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Micromagnetic simulations of MRAM cells are a computationally demanding task. Different methods exist to handle the computational complexity of the demagnetizing field, the most expensive magnetic field contribution. In this work we show how the demagnetizing field can efficiently be calculated in complex memory structures and how this procedure can be further used to simulate spin-transfer torque switching in magnetic tunnel junctions.

We demonstrate the switching of a perpendicularly magnetized free layer by spin-orbit torques. The switching is accomplished by a purely electrical, two-current pulse scheme. The first pulse realizes the selection of the cell, while the second pulse, with a lower current, completes the switching deterministically. The second current is reduced by 50% without affecting the switching performance. This current reduction results in a reduction of about 80% of the power consumption by the second pulse. Even with such a large decrease of current and power, the switching time remains robust and fast, resulting in a very efficient switching scheme.

A drift-diffusion approach to coupled spin and charge transport has been commonly applied to determine the spin-transfer torque acting on the magnetization in metallic valves. This approach, however, is not suitable to describe the predominant tunnel transport in magnetic tunnel junctions. Here we demonstrate that by introducing a magnetization dependent resistivity and adjusting the spin diffusion coefficient one can successfully apply the generalized spin and charge drift-diffusion approach also to magnetic tunnel junctions. As a unique set of equations is used for the entire structure, this paves the way to develop an efficient finite element based approach to describe the magnetization dynamics in emerging spin-transfer torque memories.

The magnetization dynamics of a free layer in spin-transfer torque MRAM is usually determined by the torque created by a position-independent current density. In circuits, the voltage, not current density, is fixed during switching. Therefore, the approximate evaluation of the torque based on the fixed current density becomes questionable in modern magnetic tunnel junctions with a tunneling magnetoresistance ratio of about 200%, where the current densities across the structure can vary by a factor of three. We compare the switching times obtained within a fixed voltage assumption with those from the approximate fixed current density approach. We demonstrate that the fixed current approach can reproduce the correct switching also in the case of high TMR, if the current is appropriately adjusted. It is shown that the correction to the current is not universal and depends on the temperature and the switching speed.

As scaling of electronic semiconductor devices - the main driving force behind an outstanding increase of computing power of modern electronic circuits - displays signs of saturation, the main focus of engineering research in microelectronics shifts towards finding new computing paradigms. The future solutions must be scalable and energy efficient while delivering high computational performance, superior to that of CMOS-based circuits. In order to benefit from the outstanding potential of highly advanced silicon processing technology, the emerging solutions must be CMOS compatible. Emerging nonvolatile memories, including magnetoresistive memories, satisfy the necessary requirements: Purely electrically addressable magnetoresistive random access memories yield all aforementioned conditions. They possess a simple structure, require fewer extra masks for fabrication, and offer endurance and speed superior to those of flash memory. Fast operation of magnetoresistive memories makes them a viable contender to static random access memory and flash memory for embedded applications in Systems on Chip. Having nonvolatile memory very close to CMOS removes the necessity of the energy-intensive data transfer from the main memory to the CPU and offers a prospect of data processing in the nonvolatile segment, where the same elements are used to store and to process the information. This opens perspectives for conceptually new low power and highperformance computing paradigms based on logic-in-memory and in-memory computing.

We demonstrate by means of numerical simulations the switching of a perpendicularly magnetized free layer by spin–orbit torques based on a two-pulse switching scheme with improved writing power efficiency. In this scheme, the first pulse selects the cell, while the second pulse completes the switching deterministically. It is shown that the magnitude of the second current pulse can be reduced to about 50% of the critical current and the switching remains reliable with a switching time of 300 ps. With such a significant current reduction the writing power required for switching decreases by 40%, which results in a very energy efficient scheme. In addition, we develop a reinforcement learning approach to optimize the pulse configuration with the goal of achieving the shortest switching time. With this approach a switching time of 146 ps has been obtained, a reduction of 50% in relation to the non-optimized configuration. These research findings confirm that reinforcement learning is a promising tool to simplify and automate the search for a faster, energy efficient scheme in the two-pulse switching approach.

Simulations of free-layer switching in spin - transfer torque MRAM are usually performed with the torque computed approximately by assuming a position-independent electric current density through the structure. For high values of the tunneling magnetoresistance, this description is not accurate anymore, and one needs to solve the spin and charge drift-diffusion equations in the whole structure self-consistently. We compute the switching time distribution obtained by the self-consistent model and compare it to the switching times from the fixed current density approach. We show that, provided the current is appropriately adjusted, the simplified model can mimic the correct switching time distribution even in the case of high TMR.

We investigate the switching of a symmetric square and an elongated rectangular perpendicular free layer by spin-orbit torque with a magnetic field-free two-pulse scheme. The switching of the layer is achieved by utilizing the in-plane shape anisotropic magnetic field. For making the switching of a symmetric square layer deterministic, an in-plane stray field created in a part of the layer is used. The combination of the shape and stray fields accelerates the switching of the free layer significantly. A switching speedup factor of 3 to 5 has been obtained. The strategy also improves the robustness of the scheme allowing fast, sub-0.5 ns switching, less sensitive to the pulses’ properties.

A long-standing problem of magnetic field-free switching of a symmetric perpendicular free layer by spin-orbit torque is resolved by employing two perpendicular consecutive current pulses. The optimal overlap of the second pulse line is found to be around 50%. The robustness of switching with respect to fluctuations of the second current pulse duration and of the overlap with the free layer is demonstrated.

We investigate the robustness of a purely electrical field-free switching of a perpendicularly magnetized free layer based on SOT. The effective magnetic field which leads to deterministic switching of a rectangular as well as of a square free layer is created dynamically by a two-current pulse scheme. It is demonstrated that the switching is very robust, being insensitive to fluctuations of the write pulses’ durations and to relatively large variations of the heavy metal wires’ dimensions. Furthermore, it remains reliable for a wide range of synchronization failures between the pulses. The combination of a rectangular free layer shape with a partial overlap with the second current line accelerates the switching of the cell allowing a fast, 0.25 ns, switching.

A magnetic field-free switching of a symmetric square free layer with perpendicular magnetization by spin–orbit torque is demonstrated based on micromagnetic modeling and numerical simulations. The field-free switching is accomplished by using a two-pulse switching scheme. An appropriate design of the cell structure yields a deterministic and fast switching, about 0.6 ns, of the magnetized free layer. It is shown that the switching is robust with respect to fluctuations of the current pulse duration and, furthermore, very robust in case of delays or overlaps between the writing current pulses.