Spin Density Wave Antiferromagnetism Research Articles

Negative spin Hall angle and large spin-charge conversion in thermally evaporated chromium thin films

Spin-to-charge conversion and the reverse process are now critically important physical processes for a wide range of fundamental and applied studies in spintronics. Here, we experimentally demonstrate effective spin-to-charge conversion in thermally evaporated chromium thin films using the longitudinal spin Seebeck effect (LSSE). We present LSSE results measured near room temperature for Cr films with thicknesses from 2 to 11 nm, deposited at room temperature on bulk polycrystalline yttrium-iron-garnet (YIG) substrates. Comparison of the measured LSSE voltage, VLSSE, in Cr to a sputtered Pt film at the same nominal thickness grown on a matched YIG substrate shows that both films show comparably large spin-to-charge conversion. As previously shown for other forms of Cr, the LSSE signal for evaporated Cr/YIG shows the opposite sign compared to Pt, indicating that Cr has a negative spin Hall angle, θsh. We also present measured charge resistivity, ρ, of the same evaporated Cr films on YIG. These values are large compared to Pt and comparable to β-W at a similar thickness. Non-monotonic behavior of both ρ and VLSSE with film thickness suggests that spin-to-charge conversion in evaporated Cr, which we expect has a different strain state than previously investigated sputtered films, could be modified by spin density wave antiferromagnetism in Cr.

Anomalous electron doping independent two-dimensional superconductivity

Transition metal (Co and Ni) co-doping effects are investigated on an underdoped Ca0.94La0.06Fe2As2 compound. It is discovered that electron doping from substituting Fe with transition metal (TM = Co, Ni) can trigger high- superconductivity around 35 K, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Remarkably, the critical temperature for the high- superconductivity remains constant against a wide range of TM doping levels. And the net electron doping density dependence of the superconducting based on the rigid band model can be nicely scaled into a single curve for Co and Ni substitutions, in stark contrast to the case of Ba(Fe1−xTMx)2As2. This carrier density independent superconductivity and the unusual scaling behavior are presumably resulted from the interface superconductivity based on the similarity with the interface superconductivity in a La2−xSrxCuO4-La2CuO4 bilayer. Evidence of the two-dimensional character of the superfluid by angle-resolved magneto-resistance measurements can further strengthen the interface nature of the high- superconductivity.

Stripe Antiferromagnetic Spin Fluctuations inSrCo2As2

Inelastic neutron scattering measurements of paramagnetic SrCo2As2 at T=5 K reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wave vector of Q(AFM)=(1/2,1/2,1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe2As2 compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by Q(AFM). SrCo2As2 has a more complex Fermi surface and band-structure calculations indicate a potential instability toward either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt-based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds.

Accurate projected augmented wave datasets for BaFe2As2

By carefully choosing parameters and including more semi-core orbitals as valence electrons, we have constructed a high-quality projected augmented wave dataset that yields results comparable to existing full-potential linearized augmented plane-wave calculations. The dataset was then applied to BaFe2As2 to study the effects of different levels of structure optimization, as well as different choices of exchange-correlation functionals. It was found that the local density approximation exchange-correlation functional fails to find the correct spin-density-wave anti-ferromagnetic (SDW-AFM) ground state under full optimization, while the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional obtains the correct state but significantly overestimates the magnetism. The electronic structure of the SDW-AFM state is not very sensitive to structure optimizations with the PBE exchange-correlation functional because the positions of the As atoms are preserved under optimizations. We further investigated the Ba atom diffusion process on the BaFe2As2 surface using the nudged elastic bands method. The Ba atom was found to be stable above the center of the squares formed by the surface As atoms, and a diffusion barrier of 1.2 eV was found. Our simulated scanning tunneling microscopy image suggests an ordered surface Ba atom structure, in agreement with Massee et al (2009 Phys. Rev. B 80 140507; van Heumen E et al 2010 arXiv:1009.3493v1).

Weak itinerant magnetism, new developments: TiBe2

Stimulated by theoretical considerations on the role of electron-phonon interaction in the weak itinerant ferromagnet ZrZn2, analogous magnetism was discovered in the system TiBe2-xCux by Matthias et al. This system is ferromagnetic for x > 0.05, while for x < 0.05 there is evidence of antiferromagnetism. Based on the analysis of the measured specific heat anomaly and of the band structure of TiBe2, it is argued that this substance is indeed a spin density wave antiferromagnet.

Large Exchange Bias due to the Spin Density Wave Antiferromagnet Cr in a Fe/Cr(100) Bilayer

Work at Korea Advanced Institute of Science and Technology (KAIST) was supported through the Creative Research Institute Project and the Cavendish-KAIST Cooperation Project of the Korean Ministry of Science and Technology and Works at SungKyunKwan University were supported by he BK21 program, the 21st Century Frontier R&D Pro- gram for Hydrogen Energy and the Basic Atomic Energy Research Institute program.

Magnetic and electrical transport properties in (Cr 99V 1) 95Fe 5 and (Cr 99V 1) 75Fe 25 alloys

DC magnetization M ( T ) and electrical resistivity ρ ( T ) have been studied in the temperature range of 4–300 K for (Cr 99V 1) 95Fe 5 and (Cr 99V 1) 75Fe 25 alloys. A minimum is observed near T N in ρ ( T ) curve for (Cr 99V 1) 95Fe 5, which indicates the spin-density wave (SDW) antiferromagnetic transition. The progressive opening up of the SDW antiferromagnetic energy gap can be well described by a temperature-dependent energy gap function as 2 Δ ∝ T N − T . A paramagnetic–ferromagnetic transition is confirmed near T C in M ( T ) curve for (Cr 99V 1) 75Fe 25, but no anomaly is observed around T C in ρ ( T ) curve that would be covered up by magnetic cluster-dependent scattering. The analyses indicated the magnetic clusters start to form at relatively high temperature above the transition temperature. T N and the SDW antiferromagnetism are rapidly depressed for Fe-poor (Cr 99V 1) 95Fe 5 while T C and the ferromagnetism are rapidly increased for Fe-rich (Cr 99V 1) 75Fe 25 by the substitution of light doping V for Cr.

Competing spin waves and superconducting fluctuations in strongly correlated electron systems

A special diagram technique recently proposed for strongly correlated electron systems is used to study the peculiarities of a spin-density wave (SDW) in competition with superconductivity. This method allows formulation of the Dyson equations for the renormalized electron propagators of the co-existing phases of SDW antiferromagnetism and superconductivity. We find the surprising result that triplet superconductivity appears provided that we have the co-existence of singlet superconductivity and SDW antiferromagnetism. A special ansatz, which takes into account the full Green's functions as well as the dynamical structure of the correlations, is used to establish the equations determining the gap functions and order parameters.

The chromium spin density wave: magnetic X-ray scattering studies with polarisation analysis

We report on X-ray magnetic diffraction studies of the spin density wave antiferromagnetism formed in the conduction electron band of chromium. Non-resonant X-ray magnetic scattering was used to directly determine that chromium has zero orbital magnetisation. Furthermore, the azimuthal dependence of this scattering provides unique evidence that chromium forms a linearly polarised wave. In the vicinity of the K absorption edge, resonant X-ray magnetic scattering was observed. A consistent model of the magnetic scattering has been derived from the resonant and non-resonant magnetic amplitudes. The enhancement of the magnetic intensity arises primarily from dipole transitions from the core 1s level to 4p states. Quadrupole transitions to the magnetic 3d states are essentially non-existent due to their sensitivity to (and the absence of) orbital moment. This effect is predicted from atomic considerations of the 3d5 ( = 0) transition metal ions.

Spin-density-wave magnetism in layered chromium studied by perturbed-angular-correlation spectroscopy

Results obtained by perturbed-angular-correlation (PAC) spectroscopy on epitaxial chromium thin films are compared with data obtained from the same samples by neutron-diffraction experiments. We report on the study of a 250-nm-thick Cr single layer and an Fe/Cr multilayer with Cr thickness of 25 nm grown on ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}$ (1102). The chromium single layer film orders below the N\'eel temperature as a longitudinal $({\mathrm{AF}}_{2})$ spin density wave antiferromagnet with the spins out of plane, while the Cr in the Fe/Cr multilayer orders as a transversal $({\mathrm{AF}}_{1})$ spin density wave antiferromagnet with the spins in plane. In addition we observe a minority volume fraction in a commensurate ${\mathrm{AF}}_{0}$ phase, stable up to at least 400 K. The present results prove that PAC probes the intrinsic magnetic features of chromium and therefore validate this technique as a complementary approach towards the understanding of the chromium magnetism in thin films and Fe/Cr multilayers.

Two-magnon Raman scattering in a spin density wave antiferromagnet

We present the results for a model calculation of resonant two-magnon Raman scattering in a spin density wave antiferromagnet. The resonant enhancement of the two-magnon intensity is obtained from a microscopic analysis of the photon-magnon coupling vertex. By combining magnon-magnon interactions with ‘triple resonance’ phenomena in the vertex function the resulting intensity line shape is found to closely resemble the measured two-magnon Raman signal in antiferromagnetic cuprates.

Two-magnon Raman scattering in a spin density wave antiferromagnet

We present the results for a model calculation of resonant two-magnon Raman scattering in a spin density wave (SDW) antiferromagnet. The resonant enhancement of the two-magnon intensity is obtained from a microscopic analysis of the photon-magnon coupling vertex. By combining magnon-magnon interactions with ‘triple resonance’ phenomena in the vertex function the resulting intensity line shape is found to closely resemble the measured two-magnon Raman signal in antiferromagnetic cuprates. Both, resonant and non-resonant Raman scattering are discussed for the SDW antiferromagnet and a comparison is made to the conventional Loudon-Fleury theory of two-magnon light scattering.

Ising expansion for the Hubbard model.

We develop series expansions for the ground state properties of the Hubbard model, by introducing an Ising anisotropy into the Hamiltonian. For the two-dimensional (2D) square lattice half-filled Hubbard model, the ground state energy, local moment, sublattice magnetization, uniform magnetic susceptibility and spin stiffness are calculated as a function of $U/t$, where $U$ is the Coulomb constant and $t$ is the hopping parameter. Magnetic susceptibility data indicate a crossover around $U\approx 4$ between spin density wave antiferromagnetism and Heisenberg antiferromagnetism. Comparisons with Monte Carlo simulations, RPA result and mean field solutions are also made.

Superconductivity and antiferromagnetism in Cr-Mo-Ru alloys

(Cr1-xMox)75Ru25 alloys with x=0, 3%, 6% or 10% exhibit superconducting properties below 3 K. For low Mo concentrations comparison with theory suggests a coexistence of spin density wave antiferromagnetism and superconductivity while the spin density wave state seems to be absent at higher concentrations. Velocity of sound measurements, fail to reveal any spin density wave antiferromagnetic transitions in the alloys. This is attributed to two possibilities: either the nesting portions of the Fermi surface are very small or, as suggested by susceptibility measurements reported recently on Cr75Ru25, an unknown magnetic phase appears at low temperatures.