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

S. E. Rowley; T. Vojta; A. T. Jones; W. Guo; J. Oliveira; F. D. Morrison; N. Lindfield; E. Baggio Saitovitch; B. E. Watts; J. F. Scott

doi:10.1103/physrevb.96.020407

arXiv · 1708.03945

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

S. E. Rowley·T. Vojta·A. T. Jones·W. Guo·J. Oliveira·F. D. Morrison·N. Lindfield·E. Baggio Saitovitch·B. E. Watts·J. F. Scott

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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.

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S. E. Rowley, T. Vojta, A. T. Jones, W. Guo, J. Oliveira, F. D. Morrison, N. Lindfield, E. Baggio Saitovitch, B. E. Watts, J. F. Scott. 2017-08-13. Quantum percolation phase transition and magneto-electric dipole glass in hexagonal ferrites. https://doi.org/10.1103/physrevb.96.020407

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