Spin Vortices, Interlayer Exchange, and Magnetocalorics in Multilayers
Time: Fri 2023-01-27 09.00
Location: 4204, Hus 3, Albanovägen 29, Albanova
Subject area: Physics, Material and Nano Physics
Doctoral student: Milton Persson , Tillämpad fysik
Opponent: Professor Coriolan Tiusan, Babes-Bolyai University
Supervisor: Professor Vladislav Korenivski, Nanostrukturfysik; Professor David B. Haviland, Nanostrukturfysik
In this thesis we study strongly coupled pairs of spin vortices, in a configurationsuch that the cores attract while external fields act to pull them apart, with potential applications in magnetic random access memory (MRAM) and oscillators.Inertial decoupling of the cores has been studied and can be achieved with weak nanosecond pulses by the help of anharmonicity. Asymmetries and defects of the vortex pairs are studied in detail for characterization as well as utilization when introduced intentionally. A certain defect is found to lift a degeneracy between topologically protected states, with potential memory applications. Fine details in measurements and micromagnetic simulations reveal the beginning of an antivortex, as the shadow of the other vortex core. A vortex and an anti-vortex forms a bimeron, and a hysteresis is found with respect to its existence as a function of the core field strength.Interlayer exchange, as used in synthetic antiferromagnets and a range of spintronics applications, is studied in detail. Significant enhancement and tunability is found in systems of dilute ferromagentic films, by the introduction of monolayer-thinferromagnetic layers. The effects of interlayer exchange on thin antiferromagneticfilms are investigated. A strong effect is seen at thicknesses that optimize the films sensitivity due to finite size effects, and a multilayer system can be constructed with tunable exchange bias.Lastly we study the magnetocaloric effect (MCE), wherein demagnetization (ormagnetization) by the removal (application) of external fields causes a decrease(increase) in the temperature of the magnetic material. An inverse effect is studied in magnetic multilayers designed such that the application of a field causes ademagnetization in the active material. The proximity effects in these strong-weakstrong ferromagnetic trilayers result in an increased MCE compared to the directeffect of the field on the active MCE material (spacer). Atomistic spin dynamics areused to investigate the internal magnetization and significant enhancement is foundusing gradient spacers. Additionally, a membrane-based nanocalorimetry setup is constructed for direct measurements of the adiabatic temperature change, as a better measure of the MCE than the typically reported isothermal entropy change.The nonzero MCE at temperatures twice the intrinsic Curie temperature of the spacer, suggests the presence of a long-range exchange due to polarized conductionelectrons. With the long interaction range unfeasible in atomistic simulations, aphenomenological numerical model is developed to recreate the experimental results.