That coil is most likely the NFC antenna. If you read the docs for some of the Samsung phones, only specific batteries allow the phone to have NFC capability. In other words, the NFC antenna is part of the battery pack.
NFC is the acronym for Near Field Communications. It is that technology that allows phones to transfer data back and forth just by touching them together.
NFC communications is extremely short range. A gap of a few millimetres will stop the communications.
It is extremely unlikely that this technology can be used to spy on people without their explicit permission - the user must make the conscious choice to place their phone in direct contact with another device.
Source:Internet