Temperature-Dependent Giant Magnetoimpedance Effect in Amorphous Soft Magnets M. KURNIAWAN, 1,4 R.K. ROY, 2 A.K. PANDA, 2 D.W. GREVE, 1 P. OHODNICKI, 1,3 and M.E. MCHENRY 1 1.—Carnegie Mellon University, Pittsburgh, PA, USA. 2.—CSIR National Metallurgical Labora- tory, Jamshedpur, India. 3.—National Energy Technology Laboratory, Pittsburgh, PA, USA. 4.—e-mail: mkurniaw@andrew.cmu.edu Giant magnetoimpedance (GMI)-based devices offer potential as next-gener- ation low-cost, flexible, ultrasensitive sensors. They can be used in applica- tions that include current sensors, field sensors, stress sensors, and others. Challenging applications involve operation at high temperatures, and there- fore studies of GMI temperature dependence and performance of soft magnetic materials are needed. We present a high-temperature GMI study on an amorphous soft magnetic microwire from room temperature to 560°C. The GMI ratio was observed to be nearly constant at 86% at low temperatures and to decrease rapidly at 290°C, finally reaching a near-zero value at 500°C. The rapid drop in GMI ratio at 290°C is associated with a reduction in the long-range ferromagnetic order as measured by the spontaneous magnetiza- tion (M) at the Curie temperature (T c ). We also correlated the impedance with the magnetic properties of the material. From room temperature to 290°C, the impedance was found to be proportional to the square root of the magnetiza- tion to magnetic anisotropy ratio. Lastly, M(T) has been fit using a Handrich– Kobe model, which describes the system with a modified Brillouin function and an asymmetrical distribution of exchange interactions. We infer that the structural fluctuations of the amorphous phase result in a relatively small asymmetry in the fluctuation parameters. Key words: Soft magnets, high temperature, giant magnetoimpedance, permeability, skin effect, amorphous INTRODUCTION Amorphous and nanocrystalline soft magnetic materials exhibit properties that render them suit- able for applications such as power transformers, motors, electromagnetic shielding, and sensing devices. 1,2 In the sensors arena, there has been growing interest in GMI-based sensing devices due to their low cost, flexibility, and superior sensitivity compared with current technologies. 3,4 While giant magnetoresistance (GMR)-based sensors have typi- cal field sensitivity of 1%/Oe, the sensitivity of GMI-based sensors can be as high as 500%/Oe. 3 For magnetic field sensing applications at high temperatures, such as directional drilling in oil and gas extraction, soft magnetic materials with good high-temperature performance are necessary. 5 The interplay between the long-range ferromagnetic order, the magnetic anisotropy, and the measured GMI response as a function of temperature is also very interesting from a fundamental point of view. Thus, there is a need to study the temperature dependence of the GMI performance of soft mag- netic materials. GMI is a phenomenon in which the electrical impedance of a conductor under an alternating- current (AC) field changes in the presence of a direct-current (DC) magnetic field. As a function of the external DC magnetic field (H), the GMI ratio is expressed as GMIð%Þ¼ DZ Z  100% ¼ ZðHÞ ZðH o Þ ZðH o Þ  100%; (1) (Received August 12, 2014; accepted October 8, 2014; published online October 25, 2014) Journal of ELECTRONIC MATERIALS, Vol. 43, No. 12, 2014 DOI: 10.1007/s11664-014-3469-7 Ó 2014 The Minerals, Metals & Materials Society 4576