18 The Temperature Behavior of Resonant and Non-resonant Microwave Absorption in Ni-Zn Ferrites

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18 
The Temperature Behavior of Resonant and 
Non-resonant Microwave Absorption
in Ni-Zn Ferrites 
Raúl Valenzuela 
Departamento de Materiales Metálicos y Cerámicos,
Instituto de Investigaciones en Materiales, 
México 
1. Introduction 
The magnetic response of Ni-Zn ferrites at microwave frequencies has been recently 
investigated by means of resonance techniques, by several authors. In this chapter, we 
present a review of recent results obtained on the resonant microwave absorption (electron 
paramagnetic resonance, EPR, and ferromagnetic resonance, FMR) in the X-band (9.5 GHz), 
of polycrystalline Ni-Zn ferrites (Zn
x
Ni
1-x
Fe
2
O
4
) for several temperature ranges. We begin at 
high temperatures in the paramagnetic state (T > T
C
, where T
C
is the Curie point); as 
temperature decreases, the onset of magnetic ordering is investigated, with its effects on the 
main FMR parameters. When experiments are carefully carried out, magnetic transitions can 
be detected as critical points in plots of the thermal behavior of the resonance line width.
We investigate also the behavior of nonresonant properties by means of the low-field 
microwave absorption (LFMA). This absorption, which occurs at applied fields of the 
same order of magnitude than the anisotropy field, H
K
, of the sample, is providing 
valuable information concerning the magnetization processes. LFMA is typically 
measured in the -1 kOe < H
DC
< +1 kOe field range. LFMA is associated with the 
nonresonant microwave absorption occurring during the magnetization processes from 
the unmagnetized state up to the approach to saturation. We provide here a short review 
of this particular measuring technique. Then, we propose to begin the study of LFMA in 
Ni-Zn ferrites also by decreasing the measuring temperature from the Curie transition. 
Clearly, LFMA is absent at T > T
C
since it depends on the magnetization processes in the 
ordered phase. For the 200 K < T < T
C
temperature range, a direct comparison of the 
anisotropy field calculated from LFMA and a calculation by using results of a direct 
measurement of H
K
on a ferrite single crystal. A very good agreement is obtained, thus 
confirming that LFMA is strongly dependent of the total anisotropy (magnetocrystalline, 
magnetoelastic and shape anisotropies) of the sample. 
We use as well a novel nonresonant microwave absorption technique known as 
magnetically modulated microwave absorption spectroscopy, MAMMAS. This technique is 
particularly well adapted to detect phase transitions of many types, as it is based on the 
change of microwave absorption regime during a change of crystalline, magnetic or 
electronic structure. MAMMAS is briefly described and applied to Ni-Zn ferrites.
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Electromagnetic Waves 
388