Figure 1

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Figure 2

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Exchange-Coupled Magnetism in Self-Organized Metal Alloy Nanochessboard Structures

Generously supported by National Science Foundation

PI: Jerry Floro
Graduate Student: Priya Ghatwai
Undergraduate Student: Eric Vetter
Project Status: ongoing

Goals: We will examine the magnetic properties, with a special interest in exchange-coupled ferromagnetism, in alloys exhibiting the self-assembled nanochessboard structure, including the Co-Pt, Fe-Pt and Fe-Pd systems.

Technical Approach: Co-Pt alloys near 60 at% Pt can undergo a eutectoidal decomposition from the FCC parent phase into a coexistence of L10 and L12 ordered phases. Under the right thermal processing conditions, a nanochessboard structure forms as a coherent array of tiled rods, in a 2+1 D structure with 20-30 nm periodicity and length > 500 nm. The L10 is a hard uniaxial ferromagnet, while L12 is paramagnetic with Tc about 288 K. Intimate coupling between hard and soft phases on 101 nm length scales can lead to exchange-coupled magnetism, enhancing the BH product through the interactions in the magnetic composite. Although the nanochessboard was first demonstrated in Co-Pt over 20 years ago*, the magnetic behavior has never been explored.

*Electron Microscopy Study of the Coherent Two-Phase Mixtures L10+L12 in Co-Pt Alloys, C. Leroux, et al., Phil Mag. B 64, 57 (1991).

Key Results: We have achieved the nanochessboard structure by slow cooling a Co0.4Pt0.6 alloy through the eutectoid isotherm, which is critical to establishing the “composition waveform” via pseudospinodal decomposition (Fig. 1). A range of process conditions, including cooling rate, temperature differential and isothermal aging time have been explored. Preliminary magnetic measurements show a coercivity up to 2200 Oe, but only modest remanence ratios. We are now undertaking recoil measurements to better understand the exchange coupling. X-ray diffraction points to a high degree of coherency strain between the L10 and L12 nanorods (Fig. 2). In work leading up to the nanochessboard, we examined alloy compositions bracketing the two-phase region, in order to assess the technical magnetic properties, phase transformation kinetics, and mechanisms for magnetization reversal. This work turned out to be quite interesting - in a partially transformed A1+L10 sample, coercivities of 4000 Oe were obtained, with Mr/Ms = 0.8, indicative of extensive exchange coupling. This coercivity could be reproduced using a domain wall pinning model based on gradients in wall energy, invoking physically reasonable parameters.