Nanostructured exchange coupled hard / soft composites: from the local magnetization profile to an extended 3D simple model

In nanocomposite magnetic materials the exchange coupling between phases plays a central role in the determination of the extrinsic magnetic properties of the material: coercive field, remanence magnetization. Exchange coupling is therefore of crucial importance in composite systems made of magnetically hard and soft grains or in partially crystallized media including nanosized crystallites in a soft matrix. It has been shown also to be a key point in the control of stratified hard / soft media coercive field in the research for optimized recording media. A signature of the exchange coupling due to the nanostructure is generally obtained on the magnetization curve $M(H)$ with a plateau characteristic of the domain wall compression at the hard/soft interface ending at the depinning of the wall inside the hard phase. This compression / depinning behavior is clearly evidenced through one dimensional description of the interface, which is rigorously possible only in stratified media. Starting from a local description of the hard/soft interface in a model for nanocomposite system we show that one can extend this kind of behavior for system of hard crystallites embedded in a soft matrix.Comment: 18 pages, 8 figures. To be published in the Journal of Magnetism and Magnetic Materials. (To be found at http://www.sciencedirect.com/science/journal/03048853

On the Ising character of the quantum-phase transition in LiHoF\u3csub\u3e4\u3c/sub\u3e

It is investigated how a transverse magnetic field affects the quantum-mechanical character of LiHoF4, a system generally considered as a textbook example for an Ising-like quantum-phase transition. In small magnetic fields, the low-temperature behavior of the ions is Ising-like, involving the nearly degenerate low-lying Jz = ± 8 doublet. However, as the transverse field increases, there is a substantial admixture of states having | Jz | \u3c 8. Near the quantum-phase-transition field, the system is distinctively non-Ising like, and all Jz eigenstates yield ground-state contributions of comparable magnitude. A classical analog to this mechanism is the micromagnetic single point in magnets with uniaxial anisotropy. Since Ho3+ has J = 8, the ion’s behavior is reminiscent of the classical limit (J = ∞), but quantum corrections remain clearly visible

Exact nucleation modes in arrays of magnetic particles

Magnetization reversal in arrays of magnetic nanoparticles with perpendicular anisotropy is investigated. Aside from domain-wall propagation effects, the reversal involves two main aspects: the nucleation behavior of individual particles and interparticle interactions. Due to magnetostatic self-interaction effects, the interparticle interaction cannot be reduced to a stray-field correction to the external field. Exact nucleation fields and explicit stray-field and self-interaction contributions are obtained for rings of equidistant dots. An exact treatment of self-interactions in various structurally inhomogeneous but rotationally symmetric wire, sphere, and thin-film nanostructures leads to renormalization of the uniaxial anisotropy. Finally, an approximate method to calculate nucleation fields is discussed

Nonzero macroscopic magnetization in half-metallic antiferromagnets at finite temperatures

Combining density-functional theory calculations with many-body Green's-function technique, we reveal that the macroscopic magnetization in half-metallic antiferromagnets does not vanish at finite temperature as for the T=0 limit. This anomalous behavior stems from the inequivalent magnetic sublattices which lead to different intrasublattice exchange interactions. As a consequence, the spin fluctuations suppress the magnetic order of the sublattices in a different way leading to a ferrimagnetic state at finite temperatures. Computational results are presented for the half-metallic antiferromagnetic CrMnZ (Z=P,As,Sb) semi-Heusler compounds.Comment: 4 pages, 2 figure

Boundary conditions and Berry phase in magnetic nanostructures

The effect of micromagnetic boundary conditions on the Berry curvature and topological Hall effect in granular nanostructures is investi- gated by model calculations. Both free surfaces and grain boundaries between interacting particles or grains affect the spin structure. The Dzyaloshinskii-Moriya interactions yield corrections to the Erdmann-Weierstrass boundary conditions, but the Berry curvature remains an exclusive functional of the local spin structure, which greatly simplifies the treatment of nanostructures. An explicit example is a model nanostructure with cylindrical symmetry whose spin structure is described by Bessel function and which yields a mean-field-type Hall-effect contribution that can be related to magnetic-force-microscopy images

Surface-induced cubic anisotropy in nanomagnets

We investigate the effect of surface anisotropy in a spherical many-spin magnetic nanoparticle. By computing minor loops, two-dimensional (2D) and 3D energyscape, and by investigating the behavior of the net magnetization, we show that in the case of not too strong surface anisotropy the behavior of the many-spin particle may be modeled by that of a macrospin with an effective energy containing uniaxial and cubic anisotropy terms. This holds for both the transverse and N\'eel's surface anisotropy models.Comment: 7 pages, 8 figure

Immobilization diffusion in R\u3csub\u3e2\u3c/sub\u3eFe\u3csub\u3e17\u3c/sub\u3e nitrides

The diffusion of nitrogen in R2Fe17 intermetallics is investigated by an approach which reconciles the solid-solution and immobilization theories of nitrogen diffusion. It turns out that two-sublattice diffusions may yield sharp concentration profiles but leave the phase structure of the nitride unchanged. Beside the reaction time and a local relaxation time there exists a global relaxation time which governs the smoothing of concentration gradients. Due to the large number of unknown energy parameters involved it is difficult to make quantitative predictions, but in general the diffusion behavior depends on factors such as the use of N2 or NH3 as a nitrogen source

Size-dependent spin-reorientation transition in Nd2Fe14B nanoparticles

Nd2Fe14B magnetic nanoparticles have been successfully produced using a surfactant-assisted ball milling technique. The nanoparticles with different size about 6, 20 and 300 nm were obtained by a size-selection process. Spin-reorientation transition temperature of the NdFeB nanoparticles was then determined by measuring the temperature dependence of DC and AC magnetic susceptibility. It was found that the spin-reorientation transition temperature (Tsr) of the nanoparticles is strongly size dependent, i.e., Tsr of the 300 nm particles is lower than that of raw materials and a significant decrease was observed in the 20 nm particles

Exact nucleation modes in arrays of magnetic particles

Magnetization reversal in arrays of magnetic nanoparticles with perpendicular anisotropy is investigated. Aside from domain-wall propagation effects, the reversal involves two main aspects: the nucleation behavior of individual particles and interparticle interactions. Due to magnetostatic self-interaction effects, the interparticle interaction cannot be reduced to a stray-field correction to the external field. Exact nucleation fields and explicit stray-field and self-interaction contributions are obtained for rings of equidistant dots. An exact treatment of self-interactions in various structurally inhomogeneous but rotationally symmetric wire, sphere, and thin-film nanostructures leads to renormalization of the uniaxial anisotropy. Finally, an approximate method to calculate nucleation fields is discussed

Magnetism of Ta Dichalcogenide Monolayers Tuned by Strain and Hydrogenation

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 figure