Cooperative Magnetic Ordering Research Articles

Layered vanadyl (IV) nitroprusside: Magnetic interaction through a network of hydrogen bonds

The hydrogen bond and π-π stacking are two non-covalent interactions able to support cooperative magnetic ordering between paramagnetic centers. This contribution reports the crystal structure and related magnetic properties for VO[Fe(CN)5NO]·2H2O, which has a layered structure. This solid crystallizes with an orthorhombic unit cell, in the Pna21 space group, with cell parameters a=14.1804(2), b=10.4935(1), c=7.1722(8) Å and four molecules per unit cell (Z=4). Its crystal structure was solved and refined from powder X-ray diffraction data. Neighboring layers remain linked through a network of hydrogen bonds involving a water molecule coordinated to the axial position for the V atom and the unbridged axial NO and CN ligands. An uncoordinated water molecule is found forming a triple bridge between these last two ligands and the coordinated water molecule. The magnetic measurements, recorded down to 2K, shows a ferromagnetic interaction between V atoms located at neighboring layers, with a Curie-Weiss constant of 3.14K. Such ferromagnetic behavior was interpreted as resulting from a superexchange interaction through the network of strong OH····OH2O, OH····NCN, and OH····ONO hydrogen bonds that connects neighboring layers. The interaction within the layer must be of antiferromagnetic nature and it was detected close to 2K.

Partial Geometric Frustration in Inorganic Supramolecular Spin Systems with One-Dimensional Trigonally Aligned Magnetic Chains ∞1(MCl4)2− (M = Fe2+, Co2+)

Exploration of new spin systems with low-dimensional subunits have been of great interest in the past decades due to their interesting physical properties and potential applications in molecular spintronics. Two inorganic supramolecular complexes, (Hg3S2)(FeCl4) and (Hg3S2)(CoCl4), with trigonally aligned 1-D infinite magnetic ∞1(FeCl4)2− or ∞1(CoCl4)2− chains have been prepared by solid-state reactions. They exhibit 3-D long-range spin order with strong field dependence and field induced metamagnetic behavior. The intrachain and interchain magnetic coupling constants were estimated by DFT+U and DFT+U+SOC calculations and the both complexes can be regarded as partially frustrated spin systems. The spin Hamiltonian was constructed, the ground state is proposed to be incommensurate spiral spin order, which differs from the commensurate 120° spin structure ground state of fully frustrated trigonal case by a little canted angle. This study shows that cooperative magnetic ordering induced by geometric frustration can be realized in inorganic supramolecular systems assembled by weak van der Waals’ interactions.

Impurity states and cooperative magnetic order in Fe-based superconductors

We study impurity bound states and impurity-induced order in the superconducting state of LiFeAs within a realistic five-band model based on the band structure and impurity potentials obtained from density functional theory (DFT). In agreement with recent experiments, we find that Co impurities are too weak produce sub-gap bound states, whereas stronger impurities like Cu do. We also obtain the bound state spectrum for magnetic impurities, such as Mn, and show how spin-resolved tunnelling may determine the nature of the various defect sites in iron pnictides, a prerequisite for using impurity bound states as a probe of the ground state pairing symmetry. Lastly we show how impurities pin both orbital and magnetic order, providing an explanation for a growing set of experimental evidence for unusual magnetic phases in doped iron pnictides.

Structure and magnetism of the β-Mn–Co solid-solution phase

The crystal structure of the β-Mn 1− t Co t solid-solution phase (0 ≤ t ≤ 0.40) has been studied with powder neutron (10 and 298 K) and single-crystal X-ray (150 K) diffraction methods. The lattice-constant ( a) isotherms at 10, 150, and 298 K go through flat maxima between t = 0.10 and 0.25. Up to t = 0.25 all Co is found to substitute at the T1 ( T1 = Mn and/or Co) site of the β-Mn crystal structure (position 8 c of space group P4 132) whereas for t = 0.40 also the T2 site (position 12 d) is partly occupied by Co (some 0.2 Mn + 0.8 Co occupancy of the T2 site). The variable positional parameters x (for T1) and y (for T2) exhibit remarkably small variations with composition ( t) and temperature. The present low-temperature powder neutron-diffraction data confirm the earlier finding that the β-Mn 1− t Co t phase does not exhibit conventional co-operative magnetic ordering. However, the appearance of diffuse scattering in the low-temperature diffraction patterns is clearly generated by short-range ordering of magnetic moments, which owing to the atomic arrangement of the β-Mn-type structure becomes geometric frustrated. The temperature dependence of the magnetic susceptibility for β-Mn 1− t Co t is re-measured. Neglecting β-Mn itself (which exhibits virtually temperature-independent paramagnetism), our magnetic susceptibility curves above some 80 K for t = 0.15, 0.25, and 0.40 can with good-will be described by the Curie–Weiss relation, indicating antiferromagnetic correlations at low temperatures. However, the thus involved paramagnetic moments and Weiss constants must indeed be stamped as unphysically large.

Heat capacity of RFexMn12−x (R = Gd,Tb and Dy) compounds: wiping out a cooperative 4f–4f exchange interaction by breakingthe 3d–4f magnetic symmetry

Using adiabatic calorimetry the heat capacity of a series ofRFexMn12−x (R = Gd,Tb and Dy) compounds has been measured from 3 to 350 K. The substitution of Fe for Mn inRFexMn12−x influences both the magnetic interactions on the 3d sublattice andthe magnetism of R (the Néel temperature doubles on going fromx = 0 to 6 and the compounds become ferromagnetic forx = 8 with Curie temperatures of around 300 K). In pureTbMn12 the heat-capacitydata shows a λ-type anomaly associated with the independent cooperative magnetic ordering of the R sublattice(∼5 K), while the anomaly related to the Mn magnetic ordering(∼100 K) is rather smooth, as observed in other itinerant magnetic systems such asYMn12. In contrast, the substitution of Fe for Mn leads, on the one hand, to a more localizedmagnetic behaviour of the 3d sublattice, and, on the other, to magnetic polarization effectsbetween the 3d and 4f sublattices, together with the disappearance of the cooperativemagnetic ordering of the R sublattice due to the breaking of the antiferromagneticsymmetry in the 3d sublattice. This is reflected in the heat-capacity curve through asmooth Schottky-like anomaly. In the case of Gd compounds the magnitude of theexchange molecular-field parameter has been deduced by fitting the magneticcontribution to the heat capacity within a simple mean-field model. From this analysiswe found that this molecular field acting on the rare-earth site increases withthe iron concentration, reaching values as large as 48 T for the concentrationx = 6. A similar analysis of the heat capacity in the ordered phase on the Tb compoundsalso leads to an enhancement of the molecular field with increasing Fe content.These results allow checking the possible crystal-field parameters for theseRFexMn12−x compounds.

From superparamagnetic to magnetically ordered state in Co-Sm-O nanocrystalline films

Transition of the system of Co nanoparticles from a superparamagnetic state into the region of cooperative magnetic ordering caused by effects of magnetic interaction between particles has been analyzed. The possibility of the preparation of materials with soft magnetic properties and higher electrical resistivity was shown.

Crystal Structure of Stoichiometric YBa2Fe3O8.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Crystal Structure of Stoichiometric YBa2Fe3O8

The oxygen-nonstoichiometry locus in YBa2Fe3O(8+w) changes with the sign of w. The singularity at w = 0 is studied by Rietveld refinement of combined high-resolution, high-intensity neutron and synchrotron X-ray diffraction data collected at room temperature on red-colored stoichiometric YBa2Fe3O8. A residual disorder of oxygen atoms is identified in a concentration of 0.03 vacancy/interstitials per formula, with corresponding anisotropy in thermal displacements of oxygen neighbors. The disorder improves accommodation of trivalent iron over the two unequal coordination polyhedra and is not identical with the charged Frenkel disorder induced thermally. The symmetry of the cooperative magnetic order is Ib'am, with opposite moments of equal magnitude (4.07 micro(B) at 295 K) at the two iron sites.

A new approach to synthesis of α-nitronyl nitroxide carboxylates. First metal complexes with α-imino nitroxide carboxylate

Abstract An effective procedure has been developed for the synthesis of the potassium salt of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide-2-carboxylic acid K(1). The salts of 4,4,5,5-tetramethyl-1-oxyl-4,5-dihydro-1H-imidazole-2-carboxylic acid X(2) are kinetically much more stable than K(1). Due to this, a series of heterospin complexes Cu(2)2(CH3OH)2, Cu(2)2(H2O)2, Cu(2)2, Co(2)2(CH3OH)2, Ni(2)2(H2O)2, Ni(2)2(py)2, and Ni(2)2(dipy) · H2O were isolated. For K(2), Cu(2)2(CH3OH)2, and Ni(2)2(py)2, crystal and molecular structure has been determined. For Cu(2)2 and Ni(2)2(bipy), cooperative magnetic ordering has been recorded at 16.6 and 4.6 K, respectively.

Magnetic interaction between superparamagnetic particles in nanogranular cobalt films

A system of cobalt nanoparticles exhibits a transition from the superparamagnetic state into the state with cooperative magnetic ordering caused by the magnetic interaction between Co particles. It is shown that this transition can be used for obtaining nanogranular materials possessing soft magnetic properties at a large electric resistivity.

Inducing Magnetism in Wide Band Gap Hosts

ABSTRACTWe have used precursor routes to prepare magnetic transition metal ion (tM) substituted wurtzite ZnO powders with up to 15% tM substitution (tM = Co and Mn) on the cation site. Careful magnetic studies reveal these samples show no cooperative magnetic ordering, and certainly no ferromagnetism. Instead, the nearest-neighbor coupling is actually antiferromagnetic. Modeling of the temperature dependence of the magnetic susceptibility indicates the difficulty in inducing ferromagnetism, in keeping with the results of density functional calculations. The alternate strategy of inducing dilute ferri magnetism in wide band gap spinel hosts with two cation sites has been more successful; dilute magnets based on tM substitution in spinel ZnGa2O4 seem promising, displaying magnetic hysteresis in nearly transparent samples.

First Examples of Long Range Magnetic Ordering of Structurally Characterized Pentachloroferrate III Di-Anions: FeCl5 2-, and Its Methanol and Aquo Adducts

Abstract A. C. susceptibility and iron-57 Mossbauer spectroscopy measurements are used to investigate the first examples of cooperative magnetic ordering phenomena for FeCl3 2- anions and the related methanol and aquo adducts. Depending on the counter cation and/or solvate identity examples of uncanted antiferromagnetic and canted (weakly ferromagnetic) three dimensional order are observed with critical temperatures - 7K.

A Mössbauer spectroscopy investigation of the Intergrowth Phases LaSr3Fe3−xMx010−δ (M = Al, Cu)

AbstractIron‐57 Mössbauer spectroscopy confirms a high sensitivity of the three‐dimensional magnetic ordering temperature (TNéel) for a series of new intergrowth phases to both oxygen stoichiometry and the partial substitution of iron by copper and aluminium in the Ruddlesden‐Popper phase LaSr3Fe3010−δ. The chemical isomer shifts suggest that significant covalent electron delocalization exists in these phases. Spectra for the paramagnetic phases indicate two distinct iron coordination environments consistent with x‐ray and neutron diffraction structure determinations. The Mössbauer spectra at 4.8 K exhibit the overlap of two magnetic hyperfine patterns corresponding to cooperative magnetic order at the iron sites with internal fields of 45 and 27 Tesla for nominal Fe3+ and Fe4+ sites respectively.

Spin reorientations in Nd 1.8Sr 0.2NiO 3.8

Nd 1.8Sr 0.2NiO 3.8 powder samples are monoclinic B112/n (C2/c) from 1.5 K to RT. The Ni sublattice becomes 3D antiferromagnetically ordered at T 1 ≈ 320 K and undergoes two additional spin reorientations at T 2≈ 130 and T 3≈ 12K. The last one coincides with the onset of the cooperative magnetic ordering of the Nd 3+ magnetic moments.

Zero and Applied Field Mossbauer Spectroscopy Studies of Strong and Weak Ferromagnetic Interactions in Inorganic and Organometallic Solids

Abstract The use of zero and applied field Mossbauer spectroscopy in the study of ferromagnetic interactions in the solid state is reviewed. The angular intensity factors relevant to magnetic hyperfine splitting and cooperative magnetic ordering for the I = ½ → I = 3-2; transition of Iron-57 are presented. The following examples are then considered: (a) very strong ferromagnetic interactions in metallic α-iron, (T critical) − 1043K, (b) very weak ferromagnetic interactions, T (critical) − 1K, for ionic fluorides of high spin iron III, (c) studies of highly anisotropic 1D chain metamagnets with direct spectroscopic observation of magnetization of ferromagnetic chains in small external fields, and finally (d) slow paramagnetic relaxation and ferromagnetism in dimers based on the low-spin iron III (spin doublet) centers of decamethylferrocenium units.

Heat capacities of synthetic hedenbergite, ferrobustamite and CaFeSi 2O 6 glass

Heat capacities have been measured for synthetic hedenbergite (9–647 K), ferrobustamite (5–746 K) and CaFeSi 2O 6 glass (6–380 K) by low-temperature adiabatic and differential scanning calorimetry. The heat capacity of each of these structural forms of CaFeSiO 6 exhibits anomalous behavior at low temperatures. The X-peak in the hedenbergite heat-capacity curve at 34.5 K is due to antiferromagnetic ordering of the Fe 2+ ions. Ferrobustamite has a bump in its heat-capacity curve at temperatures less than 20 K, which could be due to weak cooperative magnetic ordering or to a Schottky anomaly. Surprisingly, a broad peak with a maximum at 68 K is present in the heat-capacity curve of the glass. If this maximum, which occurs at a higher temperature than in hedenbergite is caused by magnetic ordering, it could indicate that the range of distortions of the iron sites in the glass is quite small and that coupling between iron atoms is stronger in the glass than in the edge-shared octahedral chains of hedenbergite. The standard entropy change, S o 298.15 − S o 0, is 174.2 ± 0.3, 180.5 ± 0.3 and 185.7 ± 0.4 J/mol·K for hedenbergite, ferrobustamite and CaFeSi 2O 6 glass, respectively. Ferrobustamite is partially disordered in Ca-Fe distribution at high temperatures, but the dependence of the configuratonal entropy on temperature cannot be evaluated due to a lack of information. At high temperatures (298–1600 K), the heat capacity of hedenbergite may be represented by the equation C o p (J/mol·K) = 3 l0.46 + 0.01257 T-2039.93 T −1 2 − 1.84604× l0 6T −2 and the heat capacity of ferrobustamite may be represented by C o p (J/mol·K) = 403.83−0.04444T+ 1.597× 10 −5T 2−3757.3T −1 2 .

Nuclear Cooperative Ordering in Single-Crystal Praseodymium at Low Temperatures

We have observed a $\ensuremath{\lambda}$ anomaly in the nuclear heat capacity of a single crystal of praseodymium known not to order down to 1.8 K. The anomaly is associated with a cooperative magnetic ordering of the hexagonal-site nuclei at 25-30 mK of the type discussed by Murao. This result is in contrast to earlier calorimetric measurements where ionic order has precluded cooperative nuclear processes. The cooperative anomaly is suppressed by a field of 1.5 T.

Peculiar anomalies in electrical resistivity of some rare earth metals at low temperatures

Step-type anomalies in the electrical resistivity between 4 and 5°K found in holmium, erbium, thulium, and lutetium, contaminated with tantalum, are described. The results are discussed in terms of the concept of super-conducting filaments formed by tantalum. Some other features of the electrical resistivity (hysteresis effects above critical temperature of tantalum, resistivity minimum) also observed in certain cases are described and are difficult to understand from the viewpoint of a superconductivity sub-structure. It is possible that in special cases some co-operative magnetic ordering occurs that is responsible for the observed effects. Some experiments which should clarify the proposed models are suggested.