All we wanna do is talk torque: Taiwanese boffins spin a better way to switch MRAM states

Platinum-class MRAM exchange bias is the key

Taiwanese spintronics boffins have made an important finding in magnetic random access memory (MRAM) tech: a faster way to switch MRAM states by using a nanometre scale layer of platinum.

Spintronics is a developing field of nanoscale electronics - and it uses not only the charge of electrons but also their spin to effect change.

Magneto-resistive or magnetic RAM (MRAM) uses magnetism to alter the resistive state of a cell from high to low, thereby signalling a binary one or zero. MRAM is non-volatile and roughly as fast as DRAM. The technology could replace DRAM, SRAM and NAND with a single chip tech.

Suppliers have been developing MRAM chip technology for more than a decade and include Avalanche, Crocus, EverSpin, Samsung, and Spin Transfer Technologies.

An MRAM cell uses magnetic tunnel junction effects, and has two magnetic layers separated by an insulating or tunnel barrier layer. One magnetic layer with high coercivity has a fixed polarity while the other with low coercivity is switched between north and south polarities. When the two layers have the same polarity the electrical tunnel magnetoresistance across the cell is lower than when they have opposing polarities, and signals a “one”.

Crocus_MRAM_diagram

Crocus MRAM diagram

The trick is switch the variable magnetic layer’s polarity as fast and as efficiently as possible. Writing data means switching the variable layer’s magnetism and this typically uses more electricity than reading the cell.

Spin Transfer-Torque Magnetic RAM (STT-MRAM) uses one of two different spin directions of electrons to help switch the polarity (“torque” the magnetic domain) of the variable magnetic layer with less electricity. However high-speed switching still needs a higher write current than low-speed switching.

A research team based at National Tsing Hua University (NTHU) in Taiwan, led by professor Chih-Huang Lai and professor Hsiu-Hau Lin, added a layer of platinum, only a few nanometers thick, underneath the two magnetic layers in the cell and manipulated the switching process better.

Electricity sent to it, with its flow of electrons, causes the spin current itself to manipulate the exchange bias of a coupled magnetic multilayer film. Exchange bias was described simply by the researchers as a phenomenon "occurring in magnetic multilayers where the antiferromagnetic layer 'fixes' the ferromagnetic layer."

The lead author told the Taipei Times the team's work marked the first time a spin current had been used to manipulate the exchange bias.

Their research has been published in Nature Materials, in a paper titled “Manipulating exchange bias by spin-orbit torque”.

The paper explains: "Exchange bias... is an integral part of spintronic devices. Here, we show that spin–orbit torque generated from spin current, a promising approach to switch the ferromagnetic magnetization of next-generation magnetic random access memory, can also be used to manipulate the exchange bias.

"Applying current pulses to a Pt/Co/IrMn trilayer causes concurrent switching of ferromagnetic magnetization and exchange bias, but with different underlying mechanisms. This implies that the ferromagnetic magnetization and exchange bias can be manipulated independently.

The researchers add that, crucially: "Our work demonstrates that spin–orbit torque in ferromagnet/antiferromagnet heterostructures facilitates independent manipulations of distinct magnetic properties, motivating innovative designs for future spintronics devices."

The technique is at least four to five years away from productisation. ®




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