Engineering skyrmion from spin spiral in transition metal multilayers

Skyrmions having topologically protected field configurations with particle-like properties play an important role in various field of science. Our present study focus on the generation of skyrmion from spin spiral in the magnetic multilayers of 4d-Fe/Ir(111) with 4d = Y, Zr, Nb, Mo, Ru, Rh. Here we investigate the impact of 4d transition metals on the isotropic Heisenberg exchanges and anti-symmetric Dzyaloshinskii-Moriya interactions originating from the broken inversion symmetry at the interface of 4d-Fe/Ir(111) multilayers. We find a strong exchange frustration due to the hybridization of the Fe-3d layer with both 4d and Ir-5d layers which modifies due to band filling effects of the 4d transition metals. We strengthen the analysis of exchange frustration by shedding light on the orbital decomposition of isotropic exchange interactions of Fe-3d orbitals. Our spin dynamics and Monte Carlo simulations indicate that the magnetic ground state of 4d-Fe/Ir(111) transition multilayers is a spin spiral in the $ab$-plane with a period of 1 to 2.5 nm generated by magnetic moments of Fe atoms and propagating along the $a$-direction. The spiral wavelengths in Y-Fe/Ir(111) are much larger compared to Rh-Fe/Ir(111). In order to manipulate the skyrmion phase in 4d-Fe/Ir(111), we investigate the magnetic ground state of 4d-Fe/Ir(111) transition multilayers with different external magnetic field. An increasing external magnetic field of $\sim$ 12 T is responsible for deforming the spin spiral into a isolated skyrmion which flips into skyrmion lattice phase around $\sim$ 18 T in Rh-Fe/Ir(111). Our study predict that the stability of magnetic skyrmion phase in Rh-Fe/Ir(111) against thermal fluctuations is upto temperature T $\leq 90$ K

Electronic structure and unconventional non-linear response in double Weyl semimetal SrSi2_2

Considering a non-centrosymmetric, non-magnetic double Weyl semimetal (WSM) SrSi$_2$, we investigate the electron and hole pockets in bulk Fermi surface behavior that enables us to characterize the material as a type-I WSM. We study the structural handedness of the material and correlate it with the distinct surface Fermi surface at two opposite surfaces following an energy evolution. The Fermi arc singlet becomes doublet with the onset of spin orbit coupling that is in accordance with the topological charge of the Weyl Nodes (WNs). A finite energy separation between WNs of opposite chirality in SrSi$_2$ allows us to compute circular photogalvanic effect (CPGE). Followed by the three band formula, we show that CPGE is only quantized for Fermi level chosen in the vicinity of WN residing at higher value of energy. Surprisingly, for the other WN of opposite chirality in the lower value of energy, CPGE is not found to be quantized. Such a behavior of CPGE is in complete contrast to the time reversal breaking WSM where CPGE is quantized to two opposite plateau depending on the topological charge of the activated WN. We further analyze our finding by examining the momentum resolved CPGE. Finally we show that two band formula for CPGE is not able to capture the quantization that is apprehended by the three band formula.Comment: 11 pages and 7 figure

Magnetization dynamics in disordered Fex_xCo1−x_{1-x} alloys : A first-principles augmented space approach and atomistic spin dynamics simulations

In this paper, we present a general method to study magnetization dynamics in chemically disordered alloys. This computationally feasible technique, which seamlessly combines three approaches : the density functional based linear muffin-tin orbitals (LMTO) for self-consistently obtaining a sparse Hamiltonian; the generalized recursion method to obtain the one and two-particle Green functions and augmented space approach to deal with disorder averaging. The same formalism applied to both spectral and response properties should make the errors compatible in different studies. %The underlying computational routines are optimized and parallelized for ease of handling. We have demonstrated a successful application to the binary chemically disordered Fe$_x$Co$_{1-x}$ alloys to explain several experimental features in magnon spectra. Our study captures significant magnon softening due to magnon-electron scattering for chemically disordered Fe$_x$Co$_{1-x}$ alloys within linear spin wave regime. As a complementary study, we have done atomistic spin dynamics simulations by solving Landau-Lifshitz-Gilbert equation with parameters obtained from ab initio multiple scattering theory to compare with the results obtained from augmented space approach.Comment: arXiv admin note: text overlap with arXiv:1102.4551, arXiv:1304.7091 by other author

Spin transport properties in a topological insulator sandwiched between two-dimensional magnetic layers

Nontrivial band topology along with magnetism leads to different novel quantum phases. When time-reversal-symmetry is broken in three-dimensional topological insulators (TIs) by applying high enough magnetic field or proximity effect, different phases such as quantum Hall or quantum anomalous Hall(QAH) emerge and display interesting transport properties for spintronic applications. The QAH phase displays sidewall chiral edge states which leads to the QAH effect. In a finite slab, contribution of the surface states depends on both the cross-section and thickness of the system. Having a small cross-section and a thin thickness leads to direct coupling of the surfaces, on the other hand, a thicker slab results in a higher contribution of the non-trivial sidewall states which connect top and bottom surfaces. In this regard, we have considered a heterostructure consisting of a TI, namely Bi2Se3, which is sandwiched between two-dimensional magnetic monolayers of CrI3 to study its topological and transport properties. Combining DFT and tight-binding calculations along with non-equilibrium Green's function formalism, we show that a well-defined exchange gap appears in the band structure in which spin polarised edge states flow. We also study the width and finite-size effect on the transmission and topological properties of this magnetised TI nanoribbon.Comment: 6 pages, 5 figures. arXiv admin note: text overlap with arXiv:2101.0625

Spin-lattice couplings in a skyrmion multilayers of Pd-Fe/Ir(111)

Pd-Fe/Ir(111) has attracted tremendous attention for next-generation spintronics devices due to existence of magnetic skyrmions with the external magnetic field. Our density functional theoretical calculations in combination with spin dynamics simulation suggest that the spin spiral phase in fcc stacked Pd-Fe/Ir(111) flips into the skyrmion lattice phase around B$_{ext} \sim$ 6 T. This leads to the microscopic understanding of the thermodynamic and kinetic behaviours affected by the intrinsic spin-lattice couplings (SLCs) in this skyrmion material for magneto-mechanical properties. Here we calculate fully relativistic SLC parameters from first principle computations and investigate the effect of SLC on dynamical magnetic interactions in skyrmion multilayers Pd-Fe/Ir(111). The exchange interactions arising from next nearest-neighbors (NN) in this material are highly frustrated and responsible for enhancing skyrmion stability. We report the larger spin-lattice effect on both dynamical Heisenberg exchanges and Dzyaloshinskii-Moriya interactions for next NN compared to NN which is in contrast with recently observed spin-lattice effect in bulk bcc Fe and CrI$_3$ monolayer. Based on our analysis, we find that the effective measures of SLCs in fcc (hcp) stacking of Pd-Fe/Ir(111) are $\sim 2.71 ( \sim 2.36)$ and $\sim 14.71 ( \sim21.89)$ times stronger for NN and next NN respectively, compared to bcc Fe. The linear regime of displacement for SLC parameters is $\leq$ 0.02 {\AA} which is 0.72\% of the lattice constant for Pd-FeIr(111). The microscopic understanding of SLCs provided by our current study could help in designing spintronic devices based on thermodynamic properties of skyrmion multilayers.Comment: 8(main text)+4(appendix) pages and 5(main text)+4(appendix) figure