Low temperature, high frequency permeability of Metglas 2714A and its potential for use as a core material for EMI filters Nicholas R. Hutzler, Konstantin I. Penanen, Talso C.P. Chui * Jet Propulsion Laboratory, Low Temperature Science and Quantum Sensor Group, California Institute of Technology, 4800 Oak Grove Drive, Mail Stop 79-24, Pasadena, CA 91109, United States Received 26 October 2006; accepted 15 March 2007 Abstract We report the measurement of the relative permeability of Metglas 2714A at 4.2 K temperature and at frequencies up to 1 MHz. We find that at 300 kHz, the real part of the relative permeability begins to drop towards unity, while the magnitude of the imaginary part increases. Our data can be used to design transformers and EMI filters for low temperature applications. Ó 2007 Published by Elsevier Ltd. Keywords: Magnetic material; Cryogenics; High-permeability; EMI filter 1. Introduction Many experiments in fundamental physics require low- frequency detection of extremely small signals. These detec- tors are often cooled to reduce thermal noise and to make use of quantum phenomena that only occur at low temper- ature. Such detectors are usually sensitive to high fre- quency electromagnetic interference (EMI), such as radio waves. The sensitivity to EMI is particularly severe when superconducting quantum interference device (SQUID) is used. It would be desirable to have an EMI filter capable of removing such interference. A simple LC low-pass filter would be sufficient. But a problem arises in finding an inductor with sufficiently high inductance at low tempera- ture. Good inductors are made by winding a coil on a core of high-permeability material. But most magnetic core materials suffer a decrease in permeability at low tempera- ture and high frequency. Therefore a material that main- tains a high-permeability at low temperature and high frequency will be desirable. Metglas 2714A, a metallic alloy manufactured by Honeywell [1], has high-permeability at high frequency at room temperature. Quach and Chui [2] have previously reported that the relative permeability remains higher than 10,000 at frequencies up to 100 kHz at 4.2 K. However, no data exists at higher frequen- cies. In this paper, we report a new measurement of the relative permeability of Metglas 2714A up to 1 MHz at 4.2 K. 2. Experimental method For measurements at frequencies <100 kHz, we use the setup shown in Fig. 1, which is similar to the setup in Ref. [2]. A sinusoidal current is injected into Coil 1, while Coil 2 is superconducting and is connected to the SQUID input coil. Let I be the current in Coil 1 and I s the induced super- conducting current in Coil 2. Let L be the self-inductance of the coils wound on the toroid and L s the inductance of the SQUID input coil, which is given by the manufac- turer [3] as 1.94 lH. The magnetic flux induced in the core by I s is in the opposite direction as that induced by I, so the net flux through the toroid is U T = L(I  I s ). Because the pickup loop is superconducting, the flux conservation requires that U T = L s I s . Solving for L we obtain 0011-2275/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.cryogenics.2007.03.004 * Corresponding author. Tel.: +1 818 354 3104; fax: +1 818 393 4878. E-mail address: talso.c.chui@jpl.nasa.gov (T.C.P. Chui). www.elsevier.com/locate/cryogenics Cryogenics 47 (2007) 279–281