February 24, 2021
Professor Kannan M. Krishnan and his post-doc, Dr. Vineeth Mohanan, have published an article in Physical Review Letters entitled "Configurable Artificial Spin Ice with Site-Specific Local Magnetic Fields."
Artificial spin ice (ASI) are two-dimensional arrays of magnetostatically interacting nanomagnets (NMs) that act as giant Ising spins. When arranged on specific lattices, they mimic the frustrated interactions found in naturally occurring pyrochlore crystals. These two-dimensional artificial magnetic systems, made possible by developments in lithography at UW, allow exploration of structures that are difficult or impossible to realize with chemically synthesized compounds. ASI is also a promising prototype for investigating emergent phenomena such as magnetic monopoles, Coulomb phase and spin fragmentation. Recently, ASI has been proposed for data storage and computational application, offering device options which integrate memory with active logic.
Most previous studies on ASI have dealt with understanding the collective behavior of arrays of identical nanomagnets, with an emphasis on different lattice geometries in order to utilize the anisotropic features of the dipolar fields. There is growing interest in designing reconfigurable arrays in which the desired magnetic configurations can be “written” by applying local magnetic fields using magnetic probes either with or without the help of global magnetic fields. These include attempts to access specific moment configurations in ASI, either by field protocols or by modifying individual nanomagnets; however, these are restricted to a small number of nanomagnetic building blocks.
In this pioneering work, Prof. Krishnan’s group has demonstrated a way to alter the properties of individual nanomagnets, in such a way that it affects their relaxational dynamics. This allows configurability of the global behavior of the lattice and provides a way to engineer a preferred magnetic texture. In particular, they demonstrate ground state tunability for a hybrid artificial spin ice composed of Fe nanomagnets which are subject to site-specific exchange-bias fields, applied in integer multiples of the lattice along one sublattice of the classic square artificial spin ice. By varying this period, three distinct magnetic textures are identified: a striped ferromagnetic phase; an antiferromagnetic phase attainable through an external field protocol alone; and an unconventional ground state with magnetically-charged pairs embedded in an antiferromagnetic matrix. Monte Carlo simulations, in collaboration with Prof. Stamps ( U. Manitoba), support the results of field protocols and demonstrate that the pinning tunes relaxation timescales and their critical behavior.
A natural extension to the present work will be to create thermally active hpa-ASI where the pinned sites remain frozen in predetermined magnetization states while the remaining array is dynamic. This will allow the investigation of energy minimization dynamics, controlled creation of magnetic monopoles and its dynamics, phase transitions in the presence of periodic pinning and further extend it to thermally driven nanomagnetic computing devices.