Low-loss ferrites for frequency applications

The discovery of new magnetic oxides called ferrites at the turn of the century initially motivated many theorists to try and explain their magnetic properties. In the 1940s, Louis Néel began to develop his theory of ferrimagnetism, which he successfully applied to all ferrites. This theory essentially describes the static magnetic properties of these materials: saturation magnetization and transition temperature. Subsequently, numerous applications were discovered for these new materials, notably for high-frequency applications made possible by their high electrical resistivity (> 1 Ω · m), which characterizes most oxides.

In addition to resistivity, the essential parameters characterizing ferrites are :

• saturation magnetization M_s: varies from 0.15 to 0.60 T ;

• the H_a anisotropy field: this characterizes the rigidity with which magnetization is maintained in preferred directions in the crystal. The more easily the magnetization can move under the action of a weak external magnetic field, the lower the material's coercive field, high permeability and low losses if the frequencies of use are not too high. A "hard" ferrite is one that is "difficult" to magnetize and has high coercivity and anisotropy fields (H_a > 100 kA/m), and a "soft" ferrite is one that is "easy" to magnetize and has low coercivity and anisotropy fields (H_a < 10 kA/m).

Ferrites with a hexagonal crystallographic structure, such as magnetoplumbite (hexaferrites), are anisotropic. Their properties in the basic plane (a,b) are very different from those along the perpendicular c-axis. They are hard magnetic materials and, in polycrystalline form, are mainly used in the production of permanent magnets. The most common hexagonal ferrites are type M strontium (barium) hexaferrites with the chemical composition SrFe₁₂O₁₉(BaFe₁₂O₁₉).

Unlike hexaferrites, soft ferrites are isotropic. They have a cubic crystallographic structure and can be classified into two groups according to their technical applications.

• The first group are the soft ferrites used for frequencies ranging from 10 kHz to 500 MHz, which we'll discuss in the opening paragraphs of this article. Their generic formula is MeFe₂O₄ where Me represents a divalent transition metal or a combination of ions (the case of a mixed ferrite). Their name spinels comes from the mineral MgAl₂O₄ with the same crystallogra-phic structure. These materials are used in vast fields such as energy conversion or signal processing. Depending on the frequency range, we use :

  • manganese-zinc (Mn-Zn) spinel ferrites from 10 kHz to 1 MHz. These ferrites are...