Resonance ferrimagnetic

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. 

There is another, quite distinct, resonance phenomenon concerned with domain wall movements occurring at approximately one-tenth of the ferrimagnetic resonance frequency. To understand this Bloch wall motion needs to be... 

The precessional motion can be maintained by a suitable radio frequency field superimposed on the steady field. For example, in Fig. 9.38(b), when a steady field Hz is applied along the z axis and a radiofrequency field //,., is applied in the x-y plane and rotates in the same sense and at the same frequency as the precession, resonance occurs. Gyromagnetic resonance as outlined above is in principle the same as ferrimagnetic resonance referred to earlier (Section 9.3.1), except that in the former case the material is magnetically saturated by a strong applied field. In practice the steady field, which determines the Larmor frequency, is made up of the externally applied field, the demagnetizing field and the anisotropy field, and is termed the effective field He. Figure 9.39 shows the He values at which resonance occurs in some of the important communications and radar frequency bands. 

The ferrimagnetic resonance phenomenon in a ceramic ferrite in the form of a strip 70 x 5 x 2 mm is to be exploited in a particular microwave device, with the static field applied along the length of the strip. If the design frequency is 2 GHz and the coercive field for the ferrite is 100 Am-1, what limiting value is placed on the saturation magnetization ... 

Sharma VK, Waldner F (1977) Superparamagnetic and ferrimagnetic resonance of ultrafine Fes04 particles in ferroflrrids. J Appl Phys 48 4298-4302... 

Dillon, J. F. Jr Earl, H. E. Jr (1959). Domain wall motion and ferrimagnetic resonance in a manganese ferrite. Journal of Applied Physics, 30, 202-13. 

Interface polarization Dipole stretching Ferroelectric hysteresis Electric domain wall resonance Electrostriction Kezoelectricity Nuclear magnetic resonance Ferromagnetic resonance Ferrimagnetic resonance... 

In conclusion, it must be remarked that in this brief outUne of work on the garnets a number of topics have necessarily been omitted. In particular, mention must be made of ferrimagnetic resonance, where the narrow lines of YIG are broadened and shifted by the inclusion of a small percentage of rare earth ions see, e.g., Kittel (1959) and Van Vleck (1961, 1964). 

Ferrimagnetic resonance studies (FMR) have been made on evaporated amorphous thin films of Ho-, Gd-, Tb- and DyFc2 by Lubitz et al. (1976) and on bulk sputtered Tb-, Gd-, and YFe2 by Bhagat and Paul (1976). The former study indicated broadened linewidths which were consistent with a Landau-Lifshitz relaxation parameter of order 10 /sec for the Gd and Ho alloys, comparable with that observed in crystalline transition metal systems. The Dy and Ho systems... 

Kubayashi, K., Kawata, H., and Mori, K. (1998). Site specification on normal and magnetic XANES of ferrimagnetic Fc304 by means of resonant magnetic Bragg scattering. J. Synch. Rad. 5, 972-4. 

Since the two sublattices of a ferrimagnet have different g factors and different moments, it might be anticipated from equation 74 that each would resonate at a different frequency in a magnetic-resonance experiment. However, the strong coupling between sublattices causes them to resonate at the same frequency and in the same sense. Therefore in place of equation 74, it is necessary to consider the pair of coupled equations... 

Pardoe, H. Clark, P.R. St Pierre, T.G. Moroz, P. Jones, S.K. (2003), A magnetic resonance imaging based method for measurement of tissue iron concentration in liver arterially embolized with ferrimagnetic particles designed for magnetic hyperthermia treatment of tumors. Magn. Reson. Imaging, 21 483-488. 

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