The search for new non-volatile memory technologies is a very intensive research area today. A promising candidate is magnetic RAM (MRAM), and several academic groups and electronics companies are using Atomistix ToolKit (ATK) from QuantumWise for different stages in the analysis of new materials for MRAM structures. ATK is a very popular tool within the entire general area of spintronics, for studies of various novel applications and devices where the information is carried not by the electron charge, but by its spin.
The QuantumWise software platform, with the first-principles engine Atomistix ToolKit (ATK) and the graphical user interface Virtual NanoLab (VNL) allows researchers to focus on the relevant points for their projects, namely the investigation of the electrical and spin properties of novel device structures, rather than spending time writing their own quantum-mechanical codes, or struggling with data import/export to visualize the results.
The unique feature of ATK to calculate ballistic tunneling current in nanostructures enables users to
- calculate the tunnel magnetoresistance (TMR) of magnetic tunnel junctions (MTJ) like Fe/MgO/Fe;
- compute the spin-dependent current/voltage characteristics when a finite bias is applied to the system;
- calculate (collinear) spin-torque transfer (STT) and intralayer exchange coupling;
- investigate the details of the spin transport mechanisms (such as barrier tunneling vs. resonant tunneling), e.g. by analyzing the k-point dependence of the transmission coefficients or scattering eigenchannels.
The calculations in ATK are based on first-principles methods, thus the software can be used to investigate novel materials, since there is no requirement for empirical input to the computational model. MRAM structures and spin filters/valves are today investigated in structures made of for instance
ATK is in general a very convenient tool for studies of basic properties of spin-dependent transport phenomena in a variety of systems, such as
- graphene and nanotubes,
- molecular spintronics,
- metallic nanowires, interfaces, and point contacts,
- etc!
Moreover, a local basis set expansion of the electron density is used, and ATK is therefore a more efficient and accurate tool for modeling local defects such as impurities or vacancies than plane wave codes. The algorithms used in ATK are also optimized to allow for simulations of large-scale systems, with up to 500-1,000 atoms.
Tools
The facilitate the study of MTJs, there is a dedicated setup tool in Virtual NanoLab, the Magnetic Tunnel Junction Builder.
- Electrodes can be of any FeCo type material (bcc [100] surfaces)
- Detailed control over layer separations in the dielectric region
- Support for buckling (shift of oxygen interface atoms)
- Fine-tune or introduce defects in other tools in Virtual NanoLab
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Tutorials
If you are interested in studying spin-depdendent systems with ATK, an excellent starting point will be the tutorials below, which cover both MRAM systems and a spin-application in graphene. Using the graphical user interface makes it efficient and easy to set up even advanced geometries and tuning the parameters to the quantum-chemical model, without the need for learning all the NanoLanguage syntax.
For other tutorials, including general ones on basic concepts in ATK etc, see the Tutorials page.
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Title & Abstract
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Resources |
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FeMgO Magnetic Tunnel Junction
Although the focus of this tutorial is to show a variety of techniques for solving convergence problems, the example system used is an FeMgO magnetic tunnel junction with anti-parallel spin-polarized electrodes, and it shows in detail how to set up and calculate such a system with ATK. All steps are explained in detail using VNL, making use of the dedicated magnetic tunnel junction builder tool which makes the geometrical setup very easy.
Download tutorial (PDF)
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femgofe_para.py
femgofe_antipara.py
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Spin Bloch States in Graphene Nanoribbons
Depending on the edge shape, graphene nanoribbons have metallic or semiconducting characteristics, but spin also plays an important role. We will use the capabilities of ATK to study the spin-dependent band structure of a zigzag ribbon. By plotting conduction and valence band Bloch states, we will see how the two spin-components are localized on opposite sides of the ribbon. We will also consider the spin polarization of the electron density.
Download tutorial (PDF)
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graphene_ribbon.py
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Examples and references
Below we illustrate some of the spintronics applications that users of Atomistix ToolKit have studied with the software. To view all references to papers studied with ATK, see the publication list.
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TMR studies of Fe/MgO/Fe
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M. Stilling, K. Stokbro & K. Flensberg, J. Comp.-Aid. Mater. Design 14, 141 (2007) and Mol. Sim. 33, 557 (2007)
Two studies of Fe/MgO/Fe MTJ structures. Among other things the influence of random perturbations to the atomic coordinates perpendicular to the Fe/MgO interface is found to lead to significant differences in the conductance. The authors also show how the conductance is lowered substantially if the Fe interfaces are oxidized (as also noticed in experiments), and discuss how inclusion of Au in the interface might reduce this problem.
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Spin-polarized transport in graphene flakes
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H. Sahin & R.T. Senger, Physical Review B 78, 205423 (2008)
Study of structural, electronic, and transport properties of hydrogen-terminated short graphene nanoribbons (graphene flakes) and their functionalization with vanadium atoms. A spin-polarized current can be produced by exploiting the spatially separated edge states using asymmetric nonmagnetic contacts. Functionalization of the graphene flake with magnetic adatoms such as vanadium also leads to spin-polarized currents even with symmetric contacts. |
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Spin-polarized transport in graphene nanoribbons with impurities
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J. Park et al., J. Chem. Phys. 130, 214103 (2009)
Investigation of the electronic transport properties and fundamental mechanism of spin polarization as a function of the location of impurities from the center to an edge of a graphene nanoribbon device with zigzag edges. The difference between center-located and edge-located impurities is discussed. For center-located impurities, the ferromagnetic ground state induces new spin states near the Fermi level which are responsible for the spin-polarized current. |
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Spin-polarized transport in a CrAs/AlAs heterojunction
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Y. Min et al., Journal of Magnetism and Magnetic Materials 321, 312 (2009)
Calculations of spin-dependent quantum transport in a CrAs(001)/AlAs(001) heterogeneous junction predicts a strong diode effect of charge and spin current. The minority spin current is inhibited when a bias voltage is applied to the terminals of both CrAs and AlAs. The majority spin current is inhibited when the bias voltage is applied to the terminal of CrAs and relaxed when the bias voltage is applied to the terminal of AlAs. A charge and spin current diode is interesting for reprogrammable logic applications in the field of spintronics.
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Ni point contact
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K. Sekiguchi et al., PRB 78, 224418 (2008)
Ab initio calculations on an atomic wire model demonstrate that the magnetic point contact is comprised of an abrupt change in magnetic moments at the contact region, drastically modifying only the flow of a spin-down current. The calculations reproduce nonlinear features observed experimentally, and the results therefore offer a method to analyze the spin transport in a magnetic point contact without a magnetic-field application, which can minimize the ambiguity in the origin of ballistic magnetoresistance. |
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Iron-cyclopentadienyl sandwich molecular wires
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L. Zhou et al., J. Am. Chem. Soc. 130, 4023 (2008)
A study on a series of novel organometallic sandwich molecular wires (SMWs), constructed with alternating iron atoms and cyclopentadienyl (Cp) rings. The wires are found to be stable and flexible, having half-metallic properties with 100% spin polarization near the Fermi level. Some wires show a nearly perfect spin filter effect when coupled between ferromagnetic electrodes. Moreover, their I-V curves exhibit negative differential resistance (NDR). The SMWs are the first half-magnetic linear molecules with showing a high spin filter effect and NDR and can be easily synthesized, suggesting that the SMWs are promising materials for application in molecular electronics. |
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(CpFeCpV)n multidecker wires
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Jian-Chun Wu et al., J. Phys. Chem. C 113, 7913 (2009)
Spin transport through (CpFeCpV)n multidecker wire sandwiched between magnetic Ni electrodes is simulated in the linear response regime. The amplitude and the sign of the spin filter efficiency can be manipulated by choosing the contact condition (e.g., anchoring groups, absorbing positions on Ni electrodes surface). The performance of the spin filter can be further manipulated by adjusting the length of the molecule wire. Various ways to realize nearly perfect spin-filter are illustrated.
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In addition, there are several spin-applications in carbon and B-N nanotubes.
   
Additional case stories
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CNT-Fe-CNT junction
N. Inoue and S. Usui, Cybernet Systems, presented at the Japanese Physical Society meeting in March 2009
Spin-filter effect in a junction between two capped (5,5) carbon nanotubes, coupled with a Fe atom. Computations of transmission and PDOS is shown, and the basic mechanisms of the spin-dependent transmission are analyzed. In addition, the influence of an applied gate voltage is presented.
Download as PDF
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Applications 
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