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

Abstract

Hexagonal ferrites do not only have enormous commercial impact (£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.

17 pages. Final version as published

References in corpus (7)

• Quantum magnetism and criticality
• Rare region effects at classical, quantum, and non-equilibrium phase transitions
• Vogel-Fulcher freezing in relaxor ferroelectrics
• Physical realization of a quantum spin liquid based on a novel frustration mechanism
• Non-d${}^0$ Electric Dipole in FeO${}_5$ bipyramid: a New Resource for Quantum Paraelectrics, Ferroelectrics and Multiferroics
• Percolation quantum phase transitions in diluted magnets
• Stacked triangular lattice: Percolation properties