MnZn ferrite

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

It is possible to systematically alter the net magnetic moment of ferrites by chemical substitutions. A very important industrial appHcation is the increase of the magnetic moment in mixed MnZn-ferrites and NiZn-ferrites. When Zn ions are introduced in Mn-ferrite or Ni-ferrite, these ions prefer to occupy A-sites. Because is nonmagnetic, the A-sublattice magnetization is reduced and consequendy the total net magnetic moment is increased. 

Figure 4 shows a typical result for as a function of temperature for a Fe -containing MnZn-ferrite. changes sign at a certain temperature Tq... 

Fig. 8. Composition regions of optimal magnetic properties ia the MnZn ferrite system (33). Values of ia units of Pq.

Fig. 9.22 Temperature dependence of the resistivity for NiZn and MnZn ferrites.

The replacement of the MnZn ferrite family by the NiZn ferrites as the application frequency rises is partly due to the higher resistivities of the NiZn... 

When excited by an applied alternating magnetic field the magnetization vector will precess around the anisotropy field as discussed more fully later (Section 9.3.4). Resonance occurs when the frequency of the applied field coincides with the natural precessional frequency, i.e. the Larmor frequency coL = yfi0HA, with the result that the permeability falls and losses increase, as shown for a family of NiZn ferrites in Fig. 9.29. The onset of such ferrimagnetic resonances restricts the use of MnZn ferrites to frequencies of less than about 2 MHz. At higher frequencies, up to about 200 MHz, compositions from the NiZn family are used. 

Fig. 9.32 The effect of dimensional resonance on permeability for two MnZn ferrite components of different sizes (a) larger and (b) smaller. Permeability values are expressed relative to those measured at 1 kHz.

There are two ferrite material properties which were not discussed in Section 9.3.1 but which are important in the inductor context they are the temperature and time stabilities of the permeability which, of course, determine the stability of the inductance. The temperature coefficient of permeability must be low, and this has been achieved for certain MnZn ferrite formulations as indicated in Fig. 9.18. A small residual temperature coefficient of inductance can be compensated by a suitable coefficient of opposite sign in the capacitance of the resonant combination. 

As expected, the NiZn ferrite system is available for the higher frequencies (up to approximately 5 MHz), whereas for frequencies up to about 100 kHz the MnZn ferrites are favoured because of their higher permeabilities. 

NiZn Ferrite 72800, 72500 MnZn Ferrite 73300 CuZn Ferrite 126800 Iron silicide Fine, Corse Wright Industries, Inc., Brooklyn, NY, USA... 

In commercial and polycrystalline ferrites processing, variables and resultant microstructures have important consequences on measured properties. Only a few will be mentioned here. For example, the addition of a few percent of cobalt to Ni-ferrite can increase its resistivity by several orders of magnitude by ensuring that the iron is maintained in the Fe " state. Similarly, it is important to sinter MnZn ferrites under reducing atmospheres to ensure that the manganese is maintained in the Mn state but not too reducing so as to convert the Fe to Fe . 

Yavuz, Ozlem M. K. Ram, M. Aldissi, P. Poddar, S. Hariharan, Synthesis and the Physical Properties of MnZn Ferrite and NiMnZn Ferrite-Polyaniline Nanocomposite Particles. /. Mater. Chem. 2005, J5,810. 

Fatemi, D.J., Harris, V.G., Browning, V.M., Kirkland, J.P. Processing and cation redistribution of MnZn ferrites via high-energy ball milling. J. Appl. Phys. 83, 6767-6769 (1998)... 

Rozman, M., Drofenik, M. Sintering of nanosized MnZn ferrite powders. J. Am. Ceram. Soc. 

FIGURE 6.1.12 Effect of oxygen content on the magnetic properties of the MnZn ferrite. 

FIGURE 6.1.13 Effect of BIiOb doping on the nonstoichiometric oxygen content of MnZn ferrite. 

FIGURE 6.1.15 Temperature dependence of core loss of MnZn ferrite for the electric car. 

FIGURE 6.1.16 The temperature dependence of saturation flux density of MnZn ferrite for electric car. 

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