Spin-lattice coupling mediated multiferroicity in (ND4)2FeCl5·D2O

Abstract

We report a neutron diffraction study of the multiferroic mechanism in (ND4)2FeCl5•D2O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at TN =7.3 K, followed by a second transition to an IC cycloidal spin state at TF E =6.8 K, the later of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The appearance of the 2nd-order harmonic coincides with the onset of the electric polarization, thereby providing unambiguous evidence that the induced electric polarization is mediated by the spin-lattice interaction. Our results for this system, in which the orbital angular momentum is expected to be quenched, are remarkably similar to those of the prototypical TbMnO3, in which the magnetoelectric effect is attributed to spin-orbit coupling.

Magnetic field-temperature phase diagram of multiferroic (NH4)2FeCl5·H2O

“…The high-field state is therefore unlikely to be magnetoelectric. Things are different at 5 T. Here, the spin configuration is quasi-collinear AFM 3 and the system is in the FE III phase with polarization along c. 24,27 Whether the spin structure, polarization, and magnetoelectric coupling at 5 T [24][25][26][27]29 are similar to those further away from equilibrium-for instance, at 27 T-awaits further study. That ferroelectricity depends so intimately upon the spin configuration is the overarching motivation for revealing the magnetic phases of (NH 4 ) 2 FeCl 5 ⋅H 2 O and unveiling the overall structure of the phase diagram.…”

“…Figure 4 displays the B-T phase diagram of (NH 4 ) 2 FeCl 5 ⋅H 2 O for B || c. We label the different regimes in accord with prior magnetization, polarization, magnetostriction, and neutron scattering. [24][25][26][27]29 Here, AFM = antiferromagnetic, PM = paramagnetic, ICC = incommensurate cycloidal, CS = cycloidal spiral, C1 = distorted cycloid, C2 = quasi-collinear, CLS = collinear sinusodial, FE = ferroelectric, and NE = non-electric. Of course, the transition to the fully polarized state (B Sat ≈ 30 T) along with two additional reorientation transitions immediately preceding it are entirely new -providing the first glimpse of the full complexity of the B-T diagram in (NH 4 ) 2 FeCl 5 ⋅H 2 O.…”

“…Although the properties and complex multiferroic phases of (NH 4 ) 2 FeCl 5 ⋅H 2 O have been extensively studied in the low-field regime, [24][25][26][27]29 it is interesting to anticipate the high-field response. In general, for type-II multiferroics, the spins need to break spatial inversion symmetry in order to create an electric polarization.…”

“…These include (i) an order-disorder transition involving NH þ 4 [T O/D ≈ 79 K], (ii) the Néel transition below which incommensurate-collinear sinusoidal magnetic order appears [T N ≈ 7.25 K], and (iii) a transition to an incommensurate-cycloidal spin state that supports a spontaneous electric polarization primarily along the a-axis [T FE ≈ 6.9 K]. [26][27][28][29] A fascinating series of spin and polarization flops occur with increasing magnetic field. 24,25 These magnetically-driven transitions will be discussed in greater detail below.…”

“…1a, b, these intermolecular interactions form a set of superexchange pathways linking the Fe 3+ S ¼ 5 2 À Á centers. [28][29][30][31] The full set of magnetic interaction pathways includes Fe-O-H⋯Cl-Fe, Fe-Cl⋯Cl-Fe, and Fe-Cl⋯O-Fe linkages. The magnetic interaction through O-H⋯Cl is the strongest and forms a zig-zag chain along the b-axis.…”