Jahn-Teller Distortion in Bimetallic Oxalates

Polyhedron, 2009

The C 3-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic oxalates splits the L ¼ 2 Fe(II) multiplet into two doublets and one singlet. In compounds that exhibit magnetic compensation, one of the doublets lies lowest in energy and carries an average orbital angular momentum L cf z that exceeds a threshold value of roughly 0.25. In a range of L cf z , a Jahn-Teller (JT) distortion enhances the splitting of the low-lying doublet and breaks the C 3 symmetry of the bimetallic planes around the ferrimagnetic transition temperature. Due to the competition with the spin-orbit coupling, the JT distortion disappears at low temperatures in compounds that display magnetic compensation. A comparison with recent measurements provides compelling evidence for this inverse, low-temperature JT transition. The size of the JT distortion is estimated using first-principles calculations, which suggest that the long-range ordering of smaller, non-C 3-symmetric organic cations can eliminate magnetic compensation.

Physical Review Letters, 2008

Because of the competition between the spin-orbit coupling and the Jahn-Teller (JT) energies in Fe(II) Fe(III) bimetallic oxalates, we theoretically predict that an undistorted phase with C 3 symmetry about each Fe site may be recovered at low temperatures. Both lower and upper JT transitions bracketing the ferrimagnetic transition temperature T c are predicted for compounds that exhibit magnetic compensation. Comparisons with recent measurements and first-principles calculations provide strong evidence for the inverse JT transition below T c .

Physical Review B, 2008

Bimetallic oxalates are layered molecule-based magnets with transition metals M͑II͒ and MЈ͑III͒ coupled by oxalate molecules ox= C 2 O 4 in an open honeycomb structure. Among the most interesting molecule-based magnets, Fe͑II͒Fe͑III͒ bimetallic compounds with spins S = 2 and SЈ =5/ 2 ferrimagnetically order at a transition temperature T c that ranges from 30 to 48 K, depending on the organic cation between the layers. In small magnetic fields, several of these compounds exhibit "giant negative magnetization" below a compensation temperature of about 0.62T c. By studying the behavior of the low-energy orbital doublet produced by a C 3-symmetric crystal field, we construct a reduced Hamiltonian that contains both the exchange and spin-orbit interactions. This Hamiltonian is used to explain almost all of the important behaviors of the Fe͑II͒Fe͑III͒ bimetallic oxalates, including the stability of magnetic order in weakly coupled layers and the magnetic compensation in compounds with high transition temperatures. In a magnetic field perpendicular to the bimetallic layers, a spin-flop transition is predicted at a field of about 3J c / B Ϸ 24 T, where J c Ϸ 0.45 meV is the nearest-neighbor antiferromagnetic exchange coupling. Holstein-Primakoff 1 / S and 1 / SЈ expansions are used to evaluate the spin-wave spectrum and to estimate the spin-wave gap ⌬ sw Ϸ 1.65 meV in compounds that exhibit magnetic compensation. We predict that the negative magnetization can be optically reversed by near-infrared light. Breaking the C 3 symmetry about each of the Fe͑II͒ ions through either a cation-induced distortion or uniaxial strain in the plane of the bimetallic layer is predicted to increase the magnetic compensation temperature.

Physical Review B, 2008

Bimetallic oxalates are layered molecule-based magnets with either ferromagnetic or antiferromagnetic interactions between transition metals M͑II͒ and MЈ͑III͒ on an open-honeycomb lattice. Some Fe͑II͒Fe͑III͒ bimetallic oxalates exhibit magnetic compensation ͑MC͒ at a compensation temperature T comp Ϸ 30 K below the ferrimagnetic transition temperature T c Ϸ 45 K. To see if MC is possible in other bimetallic oxalates, we construct a theoretical model for bimetallic oxalates that exhibit antiferromagnetic interactions. By varying the M͑II͒ and MЈ͑III͒ average orbital angular momentum, which can be controlled by the choice of interlayer cations, we find regions of MC in the families M͑II͒Mn͑III͒ with M = Fe, Co, or Ni and V͑II͒MЈ͑III͒ with MЈ = Cr or V but not in the family M͑II͒Ru͑III͒ with M = Fe or Cu.

The European Physical Journal B, 2013

We report on the magnetic properties of four isomorphous compounds of a family of quinternary oxalates down to 60 mK. In all these materials, the magnetic Fe II ions with a strong magneto-crystalline anisotropy form a distorted kagome lattice, topologically equivalent to a perfect kagome one if nearestneighbor interactions only are considered. All the compounds order at low temperature in an antiferromagnetic arrangement with magnetic moments at 120 • . A remarkable magnetic behavior emerges below the Néel temperature in three compounds (with inter-kagome-layer Zr, Sn, Fe but not with Al): the spin anisotropy combined with a low exchange path network connectivity lead to domain walls intersecting the kagome planes through strings of free spins. These produce an unfamiliar slow spin dynamics in the ordered phase observed by AC susceptibility, evolving from exchange-released spin-flips towards a cooperative behavior on decreasing the temperature. PACS. 75.25.-j Spin arrangements in magnetically ordered materials -75.60.Ch Domain walls and domain structure -75.40.Gb Dynamic properties -75.50.Ee Antiferromagnetics

Journal of Magnetism and Magnetic Materials, 2007

Two new hetero tri-nuclear oxalates (NH4)8[Fe2Co(C2O4)8]·6H2O and [Fe2Co(C2O4)2(OH)4]·2H2O have been synthesized. The compound presented opposite behaviour in ac-susceptibility measurements, different frustration level and spin–orbit coupling in magnetization ones. The Mössbauer approach pointed to local interactions governed by metal-oxalate strongly coupled unit. The data were interpreted in terms of role of positive (NH4) or negative (OH) bridging ligands on general magnetic

Spin and orbital magnetic moments of Re in AA'FeReO 6 double perovskites (A,A' = Ba, Sr, Ca) have been directly probed employing XMCD spectroscopy at the Re L 2,3 -edges. A considerable orbital magnetic moment is observed in all the studied compounds despite octahedral coordination. Relative orbital to spin contribution per Re atom rises with lattice distortion from m L /m S = -0.28 to -0.34 for AA'=Ba 2 FeReO 6 and Ca 2 FeReO 6 , respectively. A preliminary XMCD measurements at the Fe L 2,3 -edges reveals also a significant orbital moment of iron in Ca 2 FeReO 6 . The relation of the results to the magnetic properties of the compounds is discussed. 75.20.Hr, 78.70.Dm Ordered double perovskites has recently attracted great interest due to their large spin polarization and Curie temperature (Tc) much higher than room temperature. These properties are strongly desired in order to realize reasonable magnetoresistance effects at room temperature, which is not only challenging subject of fundamental science but also important phenomenon for potential application in spin electronics. Therefore the first observation of substantial magnetoresistance at room temperature in Sr 2 FeMoO 6 [1] quickly led to production of new devices like magnetic tunnel junctions and magnetoresistive potentiometers.

Soviet Physics Uspekhi, 1982

The properties of magnetic insulators containing orbitally degenerate transition metal ions (Jahn-Teller ions) are discussed. The Jahn-Teller effect in these insulators causes structural phase transitions, lowers the lattice symmetry, and gives rise to an orbital ordering. Various interactions responsible for these effects are discussed: the electron-lattice, quadrupole-quadrupole, and exchange interactions. The mutual effects of the orbital ordering and the magnetic properties of corresponding compounds are discussed. The exchange interaction in the cases of twofold and threefold orbital degeneracy is discussed. The effect of a magnetic field on the orbital and magnetic structure and the temperature dependence of the exchange interaction are studied. The properties of several representative compounds containing Jahn-Teller ions are discussed.

Journal of the American Chemical Society, 2009

The monomeric iron(II) amido derivatives Fe{N(H)Ar*} 2 (1), Ar* = C 6 H 3 -2,6-(C 6 H 2 -2,4,6-Pr i 3 ) 2 , and Fe{N(H)Ar # } 2 (2), Ar # = C 6 H 3 -2,6-(C 6 H 2 -2,4,6-Me 3 ) 2 , were synthesized and studied in order to determine the effects of geometric changes on their unusual magnetic properties. The compounds, which are the first stable homoleptic primary amides of iron(II), were obtained by the transamination of Fe{N(SiMe 3 ) 2 } 2 , with HN(SiMe 3 ) 2 elimination, by the primary amines H 2 NAr* or H 2 NAr # . Xray crystallography shows that they have either strictly linear (1) or bent (2, N-Fe-N = 140.9(2)°) iron coordination. Variable temperature magnetization and applied magnetic field Mössbauer spectroscopy studies reveal a very large dependence of the magnetic properties on the metal coordination geometry. At ambient temperature, the linear 1 displayed an effective magnetic moment in the range 7.0 to 7.50 μ B , consistent with essentially free ion magnetism. There is a very high internal orbital field component, H L ≈ 170 T which is only exceeded by a H L ≈ 203 T of Fe{C (SiMe 3 ) 3 } 2 . In contrast, the strongly bent 2 displays a significantly lower μ eff value in the range 5.25 to 5.80 μ B at ambient temperature and a much lower orbital field H L value of 116 T. The data for the two amido complexes demonstrate a very large quenching of the orbital magnetic moment upon bending the linear geometry. In addition, a strong correlation of H L with overall formal symmetry is confirmed. ESR spectroscopy supports the existence of large orbital magnetic moments in 1 and 2, and DFT calculations provide good agreement with the physical data.

Magnetochemistry

The spin crossover phenomena in Co(II) compounds are in the focus of the present paper. A microscopic theoretical approach for the description of spin transitions in mononuclear Co(II) compounds is suggested. Within the framework of this approach there are taken into account two types of interionic interactions that may be operative in the problem such as the electron-deformational interaction and the cooperative Jahn-Teller interaction arising from the coupling of the low-spin state of the Co(II) ion with the tetragonal vibrations of the nearest surrounding. The different role of these interactions in the spin transformation is demonstrated and discussed. On the basis of developed approach a qualitative and quantitative explanation of the experimental data on the temperature dependence of the magnetic susceptibility for the [Co(pyterpy)2](PF6)2, [Co(pyterpy)2](TCNQ)2⋅DMF⋅MeOH and [Co(pyterpy)2](TCNQ)2⋅MeCN⋅MeOH compounds is given.