Magnetic properties and magnetoimpedance of short CuBe/CoFeNi electroplated microtubes

The magnetic properties and, particularly the magnetoimpedance, MI, of short CuBe/CoFeNi electroplated microtube, 0.9-1.7 cm long, consisting of a CuBe metallic core surrounded by an electroplated ferromagnetic CoFeNi shell have been investigated with the final aim to be used as sensitive elements in sensor devices. From the analysis of the experimental hysteresis loops, we confirm the presence of a circular magnetic anisotropy of the ferromagnetic shell and the significant role of the inhomogeneous local distribution of the magnetization near the ends of the microtube. It is experimentally observed that the maximum MI ratio and its ohmic component depend very significantly on the microtube length while particularly the maximum value of the ohmic component is higher than that of the total MI ratio for all selected samples. These maxima are different from each other: the value of (Delta Z/Z)(max), (Delta R/R)max, the MI sensitivity, and the value of the field in which each particular MI appears. Interesting MI behavior was observed in the low excitation frequency of 2.5 MHz, where the length dependence of operation frequency was also studied. An optimal response is obtained for intermediate (critical) length sample with reduced border effect. This fact makes electroplated microtubes attractive for magnetic conditions devices and other applications. (C) 2016 Elsevier B.V. All rights reserved.