Our Group's research, examining the magnetic properties of materials, is made up of several themes including Frustrated Magnetism studying pyrochlores, SrRE2O4 materials and garnets, Low-Dimensional Magnetism including molecular magnets and spin chains, and Permanent Magnetism. We have also worked extensively on Topological Magnets including Skymions (please see our pages on the UK Skymion ProjectLink opens in a new window).
In frustrated magnetic materials, the competing interactions cannot be simultaneously satisfied leading to unusual low temperature behaviour. We study the nature of the ground states, as well as unusual excitations, in these frustrated materials.rs
Pyrochlores and spin ice
A2B2O7 pyrochlores consist of two interpenetrating networks of corner linked tetrahedra. With the A sublattice occupied by magnetic rare-earth (RE) ions, the anisotropic crystallographic environments along with quantum and dipolar interactions, lead to magnetic properties that include: order by disorder in Er2Ti2O7 [1], frustrated Heisenberg antiferromagnetism in Gd2Ti2O7 [2], and (quantum) spin ice in, for example, (Ho/Dy)2Ti2O7, [3-5] Yb2Ti2O7 [6] and Nd2Zr2O7 [7].
Spin ice adopts a highly correlated but disordered ground state where the local magnetisation fulfils the divergence free “ice rule" with two spins pointing in and two out of each tetrahedron.
Classical excitations above the spin ice manifold are defects that, by reversing the orientation of a single moment, locally violate the ice rule and so the divergence free condition.
This leads to monopole physics [7], as well as under certain circumstances, fragmentation [7].
Spin ice materials also allowed the testing of the fluctuation-dissipation relation [8].
Selected Warwick papers on pyrochlores and spin ice
[1] J. D. M. Champion et al., Phys. Rev. B 68, 020401 (2003); S. S. Sosin et al., Phys. Rev. B 82, 094428 (2010).
[2] O. A. Petrenko et al., Phys. Rev. B 70, 012402 (2004); Phys. Rev. B 85, 180412(R) (2012), J.A.M. Paddison et al. npj Quantum Mater. 6, 99 (2021).
[3] O. A. Petrenko et al., Phys. Rev. B 68, 012406 (2003).
[4] T. Fennel et al., Phys. Rev. B 70, 134408 (2004); Phys. Rev. B 72, 224411 (2005).
[5] S. Erfanifam et al., Phys. Rev. B 84 , 220404 (2011); Phys. Rev. B 90, 064409 (2014).
[6] L.-J. Chang et al., Nature Commun. 3, 992 (2012); Phys. Rev. B 89, 184416 (2014); E. Lhotel et al., Phys. Rev. B 89, 224419 (2014); A. Mostaed et al., Phys. Rev. B 95, 094431 (2017).
[7] E. Lhotel et al., Phys. Rev. Lett. 115, 197202 (2015); S. Petit et al., Nat. Phys. 12, 746 (2016); C. Paulsen et al., Nat. Phys. 12, 661 (2016); N. Martin et al., Phys. Rev. X 7, 041028 (2017), C. Paulsen et al. Nature Commun. 10, 1509 (2019); M Léger et al. Phys. Rev. Letts. 126, 247201 (2021).
[8] F. Morineau et al., Phys. Rev. Letts. 134, 096702 (2025).
The family of rare-earth (RE) strontium oxides has recently been identified as a useful addition to a small group of compounds where the combination of geometrical frustration, magnetic low dimensionality, and single-ion physics results in the stabilisation of highly unusual ground states at temperatures much lower than those expected from the strength of the magnetic interactions [1].
A wide variety of unconventional magnetic properties observed in these compounds includes a coexistence of long-range antiferromagnetic and short-range incommensurate order in SrEr2O4 [2], two types of short-range order in SrHo2O4 [3], and the absence of the longer-range magnetic correlations in SrDy2O4 down to the lowest experimentally available temperatures [4] and complex magnetic ordering in SrGd2O4 [5].
Selected Warwick papers on SrRE2O4 materials
[1] O. A. Petrenko, Low Temp. Phys. 40, 106 (2014). G. Balakrishnan et al., J. Phys.: Condens. Matter 21, 012202 (2009).
[2] O. A. Petrenko et al., Phys. Rev. B 78, 184410 (2008); T. J. Hayes et al., Phys. Rev. B 84, 174435 (2011); J. Phys. Soc. Jpn. 81, 024708 (2012).
[3] O. Young et al., J. Phys.: Conf.Ser. 391, 012081 (2012); Phys. Rev. B 88, 024411 (2013); Crystals 9, 488 (2019).
[4] T. H. Cheffings et al., J. Phys.: Condens. Matter 25, 256001 (2013), O.A. Petrenko Phys. Rev. B.95, 104442 (2017).
[5] N. Qureshi et al. Phys. Rev. B 105, 014425 (2022); Phys. Rev. B 106 , 224426 (2022).
Gadolinium Gallium Garnet Gd3Ga5O12 is a frustrated magnetic system that exhibits a spin-liquid state above a spin-glass transition at Tg ≈ 0.14 K, that is far below the anti-ferromagnetic interaction strength of ~2 K. Liquid-like magnetic diffuse scattering and persistence of strong spin fluctuations are observed in neutron scattering to low T [1-3]. Suppression of conventional magnetic order allows “hidden” order to emerge in its place [4].
Selected Warwick papers on garnets and spin liquids
[1] O. A. Petrenko et al., Phys. Rev. Lett. 80, 4570 (1998).
[2] J. A. Quilliam et al., Phys. Rev. B 87, 174421 (2013).
[3] N. d’ Ambrumenil et al., Phys. Rev. Lett. 114, 227203 (2015).
[4] J. A. M. Paddison et al., Science 350, 179 (2015).
1D and 2D magnets can be made of chains or ladders of spins, with S ≥ 1/2 and Ising, XY, or Heisenberg character. These systems offer the chance to study unusual magnetic ground states and excitations, and to compare experimental results with theories for model systems.
Molecular Magnets
Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics could make possible the design and growth of bespoke magnetic devices. Progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal.
Coordination polymers are self-organising materials consisting of arrays of metal ions linked via molecular ligands. Here, the choice of initial components dictates the form of the final product, enabling many different polymeric architectures to be obtained. These materials are thus a route to study magnetism.
We use high- and low-field magnetometry to understand the balance of competing interactions in low-dimensional magnet systems constructed from organic and molecular building blocks. Materials can be studied that are near-ideal realisations of model quantum systems, allowing the predictions of quantum theory to be tested in the laboratory.
Selected Warwick papers on molecular magnets
W. J. A. Blackmore et al., New J. Phys. 21, 093025 (2019); Inorg. Chem. 61, 141-153 (2022).
J. Brambleby et al., Phys. Rev. B 92, 134406 (2015); Phys. Rev. B 95, 024404 (2017); Phys. Rev. B 95, 134435 (2017).
V. Chandrasekhar et al., Polyhedron 72, 35 (2014).
Ca3Co2O6 is a rare example of a material where ferromagnetic 1D Ising spin chains are coupled through a much weaker antiferromagnetic (AFM) exchange on a triangular lattice [1-7]. Ca3Co2O6 orders at TN = 25 K in longitudinal amplitude-modulated spin-density-wave (SDW) along the c axis [3-6].
Below TN, there is a transition from the SDW phase to a ferrimagnetic state in low field [7]. The magnetization exhibits a significant field, temperature, and time dependence, and there is a very rare order-order transition from the SDW to a commensurate AFM phase [5]. The application of a magnetic field produces a sequence of equally spaced steps in the magnetization, that are reminiscent of quantum tunnelling of magnetization in molecular magnets.
Selected Warwick papers on Ca3Co2O6
[1] V. Hardy et al., Phys. Rev. B 70, 214439 (2004); Phys. Rev. B 70, 064424 (2004); Phys. Rev. B 70, 104423 (2004).
[2] A. Maignan et al., J. Mater. Chem. 14, 1231 (2004).
[3] O. A. Petrenko et al., Eur. Phys. J. B 47, 79 (2005).
[4] A. Bombardi et al., Phys. Rev. B 78, 100406 (2008).
[5] S. Agrestini et al., Phys. Rev. B 77, 140403 (2008); Phys. Rev. Lett. 101, 097207 (2008); Phys. Rev. Lett. 106, 197204 (2011).
[6] J. A. M. Paddison et al., Phys. Rev. B 90, 014411 (2014).
[7] V. Hardy et al., Phys Rev. B 110, 144443 (2024).
Permanent magnets are pervasive in both established and developing technologies. Found in motors and generators, transducers, magnetomechanical devices and magnetic field and imaging systems, there is a multi-billion pound worldwide market for them.
Rare Earth Transition Metal Magnets
There is a growing demand for stronger and cheaper magnetic materials. Most strong magnets are comprised of rare-earth (RE) and transition metal (TM) atoms arranged in specific crystal structures. The TM element, such as iron or cobalt, helps the ferromagnetism to persist to high temperatures and the RE component, such as neodymium or samarium, is there to generate a large magnetisation which is hard to reorientate away from an 'easy' direction specified by the crystal structure.
One well-known RE - TM permanent magnet class, based on SmCo5 has excellent high temperature performance but the cost and availability of cobalt can be a problem. There is now a concerted effort worldwide to come up with new permanent magnetic materials with improved magnetic characteristics and reduced dependence on critical elements.
Investigation of the physics underlying the principles of design of rare earth - transition metal permanent magnets: EP/M028941/1LLink opens in a new window
In our PRETAMAGLink opens in a new window research programme ran from 2016 to 2020. We used ab-initio magnetic materials modelling, applied and tested in parallel with state-of-the-art sample synthesis, characterisation and experimental investigations.
These studies were aimed at understanding the intrinsic magnetic properties and refining the design principles of RE-TM magnets.
Selected Warwick papers from the PRETAMAG project
C. E. Patrick et al., Phys. Rev. Materials 1, 024411 (2017).
C. E. Patrick et al., Phys. Rev. Lett. 120, 097202 (2018).
S. Kumar et al., Appl. Phys. Lett. 116, 102408 (2020).
S. Kumar et al., J. Phys.: Condens. Matter 32, 255802 (2020).
F. de Almeida Passos et al. Phys. Rev. B.108, 174409 (2023).
