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<p>Dear friends of neutron research,<br>
<br>
you are cordially invited to the the lecture by<br>
</p>
<h2>Prof. Dr. Pascale Deen (ESS and Nils Bohr Institut, University
of Copenhagen) on</h2>
<h2>Universality, criticality and competing classically magnetic
frustrated phases: The role of spectroscopy @ ESS</h2>
<h2>Friday, 27 February 2026, 11:00 - 12:00 CET</h2>
<p>Zoom link: <a rel="noreferrer" target="_blank"
href="https://fz-juelich-de.zoom-x.de/j/68516318781?pwd=PAHgjAtDmLWQ6J5oDfdco92fsVzbyf.1"
class="moz-txt-link-freetext">https://fz-juelich-de.zoom-x.de/j/68516318781?pwd=PAHgjAtDmLWQ6J5oDfdco92fsVzbyf.1</a><br>
Meeting-ID: 685 1631 8781<br>
Passcode: 122549</p>
<p><img src="cid:part1.gwxcgX5r.m0aovCWF@sni-portal.de" alt=""
width="121" height="144" class=""></p>
<p>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]. <br>
<br>
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].<br>
<br>
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.<br>
<br>
We present the examples of hexagonal h-YMnO<sub>3</sub> and the
quasi two-dimensional Cairo pentagonal lattice Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub>
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<sub>3</sub> 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. <br>
<br>
[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). <br>
[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) <br>
<br>
The lecture will be recorded, and there is information on data
protection at <a
class="v1moz-txt-link-freetext moz-txt-link-freetext"
href="https://www.sni-portal.de/en/files/kfn-neutron-webinar-data-protection"
target="_blank" rel="noreferrer">https://www.sni-portal.de/en/files/kfn-neutron-webinar-data-protection</a><br>
<br>
On the website <a
class="v1moz-txt-link-freetext moz-txt-link-freetext"
href="https://www.sni-portal.de/en/user-committees/committee-research-with-neutrons/neutron-webinar"
target="_blank" rel="noreferrer">https://www.sni-portal.de/en/user-committees/committee-research-with-neutrons/neutron-webinar</a>
you will find announcements of upcoming lectures.<br>
<br>
We look forward to "seeing" you there!<br>
<br>
Kind regards,<br>
<br>
Karin Griewatsch (KFN PR)<br>
Mirijam Zobel (KFN chair)<br>
Frank Schreiber and Thomas Gutberlet (Webinar organization)</p>
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