磁性穿隧接面與有機自旋閥元件中自旋相依傳輸之研究

Spin Dependent Transport in Magnetic Tunneling Junction and Organic Spin Valve Device

李愷信(Kai-Shin Li)

指導教授 ： 林敏聰

國立臺灣大學/理學院/物理研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.10654

摘要

無資料

關鍵字

磁性穿隧接面 ； 磁電容效應 ； 超導體 ； 鄰近效應、有機自旋閥 ； 非彈性穿隧 ； 自旋維持躍遷

並列摘要

The tunnel magnetocapacitance of the magnetic tunnel junction has been investigated
with a series of complex impedance spectra measured at varying magnetic
field. To avoid the circuit complication in the four-terminal measurement with high
frequency operation, two terminal approach on the complex impedance was developed
by elimination of spin independent contribution apart from the junction area
in the cross-striped geometry. A subsequent fitting process based on the difference
spectra analysis were introduced to verify the tunnel magnetocapacitance effect and
its magnitude. As a result, tunnel magnetocapacitance ratio of -0.43% with the
opposite dependence on the field as compared to the tunnel magnetoresistance of
30.67%, was extracted. This characterization technique for the tunnel magnetocapacitance
would be applied in the further development and integration of spintronics
devices. The magnetocapacitance ratio is shown to be related to the tunneling magnetoresistance.
A theoretical model, where the dynamics of the magnetization is
described by Landau-Lifishitz equation, is proposed to explain the effective capacitance
in terms of the spin-dependent chemical potential shift due to the charge
density gradient at the interfaces. Taking Julli`ere model and the experimental dimensions,
a relation between magnetocapacitance, tunneling magnetoresistance and
the interfacial density of states is obtained, giving a connection of the experimental
results to the microscopic picture. The estimated interfacial density of states of
our magnetic tunnel junction gives a reasonable agreement with the bulk density of
state.
Hybrid superconductor/magnetic tunnel junctions of Nb/CoFe/Al2O3/CoFe/NiFe
have been implemented to investigate the impact of the superconducting Nb layer
on the proximal ferromagnetic CoFe layer. A suppression of tunneling magnetoresistance
effect is observed as the Nb layer undergoes the superconducting transition.
This suppression is thought to be due primarily to the coupling of the magnetic
inhomogeneity of CoFe to the superconductivity of Nb. The magnetic structurevortex
interactions interplay each other, inducing a complex domain evolution in
the magnetization process and consequently degrading the junctions antiparallel
magnetization alignment.
Transport mechanism is one of the central issues in organic spintronic devices,
in which electrons may experience a characteristic coupling with the organic spacer
layer during tunneling between two ferromagnetic electrodes. In order to probe
the transport behavior of spin-polarized electrons in organic materials, organic spin
valves were fabricated utilizing a relatively thin organic barrier of 3,4,9,10-peryleneteracarboxylic
dianhydride (PTCDA) dusted with alumina at organic/ferromagnetic
interfaces, in which spin injection with magnetoresistance up to 12% at room temperature
was achieved. In studies of the inelastic tunneling spectrum, the observation
of characteristic peak of organic layer provides direct evidence of the interplay
between the spin-polarized electrons and the organic molecules. Combining the inelastic
tunneling results with a simple molecular vibration calculation yields further
information on the crystalline configuration of the molecular thin film and the possible
tunneling states of the spin-polarized electrons. Such interplay indicates a true
transport of spin-polarized electrons through organic material rather than through
defects or inter-diffusion compounds formed at the interfaces within the organic spin
valve. The spin dependent transport mechanism in the attractive organic semiconductors
determines the species of magnetoresistance effect. The identification of the
transport mechanism is possible through the distinctive temperature dependence of
organic spin valve conductance. Here we investigate the spin propagation process
through the elastic and inelastic transport channels depending on temperature and
bias voltage. Our experimental result distinctly demonstrates the spin-conserving
hopping transport through hybrid, CoFe/AlOx/PTCDA/AlOx/CoFe, spin valves.
We develop a spin-conserving transport model which is extended from Glazman-
Matveev theory combining with Julli`ere model to explain the spin polarized electrons
inelastic transporting in the localized states of organic barrier.

並列關鍵字

High frequency ； magnetocapacitance ； superconductor ； long range proximity effect ； inelastic tunneling ； organic spin valve ； spin conserving hopping

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