Tuning the properties of magnetic thin films by interaction with periodic nanostructures

Abstract

SummaryThe most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and out-of-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures.

Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy

“…The antidot lattices were produced by nanosphere lithography using commercial polystyrene (PS) nanospheres (Invitrogen) with a mean diameter of 210 ± 20 nm as starting material [36,37]. Close-packed monolayers of these PS spheres are deposited on Si 3 N 4 (500 nm, membranes)/Si(100) substrates by dip coating at an extraction velocity of 10 μs −1 and an angle of 60 • between the substrate surface and the air-water interface.…”

“…The deposited PS spheres were etched by an oxygen plasma (DC bias: −85 V) to reduce their diameter to 80 ± 20 nm or, alternatively, to 160 ± 20 nm. It has been shown previously that the etching process does not affect the spherical shape of the PS particles [36,37]. On top of these templates GdFe[0.36/0.36 nm] multilayer films were deposited with a 2 nm Al capping layer under UHV conditions by ion beam sputtering [38].…”

“…The pinning strength between different holes depends on the length of the domain wall at this position that is determined by the spacing between the two adjacent holes [20]. As nanosphere lithography results in a somewhat broad distribution of hole diameters and spacings [37], there is a corresponding distribution of the pinning strength at these holes. In turn, this leads to the observed distribution in coercive field H c .…”

Magnetic switching of nanoscale antidot lattices

“…For the plasma etching, an Oxford Plasma Technology Type 80Plus combined RIE (Reactive Ion Etching) and ICP (inductively coupled plasma) source was used [ 12 ]. The size reduction of the PS spheres was done in an oxygen plasma with a 25 W RIE component and 100 W ICP component at a DC bias voltage of −80 V. For PS spheres with an initial diameter of 200 nm, the achieved diameter as function of etching time has been determined in detail in advance [ 12 , 14 ]. Due to the plasma’s anisotropic component, PS particles lost their spherical shape and became oblate.…”

“…In magnetic antidot films, the nanoscale periodic structure of holes introduces an in-plane shape anisotropy to the otherwise isotropic in-plane properties of polycrystalline or amorphous thin films. Additionally, the holes may act as artificial pinning sites for domain walls, blocking or even guiding their movement in specific directions during magnetic reversal [ 13 – 14 ]. Moreover, magnetic antidot lattices eventually lead to the formation of magnetic vortex structures [ 15 ].…”