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Register a new userMagnetic switching dynamics in a ferrimagnetic two sub-lattice model including ultrafast exchange scattering
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Hans Christian Schneider
Publication date
2014
fields
Physics
and research's language is
English
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We study the heat-induced magnetization dynamics in a toy model of a ferrimagnetic alloy, which includes localized spins antiferromagnetically coupled to an itinerant carrier system with a Stoner gap. We determine the one-particle spin-density matrix including exchange scattering between localized and itinerant bands as well as scattering with phonons. While a transient ferromagnetic-like state can always be achieved by a sufficiently strong excitation, this transient ferromagnetic-like state only leads to magnetization switching for model parameters that also yield a compensation point in the equilibrium M(T) curve.
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We present a microscopic calculation of magnetization damping for a magnetic toy model. The magnetic system consists of itinerant carriers coupled antiferromagnetically to a dispersionless band of localized spins, and the magnetization damping is due to coupling of the itinerant carriers to a phonon bath in the presence of spin-orbit coupling. Using a mean-field approximation for the kinetic exchange model and assuming the spin-orbit coupling to be of the Rashba form, we derive Boltzmann scattering integrals for the distributions and spin coherences in the case of an antiferromagnetic exchange splitting, including a careful analysis of the connection between lifetime broadening and the magnetic gap. For the Elliott-Yafet type itinerant spin dynamics we extract dephasing and magnetization times T_1 and T_2 from initial conditions corresponding to a tilt of the magnetization vector, and draw a comparison to phenomenological equations such as the Landau-Lifshitz or the Gilbert damping. We also analyze magnetization precession and damping for this system including an anisotropy field and find a carrier mediated dephasing of the localized spin via the mean-field coupling.
To gain control over magnetic order on ultrafast time scales, a fundamental understanding of the way electron spins interact with the surrounding crystal lattice is required. However, measurement and analysis even of basic collective processes such as spin-phonon equilibration have remained challenging. Here, we directly probe the flow of energy and angular momentum in the model insulating ferrimagnet yttrium iron garnet. Following ultrafast resonant lattice excitation, we observe that magnetic order reduces on distinct time scales of 1 ps and 100 ns. Temperature-dependent measurements, a spin-coupling analysis and simulations show that the two dynamics directly reflect two stages of spin-lattice equilibration. On the 1-ps scale, spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. This mechanism leads to identical demagnetization of the ferrimagnets two spin-sublattices and a novel ferrimagnetic state of increased temperature yet unchanged total magnetization. Finally, on the much slower, 100-ns scale, the excess of spin angular momentum is released to the crystal lattice, resulting in full equilibrium. Our findings are relevant for all insulating ferrimagnets and indicate that spin manipulation by phonons, including the spin Seebeck effect, can be extended to antiferromagnets and into the terahertz frequency range.
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 ferrimag- nets, and demonstrate that the application of ultrashort laser pulses leads to sub-picoscond 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.
GeCo$_2$O$_4$ is a unique system in the family of cobalt spinels ACo$_2$O$_4$ (A= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of GeCo$_2$O$_4$ spinel has remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel GeCo$_2$O$_4$, which exhibits an AFM ordering below the Neel temperature $T_N$ ~21 K, followed by a cubic ($Fd{bar 3}m$) to tetragonal ($I4_{1}/amd$) structural phase transition at $T_S$ ~16 K. Our density-functional theory (DFT+U) calculations reveal that one needs to consider magnetic-exchange interactions up to the third nearest neighbors to get an accurate description of the low-temperature AFM order in GeCo$_2$O$_4$. At room temperature three distinct IR-active modes ($T_{1u}$) are observed at frequencies 680, 413, and 325 cm$^{-1}$ along with four Raman-active modes $A_{1g}$, $T_{2g}(1)$, $T_{2g}(2)$, and $E_{g}$ at frequencies 760, 647, 550, and 308 cm$^{-1}$, respectively, which match reasonably well with our DFT+U calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes, is also discussed. Noticeable changes around $T_N$ and $T_S$ are observed in the Raman line parameters of the $E_{g}$ and $T_{2g}$ modes, which are associated with the modulation of the Co-O bonds in CoO$_6$ octahedra during the excitations of these modes.
We investigate ultrafast demagnetization due to electron-phonon interaction in a model band-ferromagnet. We show that the microscopic mechanism behind the spin dynamics due to electron-phonon interaction is the interplay of scattering and the precession around momentum-dependent effective internal spin-orbit magnetic fields. The resulting magnetization dynamics can only be mimicked by spin-flip transitions if the spin precession around the internal fields is sufficiently fast (compared to the scattering time) so that it averages out the transverse spin components.
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