Switching of easy-axis to easy-plane anisotropy in cobalt(ii) complexes

A tetranuclear cubane-type complex [Co4(ntfa)4(CH3O)4(CH3OH)4] (1) with a {Co4O4} core, and a mononuclear complex [Co(ntfa)2(CH3OH)2] (2) have been rationally obtained by adjusting the ratio of the β-diketonate and Co(II) ions, with the synthetic processes being monitored by in situ microcalorimetry. Then, following synthetic conditions to obtain 2, but using three distinct N-donor coligands - 2,2'-bipyridyl (bpy), 6,6'-dimethyl-2,2'-bipyridyl (6,6-(CH3)2-bpy) and 5,5'-dimethyl-2,2'-bipyridyl (5,5-(CH3)2-bpy) - three novel mononuclear complexes have been obtained, [Co(ntfa)2(bpy)2] (3), [Co(ntfa)2(6,6-(CH3)2- bpy)2] (4) and [Co(ntfa)2(5,5-(CH3)2-bpy)2] (5). The introduction of different capping coligands - as singlecrystal X-ray crystallography ascertains - fine-tunes the structures, with changes in both the distortion degree of the coordination geometry and the intermolecular interactions, which have a direct impact on the magnetic properties of these complexes. Magnetic investigations reveal field-induced single-ion magnet behavior in all complexes with distinct energy barriers (Ueff) −39.06 (1), 36.65 (2), 36.32 (3), 28.26 (4) and 15.85 K (5). Magnetic experiments together with HF-EPR measurements and theoretical calculations demonstrate that 2 features easy-axis magnetic anisotropy (D = −60.48 cm−1), whereas 3-5 show easy-plane magnetic anisotropies − D = +70.77 cm−1 for 3, +35.71 cm−1 for 4, and +51.28 cm−1 for 5. To our knowledge, such reversal of anisotropic nature driven by coligands is unprecedented

Static and dynamical properties of the spin-5/2 nearly ideal triangular lattice antiferromagnet Ba3MnSb2O9

We study the ground state and spin excitation in Ba3MnSb2O9, an easy-plane S = 5/2 triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, which could allow for isosceles triangular exchanges and biaxial anisotropy by symmetry, we observe no deviation from the behavior expected for spin waves in the in-plane 120o state. Even the easy-plane anisotropy is so small that it can only be detected by ESR in our study. In conjunction with the quasi-two-dimensionality, our study establishes that Ba3MnSb2O9 is a nearly ideal triangular lattice antiferromagnet with the quasi-classical spin S = 5/2, which suggests that it has the potential for an experimental study of Z- or Z2-vortex excitations

Antiferromagnetic ordering of the spin-5/2 ladder compound BaMn

We have performed a first-principles study on the spin-5/2 ladder compound BaMn2O3. First, the monoclinic antiferromagnetic (AFM) ground state has been confirmed by the structure optimization. Second, in the AFM state the compound is an insulator with a band gap of ~1.3 eV. Third, the exchange interactions are estimated from the classic effective Heisenberg spin Hamiltonian model. The results show that the intra-ladder interactions (J1 ~ J2 = 52 K) are dominant, in accordance with the picture of a spin-5/2 ladder

Anisotropic exchange coupling and ground state phase diagram of Kitaev compound YbOCl

Rare-earth chalcohalide REChX (RE = rare earth; Ch = O, S, Se, Te; X = F, Cl, Br, I) is a newly reported family of Kitaev spin liquid candidates. The family offers a platform where a strong spin-orbit coupling meets a van der Waals layered and undistorted honeycomb spin lattice, which outputs highly anisotropic exchange couplings required by the Kitaev model. YbOCl is the first single crystal of the family we grew, with a size up to ∼15mm. We have performed magnetization and high magnetic field electron spin resonance measurements from 2 to 300 K. We develop the mean-field scenario for the anisotropic spin system, with which we are able to well describe the experiments and reliably determine the fundamental parameters. The self-consistent simulations give the anisotropic spin-exchange interactions of J_{±} (∼−0.3K) and J_{zz} (∼1.6K), and g factors of g_{ab} (∼3.4) and g_{c} (∼2.9). Based on the spin-exchange interactions, we employ the exact diagonalization method to work out the ground state phase diagram of YbOCl in terms of the off-diagonal exchange couplings. The phase diagram hosting rich magnetic phases including the spin-disordered one, sheds light on the novel magnetic properties of the family, particularly the Kitaev physics

Transient Magnetoelectric Coupling Induced by the Dynamic Intertwinement between Exchange Striction and Compensation in GdFeO3

In this study, we investigate the dynamic magnetoelectric (ME) coupling behaviors of GdFeO3 under pulsed magnetic fields. When a magnetic field is applied along the c-axis, and the temperature is near the compensation temperature (Tcomp = 3.5 K), we observe a subtle transition involving the reversal of Fe3+ moments at approximately 0.8 T in magnetization (M) measurements. This transition induces a corresponding jump in electrical polarization (P), which is not present in the static field measurements. The dynamic intertwining between M and P signifies a competition between antiferromagnetic (AFM) coupling between Gd3+ and Fe3+ moments and their Zeeman energies. The robust AFM coupling leads to the reversal of Fe3+ moments near Tcomp, triggering the abrupt change in P. Based on the exchange striction mechanism in the ferrimagnetic GdFeO3, we propose the possibility of achieving highly magnetic field sensitive ME coupling near the compensation temperature in ferrimagnetic multiferroic orthoferrites

High-Magnetic-Sensitivity Magnetoelectric Coupling Origins in a Combination of Anisotropy and Exchange Striction

Magnetoelectric (ME) coupling is highly desirable for sensors and memory devices. Herein, the polarization (P) and magnetization (M) of the DyFeO3 single crystal were measured in pulsed magnetic fields, in which the ME behavior is modulated by multi-magnetic order parameters and has high magnetic-field sensitivity. Below the ordering temperature of the Dy3+-sublattice, when the magnetic field is along the c-axis, the P (corresponding to a large critical field of 3 T) is generated due to the exchange striction mechanism. Interestingly, when the magnetic field is in the ab-plane, ME coupling with smaller critical fields of 0.8 T (a-axis) and 0.5 T (b-axis) is triggered. We assume that the high magnetic-field sensitivity results from the combination of the magnetic anisotropy of the Dy3+ spin and the exchange striction between the Fe3+ and Dy3+ spins. This work may help to search for single-phase multiferroic materials with high magnetic-field sensitivity

Construction of Ideal One-Dimensional Spin Chains by Topochemical Dehydration/Rehydration Route

International audienceOne-dimensional (1D) Heisenberg antiferromagnets are of great interest due to their intriguing quantum phenomena. However, the experimental realization of such systems with large spin S remains challenging because even weak interchain interactions induce long-range ordering. In this study, we present an ideal 1D S = 5/2 spin chain antiferromagnet achieved through a multistep topochemical route involving dehydration and rehydration. By desorbing three water molecules from (2,2′-bpy)FeF3(H2O)·2H2O (2,2′-bpy = 2,2′-bipyridyl) at 150 °C and then intercalating two water molecules at room temperature (giving (2,2′-bpy)FeF3·2H2O 1), the initially isolated FeF3ON2 octahedra combine to form corner-sharing FeF4N2 octahedral chains, which are effectively separated by organic and added water molecules. Mössbauer spectroscopy reveals significant dynamical fluctuations down to 2.7 K, despite the presence of strong intrachain interactions. Moreover, results from electron spin resonance (ESR) and heat capacity measurements indicate the absence of long-range order down to 0.5 K. This controlled topochemical dehydration/rehydration approach is further extended to (2,2′-bpy)CrF3·2H2O with S = 3/2 1D chains, thus opening the possibility of obtaining other low-dimensional spin lattices