The electronic and magnetic properties of a set of mononuclear terbium(III) and dysprosium(III) complexes with two tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands are reported. Two primary coordination geometries are observed, depending on the length of the linker between the 1,2-HOPO donor moieties and the resulting arrangements of the linker. Fine details of the magnetic circular dichroism (MCD) spectra of the dysprosium(III) complexes illustrate differences in the splitting of the J multiplets and allow for a thorough ligand field analysis. High frequency electron paramagnetic resonance (HF-EPR) studies of the terbium(III) complexes give insight into the composition of the ground states. Ab initio calculations are utilized to rationalize the experimental results and further illustrate the effect of the structural features on the electronic and magnetic properties of the different complexes.

Thermodynamic properties as well as low-energy magnon excitations of S = 1 honeycomb-layered Na3Ni2SbO6 have been investigated by high-resolution dilatometry, static magnetisation, and highfrequency electron spin resonance studies in magnetic fields up to 16 T. At TN = 16.5 K, there is a tricritical point separating two distinct antiferromagnetic phases AF1 and AF2 from the paramagnetic regime. In addition, our data imply short-range antiferromagnetic correlations at least up to ∼ 5·TN. Well below TN, the magnetic field BC1≈ 9.5 T is needed to stabilize AF2 against AF1. The thermal expansion and magnetostriction anomalies at TN and BC1 imply significant magnetoelastic coupling, both of which associated with a sign change of ∂L/∂B. The transition at BC1 is associated with softening of the antiferromagnetic resonance modes observed in the electron spin resonance spectra. The anisotropy gap ∆ = 360 GHz implies considerable uniaxial anisotropy. We conclude the crucial role of axial anisotropy favoring the AF1 spin structure over the AF2 one. While the magnetostriction data disprove a simple spin-flop scenario at BC1, the nature of a second transition at BC2 ≈ 13 T remains unclear. Both the sign of the magnetostriction and Grüneisen analysis suggest the short-range correlations at high temperatures to be of AF2-type.

We report the magnetic phase diagram of single-crystalline LiFePO 4 in magnetic fields up to 58 T and present a detailed study of magneto-elastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at T N in the thermal expansion coefficient α imply pronounced magneto-elastic coupling. Quantitative analysis yields the magnetic Grüneisen parameter γ mag = 6.7(5) · 10 −7 mol/J. The positive hydrostatic pressure dependence dT N /dp = 1.46(11) K/GPa is dominated by uniaxial effects along the a-axis. Failure of Grüneisen scaling below ≈ 40 K, i.e., below the peak temperature in the magneto-electric coupling coefficient [1], implies several competing degrees of freedom and indicates relevance of recently observed hybrid excitations [2].A broad and strongly magnetic-field-dependent anomaly in α in this temperature regime highlight the relevance of structure changes. Upon application of magnetic fields B||b-axis, a pronounced jump in the magnetisation implies spin-reorientation at B SF = 32 T as well as a precursing phase at 29 T and T = 1.5 K. In a two-sublattice mean-field model, the saturation field B sat,b = 64(2) T enables the determination of the effective antiferromagnetic exchange interaction J af = 2.68(5) meV as well as the anisotropies D b = −0.53(4) meV and D c = 0.44(8) meV. PACS numbers:In addition to exceptionally high applicability of lithium orthophosphates [3][4][5] for electrochemical energy storage in Li-ion batteries, competing magnetic interactions, magnetic anisotropy and coupling of spin and electric degrees of freedom yield complex magnetic behaviour in LiM PO 4 (M = Mn, Fe, Co, Ni). The rich resulting physics is, e.g., demonstrated by ferrotoroidicity in LiCoPO 4 and LiNiPO 4 [6-8]. In general, depending on the actual transition metal, LiM PO 4 develops long-range antiferromagnetic order at low temperatures and exhibits a large magneto-electric effect in the magnetically ordered phase [1, 9, 10]. The known magnetic phase diagrams of this family are rather complex, featuring incommensurate spin configurations, frustration, and usual magnetic excitations [11-17]. Magnetic phase diagrams have been reported for all lithium orthophosphates [15-17] except for LiFePO 4 . At B = 0 T, LiFePO 4 develops long-range antiferromagnetic order of S = 2 spins of the magnetic Fe 2+ -ions below T N = 50 K [18]. The ordered moment amounts to 4.09 µ B [1, 19] and the spins are mainly directed along the crystallographic b-axis (space group P nma) [19]. Notably, the ground state features a collinear rotation of the spins towards the a-axis as well as spin canting along the c-axis with an overall rotation of the ordered moments of 1.3(1) • off the b-axis [1, 2]. The observed spin canting suggests the presence of Dzyaloshinsky-Moriya (DM) interactions which may account for the magneto-electric coupling in LiFePO 4 . In particular, as spin canting is not compatible with P nma symmetry, a lower crystal symmetry might appear below T N [ 1,20]. Even in the absence of spin canting, an alternative me...

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