Scaling of nonlinear susceptibilities in artificial permalloy honeycomb lattice

TL;DR

This study investigates the nonlinear magnetic susceptibilities in a nano-structured artificial honeycomb lattice to understand the nature of the predicted spin solid phase transition, revealing unconventional behavior and questioning its thermodynamic validity.

Contribution

It provides the first analysis of nonlinear susceptibilities in ultra-small permalloy honeycomb lattices, challenging the thermodynamic nature of the spin solid phase transition.

Findings

$ ext{chi}_{3}$ changes sign at $T_s$=29 K

Nonlinear susceptibility exhibits unusual crossover behavior

Critical exponents do not follow conventional scaling

Abstract

Two-dimensional artificial magnetic honeycomb lattice is predicted to manifest thermodynamic phase transition to the spin solid order ground state at low temperature. Nonlinear susceptibilities are very sensitive to thermodynamic phase transition. We have performed the analysis of nonlinear susceptibility to explore the thermodynamic nature of spin solid phase transition in artificial honeycomb lattice of ultra-small connected permalloy (Ni$_{0.81}$Fe$_{0.19}$) elements, typical length of $\simeq$ 12 nm. The nonlinear susceptibility, $\chi_{n1}$, is found to exhibit an unusual cross-over character in both temperature and magnetic field. The higher order susceptibility $\chi_3$ changes from positive to negative as the system traverses through the spin solid phase transition at $T_s$ = 29 K. Additionally, the static critical exponents, used to test the scaling of $\chi_{n1}$, do not follow the conventional scaling relation. We conclude that the transition to the ground state is not truly thermodynamic, thus raises doubt about the validity of predicted zero entropy state in the spin solid phase.