[Paper Review] Interfacial Dzyaloshinskii-Moriya interaction in Pt/CoFeB films: effect of the heavy-metal thickness

S. Tacchi, Roberto E. Troncoso+5 more|arXiv (Cornell University)|Apr 10, 2016

Magnetic properties of thin films15 citations

TL;DR

This study investigates the thickness-dependent interfacial Dzyaloshinskii-Moriya interaction (DMI) in Pt/CoFeB ultrathin films using Brillouin light scattering (BLS) spectroscopy. It demonstrates that DMI strength increases with Pt thickness, saturating at ~0.45 mJ/m² for Pt thicknesses above ~2 nm, due to electron hopping within the heavy metal mediated by spin-orbit coupling. The findings are explained via a 3-site indirect exchange model, enabling control of chiral spin textures through heavy-metal layer engineering.

ABSTRACT

We report the observation of a Pt layer thickness dependence on the induced interfacial Dzyaloshinskii-Moriya interaction in ultra-thin Pt($d_{ ext{Pt}}$)/CoFeB films. Taking advantage of the large spin-orbit coupling of the heavy metal, the interfacial Dzyaloshinskii-Moriya interaction is quantified by Brillouin light scattering measurements of the frequency non-reciprocity of spin-waves in the ferromagnet. The magnitude of the induced Dzyaloshinskii-Moriya coupling is found to saturate to a value $0.45$ mJ$/$m${}^2$ for Pt thicknesses larger than $\sim 2$ nm. The experimental results are explained by analytical calculations based on the 3-site indirect exchange mechanism that predicts a Dzyaloshinskii-Moriya interaction at the interface between a ferromagnetic thin layer and a heavy metal. Our findings open up a way to control and optimize chiral effects in ferromagnetic thin films through the thickness of the heavy metal layer.

Motivation & Objective

• To investigate the dependence of interfacial Dzyaloshinskii-Moriya interaction (DMI) on Pt layer thickness in Pt/CoFeB heterostructures.
• To quantify the strength of DMI using Brillouin light scattering (BLS) measurements of spin-wave frequency non-reciprocity.
• To explain the observed thickness dependence using a 3-site indirect exchange mechanism involving spin-orbit coupling in Pt.
• To determine the role of the spin-diffusion length in limiting the spatial range of DMI coupling.
• To enable engineering of chiral spin textures in magnetic thin films through control of heavy-metal thickness.

Proposed method

• Prepared Si/SiO₂/Co₄₀Fe₄₀B₂₀(2 nm)/Pt(d_Pt)/Cu(3 nm) multilayers via magnetron sputtering with Pt thicknesses ranging from 0 to 6 nm.
• Performed BLS measurements using a 532 nm laser to excite and detect spin-wave modes, focusing on frequency asymmetry between counter-propagating Damon-Eshbach modes.
• Extracted DMI strength from the frequency non-reciprocity using the analytical model: Δf(k) = 2Dk sin(ka), where D is the DMI constant.
• Fitted experimental data to the model to determine D as a function of d_Pt, using parameters a = 0.25 nm, b = 0.39 nm, L₀y = 0.32 nm, and n = 2.95.
• Theoretical analysis based on the 3-site indirect exchange mechanism, where DMI arises from spin-orbit coupling in Pt mediating between CoFeB spins.
• Linked the saturation of DMI to the spin-diffusion length (~1.2–2 nm), which limits the effective range of electron-mediated exchange.

Experimental results

Research questions

• RQ1 How does the thickness of the Pt layer affect the strength of the interfacial Dzyaloshinskii-Moriya interaction in Pt/CoFeB films?
• RQ2 What is the physical origin of the observed thickness dependence of DMI in Pt/CoFeB heterostructures?
• RQ3 To what extent does the spin-diffusion length in Pt limit the range of DMI coupling in these systems?
• RQ4 Can the DMI strength be tuned and saturated by varying the Pt thickness, and what is the saturation value?
• RQ5 How does the 3-site indirect exchange mechanism explain the observed DMI thickness dependence?

Key findings

• The interfacial DMI strength increases with increasing Pt thickness and saturates at approximately 0.45 mJ/m² for Pt thicknesses above ~2 nm.
• The saturation behavior is attributed to the spin-diffusion length in Pt, which limits the effective range of electron-mediated DMI coupling.
• The DMI strength reaches a maximum value of 0.45 mJ/m² at a Pt thickness of about four monolayers (~2 nm), beyond which further thickness increases do not enhance DMI.
• The frequency non-reciprocity of spin waves, measured via BLS, provides a direct and quantitative probe of DMI, with asymmetry scaling linearly with wave vector k.
• The 3-site indirect exchange model successfully explains the experimental data, showing that DMI arises from spin-orbit coupling in Pt mediating between CoFeB magnetic atoms.
• The effective spatial range of DMI extends beyond nearest-neighbor spins, with significant coupling up to third-nearest neighbors, as confirmed by the DMI energy model h_DM(n) = -|D_ij| s_i s_j sin(kna).