The problem of unprecedented volumes of data and the need to reduce devices’ energy consumption
Globally, the demand for data storage continues to grow rapidly. The bigger concern is that the amount of data being created is growing faster than improvements in energy efficiency, making further progress in ICTs unsustainable. By 2030, almost 20% of the total energy consumption worldwide will be related to the deployment of ICTs. As a result, there is strong interest in memory technologies that can store and process information with much lower energy consumption. In magnetic materials, this is possible by replacing electric currents to manipulate magnetism with voltage (minimizing Joule heating effects).
Magneto-ionics refers to a specific mechanism of voltage-control of magnetism, where magnetic properties are tuned by driving ions (often oxygen ions) into and out of a metallic or metal oxide layer using an electric field. It has emerged as a possible path toward a memory that combines non-volatility, ultra-low energy, CMOS compatibility, analog operation (i.e., unlike conventional binary switching, ion motion can be gradual through intermediate oxygen concentrations), and high density, while speed and endurance should be improved.
If materials can be developed with fast, reversible, and durable ion motion at room temperature, magneto-ionic memories could become a compelling alternative or complement to existing technologies such as spin-transfer torque magnetoresistive random access memory (STT-MRAM) and spin-orbit torque
magnetoresistive random access memory (SOT-MRAM), particularly for ultra-low-power and neuromorphic computing applications.
Co/Pt-based stacks are important platforms for magnetic media with perpendicular anisotropy, as well as for emerging technologies such as SOT-MRAM, skyrmionics, and magneto-ionics. Their relevance arises from two complementary properties: Pt provides strong spin–orbit effects, while Co/Pt interfaces naturally exhibit strong perpendicular magnetic anisotropy (PMA).
Given the significant pressure to reduce Pt usage due to cost and materials sustainability concerns, researchers from Universitat Autònoma de Barcelona (UAB) and Singulus Technologies have joined efforts through the Nickeffect project to develop cheaper and more sustainable alternatives to Pt layers while keeping similar performance.
The approach: electrolyte-gating
Electrolyte-gating in magneto-ionics refers to the use of an electrolyte and an applied voltage to move ions into or out of a magnetic material, thereby reversibly or semi-reversibly tuning its magnetic properties through electrochemical reactions (often leading to changes in coercivity and magnetic remanence > 50%, and ultimately inducing full ferromagnetic-paramagnetic phase transformations). Propylene carbonate (PC) has been used as an electrolyte in Nickeffect for this purpose, as it is particularly well-suited and widely used electrolyte for conducting this type of study due to its favourable electrochemical stability and compatibility with ionic transport processes.
The versatility of the Co-Ni system
An MRAM cell contains a fixed (reference) layer and a free layer that stores the bit. The reference layer should never switch accidentally. Exchange coupling with natural antiferromagnets is typically used for such a purpose, but a higher stability is achieved by replacing conventional antiferromagnets with synthetic antiferromagnets, where two layers coupled antiparallel through a Ru spacer, thus making the reference layer more robust. The antiparallel alignment between the two magnetic layers through the Ru spacer can be understood through the so-called Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling. In MRAM systems with perpendicular anisotropy, multilayers comprising [Co/Pt] m /Ru/[Co/Pt] n stacks are employed in the pinned layer. By replacing Pt in the Ru-spaced bottom and upper Co/Pt stacks by Ni, we have demonstrated that electric fields can reversibly manipulate the behaviour of the RKKY stacks.
By inducing the migration of oxygen ions using low gate voltages, the magnetic exchange field can be dramatically altered post-growth (by >30%) without requiring growth of different samples. In another, more fundamental study, UAB researchers have developed a sponge-like Co-Ni oxide layer whose properties can be switched using a small electrical voltage. The tiny pores increase the surface area available for oxygen ions to move, making the material much more responsive than in its fully dense form.
Conclusion
The Nickeffect’s project findings show that carefully designing the structure of Ni–Co materials can greatly improve their performance and help bring voltage-controlled magnetic memories closer to real-world use.