Ferromagnetic/antiferromagnetic bilayers are interfaced with normal metal/ferromagnetic bilayers to form F*/AF/N/F valves. The N-spacer thickness is chosen such that it mediates strong indirect exchange [Ruderman-Kittel-Kasuya-Yosida (RKKY)] between the outer ferromagnetic layers, which varies in strength/direction depending on the N thickness and changes its direction on switching F. The system exhibits a strong modulation of the F*/AF exchange bias, oscillating in strength synchronously with the oscillation in the interlayer RKKY exchange across the normal metal spacer. The effect is explained as due to a superposition taking place within the antiferromagnetic layer of the direct-exchange proximity effect from the F*/AF interface and the indirect RKKY exchange from F penetrating AF via N. The modulation, expressed via the strength of the F*/AF bias field, reaches 400% at the first RKKY peak.

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.

We study magnetic multilayers, incorporating dilute ferromagnetic spacers between strongly-ferromagnetic layers exhibiting a proximity-enhanced magnetocaloric effect (MCE). Using magnetometry and direct measurements of the adiabatic temperature change based on a nanomembrane-calorimetry, we find that the MCE in the studied multilayer is indeed enhanced compared to that in the bulk spacer material. We develop a phenomenological numerical model of the studied trilayer and find that a long-range exchange interaction through the weakly-ferromagnetic spacer is required to adequately describe the magnetic and magnetocaloric properties of the system.

The demagnetization and associated magnetocaloric effect (MCE) in strong-weak-strong ferromagnetic trilayers, upon a reorientation of the strong ferromagnets from parallel to antiparallel (AP) magnetization, is simulated using atomistic spin dynamics. The simulations yield non-trivial spin distributions in the AP state, which in turn allows entropy to be calculated directly. The influence of longer-range spin–spin interactions and of variable strength of the external switching field are investigated. Finally, we find that the MCE in the system can be significantly improved by allowing the local exchange to vary through the spacer, which in practice can be implemented by spatially tailoring the spacer's magnetic dilution.

We demonstrate a 20-fold enhancement in the strength of the Ruderman-Kittel-Kasuya-Yosida interlayer exchange in dilute-ferromagnet/normal-metal multilayers by incorporating ultrathin Fe layers at the interfaces. Additionally, the resulting increase in the interface magnetic polarization profoundly affects the finite-size effects, sharpening the Curie transition of the multilayer, while allowing us to separately tune its Curie temperature via intra-layer magnetic dilution. These results should be useful for designing functional materials for applications in magnetocaloric micro-refrigeration and thermally assisted spin-electronics.

Effects of magnetic asymmetry on strongly coupled spin-vortex pairs with parallel core polarization and antiparallel chirality in synthetic nanomagnets are investigated. This includes vortex-core length asymmetry, biasing field asymmetry, and pinning of one of the two vortex cores. Our experimental observations as well as analytical and micromagnetic modeling show how magnetic asymmetry can be used to differentiate magneto-resistively otherwise degenerate multiple stable states of a vortex pair. These results expand the knowledge base for spin vortex arrays in nanostructures and should be useful in light of the recent proposals on coding information into multiple topological spin states, such as single and multiple vortex core/chirality states.

Spin vortices in magnetic nanopillars are used as GHz oscillators, with frequency however essentially fixed in fabrication. We demonstrate a model system of a two-vortex nanopillar, in which the resonance frequency can be changed by an order of magnitude, without using high dc magnetic fields. The effect is due to switching between the two stable states of the vortex pair, and we show that it can be done with low-amplitude fields of sub-ns duration. We detail the relevant vortex-core dynamics and explain how field anharmonicity and phase control can be used to enhance the performance.

Ferromagnetic/antiferromagnetic bilayers are interfaced with normal metal/ferromagnetic bilayers to form F*/AF/N/F valves. The N-spacer thickness is chosen such that it mediates strong indirect exchange (RKKY) between the outer ferromagnetic layers, which varies in strength/direction depending on the N thickness and in direction on switching F. The system exhibits a strong modulation of the F*/AF exchange bias, oscillating in strength synchronously with the oscillation in the interlayer RKKY exchange across the normal metal spacer. The effect is explained as due to a superposition taking place within the antiferromagnetic layer of the direct-exchange proximity effect from the F*/AF interface and the indirect RKKY exchange from F penetrating AF via N. The modulation, expressed via the strength of the F*/AF bias field, reaches 400% at the first RKKY peak.