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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