[Paper Review] Cluster magnetic octupole induced out-of-plane spin polarization in antiperovskite antiferromagnet

Yunfeng You, Hua Bai+9 more|arXiv (Cornell University)|Oct 31, 2021

Magnetic and transport properties of perovskites and related materials53 references89 citations

TL;DR

This paper demonstrates that out-of-plane spin polarization (𝛔z) in the antiperovskite antiferromagnet Mn3SnN arises from spin precession induced by carrier motion through a cluster magnetic octupole, with 𝛔z appearing only when current flows parallel to the octupole moment. The authors achieve field-free, deterministic current-induced spin-orbit torque (SOT) switching of a perpendicular ferromagnetic layer using this 𝛔z, enabling a new pathway for ultrafast, low-power spintronic devices without external magnetic fields.

ABSTRACT

Out-of-plane spin polarization {\sigma}_z has attracted increasing interests of researchers recently, due to its potential in high-density and low-power spintronic devices. Noncollinear antiferromagnet (AFM), which has unique 120{\deg} triangular spin configuration, has been discovered to possess {\sigma}_z. However, the physical origin of {\sigma}_z in noncollinear AFM is still not clear, and the external magnetic field-free switching of perpendicular magnetic layer using the corresponding {\sigma}_z has not been reported yet. Here, we use the cluster magnetic octupole in antiperovskite AFM Mn3SnN to demonstrate the generation of {\sigma}_z. {\sigma}_z is induced by the precession of carrier spins when currents flow through the cluster magnetic octupole, which also relies on the direction of the cluster magnetic octupole in conjunction with the applied current. With the aid of {\sigma}_z, current induced spin-orbit torque (SOT) switching of adjacent perpendicular ferromagnet is realized without external magnetic field. Our findings present a new perspective to the generation of out-of-plane spin polarizations via noncollinear AFM spin structure, and provide a potential path to realize ultrafast high-density applications.

Motivation & Objective

• To identify the physical origin of out-of-plane spin polarization (𝛔z) in noncollinear antiferromagnets.
• To demonstrate that 𝛔z can be generated in antiperovskite Mn3SnN via spin precession in the presence of a cluster magnetic octupole.
• To achieve external magnetic field-free switching of a perpendicularly magnetized ferromagnetic layer using 𝛔z from the antiferromagnet.
• To establish a new mechanism for generating 𝛔z based on magnetic symmetry breaking in noncollinear AFM systems.

Proposed method

• The study uses (110)-oriented Mn3SnN thin films grown on MgO substrates via magnetron sputtering at 400 °C.
• Spin-orbit torque (SOT) measurements are performed using spin-transfer FMR (ST-FMR) with microwave current injection at 2–17 GHz.
• The angular dependence of in-plane and out-of-plane torque components is extracted via line-shape fitting of the ST-FMR response, separating contributions from 𝛔z and spin Hall effect.
• Theoretical analysis confirms that 𝛔z is proportional to the cross product of the spin-orbit field (𝐇𝐬𝐨) and the cluster magnetic octupole moment (𝐓), i.e., 𝛔z ∝ 𝐇𝐬𝐨 × 𝐓.
• Device fabrication involves patterning Mn3SnN/Py and Mn3SnN/(Co/Pd)3 heterostructures into microstrips and cross-bar devices for SOT switching experiments.
• Magnetic symmetry analysis confirms that 𝛔z vanishes when current is perpendicular to the octupole moment due to preserved magnetic mirror symmetry.

Experimental results

Research questions

• RQ1 What is the physical origin of out-of-plane spin polarization (𝛔z) in noncollinear antiferromagnets like Mn3SnN?
• RQ2 How does the direction of the cluster magnetic octupole moment relative to the current affect the generation of 𝛔z?
• RQ3 Can 𝛔z in Mn3SnN enable external magnetic field-free switching of a perpendicular ferromagnetic layer?
• RQ4 What role does the magnetic symmetry of the noncollinear spin texture play in enabling or suppressing 𝛔z?

Key findings

• Out-of-plane spin polarization (𝛔z) is generated in Mn3SnN when the current is parallel to the cluster magnetic octupole moment, with 𝛔z vanishing when current is perpendicular to the octupole due to symmetry protection.
• The 𝛔z is induced by spin precession of carriers under the influence of the spin-orbit field, with the polarization proportional to the cross product of the spin-orbit field and the octupole moment (𝛔z ∝ 𝐇𝐬𝐨 × 𝐓).
• Field-free, deterministic switching of a perpendicular Co/Pd multilayer is achieved in Mn3SnN/(Co/Pd)3 heterostructures using only the current-induced 𝛔z from the antiferromagnet.
• The spin torque ratios for field-like (𝜃FL,𝑧) and antidamping (𝜃AD,𝑧) torques are quantified, confirming the contribution of 𝛔z to both torque components.
• The Néel temperature of Mn3SnN (475 K) ensures robust 𝛔z generation at room temperature, making it suitable for practical device applications.
• The mechanism is generalizable to other noncollinear antiferromagnets with 120° triangular spin textures and cluster magnetic octupole order.