[Paper Review] Quantum spin-transfer torque and magnon-assisted transport in nanostructures

Scott A. Bender, R. A. Duine+1 more|arXiv (Cornell University)|Aug 2, 2018

Quantum and electron transport phenomena3 citations

TL;DR

This paper proposes a theoretical framework for understanding quantum spin fluctuations in nanoscale magnetic junctions, showing that quantum spin fluctuations induce a distinct quantum spin-transfer torque that alters magnetoconductance independently of magnon-assisted transport. The key contribution is the identification of a temperature-dependent quantum effect that differentiates quantum from thermal spin fluctuations in charge transport.

ABSTRACT

We theoretically investigate the role of spin fluctuations in charge transport through a magnetic junction. Motivated by recent experiments that measure a nonlinear dependence of the current on electrical bias, we develop a systematic understanding of the interplay of charge and spin dynamics in nanoscale magnetic junctions, starting from a simple model for spin-dependent transport in the presence of spin fluctuations. Our model captures two distinct features arising from these fluctuations: magnon-assisted transport and spin-transfer torque alteration of the magnetoconductance. We show that as the temperature is lowered, the latter effect arises from quantum rather than thermal spin fluctuations and that this quantum spin-transfer torque is readily distinguishable from magnon-assisted transport.

Motivation & Objective

• To understand the nonlinear current-bias dependence observed in recent experiments on magnetic junctions.
• To investigate the role of spin fluctuations in charge transport at the nanoscale.
• To distinguish between magnon-assisted transport and spin-transfer torque effects arising from spin fluctuations.
• To identify the emergence of quantum spin-transfer torque at low temperatures.

Proposed method

• Develops a theoretical model for spin-dependent transport in magnetic junctions including spin fluctuations.
• Uses a systematic approach to analyze the interplay between charge and spin dynamics in nanoscale systems.
• Distinguishes between thermal and quantum spin fluctuations by examining temperature-dependent transport behavior.
• Analyzes the magnetoconductance response to identify contributions from magnon-assisted transport and spin-transfer torque.
• Applies a minimal model to isolate quantum effects from classical thermal fluctuations in spin dynamics.
• Derives conditions under which quantum spin-transfer torque becomes dominant at low temperatures.

Experimental results

Research questions

• RQ1 How do spin fluctuations influence charge transport in magnetic nanostructures?
• RQ2 What distinguishes magnon-assisted transport from spin-transfer torque effects in the presence of spin fluctuations?
• RQ3 How does quantum spin-transfer torque emerge at low temperatures compared to thermal fluctuations?
• RQ4 Can quantum spin-transfer torque be experimentally distinguished from magnon-assisted transport?

Key findings

• Quantum spin-transfer torque emerges at low temperatures due to quantum rather than thermal spin fluctuations.
• The quantum spin-transfer torque alters the magnetoconductance in a way distinct from magnon-assisted transport.
• Magnon-assisted transport and spin-transfer torque are two separate contributions arising from spin fluctuations.
• The quantum spin-transfer torque effect is identifiable through its unique temperature dependence and behavior under varying electrical bias.
• The model predicts that quantum effects dominate the spin-transfer torque at sufficiently low temperatures.
• The distinction between quantum and thermal spin fluctuations is experimentally observable in the nonlinear current-bias response.