Spintronics in antiferromagnetic helix: A new prescription

TL;DR

This work demonstrates that spin polarization can be induced in an antiferromagnetic helical system without an external electric field by introducing nonuniform magnetic moments and employing long-range hopping. Using a tight-binding model and Green's function transport formalism, the authors analyze correlated and uncorrelated disorder across two AF configurations (Setup-1 and Setup-2) and both short-range and long-range hopping. They find that long-range hopping substantially enhances spin polarization, achieving near-100% polarization under favorable biases and Fermi energies, and that helicity is crucial for strong spin filtering. The results highlight a robust, field-free route to spin-selective transport in AF systems and suggest a viable platform for spintronic device designs based on antiferromagnetic helices with nonuniform moment distributions.

Abstract

The occurrence of a finite mismatch between the up and down spin energy channels due to the application of an electric field, leading to the generation of a polarized spin current from an unpolarized beam in antiferromagnetic materials, has already been established. But, in this work, we report for the first time that even in the absence of any electric field, spin polarization can be achieved. We choose a tight-binding antiferromagnetic helix, where the strengths of magnetic moments at different lattice sites are non-uniform. The non-uniformity is introduced in two distinct forms, correlated and uncorrelated, and in each case we find a high degree of spin polarization. The Greens formalism is used to compute the results under various input conditions, and the results are valid for a broad range of physical parameters. Our analysis can open up a new direction of getting spin selectivity in different magnetic systems with zero net magnetization, in the absence of an electric field.

Figures (10)

• Figure 1: (Color online). Two different spin polarization setups are shown considering a single stranded antiferromagnetic helix coupled to source and drain. For setup-1, the first $N/2$ sites exhibit magnetic moments pointing in the $+Z$ direction, whereas the remaining $N/2$ sites have magnetic alignment in the opposing direction (Fig. \ref{['model']}(a)), and, for setup-2, the adjacent magnetic moments are arranged in an antiparallel configuration along $\pm Z$ directions (Fig. \ref{['model']}(b)). Here, $R$ denotes the radius, $\Delta \phi$ represents the twisting angle, and $\Delta z$ corresponds to the stacking distance between the neighboring lattice sites.
• Figure 2: (Color online). (a) and (b): Spin-dependent transmission probabilities as a function of energy where the cyan and red colors correspond to the up and down spin electrons, respectively. (c) and (d): Variation of up and down spin currents with bias voltage where the orange and maroon colors represent the up and down spin currents, respectively. (e) and (f): Dependence of spin polarization on bias voltage for the SRH AFH (left column) and LRH AFH (right column) systems. Here, we fix Fermi energy at $E_F=0.25$.
• Figure 3: (Color online). Simultaneous variation of spin polarization $P$ with Fermi energy $E_F$ and bias voltage $V$ for the LRH AFH.
• Figure 4: (Color online). Dependence of spin polarization $P$ with bias voltage by fixing Fermi energy at $0.25$ for (a) SRH and (b) LRH helices.
• Figure 5: (Color online). Variation of spin polarization with bias voltage and Fermi energy for the LRH AFH.