Environmental Engineering Reference
In-Depth Information
where
u
=cos(
π/
12
−
φ
)
;
v
=sin(
π/
12
−
φ
)
,
(1
.
5
.
3
b
)
and any moment deviates from the nearest
b
-axis by the
bunching angle
φ
.At4K,
φ
in Ho is 5.8
◦
, and it increases monotonically with temper-
ature towards the value 15
◦
which characterizes the uniform commen-
surable helix. An increase in temperature also causes an increase in
Q
,
but it was shown by Gibbs
et al.
(1985) that this change does not oc-
cur smoothly and continuously. Instead, the magnetic periodicity tends
to lock in to values commensurable with the lattice, and they proposed
that this is a manifestation of
spin-slip structures
, in which the moments
are arranged in a pattern in which one of the planes in regularly spaced
members of the bunched doublets of Fig. 1.20 is omitted, while the re-
maining plane of the pair orients its moments along the adjacent easy
axis. We shall discuss such structures in more detail in the next chapter.
Fig. 1.20.
The 4
f
contribution to the magnetization of Ho at 4 K,
calculated by a self-consistent mean-field theory and compared with ex-
perimental values. The zero-field structure is a bunched cone, comprising
the illustrated bunched helix in the plane, and a small moment in the
c
-direction. The value of the
c
-axis moment, deduced from neutron-
diffraction measurements, is indicated by the arrow.
Search WWH ::
Custom Search