Room-temperature in-plane ferromagnetism in Co-substituted Fe₅GeTe₂ investigated by magnetic x-ray spectroscopy and microscopy

Abstract The exploration of two-dimensional (2D) van der Waals ferromagnets has revealed intriguing magnetic properties with significant potential for spintronics applications. In this study, we examine the magnetic properties of Co-doped Fe 5 GeTe 2 using x-ray photoemission electron microscopy (XPEEM) and x-ray magnetic circular dichroism (XMCD), complemented by density functional theory calculations. Our XPEEM measurements reveal that the Curie temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mtext>C</mml:mtext> </mml:msub> </mml:mrow> </mml:math> ) of a bilayer of (Co x Fe <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>5</mml:mn> <mml:mo>−</mml:mo> <mml:mi>δ</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> GeTe 2 (with x = 0.28) reaches ∼300 K—a notable enhancement over most 2D ferromagnets in the ultrathin limit. Interestingly, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mtext>C</mml:mtext> </mml:msub> </mml:mrow> </mml:math> shows only a small dependence on film thickness (bulk <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>340</mml:mn> </mml:mrow> </mml:math> K), in line with the observed in-plane (IP) magnetic anisotropy and robust IP exchange coupling. XMCD measurements indicate that the spin moments for both Fe and Co are significantly reduced compared to the theoretical values. These insights highlight the potential of Co-doped Fe 5 GeTe 2 for stable, high-temperature ferromagnetic applications in 2D materials.