We report an unusual in-field behaviour of
SrEr_22O_44
for a magnetic field applied along two high-symmetry directions, the a
and c axes. This geometrically frustrated magnet hosts two
crystallographically inequivalent Er ions, Er1 and Er2, that are both
located on triangular zigzag ladders, but only one site, Er1, forms a
long-range magnetic order at low temperatures in a zero field. We follow
the sequence of peculiar field induced states in
SrEr_22O_44
with detailed single-crystal magnetisation and neutron diffraction
experiments. On application of an external field along the
cc axis,
the long-range antiferromagnetic order of the Er1 ions is rapidly
destroyed and replaced, in fields between 2 and 5 kOe, by a state with
shorter-range correlations. The change in correlation length coincides
with a fast increase in magnetisation during the metamagnetic transition
above which a long-range order is reestablished and maintained into the
high fields. The high-field ferromagnet-like order is characterised by
significantly different magnetic moments on the two Er sites, with the
Er1 site dominating the magnetisation process. For the field applied
parallel to the a axis, in the field range of 4 to 12 kOe, the planes of
diffuse magnetic scattering observed in zero field due to the
one-dimensional correlations between the Er2 moments are replaced by
much more localised but still diffuse features corresponding to the
establishment of an up-up-down structure associated with a one-third
magnetisation plateau. Above 14 kOe, a ferromagnet-like high-field order
is induced following another phase transition.
For this direction of the field, the Er2 moments dictate the succession
of transitions while the Er1 moments remain significantly less
polarised. A complete field polarisation of both Er sites is not
achieved even at 50~kOe for either field direction, reflecting the
strongly anisotropic nature of magnetisation process in
SrEr_22O_44.
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