Magnetoelastic coupling and uniaxial pressure dependencies of the ferromagnetic ordering temperature in the quasi-two-dimensional layered van der Waals material ${\mathrm{CrI}}_3$ are experimentally studied and quantified by high-resolution dilatometry. Clear anomalies in the thermal expansion coefficients at $T_{\mathrm{C}}$ imply positive (negative) pressure dependencies $\partial T_{\mathrm{C}}/\partial p_i$ for pressure applied along (perpendicular to) the $c$ axis. The experimental results are backed up by numerical studies showing that the dominant, intralayer magnetic coupling increases upon compression along the $c$ direction and decreases with negative in-plane strain. In contrast, interlayer exchange is shown to initially increase and subsequently decrease upon the application of both out-of-plane and in-plane compression.

The putative Kitaev material ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ has recently been proposed to enter a quantum spin disordered state when magnetic fields are applied in parallel to the honeycomb layers. In this Letter we uncover signatures of a quantum critical endpoint (QCEP) associated with the assumed order-disorder transition by means of high-resolution capacitance dilatometry. At the critical field $B_{\mathrm{C}}$ $\simeq 6 \mathrm{T}$, a sign change of the out-of-plane thermal expansion coefficient ${\alpha }_{\mathrm{c}}$ indicates the accumulation of entropy upon crossing the phase boundary. The proportional relationship between isothermal magnetization and magnetostriction signals that the QCEP can be tuned by magnetic field and pressure simultaneously. The presented results expand the material classes that exhibit metamagnetic quantum criticality to honeycomb antiferromagnets with possible Kitaev interactions.

Within the framework of 3d–4f molecular magnets, the most thoroughly investigated architecture is that of butterfly‐shaped coordination clusters as it provides an ideal testbed to study fundamental magnetic interactions. Here, we report the synthesis and characterisation of a series of isostructural V^III₂Ln^III₂ butterfly complexes, where Ln=Y (1_Y), Tb (2_Tb), Dy (3_Dy), Ho (4_Ho), Er (5_Er), Tm (6_Tm), Yb (7_Yb), which extends the previous study on isostructural butterflies with Cr^III, Mn^III and Fe^III. In zero external field, compounds 2_Tb, 3_Dy and 4_Ho show clear maxima in the out‐of‐phase component of the ac susceptibility whereas small magnetic fields are needed to suppress quantum tunnelling in 6_Tm. Combined high‐field electron paramagnetic resonance spectroscopy and magnetisation measurements unambiguously reveal an easy‐plane anisotropy of the V^III ion and antiferromagnetic Ising‐like 3d–4f exchange couplings. The strength of J_3d‐4f is shown to decrease upon variation of the 4 f ion from Tb to Ho, while increasing antiferromagnetic interaction can be observed from Ho to Tm. The exact inverse chemical trend is found for the relative angle between the 3d and 4 f main anisotropy axes, which highlights the important role of the lanthanide 4 f electron distribution anisotropy for 3d–4f exchange.

High‐frequency/high‐field electron paramagnetic resonance studies on a series of seven‐coordinate pentagonal‐bipyramidal (PBP) erbium(III) complexes Er(DAPMBH/H₂DAPS)X (H₂DAPMBH = 2,6‐diacetylpyridine bis‐4‐methoxy benzoylhydrazone, H₄DAPS = 2,6‐diacetylpyridine bis‐(salicylhydrazone)) demonstrate the effects of different apical ligands (X = (H₂O)Cl (1), (CH₃OH)N₃ (2), Cl₂ (3)) on the local magnetic anisotropy of the central Er^III ions. In particular, we report direct experimental determination of the effective g‐values and zero field splittings of the energetically low‐lying Kramers doublets. Our quantitative determination of the magnetic anisotropy highlights the relevance of an axial g‐tensor for SMM behavior and suggests that fast magnetic relaxation is mainly driven by a thermally assisted quantum tunnelling process via low‐lying excited states.