Preparation And Study Of Low-dimensional Molecular Magnet Nanomaterials

Recently, the low dimensional materials have attracted much attention for their distinctive properties and potential application.In the field of molecular magnet, Prussian blue and its analogues (A_k[B(CN)6]₁·nH2O) have attracted unusual interest for several reasons: (i) the structure of the resulting 3D network is highly symmetrical, most often a face-centered-cubic (fcc) frame of [B(CN)6] units connected by the metallic cations A in octahedral sites; a wide range of metals with different spin states and oxidation states can be substituted into the lattice, and (ii) the linear A-CN-B bridges promoted the formation of strong magnetic interactions between adjacent spin centers. These features allow considerable control over the nature and the magnitude of the local magnetic exchange interactions and many unusual properties have been found on these materials. In order to study the properties of the Prussian blue analogues in nanoscale, the iron-chromium cyanide molecular magnet Fe²⁺_1.5[Cr³⁺(CN)₆]·xH2O nanowire arrays are fabricated with the anodic aluminum oxide film template-based method, which is easy and popular for fabricating metal and metal oxide nanowire arrays by electrochemical technology. Porosity on the template is uniform and ordered. The results of morphology, structure, and magnetic properties of the iron-chromium cyanide are also described. In order to probe the properties of the molecular magnet in lower dimension, the preparation of a number of Prussian blue analogues molecular magnet nanoparticles stabilized by poly(vinylpyrrolidone) polymer matrix. The corresponding characterization and magnetic properties according to dimensional effect were reported. The main results are shown as following:1. Highly ordered Prussian blue analogue iron-chromium cyanide molecular magnet nanowire arrays with diameters of about 50 and 100 nm have been fabricated for the first time with the anodic aluminum oxide film template-based method. The curie temperature is higher than that of the Prussian blue nanowires. In Prussian blue nanowires, Fe²⁺ ions are low-spin (t_2g⁶e_g⁰, S=0) and the ferromagnetic interactions between the next-nearestneighbors Fe³⁺ (t_2g³e_g², S=5/2) is weak. Structure and chemical composition characterization results show that the iron-chromium cyanide molecular magnet nanowires process a face-centered-cubic structure in which A Fe²⁺ is in high spin with S = 2 and B Cr²⁺ is in low spin with S=3/2. The magnetic properties measurement results indicate that the magnetic ordering comes from the ferromagnetic interactions between the Fe2+ (S=1/2) and Cr²⁺ ions through the cyanide bridge.2. In order to study the magnetic properties of the Prussian blue analogues in lower dimension, Ni_1.5[Fe(CN)₆]·xH₂O nanoparticles have been synthesized for the first time using PVP as polymer matrix. The nanoparticles are highly crystalline with an fcc cubic structure in which A Ni²⁺ is in high spin with S = 1 and B Fe³⁺ is in low spin with S=l/2. Magnetic measurement results indicate that the magnetic ordering in Ni_1.5[Fe(CN)₆]·xH₂O nanoparticles come from the ferromagnetic interactions between the Ni²⁺ and Fe³⁺ ions through the cyanide bridge. The Curie temperature of Ni_1.5[Fe(CN)₆]·xH₂O nanoparticles with diameters of 75 and 20 nm is found to been reduced with respect to bulk materials, resulting from the diminution of the average number of nearest magnetic interaction neighbors and magnetic exchange interaction constants as the diameters of particles decrease. The result indicates that intrinsic magnetic will be changed because of the decreasing of the dimension.3. Because the spin centers are important for magnetic properties of the Prussian blue analogues, Fe_1.5[Cr(CN)₆]·xH₂O nanoparticles have been synthesized for the first time using PVP as polymer matrix. The nanoparticles are highly crystalline with an fcc cubic structure in which A Fe²⁺ is in high spin with S = 2 and B Cr³⁺ is in low spin with S = 3/2. Magnetic measurement results indicate that the magnetic ordering in Fe_1.5[Cr(CN)₆]·xH₂O nanoparticles come from the ferromagnetic interactions between the Fe²⁺ and Cr³⁺ ions through the cyanide bridge. The Curie temperature of Ni_1.5[Fe(CN)₆]·xH₂O nanoparticles with diameters of 93 and 41 nm is found to been reduced with respect to bulk materials, resulting from the diminution of the average number of nearest magnetic interaction neighbors and magnetic exchange interaction constants as the diameters of particles decrease. The result indicates that intrinsic magnetic will be changed because of the decreasing of the dimension.4. In order to increase the curie temperature of the Prussian blue analogues, Ni_1.5[Cr(CN)₆]·xH₂O nanoparticles have been synthesized for the first time using PVP as polymer matrix. The nanoparticles are highly crystalline with an fcc cubic structure in which A Ni²⁺ is in high spin with S = 1 and B Cr³⁺ is in low spin with S = 3/2. Magnetic measurement results indicate that the magnetic ordering in Ni_1.5[Cr(CN)₆]·xH₂O nanoparticles come from the ferromagnetic interactions between the Ni²⁺ and Cr³⁺ ions through the cyanide bridge. The Curie temperature of Ni_1.5[Cr(CN)₆]·xH₂O nanoparticles with diameters of 80 and 40 nm is found to been reduced, resulting from the diminution of the average number of nearest magnetic interaction neighbors and magnetic exchange interaction constants as the diameters of particles decrease. The result indicates that intrinsic magnetic will be changed because of the decreasing of the dimension. The curie temperature of the nanoparticles is higher than systems of Ni_1.5[Fe(CN)₆]·xH₂O and Fe_1.5[Cr(CN)₆]·xH₂O nanoparticles. The result indicates that the strongermagnetic interactions mean higher curie temperature.