Purely Antiferromagnetic Magnetoelectric Random Access Memory (AF-MERAM)
Tobias Kosub (Helmholtz-Zentrum Dresden-Rossendorf, Germany)

Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency1. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM)2 that offers a remarkable 50 fold reduction of the writing threshold compared to state-of-the-art ferromagnet-based counterparts2,3, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature [Figure 1], The basics of RAM operation - writing, storage and reading information - are demonstrated repeatedly. Furthermore, the all-electric writing and read out interfaces can be harnessed for in-depth studies of the magnetoelectric selection processes in these thin film elements, which turn out to be markedly deviant from the theory of the linear magnetoelectric effect.

While omitting the ferromagnet enables the large improvement in the writing threshold over conventional exchange biased MERAM2,3, it also eliminates all possibilities for conventional magnetoresistive read out, such as the AMR/GMR/TMR effects. Thus, a key aspect of the AF-MERAM functionality is its new all-electric read out of the pure Hall resistance4. This method is both ultra-sensitive to tiny net magnetization in metallic antiferromagnets and to boundary magnetism between nonmagnetic metals and magnetic insulators, suggesting its considerable applicability to the growing fields of antiferromagnetic spintronics and insulator spintronics.

Access to the pure Hall resistance is enabled by a new electric measurement scheme called Resistance Tensormetry, which determines electrical resistance not as a scalar quantity peculiar to the used measurement layout but instead as a tensor quantity including diagonal and off-diagonal (Hall) components. Since electrical resistance is one of the most crucial material properties for both science and technology, the more comprehensive view provided by resistance tensormetry is highly relevant for emergent topics and materials. Via µΩ level sensitivity and tensor resolution, previously unattainable figures are now open to experimental scrutiny.