2-1 Ferroelectric and Piezoelectric materials 2-1-1 basic properity
(a) Polarization mechanism [4]
The polarization is using applied electric field to arrange messy dipole moment to be ordered. Polarization mechanisms had four kinds of, there are atomic polarization, ionic polarization, dipolar polarization and space charge polarization, as shown in Fig. 1[4].
(1) atomic polarization
Applying electric field on materials to make charged particles within atoms offset relatively, such as atomic nucleus and electron. Because this mechanism is related to electron, it also is called electronic polarization.
(2) ionic polarization
The ionic polarization appeared in crystals of having ionic bonding. Because cations and anions have certain bonding rules, cations and anions offset relatively to form electric dipole moments. The ion with charges are more than atom with charges so ionic polarization is stronger than atomic polarization.
(3) dipolar polarization
The materials of permanent electric dipole moments generated easily this polarization. These materials have messy Arrange electric dipole moments. When applying electric field, electric dipole moments reversed toward the electric field direction to arrange orderly. But intensities of electric dipole moments didn’t be affected by applied electric field.
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(4) space charge polarization
When materials themselves had defects or different crystal interfaces, they would obstruct charge carriers to move. These charge carriers were bound to the different grains and generate space polarization phenomena because grain boundary and phase boundary had bounding charge carriers characteristics. Therefore it generated these polarization phenomena easily.
Fig. 1 Various polarization [4]
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(b) Ferroelectricity
The nature of the materials could be sort to 32 kinds of point groups at present.
They had 11 kinds of that didn’t have offset of anions and cations so they had centrosymmetric structure and didn’t have polarity. There were 21 kinds of non-centrosymmetric point groups. There were 20 kinds of piezoelectricity. There were 10 kinds of piezoelectricity which had characteristics of non-central symmetry and a single axis of rotation so they had spontaneous polarization phenomenon. The 32 kinds of point groups were showed in Fig. 2 .
If materials had spontaneous polarization characteristics and direction and polarization of dipole moment would change with applied electric field, it could be called ferroelectric materials. When ambient temperature increased, the spontaneous polarization of ferroelectric materials would decrease. While spontaneous polarization was zero, we called this temperature to be Curie temperature.
The ferroelectric materials had ferroelectricity, besides pyroelectricity and piezoelectricity. When ambient temperature changed, the spontaneous polarization of materials would also change which was called pyroelectricity. When materials were strained by mechanical stress, the spontaneous polarization of materials would also change which was called piezoelectricity. The relationship of them was showed in Fig.
3.
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Fig. 2 32 kinds of point group s
Fig. 3 Relationship of piezoelectricity, pyroelectricity and Ferroelectricity
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(c) Piezoelectricity
In 1880, Pierre Curie and Jacques Curie [5] discovered that some crystals were strained by mechanical stress to generate charges. This phenomenon they called piezoelectric effect. Then, they also discovered that crystals were put in an electrical field and it would strain. This reverse mechanism they called inverse piezoelectric effect. In other words, these effects could transform between mechanical energy and electrical energy.
(1) Piezoelectric effect：
The piezoelectric effect is property of transforming mechanical energy into electrical energy. When applying mechanical stress on piezoelectric materials, the spontaneous dipole moment of materials would strain by external force. In order to resist force, surface of materials would produce bound charges to keep shape of materials.
Fig. 4 piezoelectric effect
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(2) Inverse piezoelectric effect：
The inverse piezoelectric effect is property of transforming electrical energy into mechanical energy. If we applied a electric field in the same direction with bound charges, the polarization of internal electric dipole would be strong. Then piezoelectric materials would elongate. Conversely, if we applied a electric field in the reverse direction with bound charges, the polarization of internal electric dipole would be reduced. Then, piezoelectric materials would shorten.
The deformation of piezoelectric materials would change with direction and intensity of applied electric field, this effect we called inverse piezoelectric effect.
Fig. 5 inverse piezoelectric effec
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2-1-2 Piezoelectric properties of ZnO
In 2010, I. K. Bdikin et al. [6] prepared Zn and ZnO film on glass substrate by using pulsed-laser deposition, and Zn as an electrode. It formed a film structure of ZnO/Zn/glass. Author measured piezoresponse images, piezoelectric coefficients and piezoresponse hysteresis loop by using PFM measurements (Fig. 9)。
Fig. 6 shows the representative x-ray diffraction pattern of a ZnO thin film, where only two strong peaks are observed in the 2θ range between 30∘ and 40∘.
The strongest peak consists of two superimposed peaks observed at 2θ=36.322∘
and 36.762∘. They can be attributed to the (0002) plane of metallic Zn and the (1011) plane of hexagonal ZnO, respectively. The second one, less intense, is observed at 2θ=34.38∘and can be attributed to the (0002) plane of the hexagonal ZnO.
Fig. 6 X-ray diffraction pattern of ZnO/Zn/glass [6]
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Fig. 7 (a) Topography (b) OOP (c)(d) IP piezoresponse images (e) Distribution of ‘positive’ and
‘negative’ ZnO grains reconstructed by comparison of OOP and IP signals (f) Cross sections of IP and OPP images.[6]
Then, author measured longitudinal and transverse PFM, he obtained three-dimensional piezoresponse images (Fig. 7) and Summed up distribution of different ZnO grains reconstructed. In order to obtaine piezoelectric coefficients, author used known piezoelectric coefficient calibration of LiNbO3 single crystals.[7]
And he calculated for each piezoelectric coefficient of lattice planes. Fig. 8。
Fig. 8 table of ZnO piezoelectric coefficient [6]
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Fig. 9 ZnO piezoresponse hysteresis loop [6]
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2-2 Permalloy and Magnetic materials 2-2-1 Magnetic materials
The materials could be magnetized in magnetic field (H), we called magnetic materials. We usually described materials of magnetized level as magnetization intensity. We also called magnetization (M), it was defined as the magnetic moment of material per unit volume. The relationship between applied magnetic field and magnetization were showed as magnetic susceptibility, χ=M/H. If magnetization was higher, it meant that materials would be magnetized easily. The source of magnetism was caused by atom within and electron motion.
We could sort materials by magnetic susceptibility as paramagnetism, diamagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism.
(a) paramagnetism
In a paramagnetic material there are unpaired electrons, i.e. atomic or molecular orbital with exactly one electron in them. While paired electrons are required by the Pauli Exclusion Principle to have their intrinsic “spin” magnetic moments pointing in opposite directions, causing their magnetic fields to cancel out, an unpaired electron is free to align its magnetic moment in any direction. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, thus reinforcing it.
(b) diamagnetism
Diamagnetism appears in all materials, and is the tendency of a material to oppose an applied magnetic field, and therefore, to be repelled by a magnetic field. However, in a material with paramagnetic properties (that is, with a tendency to enhance an external magnetic field), the paramagnetic behavior dominates. Thus, despite its universal occurrence, diamagnetic behavior is observed only in a purely diamagnetic
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material. In a diamagnetic material, there are no unpaired electrons, so the intrinsic electron magnetic moments cannot produce any bulk effect. In these cases, the magnetization arises from the electrons' orbital motions.
(c) ferromagnetism
A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons' intrinsic magnetic moment's tendency to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered-energy state. Thus, even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to one another.
Every ferromagnetic substance has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties.
This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order.
Ferromagnetism only occurs in a few substances; the common ones are iron, nickel, cobalt, their alloys, and some alloys of rare earth metals.
(c) antiferromagnetism
In an antiferromagnet, unlike a ferromagnet, there is a tendency for the intrinsic magnetic moments of neighboring valence electrons to point in opposite directions.
When all atoms are arranged in a substance so that each neighbor is 'anti-aligned', the substance is antiferromagnetic. Antiferromagnets have a zero net magnetic moment, meaning no field is produced by them. Antiferromagnets are less common compared to the other types of behaviors, and are mostly observed at low temperatures. In varying temperatures, antiferromagnets can be seen to exhibit diamagnetic and ferrimagnetic properties.
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In some materials, neighboring electrons want to point in opposite directions, but there is no geometrical arrangement in which each pair of neighbors is anti-aligned.
This is called a spin glass, and is an example of geometrical frustration.
(d) ferrimagnetism
Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins like to point in opposite directions. These two properties are not contradictory, because in the optimal geometrical arrangement, there is more magnetic moment from the sublattice of electrons that point in one direction, than from the sublattice that point in the opposite direction.
Most ferrites are ferrimagnetic. The first discovered magnetic substance, magnetite, is a ferrite and was originally believed to be a ferromagnet; Louis Neel disproved this, however, after discovering ferrimagnetism.
2-2-2 Basic properties of permalloy
In this study, we chose Ni80Fe20 as ferromagnetic materials, it belongs to one kind of permalloy and magnetic permeability is higher. Permalloy is 35~82%
ferronickel alloy. To control ferronickel component proportion and we could change anisotropy constant ( K1) and magnetostriction constant (λs) , as shown in Fig. 10[8]. The crystal structure was showed in Fig. 11.
Most of the proportion is fcc structure. We usually could sort permalloy to three kinds of nickel content, 50%, 65% and 78% [9].
(1) The alloy of 50% nickel, it would have stronger magnetic flux density ( Bs=1.6T ) and square loop after annealing.
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(2) The alloy of 65% nickel, it would make magnetic anisotropy constant be zero after annealing.
(3) The alloy of 78% nicke, its magnetostriction constant was zero and it had good magnetic permeability.
Fig. 10 Relationship of FCC structure permalloy magnetostriction constant λs at RT [8]
Fig. 11 phase diagram of permalloy magnetostriction constant λs[8]
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Usually affecting the factor of magnetic anisotropy constant are lattice structure, mechanical force and heat treatment. In order to reduce affection, we could know when nickel content is 45%～95%, the Curie temperature could reach more than 400
℃ with observing relationship of anisotropy constant, Curie temperature, magnetostriction constant and nickel content in Fig. 12 . About 75%, magnetocrystalline anisotropy constant (Ku) is zero ; about 35%～45% and 80%, magnetostriction constant is zero so we obtain ideal permalloy in 80% nickel content.
Because magnetostriction constant is zero and anisotropy constant is low, it has good coercivity and magnetic properties.
Fig. 12 Relationship of saturated magnetization, curie temperature, magnetocrystalline anisotropy constant, magnetostriction constant and nickel content [8]
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2-3 Measured MOKE by applying voltage
In 2007, Sarbeswar Sahoo et al. [11] prepared 10nm thick Fe film on BaTiO3(100) substrate using molecular beam epitaxy. Then, author applied a electric field at 0.5mm thick BTO substrate and measured MOKE at Fe film. The electric field was changed from E=−10 kV/cm up to E=10 kV/cm in ascending steps and then descending back to E=−10 kV/cm along the arrows. We could observe coercivity Hc changing along with electric field.（Fig. 13）。
Fig. 13 Normalized Kerr magnetic loops at room temperature measured at different applied electric fields[11]
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In 2014, Wen-Chin Lin et al.[10] prepared multilayer structures of Au/Fe/ZnO/Au on Al2O3(0001) substrates to detect how voltage affected magnetism. The top and bottom Au layers were used as the electrodes when applying the voltage. The Au over-layer protects the Fe film from oxidation and contamination. The MOKE measurement was executed at RT using a magnetic field along the in-plane direction, while the applied bias voltage was gradually increased from 0 to 6 V. As shown in Fig.
14, the magnetic coercivity (Hc) of the MOKE hysteresis loops decreased as the bias voltage increased. The Hc exhibited the same variation, whether the applied voltage was positive or negative, indicating that the Hc reduction is not related to the electric field direction. And it had a reversible behavior.
Fig. 14 (a) MOKE hysteresis loops measured at RT, and various bias voltages.(b) Summarized Hc
values plotted as a function of bias voltage. The solid lines are guides for the eye.[10]
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The Hc of Fe/ZnO heterostructure was significantly enhanced by 2–3 times after applying a suitable direct heating current. This Hc enhancement is irreversible and originates from the Fe-oxidation at the Fe/ZnO interface induced by direct current heating while the bias voltage is applied. Depth-profiling XPS analysis confirmed the formation of FeO, Fe3O4, and Fe2O3 close to the interface region, depending on the Fe thickness and annealing process. To investigate whether interface conditions changed how voltage reversibly affected on Hc, author applied large voltages of 10 and 12V to generate more Fe-oxide at the interface. Fig. 15 shows that after applying 10V for 10 min, the Hc value irreversibly increased from 112Oe to 147Oe (measured when V=0).
After the 10 V-annealing, not only the Hc was enhanced but also the resistance of Fe/ZnO junction was decreased.
Fig. 15 The summarized Hc values were plotted as a function of bias voltage for as-deposited sample and the samples after applying 8V, 10V, and 12V to the Fe/ZnO junction for 10 min. The solid lines are
guides for the eye.[10]
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