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

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