Magnetic Shielding Alloy

Shielding alloys have high magnetic permeability, making them very effective at screening static or low frequency magnetic fields. Electromagnetic waves come in varying degrees of severity, which make each particular situation, unique to itself. Regardless, shielding efficiency is based upon the strength of the field in connection to type of shielding being performed. Shielding performance can be defined generally in 3 degrees: Absorption, Reflection and Multiple Reflection.

Absorption happens at surface of the shield while Reflection and Multiple Reflection happen as a result. Shielding of low frequency waves can be more difficult. To help combat this, High Permeability Alloys were introduced which created better absorption and reflection characteristics. HyMu 80, Mu Metal2 and 49 Alloy are good alloys for these applications. No matter what the application, the first approach is to understand the problem before deciding on your overall solution.

Shielding alloys are most commonly used to protect electrical components from magnetic fields and isolate wires within cables.  They are also used for transformer cores, telephone and recording devices, protecting sensitive medical equipment in an MRI facility, and increasing the quality of home theatre systems.
HYMU 80 Mu Metal Shielding Alloy is an 80% nickel-iron-molybdenum alloy with extremely high initial and maximum permeability, very low coercive force and minimum hysteresis loss, making it particularly well-suited for magnetic shielding applications.

From http://www.nealloys.com/shielding-alloys.php