The tuning of magnetic transitions in rare-earth orthoferrites is critical for developing advanced functional materials. Here, we report on the evolution of spin reorientation (SRT) and spin switching (SSW) in single crystals of Nd1-xSmxFeO3 (x = 0.1, 0.3, and 0.5), grown using an optical floating zone furnace. Structural analysis confirms a systematic lattice contraction within the orthorhombic Pbnm space group upon Sm3+ doping. This compositional engineering provides precise control over the SRT, systematically increasing its temperature while narrowing the Γ24 coexistence region. The crystals exhibit type-I spin switching, and most significantly, we demonstrate a unique way of controlling the switching temperature (TSSW). By employing a negative field-cooling protocol, an exceptionally large tunable range of nearly 158 K was achieved with a minimal cooling field of -20 Oe, the largest reported so far for this material class. These findings establish that doping in orthoferrites is a powerful strategy to create materials with tailored magnetic functionalities for advanced applications.

In Nd 1− x Sm x FeO 3 , Sm substitution shifts the Fe-sublattice spin-reorientation window to a higher temperature; in Nd 0.5 Sm 0.5 FeO 3 , a weak −20 Oe negative-field-cooling bias moves the spin-switching temperature by ∼158 K (175–333 K) at H = 20 Oe.

Two isostructural cobalt(II) chain compounds, [Co(NCX)2(py)2]n (X = S, Se) based on μ-1,3-bridging selenocyanate (1) and thiocyanate (2) ligands, were investigated to elucidate the effect of chalcogen substitution on magnetic anisotropy and exchange interactions. Both compounds form linear chains of octahedrally coordinated cobalt(II) ions with identical topology and similar magnetic exchange but different anisotropy. Heat capacity measurements reveal slightly stronger intrachain interactions and higher ordering temperatures for the selenocyanate derivative, while demonstrating a pronounced sensitivity of the critical temperature to sample handling. Frequency-domain Fourier-transform THz-EPR spectroscopy provides direct access to low-energy magnetic excitations, allowing the determination of intrachain excitation energies and effective gz values without reliance on a specific model. The selenocyanate compound exhibits both a larger chain excitation gap and enhanced axial anisotropy. Temperature-dependent THz-EPR further yields estimates of weak interchain coupling that is essential for long-range magnetic ordering. Ab initio CASSCF/CASPT2/RASSI-SO calculations reproduce the experimental trends and reveal that the enhanced anisotropy in 1 originates primarily from the softer donor character of selenium, leading to increased covalency and enhanced spin-orbit coupling at the cobalt center, while axial ligand orientation dominates the single-ion anisotropy. Together, these results demonstrate how subtle ligand substitution modulates anisotropy and exchange in ferromagnetic cobalt(II)-based Ising chains.

Manipulating the physical properties of solid matter using only photons is a major challenge in materials science. In this study, we present the photochemistry occurring in a single crystal of a simple cyanide complex, K4[MoIII(CN)7]·2H2O. Upon exposure to visible light at different wavelengths, a reversible breaking and reformation of dative bonds is triggered, resulting in a photoswitching of the MoIII coordination geometry between 6- and 7-coordinate. This transformation, in turn, induces a spin state change. The observed solid-state photochemical reactivity is robust, quantitative and occurs at a record-high temperature. It paves the way for the development of new photo-switchable high-temperature magnets and nanomagnets.

Multifunctional molecules responsive to light are highly desired as components for the construction of remotely controlled nanodevices. Here we present a DyIII single molecule magnet (SMM) comprising dithienylethene (dte) photochromic bridging ligands in the form of a pyridine (py) derivative: 1,2-bis((2-methyl-5-pyridyl)thie-3-yl)perfluorocyclo-pentene (dtepy). The title trinuclear compound {[DyIII(BHT)3]3(dtepy)2}·4C5H12 (1) was synthesized by combining the low-coordinate dysprosium complexes DyIII(BHT)3 (BHT = 2,6-di-tert-butyl-4-methylphenolate) with dtepy bridging ligands in the ‘open’ form using n-pentane as a completely inert solvent. The trinuclear molecule comprises two different DyIII centers due to its quasi-linear geometry: a central trigonal bipyramidal DyIII ion and two peripheral ones with an approximate trigonal pyramidal geometry. Thanks to that, 1 shows two types of SMM behavior which is slightly affected by the photoisomerization of the photochromic dtepy bridges. The impact of the photoisomerization on the magnetization dynamics was studied by means of alternating current (AC) magnetic susceptibility measurements for the ‘open’ and ‘closed’ forms of the molecules. The changes between the ‘open’ and ‘closed’ isomers were further investigated by IR and UV-vis spectroscopy, suggesting the co-existence of the ligand-related photochromism and single-molecule magnet behavior in 1. However, the powder X-ray diffraction studies indicate loss of structural order in the first photoisomerization step preventing in-depth studies.

The reaction of Co(OAc)(2)6H(2)O with 2,2 '-[{(1E,1 ' E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co-4(L)(2)(mu-eta(1):eta(1)-OAc)(2)(eta(2)-OAc)(2)] 1.5 CH3OH 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L](2-) and four acetate ligands. Two distinct types of Co-II centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm(-1)), while the magnetic anisotropy is the easy-plane type (D-1 = 8.8, D-2 = 0.76 cm(-1)). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec(-1), required to attain a current density of 10 mA cm(-2) and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (R-ct) of 2.927 Omega, which is comparatively lower than standard Co3O4 (5.242 Omega), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

New cobalt(II)-based complexes with [N2O2] coordination formed by two bis-chelate ligands were synthesized and characterized by a multi-technique approach. The complexes possess an easy-axis anisotropy (D<0) and magnetic measurements show a field-induced slow relaxation of magnetization. The spin-reversal barriers, i. e., the splitting of the two lowest Kramers doublets (U-ZFS), have been measured by THz-EPR spectroscopy, which allows to distinguish the two crystallographically independent species present in one of the complexes. Based on these experimental U-ZFS energies together with those for related complexes reported in literature, it was possible to establish magneto-structural correlations. U-ZFS linearly depends on the elongation parameter epsilon(T) of the (pseudo-)tetrahedral coordination, which is given by the ratio between the average obtuse and acute angles at the cobalt(II) ion, while U-ZFS was found to be virtually independent of the twist angle of the chelate planes. With increasing deviation from the orthogonality of the latter, the rhombicity (|E/D|) increases.

The reaction of Co(OAc)(2)4H(2)O, LH2, and Et3N in a 1.5:1:3 molar ratio affords a linear trinuclear complex, [Co-3(L)(2)(mu-eta(1): eta(1)-OAc)(2)(CH3CN)(2)] (1). The synthesized complex was characterized by single crystal X-ray diffraction studies, and it exhibited better electrocatalytic activity for the oxygen evolution reaction (OER) compared with the catalyst, RuO2. The complex exhibits an overpotential of 380 mV at 10 mA cm(-2) current density for the OER, which is better than that of RuO2 (480 mV) under identical experimental conditions. The Tafel slope values of the complex and RuO2 were calculated to determine the kinetics of the electrochemical reaction and were found to be 58 and 85 mV/dec, respectively, which implies its robust heterogeneous OER catalysis. The DC magnetic studies revealed that the coupling between the Co(II) ions through the dioxo bridges was found to be antiferromagnetic, and the best-fit yielded exchange interaction J = -3.077(3) cm(-1), zero field splitting parameters D-1 = 46.35(6) cm(-1) and D-2 = 5.831(7) cm(-1), g = 2.56(2), theta = 90.3(1) degrees, and sigma(chi T) = 2.4 x 10(-5). The positive values of D-2 are consistent with the orbital singlet ground term (4)A(2g) in the octahedral coordination, which suggest that the central and terminal Co(II) ions show easy-plane anisotropies.

The reaction of Co(NCS)2 with N-methylaniline leads to the formation of [Co(NCS)2(N-methylaniline)2]n (1), in which the cobalt(II) cations are octahedrally coordinated and linked into linear chains by pairs of thiocyanate anions. In contrast to [Co(NCS)2(aniline)2]n (2) reported recently, in which the Co(NCS)2 chains are linked by strong interchain N-H···S hydrogen bonding, such interactions are absent in 1. Computational studies reveal that the cobalt(II) ions in compound 1 show an easy-axis anisotropy that is lower than in 2, but with the direction of the easy axis being similar in both compounds. The high magnetic anisotropy is also confirmed by magnetic and FD-FT THz-EPR spectroscopy, which yield a consistent gz value. These investigations prove that the intrachain interactions in 1 are slightly higher than in 2. Magnetic measurements reveal that the critical temperature for magnetic ordering in 1 is significantly lower than in 2, which indicates that the elimination of the hydrogen bonds leads to a weakening of the interchain interactions. This is finally proven by FD-FT THz-EPR experiments, which show that the interchain interaction energy in the N-methylaniline compound 1 is nine-fold smaller than in the aniline compound 2.

Single crystal study of Co(NCS)2(aniline)2 reveals that this compound orders at 6.49 K into a canted AF magnetic structure that explains the presence of a hysteresis loop along the b crystal direction. Effective s=1/2 spins of Co(II) ions within the ground state doublet create an Ising spin system that can be described by J1−J2 triangular model. Temperature dependence of specific heat yields exchange interactions J1=31.3(4) K and 2J2=−2.23(7) K. Applying magnetic field ≃6 kOe perpendicular to b compensates for the AF exchange J2 decoupling ferromagnetic spin chains. This restores the one-dimensionality of the system and allows for observation of magnetic relaxation of single chains, similar to previously studied pyridine-based Co(NCS)2L2 compounds. The analysis of the relaxation time is performed taking into account single-ion relaxation mechanisms together with the Glauber relaxation. We also show, using Monte Carlo calculations of specific heat, that for ferromagnetic J1≫|J2|, the triangular Ising model can be approximated by the rectangular Ising model for which the analytical Onsager’s solution is available.

Abstract Reactions of Mn(NCS)2 with 3-bromopyridine in acetonitrile lead to the formation of Mn(NCS)2(3-bromopyridine)4 (1) and Mn(NCS)2(3-bromopyridine)2(MeCN)2 (2) that were characterized by single crystal X-ray diffraction. Compounds 1 and 2 consist of discrete complexes, in which the Mn(II) cations are octahedrally coordinated by two trans-N-bonding thiocyanate anions and four pyridine (1) or two pyridine and two acetonitrile ligands (2). Thermoanalytical measurements on 1 and 2 have shown that upon heating half of the 3-bromopyridine co-ligands from 1 or both acetonitrile ligands from 2 are removed leading to a crystalline phase with the composition [Mn(NCS)2(3-bromopyridine)2] n (3-II). From dry n-butanol a phase with the same composition was obtained (3-I) that corresponds to a polymorphic or isomeric form of 3-II. Crystal structure analysis of 3-I shows that in this form the Mn cations are linked by pairs of anionic ligands into linear chains. The results of magnetic measurements on 3-I show antiferromagnetic interactions along the chains and the analysis of the magnetic susceptibility using the Fisher model for chains gave a J value of −5.76(5) K.

The reaction of Co(NCS)2 with 3-bromopyridine leads to the formation of discrete complexes [Co(NCS)2(3-bromopyridine)4] (1), [Co(NCS)2(3-bromopyridine)2(H2O)2] (2), and [Co(NCS)2(3-bromopyridine)2(MeOH)2] (3) depending on the solvent. Thermogravimetric measurements on 2 and 3 show a transformation into [Co(NCS)2(3-bromopyridine)2]n (4), which upon further heating is converted to [{Co(NCS)2}2(3-bromopyridine)3]n (5), whereas 1 transforms directly into 5 upon heating. Compound 5 can also be obtained from solution, which is not possible for 4. In 4 and 5, the cobalt(II) cations are linked by pairs of μ-1,3-bridging thiocyanate anions into chains. In compound 4, all cobalt(II) cations are octahedrally coordinated (OC-6), as is usually observed in such compounds, whereas in 5, a previously unkown alternating 5- and 6-fold coordination is observed, leading to vacant octahedral (vOC-5) and octahedral (OC-6) environments, respectively. In contrast to 4, the chains in 5 are very efficiently packed and linked by π···π stacking of the pyridine rings and interchain Co···Br interactions, which is the basis for the formation of this unusual chain. The spin chains in 4 demonstrate ferromagnetic intrachain exchange and much weaker interchain interactions, as is usually observed for such linear chain compounds. In contrast, compound 5 shows almost single-ion-like magnetic susceptibility, but the magnetic ordering temperature deduced from specific heat measurements is twice as high as that in 4, which might originate from π···π stacking and Co···Br interactions between neighboring chains. More importantly, unlike all linear Co(NCS)2 chain compounds, a dominant antiferromagnetic exchange is observed for 5, which is explained by density functional theory calculations predicting an alternating ferro- and aniferromagnetic exchange within the chains. Theoretical calculations on the two different cobalt(II) ions present in 5 predict an easy-axis anisotropy that is much stronger for the octahedral cobalt(II) ion than for the one with the vacant octahedral coordination, with the magnetic axes of the two ions being canted by an angle of 84°. This almost orthogonal orientation of the easy axis of magnetization for the two cobalt(II) ions is the rationale for the observed non-Ising behavior of 5.

Three new hybrid organic-inorganic frameworks employing octacyanidometallates and 4,4'-bypiridine dioxide (4,4'-bpdo) as bridging molecules were prepared and characterized. The three-dimensional coordination frameworks {[FeII(μ-4,4'-bpdo)(H2O)2]2[MIV(CN)8]·9H2O}n (Fe2Mo, Fe2W and Fe2Nb; M = Mo, W and Nb) are composed of cyanido-bridged chains, which are interconnected by the organic linkers. Magnetic measurements for Fe2Nb show a two-step transition to the antiferromagnetic state, which results from the cooperation of antiferromagnetic intra- and inter-chain interactions. Fe2Mo and Fe2W, on the other hand, behave as paramagnets at 2 K because of the diamagnetic character of the corresponding octacyanidometallate(IV) building units. However, after 450 nm light irradiation they show transition to the metastable high spin MoIV or WIV states, respectively, with distinct ferromagnetic intrachain spin interactions, as opposed to the antiferromagnetic ones observed in the Fe2Nb framework.

A single crystal of [Co(NCS)2(4-methoxypyridine)2]n was obtained and investigated. The magnetic measurements performed along three perpendicular crystallographic directions are compared to the results obtained previously for a powder sample. The magnetic inter- and intrachain interactions do not differ, however, a change of the energy barrier of magnetic relaxations is obtained. For the single crystal sample the relaxation is much slower, which is attributed to the presence of longer chains, and show that below the ordering temperature the spin chains relax by the process that involves a single domain wall. Above the ordering temperature, a second relaxation process is observed, for which the relaxation time is temperature independent, indicating a negligible energy barrier. Such phenomenon was previously not observed for any of the powder samples of compounds from the [Co(NCS)2(ligand)2]n family.

M. Jurczyszyn,1 K. Maćkosz,1, 2 M. Chrobak,1, 2 J. Stępień,1 M. Rams,3 M. Waśniowska,4 A. Quer,5 M. Kallaene,5 K. Rossnagel,5 I. Miotkowski,6 V. Monteseguro-Padron,7 A. Kozłowski,2 M. Przybylski,1, 2 and M. Sikora1 1AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, Kraków, Poland 2AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Kraków, Poland 3Jagiellonian University, Faculty of Physics, Astronomy and Applied Computer Science, Kraków, Poland 4Institute of Applied Physics, University of Hamburg, Hamburg, Germany 5Universität zu Kiel, Institut für Experimentelle und Angewandte Physik, Kiel, Germany 6Purdue University, West Lafayette, United States 7European Synchrotron Radiation Facility, Grenoble, France

A large single crystal of a compound from the family of coordination polymer [Co(NCS)2(L)2]n chains was synthesized and its magnetic properties are reported. [Co(NCS)2(4-(3-phenylpropyl)pyridine)2]n is ferromagnetic with Tc = 3.39 K. Single-ion ab initio calculations predict an almost Ising-type magnetic anisotropy and the direction of the magnetic easy-axis nearly along the Co-Npy bond of the apical pyridine-based co-ligand. Both predictions are confirmed by single-crystal magnetic measurements. The magnetic relaxation of the single crystal sample significantly differs from the powder sample data, and clearly shows the presence of two separate relaxation processes. The process dominant below 3.2 K demonstrates a single chain magnet (SCM) behaviour, with a crossover between single-wall and two-wall processes, in spite of the fact that the system is ferromagnetically ordered. The faster process that dominates just below Tc is attributed to spin waves. Micromagnetic Monte Carlo simulations of the investigated compound show that the dipolar field cancels for some chains located at the border between 3-dimensional domains. Such chains are responsible for the measured ac signal, and demonstrate the SCM behaviour. The quantitative analysis of the SCM relaxation time is supported by preparing and examining a corresponding diamagnetically diluted compound, [CoxCd1-x(NCS)2(4-(3-phenylpropyl)pyridine)2]n (x = 0.013), which behaves as a field-induced single-ion magnet. The relaxation pathways for single Co(ii) spins are determined to be Raman, direct, and quantum tunneling processes, which were included in an improved approach to describe the magnetic relaxation in the Co(ii)-based SCM compound.

Reaction of Co(NCS)(2) with different coligands leads to the formation of three compounds with the general composition [Co(NCS)(2)(L)(2)](n) (L = aniline (1), morpholine (2), and ethylenethiourea (3)). In all of these compounds the cobalt(II) cations are octahedrally coordinated by two trans thiocyanate N and S atoms and the apical donor atoms of the coligands and are linked into linear chains by pairs of anionic ligands. The magnetic behavior was investigated by a combination of static and dynamic susceptibility as well as specific-heat measurements, computational studies, and THz-EPR spectroscopy. All compounds show antiferromagnetic ordering as observed for similar compounds with pyridine derivatives as coligands. In contrast to the latter, for 1-3 significantly higher critical temperatures and no magnetic single-chain relaxations are observed, which can be traced back to stronger interchain interactions and a drastic change in the magnetic anisotropy of the metal centers. These results are discussed and compared with those of the pyridine-based compounds, which provides important insights into the parameters that govern the magnetic behavior of such one-dimensional coordination polymers.

Reaction of FeCl2, KSCN, and 4-acetylpyridine in dry ethanol leads to [Fe(NCS)(2)(4-acetylpyridine)(2)](n) (1-Fe) that, in contrast to the previously reported chain isomer, consists of layers. Specific heat measurements show magnetic ordering of 1-Fe at T-c = 8.02 K. Magnetic measurements prove antiferromagnetic interactions within the layers, which is contrary to its Ni analogue. Moreover, for 1-Fe weak ferromagnetism is observed, which is due to a canted antiferromagnetic magnetic structure induced by the Dzyaloshinskii-Moriya interaction. Mixed crystals of 1-Fe with the Ni(II) analogue were prepared and investigated by a combination of powder X-ray diffraction, atomic absorption spectroscopy, and energy dispersive X-ray spectroscopy as well as magnetic and specific heat measurements. The mixed crystals behave as spin glasses due to disordered occupation of the metal crystallographic site by Ni(II) and Fe(II) and because of ferro- and antiferromagnetic exchange interactions of the homometallic compounds.

Reaction of Co(NCS)2 and Ni(NCS)2 with 4-tert-butylpyridine in ethyl acetate leads to the formation of mixed crystals of a layered compound with the composition [CoxNi1-x(NCS)2(4-tert-butylpyridine]n. The mixed crystal formation was investigated by a combination of atomic absorption spectroscopy, X-ray powder diffraction and IR spectroscopy. Magnetic and specific heat measurements prove dominating ferromagnetic exchange interactions within the layers and a ferromagnetic transition. Depending on the synthetic method, inhomogeneous samples were obtained, for which predominantly the large difference in the solubility of the homometallic compounds might be responsible. Very long reaction time leads to much better samples for which a distinct critical temperature is observed that increases smoothly with increasing Ni content.