https://www2.econ.iastate.edu/tesfatsi/DeepLearningInNeuralNetworksOverview.JSchmidhuber2015.pdf

Deep learning in neural networks

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically rich spin textures such as spin spirals and skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin texture enables extremely efficient current-driven motion. We show that spin torque from the spin Hall effect drives DWs in opposite directions in Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a Neel configuration with left-handed chirality. We directly confirm the DW chirality and rigidity by examining current-driven DW dynamics with magnetic fields applied perpendicular and parallel to the spin spiral. This work resolves the origin of controversial experimental results and highlights a new path towards interfacial design of spintronic devices.

/pdf/current-driven-dynamics-of-chiral-ferromagnetic-domain-walls-1g9i94jrpa.pdf

Current-driven dynamics of chiral ferromagnetic domain walls

Magnetic skyrmions are topologically stable spin configurations, which usually originate from chiral interactions known as Dzyaloshinskii-Moriya interactions. Skyrmion lattices were initially observed in bulk non-centrosymmetric crystals, but have more recently been noted in ultrathin films, where their existence is explained by interfacial Dzyaloshinskii-Moriya interactions induced by the proximity to an adjacent layer with strong spin-orbit coupling. Skyrmions are promising candidates as information carriers for future information-processing devices due to their small size (down to a few nanometres) and to the very small current densities needed to displace skyrmion lattices. However, any practical application will probably require the creation, manipulation and detection of isolated skyrmions in magnetic thin-film nanostructures. Here, we demonstrate by numerical investigations that an isolated skyrmion can be a stable configuration in a nanostructure, can be locally nucleated by injection of spin-polarized current, and can be displaced by current-induced spin torques, even in the presence of large defects.

Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures

Modern applications for thin film magnets involve unique requirements for the control and design of specific magnetic properties. The exchange bias effect in ferromagnet/antiferromagnet bilayers appears to be a useful feature for controlling one of the most important characteristics of a ferromagnet: coercivity. Prospects for control and enhancement of desirable effects depend upon a clear understanding of mechanisms governing exchange bias. The processes underlying the existence and properties of exchange bias are reviewed, with particular emphasis on the roles of interface structure and temperature. Results from numerical simulations are used to illustrate how exchange bias is modified by geometric structures at the interface and randomly placed defects. A general theoretical formulation of the bias problem is proposed, and an expression for the interface energy is derived. A key result is the existence of higher-order coupling terms when more than one sublattice of the antiferromagnet is present at the interface. Results from calculations of finite temperature effects on bias and coercivity are described, and the concept of viscosity in the antiferromagnet is discussed. A brief discussion is also included of how a dynamic linear response, such as ferromagnetic resonance or light scattering, can be used to determine relevant anisotropy and exchange parameters.

https://iopscience.iop.org/article/10.1088/0022-3727/33/23/201/pdf

Mechanisms for exchange bias

We demonstrate the functionality of spin-wave logic exclusive-not-OR and not-AND gates based on a Mach-Zehnder-type interferometer which has arms implemented as sections of ferrite film spin-wave waveguides. Logical input signals are applied to the gates by varying either the phase or the amplitude of the spin waves in the interferometer arms. This phase or amplitude variation is produced by Oersted fields of dc current pulses through conductors placed on the surface of the magnetic films.

/pdf/realization-of-spin-wave-logic-gates-2un8lcdrge.pdf

Realization of spin-wave logic gates

We report on magnetic domain-wall velocity measurements in ultrathin $\mathrm{Pt}/\mathrm{Co}(0.5--0.8\text{ }\text{ }\mathrm{nm})/\mathrm{Pt}$ films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, $\ensuremath{\alpha}$.

/pdf/creep-and-flow-regimes-of-magnetic-domain-wall-motion-in-1etbzj6uo4.pdf

Creep and Flow Regimes of Magnetic Domain-Wall Motion in Ultrathin Pt / Co / Pt Films with Perpendicular Anisotropy

We demonstrate the functionality of spin-wave logic XNOR and NAND gates based on a Mach-Zehnder type interferometer which has arms implemented as sections of ferrite film spin-wave waveguides. Logical input signals are applied to the gates by varying either the phase or the amplitude of the spin waves in the interferometer arms. This phase or amplitude variation is produced by Oersted fields of dc current pulses through conductors placed on the surface of the magnetic films.

/pdf/realization-of-xnor-and-nand-spin-wave-logic-gates-3y7fwwd32x.pdf

Realization of XNOR and NAND spin-wave logic gates

Exchange bias is commonly manifested as the hysteresis-loop shift observed when a ferromagnet is in contact with an antiferromagnet. Here, we report observations of exchange bias with unusual features of a ferromagnet in contact with a spin glass, demonstrating that this is a phenomenon of greater generality. The easily measured properties of the ferromagnet allow access to the internal magnetic degrees of freedom of the glass to which they are coupled. Our results show that a Co/CuMn bilayer system exhibits all the rich phenomena of coercivity enhancement, bias-field shifts and training effects associated with a conventional ferromagnet/antiferromagnet system. Nevertheless, striking differences arise, such as an orientation reversal of the bias field in a small temperature range just below the blocking temperature. We argue that all features can be understood within the context of a random-field model for long-ranged oscillatory Ruderman–Kittel–Kasuya–Yosida (RKKY) coupled spins.

Robert Stamps

Papers

Mechanisms for exchange bias

Realization of spin-wave logic gates

Creep and Flow Regimes of Magnetic Domain-Wall Motion in Ultrathin Pt / Co / Pt Films with Perpendicular Anisotropy

Realization of XNOR and NAND spin-wave logic gates

Exchange bias using a spin glass