J.D. Santos

Martensitic phase transformation in rapidly solidified Mn50Ni40In10 alloy ribbons

Heusler alloy Mn50Ni40In10 was produced as preferentially textured ribbon flakes by melt spinning, finding the existence of martensitic-austenic transformation with both phases exhibiting ferromagnetic ordering. A microcrystalline three-layered microstructure of ordered columnar grains grown perpendicularly to ribbon plane was formed between two thin layers of smaller grains. The characteristic temperatures of the martensitic transformation were MS=213K, Mf=173K, AS=222K, and Af=243K. Austenite phase shows a cubic L21 structure (a=0.6013(3)nm at 298K and a Curie point of 311K), transforming into a modulated fourteen-layer modulation monoclinic martensite.

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

We determined the magnetic entropy change and refrigerant capacity of melt spun Ni50.3Mn35.5Sn14.4 ribbons around both the structural and the magnetic transitions for a field of 20kOe. The maximum entropy changes at the structural and magnetic transitions were of 4.1 and −1.1Jkg−1K−1. Ribbons studied show a larger refrigerant capacity around the magnetic transition (46Jkg−1) than around the structural transition (26Jkg−1), suggesting that the temperature range at the magnetic transition is more adequate for a refrigerant cycle than that at the structural transition.

Thermal and magnetic field-induced martensite-austenite transition in Ni50.3Mn35.3Sn14.4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K.

Microstructure and magnetic properties of Ni50Mn37Sn13 Heusler alloy ribbons

The Heusler alloy Ni50Mn37Sn13 was successfully produced as ribbon flakes of thickness around 7–10μm melt spinning. Fracture cross section micrographs in the ribbon show the formation of a microcrystalline columnarlike microstructure, with their longer axes perpendicular to the ribbon plane. Phase transition temperatures of the martensite-austenite transformation were found to be MS=218K, Mf=207K, AS=224K, and Af=232K; the thermal hysteresis of the transformation is 15K. Ferromagnetic L21 bcc austenite phase shows a Curie point of 313K, with cell parameter a=0.5971(5)nm at 298K, transforming into a modulated 7M orthorhombic martensite with a=0.6121(7)nm, b=0.6058(8)nm, and c=0.5660(2)nm, at 150K.

Induced magnetic anisotropy in amorphous ribbons by applying a magnetic field during the quenching process

A method to induce magnetic anisotropy in amorphous magnetic ribbons has been developed. The anisotropy is induced in the high Curie temperature Fe80B20 and Co70Mn5Fe1Mo1Si14B9 amorphous magnetic ribbons by applying a magnetic field at the moment of the solidification of the melt. This method does not affect the excellent mechanical properties of these materials. The induced magnetic anisotropy has been measured by torque magnetometry and its strength and easy axis are compared with the results obtained by other authors using static magnetic annealing in similar samples.

Kinetics of crystallization of FeB-based amorphous alloys studied by neutron thermo-diffractometry

Abstract Kinetics of crystallization of two amorphous alloys, Fe 70 Cr 10 B 20 and Fe 80 Zr 10 B 10 , have been followed up by neutron thermo-diffractometry experiments performed in the two axis diffractometer D20 (Institut Laue-Langevin, Grenoble). The structural changes are directly correlated with the temperature dependence of the magnetization. Fe 70 Cr 10 B 20 crystallizes following a two-step process: an eutectic crystallization of α-Fe (BCC) and the metastable tetragonal phase (Fe 0.8 Cr 0.2 ) 3 B followed by another eutectic transformation to the stable phase (Fe 0.75 Cr 0.25 ) 2 B and more segregation of α-Fe. These tetragonal phases are magnetically anisotropic, giving rise to a large increase of the coercivity. This behavior is similar to that of Fe 80 B 20 alloys, with Cr atoms replacing the Fe positions in both crystalline phases. Fe 80 Zr 10 B 10 shows also a two-step process in which two polymorphic transformations take place.

Thermal annealing dependence of high-frequency magnetoimpedance in amorphous and nanocrystalline FeSiBCuNb ribbons.

In the present work, the effectiveness of styrene/ethylene-butylene/styrene rubbers grafted with maleic anhydride (MA) and a metallocene polyethylene (mPE) as toughening materials in binary and ternary blends with polypropylene and its nanocomposite as continuous phases was evaluated in terms of transmission electron microscopy (TEM), scanning electron microscopy (SEM), oscillatory shear flow and dynamic mechanical thermal analysis (DMA). The flexural modulus and heat distortion temperature values were determined as well. A metallocene polyethylene and a polyamide-6 were used as dispersed phases in these binary and ternary blends produced via melt blending in a corotating twin-screw extruder. Results showed that the compatibilized blends prepared without clay are tougher than those prepared with the nanocomposite of PP as the matrix phase and no significant changes in shear viscosity, melt elasticity, flexural or storage moduli and heat distortion temperature values were observed between them. However, the binary blend with a nanocomposite of PP as matrix and metallocene polyethylene phase exhibited better toughness, lower shear viscosity, flexural modulus, and heat distortion temperature values than that prepared with polyamide-6 as dispersed phase. These results are related to the degree of clay dispersion in the PP and to the type of morphology developed in the different blends.

Magnetic domain structure of amorphous Fe73.5Si13.5B9Nb3Cu1 wires under torsional stress

The evolution of the magnetic domain structure with the applied torsion has been studied by means of the Bitter technique on high magnetostrictive Fe73.5Si13.5B9Nb3Cu1 amorphous wires. A drastic change in domain patterns is observed by applying a very low torsional strain value as small as 2.4 (2πrad∕m) in the wire. The domain structure on the wire surface changes from a maze configuration, with zigzag walls, to a helical one. When the magnitude of the applied torsion is increased, both in clockwise or counterclockwise senses, a change in the values for the domain width as well as the wall angle tilting with respect to the wire axis are clearly observed, showing a saturation value of 45° for an applied torsional strain equal or higher than 10 (2πrad∕m). Also, the coercive field variation with the applied torsion shows a minimum value of 26Am−1 just for a torsional strain of 2.4 (2πrad∕m), suggesting that the internal and the applied helical anisotropies are then balanced, at least partially, resulting in ...

Very high GMI effect in commercial Vitrovac ® amorphous ribbons

In this work, we have studied the magnetic properties and magnetoimpedance behaviour in commercial Vitrovac ® 6025 low-magnetostriction ribbons as a function of the frequency and the intensity of the driving current. A very high value of the giant magnetoimpedance effect (GMI) ratio up to 420% and sensitivity of 57%/Oe have been achieved at a relatively low frequency of 3.5 MHz for the current intensity Irms = 11 mA. Although the decrease of the alternating current, up to 1.5 mA at a frequency of 3.5 MHz leads to some decay of the GMI ratio up to 300% and sensitivity of 42%/Oe, these values are still very attractive for applications in magnetic field sensors.

Annealing Effect on Martensitic Transformation and Magneto-Structural Properties of Ni-Mn-In Melt Spun Ribbons

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.