Page 41 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
38-39
Figure 2
Hysteresis loop measured on the off-diagonal neutron polarisation
matrix element Pyx of the (0 1 0.23) reflection of a (ND4)2[FeCl5(D2O)] crystal. The sample was initially cooled down to 4 K under a negative E-field of 17.5 kV cm-1 and then warmed to 6.5 K. The temperature was kept constant for the subsequent measurements as a function of
a variable E-field. Inset) Domain populations corresponding to the increasing E-field branch of the hysteresis loop. Purple and red bars refer to negative and positive chiral magnetic domains, respectively.
an incident beam polarised in the direction i. In our experimental configuration, the values of the off-diagonal components, Pyx, Pzx, of magnetic reflections like the
(0 1 0.23) depend not only on the magnetic structure
but also on the populations of the possible magnetic domains. In particular, for the same magnetic reflection the off-diagonal terms will give opposite signs for each ‘chiral’ domain, with the sign indicating the handedness (clockwise or counter-clockwise) of the ‘chiral’ magnetic domain. If both domains are equally populated, these measured terms average out and Pyx = Pzx = 0; but if
the domain populations are unbalanced, which can be achieved by cooling in an applied E-field in the direction of the ferroelectric polarisation, finite values are measured which allow the relative fraction of cycloidal domains to be determined [4].
With these measurements, we have demonstrated that the application of an electric field upon cooling results in the stabilisation of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain
with opposite magnetic chirality. Furthermore, we can tune the domain populations by varying the electric
field (figure 2), reaching a full population reversal by switching the field in a complete hysteresis loop. This provides direct proof at the microscopic level of the multiferroicity of (ND4)2[FeCl5(D2O)].
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