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Probing Spin Accumulation induced Magnetocapacitance in a Single Electron Transistor.

Lee TH, Chen CD - Sci Rep (2015)

Bottom Line: The latter is known as the magnetocapacitance effect.This dipole can effectively give rise to an additional serial capacitance, which represents an extra charging energy that the tunneling electrons would encounter.It is found that the extra threshold energy is experienced only by electrons entering the islands, bringing about asymmetry in the measured Coulomb diamond.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, National Taiwan University, Taipei 106, Taiwan.

ABSTRACT
The interplay between spin and charge in solids is currently among the most discussed topics in condensed matter physics. Such interplay gives rise to magneto-electric coupling, which in the case of solids was named magneto-electric effect, as predicted by Curie on the basis of symmetry considerations. This effect enables the manipulation of magnetization using electrical field or, conversely, the manipulation of electrical polarization by magnetic field. The latter is known as the magnetocapacitance effect. Here, we show that non-equilibrium spin accumulation can induce tunnel magnetocapacitance through the formation of a tiny charge dipole. This dipole can effectively give rise to an additional serial capacitance, which represents an extra charging energy that the tunneling electrons would encounter. In the sequential tunneling regime, this extra energy can be understood as the energy required for a single spin to flip. A ferromagnetic single-electron-transistor with tunable magnetic configuration is utilized to demonstrate the proposed mechanism. It is found that the extra threshold energy is experienced only by electrons entering the islands, bringing about asymmetry in the measured Coulomb diamond. This asymmetry is an unambiguous evidence of spin accumulation induced tunnel magnetocapacitance, and the measured magnetocapacitance value is as high as 40%.

No MeSH data available.


Fabrication and measurement process.(a,b) schematic drawings of the device in cross-section view and top view. Throughout this paper, we use blue, green, and red to indicate right electrode, center island, and left electrode, respectively. The magnetic field is applied parallel to the long-axis of the two electrodes. The right electrode is made wider to decreases the coercivity. (c) Magnetization curves for right electrode (blue), center island (green), and left electrode (red). Note that the curve for left electrode shows no field-dependence because of the large coercivity. (d) Tunnel magneto-current measured at Vb = 8.7 mV, a bias much greater than the maximum Coulomb blockade threshold voltage of 2EC/e ≈ 1 mV, so that the current is gate-independent and the TMR effect can be clearly observed. The magnetic field is swept (indicated by black arrows along the curve) sequentially from i to iv, with magnetization directions indicated by blue (right electrode), green (center island), and red (left electrode) arrows.
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f1: Fabrication and measurement process.(a,b) schematic drawings of the device in cross-section view and top view. Throughout this paper, we use blue, green, and red to indicate right electrode, center island, and left electrode, respectively. The magnetic field is applied parallel to the long-axis of the two electrodes. The right electrode is made wider to decreases the coercivity. (c) Magnetization curves for right electrode (blue), center island (green), and left electrode (red). Note that the curve for left electrode shows no field-dependence because of the large coercivity. (d) Tunnel magneto-current measured at Vb = 8.7 mV, a bias much greater than the maximum Coulomb blockade threshold voltage of 2EC/e ≈ 1 mV, so that the current is gate-independent and the TMR effect can be clearly observed. The magnetic field is swept (indicated by black arrows along the curve) sequentially from i to iv, with magnetization directions indicated by blue (right electrode), green (center island), and red (left electrode) arrows.

Mentions: To observe this extra energy cost by a single spin flip, a ferromagnetic single-electron-transistor was designed and constructed. However, the spin diffusion length in ferromagnetic materials is generally too short to sustain spin accumulation16. In order to appreciate the magnetocapacitance effect, we used a thin nonmagnetic layer to cover one of the ferromagnetic electrodes. In this way, stable charge dipole is generated inside the nonmagnetic layer. The device, as schematically shown in Fig. 1a, was fabricated using the standard electron-beam lithography technique and two-angle evaporation method. The source and drain electrodes were made of Co, whereas the island was made of 10 nm-thick permalloy (Py, Ni80%WtFe20%Wt), a ferromagnetic material with a smaller coercivity. The Py island was directly covered with a 2 nm-thick aluminum (Al) layer and then a thin tunnel barrier, which was formed by direct evaporation of alumina (Al2O3) crystal without oxidation process. Since the Al layer is much thinner than ferromagnetic Py layer, its superconductivity is suppressed by the proximity effect17181920. As illustrated in Fig. 1b, the device consists of Co/Al2O3/Al/Py/Al/Al2O3/Co with two tunnel junctions in series. The junction area A measured about 65 nm × 65 nm, corresponding to a charging energy of the order of 6 K. In addition, a gate-electrode was located about 800 nm away from the island, giving a Cg value of about 0.4 aF and a Coulomb oscillation period of about 0.5 V. All I-Vb characteristics were measured using 4-point probe technique at 120 mK.


Probing Spin Accumulation induced Magnetocapacitance in a Single Electron Transistor.

Lee TH, Chen CD - Sci Rep (2015)

Fabrication and measurement process.(a,b) schematic drawings of the device in cross-section view and top view. Throughout this paper, we use blue, green, and red to indicate right electrode, center island, and left electrode, respectively. The magnetic field is applied parallel to the long-axis of the two electrodes. The right electrode is made wider to decreases the coercivity. (c) Magnetization curves for right electrode (blue), center island (green), and left electrode (red). Note that the curve for left electrode shows no field-dependence because of the large coercivity. (d) Tunnel magneto-current measured at Vb = 8.7 mV, a bias much greater than the maximum Coulomb blockade threshold voltage of 2EC/e ≈ 1 mV, so that the current is gate-independent and the TMR effect can be clearly observed. The magnetic field is swept (indicated by black arrows along the curve) sequentially from i to iv, with magnetization directions indicated by blue (right electrode), green (center island), and red (left electrode) arrows.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4562261&req=5

f1: Fabrication and measurement process.(a,b) schematic drawings of the device in cross-section view and top view. Throughout this paper, we use blue, green, and red to indicate right electrode, center island, and left electrode, respectively. The magnetic field is applied parallel to the long-axis of the two electrodes. The right electrode is made wider to decreases the coercivity. (c) Magnetization curves for right electrode (blue), center island (green), and left electrode (red). Note that the curve for left electrode shows no field-dependence because of the large coercivity. (d) Tunnel magneto-current measured at Vb = 8.7 mV, a bias much greater than the maximum Coulomb blockade threshold voltage of 2EC/e ≈ 1 mV, so that the current is gate-independent and the TMR effect can be clearly observed. The magnetic field is swept (indicated by black arrows along the curve) sequentially from i to iv, with magnetization directions indicated by blue (right electrode), green (center island), and red (left electrode) arrows.
Mentions: To observe this extra energy cost by a single spin flip, a ferromagnetic single-electron-transistor was designed and constructed. However, the spin diffusion length in ferromagnetic materials is generally too short to sustain spin accumulation16. In order to appreciate the magnetocapacitance effect, we used a thin nonmagnetic layer to cover one of the ferromagnetic electrodes. In this way, stable charge dipole is generated inside the nonmagnetic layer. The device, as schematically shown in Fig. 1a, was fabricated using the standard electron-beam lithography technique and two-angle evaporation method. The source and drain electrodes were made of Co, whereas the island was made of 10 nm-thick permalloy (Py, Ni80%WtFe20%Wt), a ferromagnetic material with a smaller coercivity. The Py island was directly covered with a 2 nm-thick aluminum (Al) layer and then a thin tunnel barrier, which was formed by direct evaporation of alumina (Al2O3) crystal without oxidation process. Since the Al layer is much thinner than ferromagnetic Py layer, its superconductivity is suppressed by the proximity effect17181920. As illustrated in Fig. 1b, the device consists of Co/Al2O3/Al/Py/Al/Al2O3/Co with two tunnel junctions in series. The junction area A measured about 65 nm × 65 nm, corresponding to a charging energy of the order of 6 K. In addition, a gate-electrode was located about 800 nm away from the island, giving a Cg value of about 0.4 aF and a Coulomb oscillation period of about 0.5 V. All I-Vb characteristics were measured using 4-point probe technique at 120 mK.

Bottom Line: The latter is known as the magnetocapacitance effect.This dipole can effectively give rise to an additional serial capacitance, which represents an extra charging energy that the tunneling electrons would encounter.It is found that the extra threshold energy is experienced only by electrons entering the islands, bringing about asymmetry in the measured Coulomb diamond.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, National Taiwan University, Taipei 106, Taiwan.

ABSTRACT
The interplay between spin and charge in solids is currently among the most discussed topics in condensed matter physics. Such interplay gives rise to magneto-electric coupling, which in the case of solids was named magneto-electric effect, as predicted by Curie on the basis of symmetry considerations. This effect enables the manipulation of magnetization using electrical field or, conversely, the manipulation of electrical polarization by magnetic field. The latter is known as the magnetocapacitance effect. Here, we show that non-equilibrium spin accumulation can induce tunnel magnetocapacitance through the formation of a tiny charge dipole. This dipole can effectively give rise to an additional serial capacitance, which represents an extra charging energy that the tunneling electrons would encounter. In the sequential tunneling regime, this extra energy can be understood as the energy required for a single spin to flip. A ferromagnetic single-electron-transistor with tunable magnetic configuration is utilized to demonstrate the proposed mechanism. It is found that the extra threshold energy is experienced only by electrons entering the islands, bringing about asymmetry in the measured Coulomb diamond. This asymmetry is an unambiguous evidence of spin accumulation induced tunnel magnetocapacitance, and the measured magnetocapacitance value is as high as 40%.

No MeSH data available.