Wednesday, July 8, 2020

Magnetic memory combined with logical operations to revolutionize tomorrow’s computers

Modern computers that we use today have come a long way every since the transistor-based models started in 1955. A typical computer consists of an input/output unit, a central processing unit (CPU), and a storage unit. There is a clear demarcation of information storage and information processing, and this design has undergone very little change until now. Everyone would have experienced accidental data loss to some degree while working with the computer, either by a power outage or someone accidentally pulling the plug.

Data at rest in a computer is stored in a non-volatile memory such as a magnetic hard disk or a flash memory. During processing, the required data is loaded to the volatile main memory, most commonly referred to as RAM (Random Access Memory). Like the computer’s CPU, the RAM is made of current-controlled transistors. As soon as the power supply is disconnected, it resets to its original state, losing all the data.

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What if there is a solution to allow data processing within the memory without risking data loss? A team of scientists from ETH Zurich and the Paul Scherrer Institute (PSI) under the leadership of Pietro Gambardella and Laura Heyderman is hoping to change this decade-old principle and revolutionize the way computers stores and process data. Their solution is to build a fast, non-volatile memory system that can also perform logical operations (AND, OR, and NOT) on the data. The research reached a significant milestone and is described in an article in Nature.

Racetrack memory

Researchers have been working on a new type of memory called the Racetrack memory for years. Unlike the traditional hard disk-based storage where the read/write happens by mechanical means, the racetrack memory uses current pulses to move tiny magnetic regions or domains up and down in nanowires about 200 nm across and 100 nm thick. In these domains, magnetic moments are oriented in the same direction and can be used to represent the binary states. By eliminating mechanical movement, racetrack memory offers faster access times when compared to hard disk drives and also provides higher storage densities than the flash drives we have today. 

Schematic showing current-driven domain-wall inversion, which occurs as the domain-wall transfers across the in-plane region. | Image:

Despite the speed and storage density improvements, the processing still has to happen in the CPU. What the researchers have managed to do now is to perform the logical operations within this memory element directly.  

According to Pietro Gambardella. “We use an electric current to reverse the polarity of the magnetic regions, thereby performing a NOT operation on the stored data. We do this by harnessing a rather peculiar exchange interaction that occurs when we deposit a magnetic cobalt film on a platinum layer.” Due to the platinum layer’s presence, the interaction causes the magnetic regions in adjacent domains to align perpendicular to each other. 

Logical NOT

The perpendicular alignment also results in a preferred sense of rotation of the magnetization between adjacent domains. When an electrical pulse passes through the platinum layer, the flowing electrons gradually change the polarity of the atomic compass needles in the magnetic cobalt layer. This change in polarity moves the information encoded in the magnetization and creates a traveling magnetic domain. At predefined locations where the perpendicular interaction is strong, the direction of the magnetization in the traveling domain is inverted, resulting in a logical NOT operation.

The bright and dark contrast in the racetracks in the MOKE images correspond to up and down magnetization, respectively. | Image:

Other logical operations like AND and OR can be achieved by combining operations similar to NOT. The specialty of racetrack memory is that it requires current only at input and output, whereas, in a semiconductor-based circuit, every transistor requires a power supply.

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According to Gambardella, they are envisioning their technology to be used initially in low power microprocessor-based systems that require instant-on capabilities. Though it is too early to say if their technology can replace conventional semiconductor technology, the day is not too far when we have this on our computers.

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