| 其他摘要 | The (Mn1-xFex)5Ge3, CoMnSi and InMn3 compounds were prepared by
arc-melting appropriate metals in a magneto-controlled arc furnace and annealing at a
certain temperature in purified argon atmosphere. The Mn2-xZnxSb compounds were
prepared by solid state reaction method, followed by multistep heat treatment. The
La0.7Sr0.3MnO3 and Nd0.45Sr0.55MnO3 films were prepared by pulsed laser deposition
(PLD) method. The structures of crystalline compounds were studied by X-ray
diffraction (XRD), whereas the microstructures of films were investigated by means
of XRD diffraction, scanning electron microscope (SEM), transmission electron
microscope (TEM) and atomic force microscope (AFM). The magnetic, transport
properties, magneto-transport and magnetocaloric properties were measured by a
superconducting quantum interference device (SQUID) magnetometer.
In (Mn1-xFex)5Ge3 compounds ,we found a large magnetocaloric effects (MCE).
The maximum of magnetic entropy changes of 8.01 J/kg K under an external field
change of 5 T is obtained for (Mn0.9Fe0.1)5Ge3, which is the largest value in
Mn5Ge3–based solid solutions. Moreover, the Fe substitution increases the
refrigeration capacity (RC) value greatly. The largest RC value of 237 J/kg in
(Mn0.8Fe0.2)5Ge3 even compares favorably to that of many well-known magnetic
refrigeration materials, such as Gd5Ge2Si2 (200 J/kg), Gd5Ge1.9Si2Fe0.1 (235 J/kg),
MnFeP0.45As0.55 (225 J/kg), etc. Thus the Fe-containing (Mn1-xFex)5Ge3 compounds
are much-improved magnetic refrigerants for the application of room-temperature
magnetic refrigeration. The increase of the RC value is probably resulted from the
formation of magnetic nanostructure.
In addition to artificial superlattices, it has been widely realized that intermetallic
compounds with interesting mechanisms could be considered as another stream for
the research of the giant magnetoresistance (GMR) effect. A giant magnetoresistance
is observed in Mn2-xZnxSb (x < 0.3) system associated with a first-order transition
between two ferrimagnetic states, with the largest MR ratio of -37.6% in a field of 5 T
at 120 K for Mn1.9Zn0.1Sb compound. Our studied show that the influences of both
super-zone gap and spin-dependent scattering are responsible for GMR in the present
system. The butterfly loops in both the magnetization and resistivity are observed as
long as the field-induced WFI-FI transition occurs in a certain temperature region.
There were no butterfly loops in the magnetization curves when the Mn2-xZnxSb
compounds are in the stable WFI or FI state, but the butterfly loop feature persists in
resistivity, revealing a different mechanism of these two cases.
As known, the large magnetoresistance(MR) effect in almost all the intermetallic
compounds occurs in a limited temperature region around a metal-nonmetal transition
or a magnetic phase transition. However, a large MR is observed in double helical
CoMnSi compound over an entire temperature region from 5 K to the maximum
measuring temperature of 380 K, with the largest MR ratio of - 18.3% at 245 K and
the smallest MR ratio of - 5.5% at 85 K at 5 T. This phenomenon is ascribed to two
different mechanisms in different temperature regions. The suppressed spin
fluctuations of the double helical structure are responsible for the MR below 110 K.
However, in consideration of the natural multilayer superstructure of CoMnSi, the
larger MR above 110 K is ascribed to the decrease of K-space restrictions when the
change of magnetic structure from double helical order to fan order occurs.
Bulk InMn3 compound enters into the coexistence of the spin-glass-like (SGL)
phase and ferromagnetic (FM) phase. Interestingly, the giant exchange bias (EB) and
anomalous large vertical magnetization shift are observed simultaneously in InMn3.
The giant exchange bias field as large as 7.8 kOe is found at 2 K, which is resulted
from the strong unidirectional anisotropy at the SGL/FM interfaces. The anomalous
large vertical shift might be ascribed to the existence of the spin-glass-like phase that
could provide a large pinned magnetization. Moreover, the asymmetrical hysteresis
loops, directly related to this training effect, are also found and possibly resulted from
irreversible changes of multiple configurations among disorder frozen spins of
spin-glass-like phase. The clear training effect is observed, which can be interpreted
by modified Stoner-Wohlfarth model.
Ultrathin Nd0.45Sr0.55MnO3 film on (100) SrTiO3 substrate is fabricated by pulsed
laser deposition technology. The structure, microstructure and transport properties are
studied in detail. XRD indicates that the film is (100) oriented and AFM suggests the
smooth surface with the roughness of only 0.665 nm. TEM also suggests the epitaxial
growth of the film. The temperature dependence of the resistivity curve indicates,
unlike the bulk Nd0.45Sr0.55MnO3, the insulator-like behavior occurs in
Nd0.45Sr0.55MnO3 / SrTiO3 film, which is interpreted as the strong strain between the
substrate and film. After the annealing at 1000o for five hours, the transition from
insulator to metal electrical behaviors accompanied with the GMR effect occurs due
to the partly release of this strong strain. |
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