Speaker
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Multiferroics have been at the forefront of condensed-matter research for several decades due to their importance for fundamental understanding of the coupling between magnetic and electric properties, as well as their potential for the development of energy-efficient data storage technologies [1,2]. Magnetic spinels such as iron vanadate, FeV$_{2}$O$_{4}$, in which V ions occupy a geometrically frustrated pyrochlore lattice of corner-sharing tetrahedra, form a highly versatile class of systems for studying exotic phenomena such as frustration and/or electron itinerancy.
Synchrotron x-ray scattering studies [3,4] have revealed several structural phase transitions in FeV$_{2}$O$_{4}$. Two of these transitions coincide with the onset of long-range magnetic order: a collinear ferrimagnetic phase below T$_{N1}$=110K and a non-collinear ferrimagnetic structure below T$_{N2}$=60K [5,6], where the V spins adopt a 2-in-2-out configuration. Furthermore, this material is remarkable for the strong effect of applied hydrostatic pressure on both the ferromagnetic coupling and the electrical resistivity [7]. The spin dynamics at ambient pressure were previously investigated on SEQUOIA [8] and HB-3 [9], revealing anomalous spin-wave broadening in the temperature range between T$_{N1}$ and T$_{N2}$. The sequence of structural, electric, and magnetic transitions in FeV$_{2}$O$_{4}$ highlights the strong coupling between the crystal lattice and the spin-orbital degrees of freedom of both Fe and V magnetic ions.
One focus of our work is the effect of uniaxial pressure on structure and dynamics of FeV$_{2}$O$_{4}$. By deforming the crystal environment, uniaxial strain may lift the magnetic frustration of the pyrochlore lattice. In our first experiment, we monitored the order parameter of the (111)$_c$ magnetic reflection on the EIGER triple-axis spectrometer together with a recently developed in-situ uniaxial pressure device (UPD) [10]. Pressure was applied along [110] crystallographic direction with forces up to 180 N ($\sim$ 0.1 GPa). Even under such a modest force load, both magnetic ordering temperatures change – both magnetic ordering temperatures shift: T$_{N1}$ increases while T$_{N2}$ decreases. This behavior indicates that breaking the bond-length symmetry of the pyrochlore tetrahedra suppresses the 2-in-2-out ordering temperature of the V sublattice. These neutron results were confirmed with measurements of the elastocaloric effect on a FeV$_{2}$O$_{4}$ single crystal along the same direction. In a subsequent neutron experiment on the IN8 triple-axis spectrometer, we observed a narrowing of magnetic excitations near 4.5 meV at T=80K under an applied force of 70 N ($\sim$ 0.07 GPa), together with a significant reduction in intensity and a change in the critical exponent of the (200)$_c$ reflection, which contains a magnetic contribution solely from the V sublattice. Ongoing data analysis, combined with density functional theory (DFT) calculations, aims to further clarify the complex interplay between lattice distortion, frustration, and spin dynamics in this material.
[1] M. Bibes et al., Nature Materials 7, 425–426 (2008)
[2] S. Manipatruni et al., Nature 565, (2019):35-42
[3] Y. Nii et al., Phys. Rev. B 86, 125142 (2012)
[4] T. Katsufuji et al., J. Phys. Soc. Jpn. 77, 053708 (2008)
[5] G. J. MacDougall et al., Phys. Rev. B 86, 060414(R) (2012)
[6] Q. Zhang et al., Phys. Rev. B 85, 054405 (2012)
[7] A. Kismarahardja et al., Phys. Rev. Lett. 106, 056602 (2011)
[8] G. J. MacDougall et al., Phys. Rev. B 89, 224404 (2014)
[9] Q. Zhang et al., Phys. Rev. B 89, 224416 (2014)
[10] G. Simutis et al., Rev. Sci. Instrum. 94, 013906 (2023)