arXiv subjects

Gokaran Shukla

Publications and source records attributed to Gokaran Shukla.

3 recordsLinked to original sources

arXiv2023

Higher-order Hall response arises from octupole order and scalar spin chirality in a noncollinear antiferromagnet

Noncollinear antiferromagnets can generate a transverse electrical response known as the anomalous Hall effect, even though they possess almost no net magnetization. The microscopic origin of this behaviour, however, has remained unclear because conventional measurement geometries mix different contributions to the measured response. Here, we show that applying magnetic fields in selected in-plane directions allows us to disentangle the mechanisms underlying the Hall effect in a representative noncollinear antiferromagnet. By suppressing any dipole-related signal, we isolate a purely octupole-driven Hall response that exhibits a characteristic three-fold angular symmetry. At low magnetic fields, we further observe an additional Hall-like contribution that arises from the scalar spin chirality associated with noncoplanar spin textures. Combining symmetry analysis, first-principles calculations, and transport measurements, we reveal that octupole order, dipole moments, and chirality coexist and contribute in distinct field regimes. These findings establish a framework for identifying and controlling complex magnetic order parameters for spintronic applications.

cond-mat.mes-hall↗

arXiv2022

Fe- and Co-based magnetic tunnel junctions with AlN and ZnO spacers

AlN and ZnO, two wide band-gap semiconductors extensively used in the display industry, crystallise in the wurtzite structure, which can favour the formation of epitaxial interfaces to close-packed common ferromagnets. Here we explore these semiconductors as material for insulating barriers in magnetic tunnel junctions. In particular, the {\it ab initio} quantum transport code {\it Smeagol} is used to model the $X$[111]/$Y$[0001]/$X$[111] ($X=$ Co and Fe, $Y=$ AlN and ZnO) family of junctions. Both semiconductors display a valance-band top with $p$-orbital character, while the conduction band bottom exhibits $s$-type symmetry. The smallest complex-band decay coefficient in the forbidden energy-gap along the [0001] direction is associated with the $Δ_1$ symmetry, and connects across the band gap at the $Γ$ point in 2D Brillouin zones. This feature enables spin filtering and may result in a large tunnelling magnetoresistance. In general, we find that Co-based junctions present limited spin filtering and little magnetoresistance at low bias, since both spin sub-bands cross the Fermi level with $Δ_1$ symmetry. This contrasts the situation of Fe, where only the minority $Δ_1$ band is available. However, even in the case of Fe the magnitude of the magnetoresistance at low bias remains relatively small, mostly due to conduction away from the $Γ$ point and through complex bands with symmetry different than $Δ_1$. The only exception is for the Fe/AlN/Fe junction, where we predict a magnetoresitance of around 1,000\% at low bias.

cond-mat.mes-hall↗

arXiv2021

Co/AlP/Co, Co/GaN/Co as magnetic tunnel junctions

AlP and GaN are wide band-gap semiconductors (SC) uses in opto-electronic industry as light emitting diodes. Here we investigate it as future perspective candidate for insulating barrier in magnetic tunnel junctions. We employ density functional theory for ground state electronic properties and non-equilibrium Green's function method for quantum transport and examined Co/AlP/Co and Co/GaN/Co MTJs. We find that both AlP and GaN valance band maxima are predominantly made with $p_z$-type orbitals while conduction band minima are $s$-type symmetry. We find that both AlP and GaN filter $Δ_1$ symmetry of Bloch states at $Γ$-point and transmission coefficient at any energy level in-between the band-gap of materials, is mostly driven by $Δ_1$ symmetry of Bloch states tunnel via $Γ$-point in first Brillouin zones. We find large magneto-resistance $\sim$300\% in Co/AlP/Co MTJs at zero-bias. In Co/GaN/Co MTJs we find $\sim$300\% TMR at 1.25 eV below the Fermi energy ($E_{F}-1.25$)~eV, while $\sim$10\% TMR around $E_F$ in zero-bias calculations. We notice that both majority and minority $Δ_{2'}$ symmetry of Bloch states with rather different $spd$-orbitals compositions tunnel in Co[0001]/AlP[0001] MTJs and exhibit non-zero TMR, whereas in Co[111]/GaN[0001] MTJs, the both majority and minority $Δ_1$ symmetry of Bloch states with different energy-gradient tunnels at Fermi energy level along [111] transport direction. Our work accentuate the process for systematic, efficient, accurate and versatile framework to design the semiconductors based MTJs for low power electronics.

cond-mat.mtrl-sci↗