Temperature Dependence Of Magnetic Relaxation Research Articles

Field-induced oscillation of magnetization blocking barrier in a holmium metallacrown single-molecule magnet

<h2>Summary</h2> Single-molecule magnets (SMMs) exhibit magnetization blocking in the presence of strong magnetic anisotropy. By employing molecular engineering, the blocking barrier can be significantly modified, remaining independent from other external factors, such as magnetic field. Taking advantage of hyperfine coupling of electronic and nuclear spins further highlights their unique functionality. However, a poor understanding of relaxation mechanisms limits the exploitation of nuclear-spin molecular qubits. Here, we report the first observation of field-dependent oscillation of the magnetization blocking barrier in a holmium metallacrown magnet driven by the switch of relaxation mechanisms involving hyperfine interaction. First-principles calculations reveal an activated temperature dependence of magnetic relaxation, dominated either by incoherent quantum tunneling of magnetization at anti-crossing points or by Orbach-like processes at crossing points. This mechanism demonstrates that these relaxation barriers can be consecutively switched by increasing the external field, which paves a way for manipulating magnetization dynamics of SMMs using hyperfine interaction.

Dynamic Magnetic Properties of NiFe$_{2}$O$_{4}$ Nanoparticles Embedded in SiO$_{2}$ Matrix

NiFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanoparticles with average sizes of 3.5(plusmn0.4) nm, 5.0(plusmn0.5) nm, and 19(plusmn1.1) nm embedded in the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> matrix were prepared by the sol-gel method. Size and temperature dependence of magnetic relaxation of the ferrite nanoparticles were studied. A classical logarithmic distribution function was considered to analyze the thermally activated relaxation phenomenon in the nanocomposites, but it cannot describe the behavior within the whole range of temperatures and times. An alternative more general approach called <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> ln( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> /tau <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) scaling was used to generate master curves which allows one to extrapolate the relaxation behavior at times that are experimentally inaccessible. The mean energy barrier as well as the width, sigma of the energy distribution curves can also be estimated from this study. These results are in good agreement with those obtained from ac and dc susceptibility measurements.

Fabrication of superconducting joints for Bi-2212 pancake coils

To evaluate the possibility of persistent current operation for NMR/MRI using a Bi/sub 2/Sr/sub 2/CaCuO/sub 8/ (Bi-2212) superconductor at 4.2 K, the temperature dependence of magnetic relaxation of the single crystal has been measured. It was found that the operating current must be one-fourth of J/sub c/ at 4.2 K to satisfy the stability condition for NMR/MRI operation ( Delta J/J less than 0.005 ppm/h). A superconducting coil made of Bi-2212 silver-sheathed wire was fabricated by the partial melt growth process. The authors were successful in generating 1.60 T in a zero external magnetic field at 4.2 K with a coil made of silver-sheathed Bi-2212 tape which was 100- mu m thick, 10-mm wide and 150-m long. Superconducting jointed pancake coils with critical current reaching 80 A in an external magnetic field of 1.0 T at 4.2 K were also fabricated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ｂｉ２．２Ｓｒ１．８Ｃａ１．０Ｃｕ２．０Ｏｘ単結晶の磁気特性

Temperature dependence of magnetic relaxation and M-H hysteresis has been measured applying magnetic field parallel to the c-axis of the crystal for Bi2.2Sr1.8Ca1.0Cu2.0Ox single crystal which was grown by the TSFZ method. As for the magnetization decay measurement, distribution of pinning potential was calculated according to the Griessen's formalism, Obtained result shows very narrow distribution centered around 40meV, and does not have intensity at high energy. In case of hysteresis measurement, irreversibility field was obtained as a function of temperature. Irreversibility field increased exponentially at near around 25K with decreasing temperature. These peculiar phenomena may be attributed to the highly two-dimensional character of Bi-based materials.

Magnetic relaxation and critical current in an YBaCuO crystal

We have measured the temperature dependence of magnetic relaxation and critical current in an YBaCuO single crystal. Unusually strong time-logarithmic magnetic relaxation is observed. The relaxation rate increases initially with temperature, peaks at ≂25 K and slows down at higher temperatures. The results, interpreted with a thermally activated flux model, yield a low-temperature pinning energy U0≂0.02 eV. This low pinning energy results in an unusually rapid decrease of the critical current as temperature is increased.