Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling

Abstract

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn 3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn III (3,5-diCl-sal 2 (323))]BPh 4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P 1 → P 1 (1/2) . The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn 3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc , the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P 1 and P 1 → P 1 (1/2) transitions, with opposite signs. All these phases are polar, while P 1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P 1→ P 1 (1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

The effect of fluorine substituents on the crystal structure and spin crossover behavior of the cation [MnIII(3,5-diHal-sal2323)]+ complex family with BPh4 anions

“…4. 27,29,30 Unlike 4-6, which show sharp spin transformations related to structural phase transitions, in 1 no structural phase transitions are observed and the spin crossover from HS to LS is gradual. The magnetic data correlate well with the changes in the Mn-N bond lengths with decreasing temperature.…”

“…It is well known that the cation-anion non-covalent intermolecular interactions, particularly N-HÁ Á ÁX interactions between the Mn(III) cationic complex and counter anions, can play a significant role in determining the spin-state switching behavior. 6,11,[17][18][19][26][27][28][29][30] The replacement of one of the F atoms in position 5 of the phenolate nucleus by bromine leads to the appearance of shortened BrÁ Á ÁBr contacts in the dimers between cations in the structure of complex 2 (Fig. S3, ESI †).…”

“…Changes in the Mn-N bond lengths and octahedral distortion parameters with decreasing temperature are almost the same in both complexes (Table 1). A common supramolecular feature of these complexes, as well as other similar systems showing spin transformations, [27][28][29][30] is the presence in their structures of intermolecular N-HÁ Á Áp Ph Á Á ÁH-N cation-anion interactions with the formation of one-dimensional chains. In contrast to the monoclinic polymorph (2), the triclinic polymorph (3), in which the cation-anion N-HÁ Á Áp Ph interactions are only a discrete character (Fig.…”

“…[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] The development of these approaches has recently led to the synthesis of tetraphenylborates of Mn(III) complexes with a sal 2 323 ligand containing dihalide (Cl, Br) substituents on the phenolate ring: [Mn(3,5-diCl-sal 2 323)]BPh 4 (4), [Mn(3,5-Br,Cl-sal 2 323)]BPh 4 (5) and [Mn(3,5-diBr-sal 2 323)]BPh 4 (6) which showed unusual structural and magnetic properties. [27][28][29][30] All three complexes are isostructural, but have fundamentally different properties. Dichlorine-containing complex 4 shows three structural phase transitions, two of which are associated with sharp spin crossovers from HS to the HS:LS intermediate phase at 137 K and from the intermediate phase to the LS at 82 K. The nature of the dihalogen substituents (chloride and bromide) has a dramatic effect on the properties of these complexes: even the replacement of one chlorine by bromine changes the character and temperature of the phase and spin transitions.…”

“…Unlike 4, complexes 5 and 6 demonstrate only one spin switching from the HS to HS:LS intermediate state at 83 K related to the structural phase transition. It has been found that these complexes show magnetoelectric coupling and magnetic-field-induced permanent switching, 30,31 as result of cross-coupling between the spin state, structure, and electric polarization. It is also important to note that these complexes do not contain co-crystallized solvent molecules, which, firstly, increases their stability, since solvate solvents are often lost during storage of the SCO complexes, and secondly, it allows one to study the effects of ligand substitution on the magnetostructural correlations without any additional influence associated with the co-crystallized solvent.…”