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Abstract
The magnetoelectric effect is a key issue for material science and is particularly significant in the high frequency band, where it is indispensable in industrial applications. Here, we present for the first time, a study of the high frequency tunneling magneto-dielectric (TMD) effect in nanogranular FeCo-MgF films, consisting of nanometer-sized magnetic FeCo granules dispersed in an MgF insulator matrix. Dielectric relaxation and the TMD effect are confirmed at frequencies over 10MHz. The frequency dependence of dielectric relaxation is described by the Debye-Fröhlich model, taking relaxation time dispersion into account, which reflects variations in the nature of the microstructure, such as granule size, and the inter-spacing between the granules that affect the dielectric response. The TMD effect reaches a maximum at a frequency that is equivalent to the inverse of the relaxation time. The frequency where the peak TMD effect is observed varies between 12MHz and 220MHz, depending on the concentration of magnetic metal in the nanogranular films. The inter-spacing of the films decreases with increasing magnetic metal concentration, in accordance with the relaxation time. These results indicate that dielectric relaxation is controlled by changing the nanostructure, using the deposition conditions. A prospective application of these nanogranular films is in tunable impedance devices for next-generation mobile communication systems, at frequencies over 1GHz, where capacitance is controlled using the applied magnetic field.
Highlights
The magnetoelectric effect is an attractive phenomenon for material physics research, and device applications [1,2,3,4]
The relative permittivity increases with the FeCo content of the films for all frequencies considered, and decreases with increasing frequency. These results indicate that the relative permittivity in the high frequency band can be controlled using the magnetic metal concentration (FeCo content)
We observed a good fit between results from experiments and theoretical calculations in the entire frequency range, regardless of the FeCo content, suggesting that the dielectric relaxation of the nanogranular films is broadly explained by the Debye-Fröhlich model, using the dispersion of the relaxation time
Summary
The magnetoelectric effect is an attractive phenomenon for material physics research, and device applications [1,2,3,4]. Materials where the existence of the magnetoelectric effect has been reported include oxides and quantum dots [5,6,7,8]. We recently observed the magnetoelectric effect in nanogranular films. The structure of the nanogranular films comprises a complex of nanometer-sized magnetic metal granules, covered with an insulating matrix. Nanogranular films with a high concentration of magnetic granules exhibit superior soft magnetism in the high frequency band, due to the strong exchange interaction between the magnetic granules, and the high resistivity of the insulator matrix [12,13,14,15]. When the concentration of the insulator matrix is relatively high, the exchange interaction between neighboring
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