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 24.06.2011   Карта сайта     Language По-русски По-английски
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24.06.2011

Light-induced spin-crossover magnet





Journal name:

Nature Chemistry

Volume:

3,

Pages:

564–569

Year published:

(2011)

DOI:

doi:10.1038/nchem.1067


Received


Accepted


Published online




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 FeNb-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.




left


  1. 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 FeNb 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 FeNb framework, respectively.




  2. 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 dd transitions of band I (1A11T2) and band II (1A11T1) on FeII(LS). Measurement errors are included within the marks and lines.




  3. 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.




  4. Figure 4: Mechanism of light-induced spin-crossover ferromagnetism.







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  • Chen Wev .  honorary member of ISSC science council

  • Harton Vladislav Vadim  honorary member of ISSC science council

  • Lichtenstain Alexandr Iosif  honorary member of ISSC science council

  • Novikov Dimirtii Leonid  honorary member of ISSC science council

  • Yakushev Mikhail Vasilii  honorary member of ISSC science council

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