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Analytical calculations and micromagnetic simulations are used to determine the Berry curvature and topological Hall effect (THE) due to conduction electrons in small ferromagnetic particles. Our focus is on small particles of nonellipsoidal shapes, where noncoplanar spin structures yield a nonzero topological Hall signal quantified by the skyrmion number Q. We consider two mechanisms leading to noncoplanarity in aligned nanoparticles, namely flower-state spin configurations due to stray fields near corners and edges, and curling-type magnetostatic selfinteractions. In very small particles, the reverse magnetic fields enhance Q due to the flower state until the reversal occurs, whereas for particles with a radius greater than coherence radius Rcoh the Q jumps to a larger value at the nucleation field representing the transition from the flower state to the curling state. We calculate the Skyrmion density (average Berry curvature) from these spin structures as a function of particle size and applied magnetic field. Our simulation results agree with analytical calculations for both flower state and flux closure states. We showed the presence of Berry curvature in small particles as long as the size of the particle is less than the single domain limit. Using magnetic force microscopy (MFM), we also showed that in a nanodot of Co with a suitable size, a magnetic vortex state with perpendicular (turned-up) magnetization at the core is realized which can be manifested for Berry curvature and emergent magnetic field in confined geometries for single domain state at room temperature.

Topological phase in magnetic materials yields a quantized contribution to the Hall effect known as the topological Hall effect, which is often caused by skyrmions, with each skyrmion creating a magnetic flux quantum h/e. The control and understanding of topological properties in nanostructured materials is the subject of immense interest for both fundamental science and technological applications, especially in spintronics. In this work, the electron-transport properties and spin structure of exchange-coupled cobalt nanoparticles with an average particle size of 13.7 nm are studied experimentally and theoretically. Magnetic and Hall-effect measurements identify topological phase transitions in the exchange-coupled cobalt nanoparticles and were used to discover a qualitatively new type of hysteresis in the topological Hall effect namely, Berry-phase hysteresis. Micromagnetic simulations reveal the origin of the topological Hall effect namely, the chiral domains, with domain-wall chirality quantified by an integer skyrmion number. These spin structures are different from the skyrmions formed due to Dzyaloshinskii Moriya interactions in B20 crystals and multilayered thin films, and caused by cooperative magnetization reversal in the exchange-coupled cobalt nanoparticles. An analytical model is developed to explain the underlying physics of Berry-phase hysteresis, which is strikingly different from the iconic magnetic hysteresis and constitutes one aspect of 21st-century reshaping of our view on nature at the borderline of physics, chemistry, mathematics, and materials science., Comment: 29 pages and 16 images

The spin structure of exchange-coupled MnBi:Co-Fe bilayers is investigated by X-ray magnetic circular dichroism (XMCD), polarized neutron reflectometry (PNR), and micromagnetic simu-lations. The purpose of the present research is two-fold. First, the current search for new permanent-magnet materials includes hard-soft nanocomposites, and the analysis of coercivity mechanisms in these structures is an important aspect of this quest. Second, topological micro-magnetic structures such as skyrmions have recently become of intense fundamental and applied research, for example in the context of spin-based electronics. We find that the magnetization reversal of the MnBi:Co-Fe bilayer structure involves a curling-type twisting of the magnetization in the film plane. This curling in the exchange-coupled hard-soft magnetic bilayers is reminiscent of chiral spin structures known as bobbers and, in fact, establishes a new type of skyrmionic spin structure.

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all factors that can be tuned with appropriate chemistry and thermal-magnetic annealing.

The Cobalt-germanium (Co-Ge) is a fascinating complex alloy system that has unique structure and exhibit range of interesting magnetic properties which would change when reduce to nanoscale dimension. At this experimental work, the high-aspect-ratio Co5Ge3 nanoparticle with average size of 8nm was synthesized by gas aggregation-type cluster-deposition technology. The nanostructure morphology of the as-made binary Co5Ge3 nanoparticles demonstrate excellent single-crystalline hexagonal structure with mostly preferable growth along (110) and (102) directions. In contrast the bulk possess Pauli paramagnetic spin-order at all range of temperature, here we discover size-driven new magnetic ordering of as-synthesized Co5Ge3 nanoparticles exhibiting ferromagnetism at room temperature with saturation magnetization of Ms = 32.2 emu/cm3. This is first report of observing such new magnetic spin ordering in this kind of material at nano-size which the magnetization has lower sensitivity to thermal energy fluctuation and exhibit high Curie temperature close to 850 K. This ferromagnetic behavior along with higher Curie temperature at Co5Ge3 nanoparticles are attributes to low-dimension and quantum-confinement effect which imposes strong spin coupling and provides a new set of size-driven spin structures in Co5Ge3 nanoparticle which no such magnetic behavior being present in the bulk of same material. This fundamental scientific study provides important insights into the formation, structural, and the magnetic property of sub 10nm Co5Ge3 nanostructure which shall lead to promising practical versatile applications for magneto- germanide based nano-devices., Comment: Wrong Submission. My plan is to submit later

The effects of strain and hydrogenation on the electronic and magnetic properties of monolayers of Ta based dichalcogenides (TaX2; X = S, Se, Te) are investigated using density-functional theo-ry. We predict a complex scenario of strain-dependent magnetic phase transitions involving par-amagnetic, ferromagnetic, and modulated antiferromagnetic states. Covering one of the two chalcogenide surfaces with hydrogen switches the antiferromagnetic/nonmagnetic TaX2 mono-layers to a semiconductor. Our research opens new pathways towards the manipulation of mag-netic properties for future optoelectronics and spintronics applications., Comment: 13 pages, 5 figures