RESUMEN
Nanoscale artificially engineered spintronic materials could be used to enlarge the storage density of magnetic recording media. For this purpose, magnetic nanostructures such as antidot arrays exhibiting high uniaxial magnetic anisotropy are new contestants in the field of ultrahigh density magnetic data storage devices. In this context, we focus on the synthesis of nanostructured magnetic materials consisting of Dy-Fe alloyed antidot thin films, deposited onto the surface of nanoporous alumina membranes served as patterned templates. Noticeable variations of in the in-plane magnetic anisotropy have been observed by modifying the layer thickness at both microscopic and macroscopic scales. The microscopic magnetic properties have been locally studied by Nano-MOKE magnetometry. For thinner antidot samples with 15, 20 and 25 nm in thickness, a tri-axial in-plane magnetic anisotropy has been detected. Meanwhile, for thicker antidot samples (40-60 nm of layer thickness), an in-plane uniaxial magnetic anisotropy has been noted. We attribute these changes in the magnetic anisotropy to the strong correlation between the edge-to-edge distance among adjacent nanoholes, W, and the local magnetic anisotropy of antidot samples. The effective magnetic anisotropy exhibits an unexpected crossover from the in-plane to out-of-plane direction due to the increasing of the effective perpendicular magnetic anisotropy with varying the layer thickness of antidot thin films. Therefore, we detected a critical layer thickness, t = 25 nm for the Dy-Fe alloy antidot arrays, at which the appearance of the perpendicular magnetization is observed. Furthermore, an enhancement in the Curie temperature of the antidot arrays compared to the continuous thin films has been obtained. We attribute these effects to the complex magnetization reversal processes and the high thermal stability of the hexagonal structure of antidot arrays. These findings can be of high interest for the development of novel magnetic sensors and for thermo-magnetic recording patterned media based on template-assisted deposition techniques.
RESUMEN
The magnetocaloric response of Ni-Cu based multilayers has been studied with the aim of optimizing their magnetic field dependence. In contrast to the behavior of single phase materials, whose peak magnetic entropy change follows a power law with exponents close to 0.75, multilayering leads to exponents of -1 for an extended temperature span close to the transition temperature. This demonstrates that nanostructuring can be a good strategy to enhance the magnetic field responsiveness of magnetocaloric materials.
RESUMEN
The Maxwell relation, the Clausius-Clapeyron equation, and a non-iterative method to obtain the critical exponents have been used to characterize the magnetocaloric effect (MCE) and the nature of the phase transitions in Pr0.5Sr0.5MnO3, which undergoes a second-order paramagnetic to ferromagnetic (PM-FM) transition at TC ~ 247 K, and a first-order ferromagnetic to antiferromagnetic (FM-AFM) transition at TN ~ 165 K. We find that around the second-order PM-FM transition, the MCE (as represented by the magnetic entropy change, ΔSM) can be precisely determined from magnetization measurements using the Maxwell relation. However, around the first-order FM-AFM transition, values of ΔSM calculated with the Maxwell relation deviate significantly from those calculated by the Clausius-Clapeyron equation at the magnetic field and temperature ranges where a conversion between the AFM and FM phases occurs. A detailed analysis of the critical exponents of the second-order PM-FM transition allows us to correlate the short-range type magnetic interactions with the MCE. Using the Arrott-Noakes equation of state with the appropriate values of the critical exponents, the ï¬eld- and temperature-dependent magnetization [Formula: see text] curves, and hence the [Formula: see text] curves, have been simulated and compared with experimental data. A good agreement between the experimental and simulated data has been found in the vicinity of the Curie temperature TC, but a noticeable discrepancy is present for [Formula: see text]. This discrepancy arises mainly from the coexistence of AFM and FM phases and the presence of ferromagnetic clusters in the AFM matrix.