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

While the magnetic properties of nanoscaled antidot lattices in in-plane magnetized materials have widely been investigated, much less is known about the microscopic effect of hexagonal antidot lattice patterning on materials with perpendicular magnetic anisotropy. By using a combination of first-order reversal curve measurements, magnetic x-ray microscopy, and micromagnetic simulations we elucidate the microscopic origins of the switching field distributions that arise from the introduction of antidot lattices into out-of-plane magnetized GdFe thin films. Depending on the geometric parameters of the antidot lattice we find two regimes with different magnetization reversal processes. For small antidots, the reversal process is dominated by the exchange interaction and domain wall pinning at the antidots drives up the coercivity of the system. On the other hand, for large antidots the dipolar interaction is dominating which leads to fragmentation of the system into very small domains that can be envisaged as a basis for a bit patterned media.

Figure 1

SEM image of a nanoscaled hexagonal antidot lattice (antidot spacing $a=210\mathrm{nm}$ and diameter $d=160\mathrm{nm}$) in a thin GdFe film.

Figure 2

Major hysteresis loops of GdFe thin films hosting antidot lattices with large (antidot spacing $a=210\mathrm{nm}$ and diameter $d=160\mathrm{nm}$) and small (antidot spacing $a=210\mathrm{nm}$ and diameter $d=80\mathrm{nm}$) holes. The hysteresis loop of an unstructured GdFe thin film is shown for comparison as a dashed line.

Figure 3

FORC density of a hexagonal antidot lattice (antidot spacing $a=210\mathrm{nm}$ and diameter $d=160\mathrm{nm}$) in a $45\mathrm{nm}$ thin GdFe film. The major hysteresis loop is shown as inset.

Figure 4

SXM measurement series of a hexagonal antidot lattice (spacing $a=210\mathrm{nm}$ and diameter $d=160\mathrm{nm}$) in a $45\mathrm{nm}$ thin GdFe film. Images at an external applied field of (a) $240\mathrm{mT}$ (positive saturation), (b) $0\mathrm{mT}$, (c) $-8\mathrm{mT}$ (close to the coercive field), (d) $-28\mathrm{mT}$, and (e) $-240\mathrm{mT}$ (negative saturation) are shown. A part of the antidot lattice in proximity of an unstructured area was purposely chosen for direct comparison with the native film in the same frame. Enlargements of the areas marked by black rectangles 1 and 2 are shown in Figs. 5 and 6, respectively.

Figure 5

Enlargement of position 1 of the SXM measurement series of a hexagonal antidot lattice (spacing $a=210\mathrm{nm}$ and diameter $d=160\mathrm{nm}$) in a $45\mathrm{nm}$ thin GdFe film shown in Fig. 4. Images at an external applied field of $240\mathrm{mT}, 5\mathrm{mT}, 0\mathrm{mT}, -6\mathrm{mT}$, and $-240\mathrm{mT}$ are shown. Additionally, schematic sketches of the magnetization states (up or down) of the central bridge and the two adjacent nodes are shown for each field.

Figure 6

Enlargement of position 2 of the SXM measurement series of a hexagonal antidot lattice (spacing $a=210\mathrm{nm}$ and diameter $d=160\mathrm{nm}$) in a $45\mathrm{nm}$ thin GdFe film shown in Fig. 4. Images at an external applied field of $240\mathrm{mT}, 5\mathrm{mT}, -10\mathrm{mT}, -14\mathrm{mT}, -24\mathrm{mT}$, and $-240\mathrm{mT}$ are shown. Additionally, schematic sketches of the magnetization states (up or down) of the central node and the three adjacent bridges are shown for each field.

Figure 7

FORC density of a hexagonal antidot lattice (antidot spacing $a=210\mathrm{nm}$ and diameter $d=80\mathrm{nm}$) in a $29\mathrm{nm}$ thin GdFe film. The major hysteresis loop is shown as inset.

Figure 8

SXM measurement series of a hexagonal antidot lattice (spacing $a=210\mathrm{nm}$ and diameter $d=80\mathrm{nm}$) in a $29\mathrm{nm}$ thin GdFe film. Images at an external applied field of (a) $-100\mathrm{mT}$ (negative saturation), (b) $0\mathrm{mT}$, (c) $10\mathrm{mT}$, (d) $20\mathrm{mT}$ (close to the coercive field), (e) $30\mathrm{mT}$, and (f) $100\mathrm{mT}$ (positive saturation) are shown.

Figure 9

Simulation of the magnetization reversal of a hexagonal antidot lattice (spacing $a=220\mathrm{nm}$ and diameter $d=95\mathrm{nm}$) in a $28\mathrm{nm}$ thin GdFe film. Images at an external applied field of (a) $0\mathrm{mT}$, (b) $5\mathrm{mT}$, (c) $12\mathrm{mT}$, (d) $15\mathrm{mT}$ (close to the coercive field), (e) $17\mathrm{mT}$, and (f) $25\mathrm{mT}$ (positive saturation) are shown.