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Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (a-) rare earth-transition metal (a-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic a-Gd22-xTbxCo78 thin films, from x=0 to 18, and elucidate the role of Gd in HI-AOS in a-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with L=0) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing a-Gd10Tb12Co78 leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that accounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element-specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS.

In this paper, we propose helicity-dependent switching (HDS) of magnetization in Co/Pt for an energy efficient optical receiver. Designing a low-power optical receiver for optical-to-electrical signal conversion has proven to be very challenging. Current day optical receivers use a photodiode that produces a photocurrent in response to input optical signals, and power hungry transimpedance amplifiers are required to amplify the small photocurrents. These limitations can be overcome by using light helicity-induced switching of magnetization which can avoid the requirement of photodiodes and subsequent transimpedance amplification by sensing the change in magnetization with a magnetic tunnel junction (MTJ). Magnetization switching of a thin ferromagnet layer using circularly polarized laser pulses has recently been demonstrated which shows a one-to-one correspondence between light helicity and the magnetization state. We use these phenomena to directly switch the magnetization state of a thin Co/Pt ferromagnet layer at the receiver via circularly polarized laser pulses. The circular polarization is controlled in accordance with digital input data which establishes a one-to-one correspondence between the transmitted data and output magnetization state. The Co/Pt layer is used as the free layer of an MTJ, the resistance of which is modified by the laser pulses. Since the output magnetization state is controlled by the input data, the MTJ resistance is directly converted to a digital output signal. Our device-to-circuit level simulation results indicate that HDS-based optical receiver circuit consumes only 0.124 pJ/bit energy, which is much lower than existing techniques. [ABSTRACT FROM AUTHOR]

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown—the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal. [ABSTRACT FROM AUTHOR]

Synthetic ferrimagnets are composite magnetic structures formed from two or more anti-ferromagnetically coupled magnetic sublattices with different magnetic moments. Here, we report on atomistic spin simulations of the laser-induced magnetization dynamics on such synthetic ferrimagnets and demonstrate that the application of ultrashort laser pulses leads to sub-picosecond magnetization dynamics and all-optical switching in a similar manner as in ferrimagnetic alloys. Moreover, we present the essential material properties for successful laser-induced switching, demonstrating the feasibility of using a synthetic ferrimagnet as a high density magnetic storage element without the need of a write field. [ABSTRACT FROM AUTHOR]

We present a computational model of crystallographically amorphous ferrimagnetic alloys using a stochastic Landau-Lifshitz-Gilbert equation of motion for atomistic spins and an atomistic spin Hamiltonian with Heisenberg exchange. The spontaneous equilibrium magnetization is calculated and a comparison with a mean field model is made. The simulations show excellent agreement with experiments on GdFeCo using x-ray magnetic circular dichroism to determine the individual sublattice magnetizations. The calculated temperature dependence of the magnetization shows a polarization of the Gd sublattice leading to a common Curie temperature, in agreement with the experimental data. The intersublattice exchange is shown to be an important energy transfer channel for ultrafast dynamics. [ABSTRACT FROM AUTHOR]

Optical interconnect has emerged as the front-runner to replace electrical interconnect especially for off-chip communication. However, a major drawback with optical interconnects is the need for photodetectors and amplifiers at the receiver, implemented usually by direct bandgap semiconductors and analog CMOS circuits, leading to large energy consumption and slow operating time. In this letter, we propose a new optical interconnect architecture that uses a magnetic tunnel junction (MTJ) at the receiver side that is switched by femtosecond laser pulses. The state of the MTJ can be sensed using simple digital CMOS latches, resulting in significant improvement in energy consumption. Moreover, magnetization in the MTJ can be switched on the picoseconds time-scale and our design can operate at a speed of 5 Gb/s for a single link. [ABSTRACT FROM AUTHOR]

FeRh and its first-order phase transition can open new routes for magnetic hybrid materials and devices under the assumption that it can be exploited in ultra-thin-film structures. Motivated by experimental measurements showing an unexpected increase in the phase transition temperature with decreasing thickness of FeRh on top of MgO, we develop a computational model to investigate strain effects of FeRh in such magnetic structures. Our theoretical results show that the presence of the MgO interface results in a strain that changes the magnetic configuration which drives the anomalous behavior. [ABSTRACT FROM AUTHOR]

Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation mechanisms and with the latter strongly dependent on the interactions between the nanostructures. In the present work we investigate laser induced magnetization dynamics in a broadband optical resonance type experiment revealing the role of interactions between nanostructures on the relaxation processes of granular magnetic structures. The results are corroborated by constructing a temperature dependent numerical micromagnetic model of magnetization dynamics based on the Landau-Lifshitz-Bloch equation. The model predicts a strong dependence of damping on the key material properties of coupled granular nanostructures in good agreement with the experimental data. We show that the intergranular, magnetostatic and exchange interactions provide a large extrinsic contribution to the damping. Finally we show that the mechanism can be attributed to an increase in spin-wave degeneracy with the ferromagnetic resonance mode as revealed by semianalytical spin-wave calculations. [ABSTRACT FROM AUTHOR]

The article presents a analysis of the thermoelectric response from a single trapped domain wall with the use of a heated scanning probe. Topics discussed include the importance of resolving the nanoscale magnetization in addressing the underconstrained problem of determining the spin texture, symmetry of magnetothermoelectric effects, and the key features of the scanning thermoelectric microscopy technique.

The complex magnetic properties of Fe/Ir/Fe sandwiches are studied using a hierarchical multi-scale model. The approach uses first principles calculations and thermodynamic models to reveal the equilibrium spinwave, magnetization and dynamic demagnetisation properties. Finite temperature calculations show a complex spinwave dispersion and an initially counter-intuitive, increasing exchange stiffness with temperature (a key quantity for device applications) due to the effects of frustration at the interface, which then decreases due to magnon softening. Finally, the demagnetisation process in these structures is shown to be much slower at the interface as compared with the bulk, a key insight to interpret ultrafast laser-induced demagnetization processes in layered or interface materials.