Inverse magnetoelectric effects at the nanoscale for advanced magnetic memories

Magnetic memories are presently intensely researched for embedded memory applications. Especially magnetic random-access memory (MRAM) has recently been started to be commercialized in consumer appliances and it can be expected that it will become broadly used in the near future. In such magnetic memories, the information is stored in the orientation of the perpendicular magnetization in a magnetic tunnel junction. The resistance of the tunnel junction is used to read the information whereas spin-transfer torque is used to write the information. To improve the performance of the cell and to open new application fields, both the read and write speed of the MRAM cells need to be improved without affecting the memory retention. To date, much research is being devoted to improving the write performance of MRAM cells using e.g. spin-orbit torques or voltage-controlled magnetic anisotropies. By contrast, rather little attention has been paid to the read performance of MRAM cells. To improve the read performance, the current resistance-based scheme using magnetic tunnel junctions needs to be replaced by a scheme that generates a direct electrical signal. In particular voltage-based schemes using magnetoelectric materials and compounds appear promising. In recent years, magnetoelectricity has seen a renaissance due to technological and theoretical progress. While the field is currently transitioning from basic science to applied device research, many open issues remain. Several different magnetoelectric material approaches exist. Multiferroics are materials that possess simultaneously ferroelectric and ferromagnetic properties and can therefore link magnetic information and voltage signals. Among multiferroics, magnetocapacitive materials show some promise to obtain voltage signals from the state of a nanomagnet. By contrast, the most efficient magnetoelectric systems are layered compounds consisting of piezoelectric and magnetostrictive materials, which couple magnetic and electric domains via strain. To date, studies of magnetoelectric compounds and magnetocapacitive materials have mainly focused on bulk and large-scale systems with very few reports on micrometer size devices. This thesis will thus focus on the study of such magnetoelectric systems in view of their utilization for MRAM read-out schemes. This will include the patterning and the characterization of relevant materials on the nanoscale as well as the design, fabrication, and characterization of suitable test device structures.