Abstract
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.
Highlights
In SrEr2O4, the magnetic moments on the Er1 sites prefer to point along the c axis [13], and below TN = 0.75 K, they form a fully ordered antiferromagnetic state characterised by a q = 0 propagation vector as shown in Fig. 1b [14, 15]
We show below that for the two cases considered in this paper, H a and H c, the magnetisation process is mostly governed by the rearrangements of the magnetic moments on the Er2 and Er1 sites, respectively, while the contributions to the magnetisation from the other site are less significant
Positions have previously been seen in single crystals neutron diffraction, using both polarised and unpolarised neutrons [15]
Summary
Single crystal samples of SrEr2O4, grown by the floating zone technique using an infrared image furnace as previously reported [21], were studied on the WISH neutron diffractometer [22] at the ISIS facility at the Rutherford Appleton Laboratory (United Kingdom) [23,24] as well as on the diffractometers D9 and D10 [25] at the Institut Laue-Langevin, Grenoble, France. Each reflection was measured twice, with and without applied field in order to isolate the magnetic signal by direct subtraction. The cuboid-shaped single-crystal sample of approximate dimensions 2.5×1×3 mm along the principal crystallographic axes was glued to the tip of a Cu pin with the a axis vertical, i.e. parallel to the applied magnetic field. Data sets of 151 unique integer reflections were collected at 50 mK and at magnetic field values of 0, 6, 11 and 14 kOe, while a reduced data set of Figure 2: Single crystal neutron diffraction intensity maps of the (0 k l) plane measured in SrEr2O4 single crystal sample at T = 60 mK in different fields applied parallel to the a axis. The magnetic field was aligned along the a axis, which was parallel to one of the short sides of the cuboid
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