Ordering Process and Variant Selection under Magnetic Field in L10-type Co–50Pt and Fe–55Pd Alloys

Abstract

Selected formation of variants under magnetic field in a diffusional process has been investigated by using Co-50Pt (at%) and Fe-55Pd (at%) alloys exhibiting A1 (disorder)-L1 0 (order) transformations. A single variant of the ordered phase is obtained in these alloys by applying a magnetic field during the early stage of ordering. The lowest magnetic field required for obtaining the single variant is about 0.5 T for the Co-50Pt alloy, and 4 T for the Fe-55Pd alloy in the present heat-treatment conditions. Such a difference could be due to the relation between nucleation barrier for the formation of the ordered phase and magnetocrystalline anisotropy at the nucleation stage.KEY WORDS: cobalt-platinum alloy; iron-palladium alloy; Curie temperature; single variant; magnetocrystalline anisotropy.Y-, Z-variants, respectively, and these fractions as f x , f y , and f z . The fraction of each variant was determined by magnetization measurements using a SQUID magnetometer. When the specimen is highly ordered, the fraction will be obtained by using the relation f x ϭM x 0 /M s , f y ϭM y 0 /M s , f z ϭM z 0 /M s ; where M s is the spontaneous magnetization; M x 0 , M y 0 , M z 0 , are the magnetization at zero field extrapolated from the high field region measured along the X-, Y-, Z-directions, respectively. Results Disorder-Order Transformation under Zero Magnetic Field and Magnetic PropertiesIn this section, we will show the ordering process in Co-50Pt and Fe-55Pd alloys in the absence of magnetic field and determine the Curie temperatures of their disorder and order phases.First, time evolution of disorder-order transformation has been examined by measuring electrical resistivity at 273 K after performing an isothermal heat-treatment at different ordering temperatures followed by quenching into ice-water. The result is shown in Fig. 1, where the resistivity is normalized by the value of the as-quenched specimen, r A.Q. at 273 K. In the case of the Co-50Pt alloy, the resistivity saturates within 3 h by the heat-treatment at 1 023 K, while not within 200 h by the heat-treatment at 973 K, 873 K, and 773 K. These results imply that the ordering process is the fastest at 1 023 K and slowest at 773 K in the examined temperature range. Similar results were reported by Newkirk et al. 10) in a Co-48Pt (at%) alloy. In the case of Fe-55Pd alloy, the resistivity saturates within 3 h by the heat-treatment at 873 K, within 21 h at 773 K, while not within 163 h by heat-treatment at 673 K.Second, we examined the Curie temperature of the disordered phase by electrical resistivity measurement. The measurements were made by heating the disordered specimen from room temperature with a heating rate of 100 K/min, and the result is shown in Fig. 2 by solid curves. In the case of the Co-50Pt alloy (a), the resistivity heating curve exhibits a bending as indicated by an arrow at about 850 K. We can interpret this temperature as the Curie temperature of the disordered phase, T c (d) , considering the same bending appears at Curie tempera...

Electron Microscopy Study of Preferential Variant Selection in CoPt Alloy Ordered under a Magnetic Field

“…These results are consistent with the magnetization measurement. 5,7) At this stage, tweed microstructure with striations along h011i L1 0 directions is clearly observed especially in DFIs of the Y-and Z-variants in Figs. 2(f ) and 2(g).…”

Modification of preferred martensitic variant distribution by high magnetic field annealing in an Ni–Mn–Ga alloy

“…Actually, the magnetocrystalline anisotropy energy (MAE) is an important factor influencing the variant selection during martensitic transformation under a magnetic field (Kokorin & Wuttig, 2001;Ueshima et al, 2010). However, besides MAE, there may be other factors affecting the variant selection rules (Farjami et al, 2009). The exact mechanisms that account for the variant selection under a magnetic field are still not clear.…”