Thermal properties of Ti-doped Cu–Zn soft ferrites used as thermally actuated material for magnetizing superconductors

Abstract

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order-disorder phase transition in their magnetic subsystem. For applications related to the magnetization process of superconductors, however, a low value of Tc is required. Here we report and analyze in detail the thermal properties of bulk Ti-doped Cu-Zn ferrite ceramics Cu0.3Zn0.7Ti0.04Fe1.96O4 and Mg0.15Cu0.15Zn0.7Ti0.04Fe1.96O4. They are characterized by a Curie temperature in the range 120-170 K and a maximum DC magnetic susceptibility exceeding 20 for the Cu0.3Zn0.7Ti0.04Fe1.96O4 material. The temperature dependence of both the specific heat Cp and of the thermal conductivity κ, determined between 2 and 300 K, are found not to exhibit any peculiar feature at the magnetic transition temperature. The low-temperature dependence of both κ and the mean free path of phonons suggests a mesoscopic fractal structure of the grains. From the measured data, the characteristics of thermally actuated waves are estimated. The low magnetic phase transition temperature and suitable thermal parameters make the investigated ferrite ceramics applicable as magnetic wave producers in devices designed for magnetization of high-temperature superconductors. IntroductionDue to the combination of excellent magnetic properties at high frequency and low price, ferrite materials have become essential materials in a wide range of electrical engineering applications including chip components, filters, telecommunication systems, sensors [1-5] and extending to biomedical applications and energy storage [6][7][8]. This present work deals with ferrites to be inserted in an innovative low-temperature application related to the magnetization of type-II superconductors used as permanent magnets [9,10]. Unlike a classical ferromagnet where the magnetization is limited physically by the alignment of all microscopic magnetic moments, the remnant magnetization of a type-II irreversible superconductor is generated by macroscopic (resistanceless) supercurrent current loops [11,12]. When prepared in bulk form, e.g. monolithic disks of a few centimeters in diameter [13][14][15][16], superconductors are able to trap magnetic flux densities exceeding 3 teslas [11,[17][18][19], which opens up new horizons in terms of applications [20][21][22]. One of the main issues, however, is that these superconducting materials need to be magnetized before their use. In addition to the well-established magnetization techniques, -namely, zero field cooling (ZFC), field cooling (FC) and pulse field magnetization (PFM) [23] -, there is a growing interest in applying the so-called "flux pump" technology[24] to the magnetization of bulk superconductors [25,26]. Flux pumping involves creating a travelling magnetic field wave that guides magnetic flux lines toward the center of the superconducting element.The technique can be applied to films, bulk samples or coils [27]. The travelling wave can be produced by a moving magnet [26], a three-phase coil system [28] or by making use of the ...