Devices which incorporate NFC (Near Field Communications), RFID (Radio Frequency Identification), or wireless power transfer, all utilize wire coils which, when placed within close proximity of each other, can magnetically couple together and allow data and/or power to be passed between the two coils.
Image from TDK
Commonly, these coils need to be mounted in small portable devices which incorporate circuit boards, metal materials, and batteries all within close proximity. When coils are placed in these environments, magnetic fields generated not only couple between the coils, but also to the surrounding materials, causing magnetic interference, unwanted eddy currents, and reduced range.
A solution to this is to place a sheet of material with high magnetic permeability on the back side of the coil. By placing this material between the coil and the other components, the generated magnetic flux field is modified, reducing the unwanted effects previously mentioned, and actually enhancing the range of the coupled field.
Image from 3M
Digi-Key carries a number of these products which can be found here: