LOw-dimensional ferromagnetic units towards ValuablE Magneto-Electrics – LOVE-ME

Our strategy includes all stages: from Design to modeling, elaboration, crystal growth, fine structural and physical characterizations of relevant low-D inorganic compounds ... to measurements of intrinsic ME couplings and their understanding by symmetry analysis , and this until extreme condition (high pressure). Establishing the ideal chemical / structural / magnetic context for giant ME effects is a prerequisite for the development of ideal multiferroic compounds. This identification will remove both scientific and technological barriers to the implementation of multiferroics in devices. Regarding the samples already available that will be a source of inspiration, we have magnetically remarkable inorganic compounds: low-D ferromagnetic compounds, immeasurable magnetic chains, large magnetic periodicities, compounds with isolated magnetic chains, FM 2D-Ising compounds, etc. ...

A) we discovered several systems with edge-sharing Co2+ chains, in which internal exchanges are mainly ferromagnetic leading to net 1D magnets. The exchanges between them lead to metamagnetic transitions. The structural differences between the 1D-units themselves, i.e. linear versus zigzag chains, are responsible for different types of spin reversal under an external field, fitting well the intuitive hierarchy of magnetocrystalline anisotropy in the two compounds. In this tandem, the realization of spin-flop and spin-flip respectively picture ideally the intuitive previsions.
B) We were able to show magnetoelectric and multiferroic effects for the compound BaCoAs2O7, both on samples in single crystal and powder form. The coupling values depend on the applied magnetic field, and even if it remains weak is placed in the range of the majority of the phases reported to date. Finally, the “Love-ME” strategy including compounds of low dimensionality with ferromagnetic units seems fruitful for the establishment of couplings.
c) The BaM2(PO4)2 system phase diagram was investigated in this work, as well as the magnetic properties of the novel a form dealing with M= Mn2+ and Fe2+. They show particularly complex phase diagrams with ?, ?, ? variants all based on layered architectures, and weak ferromagnetism. This specificity arises from the strong tendency of large Ba2+ to favour 2D-structures while associated with oxo-anions. The ?? ?’? ß transitions are order disorder ones, suggesting a diversity of related structural polymorphs already well characterized in the M = Co2+ case.
D) The discovery of new polymorphs with the general formula BaM2X2O8 and the existence of a solid solution proved to be very useful for the scientific community, always very keen on the study of phosphate-based systems. The characterizations of these phases still remain to be done, in particular as regards the ME coupling measurements in the case of BaFe2P2O8. We were, however, able to show the MF type-I character of BaMn2P2O8, the first manganese representative in this large family.
E) Dealing with hexaferrites (famous magnets), we were able to set up a one-step synthesis protocol allowing the high purity synthesis of all of the PbFe12-xAlxO19 solid solution. Our PUND measurements and the pyroelectric current measurements clearly show the presence of an electrical polarization in this sample, the solid solution therefore exhibits type-I MF properties by simultaneous coexistence of an electrical polarization and a magnetic hysteresis, at least. up to PbFe5Al7O19.
F) Spinels of the Co5TeO8 family were synthesized. In particular, it was found that Co5TeO8 can be obtained in two structural forms: a centrosymmetric disorder Fd-3m and the other non-centrosymmetric ordinate P4332. The link between the spiral of spins and the magnetoelectric behavior has been demonstrated via small-angle scattering measurements as well as dielectric spectroscopy.

Our objective was to develop multiferroic inorganic compounds (MF), exhibiting magnetoelectric (ME) couplings through an original and thoughtful approach, by dealing with low dimensionality ferromagnetic units (0D blocks, 1D chains, 2D layers ) and the alignment of their spins under field through metamagnetic transitions. In these magnetoelectric compounds, there is a coupling between ferroelectric and magnetic properties which coexist simultaneously. Due to the existence of this coupling, these materials are of great interest, particularly for electronics (sensors, switches, measuring devices) and data storage, making it possible to take advantage of both electrical and magnetic properties. Storage capacity can thus be doubled, information can be written electrically taking advantage of low power consumption, and read magnetically non-destructively.
We thus probed the existence of ME properties among candidates from the work of our team and from some relevant compounds from the literature.
Most of the advanced mechanisms result from a magnetostrictive effect at the passage of the magnetic ordering temperature. Even if this work did not make it possible to put forward compounds exhibiting strong ME couplings at room temperature, which would be the ultimate objective in the context of an exploitation, these results make it possible in all cases to advance the scientific community on the understanding of multiferroics, which ultimately would allow the design of functional materials. At this stage, it is possible to affirm that the strategy based on low dimensionality ferromagnetic units and magnetic frustrations put forward at the origin of this project is successful. However, the reorientation of spins through metamagnetic transients does not appear to be the source of ME properties based on what has been observed on BaCoAs2O7, at least on the compounds we have studied. However, it might be interesting to extend the study of metamagnetic transitions to other compounds.

thesis Bastien Leclercq +
1-O. mentré et al. Mixed valence iron Dumortierite and its intricate topotactic exsolution at mild temperature, Inorg. Chem. 2018, 57
DOI: 10.1021/acs.inorgchem.8b02122
2-Colmont, M. et al.Compressibility of BiCu2PO6: Polymorphism against S = 1/2 Magnetic Spin Ladders
Inorg. Chem., 2018, 57, 6038
3-Nicoud,S. et al. ;Comprehensive Study of Oxygen Storage in YbFe2O4+x (x = 0.5): Unprecedented Coexistence of FeOn Polyhedra in One Single Phase, J. Am. Chem. Soc., 2017, 139, 17031
4-Leclercq, B. et al.Multiferroic BaCoX2O7 (X = P, As) Compounds with Incommensurate Structural Waves but Collinear Spin Ingredients, (2021) Advanced Quantum Technologies, 4 (1), art. no. 2000064, .
5-Nekrasova, et al.,Magnetic hexamers interacting in layers in the (Na,K)2Cu3O(SO4)3 minerals
(2020) Physical Review B, 102 (18), art. no. 184405, .
6-Leclercq, B., et al., Synthesis, structure and magnetic behavior of iron arsenites with hierarchical magnetic units, (2020) Inorganic Chemistry Frontiers, 7 (20), pp. 3987.
7-Leclercq, B.et al., Metamagnetic Transitions versus Magnetocrystalline Anisotropy in Two Cobalt Arsenates with 1D Co2+ Chains
(2019) Inorganic Chemistry, 58 (19), pp. 12609-12617.
8-Leclercq, B.,et al. , Polymorphs, phase transitions and stability in BaM2(PO4)2 M = Mn, Fe, Co systems, (2019) Inorganic Chemistry Frontiers, 7 (1), pp. 239
9-Colmont et al., Compressibility of BiCu2PO6: Polymorphism against S = 1/2 Magnetic Spin Ladders, (2018) Inorganic Chemistry, 57 (10), pp. 6038
10-Jin, Y.et al.M.,Open-framework transition metal fluorophosphates with one-dimensional antiferromagnetic Chains, (2021) Journal of Solid State Chemistry, just accepted
11-Mentré, O.et al. , S = 1/2 Chain in BiVO3F: Spin Dimers versus Photoanodic Properties, (2021) Journal of the American Chemical Society, 143 (18), pp. 6942
12-Zhu, T., et al. Cycloidal Magnetic Order Promoted by Labile Mixed Anionic Paths in M2(SeO3)F2(M = Mn2+, Ni2+), (2021) Inorganic Chemistry, online.
13- Mentre, O.et al. , Mixed-Valence Iron Dumortierite Fe 13.52.22+(As5+O 4-x)8(OH)6 and Its Intricate Topotactic Exsolution at Mild Temperatures, (2018) Inorganic Chemistry, 57 (24), pp. 15093
14- Arévalo-López, et al. The hidden story in BaNiO3 to BaNiO2 transformation: Adaptive Structural series and NiO exsolution, (2019) Chemical Communications, 55 (26), pp. 3717
15-Influence of Polymorphism on the Magnetic Properties of Co5TeO8 Spinel: Stanislav Podchezertsev, et al. Inorg. Chem. 2021, 60, 18, 13990
16-« Magnetodielectric coupling and multi-blocking effect in the Ising-chain magnet Sr2Ca2CoMn2O9 », T. Basu,et al., Journal of Applied Physics 130, 034102 (2021).
17-“Evidence of magnetoelectric effect in Co4(H2O)3(SeO3)4 “, M. Pouponet al.; Journal of Solid State Chemistry, (2019): 120962-202.0.
18- Vanishing of the incommensurate structural modulation of the one-dimensional chain compounds BaCoX2O7 (X = As, P) at high pressure. Ranjana R. Das et al In submission (2021)

Despite their immense interest in the data storage, materials science and fundamental physics, scope and applications of multiferroic compounds to date are hindered by the weakness of the polarization generated by magneto-electric coupling (ME). In this project, we propose an original approach for designing efficient multiferroic inorganic compounds associated with strong ME effects due to their unique topologies. Indeed, LOVE-ME relies on our recent discovery of inorganic compounds built on Low- Dimensional Ferromagnetic units. Dealing with isolated 1D, 2D and pseudo 3D (blocks) topologies carrying large collinear or canted macrospins, and their alignment under an external field they form the ideal playground for giant ME properties, compared to standard type I/type II multiferroics (weak ME coupling / low polarization generated via antiferromagnetic structures of cycloidal type). Our preliminary results on selected systems already suggest giant magneto-polarization. Quantification / optimization / systematic analysis of ME coupling will allow us to update the concepts behind these giants effects and to identify the key physico-chemical parameters. After the implementation of these key parameters in new low-FM D compounds dedicated, we will access to a new generation of high-performance multiferroic compounds.

Indeed, our multi-step strategy includes all stages, from the Design and modeling, Elaboration, Crystal Growth, fine Structural and Physical Characterization of relevant low-D ME, until the measurements and analysis of the intrinsic ME exchanges and responsible symmetries. The establishment of the ideal chemical/structural/magnetic context for giant ME is a prerequisite for the elaboration of optimal multiferroics. This identification will lift both the scientific and technological obstacles to the implementation of multiferroic in the devices.

Concerning the already available samples, source of inspiration, we have recently designed inorganic compounds with remarkable magnetic properties, i.e. rare examples of low-D ferromagnetic compounds with incommensurate structures, large magnetic periodicities, inorganic single-chain-magnets, and 2D-Ising FM. Their structural topologies associated with strong spin-orbit couplings (SOC) and original spin-flip-like transitions give all pre-requisites for enhanced magneto-electric (ME) couplings and electric polarization of magnetic origin (type II multiferroics). These compounds can be considered as intermediate between “molecular magnets” and “standard” inorganic magnets, and then correspond to a novel class of “full-inorganic molecular magnets”, a new paradigm in ME materials. We aim to modify and optimize the already available compounds and to produce new-inspired ones by structural design. For the conception of further compounds within an extended series, we have to handle individual magnetic units and well adapted spacers, according to incremental magnetic dimensionalities. In pseudo-3D phases ( i.e. isolated blocks), room-temperature magnetic orderings are expected, a challenging step of industrial relevance.

In fine, a new generation of advanced multiferroic materials with enhanced ME couplings will be achieved, using a rational methodology. It follows that LOVE-ME covers a broad range of concepts and techniques, including the Design and production of single-crystalline and polycrystalline materials the evidence of exacerbated ME effects across metamagnetic transitions (alignment of the microspins), their understanding by symmetry analysis and the identification of the key-ME parameters.