Page 39 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
36-37
Figure 2
Summary of schematic illustrations of magnetic structure and comparison between observed and calculated values of neutron polarisation matrix elements for the pressure-induced ferroelectric phases in multiferroic CuFeO2 [4].
anvil materials. We selected the magnetoelectric (ME) multiferroic compound delafossite CuFeO2 for the first SNP experiment under high pressure. Since the multiferroic ferrite is expected to have various types of magnetic orderings under high pressure, such as collinear spin-density-wave (SDW) and non-collinear spiral structures [2,3], we felt that it was the best candidate for studying the feasibility of SNP analysis under pressure.
First, we confirmed the collinear magnetic structure in
the ICM1 and CM1 phases, and found precise spin- canting angles of 17±2 o and 5±2 o for the ICM1 and CM1 phases, respectively [4]. The magnetic structure parameters are consistent with previously reported
values determined by unpolarised neutron diffraction studies. These results therefore demonstrate that SNP measurements are feasible under high-pressure conditions using a non-magnetic HAC.
Secondly, we succeeded in determining the magnetic structures of the pressure-induced ferroelectric phases in CuFeO2 [4]. The magnetic structures and comparison between experimental and calculated values for the polarisation matrix elements are summarised in figure 2. In the case of the ICM2 phase, an ellipsoidal proper screw
structure with magnetic point group 21’ was determined with an ellipsoidicity ratio of 0.92 ± 0.05. This magnetic symmetry is consistent with observed electric polarisation. We also ascertained the magnetic structure in the ICM3 phase, and found a spiral structure with spins confined
to the ac-plane. Since the k-vector is of triclinic symmetry
[k = (0, 0.34, 0.42)], this magnetic structure (termed the general spiral) possesses both a cycloidal modulation along the c-axis and a proper screw modulation along
the b-axis in the ICM3 phase, which has point group 11’, allowing electric polarisation along a general direction. This study also identified the existence of a phase transition between the ICM1 and ICM4 phases in the intermediate temperature region by measuring the temperature dependence of the polarisation matrix elements at several pressures. One possible magnetic structure in the ICM4 phase is presented as a canted ellipsoidal proper screw with point group 21’.
In conclusion, the present study provides evidence that SNP measurements are viable even in high-pressure conditions [4]. It is our hope that the present technique will allow researchers to elucidate pressure-induced physical phenomena associated with complex magnetic ordering.
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