The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended FeII(HS) sites in a metal–organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe–Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced FeII(HS) and neighbouring NbIV atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.
Figures at a glance
Figure 1: Crystal structure of Fe2[Nb(CN)8]·(4-pyridinealdoxime)8·2H2O. 
a, Coordination environments around Fe and Nb. The Fe atom is coordinated by two cyanide nitrogen atoms of [NbIV(CN)8] and four pyridyl nitrogen atoms of 4-pyridinealdoxime. Four CN groups of [NbIV(CN)8] are bridged to four Fe centres, and the other four remain free. Red and green ball-sticks represent the [FeN6] and [NbC8] moieties, respectively. Light blue, blue and pink balls represent C, N and O atoms in 4-pyridinealdoxime. b, Cyano-bridged Fe–Nb three-dimensional framework viewed along the c-axis. c, View from the diagonal direction. 4-Pyridinealdoxime molecules are drawn as light blue wire frames or orange sticks with planes. Zeolitic water molecules are omitted for clarity. d, Organic ligand molecules of 4-pyridinealdoxime are drawn as spheres, considering their van der Waals radii. Light grey, grey, blue and red spheres denote C, H, N and O atoms, respectively. Small red and green balls and grey sticks represent Fe and Nb atoms and the cyano-bridged Fe–Nb framework, respectively.
Figure 2: Spin-crossover transition phenomenon. 
a, Temperature T dependence of the molar magnetic susceptibility χM measured in an external field of 5,000 Oe. The decrease in the χMT value with decreasing temperature indicates the transition from the high-T form to the low-T form, which is due to the spin-crossover from FeII(HS)(S = 2) to FeII(LS)(S = 0). Squares and circles denote χMT values with decreasing (blue arrow) and increasing (black arrow) temperature, respectively. Inset: schematic illustration of the electronic configuration of FeII(HS) and FeII(LS). b, Temperature dependencies of the differential UV–vis absorption spectra from 300 K to 100 K in 20 K intervals. As the temperature decreases, the variable-temperature UV–vis absorption spectra exhibit optical absorptions around 480 nm (band I) and 650 nm (band II). Inset: schematic illustration for the ground and excited states of FeII. Arrows indicate the d–d transitions of band I (1A1 → 1T2) and band II (1A1 → 1T1) on FeII(LS). Measurement errors are included within the marks and lines.
Figure 3: Photo-induced magnetization caused by light-induced spin-crossover. 
a, Magnetization versus temperature curves at 100 Oe. Light irradiation induces a spontaneous magnetization with a Curie temperature of 20 K. b, Magnetic hysteresis curves at 2 K. After irradiation, a magnetic hysteresis loop with a coercive field of 240 Oe appeared. Blue and red circles denote measurements before and after irradiation with 473 nm light. Measurement error is included within the marks.
Figure 4: Mechanism of light-induced spin-crossover ferromagnetism. 
