Page 20 - ILL Annual Report 2019

MAGNETISM
Navid Qureshi. French
The ILL
‘After finishing my PhD thesis at the ILL in 2008,
I returned as an instrument scientist in 2014. My main research fields are frustrated magnetism, multiferroic materials and chiral structures, which I study using polarised neutrons and spherical neutron polarimetry in particular.’
Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry
Spin-polarised diffractometer D3
CuO is a magnetic system that has been extensively investigated for more than 30 years [1, 2] as a building block
of high-temperature superconductors.
It is the only known binary multiferroic compound to date and has a high transition temperature (230 K [1]) into the multiferroic state. Compared with other prototype multiferroic materials it has close analogies with the sequence of magnetic phase transitions and the nature of the phases involved. However, the nature and even the existence of
the high-temperature incommensurate— the so-called AF3 —phase has been strongly debated, both experimentally and theoretically. Here, we present the first neutron scattering study in more than three decades able to overcome the difficulties of observing this phase, which includes a very small ordered magnetic moment (~0.06 μB) and a stability range of only 0.5 K below an elevated TN.
Figure 1
Polarisation matrix elements as a function of temperature clearly revealing the magnetic phase transitions.
AUTHORS
N. Qureshi (ILL)
E. Ressouche (Grenoble University UGA and CEA, Grenoble, France) A.A. Mukhin (Russian Academy of Science, Moscow, Russia)
M. Gospodinov (Bulgarian Academy of Sciences, Sofia, Bulgaria)
V. Skumryev (University of Barcelona and Institució Catalana de Recerca, Barcelona, Spain)
ARTICLE FROM
Submitted to Science Advances
REFERENCES
[1] J.B. Forsyth, P.J. Brown and M.B. Wanklyn, J. Phys. C. Solid State Phys. 21 (1988) 2917
[2] B.X. Yang et al., Phys. Rev. B 39 (1989) 4343 [3] N. Qureshi, J. Appl. Cryst. 52 (2019) 175
The key to our success was the use of spherical neutron polarimetry, which is extremely sensitive to the direction of magnetic moments and yields conclusive results with just a few magnetic Bragg peaks. By analysing the neutron spin after the scattering process with different initial polarisation directions one can deduce the three-dimensional rotation and eventual (de)polarisation of the neutron spin, which is summarised in the polarisation matrix.
Using this technique, we first confirmed the extensively studied magnetic structures in the AF1 and AF2 phases. Then, by following specific polarisation matrix elements—which are susceptible to change in the magnetic structure—as a function of temperature, we found unambiguous proof of the existence of the elusive AF3 phase in CuO, as shown in figure 1.
In the chosen geometry with the sample’s b-axis mounted vertically, the Pyy element is -1 if the magnetic moment is aligned along b and +1 if it is in the a-c plane. The abrupt change in Pyy at 213 K is indicative of the first-order magnetic phase transition from the commensurate collinear AF1 structure to the incommensurate helical structure AF2 (see figures
2a and 2b, respectively). Note that the non-zero Pxy term in the AF2 phase states that there is an imbalance between
(0 0 0)+q
AF1 AF2 AF3
ANNUAL REPORT 2019
0.2 0 -0.2 -0.4 -0.6 -0.8
-1 200
205 210 215
220 225 230
T (K)
Pif