Millimeter-wave magnetoelectric interactions have been studied through electric field effects on magnetic excitations in bilayers of single crystal barium ferrite and lead zirconate titanate ͑PZT͒. An electric field E produces a mechanical deformation in PZT, resulting in a shift ␦f in the frequency for electromagnetic modes in the ferrite. Reflected power versus frequency profiles at 40-55 GHz for a series of bias magnetic field and E =0-10 kV/cm along the c axis of the ferrite showed an increase in ␦f to a maximum of 8 MHz. Theoretical estimates of ␦f are in agreement with the data.

We report on the first observation of nonlinear magnetoelectric effects at room temperature due to a DC current in the ferrimagnetic M-type strontium hexaferrite platelets. Utilizing microwave measurement techniques and data on the shift in magnetic mode frequencies, it was found that a DC current along the hexagonal c-axis resulted in a significant decrease in the saturation magnetization and an increase in the uniaxial magnetocrystalline anisotropy field. These changes in the magnetic order parameters were directly proportional to the square of applied electric field and were found to be much higher than variations due to Joule heating. A phenomenological theory that takes into account the current-induced magnetobielectric effects (MBE) is proposed. Expressions for coupling coefficients for MBE have been obtained and have been calculated from the variations in magnetic order parameters. The electric field E (or current) tuning of the magnetic modes in SrM reported here is orders of magnitude stronger than strain mediated E-tuning of magnetic resonance in hexaferrite-ferroelectric composites. The non-linear ME effects in the hexaferrite, therefore, open up an avenue for the realization of E-tunable broad-band microwave and millimeter wave ferrite signal processing devices such as resonators and filters.

The observation of dielectric resonance over the frequency range 40-110 GHz in single crystal yttrium iron garnet (YIG) and its magnetic field tuning characteristics are reported. The dimensions of YIG are appropriately chosen in order to have the dielectric resonance occur at a much higher frequency than the ferromagnetic resonance and avoid any hybrid spin-electromagnetic modes. The dielectric modes are magnetically tunable by 1 GHz with a magnetic field of ∼1.75 kOe. The tuning range and required bias magnetic fields, however, can be controlled with the sample dimensions (or the demagnetization factor Nzz). Theoretical calculations on magnetic field tuning characteristics for the dielectric modes are in reasonable agreement with the data. The theory also predicts a similar magnetic tuning of the dielectric modes in the sub-THz frequency range as well. The dielectric modes that can be tuned with a magnetic field are of importance for the realization of low-loss tunable devices, including resonators, isolators, and phase shifters operating in the sub-THz region.

Magnetic and dielectric resonances in the sub-terahertz (sub-THz) frequency range are observed in pure and Al-substituted hexagonal barium ferrite. A resonator based on magnetic excitations has been fabricated and its performance characteristics have been studied. The possible use of the resonator at sub-THz frequencies has been demonstrated. The resonator exhibited a loaded Q-factor of 150-330 in the frequency range 97-108 GHz. Dielectric resonances in a single-crystal barium hexaferrite are observed in the frequency range 75-110 GHz. The modes excited by circularly polarized electromagnetic waves show nonreciprocal propagation characteristics. The dielectric resonances may occur at a much higher frequency than ferromagnetic resonance. It is shown that degeneracy in the dielectric modes is lifted with an applied magnetic field and that the modes can be tuned by 10 GHz or more with . Data on frequencies of the modes versus shows hysteresis. Theoretical predictions on -tuning characteristics of the principal dielectric 11 mode are in agreement with the data. The dielectric modes are of importance for the realization of low-loss devices, including resonators, isolators and phase shifters.

The nature of the nonlinear magnetoelectric effect is investigated in platelets of single-crystal Y-type hexaferrite with a collinear ferrimagnetic structure. The effect was observed at room temperature as a shift of 1.1-to-1.4 GHz in the ferromagnetic resonance frequency of Ba2Zn2Fe12O22 (Zn2Y) rectangular resonator with the application of an in-plane DC voltage. The shift amounted to 10%–12% of the central frequency which ranged from 8 to 17 GHz (X and Ku-bands). From the experimental results, we estimated the magnetoelectric modification of effective saturation magnetization and found that it scales almost linearly with the applied DC electric power. A phenomenological model for the nonlinear magnetoelectric effect, which considers the hexaferrite magnetic symmetry, is proposed and qualitatively accounts for the observed dependence of magnetic parameters on input power. It is shown that the resonator can operate as an electrically controlled discrete phase shifter with almost π/4 phase shift and <4 dB insertion losses. These results are of importance for the use of Y-type hexaferrites in electrically tunable planar microwave signal processing devices.

Spin waves in yttrium iron garnet (YIG) nano-structures attract increasing attention from the perspective of novel magnon-based data processing applications. For short wavelengths needed in small-scale devices, the group velocity is directly proportional to the spin-wave exchange stiffness constant [Formula: see text]. Using wave vector resolved Brillouin light scattering spectroscopy, we directly measure [Formula: see text] in Ga-substituted YIG thin films and show that it is about three times larger than for pure YIG. Consequently, the spin-wave group velocity overcomes the one in pure YIG for wavenumbers k > 4 rad/ μm, and the ratio between the velocities reaches a constant value of around 3.4 for all k > 20 rad/ μm. As revealed by vibrating-sample magnetometry and ferromagnetic resonance spectroscopy, Ga:YIG films with thicknesses down to 59 nm have a low Gilbert damping ([Formula: see text]), a decreased saturation magnetization [Formula: see text] mT, and a pronounced out-of-plane uniaxial anisotropy of about [Formula: see text] mT, which leads to an out-of-plane easy axis. Thus, Ga:YIG opens access to fast and isotropic spin-wave transport for all wavelengths in nano-scale systems independently of dipolar effects.

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