Technical thesis without the help and support of many people around me, some of whom it is possible to specifically mention here. The use of the phenomenon of magnetism for information processing dates back to the end of the 19th century. Cellular automata incorporating field interaction for logic computing has been the alternative computing paradigm that offers interconnect-free design architecture, and thus, provides the scope for realizing low-power circuits in the nanoscale regime.
Quantum cellular automata (QCA), a category of cellular automata, use the Columbic interaction between electrons to realize logic functionality. Quantum dot cell automata (QCA) are considered a potential candidate to replace or complement CMOS electronic circuits at the end of the design. Molecular Cellular Automata and Atomic Cellular Automata are two other offshoots of Cellular Automata that involve field assembly computing.
Magnetic Quantum Cellular Automata (MQCA), the last member of this family to date, is the most extensively studied phenomenon that can operate at room temperature.
Motivation
The use of magnetism for information storage processing dates back to the late 19th century [14, 15]. At the beginning of the 21st century, as the semiconductor industry faces difficulties in scaling down further, several applications of magnetism for future logic and memory devices are being proposed and explored, such as Magnetic Random Access Memory (MRAM) [16], Racetrack Memory [17 ], as well as the magnetic Quantum-Dot Cellular Automata (MQCA) [18] recently called nanomagnet logic (NML). In the NML devices, the magnets are forced to be in a high energy state by a magnetic field called the clock field.
As the clock field is removed, the magnets relax into one of their two stable, low-energy states. As the nanomagnets are placed several nanometers apart, the field emanating from neighboring magnets determines their final state.
Purpose of MQCA
The predicted bias field for the nanomagnet circuit is provided locally either by a magnetic tunnel junction (MTJ) or by a bias line, which converts an electrical signal into a magnetic signal [28]. At the end of the NML circuit, the MTJ converts the magnetic signal back into an electrical signal for possible further processing. In the following subsections, we investigate the magnetic behavior properties of nanomagnets with different aspect ratios (Section 3.1) and with different shapes (Section 3.2) to show possible programmability of NML gates.
30], and it has been shown that different shapes can be differently sensitive to fabrication variations.
Symmetrically shaped nanomagnets
Even a small bias field along the easy axis can affect which magnet ground state the magnetization will select. In Figure 3-1, the blue curve shows the potential landscape of the nanomagnet immediately after the removal of the zero field. The presence of the energy barrier requires an external field stronger than the zero field to reevaluate the magnet, that is, set the magnet so that a new logic value, either up or down, can be written onto it.
The magnet retains its new state in the absence of an external field, as the size of the magnet is assumed to be larger than the superparamagnetic limit.
Symmetrically shaped nanomagnets
The external magnetic field acts from left to right, shown by the horizontal, thick, blue arrows. A schematic of an asymmetric beveled edge magnet is shown in Figure 3-2 on the left [31]. Simulations based on a single-domain model show that the asymmetry of the magnet shifts the whole.
The thin line along the diagonal of the magnet in Figure 3-2 corresponds to the effective easy axis. The horizontally applied field is not exactly in the direction of the energy maximum, so the resulting energy of the magnet is to one side of the maximum and falls to the correct ground state when the field is removed. As a result of the tilted easy axis, the nanomagnet takes on a preferred direction of magnetization, as summarized in Figure 3-3.
Nanomagnets in NML devices
We apply an external field either along the horizontal or vertical direction to set the magnetization state of each magnet; therefore, some magnets experience this field along their easy (geometric long) axis, while others experience it along their hard (geometric short) axis. Easy-axis magnetization flips the magnet into a stable, low-energy state, while hard-axis magnetization provides an unstable, high-energy state. The energy difference between the two states ranges from a few to a few hundred electron volts, depending on the size and shape of the magnet [32].
Nanomagnet Wire
The FC wires can be as long as necessary to meet other design requirements, but in the AFC wire, every other magnet inverts the carried information, i.e. the wire length must be an odd number to have the same bit value at the beginning and end of the wire. MFM scanning is performed after each magnetization to show the magnetization state of the vertical wire (Figure 3-5b and Figure 3-5c). The nanomagnets work correctly and send the information from the driver magnet through the wire according to the magnetization state of the driver magnet.
Two MFM images (Figure 3-6b and Figure 3-6c) show the magnetization of the same wire for two possible states of the driver magnet. The technology to fabricate arrays of nanomagnets is currently under development in HDD and MRAM industrial research. The deposition of the magnetic material is usually done by electroplating, sputtering or evaporation; the pattern transfer uses reactive ion etching (RIE) or low energy ion irradiation.
In Figure 4-1(b) the difference between monolayer and bilayer resistance is shown by the larger shear in the copolymer. On the one hand, this can prevent the formation of thin walls along the droplet boundary as seen in Figure 4-2(a). On the other hand, the patterns are less defined in the presence of the copolymer due to its larger shadow angle.
As a result, the vaporized material that does not arrive completely perpendicular to the surface of the substrate mostly spreads out. The elongated and flat, identically defined cobalt spots in the chains were produced (a) on single layer and (b) on double layer. In the case of single-layer resistance, there is material accumulation from the edges of the elements.
Maxwell Simulation Results
The current density as shown in the ear is found to be a maximum of 5.36×1011 A/m2 and the field associated with it is found to be a maximum of 336mT with the placement of any nanomagnetic dot over it. The simulation was performed for t = 1 sec and the maximum and the minimum values of the field were associated. Below Table 5-1 shows the comparison result for the field associated in the conductor with and without the nanomagnetic dots placed over it.
OOMMF Simulation Results
Using shape anisotropy as described in Chapter 3, Varga et.al and his team reduced the number of nanomagnets needed to make the same adder as shown in Figure 5-10, the problems associated with signal propagation. Inputs are provided by 7 bevel magnets, and the number of magnets with rounded edges is only 14 (21 magnets in total). Using the idea as proposed by Varga .et.al and his team, this paper proposes different ideas for the problems related to signal propagation with nanomagnets, the results of which have been discussed below.
The idea was to explore the phenomenal advantages of the shape anisotropy of the nanomagnet wires to solve the problems related to the signal propagation. Using the shape anisotropy one can orient the easy axis in the desired direction. Figure 5-11 shows one of the proposed methodologies that takes advantage of the shape anisotropy, referring to what Varga et al. has done in Figure 5-9 and Figure 5-10.
Domain Wall Conductor together with free-standing nanomagnets Figure 5-12 shows the fabricated structure of the Domain Wall Conductor. Micro-magnetic simulations using NIST's OOMMF package were performed to predict the behavior of inclined wires and inclined conductors. The direction of the magnetic field in the conductor is shown in the positive y direction, so the associated field in the wire is to the right.
Similarly, Figure 5-16 shows the simulation result for a driving magnet driving a wire with a field strength of 130 mT. The direction of the magnetic field in the driver here is in the negative y-direction, so the coupled field in the wire is towards the left. As mentioned earlier, the domain wall conductor has a diameter of 2.5 µm and an injection length of about 5 µm, as shown in Figure 5-12.
This work introduces new methods to solve the problems related to propagation of the signal through Nanomagnet Logic Wires. The proposed idea is able to solve the problem associated with the propagation of the signal and has been verified using Micro Magnetic NIST's proposed OOMMF software and the results have been verified.
Future Scope
The simulation with Maxwell 3D was performed to verify the magnetic field associated with the current-carrying conductor. Kogge, "Probabilistic analysis of a molecular quantum-dot cellular automata adder," Defect and Fault-Tolerance in VLSI Systems, IEEE Intl. Parkin, "Development of the Magnetic Tunnel Junction MRAM at IBM: From First Crossroads to a 16-Mb MRAM Demonstrator Chip," IBM J.
Porod, "Controlling Magnetic Circuits: How Clock Structure Implementation Will Affect Logic Regularity and Power," IEEE Symp. Imre, "Experimental study of nanomagnets for magnetic quantum dot cellular automata (MQCA) logic applications," Ph.D.
