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

Exchange through nonmagnetic insulating matrix

Exchange interactions between hard-magnetic particles in a nonmagnetic matrix are investigated by model calculations. A Landau–Ginzburg approach is developed to describe the net exchange interactions between spheres of arbitrary diameters. Introducing cylindrical coordinates and integrating over the surfaces of the adjacent spheres yields an exchange coupling which decreases with a decay length depending on interatomic exchange, intra-atomic exchange, and temperature. Typically, the decay length does not exceed a few interatomic distances. The decay is exponential but also contains a prefactor depending on the surface curvature of the grains. It increases with decreasing curvature, but this dependence is only a small correction to the leading exponential term

Exchange-coupling behavior in nanostructured FePt/Fe bilayer films

Different thicknesses of FePt/Fe bilayer films are deposited on (001) MgO substrates by sputtering Fe and Pt targets with in-situheating at 830°C. X-ray diffraction indicates a complete alignment of the FePt [001] axis with MgO [001] axis. The nucleation field Hn is estimated from hysteresis loops measured using a SQUID magnetometer. A FePt/Fe bilayer model is proposed to calculate the nucleation field Hn and compared with the experimental data. The model can explain experimental trends and gives useful predictions for nanostructure synthesis and further experiment

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

Structure and magnetic properties of nanostructured Dy/transition-metal multilayered films

We report the results of magnetic and microstructural studies for T/Dy (T=Fe, Co, Ni) compositionally modulated films prepared in a multiple-gun sputtering system. The perpendicular anisotropy and magnetization were measured systematically for X-Å Fe/Y-Å Dy and X-Å Co/Y-Å Dy films. The layer-thickness dependence of the magnetization for Co/Dy and Fe/Dy was interpreted in terms of the antiparallel coupling between transition-metal and Dy magnetic moments. For Co/Dy films the ranges of X and Y required for perpendicular anisotropy were determined. A comparision of the structural and magnetic properties of Ni/Dy, Co/Dy, and Fe/Dy is given and the origin of the perpendicular anisotropy is discussed. Journal of Applied Physics is copyrighted by The American Institute of Physics

EXTREMELY HIGH DENSITY MAGNETIC RECORDING MEDIA, WITH PRODUCTION METHODOLOGY CONTROLLED LONGITUDINAL/PERPENDICULAR ORIENTATION, GRAIN SIZE AND COERCIVITY

Disclosed is a modulated grain-composition magnetic recording material with up to terabit areal density recording capacity which, preferably, is produced by Sequential Vacuum deposition and Subsequent annealing procedures that allow Selective fabrication of magnetic material with desired grain size and coercivity, and with desired longitudinal or perpendicular magnetic particle “c-axis\u27 orientation. The preferred magnetic recording material has multiple layers of FePt/BO and/or Fe/Pt/BO, with minimum grain Size of approximately ten (10) nanometers, with perpendicularly oriented “c-axis”, and with coercivity (Hc) of up to twelve (12) K-Oe. The preferred fabrication procedure involves sequential sputter deposition of FePt and B2O3 layers, followed by an anneal step

Nonadiabatic Berry phase in nanocrystalline magnets

It is investigated how a Berry phase is created in polycrystalline nanomagnets and how the phase translates into an emergent magnetic field and into a topological Hall-effect contribution. The analysis starts directly from the spin of the conduction electrons and does not involve any adiabatic Hamiltonian. Completely random spin alignment in the nanocrystallites does not lead to a nonzero emergent field, but a modulation of the local magnetization does. As an explicit example, we consider a wire with a modulated cone angle

Structure and Magnetism of Mn5Ge3 Nanoparticles

In this work, we investigated the magnetic and structural properties of isolated Mn5Ge3 nanoparticles prepared by the cluster-beam deposition technique. Particles with sizes between 7.2 and 12.6 nm were produced by varying the argon pressure and power in the cluster gun. X-ray diffraction (XRD)and selected area diffraction (SAD) measurements show that the nanoparticles crystallize in the hexagonal Mn5Si3-type crystal structure, which is also the structure of bulk Mn5Ge3. The temperature dependence of the magnetization shows that the as-made particles are ferromagnetic at room temperature and have slightly different Curie temperatures. Hysteresis-loop measurements show that the saturation magnetization of the nanoparticles increases significantly with particle size, varying from 31 kA/m to 172 kA/m when the particle size increases from 7.2 to 12.6 nm. The magnetocrystalline anisotropy constant K at 50 K, determined by fitting the high-field magnetization data to the law of approach to saturation, also increases with particle size, from 0.4 × 105 J/m3 to 2.9 × 105 J/m3 for the respective sizes. This trend is mirrored by the coercivity at 50 K, which increases from 0.04 T to 0.13 T. A possible explanation for the magnetization trend is a radial Ge concentration gradient

Magnetically dilute metallic glasses. II. 4\u3ci\u3ef\u3c/i\u3e moments

Magnetic-susceptibility and high-field magnetization measurements are presented for amorphous Zr40Cu600-xMx (M denoting Gd and Tb), with x ranging from zero to ten. Effective moments of magnetic solutes were determined by fitting susceptibility data to the Curie-Weiss expression. The moments of Gd and Tb are very close to those expected for trivalent ions and the paramagnetic Weiss temperatures are positive. Saturation did not occur in any of the samples, even at 80 kOe and 1.3 K; however, the alloys containing Gd approached normalized magnetization values of unity. High-field hysteresis loops were used to obtain the temperature dependence of the coercive fields Hc and to determine ordering temperatures, defined as those temperatures for which Hc vanishes. The zero-field susceptibility for both the Gd and Tb alloys shows sharp peaks at low temperatures. The magnetically ordered state of the Gd alloys is characterized as spin-glass-like; that is, the spins are frozen in random directions and there are both ferromagnetic and antiferromagnetic exchange interactions, with the former dominant. The Tb alloys also have low-temperature magnetic states characterized as spin-glass-like in this sense. However, in the Tb alloys, the presence of local random anisotropy affects the high-field magnetization considerably. Attempts to fit the high-field magnetization of the Tb alloys to the local-random-anisotropy theory of Harris et al. are described

Curie temperature of multiphase nanostructures

The Curie temperature and the local spontaneous magnetization of ferromagnetic nanocomposites are investigated. The macroscopic character of the critical fluctuations responsible for the onset of ferromagnetic order means that there is only one Curie temperature, independent of the number of magnetic phases present. The Curie temperature increases with the grain size and is, in general, larger than predicted from the volume averages of the exchange constants. However, the Curie-temperature enhancement is accompanied by a relative reduction of the spontaneous magnetization. Due to the quadratic dependence of the permanent-magnet energy product on the spontaneous magnetization, this amounts to a deterioration of the magnets performance. The length scale on which an effective intergranular exchange coupling is realized (coupling length) depends on the Curie-temperature difference between the phases and on the spacial distribution of the local interatomic exchange. As a rule, it is of the order of a few interatomic distances; for much bigger grain sizes the structures mimic an interaction-free ensemble of different ferromagnetic materials. This must be compared to the magnetic-anisotropy coupling length, which is of the order of 10 nm. The difference is explained by the nonrelativistic character of the Curie-temperature problem