In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

A ferromagnetic thin-film based magnetoresistive device with a first ferromagnetic material based film having electrically conductive, ferromagnetic material nanogranules embedded in an intergranular material of a smaller electrical conductivity first nonmagnetic. The device may have an intermediate layer adjacent the first ferromagnetic material based film and a second film is on the other side of the intermediate layer of a substantially ferromagnetic material. The first film is less than 1.0 μm thick.

Magnetic devices using nanocomposite materials

Excellent soft magnetic properties have been achieved in a wide metal volume fraction (x) range for as-deposited (Fe65Co35)x(SiO2)1?x granular films fabricated by magnetron sputtering. With the decrease in x from 0.7 to 0.5, the films exhibit small coercivity Hc not exceeding 4?Oe and high electrical resistivity ? up to 5.48 ? 103????cm. A minimum Hc value of 1.7?Oe was obtained for the sample of x = 0.57 with ? = 2.86 ? 103????cm. At a frequency lower than 1.3?GHz, the real part ?' of complex permeability of this sample is about 170 and then decreases with frequency while the imaginary part ?'' is visible when f > 1?GHz, which may be caused by anisotropy dispersion. The FMR frequency is as high as 2.4?GHz, implying a high cut-off frequency for high frequency applications. A study of high resolution transmission electronic microscope indicates that the samples consist of Fe65Co35 particles uniformly embedded in an insulating SiO2 matrix. The excellent soft magnetic property is ascribed to exchange coupling among magnetic particles. The investigation of the ?m?H curve evidences the existence of this inter-granule interaction.

http://iopscience.iop.org/article/10.1088/0022-3727/40/12/016/pdf

Microstructure and magnetism of FeCo?SiO2 nano-granular films for high frequency application

(Fe3O4)1−x–(SiO2)x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by log ρ∝T−1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH−ρ0)/ρ0} of about −4.7% for Fe3O4 films and −1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x⩾0.4.

Structure and magnetotransport properties of Fe3O4–SiO2 composite films reactively sputtered at room temperature

A systematic study on the structural, magnetic, and transport properties of an all-oxide granular system consisting of Fe3O4 grains dispersed in an insulating Al2O3 matrix is presented. X-ray diffraction and transmission electron microscopy analyses provide the structure of nanosized Fe3O4 grains embedded in an amorphous Al2O3 matrix and confirm that the Fe3O4 grain size decreases with increasing Al2O3 volume fraction x. Magnetic measurements further show that the coercivity of the granular films decreases with increasing x due to decreasing Fe3O4 grain size, with the magnetization curves exhibiting typical superparamagnetic characteristics for x0.19. Temperature dependence of resistance and nonlinearity in current-voltage characteristics confirms that the spin dependent tunneling of conduction electrons across grains dominates the transport properties. This tunneling conductance depends on the relative orientation of magnetization between Fe3O4 grains, which results in an isotropic granular tunneling magnetoresistance TMR effect. We also observed that the granular TMR ratio has strong temperature dependence and decreases with increasing temperature for all the granular films.

https://www.ece.nus.edu.sg/stfpage/eleaao/Adekunle_Adeyeye/funcmat/funcmat1.pdf

Magnetic and tunneling magnetoresistive properties of an all-oxide Fe 3 O 4 -Al 2 O 3 granular system

The frequency dependence of the magnetic permeability as well as of the electrical impedance have been investigated for soft-magnetic granular FeHfO thin films. The impedance measurements indicate that capacitive effects resulting from the inhomogeneous structure of the layers are of no importance for the roll off of the permeability of the present films. The frequency behavior at various FeHfO thicknesses shows that eddy current effects start to play a role above thicknesses of 10 μm. Below this thickness ferromagnetic resonance dominates the roll off.

/pdf/on-the-frequency-dependence-of-the-magnetic-permeability-of-2v65rhub95.pdf

On the frequency dependence of the magnetic permeability of FeHfO thin films

We have studied the tunneling resistivity and magnetoresistance of reactive sputter deposited FeHfO and FeHfSiO thin granular films Maximum magnetoresistance ratios at room temperature of 2% and 32% were observed for films with compositions of Fe47Hf10O43 and Fe40Hf6Si6O48, respectively The magnetoresistance shows a decrease with temperature, which cannot be explained by spin-dependent tunneling only We propose that spin-flip scattering in the amorphous FeHf(Si)O matrix causes this decrease as function of temperature A two current model for the tunnel magnetoresistance, taking into account spin-flip scattering, is presented which can describe the observed temperature dependence of the magnetoresistance

/pdf/temperature-dependence-of-the-resistivity-and-tunneling-3i5u5wsntf.pdf

B Bas Rulkens

Papers

On the frequency dependence of the magnetic permeability of FeHfO thin films

Temperature dependence of the resistivity and tunneling magnetoresistance of sputtered FeHf(Si)O cermet films