Magnetically tuning the loss tangent in La(Al_{1 - X}Fe_x)O₃using low field electron paramagnetic resonance transitions

In 2012, we demonstrated that microwave loss in practical microwave dielectrics is dominated by electron paramagnetic resonance transitions at cryogenic temperatures. We later used this understanding to develop "smart"materials that switch Fe-doped Al2O3 (ϵr = 9.8) dielectric ceramics between a low-loss "on state"and a high-loss "off state"at frequencies of ∼12 and ∼19 GHz with a small magnetic field (<100 G). In this report, we extend our work on smart materials to the large dielectric constant (ϵr = 24) host La(Al1-xFex)O3 so that it can be used in compact resonator and filter designs operating at ∼4 GHz to ∼7 GHz. The Fe3+ ions' zero-field splitting energies are determined by the crystal-field parameters D = 1.55 GHz and E = 0 GHz, along with significant contributions from the higher-order terms, B 4 0(-6.467 MHz) and B 4 3 (160 MHz). These switchable dielectrics may have applications in future communication and Doppler technology.