Among other things, the disclosure features a system comprising a sensor, a DCmagnetic field source, an AC magnetic field source, and a receiver. The sensorhas anaspect ratio of 10:1 or higher and comprises a ferromagnetic material. Theferromagneticmaterial has a non-linear magnetization response, and the response contains amaximumpoint of non-linearity. The DC magnetic field source is adjustable forproviding amagnetic excitation field to excite a magnetic field within the sensor. Theprovidedmagnetic excitation field has a range such that the excited magnetic fieldwithin thesensor is near the maximum point of non-linearity. The AC magnetic fieldsource isconfigured to generate an AC magnetic field to cause the sensor to generateevenharmonics. The receiver is configured to receive the even harmonics from thesensor fordetermining a position of the sensor.
