[Neutron] Webinar on 27 Feb. on magnetic frustrated phases and the role of spectroscopy @ ESS by Pascale Deen

[Karin Griewatsch]

Dear friends of neutron research,

you are cordially invited to the the lecture by


    Prof. Dr. Pascale Deen (ESS and Nils Bohr Institut, University of
    Copenhagen) on


    Universality, criticality and competing classically magnetic
    frustrated phases: The role of spectroscopy @ ESS


    Friday, 27 February 2026, 11:00 - 12:00 CET

Zoom link: 
https://fz-juelich-de.zoom-x.de/j/68516318781?pwd=PAHgjAtDmLWQ6J5oDfdco92fsVzbyf.1
Meeting-ID: 685 1631 8781
Passcode: 122549

A primary focus of condensed matter physics has been on understanding 
the ordered states within materials.  Associated phase transitions are 
often described by Ginzburg–Landau and renormalisation group theory in 
which a compound is expected to follow a spontaneous symmetry breaking 
across a transition temperature [1,2].

Some systems do not order so easily due to competing interactions that 
cannot be simultaneously satisfied. These conflicting interactions are 
collectively referred to as frustration. This scenario leads to the 
emergence of phases of matter where conventional ordering phenomena are 
significantly suppressed or may never occur, while allowing for the 
emergence of other unusual order and disorder phenomena [3].

We turn our attention to classical magnetic compounds that show a strong 
frustration index and order at a reduced temperature, with respect to 
the Curie Weiss temperature. Novel emergent states of matter are 
observed across a broad temperature range developing below the 
transition temperatures and extend deep into the paramagnetic regime, 
contrary to the theoretical framework of phase transitions.

We present the examples of hexagonal h-YMnO_3 and the quasi 
two-dimensional Cairo pentagonal lattice Bi_2 Fe_4 O_9 as two frustrated 
magnetic compounds that reveal diffuse but correlated magnetic 
correlations and continuous fluctuations below TN and deep into the 
paramagnetic regime. We present the relevant exchange interactions in 
the ordered phase that drive magnetic frustration and the diffuse 
spin-wave spectrum that develops for T < TN and extends for T >> TN 
[4-8]. The neutron scattering signatures can be understood using a 
combination of first-principles density functional calculations and 
spin-dynamics simulations to reveal clusters of emerging order. In the 
case of h-YMnO_3 the magnetic order can be visualised in terms of 
composite trimer magnetoelectric monopoles and toroidal moments, instead 
of individual spins [9]. These results provide new insight into the 
magnetic phase transitions of frustrated systems and the requirements on 
neutron spectroscopy and data analysis for our future instruments to 
access these unusual phases.

[1] K. G. Wilson, Rev. Mod. Phys. 47, 773 (1975). [2] Modern Theory Of 
Critical Phenomena (2018): Taylor & Francis. Shang-keng Ma. [3] C. 
Lacroix, P. Mendels, F. Mila (eds.), Introduction to Frustrated 
Magnetism, Springer (2011).
[4 ] S. Holm et al. Phys. Rev. B 97, 134304 (2018) [5] S. Janas et al. 
Phys. Rev. Lett. 126, 107203 (2021) [6] K. Beauvois. et al. Phys. Rev. 
Lett. 124, 127202 (2020) [7] Manh Duc Le  et al. Park. Phys. Rev. B 104, 
104423 (2021) [8] E. Y. Lenander et al. Phys. Rev. B 113, 014424 (2026) 
[9] T. Tosic et al. Phys. Rev. Lett. 6, L042037 (2024)

The lecture will be recorded, and there is information on data 
protection at 
https://www.sni-portal.de/en/files/kfn-neutron-webinar-data-protection 
<https://www.sni-portal.de/en/files/kfn-neutron-webinar-data-protection>

On the website 
https://www.sni-portal.de/en/user-committees/committee-research-with-neutrons/neutron-webinar 
<https://www.sni-portal.de/en/user-committees/committee-research-with-neutrons/neutron-webinar> 
you will find announcements of upcoming lectures.

We look forward to "seeing" you there!

Kind regards,

Karin Griewatsch (KFN PR)
Mirijam Zobel (KFN chair)
Frank Schreiber and Thomas Gutberlet (Webinar organization)
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