Realization of random-field dipolar Ising ferromagnetism in a molecular magnet

TL;DR

This study demonstrates that a molecular magnet exhibits ferromagnetism driven by dipolar interactions, with a quantum critical point influenced by randomness from local easy axis tilts, aligning with a random-field Ising model in a transverse field.

Contribution

It provides experimental evidence for random-field dipolar Ising ferromagnetism in a molecular magnet, highlighting the role of intrinsic disorder and external fields in quantum phase transitions.

Findings

Curie-Weiss law observed in susceptibility indicating ferromagnetic transition.

Transition temperature decreases rapidly with transverse field, reaching a quantum critical point.

Results align with a random-field Ising model in a transverse field.

Abstract

The longitudinal magnetic susceptibility of single crystals of the molecular magnet Mn$_{12}$-acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase due to dipolar interactions. With increasing magnetic field applied transverse to the easy axis, the transition temperature decreases considerably more rapidly than predicted by mean field theory to a T=0 quantum critical point. Our results are consistent with an effective Hamiltonian for a random-field Ising ferromagnet in a transverse field, where the randomness is induced by an external field applied to Mn$_{12}$-acetate crystals that are known to have an intrinsic distribution of locally tilted magnetic easy axes.