Quantum percolation phase transition and magneto-electric dipole glass in hexagonal ferrites
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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 PbFe12-xGaxO19 to zero by chemical substitution x. The phase transition boundary is found to vary as TN~(1 - x/xc)2/3 with xc 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 unique 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 Tm has a Vogel-Fulcher dependence with Tm finite, or suppressed below zero in the zero frequency limit, depending on 2 of 18 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.
Rowley , S E , Vojta , T , Jones , A , Guo , W , Oliveira , J , Morrison , F D , Lindfield , N , Baggio-Saitovitch , E , Watts , B E & Scott , J F 2017 , ' Quantum percolation phase transition and magneto-electric dipole glass in hexagonal ferrites ' Physical Review. B, Condensed matter and materials physics , vol 96 , no. 2 , 020407(R) . DOI: 10.1103/PhysRevB.96.020407
Physical Review. B, Condensed matter and materials physics
© 2017 American Physical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1103/PhysRevB.96.020407
SER and EBS acknowledge support from a CONFAP Newton grant. T.V. acknowledges support from the NSF under Grant No. DMR-1506152.
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