Microwave ferrites, part 2: passive components and electrical tuning

Low-loss ferrimagnets are the basis for passive microwave components operating in a wide range of frequencies. The magnetic resonances of passive components can be tuned using static magnetic fields over a wide frequency range, where higher operation frequencies require higher magnetic bias unless hexaferrites with large crystalline anisotropy are used. However, electrical tuning of the operation frequency, which can be achieved if the magnetic property of the material is sensitive to the field through magnetoelectric (ME) coupling, is more attractive than magnetic tuning. In the so-called multiferroic materials such as TbMnO3, TbMn2O5, BiFeO3, Cr2O3, and BiMnO3, which possess simultaneously both the ferroelectric and ferromagnetic properties, ME coupling is very small to be practical. The ME effect, however, can be significantly enhanced in the case of bilayer/multilayer structures with one constituent highly piezoelectric, such as Pb(Zr1 - x Ti (x) )O-3 (PZT) and 0.7Pb(Mg1/3Nb2/3)O-3-0.3PbTiO(3) (PMN-PT), and the other highly ferromagnetic, opening up the possibility for a whole host of tunable microwave passive components. In such structures, the strain induced by the electric field applied across the piezoelectric material is transferred mechanically to the magnetic material, which then experiences a change in its magnetic permeability through magnetostriction. Additionally, electrical tuning coupled with high dielectric permittivity and magnetic susceptibility could lead to miniature microwave components and/or make operation at very high frequencies possible without the need for increased size and weight common in conventional approaches. In Part 1 of this review, fundamentals of ferrite materials, interconnecting chemical, structural, and magnetic properties with the treatment of various types of ferrites used in microwave systems are discussed. Part 2 discusses the basis for coupling between electrical and magnetic properties for highly attractive electrical tuning of passive components by combining piezoelectric materials with ferrites and various device applications of ferrites.