[Paper Review] Mn4N ferrimagnetic thin films for sustainable spintronics

Toshiki Gushi, Klug, M. Jovičević+7 more|arXiv (Cornell University)|Jan 21, 2019

ZnO doping and propertiesMaterials Science8 references3 citations

TL;DR

This paper demonstrates that epitaxial Mn4N thin films on SrTiO3 substrates are a sustainable alternative to rare-earth and heavy-metal-based spintronic materials, exhibiting perpendicular magnetization, a high extraordinary Hall angle (2%), and highly mobile domain walls driven by spin-polarized currents without spin-orbit torques. The key result is record domain wall velocities (~1 km/s) enabled by strong adiabatic spin transfer torque due to high spin polarization and reduced magnetization.

ABSTRACT

Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that Mn4N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn4N thin films grown epitaxially on SrTiO3 substrates possess remarkable properties, such as a perpendicular magnetisation, a very high extraordinary Hall angle (2%) and smooth domain walls, at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin polarised currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetisation and a large spin polarisation. Finally, we show that the application of gate voltages through the SrTiO3 substrates allows modulating the Mn4N coercive field with a large efficiency.

Motivation & Objective

• To develop a sustainable alternative to critical materials like Co, heavy metals, and rare-earths in spintronic devices.
• To investigate the magnetic and transport properties of epitaxial Mn4N thin films for spintronic applications.
• To demonstrate electric field control of magnetic anisotropy via gate voltage in Mn4N/SrTiO3 heterostructures.
• To evaluate domain wall dynamics under spin-polarized current injection without spin-orbit torques.

Proposed method

• Epitaxial Mn4N thin films were grown on SrTiO3 (001) substrates using pulsed laser deposition.
• Magnetic anisotropy and magnetization were characterized using SQUID magnetometry and vibrating sample magnetometry.
• The extraordinary Hall effect was measured to determine the Hall angle and spin polarization.
• Domain wall motion was probed using spin-polarized current injection and real-time imaging techniques.
• Gate voltage was applied through the SrTiO3 substrate to modulate the coercive field of Mn4N.
• Theoretical analysis based on the adiabatic spin transfer torque model was used to explain high domain wall velocities.

Experimental results

Research questions

• RQ1 Can Mn4N thin films exhibit perpendicular magnetic anisotropy suitable for spintronic devices?
• RQ2 What is the magnitude of the extraordinary Hall angle in Mn4N, and what does it imply about spin polarization?
• RQ3 Can domain walls in Mn4N be moved at high speeds using only spin-polarized current, without spin-orbit torques?
• RQ4 To what extent can the coercive field of Mn4N be tuned via gate voltage applied through a SrTiO3 substrate?
• RQ5 How does the combination of reduced magnetization and high spin polarization in Mn4N enhance adiabatic spin transfer torque efficiency?

Key findings

• Mn4N thin films exhibit perpendicular magnetic anisotropy with a well-defined out-of-plane magnetization.
• The extraordinary Hall angle reaches 2%, indicating high spin polarization in the material.
• Domain walls in Mn4N can be moved at record speeds of up to ~1 km/s under spin-polarized current injection, without requiring spin-orbit torques.
• The high domain wall mobility is attributed to strong adiabatic spin transfer torque, enabled by the conjunction of high spin polarization and reduced magnetization.
• Gate voltage applied through the SrTiO3 substrate modulates the Mn4N coercive field with high efficiency, demonstrating electric field control of magnetic anisotropy.
• The system shows smooth domain walls at the millimeter scale, indicating low pinning and high material quality.