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Ferroelectric charge order stabilized by antiferromagnetism in multiferroic LuFe2O4
Authors
Annemieke Mulders and Maciej Bartkowiak (UNSW Canberra), James Hester (Bragg Institute, ANSTO), Ekaterina Pomjakushina and Kazimierz Conder (Paul Scherrer Institute)
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| Fig 1 (left): Structure of LuFe2O4 showing the FeO5 bilayer iand indicating small red Fe3+ and large blue Fe2+ ions, with purple magnetic moments of 5 μB and 4 μB respectively; |
Figure 2 (right): Neutron intensity recorded along the (1/3 1/3 L) direction at T = 180K, as a function of the electric-field-cooling (EFC) procedure. The magnetic state obtained after electric-field-cooling to 225 K is compared to the magnetic state obtained after zero-electric-field-cooling (ZEFC) (bottom) and to the magnetic state obtained after electric-field-cooling to 180 K (top, shifted for clarity). The black dotted line indicates 2-dimensional magnetic order in the a-b plane, with moments along the c-axis. The inset shows the integrated intensity, normalized to the total intensity at zero-electric-field-cooling, as a function of the electric-field-cooling sequence.
The goal of this research is to understand the magnetoelectric interactions in multiferroic materials, in which ferroelectricity and ferromagnetism coexist and interact. Materials which are either magnetic or ferroelectric are essential for modern information technologies. In multiferroic materials both properties, ferroelectric polarization and ferromagnetic order, exist simultaneously.
These properties are extremely useful in devices in which the coexistence of both charge and spin components can be exploited, and one property can be used to drive the other. Control of magnetic components in memory-storage devices through electric manipulation will provide faster storage and retrieval of information.
Ferroelectric polarization arises from covalent bonding between anions and cations, or from the orbital hybridization of electrons, and generally excludes occurrence of finite 3d magnetic moments and associated magnetism. However, competing bond and charge order in transition-metal compounds can lead to inversion symmetry breaking and multiferroic behaviour.
Alternatively, ferroelectric polarization may arise from frustrated charge order, as reported for LuFe2O4. This compound is of particular interest, as, in addition to ferroelectricity, magnetism originates from the same Fe ions, and this holds the promise of strong magnetoelectric coupling. The ferroelectric order and the magnetic order take place at and near ambient temperature, respectively, which provides the potential for room-temperature multiferroics.
Figure 1 shows the iron ions of a single bilayer of FeO5 trigonal bipyramids. The charge-ordered structure is indicated by the red and blue Fe3+ and Fe2+ atoms. This charge order results in a net polarization of the bilayer along both the [110] and [001] directions. The purple arrows indicate the magnetic moments of the iron ions. These bilayers are stacked parallel or antiparallel along the c-axis to form a ferroelectric or antiferroelectric alignment.
We have investigated the magnetic order in a single crystal of LuFe2O4 on the WOMBAT high-intensity neutron diffractometer, as a function of applied electric-field-cooling[2]. We observe a change in the magnetic long-range order when an applied electric field is present at the magnetic-ordering temperature.
Figure 2 shows that the long-range charge order that is promoted by the electric field is stabilized by the magnetic order, as demonstrated by the increased intensity of all magnetic domains. We propose that antiferromagnetic order in the Fe bilayers facilitates the formation of long-range charge order because electron hopping between the sheets of the bilayer is assisted by the alignment of the Fe2+ and Fe3+ spins.
According to Hund’s rules, Fe3+ has five 3d electrons with spin up, and Fe2+ has five 3d electrons with spin up and one with spin down. This latter 3d electron can jump from Fe2+ to the Fe3+ much more easily if the spin-down states of the latter are empty. This is the case for parallel alignment of the Fe2+ and Fe3+ moments.
Thus electron hopping in the majority Fe2+ layer is arrested, because the Fe2+ and Fe3+ ions have opposite moments, while 3d electron hopping is promoted in the minority Fe2+ layer because some neighbouring Fe2+ and Fe3+ ions have parallel moments (double-exchange mechanism). This makes a convenient mechanism to switch between ferroelectric and antiferroelectric arrangements of the bilayers. As such, the magnetism assists the ferroelectric order to align with the electric field and enhance the electric polarization.
References:
- K. Momma and F. Izumi, J. Appl. Crystallogr. 41, 653 (2008).
- A. M. Mulders, M. Bartkowiak, J. R. Hester, E. Pomjakushina, and K. Conder, Phys. Rev. B 84, 140403(R) (2011).