When entities in a system, forced to get along within given lattice constraints, are hindered to simultaneously satisfy their local interactions, frustration arises. In magnetism, this usually involves interaction between the spins or magnetic moments. Magnetically frustrated systems are fascinating because they exhibit an extensive ground state degeneracy and hard-to-predict behavior, in addition to the emergence of novel states of matter. Artificial spin ice systems consisting of single-domain Ising- or XY-type nanomagnets that are lithographically arranged onto a variety of two-dimensional geometries have risen as an attractive playground to directly image and visualize frustration-driven phenomena [1--5]. In this colloquium, I will start with an introduction to the main basic concepts behind the creation of such artificial frustrated systems and examples of emergent phenomena one can hope to achieve in them. Then, in the second part I will discuss two successful demonstrations, namely the direct observation of emergent magnetic monopole dynamics in macroscopically degenerate artificial spin ice [6] and the emergence of polaronic states in the so-called dipolar dice lattice [7]. I will then conclude with an outlook where research activities in the field might be heading next.

[1] S. H. Skjærvø, C. H. Marrows, R. L. Stamps, and L. J. Heyderman, Nature Reviews Physics 2, 13 (2020).
[2] A. Farhan, P. M. Derlet, A. Kleibert, A. Balan, R. V. Chopdekar, M. Wyss, L. Anghinolfi, F. Nolting, and L. J. Heyderman, Nature Physics 9, 375 (2013).
[3] A. Farhan, C. F. Petersen, S. Dhuey, L. Anghinolfi, Q. H. Qin, M. Saccone, S. Velten, C. Wuth, S. Gliga, P. Mellado, et al, Nature Communications 8, 995 (2017).
[4] M. Saccone, K. Hofhuis, D. Bracher, A. Kleibert, S. van Dijken, and A. Farhan, Nanoscale 12, 189 (2020).
[5] I. Gilbert, Y. Lao, I. Carrasquillo, L. O’Brien, J. D. Watts, M. Manno, C. Leighton, A. Scholl, C. Nisoli, and P. Schiffer, Nature Physics 12, 162 (2015).
[6] A. Farhan, M. Saccone, C. F. Petersen, S. Dhuey, R. V. Chopdekar, Y. Huang, N. Kent, Z. Chen, M. J. Alava, T. Lippert, et al, Sci Adv 5, eaav6380 (2019).
[7] A. Farhan, A. Scholl, C. F. Petersen, L. Anghinolfi, C. Wuth, S. Dhuey, R. V. Chopdekar, P. Mellado, M. J. Alava, and S. van Dijken, Nature Communications 7, 12635 (2016).
 

Dr. Farhan obtained his doctoral degree in 2014 at the University of Basel with a research project on accessing and observing low-energy state in two-dimensional artificial spin ice systems via thermal annealing. He was then awarded mobility fellowships by the Swiss National Science Foundation, which allowed him to pursue independent research with a primary focus on exploring emergent magnetic monopoles and other emergent phenomena in artificial frustrated spin systems. After a short Return Fellowship, he moved as postdoctoral researcher to Aalto University, where he started to work on employing complex network science in the quest for the realization of artificial spin glasses. In addition, he started to pursue research on complex oxide and multiferroic thin films.

To request for meeting link, please write to phweb@ust.hk.