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, B40(−6.467 MHz) and B43 (160 MHz). These switchable dielectrics may have applications in future communication and Doppler technology.
Spin polarized neutron reflectometry measurements were performed on Ni80Co20/Cu multilayers with a Cu spacer thickness of 20 Å, corresponding to the second oscillation peak in the magnetoresistance of the NiCo/Cu multilayer system. Measurements in a 15 Oe field indicate a nearly perfect antiferromagnetic stacking of the magnetic moments in successive Ni80Co20 layers. The existence of a small magnetic anisotropy in these magnetically soft multilayers leads to the canting of the magnetic moments at an angle of ∼70° with respect to the neutron spin polarization. This interlayer antiferromagnetic coupling can be suppressed by an applied field of ∼200 Oe.
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