Exchange stiffness in ultrathin perpendicularly-magnetized CoFeB layers determined using spin wave spectroscopy

TL;DR

This study measures the exchange stiffness of ultrathin perpendicularly magnetized CoFeB layers using spin wave spectroscopy in nanopillars, revealing a slightly reduced exchange stiffness compared to bulk material, with implications for ultrathin film magnetization modeling.

Contribution

The paper introduces a method to determine exchange stiffness in ultrathin magnetic layers using spin wave spectroscopy and nanopillar devices, providing new quantitative insights.

Findings

Exchange stiffness for 1 nm layers is approximately 20 pJ/m.

Ultrathin layers exhibit slightly lower exchange stiffness than bulk.

Thickness dependence affects magnetic modeling of ultrathin films.

Abstract

We measure the frequencies of spin waves in nm-thick perpendicularly magnetized FeCoB systems, and model the frequencies to deduce the exchange stiffness of this material in the ultrathin limit. For this, we embody the layers in magnetic tunnel junctions patterned into circular nanopillars of diameters ranging from 100 to 300 nm and we use magneto-resistance to determine which rf-current frequencies are efficient in populating the spin wave modes. Micromagnetic calculations indicate that the ultrathin nature of the layer and the large wave vectors used ensure that the spin wave frequencies are predominantly determined by the exchange stiffness, such that the number of modes in a given frequency window can be used to estimate the exchange. For 1 nm layers the experimental data are consistent with an exchange stiffness A= 20 pJ/m, which is slightly lower that its bulk counterpart. The thickness dependence of the exchange stiffness has strong implications for the numerous situations that involve ultrathin films hosting strong magnetization gradients, and the micromagnetic description thereof.