Würzburg ToCoTronics Colloquium

Glasses are characterized by an amorphous structure and show dynamic heterogeneity, which refers to a spatial coexistence of frozen and active regions, where relaxation times among regions can vary by orders of magnitude [1].

Their magnetic counterparts are spin glasses, which possess randomly oriented spins that continuously evolve, which is known as aging [2]. Yet, a microscopic understanding of aging in terms of spin configurations is still lacking. Partly, because much of what is known experimentally about spin glasses is based on ensemble thermodynamic measurements. Hence, there is a demand for experimental methods, which can study the magnetization and its ensuing dynamics, as it relates to aging, on the nanoscale.

In this talk, I will present the discovery of the low temperature magnetic ground state of Nd(0001) to be a self-induced spin glass state [3]. Using spin-polarized scanning tunneling microscopy as a function of magnetic field and temperature, we show that the magnetic state exhibits both short-range order as well as aging behavior. Interestingly, as temperature increases, we observe an unconventional magnetic phase transition from glassiness to long-range magnetic order, which serves as a counterexample to the common thermodynamic understanding of temperature and disorder being synonymous [4].

Lastly, we explore the aging dynamics in elemental neodymium by inducing dynamics through magnetic field cycles and tracking the local order in space and time. We find a coexistence of regions with slow and fast dynamics providing evidence for dynamic heterogeneity in a spin glass system. This observation provides a natural bridge between the description of spin glasses and its non-magnetic counterparts.

[1] Berthier et al., Dynamical Heterogeneities in Glasses, Colloids, and Granular Media, Oxford University Press (2011)
[2] Binder et al., Rev. Mod. Phys. 58, 801-976 (1986)
[3] Kamber et al., Science 368, 6494 (2020)
[4] Verlhac et al., Nature Physics 18, 905-911 (2022)

Self-induced spin glass state of Nd(0001). (a) Large scale magnetization image illustrating the lack of long-range magnetic order in elemental neodymium (T = 1.3 K, I_t = 200 pA, scale bar: 50 nm, imaged with an out-of-plane sensitive Cr bulk tip). (b) Regions of distinct locally ordered magnetic patterns as marked in a (scale bar: 10 nm). (c) Fourier transform of the magnetization image in a reveals a broad distribution of spectral weight resulting from magnetic frustration (scale bar: 3 nm^-1).