We demonstrate a simple method to tailor the magnetization reversal mechanisms of Co/Pt multilayers by depositing them onto large area nanoporous anodized alumina (AAO) with various aspect ratios, A = pore depth/diameter. Magnetization reversal of composite (Co/Pt)/AAO films with large A is governed by strong domain-wall pinning which gradually transforms into a rotation-dominated reversal for samples with smaller A, as investigated by a first-order reversal curve method in conjunction with analysis of the angular dependent switching fields. The change of the magnetization reversal mode is attributed to topographical changes induced by the aspect ratio of the AAO templates.
Co ∕ Pt multilayers deposited on anodized alumina substrates are introduced as percolated perpendicular media. The pores act as pinning sites due to variations of thickness and anisotropy direction around the perimeters. Coercivity, domain size, and switching field can be engineered by controlling pore density. The media exhibit strong perpendicular anisotropy and the switching field remains unchanged at its minimum up to an angular deviation of 50° from the easy axis. A better tolerance of switching-field distributions can thus be achieved, which may help to achieve a high signal-to-noise ratio. The thermal stability of the proposed media is investigated by micromagnetic simulations.
Large-area, over several square centimeters, mesoporous array of magnetic nanostructure with perpendicular anisotropy is prepared by depositing Co/Pt multilayers (MLs) on a mesopore array of anodized alumina (AAO) fabricated on Si wafers. The MLs are mainly deposited on the top of AAO walls and perimeters of the pores; very small amounts of magnetic material reach the bottom due to the high aspect ratio of AAO. Consequently, ordered pores are present in the magnetic MLs. The mean pore diameter of the fabricated mesoporous array is 8.83 nm with a standard deviation of 3.16 nm and density of about 2.1 × 10(11) cm(-2). The Co/Pt MLs deposited on AAO and Si both exhibit strong perpendicular magnetic anisotropy, but the perpendicular coercivity (H(c)) increases by 15 times on AAO compared to that on Si. On the other hand, the magnetic cluster size decreases from 1000 nm (on Si) to 100 nm due to the presence of high-density pores. The dramatic increase in H(c) and the decrease in magnetic cluster size suggest that the pores behave as effective pinning sites. The magnetization-switching characteristics of the fabricated porous structure are different from those of the continuous films or Stoner-Wohlfarth-type (S-W) particles. One of the potential applications of this mesoporous structure may be in the field of high-density magnetic data storage.
The exchange bias field (Hex) of IrMn/CoFe deposited on a large-area nanoporous anodized alumina (AAO) is tailored by varying the pore density (D) and the network width (w) of AAO. The Hex increases with increasing D and reaches its maximum at D=5.6×1010 cm−2 and w∼28 nm. The enhancement in Hex, twice larger than that of the continuous film, is attributed to the reduction in antiferromagnetic (AFM) and ferromagnetic (FM) domain sizes. The suppression of Hex is observed for further increase in D, which may result from the excessive misalignment of AFM and FM spins and weakened AFM anisotropy.
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