Heusler materials with perpendicular magnetic anisotropy. Thin films for spintronics

Niesen A. Heusler materials with perpendicular magnetic anisotropy. Thin films for spintronics. Bielefeld: Universität Bielefeld; 2019.Diese Arbeit befasst sich mit der Untersuchung von neuen Materialien für zukünftige spintronische Anwendungen. Heusler - Legierungen sind vielversprechend aufgrund von ihrer vorhergesagten hohen Spinpolarisation, sowie ihrer Vielfältigkeit und Mannigfaltigkeit an magnetischen und elektrischen Eigenschaften. Die in dieser Arbeit untersuchten Mn-basierten Verbindungen zeigen hohes Potential aufgrund von ihrer hohen senkrechten magnetischen Anisotropie als intrinsische Eigenschaft, sowie der niedrigen Magnetisierung. Die untersuchten Co-basierten Verbindungen wurden auf einem neuartigen Puffermaterial, dem TiN, realisiert. In diesem Fall, konnte die senkrechte magnetische Anisotropie durch eine Reduktion der Schichtdicke bis auf 1 nm, sowie durch nachträgliche Hitzebehandlung der Schichten bei Temperaturen bis zu 500 °C, erreicht werden. Es wurde nachgewiesen, dass durch eine geeignete Anpassung des Herstellungsprozesses, die Materialeigenschaften optimiert und je nach Anforderungen für die jeweilige Anwendung, angepasst werden können. Beide untersuchten Materialgruppen zeigen nachgewiesenermaßen ein hohes Potential für die Implementation in zukünftige spintronische Anwendungen

Titanium Nitride as a Seed Layer for Heusler Compounds

Titanium nitride (TiN) shows low resistivity at room temperature, high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by X-ray diffraction and 4-terminal transport measurements. Element specific X-ray absorption spectroscopy revealed pure TiN in the bulk. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode, an out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device (SQUID) and anomalous Hall effect (AHE) for Mn2.45Ga. Magneto optical Kerr effect (MOKE) measurements were carried out to investigate the magnetic properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher coercivity and squareness ratio compared to unbuffered samples. The Heusler compound Co2FeAl showed already good crystallinity when grown at room temperature

Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes

Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some {\mu}V, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co$_2$FeAl and Co$_2$FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B based junctions.Comment: 9 pages, 4 figure

Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co2FeAl/MgO interface upon annealing

Conca A, Niesen A, Reiss G, Hillebrands B. Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co2FeAl/MgO interface upon annealing. JOURNAL OF PHYSICS D-APPLIED PHYSICS. 2018;51(16): 6.We investigate a series of films with different thickness of the Heusler alloy Co2FeAl in order to study the effect of annealing on the interface with a MgO layer and on the bulk magnetic properties. Our results reveal that while the perpendicular interface anisotropy constant K-S(perpendicular to) is zero for the as-deposited samples, its value increases with annealing up to a value of 1.14 +/- 0.07 mJ m(-2) for the series annealed at 320 degrees C and of 2.01 +/- 0.7 mJ m(-2) for the 450 degrees C annealed series owing to a strong modification of the interface during the thermal treatment. This large value ensures a stabilization of a perpendicular magnetization orientation for an extrapolated thickness below 1.7 nm. The data additionally shows that the in-plane biaxial anisotropy constant has a different evolution with thickness in as-deposited and annealed systems. The Gilbert damping parameter a shows minima for all series for a thickness of 40 nm and an absolute minimum value of 2.8 +/- 0.1 x 10(-3). The thickness dependence is explained in terms of an inhomogeneous magnetization state generated by the interplay between the different anisotropies of the system and by the crystalline disorder

Low damping magnetic properties and perpendicular magnetic anisotropy in the Heusler alloy Fe1.5CoGe

We present a study of the dynamic magnetic properties of TiN-buffered epitaxial thin films of the Heusler alloy Fe1.5CoGe. Thickness series annealed at different temperatures are prepared and the magnetic damping is measured, a lowest value of α = 2.18 × 10−3 is obtained. The perpendicular magnetic anisotropy properties in Fe1.5CoGe/MgO are also characterized. The evolution of the interfacial perpendicular anisotropy constant K⊥S with the annealing temperature is shown and compared with the widely used CoFeB/MgO interface. A large volume contribution to the perpendicular anisotropy of (4.3 ± 0.5) × 105 J/m3 is also found, in contrast with vanishing bulk contribution in common Co- and Fe-based Heusler alloys

Structural and Magnetic Properties of Sputter-Deposited Mn-Fe-Ga Thin Films

Niesen A, Sterwerf C, Glas M, Schmalhorst J-M, Reiss G. Structural and Magnetic Properties of Sputter-Deposited Mn-Fe-Ga Thin Films. IEEE Transactions on Magnetics. 2016;52(7): 2600404.We investigated the structural and magnetic properties of sputter-deposited Mn-Fe-Ga compounds. The crystallinity of the Mn-Fe-Ga thin films was confirmed using X-ray diffraction. X-ray reflection and atomic force microscopy measurements were utilized to investigate the surface properties, roughness, thickness, and density of the deposited Mn-Fe-Ga. Depending on the stoichiometry, as well as the used substrates [SrTiO3 (001) and MgO (001)] or buffer layer (TiN), Mn-Fe-Ga crystallized in the cubic or the tetragonally distorted phase. Anomalous Hall effect and alternating gradient magnetometry measurements confirmed strong perpendicular magnetocrystalline anisotropy. Hard magnetic behavior was reached by tuning the composition. TiN-buffered Mn2.7Fe0.3Ga revealed sharper switching of the magnetization compared with the unbuffered layers

Perpendicular magnetic anisotropy of TiN buffered Co2FeAl/MgO bilayers

Niesen A, Ludwig J, Glas M, et al. Perpendicular magnetic anisotropy of TiN buffered Co2FeAl/MgO bilayers. Journal of Applied Physics. 2017;121(22): 223902