Rhombic Anisotropy Research Articles

Are Actinyl Cations Good Probes for Structure Determination in Solution by NMR?

We report on NMR spectroscopy, CAS-based method calculations, and X-ray diffraction of AnV and AnVI complexes with a neutral and slightly flexible TEDGA ligand. After checking that pNMR shifts mainly arise from pseudocontact interactions, we analyze pNMR shifts considering the axial and rhombic anisotropy of the actinyl magnetic susceptibilities. The results are compared to those of a previous study performed on [AnVIO2]2+ complexes with dipicolinic acid. It is shown that 5f2 cations (PuVI and NpV) make very good candidates for determining the structure of actinyl complexes in solution by 1H NMR spectroscopy as shown by the invariance of the magnetic properties to the equatorial ligands, as opposed to the NpVI complexes with a 5f1 configuration.

Dynamically generated quadrupole polarization using Floquet adiabatic evolution

We investigate the nonequilibrium dynamics of the $S=1$ quantum spin chain subjected to a time-dependent external drive, where the driving frequency is adiabatically decreased as a function of time (``Floquet adiabatic evolution''). We show that when driving the rhombic anisotropy term (known as the ``two-axis countertwisting'' in the context of squeezed spin states) of a N\'eel antiferromagnet, we are able to induce an overall enhancement in the quadrupole polarization, while at the same time suppressing the staggered magnetization order. The system evolves into a new state with a net quadrupole moment and antiferroquadrupolar correlations. This state remains stable at long times once the driving frequency is kept constant. On the other hand, we find that we cannot achieve a quadrupole polarization for the symmetry-protected Haldane phase, which remains robust against such driving.

Series of Benzoquinone-Bridged Dicobalt(II) Single-Molecule Magnets.

Mononuclear complexes within a particular coordination geometry have been well recognized for high-performance single-molecule magnets (SMMs), while the incorporation of such well-defined geometric ions into multinuclear complexes remains less explored. Using the rigid 2-(di(1H-pyrazol-1-yl)methyl)-6-(1H-pyrazol-1-yl)pyridine (PyPz3) ligand, here, we prepared a series of benzoquinone-bridged dicobalt(II) SMMs [{(PyPz3)Co}2(L)][PF6]2, (1, L = 2,5-dioxo-1,4-benzoquinone (dhbq2-); 2, L = chloranilate (CA2-); and 3, L = bromanilate (BA2-)), in which each Co(II) center adopts a distorted trigonal prismatic (TPR) geometry and the distortion increases with the sizes of 3,6-substituent groups (H (1) < Cl (2) < Br (3)). Accordingly, the magnetic study revealed that the axial anisotropy parameter (D) of the Co ions decreased from -78.5 to -56.5 cm-1 in 1-3, while the rhombic one (E) increased significantly. As a result, 1 exhibited slow relaxation of magnetization under a zero dc field, while both 2 and 3 showed only the field-induced SMM behaviors, likely due to the increased rhombic anisotropy that leads to the serious quantum tunneling of the magnetization. Our study demonstrated that the relaxation dynamics and performances of a multinuclear complex are strongly dependent on the coordination geometry of the local metal ions, which may be engineered by modifying the substituent groups.

A C,S bonded quasi-two-coordinate chromium(II) complex showing field-induced slow magnetic relaxation behaviour.

A C,S bonded quasi-two-coordinate Cr(II) complex, Cr(SAr*)2 (HSAr* = HSC6H3-2,6(C6H2-2,4,6-Pri3)2), has been synthesized according to literature precedent. Magnetic measurements, high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and ab initio calculation studies show that the field-induced slow magnetic relaxation behaviour is caused by relatively weak axial magnetic anisotropy.

Heteroleptic complexes of Ni(II) with 2,2′-bipyridine and benzoato ligands. Magnetic properties of [Ni(bpy)(Bz)2

Under various experimental conditions seven complexes have been crystallized from the systems Ni(II) – HBz – bpy (HBz = benzoic acid, bpy = 2,2′-bipyridine). The use of freshly prepared Ni(OH)2 as starting reagent leads to isolation of [Ni(bpy)(Bz)2] (1) and [Ni(bpy)3](Bz)2·5H2O (2). On the other hand, the use of nickel(II) nitrate as starting salt yielded 1 and also resulted in the formation of additional complexes in which the nitrate group entered into the final compositions, namely [Ni(bpy)2(Bz)](NO3)·2HBz (3), [Ni(bpy)2(Bz)](NO3)·1.25H2O (4), [Ni(bpy)2(H2O)(NO3)](NO3)·0.5HBz (5), [Ni(bpy)2(NO3)](NO3) (6) and [Ni(bpy)3](NO3)2·nH2O (n = 4.25) (7). The newly prepared complexes were characterized chemically and spectroscopically. The results of X-ray structure analyses of 1, 3–6 have shown that in all complexes, the Ni(II) atoms are hexacoordinated. Complex 1 exhibits a mononuclear molecular structure, while the ionic structures of 3–6 are built up of mononuclear complex cations with a heteroleptic coordination sphere around the Ni(II) atoms and by nitrate anions. In addition, the structures of 3–5 contain HBz solvate molecules or water molecules. The magnetic properties of 1 were studied in the temperature range 1.8–300 K showing that 1 is characterized by the presence of single-ion anisotropy of the easy-axis type with a value of D/kB = −6.32 K and a strong rhombic term with E/D = 0.327. Theoretical calculations using the ORCA package also confirm a significant influence of rhombic anisotropy.

Single Ion Anisotropy of CrIII and FeIII in a Series of {Ti7M} Rings

A series of heterometallic [TiIV7MIII] rings are reported which contain a single paramagnetic metal centre, either FeIII or CrIII. The structures contain an octagon of metals bridged by oxides within the ring and pivalate ligands outside the ring. The structures encapsulate a single cation, either a secondary ammonium cation or a cesium. Multi-frequency EPR spectroscopy was used to measure the zero-field splitting parameters for the paramagnetic metal sites. The FeIII sites have rhombic anisotropy, with D around − 0.18 cm−1 and λ (= D/E) approximately 0.25. The CrIII have a more axial anisotropy, with D around − 0.5 cm−1 and λ around 0.05. Attempts to find a magneto-structural correlation are vitiated by the difficulty of refining the challenging crystal structures.

Powder Sample Susceptibility for Single Ion Magnets with S=1,3/2 with Rhombic Anisotropy

Powder Sample Susceptibility for Single Ion Magnets with S=1,3/2 with Rhombic Anisotropy

Tuning Magnetic Anisotropy in a Class of Co(II) Bis(hexafluoroacetylacetonate) Complexes.

Tuning the magnetic anisotropy of metal ions remains highly interesting in the design of improved single-molecule magnets (SMMs). We herein report synthetic, structural, magnetic, and computational studies of four mononuclear CoII complexes, namely [Co(hfac)2 (MeCN)2 ] (1), [Co(hfac)2 (Spy)2 ] (2), [Co(hfac)2 (MBIm)2 ] (3), and [Co(hfac)2 (DMF)2 ] (4) (MeCN=acetonitrile, hfac=hexafluoroacetylacetone, Spy=4-styrylpyridine, MbIm=5,6-dimethylbenzimidazole, DMF=N,N-dimethylformamide), with distorted octahedral geometry constructed from hexafluoroacetylacetone (hfac) and various axial ligands. By a building block approach, complexes 2-4 were synthesized by recrystallization of the starting material of 1 from various ligands containing solution. Magnetic and theoretical studies reveal that 1-4 possess large positive D values and relative small E parameters, indicating easy-plane magnetic anisotropy with significant rhombic anisotropy in 1-4. Dynamic alternative current (ac) magnetic susceptibility measurements indicate that these complexes exhibit slow magnetic relaxation under external fields, suggesting field-induced single-ion magnets (SIMs) of 1-4. These results provide a promising platform to achieve fine tuning of magnetic anisotropy through varying the axial ligands based on Co(II) bis(hexafluoroacetylacetonate) complexes.

Field-induced slow magnetic relaxation in pseudooctahedral cobalt(ii) complexes with positive axial and large rhombic anisotropy.

The preparation, X-ray crystal structure, spectroscopic and variable-temperature dc and ac magnetic properties of two six-coordinate cobalt(ii) complexes of formula [Co(bim)4(tcm)2] (1) and [Co(bmim)4(tcm)2] (2) (bim = 1-benzylimidazole, bmim = 1-benzyl-2-methylbenzimidazole and tcm- = tricyanomethanide ion) are reported. 1 and 2 crystallize in the monoclinic P21/n and C2/c space groups with the asymmetric units composed of one tcm- ion and half the [Co(bim)4]2+ and [Co(bmim)4]2+ complex cations, respectively. Their cobalt atoms are in compressed (1)/rhombic (2) CoN6 octahedral environments, the axial positions being occupied by monodentate tricyanomethanide anions. The neutral molecules in 1 are linked through weak C-HN type interactions into supramolecular chains, which are further interconnected into supramolecular 2D motifs by C-Hπ stacking. No short intermolecular interactions occur in 2. The values of the shortest intermolecular cobalt-cobalt separation are 10.901(1) (1) and 10.577(3) Å (2). Detailed ac and dc magnetic studies indicate that 1 and 2 are field-induced single-ion magnets (SIMs) with D = +46.1 (1)/+80.1 cm-1 (2) thus presenting new examples of SIMs with transversal magnetic anisotropy. Theoretical calculations by CASSCF/NEVPT2 support these results and suggest that the relaxation of the magnetization occurs in the ground state under applied fields through two Orbach processes possibly bound to low-lying vibrational modes. Q-band EPR study for polycrystalline samples 1 and 2 at low temperatures confirms the positive sign of D, allows the rough estimation of the E/D ratio [0.144 (1) and 0.180 (2)] and reveals the occurrence of a strong asymmetry in the g-tensors. The values found for the spin-reversal barrier, Ea ≈ 28 and 11 cm-1 (1) and 20 and 9 cm-1 (2), are within the range of those found in other cobalt(ii) field-induced SIMs with a pseudooctahedral trans-CoN4N'2 chromophore.

Synthesis, Crystal Structures, and EPR Studies of First MnIIILnIII Hetero-binuclear Complexes.

A new family of binuclear complexes [MnIIILnIII(dpm)4(MeO)2(MeOH)2] is reported (where Ln = LaIII (1), PrIII (2), and EuIII(3)). These compounds were obtained from a one-pot reaction between 2,2,6,6-tetramethyl-3,5-heptanodione (Hdpm), MnII, and the respective LnIII salt in the presence of sodium methoxide. The derivative containing the diamagnetic ion LaIII has been synthesized in order to characterize the local anisotropy of the MnIII ion. High-field electron paramagnetic resonance (HFEPR) spectroscopy shows that the MnIII ion, with an elongated octahedral geometry in all compounds, has a significant axial zero-field splitting and a small rhombic anisotropy. Additionally, the HFEPR measurements indicate that there is almost no exchange between the spin carriers in these compounds, all of which exhibit field-induced slow relaxation of the magnetization.

Effects of uniaxial pressure on the quantum tunneling of magnetization in a high-symmetry Mn12 single-molecule magnet

The symmetry of single-molecule magnets dictates their spin quantum dynamics, influencing how such systems relax via quantum tunneling of magnetization (QTM). By reducing a system's symmetry, through the application of a magnetic field or uniaxial pressure, these dynamics can be modified. We report measurements of the magnetization dynamics of a crystalline sample of the high-symmetry $[\mathrm{M}{\mathrm{n}}_{12}{\mathrm{O}}_{12}{({\mathrm{O}}_{2}\mathrm{CM}\mathrm{e})}_{16}{(\mathrm{M}\mathrm{e}\mathrm{OH})}_{4}]\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{M}\mathrm{e}\mathrm{OH}$ single-molecule magnet as a function of uniaxial pressure applied either parallel or perpendicular to the sample's ``easy'' magnetization axis. At temperatures between 1.8 and 3.3 K, magnetic hysteresis loops exhibit the characteristic steplike features that signal the occurrence of QTM. After applying uniaxial pressure to the sample in situ, both the magnitude and field position of the QTM steps changed. The step magnitudes were observed to grow as a function of pressure in both arrangements of pressure, while pressure applied along (perpendicular to) the sample's easy axis caused the resonant-tunneling fields to increase (decrease). These observations were compared with simulations in which the system's Hamiltonian parameters were changed. From these comparisons, we determined that parallel pressure induces changes to the second-order axial anisotropy parameter as well as either the fourth-order axial or fourth-order transverse parameter, or to both. In addition, we find that pressure applied perpendicular to the easy axis induces a rhombic anisotropy $E\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}D/2000\phantom{\rule{0.16em}{0ex}}\mathrm{per}\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$ that can be understood as deriving from a symmetry-breaking distortion of the molecule.

Magneto-Structural Correlations in Pseudotetrahedral Forms of the [Co(SPh)4]2- Complex Probed by Magnetometry, MCD Spectroscopy, Advanced EPR Techniques, and ab Initio Electronic Structure Calculations.

The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co(SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4]2- upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4]2-: (PPh4)2[Co(SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)4]2- shows strong axial magnetic anisotropy in 1, with D = -55(1) cm-1 and E/D = 0.00(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm-1 and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.

Supramolecular complexes [M{NH(CH2)4O}{S2CN(C2H5)2 2] ∙ CCl4 (M = Zn or 63Cu(II)): Synthesis, structures, spectral and thermal properties

Solvated adducts of diethyldithiocarbamate complexes of zinc and copper(II) of the formula [M{NH(CH2)4O}{S2CN(C2H5)22] ∙ CCl4 (M = Zn (I) and 63Cu (II)) were obtained. 13C MAS NMR experiments revealed magnetic nonequivalence in the dithiocarbamate moieties of the adduct isomers, the morpholine heterocycles, and the outer-sphere solvate molecules. The rhombic anisotropy of the EPR parameters of magnetically diluted isotope-substituted complex II is due to the copper polyhedron geometry, which is intermediate between a tetragonal pyramid and a trigonal bipyramid, with the ground state of the unpaired electron resulting from the mixing \(3{d_{{x^2} - {y^{2 - }}}}\) of and \(3{d_{{z^{2 - }}}}\) of copper(II). According to X-ray diffraction data, complex I is a supramolecular complex combining structurally nonequivalent adduct molecules (A and B) and “guest” molecules (CCl4). In addition, the crystal lattice has an array of channels occupied by outersphere solvate CCl4 molecules (a structural type of lattice clathrates). An STA study of the thermal properties revealed three main thermolysis steps: desorption of the solvate CCl4 molecules, elimination of coordinated morpholine molecules, and thermolysis of the dithiocarbamate moiety of the adduct.

Application of orthorhombic standardization in magnetic susceptibility studies of localized spin models with S=1, 3/2, 2, 5/2

The standardization idea is nowadays tacitly accepted in EMR area, however, its usefulness in magnetism studies has not been fully recognized as yet. This idea arises due to intrinsic features of orthorhombic Hamiltonians of any physical nature, including the crystal (ligand) field (CF/LF) Hamiltonians or the zero-field splitting (ZFS) ones. Standardization limits the ratio of the orthorhombic parameter to the axial one to a fixed range between 0 and a specific value that depends on the notation used. For the ZFS parameters expressed in the conventional spin Hamiltonian (SH) notation the ratio λ=E/D can always be limited to the range (0, ±1/3) by appropriate choice of coordinate system. Implications of standardization of orthorhombic spin Hamiltonians for interpretation of experimental magnetic susceptibility data are considered. Using a numerical example, we show the existence of alternative solutions for ZFS parameters potentially obtainable from fitting experimental magnetic data and discuss their importance. For the first time algebraic applications of the standardization to the expressions for magnetic susceptibility tensor derived earlier for localized spin models with S=1, 3/2, 2, 5/2 and with rhombic anisotropy are explored. The numerical and algebraic results allow us to formulate an 'invariance principle'. These considerations facilitate interpretation of experimental magnetic data and provide an additional check of correctness of analytical magnetic susceptibility expressions.

Связанная динамика 180-градусных доменных стенок одинаковой полярности в двухосных ферромагнетиках

The bound dynamics of two same polarity 180° domain walls with the opposite topological charges in a biaxial ferromagnet with a large quality factor is theoretically investigated. The domain walls moving towards each other under the influence of an external magnetic field with regard to dissipation are considered. The case of a large size ferromagnet with small external magnetic fields and attenuations has been studied. An inhomogeneous exchange interaction, uniaxial and rhombic anisotropy are taken into account in the magnetic energy density. A one-dimensional in coordinates soliton model in which the interacting domain walls are regarded as a two-soliton solution is used. A system of ordinary differential equations describing the evolution of the soliton solution parameters under the influence of the magnetic field and attenuation is obtained. The numerical study of differential equations shows a strong dependence of walls interaction in the magnetic field on the attenuation. It was found that on the interaction of two 180-degree walls of the same polarity at a low attenuation and specific direction of the external magnetic field there occurs an annihilation of the walls, accompanied by multiple scattering process and the walls polarity change. With the increase of the attenuation parameter the annihilation of soliton-antisoliton pair happens as a result of a single scattering process. In the case of the field opposite direction same polarity 180-degree domain walls while moving come close at the minimum distance, and then the repulsion and subsequent divergence from each other occur.

Experimental and Theoretical Study of Stripe Magnetic Domain Structure Drift in Iron Garnet Crystals

The results of experimental and theoretical study of magnetic domain structure drift in low frequency oscillating magnetic eld oriented perpendicular to the sample plate are presented. Experimental study was performed on uniaxial iron garnet (TbErGd)3(FeAl)5O12 (111) plate with rhombic anisotropy for the case when orientation of domain walls of stripe domains is preserved. Dynamic domain structure was revealed by means of magnetooptic Faraday e ect and registered by high speed digital camera at the speed equal to 1200 fps. Theoretical model based on the motion equations for coupled harmonic oscillators that takes into account attenuation and eld inhomogeneity along the plate is proposed.

Field-induced slow relaxation in a monometallic manganese(III) single-molecule magnet.

High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted Mn(III) ion in Na5[Mn(L-tart)2]·12H2O (1; L-tart = L-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (∼1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated Mn(III) centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field.

Slow magnetic relaxation in octahedral cobalt(II) field-induced single-ion magnet with positive axial and large rhombic anisotropy.

Pseudooctahedral mononuclear cobat(II) complex [Co(abpt)2(tcm)2] (1), where abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole and tcm = tricyanomethanide anion, shows field-induced slow relaxation of magnetization with U = 86.2 K and large axial and rhombic single-ion zero-field-splitting parameters, D = +48(2) cm(-1) and E/D = 0.27(2) (D = +53.7 cm(-1) and E/D = 0.29 from ab initio CASSCF/NEVPT2 calculations), thus presenting a new example of a field-induced single-ion magnet with transversal magnetic anisotropy.

A New Family of Trinuclear Nickel(II) Complexes as Single‐Molecule Magnets

Three new trinuclear nickel (II) complexes with the general composition [Ni3 L3 (OH)(X)](ClO4 ) have been prepared in which X=Cl(-) (1), OCN(-) (2), or N3(-) (3) and HL is the tridentate N,N,O donor Schiff base ligand 2-[(3-dimethylaminopropylimino)methyl]phenol. Single-crystal structural analyses revealed that all three complexes have a similar Ni3 core motif with three different types of bridging, namely phenoxido (μ2 and μ3 ), hydroxido (μ3 ), and μ2 -Cl (1), μ1,1 -NCO (2), or μ1,1 -N3 (3). The nickel(II) ions adopt a compressed octahedron geometry. Single-crystal magnetization measurements on complex 1 revealed that the pseudo-three-fold axis of Ni3 corresponds to a magnetic easy axis, being consistent with the magnetic anisotropy expected from the coordination structure of each nickel ion. Temperature-dependent magnetic measurements indicated ferromagnetic coupling leading to an S=3 ground state with 2J/k=17, 17, and 28 K for 1, 2, and 3, respectively, with the nickel atoms in an approximate equilateral triangle. The high-frequency EPR spectra in combination with spin Hamiltonian simulations that include zero-field splitting parameters DNi /k=-5, -4, and -4 K for 1, 2, and 3, respectively, reproduced the EPR spectra well after a anisotropic exchange term was introduced. Anisotropic exchange was identified as Di,j /k=-0.9, -0.8, and -0.8 K for 1, 2, and 3, respectively, whereas no evidence of single-ion rhombic anisotropy was observed spectroscopically. Slow relaxation of the magnetization at low temperatures is evident from the frequency-dependence of the out-of-phase ac susceptibilities. Pulsed-field magnetization recorded at 0.5 K shows clear steps in the hysteresis loop at 0.5-1 T, which has been assigned to quantum tunneling, and is characteristic of single-molecule magnets.

Cu(en)2SiF6 and [Cu(dmen)2(H2O)]SiF6 (en = 1,2-diaminoethane; dmen = N,N-dimethyl-1,2-diaminoethane): Preparations, crystal structures, spectroscopic properties and hydrogen bonding mediated magnetism

Crystals of Cu(en)2SiF6 (en=1,2-diaminoethane) (1) and [Cu(dmen)2(H2O)]SiF6 (dmen=N,N-dimethyl-1,2-diaminoethane) (2) were isolated from aqueous–ethanolic systems Cu2+-en or dmen-SiF62−. The chain-like crystal structure of 1 is built up of [Cu(en)2]2+ complex cations linked by bridging μ2-SiF62− anions with trans positions of the bridging fluorine atoms. Copper ion exhibits usual distorted octahedral coordination; there are two chelate coordinated en ligands in the equatorial plane with CuN1 bonds of 1.9994(14)Å (4x), while the axial positions are occupied by fluorine atoms from SiF62− anions with CuF2 bond of 2.6065(14)Å (2x). The individual chains are linked by NH···F type hydrogen bonds. In 2 the coordination polyhedron of the Cu(II) atom can be described as slightly deformed square pyramid (τ=13.0%) with four N atoms from two chelate dmen ligands placed in the basal plane (CuN bonds are from the range 2.0164–2.0599Å) while the apical position is occupied by an aqua ligand at CuO distance is 2.293(2)Å. The structure of 2 is stabilized by OH···F and NH···F hydrogen bonds between complex cations and the hexafluoridosilicate anions. The EPR spectra show the rhombic anisotropy of g-factor in both complexes. Magnetic measurements indicate the presence of two-dimensional antiferromagnetic structure in both complexes. A weak exchange coupling J/kB=−0.85K was found in 1. Much stronger exchange coupling J/kB=−3.69K present in 2 allowed us to identify spatial anisotropy of the exchange coupling, α=0.2, and to describe 2 by rectangular antiferromagnetic lattice model.