Probing Spin Accumulation induced Magnetocapacitance in a Single Electron Transistor

At a Glance

Metadata	Details
Publication Date	2015-09-08
Journal	Scientific Reports
Authors	Teik Hui Lee, Chii Dong Chen, Teik Hui Lee, Chii Dong Chen
Institutions	Institute of Physics, Academia Sinica, Academia Sinica
Citations	14
Analysis	Full AI Review Included

Technical Documentation & Analysis: Spin Accumulation Induced Magnetocapacitance in SETs

This document analyzes the research paper “Probing Spin Accumulation induced Magnetocapacitance in a Single Electron Transistor” to provide technical specifications and align the findings with 6CCVD’s advanced MPCVD diamond material solutions for spintronics and quantum electronics.

Executive Summary

Core Mechanism Demonstrated: The study successfully demonstrated Tunnel Magnetocapacitance (TMC) induced by non-equilibrium spin accumulation in a Ferromagnetic Single-Electron Transistor (SET).
Physical Origin: Spin accumulation/depletion at the anti-parallel (AP) interface forms a tiny charge dipole, which acts as an extra serial capacitance ($C_{s}$).
Device Architecture: The device utilized a double magnetic tunnel junction SET structure: Co/Al${2}$O${3}$/Al/Py/Al/Al${2}$O${3}$/Co, fabricated via electron-beam lithography.
Key Achievement: A high magnetocapacitance value ($\Delta_{TMC}$) of 40% was measured, which is the highest reported among AlO$_{x}$-based magnetic tunnel junctions.
Energy Implication: The extra serial capacitance corresponds to an additional charging energy required for a single spin flip event during sequential tunneling in the AP configuration.
Verification Method: The TMC effect was unambiguously confirmed by observing asymmetry in the measured Coulomb diamond stability diagrams under AP alignment.
Future Relevance: This work provides a clear microscopic mechanism for TMC, crucial for developing next-generation magnetic-read memory and magnetic-field sensors.

Technical Specifications

The following hard data points were extracted from the experimental results and device parameters:

Parameter	Value	Unit	Context
Maximum Magnetocapacitance ($\Delta_{TMC}$)	40	%	Observed in AP configuration, highest for AlO$_{x}$ MTJs
Charging Energy ($E_{c}$)	510	µeV	Determined from All-P Coulomb diamond
Measurement Temperature	120	mK	Cryogenic environment for SET operation
Island Material Thickness	10	nm	Permalloy (Py, Ni${80%wt}$Fe${20%wt}$)
Nonmagnetic Al Layer Thickness	2	nm	Used to cover the Py island
Junction Area ($A$)	65 x 65	nm$^{2}$	Estimated device area
Gate Capacitance ($C_{g}$)	0.4	aF	Calculated value
Coulomb Oscillation Period	0.5	V	Gate voltage period
Left Junction Capacitance ($C_{Left}$)	101.8	aF	Derived from All-P configuration
Right Junction Capacitance ($C_{Right}$)	55.1	aF	Derived from All-P configuration
Magnetic Field Range (Ramping)	±1000	Oe	Used to manipulate magnetization alignment

Key Methodologies

The experiment relied on precise nanoscale fabrication and ultra-low temperature electrical characterization:

Fabrication Technique: The device was constructed using standard electron-beam lithography and a two-angle evaporation method.
Material Deposition: Source and drain electrodes were made of Cobalt (Co). The center island was 10 nm thick Permalloy (Py).
Tunnel Junction Formation: The Py island was covered sequentially by a 2 nm thick Aluminum (Al) layer and a thin Alumina (Al${2}$O${3}$) tunnel barrier, formed by direct evaporation without oxidation.
Device Structure: The final device configuration was a double tunnel junction SET: Co/Al${2}$O${3}$/Al/Py/Al/Al${2}$O${3}$/Co.
Electrical Bias: The device was symmetrically voltage biased at +$V_{b}$/2 and -$V_{b}$/2.
Magnetic Control: The magnetic field ($H$) was applied parallel to the long axis of the Co electrodes, allowing the island and electrodes to be aligned in Parallel (P) or Anti-Parallel (AP) configurations.
Capacitance Extraction: Junction capacitances were accurately determined by analyzing the slopes of the Coulomb diamond stability diagrams in various magnetic configurations (All-P, Left-AP, Right-AP, Both-AP).

6CCVD Solutions & Capabilities

This research demonstrates the critical role of material interfaces and nanoscale geometry in spintronic devices. While the paper uses conventional metal/oxide stacks, 6CCVD offers advanced MPCVD diamond materials that provide superior platforms for replicating or extending this research, particularly for applications requiring extreme stability, high thermal management, or integration with quantum defects.

Applicable Materials

To replicate or extend this research into high-performance or quantum spintronic applications, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Ideal as a substrate for high-purity, low-defect density spintronic devices. Its atomically smooth surface (Ra < 1 nm) is critical for subsequent e-beam lithography and thin film deposition (Co, Py, Al${2}$O${3}$), ensuring optimal interface quality for the tunnel barrier.
Heavy Boron-Doped Diamond (BDD): Highly recommended for creating conductive electrodes or islands. BDD offers tunable metallic conductivity and superior chemical stability compared to conventional metals, enabling the creation of robust, non-oxidizing ferromagnetic/non-magnetic interfaces.
Polycrystalline Diamond (PCD) Wafers: Available in large formats (up to 125 mm) for high-throughput research or industrial scaling of TMC-based memory devices.

Customization Potential

The fabrication of the SET device required precise control over material thickness and geometry (65 nm x 65 nm junction area). 6CCVD’s capabilities directly address these requirements:

Paper Requirement	6CCVD Customization Service	Relevance to TMC Research
Precise Thin Film Stacks (Co, Py, Al, Al${2}$O${3}$)	Custom Metalization: We offer in-house deposition of Au, Pt, Pd, Ti, W, and Cu stacks.	Allows researchers to integrate custom ferromagnetic layers or contact pads directly onto the diamond substrate, streamlining device fabrication.
Need for Ultra-Smooth Interfaces	Advanced Polishing: SCD surfaces polished to Ra < 1 nm; inch-size PCD polished to Ra < 5 nm.	Minimizes interface roughness, which is crucial for maintaining the integrity of the thin (2 nm) Al layer and the Al${2}$O${3}$ tunnel barrier, directly impacting spin diffusion and TMC magnitude.
Custom Electrode Geometry	Precision Laser Cutting: Custom dimensions and shapes for plates/wafers up to 125 mm.	Supports the creation of specific electrode geometries necessary for controlling shape anisotropy and achieving stable magnetic configurations (P and AP alignment) as demonstrated in the paper.
SCD/PCD Thickness Control	Thickness Range: SCD (0.1 µm - 500 µm), PCD (0.1 µm - 500 µm), Substrates (up to 10 mm).	Provides flexibility for integrating diamond layers as active components (e.g., BDD electrodes) or as robust, thick substrates for cryogenic measurements (120 mK).

Engineering Support

The successful observation of TMC relies on complex material science, precise interface engineering, and cryogenic measurement expertise. 6CCVD’s in-house PhD team specializes in the growth and characterization of diamond for advanced electronic and quantum applications. We can assist researchers with material selection, doping profiles (for BDD), and surface preparation protocols for similar Spintronic Single-Electron Transistor (SET) projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

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%.

Tech Support

Original Source

DOI: https://doi.org/10.1038/srep13704