General Landau theory of non-symmetry-breaking and symmetry-breaking spin transition materials

Abstract

Spin-transition materials, including the families of spin-crossover and charge-transfer systems, and more generally molecular-based materials exhibiting electronic and/or structural bistability, may undergo various types of phase transitions. The change of electronic state is stabilized by molecular reorganizations and both phenomena, which are usually non-symmetry breaking, can be described through the evolution of an order parameter q. Due to symmetry, q linearly couples to volume change. It is known that such elastic interactions are responsible for cooperative phenomena in non-symmetry-breaking spin-transitions. However, spin-transition materials may also exhibit symmetry-breaking phenomena related to various types of orders such as structural order as well as spin-state concentration waves. The universal framework of the Landau theory of phase transition is relevant for describing such ordering processes through the evolution of a symmetry-breaking order parameter . The simultaneous or sequential occurrence of spin transition and symmetry breaking phenomena are reported for numerous spin-transition materials and the coupling between these two types of instabilities is responsible for the emergence of various types of functions. In this work, we use the Landau approach to describe both symmetry-breaking phenomena and non-symmetry-breaking spin transition. We discuss how their coupling can generate sequences of phase transitions, from simple spin crossover to spin transition, continuous or discontinuous symmetry breaking, including ferroelasticity or stepwise spin transitions.

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

“… 51 A formalism from Landau theory provides a convenient phenomenological means of analyzing this behavior in terms of elastic coupling between the order parameter associated with SCO and symmetry-breaking order parameters due to ordering of HS and LS atoms. 52 , 53 In the limiting case of SCO occurring without any change in crystallographic symmetry, the evolution of a single order parameter monitoring the global evolution of the spin state is sufficient to characterize the evolution of properties such as magnetic susceptibility, unit cell volume, and elastic moduli. This parameter has the symmetry of the identity representation of the space group and is therefore a non-symmetry-breaking order parameter q spin , as it preserves spatial symmetry.…”

Elastic Modeling of Two-Step Transitions in Sterically Frustrated 1D Binuclear Spin-Crossover Chains

“…[62], we investigated the case of 1D frustrated elastic SCO mononuclear chains, in which the frustration was introduced through conflicting interactions between the nn and nn equilibrium distances. We have shown in this study that according to the strength of the frustration, several thermodynamic behaviors can be reproduced, among which multi-step [30,[49][50][51][52][63][64][65][66][67][68][69] and incomplete spin transitions are retrieved.…”

Thermal and Magnetic Field Switching in a Two‐Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings

“…The SCO corresponds to an evolution of the non‐symmetry‐breaking order parameter , where and denote the number of sites in HS or LS states. Therefore, in the fully HS state and in the fully LS state [5, 6] . The elastic coupling between spin centres and the crystalline lattice results in a volume strain that can be weak, which would lead to a continuous SCO, or strong enough to drive a cooperative first‐ (discontinuous) or second‐order (continuous) phase transition [7–11] .…”

“…Thus, the nature of the SB can be very different, for example, cell doubling/reconstructive ordering and crystal system changes (ferroelastic phase transitions) can both result from the loss/gain of symmetry operators between two phases with a group/subgroup relation [7, 12] . The coupling of non‐SB ( q ) and SB ( η ) order parameters to the volume strain results in the linear quadratic coupling D qη 2 ( D being a measure of coupling strength), as described in the framework of Landau theory [5, 18] . In this case, a first‐order, SB structural phase transition and a SCO can happen simultaneously.…”

“…This allows for hysteretic switching and trapping of an excited spin state which sparks interest for applications such as memory effects [3, 6, 19] . The bi‐quadratic coupling E q 2 η 2 can stabilize half conversion SCO (around q =0) [5, 16] …”