magnetocrystalline anisotropy energy at around 0.3 MJ/m^3. A calculation of the lattice constant for these materials were attempted but failed due to the emergence of local minima and the calculations of magnetic properties were instead done using lattice parameters interpolated between known experimental values.

The magnetic and physical properties of oxide-free, ligand passivated, iron nanoparticles were studied using superconducting quantum interference device (SQUID) magnetometry and synchrotron based X-ray radiation. Particles used for this study ranged in diameter between 2 and 10 nm, which made it possible to distinguish between bulk and surface effects in the nanoparticles’ properties. Additionally, the effects of two different weakly interacting ligands (2,4-pentanedione and hexaethylene glycol monododecylether) on the nanoparticles’ behavior were studied. The results of this study were compared to theoretical predictions of magnetic transition metal behavior in both thin films and nanoparticles, as well as experimental results from measurements of transition metal clusters formed in an inert carrier gas and measured with a Stern-Gerlach magnet. Magnetometry revealed that the iron nanoparticles have a magnetocrystalline anisotropy an order of magnitude greater than bulk iron. At the same time, these particles exhibit a saturation mass magnetization up to 209 Am2/kg, which is only slightly lower than bulk iron. The structural properties of these particles were characterized using high energy X-ray diffraction analyzed using the atomic pair distribution function method (PDF). The PDF analysis indicates that the Fe particles have a distorted and expanded form of the bcc lattice, which could, at least in part, explain the magnetocrystalline anisotropy of these nanoparticles. X-ray absorption fine structure (XAFS) was used to study the surface properties of the iron nanoparticles and further characterize their structural properties. XAFS showed that oxidized species of iron exist at the nanoparticles’ surface and can be attributed to iron/ligand interactions. The percentage of oxidized species scales with the surface to volume ratio of the nanoparticles, and therefore appears limited to the nanoparticle surface. The layer of Fe(II) species present at the nanoparticles’ surface accounts for the reduction in saturation mass magnetization values (when compared to bulk iron) observed in these particles. XAFS analysis also provided further confirmation of the nanoparticles’ expanded crystalline lattice. / text

Magnetic materials have many applications in technology. The magnetic properties of materials are therefore important to catalogue for future use. In this project, the magnetic properties of thin films of permalloy are investigated. Specifically, the goal is to find the perpendicular magnetic anisotropy energy (PMAE) of thin film geometries of permalloy of varying film thickness. The PMAE is computed with powerful parallel computers using density functional theory (DFT) as implemented in the open-source DFT package OpenMX. The project consists of two parts: Computations on the bulk system and computations on six thin film systems of varying thickness. The thin films are periodic in the basal plane (the permalloy has a tetragonal crystal structure). The easy axis of magnetization was found to be along the c-axis of the tetragonal structure, both for bulk and thin film geometries. For the thin film geometries, this corresponds to an out-of-plane easy axis. The PMAE of the thinnest thin film geometries (4-5 atomic layers) were several times greater than that of the bulk system. Thin films with one more layer of Fe than Ni have especially great PMAE. When comparing the results to another study, the magnetocrystalline anisotropy as computed in this project turned out to be more than two orders of magnitude greater than in the previous study. It is hypothesised that this is because of the differing crystal structure of permalloy used in the study.

<p>Magnetic properties of selected compounds and superlattices have been experimentally studied using SQUID (superconducting quantum interference device) and VSM (vibrating sample magnetometer) magnetometry, neutron diffraction and Mössbauer spectroscopy measurements combined with theoretical <i>ab initio</i> calculations. </p><p>The magnetic compounds (Fe_1-xM_x)₃P, M=Co or Mn have been studied extensively. It was found that Co can substitute Fe up to <i>x</i>=0.37. Increasing the Co content leads to a reduction of the Curie temperature and the magnetic moment per metal atom. Mn can substitute Fe up to<i> x</i>=0.25 while Fe can be substituted into Mn₃P to 1-<i>x</i>=0.33. On the iron rich side, the drop in Curie temperature and magnetic moment when increasing the Mn content is more rapid than for Co substitution. On the manganese rich side an antiferromagnetic arrangement with small magnetic moments was found. </p><p>The interlayer exchange coupling and the magnetocrystalline anisotropy energy of Fe/V superlattices were studied. The coupling strength was found to vary with the thickness of the iron layers. To describe the in-plane four-fold anisotropy, the inclusion of surface terms proved necessary. </p><p>The in-plane four fold anisotropy was also studied in a series of Fe/Co superlattices, where the thickness of the Co layers was kept thin so that the bcc structure could be stabilized. Only for samples with a large amount of iron, the easy axis was found to be [100]. The easy axis of bulk bcc Co was therefor suggested to be [111]. </p>

The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.

Magnetic properties of selected compounds and superlattices have been experimentally studied using SQUID (superconducting quantum interference device) and VSM (vibrating sample magnetometer) magnetometry, neutron diffraction and Mössbauer spectroscopy measurements combined with theoretical ab initio calculations. The magnetic compounds (Fe1-xMx)3P, M=Co or Mn have been studied extensively. It was found that Co can substitute Fe up to x=0.37. Increasing the Co content leads to a reduction of the Curie temperature and the magnetic moment per metal atom. Mn can substitute Fe up to x=0.25 while Fe can be substituted into Mn3P to 1-x=0.33. On the iron rich side, the drop in Curie temperature and magnetic moment when increasing the Mn content is more rapid than for Co substitution. On the manganese rich side an antiferromagnetic arrangement with small magnetic moments was found. The interlayer exchange coupling and the magnetocrystalline anisotropy energy of Fe/V superlattices were studied. The coupling strength was found to vary with the thickness of the iron layers. To describe the in-plane four-fold anisotropy, the inclusion of surface terms proved necessary. The in-plane four fold anisotropy was also studied in a series of Fe/Co superlattices, where the thickness of the Co layers was kept thin so that the bcc structure could be stabilized. Only for samples with a large amount of iron, the easy axis was found to be [100]. The easy axis of bulk bcc Co was therefor suggested to be [111].

The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.

Theoretische Berechnungen sagen für tetragonal gedehntes Fe-Co eine hohe magnetokristalline Anisotropie voraus, wie sie für seltenerdfreie Dauermagnetwerkstoffe vorteilhaft wäre. In dieser experimentellen Arbeit werden epitaktische Fe-Co-Schichten strukturell und magnetisch charakterisiert. Zur Untersuchung der Dehnung in diesen Schichten eignen sich AuxCu100-x-Pufferschichten besonders, da über die Stöchiometrie (x) deren lateraler Gitterparameter eingestellt werden kann. Wird Fe-Co auf einer solchen Pufferschicht abgeschieden, erfolgt aufgrund dessen hoher elastischer Energie schon in den ersten Monolagen eine vollständige Relaxation der pufferinduzierten Dehnung. In ternären Fe-Co-X-Schichten, in denen kleine X-Atome (X = B, C oder N) Oktaederlücken besetzen, wird jedoch eine spontane tetragonale Dehnung c/a bis zu 1,05 beobachtet. Entlang der gedehnten c-Achse tritt eine uniaxiale magnetokristalline Anisotropie auf, die für B- oder C-Zulegierungen von 2 at% eine maximale Anisotropiekonstante von 0,4 MJ/m³ zeigt. Wird der X-Gehalt weiter erhöht, nehmen die Kristallinität der Schichten und die magnetische Anisotropie ab. Neben der magnetokristallinen Anisotropie des Schichtvolumens wird an den Fe-Co(-X)-Schichten eine hohe Grenzflächenanisotropie beobachtet. Der Beitrag der freien Oberfläche übersteigt den der Au-Cu-Grenzfläche dabei deutlich. / Theoretical calculations predict a high magnetocrystalline anisotropy for tetragonally strained Fe-Co, which would be beneficial for rare-earth free permanent magnet materials. In this experimental work, epitaxial Fe-Co films are investigated structurally and magnetically. AuxCu100-x buffer layers are very suitable to study the strain in these films since their in-plane lattice parameter can be tailored via the applied stoichiometry (x). However, when Fe-Co is deposited on such a buffer layer, the induced strain of the Fe-Co lattice relaxes completely within the first monolayers, due to its high elastic energy. In ternary Fe-Co-X films, where small atoms X like B, C or N occupy octahedral interstitial sites, a spontaneous strain c/a up to 1.05 is observed. A uniaxial magnetocrystalline anisotropy along the strained c axis appears. Their maximum anisotropy constant is 0.4 MJ/m³ for B or C contents of 2 at%. If the X content is further increased, the crystallinity and thus, the magnetic anisotropy of the films degrade. Together with the magnetocrystalline anisotropy of the films’ volumes, a high interface anisotropy is observed for the Fe-Co(-X) films. The contribution of the free surface clearly exceeds the contribution of the Au-Cu interface.

The large magneto-crystalline anisotropy energy of the L10 phase has pushed the interest to the FePt nanoparticles to get smallest possible not superparamagnetic particles for magnetic data storage media. The DC magnetron sputtering process, in an inert gas atmosphere and subsequently ejection into high vacuum via differential pumping in addition with a newly constructed light furnace, allows us to have a predeposition annealing of FePt nanoparticles. The advantage compared to wet chemical process route is, that we can prevent the growing of particles on a substrate. In order to determine the experimentally hardly accessible temperature of the particles, the thermal history of the particles is rather calculated from the interaction with the light field along the flight path through the light furnace used for the in-flight annealing. The results obtained for the particle temperature are corroborated by experimental findings on the sintering of agglomerated particles and change in magnetic properties due to heating over the L10 stability temperature. The experiments reveal that the effect of the thermal treatment on both the structural and magnetic properties of the FePt nanoparticles strongly depends on the particles’ crystal structure. The magnetic behavior shows a size depending effective uniaxial magnetic anisotropy constant. This behavior is strongly correlated to the structure of the 5 nm to 8 nm L10 FePt particle.

Mit Hilfe der Dichtefunktionaltheorie wurden magnetische Eigenschaften an 3d Elementen und Legierungen und 5f Verbindungen untersucht. Dabei wurde auf die Wichtigkeit von Orbitalpolarisationskorrekturen eingegangen und diese näher erörtert. Im ersten Anwendungsteil wurden magnetische Momente und die Magnetokristalline Anisotropie Energie an 3d Elementen untersucht. Des Weiteren wurden FeCo Legierungen als mögliche Bestandteile in der Festplattenindustrie diskutiert. Im letzten Abschnitt wurden Uranverbindungen in Hinsicht auf Ihre Orbitalpolarisation untersucht.