Investigation of the monopole magneto-chemical potential in spin ices using capacitive torque magnetometry

TL;DR

This study uses capacitive torque magnetometry to explore the magneto-chemical potential in spin ices, revealing sublattice-dependent effects and the importance of long-range interactions in modeling spin-ice behavior.

Contribution

It demonstrates the effectiveness of torque magnetometry in characterizing monopole-related potentials and highlights the need for extended models including long-range interactions.

Findings

Magneto-chemical potential varies with spin-sublattice in Ho2Ti2O7.

Torque magnetometry is sensitive to effective spin correlation strength ($J_{eff}$).

Long-range exchange interactions are necessary for accurate modeling.

Abstract

The single-ion anisotropy and magnetic interactions in spin-ice systems give rise to unusual non-collinear spin textures, such as Pauling states and magnetic monopoles. The effective spin correlation strength ($J_{eff}$) determines the relative energies of the different spin-ice states. With this work, we display the capability of capacitive torque magnetometry in characterizing the magneto-chemical potential associated with monopole formation. We build a magnetic phase diagram of Ho$_2$Ti$_2$O$_7$, and show that the magneto-chemical potential depends on the spin-sublattice ($\alpha$ or $\beta$), i.e., the Pauling state, involved in the transition. Monte-Carlo simulations using the dipolar-spin-ice Hamiltonian support our findings of a sublattice-dependent magneto-chemical potential, but the model underestimates the $J_{eff}$ for the $\beta$-sublattice. Additional simulations, including next-nearest neighbor interactions ($J_2$), show that long-range exchange terms in the Hamiltonian are needed to describe the measurements. This demonstrates that torque magnetometry provides a sensitive test for $J_{eff}$ and the spin-spin interactions that contribute to it.