Dr. Michael Coey

Abstract:

Intermetallic compounds of 4f and 3d elements, especially Fe and Co were intensively investigated in the 20^th century, and the roles of crystal structure, exchange and crystal field  were elucidated.  Important consequences were  rational design permanent magnets with strong uniaxial anisotropy using an appropriate light rare earth (SmCo₅, Nd₂Fe₁₄B), uniaxial ferrimagnets with compensation (TbCo₃) and cubic ferrimagnets with strong magnetostriction but no net anisotropy (Tb_0.3Dy_0.7)Fe₂. Analogous compounds with the nonmagnetic rare earth yttrium were invaluable for isolating the 3d contribution to the magnetism.  When high-quality metallic thin films began to be produced by sputtering 1970s, it was found that some 4f-3d binaries could be deposited as amorphous films which exhibited perpendicular magnetic anisotropy. Ferrimagnetic Gd-Fe-Co magneto-optic recording media with compensation point writing were an interesting, if commercially limited development [1]. Amorphous alloys with strongly anisotropic rare earth elements tend to have magnetic ground-states where the rare earth moments freeze along randomly-oriented axes, with a net magnetic moment — parallel or antiparallel to that of Fe or Co [2], and a new series of random noncollinear magnetic structures was discovered.

A revival of interest in these materials has been spurred by several developments. One is the reappraisal of transverse magnetotransport  (anomalous, spin and orbital Hall effects) in terms of real- or reciprocal-space Berry curvature. Atomic-scale simulations of atomic and magnetic structures have improved greatly in the past 50 years. Also, the observation in 2013 of ultra-fast single-pulse all-optical toggle switching in thin films of perpendicular ferrimagnetic amorphous Gd_x(FeCo)_1-x with x ≈ 0.25 opened new perspectives for magneto-optic applications, and understanding of the transient collapse of magnetization and anisotropy [3]. Proposals for using these thin films for as magnetic switches modulate an optical signal at frequencies of up 100 GHz raises the possibility of multiplexing the existing  global fibre-optic communication system and increasing its capacity by a factor of five. 

A new study of spin and orbital magnetism and magnetotransport in amorphous R­­_1-x­Co_x will be presented, which allows a reassessment of the noncollinear magnetic structures of amorphous alloys with a heavy rare earth, Th, Dy or Er and its temperature dependence and field dependence in the vicinity of the magnetic compensation point T_comp. The corresponding amorphous yttrium alloys are remarkably soft ferromagnets despite the strong random anisotropy experienced by the cobalt, which is exchange-averaged to vanishing point on account of the exchange field and extrapolated Curie temperatures in excess of 1000 K  for x ≈ 0.9. This makes the amorphous Y_1-x­Co_x alloys ideal materials with which to explore new ideas of orbital electronics, where the orbital polarization of the electrons is the key to magnetoelectronics, via the orbital Hall effect (OHE), rather than the spin polarization as in conventional spintronics [5] and effects may be are much larger. The orbital moment in random close-packed amorphous cobalt may exceed 0.5 Bohr magnetons per atom. The quantities of cobalt and rare earth metals required for the new thin-film functionality are miniscule, of order a milligram for a wafer with a trillion orbital switches.