Magnetism of noncolinear amorphous DyCo3 and TbCo3 thin films
arxiv(2023)
摘要
The magnetization of amorphous DyCo3 and TbCo3 is studied by magnetometry,
anomalous Hall effect and magneto-optic Kerr effect to understand the
temperature-dependent magnetic structure. A square magnetic hysteresis loop
with perpendicular magnetic anisotropy and coercivity that reaches 3.5 T in the
vicinity of the compensation temperature is seen in thin films. An anhysteretic
soft component, seen in the magnetization of some films but not in their Hall
or Kerr loops is an artefact due to sputter-deposition on the sides of the
substrate. The temperature-dependence of the net rare earth moment from 4-300K
is deduced, using the cobalt moment in amorphous YxCo1-x. The single-ion
anisotropy of the quadrupole moments of the 4f atoms in the randomly-oriented
local electrostatic field gradient overcomes their exchange coupling to the
cobalt subnetwork, resulting in a sperimagnetic ground state where spins of the
noncollinear rare-earth subnetwork are modelled by a distribution of rare earth
moments within a cone whose axis is antiparallel to the ferromagnetic axis z of
the cobalt subnetwork. The reduced magnetization (Jz)/J at T=0 is calculated
from an atomic Hamiltonian as a function of the ratio of anisotropy to exchange
energy per rare-earth atom for a range of angles between the local anisotropy
axis and -z and then averaged over all directions in a hemisphere. The
experimental and calculated values of (J-z)/J are close to 0.7 at low
temperature for both Dy and Tb. On increasing temperature, the magnitude of the
rare earth moment and the local random anisotropy that creates the cone are
reduced; the cone closes and the structure approaches collinear ferrimagnetism
well above ambient temperature. An asymmetric spin flop of the exchange-coupled
subnetworks appears in the vicinity of the magnetization compensation
temperatures of 175K for amorphous Dy0.25Co0.75 and 200 K for amorphous TbCo3.
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