Magnetic Properties and MCE in Heusler-Type Glass-Coated Microwires

Abstract

We prepared Heusler-type Ni–Mn–Ga glass-coated microwires with total diameters, D, from 16 until 65 μm consisted of a metallic nucleus with diameters, d, between 10 and 31 μm and surrounded by the glass coating by the Taylor–Ulitovsky technique and measured the magnetocaloric effect, MCE, by the recently introduced precise method allowing the detection of a change in the temperature, ΔT, with an accuracy of below 10−3 K. As-prepared microwires did not show ferromagnetic ordering at least near room temperature. Annealing above 773 K even for few minutes results in drastic change of magnetic properties: annealed sample show magnetization versus temperature dependence typical for ferromagnetic behavior with the Curie temperature about 315 K. Analysis of the X-ray diffraction allows us to identify presence of cubic phase in as-prepared state with the space group Fm3m and some amount of the tetragonal phase with space group I4/mmm. We measured directly the MCE, ΔT, in annealed samples. Before glass removal, we observed ΔT≈0.06 K for sample 1 and 0.08 K for sample 2. After glass removal of sample 1, ΔT increased until 0.22 K. Observed MCE is associated with magnetic (paramagnetic–ferromagnetic) and probably structural (austenite–martensite) phase transitions. Temperatures of the peak values of MCE were found to be of ∼318 K and ∼309 K, respectively Use of the glass-coating fabrication technique allows fabrication of composite thin wires from the brittle Ni–Mn–Ga alloy that cannot be cold-drawn to create fibers by conventional methods.