Demagnetizing tape heads and recorder parts is a ritual of magnetic recording. If any part that touches the tape is magnetized beyond a certain level it will begin degrading the tapes played on it.

While early machines may have had an issue with magnetization, most late-model machines rarely become magnetized. The source for real information on this is Jay McKnight’s Magnetic Reference Lab Web site.

Demagnetizing a tape recorder discusses the issues and the need to demagnetize.

Field Strength for Partial Erasure of Magnetic Tape discusses the minimum strength that constitutes a risk to a magnetic record. The challenge in analyzing a tape recorder’s magentization is that it is very difficult to measure a small part since the area of magnetization is generally far smaller than most sensors, thereby providing a faulty (i.e. too-low) reading.

I use the Annis Han-D-Mag, available here. While I bought the kit, the Pocket Magnetometer is not that useful due to the relatively large size of the sensor area. I often demagnetize head assemblies on a bulk tape eraser.