(NH₄)₂[FeCl₅(H₂O)] is a rare molecular magnet exhibiting coupled magnetic and ferroelectric properties as a function of temperature and applied magnetic field [1-4]. Unlike its counterpart compounds where NH₄ group is replaced by K, Cs, and Rb, (NH₄)₂[FeCl₅(H₂O)] is the only system in this family that exhibits magnetically induced ferroelectricity at low temperature, suggesting that NH₄ plays a critical role in the unusual properties of (NH₄)₂[FeCl₅(H₂O)]. Neutron scattering is a powerful tool to study the magnetism of a materials. In this talk, I will present results of neutron scattering studies on deuterated (NHD₄)₂[FeCl₅(D₂O)] single crystals that provide insights on the nature of the coupled phenomena. Both elastic and inelastic neutron scattering experiments were performed at the High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory to determine the magnetic structures and investigate the dynamics in this material. Our inelastic neutron scattering results also reveal the role the ion played in the intriguing properties observed in (NH₄)₂[FeCl₅(H₂O)].

[1] M Ackermann et al, New Journal of Physics 15, 123001 (2013).

[2] Jose Alberto Rodriguez-Velamazan, et al, Scientific Reports, 5:14475, DOI:10.1038/srep14475; Phys. Rev. B 95, 174439 (2017).

[3] W. Tian et al, Phys. Rev. B 94, 214405 (2016); Phys. Rev. B 98, 054407 (2018).

[4] Amanda J. Clune et al, npj Quantum Materials 4:44 (2019)

Acknowledgments: Research conducted at ORNL's Spallation Neutron Source and High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy.

In topological materials science, the aim is to find pronounced phenomena rooted in the concepts of topology in new materials, and harness them for novel and robust functions. Promising materials classes include magnetic materials hosting nanoscale magnetic skyrmions, or Dirac and Weyl semimetals, which are hallmarked by topological invariants in real- or reciprocal spaces, respectively. With recent attention focused on magnetic topological materials, here we consider the question if novel functionalities may be found in systems with electronic and magnetic structures that are both topologically nontrivial, and where they coexist and may be coupled.

In this context, I will present our recent experimental work on the polar tetragonal magnet CeAlGe [1]. This system was predicted recently to be an easy-plane ferromagnetic type-II Weyl semimetal, with the magnetic and electrical properties little-explored. We combine magnetometry, neutron scattering and electrical transport measurements to reveal CeAlGe as a host of incommensurately-modulated multi-k magnetic phases with a nanometric length-scale. Application of modern magnetic symmetry analysis methods for refining neutron diffraction data reveals the ground state magnetic structure contains topological merons and antimerons, which can be thought of as 'half-skyrmions' carrying half-integer topological charge. While the ground state carries no topological Hall effect, the effect emerges for a phase induced by an intermediate field along the polar c-axis, which may be generated by a magnetic structure containing anti-meron pairs. We discuss the implication for the existence of such magnetic phases in Weyl semimetals and the possibilities for new functionalities.

[1]. P. Puphal et al... and J.S. White, Phys. Rev. Lett. 124, 017202 (2020)

The magnetetoelectric CaBaCo₄O₇ compound offers an interesting scenario to study frustrated magnetic configurations. The Co²⁺ and Co³⁺ ions in tetrahedral oxygen coordination form a three-dimensional framework of interconnected triangular and kagome layered arrangements [1]. The compound becomes ferrimagnetic below 60 K, and displays a strong increase of electric polarization of 17 000 μC/cm², driven by exchange-striction. In this work, we present our results on the thermal evolution of magnetic and crystallographic properties of powder samples of Ca_1-xSr_xBaCo₄O₇ (x = 0, 0.02, 0.05, 0.07) to study the effect of substitution at the Ca site. We will show that low doping levels (<10 at.%) change quite dramatically the magnetic behavior of the compound, as observed in magnetization vs. temperature measurements. Combined with extensive use of Neutron Power Diffraction we analysed the evolution of the magnetic order as a function of temperature and composition of the samples. The reported non collinear ferrimagnetic order of the parent compound is only retained for the lowest doping level x = 0.02 and is accompanied by a strong unit cell distortion. In turn, further Sr doping blurs this distortion and favors other magnetic arrangements. In the temperature range 62 K < T < 82 K, samples with x ≥ 0.02 show a plateau in the magnetization. By using the superspace group theory and its implementation in the Rietveld refinement of neutron diffraction data, we have solved the incommensurate magnetic structure that appears at these intermediate temperatures. The magnetic order has a propagation vector k = (1/2, 1/2, g) with g ≈ 0.02 and it belongs to the superspace group Pna2₁1’(1/2, 1/2, g)qq0s. This phase corresponds to a modulated spin structure with distinct behaviors of the triangular and kagome cobalt sites and could explain previous findings reported in the literature for other substitution sites in the CaBaCo₄O₇ family.

[1] V. Caignaert, V. Pralong, A. Maignan, B. Raveau. Solid State Communications 149 ,453 (2009)

In crystal engineering hydrogen and halogen bonds have proven to be very valuable crystal engineering tool for design of supramolecular architectures by self-assemblies of small building blocks, shaping their final architectures and determine the resultant topology and ultimately controlling many physical properties. [1] A number of supramolecular synthetic strategies to harness their potential have already been developed, but only for purely organic system. Although metal-organic supramolecular assemblies exhibit many technologically important properties, their design is often difficult to predict because introduction of metal cations and charge-balancing entities into metal-free solids commonly disrupt well-established connectivity of the key functional groups.[2] This is especially pronounced for magnetic metal-organic systems where magnetic behaviour not only depends on fine tuned parameters in the crystal packing but as well on the functional group, nature of the acceptor (A) and donor (D) atoms, lengths and angles of non-covalent interactions. When all of this is taken into account, targeting supramolecular architectures with desired magnetic properties becomes even more difficult and multiplex. Therefore, in those systems hydrogen and halogen bonds are rarely explored as magnetic exchange pathways or as a crystal engineering tool for directing magnetic behaviour. As well as molecular interactions, in field of molecular magnetism, metal-organic systems are not even approximately investigated as a miscellaneous copper oxide compounds, especially compounds with pyrazine and pyridine based ligands in which copper is bridged by halogen element. So far it is known that pyrazine and pyrazine derivates can be mediators of magnetic exchange within dimers, linear chains and two-dimensional lattices, and they are used in preparation of low-dimensional magnetic materials.[3] However, some insight in functional group effects on magnetic exchange of these systems in literature is not observed.

In order to understand the magnetic behaviour of crystalline coordination compounds with general formula (n-Rpz/pym/py)CuX₂and correlate structural features (in particular, functional groups, chemical linkages, bond length and angles) to magnetic exchange, we presented statistical and magneto-structural analysis of crystallography database and prepared a series of 1D polymeric chain copper(II) halides with pyrazine-, pyrimidine- and pyridine based ligands bearing the lactam or halogen functionality as a supramolecular synthetic vector. For all obtained coordination compounds ([CuCl₂(2-NH₂pz)₂]_n, [CuCl₂(2-pyz)₂]_n, [CuCl₂(4-pym)₂]_n, [CuBr₂(4-pym)₂]_n, [CuBr₂(3-Clpy)₂]_n, [CuBr₂(3-Brpy)₂]_n and [CuBr₂(3-Ipy)₂]_n) temperature dependence of magnetization M(T) was measured using SQUID magnetometer in the temperature range 2‒300 K. Linear dependence between magnetization and magnetic field allows usage of the linear magnetic susceptibility, χ. In accordance with crystal structure, we applied approach of Bonner–Fischer and modelled entire M(T) curves for all obtained compounds using spin chain of antiferromagnetically interacting neighbouring Cu²⁺ ions along structural chains. [3] These results are compared and discussed within structural features influence on magnetic superexchange J.

[1] Bernstein J.; Crystal growth, polymorphism and structure-property relationships in organic crystals properties, J. Phys. D: Appl. Phys. 1993, 26, B66

[2] Desiraju, G.R. Crystal engineering: a holistic view, Angew. Chem. Int. Ed. 2007, 46, 8342- 8356

[3] Herringer S. N.; Longendyke A. J.; Turnbull M. M.; Landee C. P.; Wikaira J. L.; Jameson G. B.; Telfer S. G. Synthesis, structure, and magnetic properties of bis(monosubstituted- pyrazine)dihalocopper(ii) Dalton Trans. 2010, 39, 2785–2797 [4] O. Kahn, Molecular magnetism, Wiley-VCH, 1992.

Keywords: supramolecular assemblies of copper(II) complexes, antifferomagnetic spin chains, intermolecular interactions, magneto-structural correlations

Lacunar spinel is a class of compounds that are derivative of the spinel family, AB₂X₄, with some vacancies at the A-site. They are very interesting both crystallographically and with respect to the physical properties as several members exhibit structural phase transition from F-43m to R3m and long-range magnetic ordering at low-temperature. Having R3m (C_3v symmetry) space group along with long-range magnetism make these compounds interesting in the aspect of spintronics, as they may host Néel-type skyrmions. One such very well-studied compound is GaV₄S₈ that hosts skyrmion with individual size of 22 nm. Here, we report a study on a different member of the lacunar spinel family, GaMo₄Se₈ that is expected to have smaller skyrmions size. We performed high-resolution powder neutron diffraction across the structural phase transition (T_S = 51 K). Through Rietveld refinement, it is found out that there are two coexisting low-temperature crystal structures with space group R3m (major phase) and Imm2 (minor phase), which is very unique only for GaMo₄Se₈. We propose an explanation for the coexisting of both crystal structures through mode-crystallographic and bond-valence sum analysis and postulate that the large strain in the rhombohedral structure is alleviated by the formation of the orthorhombic phase with larger displacive distortion amplitude. Furthermore, we have carried out magnetization measurements and performed magnetic critical behavior analysis. We find that the magnetic transition in GaMo₄Se₈ is close to a tricritical mean-field model, and the analysis of the magnetic phase diagram using magneto-entropic map revealed a positive phase-field which might be an indication of the presence of complex magnetic structures such as cycloid or skyrmions states.

The orthorhombic iron- and chromium-based perovskites (orthoferrites RFeO₃ and orthochromites RCrO₃, where R is a lanthanide) have been studied for a long time for their wide variety of magnetic properties [1, 2]. Given the flexibility in chemical composition allowed within the perovskite structure, there are plenty of opportunities for cation substitutions in the search for novel properties. In this work, several new quaternary perovskites were studied in an attempt to tune different magnetic properties. Most of these materials display a magnetic transition called spin reorientation (SR), which is outlined on Fig. 1. To evaluate the diverse magnetic transitions, neutron powder diffraction (NPD) experiments were performed in the instruments HRPT (Paul Scherrer Institute) and D1B and D2B (Institut Laue Langevin).

Among the studied compounds, the perovskites RCr_0.5Fe_0.5O₃ (R = Tb, Dy, Ho, Er, Tm, Yb, Lu) display magnetic properties which are mainly determined by the lanthanide cation, particularly at low temperatures. These materials also retain similarities with the corresponding orthochromites and orthoferrites, providing a framework to understand their magnetic properties. Other interesting findings in these perovskites include negative thermal expansion, metamagnetic transitions and magnetization reversal (MR) [3, 4]. The next step was assessing different strategies for the tuning of the magnetic transition temperatures, with substitutions in the A and B sites of the perovskite structure (Sm_1-xTm_xFeO₃ and TmCr_1-xFe_xO₃, respectively). Both systems enabled the tuning of their magnetic transitions as a function of composition. In the former, the SR transition was successfully shifted to room temperature, while in the latter, three different magnetic transition temperatures (T_SR, T_compensation of MR and T_Néel) could be tuned.

This work covers a wide compositional space within the mixed orthochromite-orthoferrite system, exploring many interesting and puzzling magnetic properties. In all cases, NPD was used along extensive magnetization measurements to understand the different magnetic transitions in detail.