Daniel M. Pajerowski

Persistent Photoinduced Magnetism in Heterostructures of Prussian Blue Analogues

Heterostructured ABA thin films consisting of two different Prussian blue analogues, where A is a ferromagnet and B is a photoinducible ferrimagnet, have been fabricated for the first time. This novel arrangement allows the magnetization to be decreased by irradiation with white light and significantly increases the ordering temperature of the photoinduced magnetism from 18 to 75 K.

Superparamagnetic Fe3O4/SiO2 Nanocomposites: Enabling the Tuning of Both the Iron Oxide Load and the Size of the Nanoparticles

Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 nm containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.

Photoinduced Magnetism in Core/Shell Prussian Blue Analogue Heterostructures of KjNik[Cr(CN)6]l·nH2O with RbaCob[Fe(CN)6]c·mH2O

Core/shell and core/shell/shell particles comprised of the Prussian blue analogues K(j)Ni(k)[Cr(CN)(6)](l)·nH(2)O (A) and Rb(a)Co(b)[Fe(CN)(6)](c)·mH(2)O (B) have been prepared for the purpose of studying persistent photoinduced magnetization in the heterostructures. Synthetic procedures have been refined to allow controlled growth of relatively thick (50-100 nm) consecutive layers of the Prussian blue analogues while minimizing the mixing of materials at the interfaces. Through changes in the order in which the two components are added, particles with AB, ABA, BA, and BAB sequences have been prepared. The two Prussian blue analogues were chosen because B is photoswitchable, and A is ferromagnetic with a relatively high magnetic ordering temperature, ~70 K, although it is not known to exhibit photoinduced changes in its magnetic properties. Magnetization measurements on the heterostructured particles performed prior to irradiation show behavior characteristic of the individual components. On the other hand, after irradiation with visible light, the heterostructures undergo persistent photoinduced changes in magnetization associated with both the B and A analogues. The results suggest that structural changes in the photoactive B component distort the normally photoinactive A component, leading to a change in its magnetization.

Size dependence of the photoinduced magnetism and long-range ordering in Prussian blue analogue nanoparticles of rubidium cobalt hexacyanoferrate

Nanoparticles of rubidium cobalt hexacyanoferrate (RbjCok[Fe(CN)6]l?nH2O) were synthesized using different concentrations of polyvinylpyrrolidone (PVP) to produce four different batches of particles with characteristic diameters ranging from 3 to 13?nm. Upon illumination with white light at 5?K, the magnetization of these particles increases. The long-range ferrimagnetic ordering temperatures and the coercive fields evolve with nanoparticle size. At 2?K, particles with diameters less than approximately 10?nm provide a Curie-like magnetic signal.

Tuning the Sign of Photoinduced Changes in Magnetization: Spin Transitions in the Ternary Metal Prussian Blue Analogue NaαNi1−xCox[Fe(CN)6]β·nH2O

Tuning the composition of the ternary transition-metal Prussian blue analogue Na(alpha)Ni(1-x)Co(x)[Fe(CN)(6)](beta) x nH(2)O allows the sign of the photoinduced change in magnetization to be controlled. The parent cobalt hexacyanoferrate material is well-known to display photoinduced and thermal charge-transfer-induced spin transitions (CTISTs). Upon partial replacement of Co ion sites with Ni(II), irradiation with halogen light can cause either an increase or a decrease in magnetization, depending upon the extent of Ni(II) substitution, the applied field, and the temperature. For all compositions with x > 0, photoexcitation generates new moments according to the same mechanism observed for the parent x = 1 compound. However, the presence of Ni(II) introduces a superexchange of opposite sign, providing a mechanism for controlling the sign of the change in magnetization with applied light. Additionally, dilution of the spin-crossover material reduces the magnitude and hysteresis of the thermal CTIST effect. These effects can be qualitatively explained by simple mean-field models.

Anisotropic magnetism in Prussian blue analogue films

A series of Prussian blue analogue (PBA) thin films with thicknesses ranging from 25 nm to 2 μm and with the chemical formula RbjM1k[M2(CN)6]l·nH2O have been synthesized using sequential adsorption techniques (where M1 = Co2+, Ni2+, Cu2+, or Zn2+and M2 = Cr3+ or Fe3+). Since the resulting materials possess the PBA structure, changing the metal ions effectively tunes the magnetization density because SCo = 3/2, SNi = 1, SCu = 1/2, and SZn = 0 on the divalent metal sites and SCr = 3/2 and SFe = 1/2 on the trivalent metal sites. The effect of changing metal ions on the magnetic susceptibility, and specifically the difference between parallel and perpendicular orientations of the films with respect to the applied magnetic field, was investigated with SQUID magnetometry, and all samples, except the ZnFe PBA films, showed magnetic anisotropy. The films were also characterized with infrared spectroscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, and scanning electron microscopy.

Metal Monophosphonates M{(2-C5H4NO)CH2PO3}(H2O)2 (M = Co, Ni, Mn, Cd): Synthesis, Structure, and Magnetism

Four isostructural metal monophosphonates, M{(2-C(5)H(4)NO)CH(2)PO(3)}(H(2)O)(2) with M = Co (1), Ni (2), Mn (3), and Cd (4), were synthesized and structurally characterized. These compounds show a double-chain structure in which the M(2)(μ-O)(2) dimers are connected by O-P-O bridges. The magnetic responses of 1-3 were investigated over a wide range of magnetic fields (up to 10 T) and temperatures (down to 50 mK). Except for 4, which is weakly diamagnetic from 2 K to room temperature, the dominant magnetic interactions are antiferromagnetic. Isothermal magnetic field sweeps at 50 mK provide signatures in the magnetic responses that are associated with antiferromagnetic to field-induced fully polarized (magnetically saturated) transitions. Analysis of the magnetic data indicates that 1 and 2 form magnetic dimer-like clusters with weak dimer-dimer interactions present. Contrastingly, the magnetic interactions present in 3 are significantly weaker, so a definitive description of the magnetism of this compound is elusive.

Synthesis and Size Control of Iron(II) Hexacyanochromate(III) Nanoparticles and the Effect of Particle Size on Linkage Isomerism

The controlled synthesis of monodisperse nanoparticles of the cubic Prussian blue analogue iron(II) hexacyanochromate(III) is reported along with a kinetic study, using cyanide stretching frequencies, showing the variations of the activation energy (E(a)) of the linkage isomerism as a function of the particle size. Highly reproducible, cubic-shaped iron(II) hexacyanochromate(III) nanocrystals, with sizes ranging from 2 to 50 nm, are synthesized using a microemulsion technique, whereas a bulk synthesis yields nonuniform less monodisperse particles with sizes greater than 100 nm. Monitoring the cyanide stretching frequency with FTIR spectroscopy shows that the rate of isomerization is faster for smaller particles. Moreover, a kinetic analysis at different temperatures (255 K ≤ T ≤ 321 K) gives insight into the evolution of E(a) with the particle size. Finally, time-dependent powder X-ray diffraction and net magnetization confirm the FTIR observations. The data are interpreted within the concept of a simple two-component model with different activation energies for structures near the surface of the solid and within the bulk.

Magnetic anisotropy in thin films of Prussian blue analogues

The magnetic anisotropy of thin (~ 200 nm) and thick (~ 2

Magnetic neutron scattering of thermally quenched K-Co-Fe Prussian blue analog photomagnet

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