[Paper Review] Tunable sign change of spin Hall magnetoresistance in Pt/NiO/YIG structures

Dazhi Hou, Zhiyong Qiu+8 more|White Rose Research Online (University of Leeds, The University of Sheffield, University of York)|Oct 24, 2016

Magnetic properties of thin films4 citations

TL;DR

This study demonstrates tunable sign reversal of spin Hall magnetoresistance (SMR) in Pt/NiO/YIG trilayers by controlling NiO thickness and temperature. The negative SMR at low temperatures arises from a spin-flop coupling between NiO's Néel order and YIG's magnetization, which competes with YIG's positive SMR contribution at higher temperatures, enabling reversible switching of SMR sign via temperature or NiO thickness.

ABSTRACT

Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature which increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to Pt/YIG bilayer which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of NiO layer. We explain the negative SMR by the NiO 'spin-flop' coupled with YIG, which can be overridden at higher temperatures by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

Motivation & Objective

• To investigate the origin of anomalous negative spin Hall magnetoresistance (SMR) in Pt/NiO/YIG heterostructures, contrary to conventional SMR theory.
• To determine how NiO thickness and temperature influence the sign of SMR, challenging the assumption that antiferromagnets only modulate spin transmission efficiency.
• To establish a phenomenological model explaining the competition between negative SMR from NiO and positive SMR from YIG, leading to sign reversal.
• To validate the model using spin pumping and SMR measurements, linking the temperature-dependent transparency of NiO to spin current transmission.

Proposed method

• Proposed a phenomenological model for interfacial spin current in Pt/NiO/YIG, incorporating both NM/AFM and NM/FI contributions via a linear combination.
• Introduced a temperature-dependent transparency function $ t(T) riangleq G_F(e^{aT} - 1) $, derived from spin pumping data, to describe NiO's spin transmission efficiency.
• Used the diffusion equation and Onsager's principle to calculate SMR, with the resistivity change expressed in Eq. (2) involving spin Hall angle $ heta_{ ext{SHE}} $, spin diffusion length $ ho_N $, and interface conductances.
• Fitted the SMR data using fixed parameters ($ heta_{ ext{SHE}} = 0.05 $, $ d_N = 4.0 $ nm, $ ho_0 = 860 $ $ ext{nm} imes ext{nm} $, $ au_N = 1.5 $ nm) and extracted $ G_{AF} $ and $ G_F $ from experimental data.
• Excluded data near and above NiO's Néel temperature to ensure exponential fitting validity, focusing on intermediate temperature regime.
• Validated the model by reproducing the NiO thickness-dependent SMR sign change across multiple samples with varying $ d_{ ext{NiO}} $.

Experimental results

Research questions

• RQ1 Why does Pt/NiO/YIG exhibit negative SMR at low temperatures, contrary to the predicted positive SMR in conventional Pt/YIG bilayers?
• RQ2 How does the NiO thickness influence the temperature at which the SMR sign reverses?
• RQ3 What is the origin of the negative SMR contribution, and why does it persist even when NiO blocks direct spin transmission between Pt and YIG?
• RQ4 How does the competition between SMR contributions from NiO and YIG lead to a tunable sign change in SMR?
• RQ5 Can the observed SMR behavior be quantitatively modeled using a phenomenological framework incorporating temperature-dependent spin transparency?

Key findings

• The SMR sign reverses from negative to positive with increasing temperature, and the transition temperature increases with NiO thickness.
• The negative SMR persists even when NiO fully blocks direct spin transmission from Pt to YIG, indicating it originates from NiO's intrinsic spin current response.
• The negative SMR is attributed to a spin-flop coupling between the Néel order of NiO and the magnetization of YIG, which dominates at low temperatures.
• The positive SMR contribution from YIG becomes dominant at higher temperatures, overcoming the negative contribution from NiO, leading to a sign crossover.
• The phenomenological model with $ t(T) = G_F(e^{aT} - 1) $ successfully reproduces the NiO thickness-dependent SMR sign change across all measured samples.
• Fitted parameters show that $ G_{AF} $ (NiO interface conductance) decreases with increasing NiO thickness, while $ G_F $ (YIG conductance) also decreases, indicating reduced spin current transmission efficiency with thicker NiO layers.