resonators magnetically tunable over a broad rf frequency range capable of operation at relatively low rf frequencies are disclosed as well as tunable devices based on them . the resonators comprise a resonator body made of a ferrimagnetic single - crystal material which is generally flat , and is generally rotationally symmetrical with respect to an axis of rotation , for example disk - like or oblate spheroidal - like . the axis of rotation of the resonator body is oriented with respect to the crystalline lattice in a particular way depending on the anisotropy . the same orientation is required for the dc field whose magnitude determines the resonance frequency . the resonators are particularly useful in broadband tunable rf and microwave filters and oscillators , and can be tuned by means of the dc field to resonate from about 50 mhz to several ghz .