Photoinduced magnetization on Mo ion in copper octacyanomolybdate : an x-ray magnetic circular dichroism investigation

Marie-Anne Arrio (1) , Jérôme Long (2) , Christophe Cartier Dit Moulin (2) , Anne Bachschmidt (2) , Valerie Marvaud (2) , Andrei Rogalev (3) , Corine Mathonière (4) , Fabrice Wilhelm (3) , Philippe Sainctavit (1)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 IPCM - Institut Parisien de Chimie Moléculaire
3 ESRF - European Synchrotron Radiation Facility
4 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux

Recently synthesized copper octacyanomolybdate molecules present interesting photomagnetic properties. Before irradiation, the molecules behave as noncoupled Cu^II paramagnetic ions (with central diamagnetic Mo^IV ion), whereas after irradiation (406 nm) they behave as superparamagnetic molecules with Cu^II ions coupled to the molybdenum ion. The proposed mechanism to explain these photomagnetic properties is based on the photoinduced charge transfer from Mo^IV (S = 0) to Cu^II (S = 1/2) leading to the formation of Mo^V (S = 1/2) and Cu^I (S = 0) with strong ferromagnetic coupling between Mo^V and the other Cu^II spin carriers. This paper presents X-ray magnetic circular dichroism (XMCD) measurements at the molybdenum L_2,3 edges. Two bimetallic molecules have been investigated: [Mo(CN)₂(CN-CuL)₆]⁸⁺, L being tris(2-amino)ethylamine and [Mo(CN)₆(CN-CuL′₂)₂], with L′ being N,N′,dimethyl ethylene diamine (labeled MoCu₂-Meen). In both cases, before irradiation the XMCD signal is null as expected for diamagnetic Mo^IV (S = 0). After irradiation, an XMCD signal appears, which directly demonstrates the formation of spin density on the Mo ion. After reaching room temperature, the photoinduced spectral features disappear, indicating the reversibility of the photoinduced process. In the case of MoCu₂-Meen, the XMCD experiments allow the observation of an X-ray-induced metastable state made of high-spin Mo^IV ion (S = 1). The application of the sum rule for isotropic spectra shows that there is no variation of the molybdenum oxidation state during the X-ray-induced magnetic process. From the Mo L_2,3 XMCD signal of the X-ray-photoexcited MoCu₂-Meen, we obtain an orbital magnetic moment equal to 0.13 μ_B and a spin magnetic moment equal to 1.22 μ_B at T = 10 K and H = 6 T. These results demonstrate that the Mo ion in the X-ray-photoinduced excited state of the MoCu₂-Meen complex is high-spin Mo^IV (S = 1).

Domaines: Matériaux

Marie-Anne Arrio, Jérôme Long, Christophe Cartier Dit Moulin, Anne Bachschmidt, Valerie Marvaud, et al.. Photoinduced magnetization on Mo ion in copper octacyanomolybdate : an x-ray magnetic circular dichroism investigation. Journal of Physical Chemistry C, 2010, 114 (1), pp.593-600. ⟨10.1021/jp9087699⟩. ⟨hal-00467036⟩