Structure-Correlated Exchange Anisotropy in Oxidized Co₈₀Ni₂₀ Nanorods

Abstract

Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co 80 Ni 20 nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic caxis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co 80 Ni 20 alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature.H ighly elongated nanomagnets like nanorods and nanowires have recently attracted huge interest due to possible applications in magnetic memories, 1 sensors, 2 and microwave devices 3 or the use as permanent magnets. 4 The magnetic properties of such nanorods with a length-to-diameter aspect ratio larger than 5 are strongly governed by the shape anisotropy, and thus, the easy axis of magnetization aligns parallel to the long axis of a nanorod. If the preparation technique or the synthetic method exerts control over the crystallographic orientation of the material along the nanorod geometry, one expects large coercive fields and high remanent magnetization stemming from the magnetocrystalline easy axis aligned parallel to the axis of the rod. Additionally, besides the shape and magnetocrystalline, any further increase of magnetic anisotropy, e.g., by exchange coupling to an antiferromagnetic material (exchange bias), will increase the energy product of the magnetic hysteresis of these nanostructures. 5,6 Systems matching the above criteria are very interesting for permanent magnet applications as already suggested in the late 1950s. 7−9 Nowadays, strong permanent magnets for various applications usually contain a significant amount of rare earth elements like Nd or Sm. 10 For better sustainability, availability, and lower prices, the above idea of exploiting the combination of shape, magnetocrystalline, and exchange anisotropy in rare earth-free nanomagnets has received renewed attention over the last years. 11,12 Due to their low magnetocrystalline anisotropy energy density (MAE), purely metallic Fe or Ni nanorods do not have the potential to fulfill the needs of permanent magnets. Co crystallized in the hcp structure has about 1 order of magnitude larger MAE, rendering it a ...

Tunable High-Field Magnetization in Strongly Exchange-Coupled Freestanding Co/CoO Core/Shell Coaxial Nanowires

“…Actually, the study of one dimensional (1D) structures (e.g., nanowires, nanorods, or nanotubes), has attracted ample attention during the last few years, not only from the basic [15][16][17]36,38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] The exchange coupling between the FM core and the AFM or FiM shell gives rise to phenomena similar to the ones observed in thin films and nanoparticles, e.g., loop shifts and coercivity enhancement. [15][16][17]36,38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] However, new effects could emerge if the exchange coupling is induced in such a way that it directly competes with the shape anisotropy.…”

“…[15][16][17]36,38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] However, new effects could emerge if the exchange coupling is induced in such a way that it directly competes with the shape anisotropy.…”

Co@CoSb Core–Shell Nanorods: From Chemical Coating at the Nanoscale to Macroscopic Consolidation

“…11 Recently, several groups have worked on the bottom-up approach of permanent magnets prepared with magnetic nanorods and nanowires. 12,13,14,15 Permanent magnets are a special class of magnetic material that exhibits both high coercivity and high magnetization. 16,17 In particular, cobalt nanorods (Co NRs) with a hcp structure are interesting for this application because when their long axis is parallel to the hcp c axis, they exhibit both shape and magnetocrystalline anisotropies.…”

“…(b) Evolution of the (0002) and (10-10) crystallite sizes of the hcp Co and (10-11),(10)(11)(12) and(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) crystallite sizes of the β-CoSb as a function of the Sb content.…”

“…Crystallite size of the Co 50 @(Co 0.5 Sb 0.5 ) 50 particles measured after annealing under forming gas at different temperature for the Co hcp phase perpendicular and parallel to the c axis calculated from the (10-10) and (0002) line broadening, respectively, and for the (10-11),(10)(11)(12) and(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) reflections of the β-CoSb phase. The fit of the corresponding XRD patterns is provides in the supplementary materials(Fig.…”