Quantum percolation phase transition and magneto-electric dipole glass in hexagonal ferrites

TL;DR

This study explores the quantum phase transition in hexagonal ferrites, revealing a geometrically driven magnetic transition and the emergence of a magneto-electric dipole glass near zero temperature, with potential quantum technology applications.

Contribution

It demonstrates a compositionally-tuned quantum critical point in PbFe$_{12-x}$Ga$_x$O$_{19}$ and links it to geometric percolation, introducing new insights into quantum properties of hexaferrites.

Findings

Magnetic transition temperature varies as (1-x/x_c)^{2/3}

Zero-temperature phase transition is geometrically driven

Emergence of a magneto-electric dipole glass near criticality

Abstract

Hexagonal ferrites do not only have enormous commercial impact ({\pounds}2 billion/year in sales) due to applications that include ultra-high density memories, credit card stripes, magnetic bar codes, small motors and low-loss microwave devices, they also have fascinating magnetic and ferroelectric quantum properties at low temperatures. Here we report the results of tuning the magnetic ordering temperature in PbFe$_{12-x}$Ga$_x$O$_{19}$ to zero by chemical substitution $x$. The phase transition boundary is found to vary as $T_N \sim (1-x/x_c)^{2/3}$ with $x_c$ very close to the calculated spin percolation threshold which we determine by Monte Carlo simulations, indicating that the zero-temperature phase transition is geometrically driven. We find that this produces a form of compositionally-tuned, insulating, ferrimagnetic quantum criticality. Close to the zero temperature phase transition we observe the emergence of an electric-dipole glass induced by magneto-electric coupling. The strong frequency behaviour of the glass freezing temperature $T_m$ has a Vogel-Fulcher dependence with $T_m$ finite, or suppressed below zero in the zero frequency limit, depending on composition $x$. These quantum-mechanical properties, along with the multiplicity of low-lying modes near to the zero-temperature phase transition, are likely to greatly extend applications of hexaferrites into the realm of quantum and cryogenic technologies.