Papers by Author: Reiko Sato Turtelli

Abstract: We report the effect of a short-time vacuum annealing (1073 K during 10 minutes) on structural phase transition temperatures and magneto-structural properties of as-quenched ribbons of the Heusler alloy Ni50.6Mn34.5In14.9. This alloy crystallizes in a single phase cubic B2-type austenite with a Curie point of TCA=284 K that with the lowering in temperature transforms into a martensite with TCM185 K. The direct and reverse martensitic phase transition temperatures were MS=257 K, Mf = 221 K, AS = 239 K, and Af = 266 K. After annealing austenite shows the highly ordered L21-type structure while the average chemical composition as well as the structural and magnetic transition temperatures were shifted to Ni50.2Mn34.3In15.5 and MS = 253 K, Mf = 238 K, AS = 257 K, Af = 265 K, ΔT = 13 K, TCA = 299 K and TCM207 K. In the annealed samples the magnetization changes associated to the magnetic and structural transitions are more abrupt and magnetization isotherms in both the austenitic and martensitic existence region show higher initial magnetic susceptibility and faster approach to saturation. Field-cooled hysteresis loops at 10 K were shifted along the negative H-axis for both samples, but a significant anomaly was evident on the left side of the hysteresis loop for as-quenched ribbons.

Abstract: Nanocrystalline YCo5 powders with high coercivity were prepared by mechanical milling and subsequent heat treatment at 820 °C for different annealing times, ta = 2.5, 3.0, 3.5 and 4.5 min, obtaining average crystallite sizes of  17, 19, 32 and 39 nm., respectively. The coercivity values were determined from the hysteresis loops measured at maxima fields of Hm = 5 and 20 T. The highest coercivity was obtained for the sample exhibiting  19 nm, where at room temperature and Hm = 5 T, the coercivity value is of 9.0 kOe. At 77 K and Hm = 5 T, the coercivity increase to 11.8 kOe and for Hm = 20 T, a higher value such as 13.1 kOe was found. The Ms/Mr ratio is enhanced to 0.62 indicating the occurrence of exchange interaction among nanocrystalline magnets.

Abstract: A new system for measuring magnetic viscosity in bulk hard magnetic materials base on a pulsed field method is presented. After the magnetizing field pulse, the stray field of the sample, which is proportional to the magnetic moment, is measured with a compensated set of two Hall probes. The set of Hall probes is driven with an AC voltage source and the Hall voltage is detected using a lock-in amplifier. By this method the system is able to measure the time dependence of the magnetization (viscosity). The magnetic viscosity coefficient (S) is obtained through a linear fitting of the magnetization decay versus ln t. Two typical Nd-Fe-B samples were measured and good agreement with the results from the traditional viscosity experiment was obtained.