Dc Magnetic Studies Research Articles

Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La0.5Ca0.5MnO3

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x=0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles.

Spin glass behavior and critical analysis across the magnetic phase transition in Gd2CoMnO6: dc magnetization and ac susceptibility study

We report here on critical analysis across magnetic phase transition and spin dynamics in Gd2CoMnO6. We found that this material behaves differently below and above the applied magnetic field of 20 kOe. The magnetic phase transition switches from nearly mean-field type to unusual class and Tc shifts towards the high temperature above 20 kOe field. The nature of the magnetic phase transition is explored by carrying out critical analysis at low as well as at high magnetic field. The critical exponents obtained at low field using Kouvel-Fisher method are β = 0.65 (2) γ = 0.90 (2), δ = 2.43 and Tc = 120 K. Apparently, these values of critical exponents appear close to mean-field model. For high field the critical exponents are β = 1.24 (2) γ = 0.64 (5), δ = 1.51 (3) and Tc = 128 K. The critical exponents show significant deviation from any universal class. This switchover in the nature of the magnetic phase transition is unique and not seen in many compounds. The formation of non-Griffiths-like clusters in this compound can be a reason for such unique behavior. Further, ac susceptibility has been measured to understand the spin dynamics in detail. The dispersion of frequency-dependent χac below Tc confirms a spin glass state in this material. The observed value of τo and To indicate the slow dynamic spin which is caused by co-existence of Co/Mn spin magnetic moments. The magneto-caloric effect is also presented for Gd2CoMnO6 in this study. The magnetic study and critical analysis across the phase transition reveal a switchover in the nature of phase transition in this material. A non-Griffiths like cluster formation above Tc is found and dynamic susceptibility study reveals a spin glass state below Tc in Gd2CoMnO6.

Halo‐Substituted Blatter Radicals and Their Role in Modulating Magnetic Interaction in Metal Complexes: A Combined Experimental and Theoretical Study

AbstractHerein, we report the synthesis, structure, and magnetic properties of three new halo‐substituted benzotriazinyl radicals (Rad3a‐3c where a=Cl, b=Br, c=I) and two metal complexes [M(hfac)2(Rad3c)⋅CHCl3](M=Cu(II) (1), Zn(II) (2) and hfac: hexafluoroacetylacetonate) as representative examples. Structural analyses disclose that all the radicals are isostructural. Compared to the reported benzotriazinyl radicals, the N4 atom of pyridine flips opposite to the spin bearing (N1) centre, and it is stabilized by secondary C−H⋅⋅⋅N interaction. The π⋅⋅⋅π stacking interaction between the spin bearing centre and pyridyl ring decreases as the size of the halogen increases from ‐Cl to ‐I. The stacking distance is substantially longer than the other coordinating benzotriazinyl radicals reported so far. The dc magnetic studies reveal the presence of strong intermolecular antiferromagnetic exchange interaction between the radical dimers, which decreases in the order of Rad3a→Rad3b→Rad3c. Complex 1 shows a strong intramolecular through‐bond metal‐radical ferromagnetic interaction, whereas a weak intermolecular through‐space radical⋅⋅⋅radical antiferromagnetic exchange interaction is observed in complex 2. Furthermore, the broken symmetry (BS)‐DFT calculations were performed to get more insight into the exchange interaction for all the radicals and metal complexes.

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2.

The preparation of three new heterometallic clusters [Ce6Mn12O17(O2CPh)26] (1), [Ce10Mn14O24(O2CPh)32] (2), and [Ce23Mn20O48(OH)2(tbb)46(H2O)4](NO3)2 (3; tbb- = 4-tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral CeIV-oxo core: a Ce6 in 1, two edge-fused Ce6 giving a Ce10 bioctahedron in 2, or a larger Ce19 octahedron in 3. Complex 1 is the first Ce6 cluster with a central μ6-O2-. 2 and the cation of 3 are molecular nanoparticles of CeO2 (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce19O32} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO2 lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mnn ions disposed as four Mn3, two Mn7, and four Mn5 units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO2/MnOx mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO2 phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = 1/2, and S = 3/2 ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn3 unit with no interaction between the end MnIII ions gave an excellent fit with the following values: J1 = 5.2(3) cm-1, J2 = -7.4(3) cm-1, and g = 1.96(2).

Enhanced B-site cation ordering and improved magnetic properties: Role of A-site Bi substitution in La2NiMnO6 double perovskites

The influence of A-site Bi substitution on B-site cation ordering, crystal structure, and magnetic properties of La2-xBixNiMnO6 double perovskites has been studied. Compounds with and without Bi substitution adopt monoclinic crystal structure with the P21/n space group as determined from the Rietveld refinement of x-ray diffraction data. X-ray photoelectron spectroscopy indicated an increase in Mn4+/Mn3+ and Ni2+/Ni3+ ratios in Bi substituted sample, and hence emphasized improved Ni and Mn ordering. This has been well supported by bond length analysis which suggested a compression in<Mn–O>and elongation in<Ni–O>bond lengths. The Bi substituted compound showed a relatively larger moment as compared to the parent sample, which was attributed to the suppression of antisite disorder i.e., enhanced Ni-Mn cation ordering due to Bi substitution. The temperature-dependent dc magnetization studies reveal a ferromagnetic to paramagnetic phase transition, and the Curie temperature has been observed to increases with Bi substitution. The critical exponent analysis around phase transition also confirmed the increase in the critical temperature for the Bi substituted compound. A detailed analysis of the critical behavior has been performed using different models, which suggested a long-range ferromagnetic ordering for both pure and Bi substituted samples.

Mono- and tetranuclear Fe(II,III) complexes with primary 1,3-diaminopropane: Synthetic aspects, magnetic properties and thermal behavior

New iron carboxylate complexes with a aliphatic diamine, 1,3-diaminopropane (dap), were obtained under various synthesis conditions: in the air, [FeIII4O2(Piv)8(dap)2] (1, Piv is the pivalate anion), and in an inert atmosphere, [FeII(Piv)2(dap)2] (2). The structure of the compounds was determined by single crystal X-ray diffraction analysis. In mononuclear complex 2, the iron center coordinates two carboxylate ligands and two chelate dap. In the tetranuclear complex 1, the coordination environment of the metal ions is different, and only two ions coordinate the chelate dap. According to Mössbauer spectroscopy data, the Fe3+ ions in 1 and Fe2+ ion in 2 are in a high-spin state and in an octahedral environment of donor atoms. A DC magnetic susceptibility study and quantum-chemical analysis of intramolecular pathways using broken-symmetry density functional theory (DFT) showed that the magnetic exchange coupling between the Fe3+ ions of the central Fe2(μ-O)2 unit is weak (|J| ≤ 7 cm−1), while each of these ions is coupled by fairly strong antiferromagnetic exchange interactions with two other Fe3+ ions (J = −32.9/−43.2 cm−1). The thermal behavior of 1 and 2 was studied by simultaneous thermal analysis (STA).

A study on magnetic and spin polarized electric transport properties of cation doped LaCaMnO3perovskite ceramic system

Thepoly-crystallineLa0.5Nd0.15Ca0.35MnO3 has been synthesized by a conventional solid reaction method through the cation doping in LaCaMnO3. The analysis of the powder XRD of the composites revealed the formation of an orthorhombic structure in Pbnm space group. The cell parameters of the formed system has obtained and confirmed through Reitveld analysis and it is found to be a=5.4505,b=5.4457,c=7.6876.The lattice parameters and the spin ordering temperature TC of this system are decreasing due to Nd substitution. The ac and dc magnetization studies and resistivity measurements in ZFC and FC mode gives good understanding to the magnetic and electric transport properties of the doped system. The magnetization studies show a paramagnetic to ferromagnetic transition at a temperature below the critical temperature TC.The resistivity measurements reveals semiconducting to metallic transition below the temperature TMS. The temperature dependent transport nature shown in this system is one type of the correlated polaron hopping mechanism and it can be modeled by an adiabatic small polaron hopping(SPH) mechanism.

Rare nuclearities in Mn/oxo cluster chemistry: Synthesis and characterization of a mixed-valence {MnII/III11} complex bearing acetate and salicylhydroximate(-3) bridging/chelating ligands

The one-pot reaction between Mn(O2CMe)2·4H2O, salicylhydroxamic acid (shaH2) and Me4NOH·5H2O in a molar ratio of 2:1:2, and a solvent mixture comprising MeCN and MeOH, affords the new mixed-valence cluster compound (Me4N)4[MnII2MnIII9O4(O2CMe)9(shi)6] (1) in moderate yields. The anion of the tetradentate (N,O,O,O) chelating/bridging ligand shi3− resulted from the in-situ metal ion-assisted amide-iminol tautomerism of shaH2. The [MnII2MnIII9(μ3-O)4(μ-OR)3(μ3-NO)6]14+ core of 1 comprises four vertex-sharing {MnIII3(μ3-O2−)}7+ triangular subunits of different types, which are linked to each other and the MnII atoms through the oximato arms and the carboxylate O-atoms of six η1:η1:η1:η2:μ3 shi3− and three η1:η2:μ3 MeCO2− groups, respectively. Peripheral ligation about the core is provided by six η1:η1:μ MeCO2− groups and the chelating parts of the shi3− ligands. Solid-state direct-current (dc) magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the Mn centers, mainly promoted by the coplanarity of the μ3-O2− within the {Mn3} triangular planes and the insignificant distortion of the MnNOMn bridging units. Low-lying excited states precluded obtaining a good fit from the magnetization data, and the ground state was instead determined from the alternating-current (ac) data, which indicated an S = 3 ground state for 1. The combined results emphasize the continuing supply from the use of the salicylhydroxime chelating/bridging ligand of new 3d-metal clusters of very rare nuclearities and beautiful molecular structures.

Evolution from sinusoidal to collinear A-type antiferromagnetic spin-ordered magnetic phase transition in Tb1−x Pr x MnO3 solid solution

The present study reports on the structural and magnetic phase transitions in Pr-doped polycrystalline Tb0.6Pr0.4MnO3, using high-resolution neutron powder diffraction (NPD) collected at SINQ spallation source, to emphasize the suppression of the sinusoidal magnetic structure of pure TbMnO3 and the evolution to a collinear A-type antiferromagnetic ordering. The phase purity, Jahn–Teller distortion, and one-electron bandwidth for eg orbital of Mn3+ cation have been calculated for polycrystalline Tb0.6Pr0.4MnO3, in comparison to the parent materials TbMnO3 and PrMnO3, through the Rietveld refinement study from x-ray diffraction data at room temperature, which reveals the GdFeO3 type orthorhombic structure of Tb0.6Pr0.4MnO3 having Pnma space group symmetry. The temperature-dependent zero field-cooled and field-cooled dc magnetization study at low temperature down to 5 K reveals a variation in the magnetic phase transition due to the effect of Pr3+ substitution at the Tb3+ site, which gives the signature of the antiferromagnetic nature of the sample, with a weak ferromagnetic component at low temperature-induced by an external magnetic field. The field-dependent magnetization study at low temperatures gives the weak coercivity having the order of 2 kOe, which is expected due to the canted-spin arrangement or ferromagnetic nature of Terbium ordering. The NPD data for Tb0.6Pr0.4MnO3 confirms that the nuclear structure of the synthesized sample maintains its orthorhombic symmetry down to 1.5 K. Also, the magnetic structures have been solved at 50 K, 25 K, and 1.5 K through the NPD study, which shows an A-type antiferromagnetic spin arrangement having the magnetic space group Pn′ma′.

Tuning of multi-magnetic phase and exchange bias effect by antisite disorder in Ca-doped La2CoMnO6 double perovskites

The exchange bias effect at the magnetic interfaces and multi-magnetic phases strongly depends on the antisite disorder (ASD) driven spin configuration in the double perovskite systems. The percentage of ASD in double perovskites is extensively accepted as a key for designing diverse new nanospintronics with tailored functionalities. In this regards, we have investigated such ASD driven phenomena in Ca2+ doped bulk and polycrystalline La2−x Ca x CoMnO6 (0 ⩽ x ⩽ 1) series of samples. The structural and Raman studies provide evidence of an increase in the disorder due to the increment of Ca concentration in the parent compound (x = 0). The enhancement of disorder in the doped system induces various magnetic orderings, magnetic frustration and cluster glass-like behavior, which have been confirmed from AC and DC magnetic studies and neutron diffraction studies. As a result, significantly large exchange bias effects, namely zero-field cooled (spontaneous) and field-cooled (conventional) exchange bias, are found. These results reveal the tuning of ASD by doping, which plays an active role in the spin configuration at the magnetic interfaces.

Growth and characterization of Dy1-xYxMnO3 single crystals by optical floating zone technique: A combined X-ray diffraction and DC magnetization study

Large size single crystals of DyMnO3, YMnO3 and their solid solutions, Dy1-xYxMnO3, have been grown by the optical floating zone technique. It is shown that pristine DyMnO3 crystals grown in air atmosphere correspond to the orthorhombic phase in the Pnma space group symmetry while growth in the argon atmosphere leads to the hexagonal phase in the P63cm space group symmetry. The crystals grown in air atmosphere for the composition range 0.10 ≤ x ≤ 0.50 show coexistence of orthorhombic and hexagonal phases with increasing hexagonal phase fraction until at x ~ 0.60, the orthorhombic phase completely transforms to the hexagonal phase even in the air atmosphere. On the other hand, all the crystals grown in argon atmosphere correspond to the hexagonal phase only. Our Rietveld refinements using X-ray powder diffraction data on powders obtained after crushing the crystals further confirm the hexagonal structure in the P63cm space group symmetry for all crystals grown in argon atmosphere. The unit cell volume and the lattice parameters obtained by Rietveld refinement are shown to decrease with increasing Y3+ substitution at the Dy3+ site in DyMnO3. DC magnetisation measurements on h-Dy1-xYxMnO3 single crystals reveal that Y3+ substitution marginally increases the antiferromagnetic ordering temperature (TN) from 67 K for h-DyMnO3 to 72 K for h-YMnO3. However, the first spin reorientation (SR) transition temperature (TSR1) of Mn3+ decreases with increasing Y3+ substitutions as per TSR1 ~ k(x-xc)0.3, where k is a proportionality constant and xc is the critical composition for which this transition temperature goes to 0 K. The remaining two magnetic transition temperatures related to Dy3+ reordering (TDy3+)/spin-glass transition (TSG) and second spin reorientation transition (TSR2) show very weak dependence on Y3+ substitution before going to 0 K around x = xc ~ 0.90 composition. A magnetic phase diagram of h-Dy1-xYxMnO3 showing stability field regions in respect of different magnetic phases is also presented.

Hydrothermal synthesis of Bi2Fe4O9 nanochains and study of their multiferroic coupling

We report synthesis of self-assembled Bi2Fe4O9 nanochains by hydrothermal method using Oleic acid as a surfactant which plays an important role in the self-assembled nanochain formation. Temperature dependent dc magnetization study suggests downward shift of the Neel temperature TN ~ 63 K for the particles of size ~ 22 nm while it is reported to be ~ 260 K in the bulk sample. The magnetic hysteresis (M−H) loops offer evidence of weak ferromagnetism across the entire temperature range 10–300 K. Exchange coupling across the interface between coexisting ferromagnetic and antiferromagnetic orders gives rise to a small amount of exchange bias at low temperature. The sample also exhibits substantial magnetoelectric multiferroic coupling at room temperature with ~77% suppression of ferroelectric polarization under 10 kOe magnetic field.

ZnII and CuII-Based Coordination Polymers and Metal Organic Frameworks by the of Use of 2-Pyridyl Oximes and 1,3,5-Benzenetricarboxylic Acid.

The simultaneous use of 2-pyridyl oximes (pyridine-2 amidoxime, H2pyaox; 2-methyl pyridyl ketoxime, Hmpko) and 1,3,5-benzenetricarboxylic acid (H3btc) provided access to five new compounds, namely [Zn(H2btc)2(H2pyaox)2]•2H2O (1•2H2O), [Zn(Hbtc)(H2pyaox)2]n (2), [Cu(Hbtc)(H2pyaox)]n (3), [Cu(Hbtc)(HmpKo)]n (4) and [Cu2(Hbtc)2(Hmpko)2(H2O)2]•4H2O (5•4H2O). Among them, 3 is the first example of a metal-organic framework (MOF) containing H2pyaox. Its framework can be described as a 3-c uninodal net of hcb topology with the layers being parallel to the (1,0,1) plane. Furthermore, 3 is the third reported MOF based on a 2-pyridyl oxime in general. 2 and 4 are new members of a small family of coordination polymers containing an oximic ligand. 1–5 form 3D networks through strong intermolecular interactions. Dc magnetic susceptibility studies were carried out in a crystalline sample of 3 and revealed the presence of weak exchange interactions between the metal centres; the experimental data were fitted to a theoretical model with the fitting parameters being J = −0.16(1) cm−1 and g = 2.085(1). The isotropic g value was also confirmed by electronic paramagnetic resonance (EPR) spectroscopy. Reactivity studies were performed for 3 in the presence of metal ions; the reaction progress was studied and discussed for Fe(NO3)3 by the use of several characterization techniques, including single crystal X-ray crystallography and IR spectroscopy.

Expanding the NUIG MOF family: synthesis and characterization of new MOFs for selective CO2 adsorption, metal ion removal from aqueous systems, and drug delivery applications.

Metal organic frameworks (MOFs) have attracted considerable attention in recent years due to their use in a wide range of environmental, industrial and biomedical applications. The employment of benzophenone-4,4'-dicarboxylic acid (bphdcH2) in MOF chemistry provided access to the 3D mixed metal MOFs [CoNa2(bphdc)2(DMF)2]n (NUIG2) and [ZnK2(bphdc)2(DMF)2]n (NUIG3), and the 2D homometallic MOF [Co2(OH)(bphdcH)2(DMF)2(H2O)2]n(OH)·DMF (1·DMF). 1·DMF is based on a dinuclear SBU and consists of interpenetrating networks with an sql topology. Dc magnetic susceptibility studies were carried out in 1·DMF and revealed the presence of weak antiferomagnetic exchange interactions between the metal centres. NUIG2 and NUIG3 are structural analogues of [ZnNa2(bphdc)2(DMF)2]n (NUIG1), which has shown an exceptionally high encapsulation for ibuprophen (Ibu), NO and metal ions. Both NUIG2 and NUIG3 display high metal ion (CoII, NiII, CuII) adsorption capacity, comparable to that of NUIG1, with NUIG2 exhibiting good performance in Ibu uptake (780 mg Ibu per g NUIG2). Monte Carlo simulations were conducted in NUIG1 in order to assess its adsorption capacity for other guest molecules, and revealed that it possesses an outstanding CO2 uptake at ambient pressure, which is larger than that of the previously reported best functioning species (104 vs. 100 cm3 (stp) per cm3). Furthermore, NUIG1 exhibits high selectivity for CO2 over CH4.

Magnetic ordering of Ho and its role in the magnetization reversal and coercivity double peaks in the Ho3Fe5O12 garnet

Rare-earth based garnet system, Ho3Fe5O12 shows magnetization compensation phenomenon at Tcomp = 138 K and a magnetic transition at 50 K. In the present work, we reveal the physics of these low temperature magnetic transitions by carrying out detailed dc magnetization, ac susceptibility, neutron depolarization, and neutron diffraction studies. Interestingly, our dc magnetization study under low (≲ 50 Oe) magnetic fields has shown a previously unknown sign reversal of magnetization (below the Tcomp) that persists down to 5 K, the lowest measured temperature. Magnetic measurements under moderate fields between 50 Oe and 1 kOe reveal two compensation temperatures (in the range of 105 – 150 K depending upon the applied field value) leading to double magnetization reversals. Interestingly, for magnetic fields ≥ 1 kOe, these two compensation temperatures merge to a single Tcomp (=138 K). A modified Stoner-Wohlfarth model has been used to explain the asymmetrical nature of double peak around the Tcomp in the temperature dependent coercivity data. The ac susceptibility study supports the existence of both the transitions with an evidence of deep minimum at the Tcomp (138 K) and a broad peak at 50 K. Mesoscopic neutron depolarization study has revealed a zero domain-magnetization state at the Tcomp. Temperature dependent neutron diffraction study has revealed that Ho carries an induced moment above the Tcomp due to polarizing effect under the internal magnetic field of the two magnetically ordered Fe sublattices at 567 K, whereas a single umbrella type ordering of Ho sublattice moments appears below the Tcomp, resulting in the distortion of magnetic unit cell from cubic to rhombohedral symmetry. However, below 50 K, Ho3+ sites are divided into two inequivalent magnetic sublattices, having different moments and canting angles, and it leads to double umbrella type magnetic structure. Neutron diffraction study has revealed an asymmetric variation of the tetrahedral, octahedral, and dodecahedral sublattice moments resulting in a magnetization reversal at ~ 138 K which is very well supported by the mean field theory calculations. In the present study, we have resolved the ambiguity of magnetic ordering of the rare-earth (Ho3+) sublattice and its role in multiple magnetic transitions. The utility of such compensated ferrimagnetic materials in spin polarizers/analyzers and fast switching memory devices has been emphasized.

Selective Coordination of Self-Assembled Hexanuclear [Ni4Ln2] and [Ni2Mn2Ln2] (Ln = DyIII, TbIII, and HoIII) Complexes: Stepwise Synthesis, Structures, and Magnetic Properties.

The versatility for a unique aggregation of heterometallic 3d-4f and 3d-3d'-4f ions by the new Schiff base ligand 2-{[(2-hydroxy-3-methoxybenzyl)imino]methyl}phenol (H2L) providing O3N donors has been examined. A series of complexes having the general formula [Ln2Ni4(L)4(μ1,3-CH3CO2)2(μ3-OH)4(MeOH)2]·xCH3OH·yH2O [where Ln = DyIII, x = 4, y = 0 (1), Ln = TbIII, x = 0, y = 4 (2), and Ln = HoIII, x = 4, y = 0 (3)] were obtained from the sequential use of lanthanide(III) nitrate salts and Ni(CH3CO2)2·4H2O. The incorporation of two different 3d ions and one 4f ion in the same coordination aggregate was achieved through the isomorphic replacement of two NiII centers by MnIII ions as second group of cationic complexes, [Ln2Ni2Mn2(L)4(μ1,3-CH3CO2)2(μ3-OH)4(MeOH)2](NO3)2·2CH3OH [where Ln = DyIII (4), TbIII (5), and HoIII (6)]. Direct-current (dc) magnetic susceptibility studies hint to the possibility of ferromagnetic interactions occurring in the aggregates, whereas alternating-current susceptibility measurements find both the DyIII analogues, 1 and 4, to show out-of-phase components at zero applied dc field, characteristic of single-molecule-magnet (SMM) behavior. Micro-SQUID studies reveal open hysteresis loops for 1, corroborating its SMM character. Further detailed complete-active-space self-consistent-field and density functional theory calculations were also performed, supporting the experimental findings in complexes 1 and 4.

Investigation of magnetic ordering and origin of exchange-bias effect in doped manganite, Sm0.4Ca0.6MnO3

We have carried out detailed investigations of magnetic ordering and the origin of exchange-bias phenomenology in polycrystalline 154Sm0.4Ca0.6MnO3 perovskite compound by employing dc and ac magnetizations, neutron depolarization, and neutron diffraction techniques. The neutron diffraction study reveals a broad charge ordering transition in the temperature range of 275–150 K, manifested by gradually varying structural distortions. DC magnetization and neutron diffraction studies infer a magnetic ordering around 110 K. Below ∼40 K, this compound exhibits a large enhancement of the magnetization and an exchange-bias effect (∼18 kOe at 5 K). The exchange-bias effect shows a characteristic temperature and cooling field dependence and a training effect. The compound undergoes a CE-type (checker-board type) antiferromagnetic (AFM) ordering of Mn3+/Mn4+ spins below 110 K. The large magnetization below ∼40 K arises from the ferrimagnetic (FIM) ordering of a minority (∼5 vol. %) non-perovskite phase of Mn3O4. The appearance of exchange bias is ascribed due to the presence of an interface, formed between the FIM (Mn3O4) phase and the main AFM perovskite phase. Our study has implications in giving new directions in the search of large exchange-bias effects in mixed FIM/AFM bulk systems.

From 1D Coordination Polymers to Metal Organic Frameworks by the Use of 2-Pyridyl Oximes.

The synthesis and characterization of coordination polymers and metal–organic frameworks (MOFs) has attracted a significant interest over the last decades due to their fascinating physical properties, as well as their use in a wide range of technological, environmental, and biomedical applications. The initial use of 2-pyridyl oximic ligands such as pyridine-2 amidoxime (H2pyaox) and 2-methyl pyridyl ketoxime (Hmpko) in combination with 1,2,4,5-benzene tetracarboxylic acid (pyromellitic acid), H4pma, provided access to nine new compounds whose structures and properties are discussed in detail. Among them, [Zn2(pma)(H2pyaox)2(H2O)2]n (3) and [Cu4(OH)2(pma)(mpko)2]n (9) are the first MOFs based on a 2-pyridyl oxime with 9 possessing a novel 3,4,5,8-c net topology. [Zn2(pma)(H2pyaox)2]n (2), [Cu2(pma)(H2pyaox)2(DMF)2]n (6), and [Cu2(pma)(Hmpko)2(DMF)2]n (8) join a small family of coordination polymers containing an oximic ligand. 9 exhibits selectivity for FeIII ions adsorption, as was demonstrated by a variety of techniques including UV-vis, EDX, and magnetism. DC magnetic susceptibility studies in 9 revealed the presence of strong antiferromagnetic interactions between the metal centers, which lead to a diamagnetic ground state; it was also found that the magnetic properties of 9 are affected by the amount of the encapsulated Fe3+ ions, which is a very desirable property for the development of magnetism-based sensors.

Synthesis, structural variation, and magnetic properties of two polynuclear Mn-based complexes derived from 4,5-Bis(hydroxymethyl)-2-methylpyridin-3-ol

The syntheses, crystal structures and magnetochemical characterization are reported for two novel high-valent manganese complexes, (NMe4)2(Mn3O(PNH)3(PN)3) (1) and, (NH4)[Mn7(PN)6(PNH)2(MeO)2(O)2(CMe3CH2CO2)2] (2) incorporating the anion of 4,5-Bis(hydroxymethyl)-2-methylpyridin-3-ol. Complexes were characterized by elemental and thermogravimetric analyses, FT-IR spectroscopy, and single-crystal X-ray diffraction studies. Variable temperature, solid-state direct current (dc) magnetization studies were carried out on both the complexes 1 and 2 in the 5.0–315 K range, and they established that both complexes to have ground state spin values of S = 5.

Magnetization spin reversal and neutron diffraction study of polycrystalline Tb0.55Sr0.45MnO3

We have investigated the structural and magnetic phase transitions in Sr doped polycrystalline Tb0.55Sr0.45MnO3, using temperature-dependent high-resolution neutron powder diffraction in the HRPT diffractometer at PSI to address the origin of magnetization reversal at low temperature. The solid solution Tb0.55Sr0.45MnO3 crystallizes in O′ type orthorhombic structure having the Pnma symmetry. The substitution of the divalent cation of Sr2+ at the site of Tb3+ dilutes the Mn–Tb interaction and affects the magnetic structure, which was observed by the field and temperature-dependent dc magnetization and neutron diffraction study. The temperature-dependent zero field-cooled and field cooled dc magnetization study reveals the indication of spin reversal phenomena, which is the nature of canted antiferromagnetic or ferrimagnetic transition at 100 Oe because of Mn–Mn sub-lattices interaction below 65 K. By increasing the applied magnetic field up to 20 kOe, a weak ferromagnetic type of behaviour is observed. The field-dependent magnetization shows weak coercivity due to the canted spin structure at low temperatures. The low-temperature neutron diffraction study of polycrystalline Tb0.55Sr0.45MnO3 reveals the non-collinear canted antiferromagnetic structure due to Tb ordering at 1.5 K, which turns into a non-collinear ferromagnetic structure along with spin canting followed by the spin reversal phenomena at 25 K.