264
M. Becherer
secutive Ar ion etching. For the definition of the artificial nucleation center (ANC),
two methods are applied, either in a one step process during FIB lithography with
increased areal dose at the ANC spots, or in a second alignment step after island etching. The ANC itself is most commonly not visible in scanning electron microscopy
imaging and hence, highlighted by red circles in the SEM image of Fig. 2b. ANC
creation and its importance for field-coupling is summarized in Fig. 2c. Basically,
the switching field (the magnetic field that has to be applied for magnetization reversal) is controlled by the dose, species and energy of ion irradiation. But also spatial
extension and the position of the FIB ANC is vital for NML operation. It is indeed
a complex process and still a matter of current research. Overall, fabrication of 2D
pNML devices can be subdivided in three major steps:
1. Film deposition and characterization for viable film composition,
2. High resolution lithography and island definition,
3. ANC creation during lithography or in a consecutive, aligned FIB irradiation step.
2.2 pNML Operation Principles
The basic elements for both computation and signal propagation are lithographically
patterned –in at least one dimension– sub-micron sized, ferromagnetic islands. As
explained in the previous section, two magnetization directions are preferred, both
perpendicular to the film plane. The binary logic states ‘0’ and ‘1’ are represented by
the down- or up-magnetization direction of the islands, pointing either ‘in’ or ‘out’ of
the surface as depicted in Fig. 3a. The magnetic anisotropy, which defines the direction of lowest energy for the magnetization vector, is governed by an interplay of
crystalline and interfacial anisotropy. Hence, for thin film media with perpendicular
magnetization, various shapes of magnetic islands are possible, ranging from squares
or circles to elongated wires and stripes, the latter also referred to as domain-wall
(DW) conductors. As a matter of fact, shape anisotropy is in a first order approximation constant over the 2D plane and hence, negligible, providing many degrees of
freedom in the design of logic gates. However, it is worthwhile to mention, that the
position of the ANC is vitally important for logic operation.
In order to reach a lowest energy state, magnetic coupling fields are acting strongly
between next neighbor magnets and can be used for logic operations as shown in
the energy diagram of Fig. 3a. For two neighboring magnets, there are 4 possible
stable states, from which the 2 anti-parallel states are of lower energy. The principle
mechanism behind nanomagnetic computation is to switch the magnets with externally applied field pulses. At the same time they relax to the lowest energy state
by summing the external clocking field with the stray fields of the next neighbor
magnets right within the ANC.
Coupling fields of magnetic islands decay with H c ∝ 1/r
n and 1 ≤ n ≤ 3, where
H c is the coupling field, r the spatial distance and n a rational number depending
on the spatial geometry of the arrangement. Coupling is intrinsically symmetric, i.e.
M. Becherer
secutive Ar ion etching. For the definition of the artificial nucleation center (ANC),
two methods are applied, either in a one step process during FIB lithography with
increased areal dose at the ANC spots, or in a second alignment step after island etching. The ANC itself is most commonly not visible in scanning electron microscopy
imaging and hence, highlighted by red circles in the SEM image of Fig. 2b. ANC
creation and its importance for field-coupling is summarized in Fig. 2c. Basically,
the switching field (the magnetic field that has to be applied for magnetization reversal) is controlled by the dose, species and energy of ion irradiation. But also spatial
extension and the position of the FIB ANC is vital for NML operation. It is indeed
a complex process and still a matter of current research. Overall, fabrication of 2D
pNML devices can be subdivided in three major steps:
1. Film deposition and characterization for viable film composition,
2. High resolution lithography and island definition,
3. ANC creation during lithography or in a consecutive, aligned FIB irradiation step.
2.2 pNML Operation Principles
The basic elements for both computation and signal propagation are lithographically
patterned –in at least one dimension– sub-micron sized, ferromagnetic islands. As
explained in the previous section, two magnetization directions are preferred, both
perpendicular to the film plane. The binary logic states ‘0’ and ‘1’ are represented by
the down- or up-magnetization direction of the islands, pointing either ‘in’ or ‘out’ of
the surface as depicted in Fig. 3a. The magnetic anisotropy, which defines the direction of lowest energy for the magnetization vector, is governed by an interplay of
crystalline and interfacial anisotropy. Hence, for thin film media with perpendicular
magnetization, various shapes of magnetic islands are possible, ranging from squares
or circles to elongated wires and stripes, the latter also referred to as domain-wall
(DW) conductors. As a matter of fact, shape anisotropy is in a first order approximation constant over the 2D plane and hence, negligible, providing many degrees of
freedom in the design of logic gates. However, it is worthwhile to mention, that the
position of the ANC is vitally important for logic operation.
In order to reach a lowest energy state, magnetic coupling fields are acting strongly
between next neighbor magnets and can be used for logic operations as shown in
the energy diagram of Fig. 3a. For two neighboring magnets, there are 4 possible
stable states, from which the 2 anti-parallel states are of lower energy. The principle
mechanism behind nanomagnetic computation is to switch the magnets with externally applied field pulses. At the same time they relax to the lowest energy state
by summing the external clocking field with the stray fields of the next neighbor
magnets right within the ANC.
Coupling fields of magnetic islands decay with H c ∝ 1/r
n and 1 ≤ n ≤ 3, where
H c is the coupling field, r the spatial distance and n a rational number depending
on the spatial geometry of the arrangement. Coupling is intrinsically symmetric, i.e.