Ferrites permeability

Many of the specific applications of ferrites depend on their behaviour at high frequencies. When subjected to an ac field, ferrite permeability shows several dispersions as the field frequency increases, the various magnetisation mechanisms become unable to follow the field. The dispersion frequency for each mechanism is different, since they have different time constants. Fig. 4.59. The low-frequency dispersions are associated with domain wall dynamics and the high-frequency dispersion, with spin resonance the latter, usually in the GHz range, is discussed in Section 4.6.2. 

A ferrite toroid or E core can be used for a drive transformer. No gap is needed since the input coupling capacitor guarantees that the core will operate in a bipolar fashion. A high permeability core is also suitable for this purpose. The wire that is going to be used will be in the range of 32 to 36 AWG. The core size will be approximately 0.4 to 0.6 inches (10 to 15mm). 

A high permeability ferrite is used such as the W material from Magnetics, Inc. which has a permeability of 10,000. 

Chemical reagents are primarily concerned with dielectric liquids or solids. For metal oxides such as ferrites, however, magnetic losses occur in the microwave region. As for a dielectric material, a complex magnetic permeability is defined as given by Eq. (16) ... 

The thus obtained high-density Mn-Zn ferrite was investigated in detail from the view of physical and mechanical properties, that is, the relationships between the composition of metals (a,) ) and <5 the magnetic properties such as temperature and frequency dependence of initial permeability, magnetic hysteresis loss and disaccommodation and the mechanical properties such as modulus of elasticity, hardness, strength, and workability. Figures 3.13(a) and (b) show the optical micrographs of the samples prepared by the processes depicted in Fig. 3.12(a) and (b), respectively. The density of the sample shown in Fig. 3.13(a) reached up to 99.8 per cent of the theoretical value, whereas the sample shown in Fig. 3.13(b) which was prepared without a densification process, has many voids. 

Soft magnetic materials are characterized hy high permeability and low cnercivily. There are six principal groups of commercially important soft magnetic materials iron and low carbon sieels, iron-silicon alloys, iron-aluminum and iron-aluminuni-silicon alloys, nickel-iron alloys, iron-coball alloys, and ferrites. In addition, iron-boron-hased amorphous soft magnetic alloys are commercially available. Table I summarizes tile properties of some of these materials. Table 2 summarizes properties of some ferrites. Properties of amorphous soft magnetic alloys arc listed in Table 3. 

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. 9.31 Dependence of the permeability of the ferrite Ni0 36Zn0 64Fe204 on frequency and microstructure S, normally sintered HP, hot-pressed (after [9]).

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.

To understand the principle of operation of important non-reciprocal (see below) microwave devices, consider what occurs when a plane-polarized microwave is propagated through a ferrite in the direction of a saturating field Ht. The wave can be resolved into two components of equal amplitude but circularly polarized in opposite senses, i.e. into a right-polarized and a left-polarized component. These two components interact very differently with the material, leading to different complex relative permeabilities H r+ = n r+ - j/r" i and /r - = /T- — j/r"-, as shown in Fig. 9.40. Because of the... 

For hard ferrites for use as permanent magnets relative permeability is unimportant and is usually near to unity in the magnetized state. The important parameters are a high coercive field, a high remanent magnetization and the maintenance of a suitable combination of these properties over the operational... 

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. 

A soft ferrite with complex relative permeability fi T = 2000 — 7j is in the form of a toroid of cross-sectional area 0.5 cm2 and inner radius 3 cm. A primary winding... 

Calculate the effective permeability and loss tangent of the ferrite and the e.m.f. when a 0.1 mm gap is introduced into the toroid. 

Certain ferromagnetic materials, notably iron, steels, and some ceramics (ferrites), are far more receptive ( lOOx) to magnetic flux than is air. Permeability is a measure of the receptiveness of the material to having magnetic flux set up in it [15],... 

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