ABSTRACT
We studied (i) a set of three Co : Au continuous films, grown by sputtering co-deposition (â¼80 nm thick) with concentration ratios of 2 : 1, 1 : 1 and 1 : 0 (i.e., a pure Co film was also included), and (ii) a corresponding set of antidot arrays, produced by nanosphere lithography with the same hexagonal pattern (nominal lattice periodicity â¼520 nm). The samples were investigated by atomic and magnetic force microscopy and SQUID magnetometry. A twofold aim was fulfilled: to gain information on the magnetism of the CoAu compound (saturation magnetization, effective in-plane and out-of-plane anisotropy, exchange stiffness constant and magnetostrictive behavior) and to compare the magnetic behavior of the continuous and patterned samples. The continuous films exhibited a variety of hysteretic behaviours and magnetic configurations, ruled by the interplay between different magnetic anisotropy terms (magnetocrystalline, magnetoelastic and shape). The Co1Au1 film was anisotropic in the plane, whereas Co2Au1 and Co were isotropic and had an out-of-plane magnetization component; stripe domains were observed in Co2Au1, resulting in a transcritical hysteresis loop. A key role in determining these properties was ascribed to the magnetoelastic anisotropy term. Unlike the continuous films, the antidot arrays showed a similar hysteretic behavior and important similarities in the spin configuration were pointed out, despite the different compositions. We argue, also based on micromagnetic simulations, that this occurred because the nanopatterning enabled a local modification of the shape anisotropy, thus smoothing out the differences observed in the continuous films.
ABSTRACT
Ordered magnetic nanodot arrays with extremely high density provide unique properties to the growing field of nanotechnology. To overcome the size limitations of conventional lithography, a fine-tuned sputtering deposition process on mesoporous polymeric template fabricated by diblock copolymer self-assembly is herein proposed to fabricate uniform and densely spaced nanometer-scale magnetic dot arrays. This process was successfully exploited to pattern, over a large area, sputtered Ni80Fe20 and Co thin films with thicknesses of 10 and 13 nm, respectively. Carefully tuned sputter-etching at a suitable glancing angle was performed to selectively remove the magnetic material deposited on top of the polymeric template, producing nanodot arrays (dot diameter about 17 nm). A detailed study of magnetization reversal at room temperature as a function of sputter-etching time, together with morphology investigations, was performed to confirm the synthesis of long-range ordered arrays displaying functional magnetic properties. Magnetic hysteresis loops of the obtained nanodot arrays were measured at different temperatures and interpreted via micromagnetic simulations to explore the role of dipole-dipole magnetostatic interactions between dots and the effect of magnetocrystalline anisotropy. The agreement between measurements and numerical modelling results indicates the use of the proposed synthesis technique as an innovative process in the design of large-area nanoscale arrays of functional magnetic elements.
ABSTRACT
Ferrimagnetic glass-ceramics are potential candidates for magnetic induction hyperthermia, which is one form of inducing deep-regional hyperthermia, by using a magnetic field. The aim of this work was to analyse the influence of the amount of crystallised magnetite on the magnetic properties of glass-ceramic samples. Thus, two different ferrimagnetic glass-ceramics with the composition of the system Na(2)O-CaO-SiO(2)-P(2)O(5)-FeO-Fe(2)O(3) were prepared by melting at 1500 degrees C for 30 min of the coprecipitation-derived starting products. The X-ray diffraction patterns show the presence of nanometric magnetite crystals in a glassy matrix after cooling from melting temperature. The estimated amount of crystallised magnetite varies between 20 and 45 wt.%, as a function of the chemical composition. The morphology of the crystals was studied by scanning electron micrography and transmission electron micrography. Glass transition temperature and thermal stability were investigated by differential thermal analysis. Magnetic hysteresis cycles were analysed using a vibrating sample magnetometer with a maximum applied field of 17 kOe, at room temperature, in quasi-static conditions. Calorimetric measurements were carried out using a magnetic induction furnace. The power losses estimated from calorimetric measurements under a magnetic field of 40 kA/m and 440 kHz are 65 W/g for the glass-ceramic with lower iron oxides content and 25 W/g for the glass-ceramic with higher iron oxide content.
