Results of Experiments
[6] Experiments were performed on 10- to 13-mg monolithic pieces of synthetic
titanomagnetites (Fe3-xTixO4, where
x = 0.2, 0.4, and 0.6) placed
in the central part of a 1-cm kaolin cube. Note that, precisely on duplicates
of these samples, partial TRM self-reversal was fixed experimentally in products
of multiphase oxidation of titanomagnetites of the given compositions
[Gapeev and Gribov, 2002b].
[7] In the present study, initial samples were preliminarily demagnetized by an
alternating magnetic field ( H = 0.2 T) along three mutually orthogonal
directions, after which they were subjected to multiphase oxidation in air in
a thermomagnetometer at 500oC over exposure times ( t ) of 2 min to 240 h
in a constant magnetic field. The CRM forming in these experiments was measured
at the temperature of its acquisition in the thermomagnetometer shields with an
uncompensated field being no more than 10 nT. After removal of the field, the
samples were cooled in the same shields to room temperature ( T0 = 20o C).
A self-reversal was determined from anomalous behavior in curves of cooling
(at
H = 0) and subsequent heating (also in the absence of the field) of
laboratory CRMs.
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Figure 1
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[8] Behavior of
J<undef>rc (500oC,
t ). Figures 1a, 1b, and 1c (curves 1) plot
the time dependence of
Jrc at 500oC measured on titanomagnetite samples
of the respective compositions
x = 0.2, 0.4, and 0.6. All plots clearly display
the following pattern in the behavior of
Jrc (500oC,
t ): an initial
rise in the CRM to its maximum value, its subsequent decrease and, with the
further exposure of the sample at the given temperature, a new rise followed by
a decrease if the annealing time is sufficiently long. As was shown in
[Gapeev and Gribov, 2002b;
Gribov, 2004],
such a behavior pattern of
Jrc (500oC,
t ) clearly reflects
titanomagnetite alteration stages: the initial rise in CRM is associated with
single-phase oxidation (commonly at cracks and edges of grains) of the initial
titanomagnetite, and the subsequent changes in
Jrc (500oC,
t ) are
related to the transformation of the spinel phase from a metastable (cation-deficient)
into a two-phase state through nucleation and growth of hemoilmenite lamellae
coherently associated with the titanomagnetite matrix.
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Figure 2
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Figure 3
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[9] Behavior of
J<undef>rc (20oC,
t ). In the given layout of experiment, a positive
magnetization (i.e. directed along the external magnetizing field
H0 ) could
have been acquired in the process of titanomagnetite annealing only by spinel
phase regions that had a blocking temperature ( Tb ) above 500o C. The fraction
of such regions for low-Ti titanomagnetites is larger compared to high-Ti
varieties, and the fraction of ferrimagnetic phases with
Tc<500oC, on the
contrary, smaller. Therefore, one might expect that, upon the transition from
the
x = 0.2 titanomagnetite to the
x = 0.6 titanomagnetite, the effects of
anomalous behavior of
Jrc ( T)
during cooling of a sample in the absence of
a field will only be enhanced due to an increase in the relative amount of the
negative (with respect to
H0 ) component of
Jrc(T0) produced by lower
temperature ferrimagnetic regions of a grain. As is evident from Figure 1 (curves 2)
and Figure 2 (curves 1), the given experiment completely confirmed this
suggestion. Actually, samples of titanomagnetites with
x = 0.2 and 0.4 that
experienced multiphase oxidation in the field
H0 exhibit partial self-reversal
of
Jrc(T, H = 0)
during their subsequent cooling, while the
Jrc(T, H = 0) self-reversal is complete in samples of the
x = 0.6 initial titanomagnetite
(i.e. the values
Jrc (20oC) were always negative (Figure 1c, curve 2),
although the anomalous behavior of
Jrc(T, H = 0) does not become more pronounced
with an increase in
Jrc (500oC) (Figure 3)). It is natural to suppose that,
during titanomagnetite oxidation at different temperatures, the relation between grain
regions magnetized along the field and in the opposite direction will change, as
will also be true of the relation between normally and reversely magnetized
components of
Jrc (20o C).
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Figure 4
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[10] Properties of
J<undef>rc (20oC,
t ). Figure 4 shows that distinctions are also
observed in AF demagnetization curves of
Jrc( H, 20oC) (created in
multiphase-oxidized (at 500o C) titanomagnetite samples with different initial
compositions cooled to room temperature in the absence of a field). Thus, in the
case of low-Ti samples, the destruction of a viscous component during
H -demagnetization
is followed by demagnetization of the negative low-temperature component
Jrc (20o C), bringing about a rise in
Jrc( H) observed in Figure 4a (curves 1-4).
The further
H -demagnetization of the sample leads to the destruction of
magnetically more rigid positive component and, as a result, to a drop in CRM.
On the other hand, an increase in the time of oxidation of the given
titanomagnetites leads to a gradual disappearance of ferrimagnetic regions with
Tc< 500o C in a grain. Accordingly, the amplitude of
Jrc( H, 20oC) gradually
decreases, and only a decrease in
Jrc( H, 20oC) (curve 5 in Figure 4a) is
observed at
t> 11 h; therefore, this decrease can be related to the fact that
the state with
Tb> 500oC is attained in all grain regions.
[11] In the case of high-Ti samples, with a predominant contribution to
Jrc (20oC)
of the softer reversely magnetized low-temperature component, AF demagnetization
of the latter gradually decreases its value, up to the transition into the
region of negative values (Figure 4b).
[12] We should note that median fields ( Hm ) of AF destruction of
Jrc (20oC) in
both examples considered above will reflect the rigidity of its different
components. Thus, in the first case (low-Ti samples), the value
Hm reflects
the magnetic rigidity of the
Jrc (20o C) component magnetized along the field
H0, whereas in the second case (high-Ti samples) the corresponding component
is directed in the opposite direction. Results of
Hm measurements presented in
the Table 1 show that the rigidity of the along-field component is two to three
times higher than the rigidity of the reversely magnetized component of CRM. For
comparison, the table also presents the values of median fields of AF
destruction of pTRMs acquired in the field
H0 during cooling of multiphase-oxidized
samples (of course, with the use of duplicates) from 500o C to 20oC,
i.e. in the temperature interval in which the reversely magnetized component
Jrc (20oC) forms. As expected, the resulting values of
Hm for pTRM and
the low-temperature CRM component are commensurate.
[13] Further experiments were intended to asses the role of CRM in the self-reversal
of the resulting magnetization
Jrc+rpt. For this purpose, samples of
multiphase-oxidized titanomagnetites that acquired
Jrc (500oC,
H0 )
during annealing were cooled to room temperature in the magnetic field
H0.
Results are presented in Figure 2 (curves 2). It is well seen that the superposition
of the partial remanence
Jrc (500o C, 20oC;
H0 ) on
Jrc (500o C)
make the heating curve
Jrc+rpt(T, H = 0) anomalous as well, but negative values
of the total magnetization are not observed.
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Figure 5
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[14] Figure 5 presents transformations of the curves
Jrc+rpt(T) for multiphase-oxidized
(at 500o C during 168 h in a field of 0.1 mT) titanomagnetite samples
(initially with
x = 0.6) under different conditions of their subsequent cooling
also in a field of 0.1 mT. The anomalous behavior of
Jrc+rpt(T) is clearly
seen to become more pronounced with a decrease in the cooling rate. It is
natural to suppose that this is associated with an increase in the contribution
of the magnetically soft viscous component of the resulting magnetization.
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Figure 6
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[15] To gain constraints on the relative contributions of magnetically rigid and soft
components to the remanence
Jrc+rpt(T0), we constructed the dependences
Jrc+rpt( H, T0) shown in Figure 6. The curves
Jrc+rpt( H) are seen to be
multicomponent curves. With a decrease in the cooling rate of samples, the
fraction of the magnetically soft component of
Jrc+rpt reflecting an
increasing contribution just of its viscous component increases. However, the
measured integral value
Hm is always evidence for the contribution of
magnetically rigid regions in a grain.
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