Effect of solvent on the magnetic properties of the high-temperature V[TCNE]x molecule-based magnet

Journal ArticleThe different magnetic behaviors of the V[TCNE]x (TCNE=tetracyanoethylene) magnet prepared via the reaction of TCNE and V(CO)6 in CH2Cl2 and the solvent-free chemical vapor deposition (CVD) of TCNE and V(CO)6 onto a glass substrate were determined. It was shown that the presence of noncoordinating CH2Cl2 solvent molecules in the structure of the V[TCNE]x magnet does not substantially influence the magnetic ordering temperature; however, it creates additional structural disorder. This results in a correlated spin-glass behavior in low magnetic fields (H<100 Oe) at helium temperatures, small remanence, and vanishing coercive fields at room temperature. In contrast, the CVD-prepared film has increased structural order. Quantitatively, the degree of disorder was analyzed in the framework of the random magnetic anisotropy model

Theory of Room Temperature Ferromagnet V(TCNE)_x (1.5 < x < 2): Role of Hidden Flat Bands

Theoretical studies on the possible origin of room temperature ferromagnetism (ferromagnetic once crystallized) in the molecular transition metal complex, V(TCNE)_x (1.5<x<2) have been carried out. For this family, there have been no definite understanding of crystal structure so far because of sample quality, though the effective valence of V is known to be close to +2. Proposing a new crystal structure for the stoichiometric case of x=2, where the valence of each TCNE molecule is -1 and resistivity shows insulating behavior, exchange interaction among d-electrons on adjacent V atoms has been estimated based on the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that Hund's coupling among d orbitals within the same V atoms and antiferromagnetic coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to hybridization result in overall ferromagnetism (to be precise, ferrimagnetism). This view based on localized electrons is supplemented by the band picture, which indicates the existence of a flat band expected to lead to ferromagnetism as well consistent with the localized view. The off-stoichiometric cases (x<2), which still show ferromagnetism but semiconducting transport properties, have been analyzed as due to Anderson localization.Comment: Accepted for publication in J. Phys. Soc. Jpn. Vol.79 (2010), No. 3 (March issue), in press; 6 pages, 8 figure

Magnetic Bistability and Nucleation of Magnetic Bubbles in a Layered 2D Organic-Based Magnet [Fe(TCNE)(NCMe)(2)][FeCl(4)]

The 2D layered organic-based magnet [Fe(TCNE)(NCMe)2][FeC 14] (TCNE=tetracyanoethylene) exhibits a unique macroscopic magnetic bistability between the field-cooled and zero-field-cooled states, which cannot be explained by either superparamagnetic behavior or spin freezing due to spin glass order. This magnetic bistability is described through consideration of the ensemble of uncoupled 2D Ising layers and their magnetization reversal initiated by a field-induced nucleation of magnetic bubbles in individual layers. The bubble nucleation rate strongly depends on the external field and temperature resulting in anomalous magnetic relaxation.open7

Coordination polymers, metal–organic frameworks and the need for terminology guidelines

Coordination polymers (CPs) and metal–organic frameworks (MOFs) are among the most prolific research areas of inorganic chemistry and crystal engineering in the last 15 years, and yet it still seems that consensus is lacking about what they really are, or are not

Suppression of a charge density wave ground state in high magnetic fields: spin and orbital mechanisms

The charge density wave (CDW) transition temperature in the quasi-one dimensional (Q1D) organic material of (Per)$_2$Au(mnt)$_2$ is relatively low (TCDW = 12 K). Hence in a mean field BCS model, the CDW state should be completely suppressed in magnetic fields of order 30 - 40 T. To explore this possibility, the magnetoresistance of (Per)$_2$Au(mnt)$_2$ was investigated in magnetic fields to 45 T for 0.5 K < T < 12 K. For fields directed along the Q1D molecular stacking direction, TCDW decreases with field, terminating at about ~ 37 T for temperatures approaching zero. Results for this field orientation are in general agreement with theoretical predictions, including the field dependence of the magnetoresistance and the energy gap, $\Delta_{CDW}$. However, for fields tilted away from the stacking direction, orbital effects arise above 15 T that may be related to the return of un-nested Fermi surface sections that develop as the CDW state is suppressed. These findings are consistent with expectations that quasi-one dimensional metallic behavior will return outside the CDW phase boundary.Comment: 12 pages, 5 figure

Dielectric properties of BaMg1∕3Nb2∕3O3 doped Ba0.45Sr0.55Tio3 thin films for tunable microwave applications

Ba(Mg1∕3Nb2∕3)O3 (BMN) doped and undoped Ba0.45Sr0.55TiO3 (BST) thin films were deposited via radio frequency magnetron sputtering on Pt/TiO2/SiO2/Al2O3 substrates. The surface morphology and chemical state analyses of the films have shown that the BMN doped BST film has a smoother surface with reduced oxygen vacancy, resulting in an improved insulating properties of the BST film. Dielectric tunability, loss, and leakage current (LC) of the undoped and BMN doped BST thin films were studied. The BMN dopant has remarkably reduced the dielectric loss (∼38%) with no significant effect on the tunability of the BST film, leading to an increase in figure of merit (FOM). This is attributed to the opposing behavior of large Mg2+ whose detrimental effect on tunability is partially compensated by small Nb5+ as the two substitute Ti4+ in the BST. The coupling between MgTi″ and VO•• charged defects suppresses the dielectric loss in the film by cutting electrons from hopping between Ti ions. The LC of the films was investigated in the temperature range of 300–450K. A reduced LC measured for the BMN doped BST film was correlated to the formation of defect dipoles from MgTi″, VO•• and NbTi• charged defects. The carrier transport properties of the films were analyzed in light of Schottky thermionic emission (SE) and Poole–Frenkel (PF) emission mechanisms. The result indicated that while the carrier transport mechanism in the undoped film is interface limited (SE), the conduction in the BMN doped film was dominated by bulk processes (PF). The change of the conduction mechanism from SE to PF as a result of BMN doping is attributed to the presence of uncoupled NbTi• sitting as a positive trap center at the shallow donor level of the BST