Download miễn phí Luận văn Study of perpendicular exchange bias mechanism in MnPd/Co multilayers



CONTENTS
Preface 1
Chapter 1 Introduction
1.1 Background 3
1.2 Overview on exchange bias 6
1.3 Previous studies on perpendicular exchange bias 12
Chapter 2 Experimental
2.1 Introduction 15
2.2 Sample preparation 15
2.3 Experimental techniques 18
2.3.1 Glancing incident X-ray diffraction 18
2.3.2 Field emission scanning electron microscope 18
2.3.3 Stylus-method profilemetry 19
2.3.4 Energy dispersive X-ray spectrometer 19
2.3.5 Wavelength dispersive X-ray spectrometer 20
2.3.6 Magnetization hysteresis loops 21
2.3.7 Magnetization – temperature curve 22
2.3.8 Magnetic force microscope & atomic force microscope 22
Chapter 3 Experimental results
3.1 Introduction 23
3.2 Crystallographic structure 23
3.2.1 Glancing incident X-ray diffraction 23
3.2.2 Cross-section observation 25
3.3 Magnetic properties 25
3.3.1 Domain observation 26
3.3.2 Magnetization hysteresis loops at low temperature 26
3.3.3 Magnetization hysteresis loops at room temperature 30
3.3.4 Temperature dependence of magnetization in MnPd/Co multilayers 36
Chapter 4 Discussions
4.1 Introduction 37
4.2 Crystallographic structure 37
4.2.1 Glancing incident X-ray diffraction 37
4.2.2 Cross-section observation 38
4.3 Magnetic properties 38
4.3.1 Domain observation 39
4.3.2 Thickness dependence of exchange bias 39
4.3.2.1 Co thickness dependence of exchange bias 39
4.3.2.2 MnPd thickness dependence of exchange bias 41
4.3.3 Perpendicular magnetic anisotropy in MnPd/Co multilayers 43
4.3.3.1. Perpendicular anisotropy at low temperature 44
4.3.3.2. Perpendicular anisotropy at roomtemperature 46
4.3.3.3. Effect of annealing on perpendicular anisotropy 46
4.3.3.4. Anomalous field induced anisotropy 50
4.3.4 Temperature dependence of magnetization in MnPd/Co multilayers 51
4.4 Explanation of exchange bias coupling mechanism 52
Conclusions and further direction 56
References 58


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ductively to it. In the present thesis,
the area compositions of Mn and Pd on the target were about 60:40,
respectively. Meanwhile, a circular Co target was used to prepared
ferromagnetic layers.
The RF sputtering system has two power sources used for two targets. The
targets were placed in its positions in the deposition chamber and after that,
the deposition chamber was pumped out until the pressure inside was less
than 5 × 10-6 mbar. Samples were fabricated in Ar gas. The gas flow was
regulated by a mass flow controller and kept at a constant rate during the
deposition. Sputtering process was carried out in the condition of the Ar
pressure kept at about 5 × 10-3 mbar. No external magnetic field was applied
in the deposition and substrates were at ambient temperature.
The samples used in the present thesis are Si/[MnPd/Co]10 multilayer thin
films. The MnPd and Co layers were deposited alternately onto single crystal
Si(111) substrates (see Fig. 2-2) at the power of 150 W for the MnPd target
and 300 W for the Co target. The corresponding deposition rates for MnPd
and Co layers are 2.3 × 10-2 nm/s and 2.8 × 10-2 nm/s.
The compositions of MnPd layer were determined using a wavelength
dispersive X-ray spectrometer (WDS) and an energy dispersive X-ray
spectrometer (EDS). The results showed the Mn and Pd compositions are 11:
89, respectively.
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N bilayers x = 2.5 – 10 nm
y = 3.5 – 30 nm
N = 10 bilayers
MnPd y (nm)
Co x (nm)
Co x (nm)
Co x (nm)
Co x (nm)
MnPd y (nm)
MnPd y (nm)
MnPd y (nm)
Si substrate
Fig. 2-2. Schematic view of [MnPd/Co]N multilayer structure used in
the present thesis.
- 18 -
2.3 Experimental techniques
2.3.1 Glancing incident X-ray diffraction
In order to analysis the structure of sample, θ/2θ scan X-ray diffraction
was carried out using a PANalytical-Philips X’pert Pro system at Hanoi
University of Technology. A Cu target is used as the X-ray source. A double-
crystal monochromator is used to obtain monochromatic and collimated Cu
Kα1 radiation (λ=0.154056). The incident X-ray and the sample were fixed.
The incident angle of the X-ray beam was of 1 degree with respect to the
sample surface. Meanwhile, the detector rotated so that the θ/2θ scan
configuration was preserved during the measurements. In the present thesis,
the angle 2θ was from 25 to 70 degrees.
Diffracted beam
Sample
Incident beam
Fig. 2-3. Schematic diagram of glancing incident θ/2θ
scan X-ray diffraction configuration.
2.3.2 Field emission scanning electron microscope
Cross-section images were observed by a field emission scanning electron
microscope (FESEM). The best resolution of the system is up to 2 nm
(standard mode), 3 to 5 times better than conventional SEM. Because a field
emission source provides narrower probing electron beams at low temperature
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and high energy with acceleration voltage from 0.5 to 30kV (variable at 0.1
kV/step). The magnification of the system is in the range of X 20 - X 800000.
In present thesis, observations of cross-section were carried out by a
Hitachi FE-SEM S4800 microscope system at the Institute of Materials
Science, Vietnamese Academy of Science and Technology. After the sample
was broken into half, they were immediately used to view.
2.3.3 Stylus-method profilemetry
The stylus method consists of measuring the mechanical movement of a
stylus as it is made to trace the topography of a film-substrate step. The film
thickness is directly read out as the height of the resulting step-contour trace.
The profilemeter used in this thesis is called Alpha-step model with the
vertical resolution of about 1 Å. The Alpha-Step IQ is guaranteed step height
repeatability which makes it easier to precisely determine the thickness of thin
films, roughness, etch depth in a wide extending below 8 nm and tall step
height up to 2 mm. The performance is due to modern ultra-low noise
electronics and precision mechanical components. The stylus scanning motion
provides exceptional stability for extremely repeatable measurements.
To determine deposition rate for a material, a single layer with a film-
substrate step was prepared in a specific time. The single layer was measured
for three times in order to receive the mean thickness. Hence, one can
calculate the deposition rate for the material. In this thesis, the deposition
rates for MnPd and Co are respectively 2.3 × 10-2 nm/s and 2.8 × 10-2 nm/s.
These thickness measurements were carried out at the Institute of Materials
Science, Vietnamese Academy of Science and Technology.
2.3.4 Energy dispersive X-ray spectrometer (EDS)
- 20 -
X-rays emitted from a sample under electron bombardment are collected
with a liquid nitrogen-cooled solid state detector and analyzed via computer
according to their energy. Typically, the computer programs used in EDS will
display a real time histogram of number of X-rays detected per channel
(variable, but usually 10 electron volts/channel) versus energy expressed in
KeV.
Using EDS, all of the energies of the characteristic X-rays incident on the
detector are measured simultaneously and data acquisition is therefore very
rapid across the entire spectrum. However, the resolution of an EDS detector
is considerably worse than that of a WDS spectrometer. Besides, it is very
difficult to determine precisely elements and its compositions if there is only a
small amount of one of the overlapped elements.
In practice, EDS is most often used for qualitative elemental analysis,
simply to determine which elements are present and their relative abundance.
Depending on the specific needs of the investigations, quantitative results
may be advised to use the electron microprobe. In some instances, however,
the area of interest is simply too small and must be analyzed by TEM (where
EDS is the only option) or high resolution SEM (where the low beam currents
used preclude WDS, making EDS the only option). In this thesis, the sample
composition was analyzed by a Hitachi FESEM S4800 microscope system
integrated EDS at the Institute of Materials Science, Vietnamese Academy of
Science and Technology.
2.3.5 Wavelength dispersive X-ray spectrometer (WDS)
WDS was the original technique developed to precisely and accurately
determine chemical compositions of micro-volumes (a few cubic microns) of
"thick" specimens, and the instrument used is the electron microprobe. The
- 21 -
key feature of the electron microprobe is a crystal-focusing spectrometer, of
which there are usually 3-5 different diffracting crystals.
The WDS spectrometer can acquire the high count rate of X-rays produced
at high beam currents, because it measures a single wavelength at a time. This
is important for trace element analysis. In practice, it is advantageous to use
the speed of EDS for an initial survey of an unknown sample because major
elements will be rapidly identified. However, if trace elements are present
they will not be identified, and it may be difficult to interpret complex
overlaps which are common in EDS analysis. Following the initial energy
dispersion survey, wavelength dispersion can be used to check for overlaps
and to increase sensitivity for trace elements. A Jeol JXA 8800R electron
probe microanalyzer at the Institute of Geology and Minerals was used in the
present study.
2.3.6 Magnetization hysteresis loops
Magnetization curve provides basic magnetic properties of a magnetic
material. From the curve, one can estimate the saturation magnetization MS,
the coercitivity HC, the magnetic anisotropy K, the magnetization remanence
MR and the exchange bias field HE. Magnetic behavior can also be understood
of microscopic structural properties. In the present study, measurements of
magnetization curves were performed using a DMS 880 VSM system at the
ITIMS. The magnetic field used in the present study was up to 13.5 kOe along
both the parallel and perpendicular directions. For these measurements, the
background resulted from any source such as the sample holder and the
substrate was subtracted. Before each measurement, a standard Ni sample
(with total magnetic moment of 3.799 emu) was always used to calibrate the
system. For the measurement of the hysteresis loops at low temperatures, a
tube attached in the VSM and a thermocouple is placed inside the tube
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together with the heating coil. By evaporating liquid nitrogen and
simultaneously adjusting the current for the heating coil, one can control the
system with the temperature accuracy of about 5 degrees.
2.3.7 Magnetization-temperature curve
Magnetization-temperature curve were carried out by a VSM system
(described in the pre...

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