Novel Mechanism of Photoinduced Reversible Phase Transitions in Molecule-Based Magnets

Abstract

A novel microscopic mechanism of bi-directional structural changes is proposed for the photoinduced magnetic phase transition in Co-Fe Prussian blue analogues on the basis of ab initio quantum chemical cluster calculations. It is shown that the local potential energies of various spin states of Co are sensitive to the number of nearest neighbor Fe vacancies. As a result, the forward and backward structural changes are most readily initiated by excitation of different local regions by different photons. This mechanism suggests an effective strategy to realize photoinduced reversible phase transitions in a general system consisting of two local components. 78.90.+t,71.35.Lk, Repeatable switching-on and -off of magnetization by external stimuli such as light is one of the most fascinating phenomena with potential applications in next generation's information storage and processing. A bi-directional photo-induced magnetization was first discovered in a cobalt-iron Prussian blue analogue,Illumination of visible light (500 -700 nm) at low temperature induces a bulk magnetization (presumably, ferrimagnetism), which can be eliminated by illumination of near-IR light (∼1300 nm). In spite of various experimental and theoretical efforts [1,2,[4][5][6][7][8], the microscopic mechanism of the reversible magnetization is still not clear. In this Letter we report ab initio quantum chemical cluster calculations for the Co-Fe Prussian blue analogue, unveiling the microscopic mechanism of the bi-directional photo-induced local structural changes that trigger the phase transitions.The crystal of a Prussian blue analogue K 1−2x Co 1+x Fe(CN) 6 is composed of two metallic sites located on vertices of the cubic lattice and each surrounded by six cyano moieties, as shown in Fig. 1. The d-orbitals of transition metals split into t 2g and e g orbitals by the ligand field. In the case of x =0, there are vacancy sites with replacement of CN by H 2 O as shown in Fig. 1(b). Various fascinating phenomena including a room temperature magnet [9], electrochemically tunable magnets [10], transparent and colored magnetic thin films [11], and photo-induced magnetic dipole inversion [12] have been observed in such non-stoichiometric compounds. We will show that this non-stoichiometric aspect is essential for the reversible photo-induced magnetization.The low spin (LS) configuration of the ground nonmagnetic state and the high spin (HS) configuration of the meta-stable magnetic state of K 0.4 Co 1.3 Fe(CN) 6 · 5H 2 O are most likely Co III (dε 6 , S = 0)Fe II (dε 6 , S = 0) and Co II (dε 5 dγ 2 , S = 3/2)Fe III (dε 5 , S = 1/2), respectively [1,2]. These are depicted in Fig.2 as LS0 and HS0 states. Figure 2 schematically represents the most plausible elementary processes in the cycle of the photo-induced structural change. The LH0 and HS0 states are converted to the intermediate states LS1 and HS1, respectively, by photo-induced charge transfer (CT) between iron and cobalt atoms, and then to the final states HS0 and LS0 by intersystem crossing due t...

Reversible mechanism for spin crossover in transition-metal cyanides

“…23 In contrast, previous simulations of PBA clusters (rather than crystals) were interpreted to posit a significant and essential role of H 2 O in PBA spin transitions. 19 Thus, our current findings are in better agreement with available experiments and with recent calculations for anhydrous and watercontaining Prussian blue crystals in the LS state. 23 This correspondence is due ostensibly to the fact that we have modeled the actual crystal and found the crystal symmetry distortion to couple strongly to the spin transition mechanisms.…”

“…1,2,12,13 However, neither the microscopic transition mechanisms nor the effects of Fe-vacancies, water, and alkali metals on these mechanisms are completely understood, despite considerable experimental and theoretical effort. [16][17][18][19] 6 ] does not influence the charge transfer excitation energy. 23 Here we investigate the microscopic spin transition mechanisms via an ab initio lattice model for CoFe-PBA.…”

“…Moreover, as this spin crossover is realized in the anhydrous form of this crystal, we find that incorporated H 2 O molecules are not essential prerequisites of the transition as had been proposed previously. 19 …”

Muon spin relaxation study of the magnetism in unilluminated Prussian Blue analogue photomagnets

“…The proposed origin of the photomagnetic effect is a light-induced charge transfer from the state: Fe II (t 6 2g , S = 0)-CNCo III (t 6 2g , S = 0) to the meta-stable self-trapped state: Fe III (t 5 2g , S = 1/2)-CN-Co II (t 5 2g e 2 g , S = 3/2) [9,14], effectively increasing the concentration of magnetic moments. The non-stoichiometry is believed essential for the photoinduced magnetization [14]. However, in spite of considerable experimental and theoretical effort, the microscopic mechanism of the photomagnetic effect and the nature of magnetic ordering remain unclear, often because conclusions are drawn solely on the basis of macroscopic magnetization measurements [12].…”

“…The proposed origin of the photomagnetic effect is a light-induced charge transfer from the state: Fe II (t 6 2g , S = 0)-CNCo III (t 6 2g , S = 0) to the meta-stable self-trapped state: Fe III (t 5 2g , S = 1/2)-CN-Co II (t 5 2g e 2 g , S = 3/2) [9,14], effectively increasing the concentration of magnetic moments. The non-stoichiometry is believed essential for the photoinduced magnetization [14].…”