With advances in lithography, scientists today can create arrays of coupled ferromagnetic structures (dipoles) that mimic atomic spins. These artificial spin lattices (ASLs) can provide insight into collective spin motions and dipole interactions, which often play important roles in determining the ground states of quantum materials and could prove crucial for implementing advanced computing and data-storage strategies. Understanding such systems is challenging due to low phase-transition temperatures and lack of control in the engineering of atomic systems.
In this work, researchers fabricated two-dimensional, square, permalloy (Ni0.8Fe0.2) ASLs that undergo a paramagnetic-to-antiferromagnetic (AFM) phase transition with decreasing temperature. The presence of antiferromagnetic superdomains below the transition temperature was confirmed using photoemission electron microscopy (PEEM) at ALS Beamline 11.0.1.
Using coherent x-rays at ALS Beamline 12.0.2 and NSLS-II Beamline 23-ID-1, researchers made the first-ever observation of spontaneous, millisecond-scale fluctuations in magnetic domain walls. In the technique they used—x-ray photon correlation spectroscopy (XPCS)—coherent x-ray diffraction from magnetic superdomains produces a complex interference (speckle) pattern related to real-space magnetic textures. By tracking the time-dependent speckle motion, the researchers discovered that superdomain walls have two distinct regimes of motion as the ASL goes through the AFM phase transition: low-temperature ballistic and high-temperature diffusive.
Such studies of the dynamical properties of magnetic domains are critical to understanding ordering mechanisms in ASLs, and ultimately for implementing real-world applications based upon artificial spin systems. The XPCS method and model used in this work can be readily applied to future studies of fluctuations in various quantum materials.
X.M. Chen, B. Farmer, J.S. Woods, S. Dhuey, W. Hu, C. Mazzoli, S.B. Wilkins, R.V. Chopdekar, A. Scholl, I.K. Robinson, L.E. De Long, S. Roy, and J.T. Hastings, “Spontaneous Magnetic Superdomain Wall Fluctuations in an Artificial Antiferromagnet,” Phys. Rev. Lett. 123, 197202 (2019), doi:/10.1103/PhysRevLett.123.197202