The type of magnetic coupling between the atoms of a solid, i.e. whether the material is a ferromagnet or anti-ferromagnet, is one of the most important properties of any magnet. It is normally governed by the exchange interaction, associated with a characteristic time scale at which spin-flip scattering processes occur that is usually of the order a few hundred femtoseconds. Recent technological advances in laser pulse design have allowed researchers to manipulate by laser light magnetic structure at these ultra-short time scales, and have unveiled rich underlying mechanisms that could contribute towards femtosecond control of spin structure. Such advances in the control of magnetic structure by laser light hold out the promise of a revolution in magnetic storage devices.
Motivated by this, a team of researchers led by Dr. Sangeeta Sharma from the Max Planck Institute for Microstructure Physics in Germany investigated multi-sub-lattice magnetic materials, demonstrating an unprecedented control over spin structure is possible at the ultra-short times scales governed by purely electronic processes. They uncovered a rich phenomena of ultra-short time spin manipulation, including even changing the magnetic order of a material from antiferromagnetic to ferromagnetic on femtosecond time scales. Their work is now published in the research journal, Nano Letters.
Their research method employed state-of-the-art ab-initio method called time dependent density functional theory to study the process of OISTR in materials. To perform these simulations the team at MPI extended time dependent density functional theory for a fully non-collinear case and implemented it within a highly accurate electronic structure code called Elk.
The authors observed that the laser pulses induced a spin-selective charge flow that was seen to generate dramatic changes in the magnetic structure of materials, including a switching of magnetic order from anti-ferromagnetic to transient ferromagnetic in multi-sub-lattice systems. Moreover, the researchers noted that the microscopic mechanism underpinning the ultra-fast switching of magnetic order was dominated by spin-selective charge transfer from one magnetic sub-lattice to another. Lastly, the authors made the vital observation in that the spin modulation was observed to be purely optical in nature and so represents one of the fastest means of manipulating spin by light. Dr. Sangeeta Sharma and colleagues study has successfully demonstrated that the local density of states is critical in determining the optically induced charge transfer between the majority and minority channels. To break this down even further, the researchers refined their findings into three simple rules that govern ultra-short time laser-induced spin-dynamics and encapsulate early-time magnetization dynamics of multi sub-lattice systems. Altogether, their mechanism is universally applicable to anti-ferromagnetic, transient ferromagnetic, and ferri-magnets in both multilayer and bulk geometry.
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