# Seed Layer Impact on Structural and Magnetic Properties of [Co/Ni] Multilayers with Perpendicular Magnetic Anisotropy

## TL;DR

This study investigates how different seed layers affect the structural and magnetic properties of [Co/Ni] multilayers with perpendicular magnetic anisotropy, especially after annealing, revealing diffusion as a key factor in property changes.

## Contribution

It provides new insights into how seed layer materials influence PMA and damping in [Co/Ni] multilayers, highlighting diffusion effects during annealing.

## Key findings

- PMA maintained on all seeds after annealing up to 400°C.
- NiCr seed yields the highest effective perpendicular anisotropy after annealing.
- Diffusion from seed layers significantly impacts magnetic property evolution.

## Abstract

[Co/Ni] multilayers with perpendicular magnetic anisotropy (PMA) have been researched and applied in various spintronic applications. Typically the seed layer material is studied to provide the desired face-centered cubic (\textit{fcc}) texture to the [Co/Ni] to obtain PMA. The integration of [Co/Ni] in back-end-of-line (BEOL) processes also requires the PMA to survive post-annealing. In this paper, the impact of NiCr, Pt, Ru, and Ta seed layers on the structural and magnetic properties of [Co(0.3 nm)/Ni(0.6 nm)] multilayers is investigated before and after annealing. The multilayers were deposited \textit{in-situ} on different seeds via physical vapor deposition at room temperature. The as-deposited [Co/Ni] films show the required \textit{fcc}(111) texture on all seeds, but PMA is only observed on Pt and Ru. In-plane magnetic anisotropy (IMA) is obtained on NiCr and Ta seeds, which is attributed to strain-induced PMA loss. PMA is maintained on all seeds after post-annealing up to 400$^{\circ}$C. The largest effective perpendicular anisotropy energy ($K_U^{\mathrm{eff}}\approx 2\times10^5$J/m$^3$) after annealing is achieved on NiCr seed. The evolution of PMA upon annealing cannot be explained by further crystallization during annealing or strain-induced PMA, nor can the observed magnetization loss and the increased damping after annealing. Here we identify the diffusion of the non-magnetic materials from the seed into [Co/Ni] as the major driver of the changes in the magnetic properties. By selecting the seed and post-annealing temperature, the [Co/Ni] can be tuned in a broad range for both PMA and damping.