Page 44 - ILLS Annual Report 2018

MAGNETISM
David Boldrin. British
Imperial College London, UK
‘After my PhD in Chemistry I moved to a post-doc in the Physics department of Imperial College London. I’m interested in all things magnetic, from magnets with exotic quantum effects to
those for energy efficient refrigeration, and in
using neutrons to study them. I love using chemistry to synthesise new things, be it in a lab or in a brewery!’
Exchange anisotropy and multi-k magnetic order in a spin-1/2 third-neighbour kagome antiferromagnet
Time-of-flight spectrometers IN4 and IN5
When quantum spins form networks of corner- sharing triangles—the so-called kagome lattice—conventional magnetic ordering
is suppressed by geometric frustration and quantum fluctuations, leaving room for exotic magnetic states such as quantum spin liquids
or unconventionally ordered states. Nature still provides the best examples of these systems
and therefore remains the best resource when searching for quantum frustration. Inelastic neutron scattering measurements on IN4 and IN5 on the natural mineral vesignieite, which forms a spin-1/2 kagome lattice, show that
the antiferromagnetic exchange interaction between third-nearest neighbour spins dominates all other interactions. The opening of a gap
in one of the spin-wave branches reveals
a tiny symmetric exchange anisotropy that favours an unconventional coplanar multi-k magnetic structure. This structure is energetically degenerate with the octahedral state predicted by theory for isotropic Heisenberg exchange.
Figure 1
The kagome lattice of vesignieite showing the ordered arrangement of the Cu2+ S = 1⁄2 spins (coloured arrows) of the hexagonal magnetic structure. The black line is the crystallographic unit cell and the green line the magnetic unit cell. The top right inset illustrates how the spins point towards the vertices of a hexagon.
AUTHORS
D. Boldrin and A.S. Wills (University College, London, UK) H.C. Walker, P. Manuel and D.D. Khalyavin (ISIS, UK)
B. Fåk, E. Canévet and J. Ollivier (ILL)
ARTICLE FROM
Phys. Rev. Lett. (2018)—doi: 10.1103/PhysRevLett.121.107203
REFERENCES
[1] L. Balents, Nature 464 (2010) 199
[2] L. Messio, C. Lhuillier and G. Misguich, Phys. Rev. B 83 (2011)
184401
[3] T.-H. Han, J.S. Helton, S. Chu, D.G. Nocera, J.A. Rodriguez-Rivera,
C. Broholm and Y.S. Lee, Nature 492 (2012) 406
[4] B. Fåk, E. Kermarrec, L. Messio, B. Bernu, C. Lhuillier, F. Bert,
P. Mendels, B. Koteswararao, F. Bouquet, J. Ollivier, A.D. Hillier, A. Amato, R.H. Colman and A.S. Wills, Phys. Rev. Lett. 109 (2012) 037208
In geometrically frustrated magnets, the topology of the lattice prevents conventional long-range order. Instead, exotic and degenerate ground states are expected, such as quantum spin liquids [1] and unconventional non-collinear multi-k magnetic structures [2]. In the kagome lattice (figure 1), the most frustrated of the simple two-dimensional lattices, theoretical work focused initially on the nearest-neighbour Heisenberg model with isotropic spins, as realised in the emblematic mineral Herbertsmithite, γ-ZnCu3(OH)6Cl2 [3]. More recent work has addressed the case of anisotropic interactions as well as interactions beyond first-neighbours. An example is the mineral kapellasite, α-ZnCu3(OH)6Cl2, where competition between first- and third-neighbour exchange leads to a
chiral quantum spin liquid state with fluctuations born of an unconventional but classic cuboctahedral cuboc2 phase [4].
We studied the natural mineral vesignieite, BaCu3V2O8(OH)2, a rare realisation of a fully stoichiometric kagome lattice of quantum spins S = 1⁄2. The material orders antiferromagnetically at TN = 9 K, ten times below the Weiss temperature, showing the high degree of magnetic frustration. Inelastic neutron scattering measurements were performed on a fully deuterated powder sample using the IN5 and IN4 time-of-flight spectrometers. In addition to the spin excitations shown in figure 2a, magnetic Bragg peaks were
observed with the unusual propagation vector k = (1⁄2,0,0),
ANNUAL REPORT 2018
J Jd J2 3 J1
b
a
Cu1 Cu2