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

4.3 Low temperatures (T << T
C
)
Ni-Zn ferrites present other interesting phenomena at T below room temperature. These 
appear as small “bumps” in the thermal behavior of both the resonance field and the 
linewidth, at about 240 K. These phenomena are more evident in LFMA (Low-Field 
Microwave Absorption). In this absorption mode (see Section 3.), the system is far from the 
resonance conditions as stated in the Larmor relation, and it can be thought as the 
interaction between the microwave field and the ordered spins in the material as the 
magnetization state progresses from the demagnetized state toward magnetic saturation. In 
the simple case, LFMA appears as an antisymmetric signal at both sides of the H = 0, Fig. 
4.4. LFMA also exhibits hysteresis by cycling the application of the magnetic field.
-1.0
-0.5
0.0
0.5
1.0
Signal-2
154 K
180 K
205 K
216 K
239 K
d
P
/d
H
(a
.u
.)
H
(kOe)
Signal-1
381 K
365 K
397 K
413 K
430 K
Fig. 4.4. LFMA measurements of Ni
0.35
Zn
0.65
Fe
2
O
4
ferrites, for selected temperatures. The 
temperature range in the upper graph is 300-430 K, while in the lower one it is 154-239 K 
(adapted from Alvarez et al 2010). 
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The Temperature Behavior of Resonant and Non-resonant Microwave Absorption in Ni-Zn Ferrites 
399 
Several features are significant in these plots. Beginning with the high temperatures (upper 
part of Fig. 4.4), it is evident that the amplitude of the signal at both sides of H = 0 decreases as 
T
increases, leading to a flat response for T 
≥ 430 K, which is the Curie point. It is therefore 
confirmed that LFMA is associated with magnetization processes in the ordered phase.
Also, it can be observed that the field corresponding to the peak to peak magnetic field 
values, maxima (for negative fields) and the minima (for positive fields) increases as T 
decreases. By comparing with a direct calculation of the anisotropy field, H
K
, Valenzuela et 
al (2011) were able to show that the amplitude between maxima-minima in LFMA is directly 
associated with H
K
, upper part of Fig. 4.5. 
By a comparison between the two sets of curves separated by T ~ 250 K, it appears that there is 
a continuous evolution of the antisymmetrical signal, from high T to low T, from a signal with 
the same phase as the FMR signal (Fig. 4.1) for T > 250 K, to the opposite, also antisymmetric, 
but minimum-maximum (Min-Max), or out-of-phase signal, which is clearly reached at T 
≤ 150 
K. Both signals are centered on H = 0. The presence of such out-of-phase signal has been 
correlated with the occurrence of a ferromagnetic ordering (Owens 2001, 2005). In fact, an-out-
of phase LFMA signal has been observed in many ferromagnetic systems (Montiel et al 2005, 
2008, de Cos et al 2008). It can be assumed that a parallel, ferromagnetic arrangement of spins 
is related with this signal, while an antiparallel, ferrimagnetic structure leads to the opposite 
result (in phase signal). In the present case, the evolution of the signal when decreasing 
temperature should be associated with the appearance of a parallel arrangement of spins for T 
≤ 150 K. Due to the Yafet-Kittel triangular structure, Fig. 2.5 (c), there is effectively a 
ferromagnetic arrangement simply by considering the components of the canted spins of 
cations on B sites. Figure 4.5 shows the evolution of Curie temperature and Yafet-Kittel 
transition for NiZn ferrites; the latter was determined by neutron diffraction (Satya Murthy et 
al, 1969). The results on Fig. 4.4 were obtained for x = 0.65, leading to a Yafet-Kittel transition, 
T
YK
, about 250 K, which is in very good agreement with these results.
The transition from the collinear arrangement to the Yafet-Kittel triangular structure can be 
detected (as temperature decreases) by means of MAMMAS experiments, as shown in Fig. 
150
200
250
300
350
400
450
500
0.0
0.2
0.4
0.6
0.8
1.0
Δ
H 
(kOe
)
T (K)
Fig. 4.5. Thermal variations of the peak-to-peak magnetic field of LFMA spectra for both 
signals.
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Electromagnetic Waves 
400 
0.0
0.2
0.4
0.6
0.8
0
200
400
600
800
triangular (Yafet-Kittel)
ferrimagnetic (colineal)
paramagnetic
T
(K
)
x
Ni
1-x
Zn
x
Fe
2
O
4
Fig. 4.6. Curie temperature and Yafet-Kittel temperature for NiZn ferrites (Adapted from 
Valenzuela 2005a and Satya Murthy et al 1969). 
4.7. As explained in Section 3, the sample is subjected to a small magnetic field, and its 
microwave absorption is monitored as temperature is slowly changed. The MAMMAS 
response exhibits, from room temperature, a continuous decrease to a minimum value at 
about 240 K. Then, the absorption increases again as the temperature keeps decreasing. 
These features point to a change in the microwave absorption regime due to a change in the 
material structure. In this case, all evidence is associated with the transition from the 
150
175
200
225
250
275
300
-0.4
-0.2
0.0
0.2
0.4
M
A
M
M
A
S res
pons
e (a.
u
.)
T  (K)
f = 9.4 GHz
Fig. 4.7. Microwave absorption response of Ni
0.35
Zn
0.65
Fe
2
O
4
ferrites in the MAMMAS 
experiment (Alvarez et al 2010). 
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The Temperature Behavior of Resonant and Non-resonant Microwave Absorption in Ni-Zn Ferrites 
401 
collinear ferromagnetic structure with iron in A sites of the spinel coupled by a 
superexchange interaction with iron cations (and nickel cations) on B sites, for T > 240 K, to 
the triangular structure, where spins are no more collinear. Due to the weakening of the A-
O-B interaction (as A sites become increasingly populated by zinc non magnetic ions), it 
becomes comparable to the B-O-B interaction which tends to establish an antiparallel 
geometry on the spins of ions on B sites.