Y. Yeshurun

Nanophase formation of strontium hexaferrite fine powder by the sonochemical method using Fe(CO)5

Abstract A fine strontium hexaferrite powder has been successfully synthesized by a sonochemical method employing Fe(CO)5 and SrCO3. SrCO3 was first synthesized using strontium nitrate and urea, and it was found that applying ultrasound radiation during this process results in the uniform formation of SrCO3 hexagonal rods. These rods were then dispersed with in situ generated amorphous Fe2O3, using Fe(CO)5 as the source. The resultant precursor was then calcined at 900°C, which is lower than the conventional solid-state reaction of applying 1300°C to get the strontium hexaferrite fine powder. This sonochemically derived ferrite exhibited an intrinsic coercivity of ∼4600xa0Oe and a saturation magnetization of ∼60xa0emu/g at 20xa0K and ∼32xa0emu/g at 300xa0K.

Sonochemical synthesis of nanocrystalline LaFeO3

Nanocrystalline perovskite-type LaFeO3 with particle size of about 30 nm was prepared by a sonochemical method using iron pentacarbonyl and lanthanum carbonate as starting materials. The overall process involves three steps: formation of lanthanum carbonate using lanthanum nitrate and urea; reaction of the so-formed lanthanum carbonate with iron pentacarbonyl resulting in the formation of a precursor; calcination of the precursor to obtain nanocrystalline particles of LaFeO3. Transmission electron microscopy revealed the particles to have a mean size of about 30 nm. Study of the magnetic properties of nanocrystalline LaFeO3 particles shows a coercivity of ∼250 Oe, while the saturation magnetization is ∼40 memu g−1.

Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three

Annealing study of Fe2O3 nanoparticles: Magnetic size effects and phase transformations

{mathrm{Fe}}_{2}{mathrm{O}}_{3}

Remanent magnetization, lower critical fields and surface barriers in an YBa2Cu3O7 crystal

samples composed of regular amorphous, acicular amorphous, and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time-dependent barrier yields a natural explanation for the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define an energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data.

Magnetic properties of YNi2B2C superconductor

Sonochemically synthesized Fe2O3 nanoparticles were annealed in air or in vacuum while their magnetization was continuously recorded. Annealing in vacuum at temperatures Ta between 240 and 450u200a°C produced nanophases of γ-Fe2O3 with average particle size ranging from 4 to 14 nm, depending on Ta. Phase transformation into α-Fe2O3 occurred directly by annealing in air, or via an intermediate Fe3O4 phase by annealing in vacuum at temperatures higher than 450u200a°C. Mapping the correlation between the magnetic properties and the annealing conditions, enables control of the annealing process to obtain nanocrystals of γ-Fe2O3, α-Fe2O3, or Fe3O4 with different particle size and magnetic properties.

Tunneling and enhanced magnetoresistance in Nd2/3Sr1/3MnO3 thin films with microcracks

The isothermal remanent magnetization Mrem of an YBa2Cu3O7 crystal was measured as function of the maximum applied field 10 Oe⩽Hm ⩽40 kOe, for several isotherms 4.2K⩽ T⩽65K, for Hm⊥c and Hm∥c. Above an onset field for flux penetration Hp, Mrem initially increases sharply with Hm and then crosses over to a saturation value. An extended Bean model is used to fit the Mrem(H) data. The derived onset field for T>50K is approximately linear with T and is in agreement with the lower critical fields Hc1(T) measured by other techniques. At lower temperatures, however, Hp continues to increase. We incorporate surface barriers in the extended Bean model and consider the possibility that the low-temperature increase in Hp might be due to such barriers.

Magnetic and magneto-optical properties of oxide glasses containing Pr3+, Dy3+ and Nd3+ ions

Abstract We report on a study of the magnetic properties of the intermetallic superconductor YNi 2 B 2 C in a wide range of magnetic fields (up to 8 T) and temperatures (4.4−16 K). The results yield the characteristic length scales and fields for this compound: λ (0) ≈ (3.5 ± 0.5) × 10 −5 cm, ξ(0) ≈ (10 ±2) × 10 −7 cm, H c1 (0) ≈ 83 G and H c2 (0)≈ 47600 G. We compare our results with those obtained by other groups and add new information on thermodynamic properties, in particular on the lower critical field, and on irreversible magnetic features such as the critical current, pinning force and magnetic relaxation. We conclude that YNi 2 B 2 C is a type-II superconductor with relatively weak pinning centers.

Magnetic and structural properties of Nd2Fe17-xMx (M = Cu, Cr, V, Nb, and Zr) compounds

We found that microcracks in thin Nd2/3Sr1/3MnO3 films create a series of intrinsic break junctions which are ideal for investigating tunneling phenomena in this system. A comparison of films with and without cracks, which have similar ferromagnetic Curie temperature (Tc) of 140–150 K, shows that the cracked film has a lower insulator to metal transition temperature TM (97 K vs 140 K), three orders of magnitude higher resistivity at TM, and two times larger magnetoresistance at 1 T near TM. At T>TM we observed that lnu2009ραT−1/4 in the uncracked film while in the cracked film a lnu2009ραT−1/2 dependence was found. This indicates that the conductivity in the first case is due to variable range hopping in three dimension, while in the second case it is dominated by thermally activated tunneling across the insulating barriers (the microcracks).

Remanent nonlinear magnetic response in superconducting Y‐Ba‐Cu‐O

Abstract Magnetization of a number of oxide glasses containing Pr 3+ , Dy 3+ and Nd 3+ ions has been measured as a function of the magnetic field (0–50xa0kOe) and temperature (5–300xa0K). Magneto-optical Faraday rotation (FR) in the same glasses has been measured as a function of the light wavelength (240–1000xa0nm) and temperature (80–350xa0K). Dispersion of FR has permitted us to find the positions of the effective transitions responsible for FR and their magneto-optical activities. Temperature dependencies of both FR and the magnetization at high temperatures obey the Curie–Weiss law with negative Weiss constants. High temperature magnetic moments of the rare-earth ions in the glasses are rather close to those of the free ions. At low temperatures, all studied glasses reveal downward curvature of the reciprocal magnetization variation with decreasing temperature. The origin of this behavior is discussed. Formation of antiferromagnetically ordered clusters in Dy glasses is supposed.