Effect of the substrate temperature on the Gilbert damping and two-magnon scattering contributions to ferromagnetic resonance linewidth in Co ₂ FeAl _0.5 Si _0.5 Heusler alloy thin films

Abstract Sputtered Co 2 FeAl 0.5 Si 0.5 (CFAS) thin films, grown at different substrate temperatures ( T S = 27 °C–550 °C) with varying degree of anti-site disorder (ASD), are used to investigate the effect of T S (and hence of ASD) on the magnetization dynamics. Regardless of the magnitude of T S (or equivalently, the strength of ASD), all the CFAS thin films exhibit the in-plane (IP) and out-of-plane (OP) uniaxial magnetic anisotropy; the anisotropy field, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>H</mml:mi> <mml:mi>k</mml:mi> <mml:mi mathvariant="normal">⊥</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> , in the OP case, is an order of magnitude higher than IP— <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>H</mml:mi> <mml:mi>k</mml:mi> <mml:mo>∥</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> . Both the angular dependent cavity-based and broadband ferromagnetic resonance (FMR) data unambiguously establish that, apart from the intrinsic Gilbert damping, the extrinsic two-magnon scattering significantly contributes to the broadening of FMR linewidth. The TS500 film, deposited at T S = 500 °C, with the least ASD, exhibits the highest magnetization ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mtext>s</mml:mtext> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1107</mml:mn> <mml:mi>G</mml:mi> </mml:mrow> </mml:math> ), lowest IP- and OP- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>H</mml:mi> <mml:mi>k</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> and an exceptionally small Gilbert damping constant ( α = 1.35 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>×</mml:mo> </mml:mrow> </mml:math> 10 −4 ). Such a low value of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:math> is largely a consequence of the reduced minority spin density of states at the Fermi level. Another important aspect of the present work is the observation of a linear relation between the IP uniaxial anisotropy energy density, ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>u</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> (T)), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mo>[</mml:mo> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mtext>S</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>T</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> <mml:mo>]</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> , revealed by the elaborate data analysis of the temperature-dependent FMR spectra. Thus, varying ASD by T S in CFAS thin films provides a means of realizing magnetic properties conducive for applications in the spin-transfer torque and magnetic random-access memory devices.