Hysteretic characteristics of giant magnetoimpedance due to the exchange coupling in annealed amorphous materials

Abstract

Experiments of the giant magnetoimpedance (GMI) profile have been performed in annealed amorphous Co66Fe4B15Si15 ribbons in open air to characterize the role of the bias field on the GMI. The GMI ratio profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called GMI valve, in an 8 h annealed sample at 380 °C. The GMI valve is related to exchange coupling of the bias field with magnetization of the soft amorphous phase, where the bias field is caused by hcp-Co and fcc-Co crystalline phases on the surface B and Si depleted layer. The bias field is stable for an external field less than 100 Oe, but its direction is entirely changed according to the external field over 400 Oe. The GMI profile begins to reveal hysteresis for increasing and decreasing cyclic fields when the maximum field strength exceeds 200 Oe. The peak for the increasing field is positioned in the negative field region, but in the positive field region for decreasing field. This behavior is opposite to general magnetic hysteretic characteristics, suggesting that there is an antiferromagnetic coupling of the bias field with magnetization of the inside amorphous phase.

Generation of second harmonic in off-diagonal magneto-impedance in Co-based amorphous ribbons

“…The crystalline layers have the unidirectional anisotropy induced by the annealing field. The value of the unidirectional anisotropy field H u in the crystalline layers is sufficiently high and attains several hundreds of Oersted [27,29]. The unidirectional anisotropy field in the outer layers makes the angle ϕ with respect to the transverse direction (see figure 3).…”

Magnetotransport at High Frequency of Soft Magnetic Amorphous Ribbons

“…8 (c)), also submitted to a stress annealing after a pre-annealing, Z zz (H, f) curves present an asymmetric character. It is worthy to mention that the asymmetric GMI has been previously observed in Co-based amorphous ribbons annealed in air and in the presence of a weak magnetic field [24,25,26], which was attributed to the surface crystallization that takes place at this annealing temperature (at 380 ºC). Forsooth, this type of annealing produces an asymmetric hysteresis loops in amorphous ribbons due to the exchange interaction of amorphous bulk with the magnetically harder crystalline surface layers [23].…”

“…Hence, the GMI effect becomes asymmetric with regard to the mentioned pre-annealing treatment, and this asymmetry can be assigned to the inelastic stress-induced anisotropy component to be rather different in the two ribbon surfaces, owing to the crystallization effect starts to be significant. Therefore, this GMI behavior could result from magnetization of the amorphous ribbon, suggesting that there is an antiferromagnetic coupling of these two phases in the ribbon surface layer between the magnetization of the crystalline one with the magnetization of the inner bulk amorphous phase [24].…”

Asymmetric off-diagonal magnetoimpedance in field-annealed amorphous ribbons: Analysis of bias current effect

“…The composition-depth profile measured by Auger electron spectroscopy has shown that the Co-rich crystalline layer is developed between the oxidation layer and the amorphous bulk [20,23,24]. Furthermore, the differential thermal analysis profile of the ribbon annealed in air has demonstrated that the surface layer has the crystallization temperature about 330 1C, much lower than the crystallization temperature of the amorphous bulk [28]. The existence of the surface crystalline phase has been confirmed also in the study of the surface magnetic properties of the ribbons by means of the magneto-optical Kerr effect [29][30][31].…”