Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU
Professor J.M.D. Coey, School of Physics and CRANN, Trinity College Dublin
Prof Hariom Jani
Abstract
Amorphous yttrium-cobalt thin films a-YxCo1-x , where nonmagnetic yttrium serves to stabilize cobalt in an amorphous structure, combine ultra-soft, anhysteretic wandering axis ferromagnetism due to exchange averaging of the local random uniaxial anisotropy at Co sites, with a remarkably high (but inaccessible) Curie temperature thought to approach 2000 K as x —> 0. The thin films with x < 0.25 may be ideal ferromagnets for use in future room-temperature orbitronic devices based on the orbital Hall effect
Thin films where Y is replaced by Gd, and some Co is replaced by Fe are compensating ferrimagnets that combine best properties of ferromagnets for spintronics (spin polarization, AHE, Kerr effect) with the best properties of antiferromagnets (high magnetic resonance frequency, insensitivity to stray fields). The amorphous films were once used as magneto-optical media for compensation-point or Curie-point writing. More recently, they were found to exhibit single-pulse all-optical toggle switching following ultra-fast demagnetization by unpolarized picosecond laser pulses. Only S-state Gd- (or Mn-) based ferrimagnetic metals are known to are known to exhibit this effect. Related Dy or Tb alloys with a large 4f orbital moment only exhibit a few cycles of limited partial switching
The a-DyxCo1-x and a-TbxCo1-x materials are random easy-axis sperimagnets with negative 4f quadrupole moments and local magnetic moments scattered in orientation but coupled antiparallel to a ferromagnetic cobalt subnetwork whose coercivity diverges at compensation. However random second-order axial crystal-field theory is inadequate to explain the low-temperature magnetization of these two sperimagnets, or the differences between them that arise from fourth order crystal field terms of opposite sign due to the opposite the sign of the 4f hexadecapole moments of Dy and Tb.
Sperimagnetiism in random hard axis a-TmxCo1-x (Tm has a positive 4f quadrupole moment) is quite different; coercivity is always small and does not diverge at compensation. Surprisingly, the low-temperature reduced rare earth moment 〈m〉/m0) is bigger for random hard axes than random easy axes.
Refreshments will be served after the Seminar.