| Summary: | 博士 === 國立中山大學 === 物理學系研究所 === 98 === In this thesis, we have performed systematical study of the complex impedance
spectra(CIS) with the manganeses oxide thin films by the equivalent circuit
model(ECM) composed of resistance and capacitance. The ECM has been utilized
in analog of the electrical and dielectric properties of the granular films. The purpose
of this research is to understand how the electrical- and magneto-transport properties
in La0.67Ca0.33MnO3(LCMO),La0.8Ba0.2MnO3(LBMO),La0.67Sr0.33MnO3(LSMO(113))
and La0.67Sr1.33MnO4 (LSMO(214)) thin films, at various magnetic fields and
temperatures.
First of all, we demonstrate that the LSMO(214) and LSMO(113) can be
sensitively affected by magnetic states on the manganite films. Our result provides
further understanding of the dielectrics variation during the phase transition from an
AFM insulating phase and/or a ferromagnetic metallic phase to a paramagnetic PM
metallic phase. It is known that the strong correlation between the itinerant carriers
and the local magnetic moments is the mechanism for FM/PM phase transition for
LSMO(113), while the direct magnetic exchange coupling governed the AFM/PM
phase transition and an indirect coupling to the status of intrinsic carriers for
LSMO(214) films. These transitions can not be concludes directly by using a dc
resistance measurement but can be clearly distinguished by the CIS measurement.
On the other hand, the dc resistance (Rdc) and the relaxation time(τ) have the same
tendency that this indicates the changes of τ matches to the electric transport
properties for LCMO_90min and LSMO(214) thin films.
We focus on the the dielectric properties of both samples are insensitive to
temperature, revealing that the dielectric behavior is independent of magnetic phase transition but strongly associated with the transport properties. Therefore, the
magnetic transitions can be most thoroughly investigated by combining CIS
measurements and RC ECM, as well as by making dc resistance measurements.
Moreover, the relative change of Mχ(ac) is nearly larger than the dc resistive
variation. This phenomenon, called giant magneto-impedance effect (GMI), implies that
thehigh-frequency magnetotransport effect may enhance the performance of these
manganese oxides for sensing the magnetic field. The CMI, have been analyzed by
ECM, including two sets of parallel R and capacitance (C) components in series. The
analyzing results the specific feature of grain boundaries(GBs) can be attributed to the
interplay of magnetic moment spin disorder to ordering. The grain boundary (GB)
effect can enhance low field magnetoresitance (LFMR) for artificial GBs, but shows
very limited enhancement for those GBs in epitaxial films. This study finds that
artificial GBs, which exhibit large LFMR, can be modeled as a non-conductive layer
which disconnects the lattice periodicity of adjacent grains and contains no magnetic
ions. The GBs in the present fully strained epitaxial film, which shows a relatively
smaller LFMR, are more similar to a semi-continuous grain with continuous
distribution of magnetic ions that align loosely parallel to the grain magnetic moment.
In addition, we report in this study the high frequency magneto-transport
properties, based on the classical model, of La0.8Ba0.2MnO3 and La0.67Ca0.33MnO3 thin
films around their ferromagnetic transitions and under an external magnetic field. It
is found that the specific features of magneto-impedance can be correlated with the
complex magnetization response and the dielectric relaxation in corresponding phase
states. The fast dielectric relaxation time, τE, and the slow magnetic response, τH,
reflect the interplay of itinerant carriers and the magnetic coupling to the ac
electromagnetic wave, indicating that the double exchange, or hopping, of carriers
between O 2P and Mn 3d-eg states occur prior to the indirect magnetic coupling of adjacent Mn ions via strong Hunt’s rules. Applied magnetic field enhances both
electric and magnetic dipoles are now responding faster to the electromagnetic wave.
The results of our work may provide a fundamental understanding of high frequency
magnetic and electrical properties of the manganite films, and imply tips for device
application of the films.
|
|---|
