The micromagnetic exchange stiffness is a critical parameter in numerical modeling of magnetization dynamics and reversal processes, yet the current literature reports a wide range of values even for such simple and widely used material as Cobalt.With use of ab-intio estimated Heisenberg parameters we calculate the low temperature micromagnetic exchange stiffness parameters for hexagonal-close-packed (HCP) and face-centred cubic Cobalt without previous and our own. For HCP Co they are slightly different in the directions parallel and perpendicular to the c-axis. We establish the exchange stiffness scaling relation with magnetization A(m) ∼ m 1.8 valid for all sets of parameters for a wide range of temperatures. For HCP Co we find an anisotropic domain wall width in the range 24 − 29 nm which increases slowly with temperature. The results form a critical input for large-scale temperature-dependent micromagnetics simulations and demonstrate the importance of correct parameterization for accurate simulation of magnetization dynamics.

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Fe 3 O 4 nanoparticles are one of the most promising candidates for biomedical applications such as magnetic hyperthermia and theranostics due to their bio-compatibility, structural stability and good magnetic properties. However, much is unknown about the nanoscale origins of the observed magnetic properties of particles due to the dominance of surface and finite size effects. Here we have developed an atomistic spin model of elongated magnetite nanocrystals to specifically address the role of faceting and elongation on the magnetic shape anisotropy. We find that for faceted particles simple analytical formulae overestimate the magnetic shape anisotropy and that the underlying cubic anisotropy makes a significant contribution to the energy barrier for moderately elongated particles. Our results enable a better estimation of the effective magnetic anisotropy of highly crystalline magnetite nanoparticles and is a step towards quantitative prediction of the heating effects of magnetic nanoparticles. arXiv:1909.02470v1 [cond-mat.mes-hall] 5 Sep 2019 2/16 10/16 16/16

Understanding the domain wall dynamics is an important issue in modern magnetism. Here we present results of domain wall displacement in curved cylindrical nanowires at a constant magnetic field. We show that the average velocity of a transverse domain wall increases with curvature. Contrary to what it is observed in stripes, in a curved wire the transverse domain wall oscillates along and rotates around the nanowire with the same frequency. These results open the possibility of new oscillation-based applications.

We have developed a new method to study the oxygen surface exchange kinetics in oxide materials in the form of epitaxial thin films by analyzing subtle cell parameter variations induced by changes in the oxygen stoichiometry of the material. The method consists of continuously analyzing the X-ray diffraction pattern of particular film reflections with a linear X-ray fast detector in a static position, while exposing the sample to sudden changes in the pO 2 of the atmosphere at elevated temperatures. With this method, we have been able to follow cell parameter changes as small as 2.10 −4 Å in time intervals as short as 10 s in La 2 NiO 4+δ epitaxial films and La 2 NiO 4+δ /LaNiO 3−δ bilayers. This method provides a simpler and contactless tool for dynamically analyzing oxygen surface exchange kinetics and diffusion in transition metal oxide compounds, and complements other currently used techniques such as Electric Conductivity Relaxation (ECR) and Isotopic Exchange depth profiling (IEDP). In addition, this method is a unique tool to address oxygen transport across solid−solid interfaces in thin film heterostructures.

Based on magnetization, specific heat, magnetostriction and neutron-diffraction studies on single-crystal TbCo(2)B(2)C, it is found out that the paramagnetic properties, down to liquid nitrogen temperatures, are well described by a Curie-Weiss behavior of the Tb(3+) moments. Furthermore, below T(c) = 6.3 K, the Tb sublattice undergoes a ferromagnetic (FM) phase transition with the easy axis being along the (100) direction and, concomitantly, the unit cell undergoes a tetragonal-to-orthorhombic distortion. The manifestation of an FM state in TbCo(2)B(2)C is unique among all other isomorphous borocarbides, in particular TbNi(2)B(2)C (T(N) = 15 K, incommensurate modulated magnetic state) even though the Tb ions in both isomorphs have almost the same crystalline electric field properties. The difference among the magnetic modes of these Tb-based isomorphs is attributed to a difference in their exchange couplings which are in turn caused by a variation in their lattice parameters and in the position of their Fermi levels.

In all cases the values correspond to their particular defect equilibrium and degree of charge localization. The oxygen surface exchange kinetics was also evaluated from in-situ time-resolved analyses of the cell parameter variations. LSC, LNO and GBCO films show fast oxygen reduction kinetics, k chem = 5 · 10 −6 , 3 · 10 −6 , and 2 · 10 −7 cm/s at 700 • C, respectively, in relative agreement with reported values, while BSCF films show much slower kinetics than expected, below k chem = 10 −7 cm/s at 650 • C, related to the degradation process observed in the films.

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