Finding order in disorder: Magnetic coupling distributions and competing anisotropies in an amorphous metal alloy

Abstract

Amorphous metals have unusual magnetic properties that arise due to the disordered atomic arrangement. We show that Co x(Al70Zr30)100− x (65 < x < 92 at. %) amorphous alloys have a distribution in the local magnetic coupling and ordering temperature, which can be explained by nanoscale composition variations. We use competing anisotropies induced by the substrate and an applied field during growth to probe the Co concentration distribution. Only regions with high enough Co concentration develop a magnetic anisotropy along the magnetic field during growth, whereas regions of low Co concentration have an anisotropy dictated by the substrate. A Gaussian distribution in the Co concentration of width 5.1 at. % is obtained from the variation in anisotropy. The results demonstrate the importance of composition variations for emergent magnetic properties and have far reaching implications for the properties of disordered materials in general.

Structural and magnetic changes in CoAlZr thin films upon post annealing

“…The samples were deposited in a uniform in-plane 130 mT magnetic field, from four SmCo magnets fixed on either side of the samples. This has been shown to induce a uniaxial anisotropy with an easy axis along the field direction [2]. The sample holder was rotated continuously during sputtering, to ensure that anisotropy is defined solely by the applied field and not by the sputter geometry (see e.g.…”

“…The sample holder was rotated continuously during sputtering, to ensure that anisotropy is defined solely by the applied field and not by the sputter geometry (see e.g. [2], [3]). The thickness of all the CoAlZr films was 60 nm.…”

“…The thickness of all the CoAlZr films was 60 nm. The precise composition was Co 0.85 (Al 0.7 Zr 0.3 ) 0.15 , chosen for its high Curie temperature of 700-850 K, and for its amorphous structure [2] facilitated by the amorphous wetting layer and the composition chosen.…”

“…Static and dynamic magnetic properties were measured by magneto-optic Kerr effect (MOKE) magnetometry, vibrating sample magnetometry (VSM), and broadband ferromagnetic resonance (FMR) [2], [6]- [8]. The samples analyzed by FMR were circular disks with a diameter of 4 mm, in order to reduce the occurrence of domain formation in corners and to exclude in-plane shape anisotropy effects.…”

“…CoAlZr amorphous alloys have been shown to have highly tuneable magnetic properties. They are ultrasoft, with coercivity of the order of 0.2 mT, their ordering temperature and magnetization can be controlled over a large continuous range by composition and they have been shown to sustain large magnetic proximity effects in heterostructures [1], [2]. Many of these features are rooted in the disordered atomic arrangement.…”