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Abstract: Epitaxial films of the ferromagnetic manganite La0.7⁢Sr0.3⁢MnO3 on substrates of the ferroelectric perovskite BaTiO3 are known to display sharp magnetic changes and large magnetoelectric effects when the film is strained by the substrate undergoing thermally driven structural transitions and ferroelectric domain switching, respectively. However, only a single component of the in-plane magnetization has been hitherto imaged. Here we present magnetic vector maps—obtained from photoemission electron microscopy images with magnetic contrast from x-ray magnetic circular dichroism—to show that the electrically and thermally driven changes of local and global magnetization are deterministically influenced by the state of the substrate while also being complex and sample dependent. Our findings, supported by ferromagnetic resonance data and vibrating sample magnetometry, reveal that the behavior of La0.7⁢Sr0.3⁢MnO3 films on BaTiO3 substrates is not well predicted from knowledge of each system, probably due to long-range strain between BaTiO3 domains. In the future, it would be interesting to reduce complexity by patterning the film into regions between which magnetic communication is negligible.

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Abstract: he pyrochlore vanadates are compelling candidates for next-generation dissipationless devices. and are ferromagnetic insulators (T 70 K) that are believed to exhibit the magnon Hall effect and are expected to host topological magnons. Their completely dissipationless magnon edge states could be harnessed to realize low-power information transport in spintronic or magnonic devices. As a crucial step in the realization of devices, we synthesize the first thin films of pyrochlore on isostructural substrates and explore the evolution of their magnetic properties down to the ultrathin limit. All films are insulating ferromagnets with transition temperatures of up to the bulk value (T 68 K) that decrease with thickness according to finite-size effects. Our films also exhibit a change in anisotropy from in-plane to out-of-plane easy axis coincident with the development of partial strain relaxation and nonzero magnetic hysteresis in an applied field. This evolution demonstrates the impact of strain on magnetic anisotropy and paves the way to tunable magnon topology.

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Abstract: Heterostructures composed of heavy metal and van der Waals (vdW) magnets serve as platforms to investigate magnetotransport properties, enabling the electric readout of the spin-flop transition in the vdW antiferromagnet. We investigate the spin and orbital contributions to magnetism in Pt/exfoliated multilayer CrPS4 heterostructure using the synchrotron-radiation based x-ray magnetic circular dichroism technique measured in the total electron yield (TEY) mode. The TEY detection, with probing depth of 5–10 nm, mainly reflects the interfacial magnetic behavior near the Pt/CrPS4 boundary. A spin-flop transition appears near 0.7 T in both the CrPS4 single crystal and the Pt/CrPS4 heterostructures. The total Cr moment remains ∼2 μB/f.u. in both systems at 14 T and 6 K. In Pt/CrPS4, the orbital moment is strongly modulated by Pt, as manifested in the enhancement from ∼0.1 μB/f.u. in CrPS4 to ∼0.5 μB/f.u. in Pt/CrPS4, an effect attributable to the strong spin–orbit coupling with Pt. At 25 K, the total Cr moment reduces to ∼1.1 μB/f.u. in both systems. The Cr orbital moment in CrPS4 remains low ∼0.1 μB/f.u., whereas in Pt/CrPS4 it remains high ∼0.5 μB/f.u. These findings provide qualitative evidence of robust spin–orbit coupling and orbital hybridization at Pt/CrPS4 interface, and highlight the potential of heavy metal/vdW antiferromagnet heterostructures for spin-orbitronic device applications.

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Abstract: Cobalt ferrite nanoparticles are a benchmark among low-to-medium energy alternatives to rare-earth permanent magnets, although their intrinsic behavior is often obscured by surface disorder, finite-size effects, and superparamagnetic relaxation. Here, we overcome these limitations by synthesizing large, highly crystalline cobalt-doped ferrite nanoparticles (≈ 25 nm), which remain blocked at room temperature and thus provide a clean platform to disentangle the fundamental role of cobalt in the spinel lattice. By systematically varying the cobalt content, we reveal a complex interplay between cation distribution, oxygen vacancy formation, and magnetic response. Structural and compositional analysis confirms predominant Co2+ occupancy at octahedral sites, accompanied by a redistribution of Fe2+/Fe3+ and non-linear oxygen vacancy generation. We find that while saturation magnetization is largely governed by defect chemistry, the coercivity and effective anisotropy are primarily controlled by cobalt incorporation and saturate at intermediate compositions. In contrast, thermomagnetic analysis reveals an anomalous evolution of magnetization at intermediate temperatures for specific cobalt contents. This behavior is consistent with a change in the anisotropy landscape, suggestive of a growing contribution from higher-order anisotropy terms, rather than a simple uniform increase in magnetocrystalline anisotropy. These results indicate that cobalt doping tunes the balance between different anisotropy contributions in a composition- and temperature-dependent manner. Overall, our findings highlight the subtle interplay between cation distribution, anisotropy landscape, and thermal stability in spinel ferrites, providing fundamental insight for the design of high-coercivity rare-earth-free nanomagnets.

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Abstract: Superfluid helium droplets represent a unique state of matter, which are large clusters of helium typically containing approximately 103-1011 helium atoms and exhibit remarkable properties such as superfluidity, a very low temperature (0.37 K) and high thermal conductivity. This PhD project investigates two novel aspects of superfluid helium droplets: the use of superfluid helium nanodroplets as the nanoreactors to grow magnetic nanoparticles and the generation and exploration of quantum vortices in a controlled manner. In the first part, we exploit the very low temperature and rapid cooling to develop a new approach for fabricating magnetic nanoparticles. For the very low temperature and the ultrahigh thermal conductivity, superfluid helium can suppress thermal effects during the atom-by-atom growth of magnetic nanoparticles, making the relatively weak exchange interactions (compared with metallic bonding) the driving force. As a result, the atomic spins align for ferromagnetic elements and thus the magnetic moments of nanoparticles are maximized. In particular, we focus on iron nanoparticles coated by a gold shell and investigate their properties by electron microscopy (for structural investigation) and x-ray circular dichroism (XMCD), for magnetic property measurement) at the Diamond Light Source. We first study mass spectrometry of small iron clusters and observed abnormal behaviours. Unlike other molecular clusters formed in helium droplets such as water, gold and silver, which typically follow a Poisson distribution, Fe+ channel was found to be far greater than that of FeN+ (N = 2-8) clusters. We postulate this as an indicator for the formation of high-spin iron clusters inside superfluid helium and attempt to provide an interpretation based on DFT calculations. However, XMCD showed an expected low magnetisation for Fe/Au core-shell nanoparticles which is even lower than iron oxide nanoparticles, indicating that the neutral Fe atoms are oxidized into Fe2+ within the nanoparticles which is magnetically inert. This is accounted by the very high electron negativity of Au atoms and the alloying effect during the growth of nanoparticles, which dismisses the magnetic properties. Our work shows that the choice of protective shell is important to maintain the magnetic properties of iron nanoparticles and points the direction for the next-step research. The second part presents a breakthrough in quantum vortex research. We demonstrate a novel method for generating controlled vortex arrays in superfluid helium droplets through collisions with cesium ions. Subsequential addition of metal atoms (Ag and Au) to helium droplets and the formation of nanodroplets allow the vortex lattices to be imaged after the nanoparticles are deposited onto a solid surface. By this approach we have revealed a record-high vortex density of 5.6×10¹⁴ m⁻², exceeding previous observations in bulk superfluid helium by more than six orders of magnitude. This unprecedented vortex density opens new possibilities for studying quantum hydrodynamics at extreme angular momenta and investigating quantum turbulence in previously inaccessible regimes. Through detailed theoretical analysis and experimental characterization, this work establishes superfluid helium droplets as a versatile platform for both materials synthesis and fundamental research. Our findings not only advance the understanding of superfluidity but also provide a new pathway for developing high-performance magnetic nanomaterials that can potentially revolutionize biomedical science and technologies.