High-Field Magnetoelectric and Spin-Phonon Coupling in Multiferroic (NH4)2[FeCl5·(H2O)]

Kendall D. Hughey, Minseong Lee, Jisoo Nam, Amanda J. Clune, Kenneth R. O’Neal, Wei Tian, Randy S. Fishman, Mykhaylo Ozerov, Jun Hee Lee, Vivien S. Zapf, Janice L. Musfeldt

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Abstract

We combine high field polarization, magneto-infrared spectroscopy, and lattice dynamics calculations with prior magnetization to explore the properties of (NH4)2[FeCl5·(H2O)]─a type II molecular multiferroic in which the mixing between charge, structure, and magnetism is controlled by intermolecular hydrogen and halogen bonds. Electric polarization is sensitive to the series of field-induced spin reorientations, increasing linearly with the field and reaching a maximum before collapsing to zero across the quasi-collinear to collinear-sinusoidal reorientation due to the restoration of inversion symmetry. Magnetoelectric coupling is on the order of 1.2 ps/m for the P∥c, H∥c configuration between 5 and 25 T at 1.5 K. In this range, the coupling takes place via an orbital hybridization mechanism. Other forms of mixing are active in (NH4)2[FeCl5·(H2O)] as well. Magneto-infrared spectroscopy reveals that all of the vibrational modes below 600 cm-1 are sensitive to the field-induced transition to the fully saturated magnetic state at 30 T. We analyze these local lattice distortions and use frequency shifts to extract spin-phonon coupling constants for the Fe-O stretch, Fe-OH2 rock, and NH4+ libration. Inspection also reveals subtle symmetry breaking of the ammonium counterions across the ferroelectric transition. The coexistence of such varied mixing processes in a platform with intermolecular hydrogen- and halogen-bonding opens the door to greater understanding of multiferroics and magnetoelectrics governed by through-space interactions.

Original languageEnglish
Pages (from-to)3434-3442
Number of pages9
JournalInorganic Chemistry
Volume61
Issue number8
DOIs
StatePublished - Feb 28 2022

Funding

Research at Tennessee is supported by the National Science Foundation (DMR-1707846). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida. V.S.Z., W.T., and R.S.F. acknowledge funding from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, Condensed Matter Experiment and Theory Programs. J.H.L. and J.N. at UNIST were supported by Basic Research Laboratory (NRF2017R1A4A1015323), Midcareer Researcher (2020R1A2C2103126), and Creative Materials Discovery (2017M3D1A1040828) programs through the National Research Foundation of Korea, the MOTIE (Ministry of Trade, Industry and Energy; No. 10080657), and KRSC (Korea Semiconductor Research Consortium) support program. We thank Mark Turnbull for useful conversations.

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