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

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.

Pressure–Temperature–Magnetic Field Phase Diagram of Multiferroic (NH4)2FeCl5·H2O

“…38 Electric polarization is similar to the magnetic response with a series of low-field polarization flops (from a to a′ to c) across the spin flop transition 29,37,39 above which it grows linearly with increasing field, 37,40 reaching a maximum near 24 T before plunging to zero as the inversion center is lost near 27 T�consistent with expectations for a Type II multiferroic. 4,40 Multiferroicity in the commensurate quasi-collinear state above 5 T is described by the spindependent p−d hybridization model, 36 and recent work to 60 T shows that the system continues to display an orbital hybridization mechanism. 37,40 41 and the Ni analogue 42 is the fully molecular character of our target system, the fact that amine ordering does not immediately trigger ferroelectricity, and the manner in which intermolecular hydrogen and halogen bonding (rather than superexchange ligands) support magnetic interactions.…”

“… Spin density calculations reveal how intermolecular interactions support magnetic exchange as well as changes in these pathways across the transition to the fully saturated state . Electric polarization is similar to the magnetic response with a series of low-field polarization flops (from a to a′ to c ) across the spin flop transition ,, above which it grows linearly with increasing field, , reaching a maximum near 24 T before plunging to zero as the inversion center is lost near 27 Tconsistent with expectations for a Type II multiferroic. , Multiferroicity in the commensurate quasi-collinear state above 5 T is described by the spin-dependent p – d hybridization model, and recent work to 60 T shows that the system continues to display an orbital hybridization mechanism. , Therefore, (NH 4 ) 2 FeCl 5 ·H 2 O hosts two distinct magnetoelectric coupling mechanisms rather than a single overarching model. To the best of our knowledge, a magnetic field-dependent coupling mechanism has not been observed in any other multiferroic.…”

“…4,40 Multiferroicity in the commensurate quasi-collinear state above 5 T is described by the spindependent p−d hybridization model, 36 and recent work to 60 T shows that the system continues to display an orbital hybridization mechanism. 37,40 41 and the Ni analogue 42 is the fully molecular character of our target system, the fact that amine ordering does not immediately trigger ferroelectricity, and the manner in which intermolecular hydrogen and halogen bonding (rather than superexchange ligands) support magnetic interactions. As a result, (NH 4 ) 2 [FeCl 5 •(H 2 O)] hosts exotic noncollinear magnetic states, a complex magnetic field−temperature phase diagram, and two different types of magnetoelectric coupling mechanisms in magnetic field as described above.…”

Structural Diversity and Tunable Ferroelectricity in a Family of Hybrid Perovskites: AM(NO3)3

“…As one of the most important properties of perovskite materials, ferroelectricity was also realized in hybrid perovskites. These hybrid perovskites offer the opportunity to combine ferroelectricity with other properties, which gives rise to novel properties, such as magnetoferroelectric properties, photoferroelectric properties, and semiconducting ferroelectric properties. − The last decade has witnessed the rapid renaissance of hybrid perovskite materials because of their easy process, light weight, mechanical flexibility, and even excellent ferroelectric properties. Although HOIPs can have a diverse range of compositions, including halides, formates, cyanides, dicyanamides, azides, and dicyanometallates, ferroelectricity is realized mainly in alkaline metal/NH 4 + halides and metal formates.…”

Recent Progress of Multiferroicity and Magnetoelectric Effects in ABX3‐Type Perovskite Metal–Organic Frameworks

“…[ 45 ] Similarly, the ordering of hydrogen bonds plays a vital role in inducing the paraelectric to ferroelectric phase transition in multiferroic (NH 4 ) 2 [FeCl 5 (H 2 O)]. [ 77,78 ] Besides this, resonant quantum tunneling of magnetization which is well interpreted based on a selective long‐distance super exchange model, has been observed in DMA‐Fe (Figure 10e). This peculiar magnetic behavior is due to the exchange interaction between transition metal ions through an organic linker depends on the position of hydrogen bonds.…”