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Future embedded and standalone magnetic random access memory (MRAM) solutions require materials possessing bulk magnetocrystalline anisotropy at sub-10 nm feature sizes. FePd in its L10-ordered phase delivers MJ/m3 bulk anisotropy energy density (Ku) 1, large interlayer exchange coupling through Ru 2, Gilbert damping (α) below 0.008 3 and robust compatibility across a range of Pt-group buffer layers 4. Transitioning FePd films into mature spintronic applications requires a higher level of process control realize highly uniform microstructural, magnetic and magnetotransport properties.
We demonstrate control of the orientation distribution of the c-axis in highly (001)-oriented L10 FePd. Using direct current magnetron sputtering on (001)-cut, polished MgO substrates, we explore an N2 substrate anneal, variation in the individual thicknesses of the Cr/Pt underlayer and post-anneal modules between layers. Each sample had an 8-nm-thick FePd layer grown at 350 °C and post-annealed at 500 °C, leading to a high degree of L10 ordering, with Ku ranging between 0.8 MJ/m3 and 1.1 MJ/m3 and α ranging from 0.0067 to 0.0133. Despite the relatively narrow spread in these key parameters, we observed a wide range of inhomogeneous linewidth broadening (µ0ΔH0) from ferromagnetic resonance measurements, ranging from below 0.01 T to nearly 0.2 T (Fig. 1(a)). Variation in µ0ΔH0 correlates with the FePd grain orientation distribution, as measured by x-ray diffraction rocking curve scans of the (002) Bragg peak (Fig. 1(b)). Our process controls enable varying the full-width-half-maximum (FWHM) of the (002) peak from 14 degrees down to nearly 2 degrees, leading to narrowing of the local anisotropy distribution and reduction in the µ0ΔH0 (Fig. 1(b) inset). These process improvements will deliver the large-scale uniformity needed to implement FePd-based MRAM solutions.
References:
1 M. Futamoto et al., AIP Advances 6, 085302 (2016)
2 D.-L. Zhang et al., Phys. Rev. Appl. 9, 044028 (2018)
3 D.-L. Zhang et al., Appl. Phys. Lett. 117, 082405 (2020)
4 X. Wang et al., AIP Advances 11, 025106 (2021)
Figure 1: (a) FMR Linewidth versus frequency for three samples with inhomogeneous line broadening of 0.178 T (blue), 0.075 T (green) and 0.015 T (red). (b) Inhomogeneous line broadening versus FWHM of FePd(002) (Δω), with (inset) exemplary FePd(002) rocking curve scans of the red, blue and green star samples highlighted with colored markers in (a)
