Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor VTCNE x (x 2; TCNE tetracyanoethylene; magnetic ordering temperature T c 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in VTCNE x are proposed to originate from structural changes induced by internal excitation in TCNE ÿ anions, which lead to relaxation to a long-lived metastable state. DOI: 10.1103/PhysRevLett.97.247205 PACS numbers: 75.50.Xx, 75.50.Lk, 75.90.+w, 78.90.+t Materials possessing novel combinations of magnetic and electrical properties, such as multiferroic oxides, and dilute magnetic semiconductors (DMS), have received great attention in recent years, especially in the area of spintronics, which exploits both the spin and electrical properties of charge carriers in a material [1][2][3]. Organicbased magnets provide an alternative pathway in achieving novel characteristics of materials with a tunability of magnetic and electronic properties via organic methodologies. VTCNE x , (x 2), is a particularly interesting system because it features a magnetic ordering temperature above room temperature (T c 400 K) with a fully spin-polarized conduction band [4 -7]. Meanwhile, the control of magnetic properties by light has also been an active area of research that has attracted considerable attention in recent years. Extensive studies have been performed on a variety of systems that display magnetic bistability and/or photoinduced magnetism (PIM). These include cyanometalatebased magnets [8][9][10], spin crossover complexes [11,12] . Challenges towards multiple functionality in a single system include design and preparation of materials that exhibit both long-range magnetic order and photoinduced magnetization as well as light-tunable transport properties.We report here unusual multiple photonic phenomena in CVD-deposited films of the organic-based magnetic semiconductor VTCNE x . Photoinduced magnetism and conductivity are observed and correlate with each other and suggest reversible trapping of a photoexcited metastable state induced by structural change, as indicated in photoinduced IR absorption spectra.The organic-based magnet VTCNE x is a member of the MTCNE x family (M V, Fe, Mn, Co, Ni) [16] of magnets. The magnetic ordering in this class of magnet is based on direct exchange between the unpaired electrons in TCNE ÿ orbitals, whose spin (S 1=2) is distributed over the entire anion [17], and the spin in the transition metal ions. The VTCNE x is a ferrimagnet of uncompensated antiparallel TCNE ÿ (S 1=2) and V 2 (S 3=2) spins with a T c 400 K [4]. Extended x-ray absorption fine structure (EXAFS) studies indicate that V 2 has octahedral coordination with N [18]. The conductivity is semiconductorlike and varies with preparation in the range 10 ÿ2 -10 ÿ4 S cm ÿ1 ...

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L{2,3} and C and N K edges reveal bonding and backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE]x approximately 2. In V spectra, d{xy}-like orbitals are modeled assuming V2+ in an octahedral ligand field, while d{z{2}} and d{x{2}-y{2}} orbitals involved in strong covalent sigma bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE]x approximately 2 associated with weaker pi bonding interactions that yield its novel properties.

The V TCNE x , x 2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V TCNE x film. Upon optical excitation ( 457:9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V TCNE x induced by the increased structural disorder in the system. DOI: 10.1103/PhysRevLett.99.157205 PACS numbers: 75.90.+w, 75.50.Lk, 75.50.Xx, 76.50.+g Recently, molecule-based magnets have received growing attention for their new science and also possible application to various spin related devices [1]. A variety of molecule-based magnetic systems display magnetic bistability due to their inherent structural nature and/or their flexibility, and often their bistability can be controlled by optical stimulus [2 -4]. A particularly interesting phenomenon is the coexistence of photoinduced magnetism (PIM) and long-range magnetic order, which so far is mainly investigated in two classes of systems: cyanometalate compounds [2,5,6] and the family of M TCNE x (M Mn, V; x 2; TCNE tetracyanoethylene) magnets [4,7]. Extensive research revealed that PIM in cyanometalate compounds originates from charge transfer induced changes in spin values within the cluster glass model [5,8]. On the other hand, the lack of change in saturation magnetization in M TCNE x by the illumination [4,7,9] suggests a completely different mechanism for PIM. Recent theoretical study suggested that the origin of PIM in Mn TCNE 2 is a result of change of relative strengths of the double-exchange and superexchange couplings by the charge transfer between metal and ligand due to optical excitation [10]. However, the charge transfer and the change of exchange couplings in this model imply change in saturation magnetization [10].In this study, we employed photoinduced ferrimagnetic resonance (PIFMR) to investigate the mechanism of PIM in V TCNE x films prepared by chemical vapor deposition (CVD). V TCNE x is a ferrimagnet of antiparallel TCNE ÿ (S 1=2) and V 2 (S 3=2) spins with magnetic ordering temperature T c 400 K [11]. Detailed transport studies indicate that the material is a semiconductor with an energy gap 0:5 eV between spin polarized valence and conduction bands [12 -15]. It was reported that V TCNE x exhibits concomitant photoinduced magnetic and electrical phenomena upon optical stimulus [7]. Both the magnetization and conductivity show persistent and thermally reversible change induced by the ! excitation in TCNE ÿ anions [7]. Under illumination, there is a substantial decrea...

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We describe how the composition of an organic - based magnet can be controlled by varying the Chemical Vapor Deposition (CVD) conditions. A study was conducted for the Co2(CO)8/ TCNE system to form cobalt tetracyanoethylene [Co(TCNE)x, x∼2, a paramagnetic material], and for the V(CO)6/ TCNEx system to form vanadium tetracyanoethylene [V(TCNE)x, x∼2, a ferrimagnetic material]. Thin V(TCNE)x, x∼2 films (∼0.05-0.5 μm) with room temperature conductivity of 10-4<σRT<10-3S/cm and magnetic ordering temperature Tc of up to ∼400K were deposited. The V(TCNE)x, x∼2 thin films have the potential for incorporation in a spin-valve device as one of the magnetic contacts, and are promising candidates to form optically controlled magnetic-based structures.