Hexagonal ferrites not only have enormous commercial impact (£2 billion/year in sales) due to applications that include ultrahigh-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 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 magnetoelectric coupling. The strong frequency behavior 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 composition x. These quantum-mechanical properties, along with the multiplicity of low-lying modes near the zero-temperature phase transition, are likely to greatly extend applications of hexaferrites into the realm of quantum and cryogenic technologies.
S. E. Rowley et al., "Quantum Percolation Phase Transition and Magnetoelectric Dipole Glass in Hexagonal Ferrites," Physical Review B, vol. 96, no. 2, American Physical Society (APS), Jul 2017.
The definitive version is available at https://doi.org/10.1103/PhysRevB.96.020407
Keywords and Phrases
Ferrite; Saturation magnetization; Strontium hexaferrite; Dielectric properties; Ferrimagnetism; Ferroelectricity; Magnetic phase transitions; Percolation; Quantum phase transitions
International Standard Serial Number (ISSN)
Article - Journal
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