Nonlinear temperature variation of magnetic viscosity in nanoscale FeOOH particles

Fluorine-Induced Surface Metallization for Ammonia Synthesis under Photoexcitation up to 1550 nm

The first observation of surface metallization of TiO_2-x induced by fluoride ions is presented. The emerging metallic states are contributed by the 3d orbital of surface Ti and the 2p orbital of surface bridging F, which are intrinsically originated from the strong electron repulsion between F^- and adjacent Ti³⁺ . The metalized TiO_2-x with reduced work function and downward band bending possesses high electron-donating power to supported Ru species via atomic-scale ohmic contacts, exhibiting unprecedented photocatalytic performances for ammonia synthesis across the entire solar spectrum region (200-1550 nm) at room temperature. Mechanism and kinetic analysis revealed that the loaded Ru could behave as efficient electron sinks to accumulate photogenerated electrons and that the metallic surface markedly enhanced the dissociation of H₂ and N₂ by the hot electrons generated by the visible or even infrared light irradiation.

Magnetic states of nanostructures containing Ni2+ ions at the surface of SiO2 nanospheres

Ultra-small magnetic particles containing Ni²⁺ ions were grown at the surface of SiO₂ spheroidal nanoparticles (typical diameter: 50 nm) starting from NiCl₂ solutions. Depending on preparation details, two samples characterized by magnetic sub-nanostructures or lamellar sub-nanoparticles at the SiO₂ nanosphere surface were obtained. The decorated SiO₂ nanospheres were submitted to physico-chemical and magnetic characterization. In both samples, a magnetically blocked phase is observed at low temperature. Below 5 K, discontinuities in isothermal magnetization loops and magnetic relaxation effects suggest the onset of coherent quantum tunneling of nanoparticle magnetization (QTM). Relaxation effects give are described by a field- and temperature-dependent magnetic viscosity S_V(H,T); the total spin number of magnetic units is estimated by fitting the isothermal S_V(H) curve to a model for an assembly of particles with random anisotropy axes. The mean number of aligned spins involved in the low-temperature relaxation is 32 and 15 in the two considered samples. Phonon-assisted QTM plays an increasingly important role with raising temperature and the quantum regime gradually merges with the classical behavior. Above the blocking temperature the magnetic units behave as classical superparamagnetic particles. When the intra-particle ferromagnetic order disappears the Ni²⁺ ions respond individually to the magnetic field.

The glassy behaviour of poorly crystalline Fe2O3 nanorods obtained by thermal decomposition of ferrous oxalate

Nanorod ferrous oxalate dihydrate (FeC2O4 × 2H2O) which had been synthesized by the microemulsion method, was used as a precursor in the thermal decomposition process performed in air atmosphere. The formation of nanocrystalline hematite as the final product was preceded by the appearence of an intermediate product. Comprehensive study comprising several complementary techniques (x-ray diffraction, transmission electron microscopy, selected area electron diffraction, thermogravimetric/differential thermal analyses and SQUID magnetometry) confirmed that the intermediate product corresponds to the poorly crystalline Fe2O3. Due to the specific nanorod shape and poorly crystalline structure, the investigated Fe2O3 showed high coercive field value of ~0.5 T at 5 K. Special attention in this study was devoted to the peculiar magnetic properties of poorly crystalline Fe2O3, which were thoroughly investigated by employing sophisticated experimental procedures such as relaxation of thermoremanent magnetization for different cooling fields, zero field and field cooled memory effects as well as aging experiments for different waiting times. At low temperatures and weak applied magnetic fields, the investigated system behaves similarly to spin glasses, manifesting slow, collective relaxation dynamics of magnetic moments through memory, rejuvenation and aging effects.

Quantum tunneling of magnetization in ultrasmall half-metallic V3O4 quantum dots: displaying quantum superparamagnetic state

Quantum tunneling of magnetization (QTMs), stemming from their importance for understanding materials with unconventional properties, has continued to attract widespread theoretical and experimental attention. However, the observation of QTMs in the most promising candidates of molecular magnets and few iron-based compounds is limited to very low temperature. Herein, we first highlight a simple system, ultrasmall half-metallic V(3)O(4) quantum dots, as a promising candidate for the investigation of QTMs at high temperature. The quantum superparamagnetic state (QSP) as a high temperature signature of QTMs is observed at 16 K, which is beyond absolute zero temperature and much higher than that of conventional iron-based compounds due to the stronger spin-orbital coupling of V(3+) ions bringing high anisotropy energy. It is undoubtedly that this ultrasmall quantum dots, V(3)O(4), offers not only a promising candidate for theoretical understanding of QTMs but also a very exciting possibility for computers using mesoscopic magnets.

Hysteresis Loop Shifts in Magnetic Field Cooled FeOOH Nanoparticles

Measurements of the magnetization M as a function of temperature (5K - 300K) and applied magnetic field H (up to 50 kOe) in 30 Å particles of FeOOH are reported. M increases with decreasing T, peaking at TB = 65 K below which the ZFC (zero-field-cooled) and the FC (field-cooled) data separate. Hysteresis loop measured at 10 K for ZFC shows an open loop up to 40 kOe with coercivity = 2 kOe. For the FC case, the loop shifts and the loop-shift increases with the cooling field ItL, approaching saturation above Hc = 20 kOe. From the variation of M vs H above TB, a magnetic moment/particle μp = 300 μB is determined. These results suggest that the FeOOH nanoparticles have an antiferromagnetically ordered core with uncompensated surface spins yielding μp and the surface spins order in a spin-glass-like state below TB, possibly due to interparticle interactions.

Dynamic susceptibility evidence of surface spin freezing in ultrafine NiFe2O4 nanoparticles

We investigated the dynamic behavior of ultrafine NiFe2O4 nanoparticles (average size D = 3.5 nm) that exhibit anomalous low temperature magnetic properties such as low saturation magnetization and high-field irreversibility in both M(H) and ZFC-FC processes. Besides the expected blocking of the superspin, observed at T1 approximately 45 K, the system undergoes a magnetic transition at T2 approximately 6 K. For the latter, frequency- and temperature-resolved dynamic susceptibility data reveal characteristics that are unambiguously related to collective spin freezing: the relative variation (per frequency decade) of the in-phase susceptibility peak temperature is approximately 0.025, critical dynamics analysis yields an exponent znu = 9.6 and a zero-field freezing temperature T(F) = 5.8 K, and, in a magnetic field, T(F)(H) is excellently described by the de Almeida-Thouless line delta T(F) = 1 - T(F)(H)/T(F) alpha H(2/3). Moreover, out-of-phase susceptibility versus temperature datasets collected at different frequencies collapse on a universal dynamic scaling curve. All these observations indicate the existence of a spin-glass-like surface layer that surrounds the superparamagnetic core and undergoes a transition to a frozen state upon cooling below 5.8 K.

Temperature and time dependent magnetic phenomena in a nearly stoichiometric Ni(2)MnGa alloy

In this work, the temperature and time dependence of the magnetic properties of a polycrystalline Ni(49.7)Mn(24.1)Ga(26.2) alloy is analysed. The law of approach to magnetic saturation has been employed to estimate the magnetic anisotropy in the three structural phases of the alloy (martensitic, pre-martensitic and austenitic). The temperature dependences of magnetic parameters, such as the magnetic susceptibility and coercive field, are interpreted in terms of the changes in the magnetic anisotropy taking place with the structural transformations. The strong magnetocrystalline anisotropy is confirmed to mainly control the magnetic response of the low temperature martensitic phase. Furthermore, magnetic relaxation studies (magnetic after-effect) have been employed to analyse the main differences between the magnetization processes in the three characteristic structural phases. The time decay of the magnetization displays a distinctive response in the pre-martensitic state. The results (logarithmic time decay of the remanent magnetization and field dependence of the magnetic viscosity) indicate the thermally activated nature of the relaxation process.

CLAYS AND CLAY INTERCALATION COMPOUNDS:Properties and Physical Phenomena

▪ Abstract  The materials properties and physical phenomena exhibited by layered silicate clays and clay intercalation compounds, a subgroup of the general class of layered solids, are reviewed. The importance of layer rigidity is emphasized. Clays are compared and contrasted with the more familiar layered solids such as graphite and dichalcogenides. Some of the unusual structural features of clays including interstratification, swelling, and the lack of staging are discussed and explained qualitatively and quantitatively. Novel magnetic phenomena such as that associated with a disordered two-dimensional kagomé antiferromagnet formed in synthetic clays and the effect of co-intercalated water on the crystal field–induced magnetic ordering in natural clays are described and analyzed. The vibrational excitations in clays are addressed in terms of lattice dynamical models for the phonon dispersion curves. The theoretical models are compared with experimental measurements including neutron scattering and Raman spectroscopy.