The intercalation reaction of pyridine with manganese thiophosphate, MnPS3

The intercalation of pyridine in the layered manganese thiophosphate, MnPS3, has been examined in detail by a variety of techniques. The reaction is interesting since none of the anticipated changes in optical and electrical properties associated with intercalation of electron donating molecules is observed. The only notable change in the properties of the host lattice is in the nature of the low-temperature magnetic ordering; while MnPS3 orders antiferromagnetically below 78 K, the intercalated compound shows weak ferromagnetism probably due to a canted spin structure. Vibrational spectra clearly show that the intercalated species are pyridinium ions solvated by neutral pyridine molecules. The corresponding reduced sites of the host lattice, however, were never observed. The molecules in the solvation shell are exchangeable. Although the reaction appears to be topotactic and reversible, from XRD, a more detailed analysis of the products of deintercalation reveal that it is not so. The intercalation proceeds by an ion exchange/intercalation mechanism wherein the intercalated species are pyridinium ions solvated by neutral molecules with charge neutrality being preserved not by electron transfer but by a loss of Mn2+ ions from the lattice. The experimental evidence leading to this conclusion is discussed and it is shown that this model can account satisfactorily for the observed changes (or lack of it) in the optical, electrical, vibrational, and magnetic properties.

Charge-ordering in manganates

Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln(1-x)A(x)MnO(3) (Ln rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulating properties. This phenomenon is to be contrasted with the ferromagnetic metallic behavior that occurs when double-exchange between the Mn3+ and Mn4+ ions predominates. Two distinct types of charge-ordering can be delineated. In one, a ferromagnetic metallic (FMM) state transforms to the charge-ordered (CO) state on cooling. In the other scenario, the CO state is found in the paramagnetic ground stale and there is no ferromagnetism down to the lowest temperatures. Magnetic fields transform the CO state to the FMM state, when the average radius of the A-site cations is sufficiently large ([r(A)] > 1.17 Angstrom). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also found only when [r(A)] greater than or similar to 1.17 Angstrom. The effect of the size of the A-cations on the Mn-O-Mn angle is not enough to explain the observed variations of the charge-ordering temperature as well as the ferromagnetic Curie temperature T-c. An explanation based on a competition between the Mn and A-cation orbitals for sigma-bonding with the oxygen rho(sigma) orbitals is considered to account for the large changes in T-c and hence the true bandwidth, with [r(A]). Effects of radiation, electric field, and other factors on the CO state are discussed along with charge-ordering in other manganate systems. Complex phase transitions, accompanied by changes in electronic and magnetic properties, occur in manganates with critical values of(rA) Or bandwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-delta.

A new class of magnetic materials: Sr2FeMoO6 and related compounds

Ordered double perovskite oxides of the general formula A2BB′O6 have been known for several decades to have interesting electronic and magnetic properties. However, a recent report of a spectacular negative magnetoresistance effect in a specific member of this family, namely Sr2FeMoO6, has brought this class of compounds under intense scrutiny. It is now believed that the origin of the magnetism in this class of compounds is based on a novel kinetically-driven mechanism. This new mechanism is also likely to be responsible for the unusually high temperature ferromagnetism in several other systems, such as dilute magnetic semiconductors, as well as in various half-metallic ferromagnetic systems, such as Heussler alloys.

Novel Mn-doped chalcopyrites

Heavily Mn-doped II-VI-V-2 semiconductors, such as CdGeP2 and ZnGeP2 have been prepared by depositing Mn on single crystalline substrate at nearly 400 T in an ultra high vacuum chamber. Well-defined ferromagnetic hysteresis with a saturation behavior appears in the magnetization curve up to above room temperature. The chemical states of the ZDGeP(2):Mn interface has been clarified by a careful in situ photoemission spectroscopy. The as-prepared surface consists of Ge-rich, metallic Mn compound. In and below the sub-surface region, dilute divalent Mn species as precursors of the DMS phase exist. No MnP phase was observed at any stage of the depth profile. Theoretical band-calculation suggests that the system with vacancies (Cd, V-c, Mn)GeP2 or a non-stoichiometric (Cd, Ge, Mn)GeP2 are ferromagnetic and energetically stable although ferromagnetism is not stable in a stoichiometric compound (Cd, Mn)GeP2. (C) 2003 Elsevier Ltd. All rights reserved.

Experimental evidence of Ga-vacancy induced room temperature ferromagnetic behavior in GaN films

We have grown Ga deficient GaN epitaxial films on (0001) sapphire substrate by plasma-assisted molecular beam epitaxy and report the experimental evidence of room temperature ferromagnetic behavior. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm(-1) in Raman spectra confirms the existence of Ga vacancies. The x-ray photoelectron spectroscopic measurements further confirmed the formation of Ga vacancies; since the N/Ga is found to be >1. The ferromagnetism is believed to originate from the polarization of the unpaired 2p electrons of N surrounding the Ga vacancy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3654151]

Growth, magnetotransport, and magnetic properties of ferromagnetic (In,Mn)Sb crystals

In1−xMnxSb crystals are grown with different Mn doping concentrations (x = 0.006, 0.01, 0.02, and 0.04) beyond the equilibrium solubility limit by the horizontal Bridgman technique. Structural, magnetic, and magnetotransport properties of the grown crystals are studied in the temperature range 1.4–300 K. Negative magnetoresistance and anomalous Hall effect are observed below 10 K. The anomalous Hall coefficient is found to be negative. The temperature dependence of the magnetization measurement shows a magnetic ordering below 10 K, which could arise from InMnSb alloy formation. Also, the saturation in magnetization observed even at room temperature suggests the existence of ferromagnetic MnSb clusters in the crystals, which has been verified by scanning electron microscopy studies. The carrier concentration increases with Mn doping, and this results in a decrease of resistivity. The carrier concentration and mobility at room temperature for the doped crystals are ∼ 2×1019 cm−3 and ∼ 200 cm2/V s, respectively. The observed anomalous Hall effect suggests the carrier mediated ferromagnetism below 10 K in In1−xMnxSb crystals.

Magnetization in electron- and Mn- doped SrTiO3

Mn- doped SrTiO3.0, when synthesized free of impurities, is a paramagnetic insulator with interesting dielectric properties. Since delocalized charge carriers are known to promote ferromagnetism in a large number of systems via diverse mechanisms, we have looked for the possibility of any intrinsic, spontaneous magnetization by simultaneous doping of Mn ions and electrons into SrTiO3 via oxygen vacancies, thereby forming SrTi1-xMnxO3-delta, to the extent of making the doped system metallic. We find an absence of any enhancement of the magnetization in the metallic sample when compared with a similarly prepared Mn doped, however, insulating sample. Our results, thus, are not in agreement with a recent observation of a weak ferromagnetism in metallic Mn doped SrTiO3 system.

Physical and magnetic properties of (Co, Ag) doped ZnO nanoparticles

Undoped and (Co, Ag) co-doped ZnO nanostructure powders are synthesized by chemical precipitation method without using any capping agent and annealed in air ambient at 500 A degrees C for 1 h. Here, the Ag concentration is fixed at 5 mol% and Co concentration is increased from 0 to 5 mol%. The X-ray diffraction studies reveal that undoped and doped ZnO powders consist of pure hexagonal structure and nano-sized crystallites. The novel Raman peak at 530 cm(-1) has corroborated with the Co doped ZnO nanoparticles. Moreover, the PL studies reveal that as the Co doping concentration increases and it enters into ZnO lattice as substitutional dopant, it leads to the increase of oxygen vacancies (Vo) and zinc interstitials (Zn-i). From the magnetization measurements, it is noticed that the co-doped ZnO nanostructures exhibit considerably robust ferromagnetism i.e. 4.29 emu g(-1) even at room temperature. These (Co, Ag) co-doped ZnO nanopowders can be used in the fabrication of spintronic and optoelectronic device applications.

Effect of size reduction on magnetic ordering in Bi0.2Sr0.8MnO3

We present a comparative study of the temperature dependent magnetic properties and electron paramagnetic resonance parameters of nano and bulk samples of Bi0.2Sr0.8MnO3 (BSMO). Bulk BSMO is known to have a high T-N similar to 260K and robust charge ordering (T-CO similar to 360 K). We confirm that the bulk sample shows an antiferromagnetic transition around similar to 260K and a spin-glass transition similar to 40 K. For the nano sample, we see a clear ferromagnetic transition at around similar to 120 K. We conclude that spin glass state, which is present due to the co-existence of antiferromagnetic and ferromagnetic states in the bulk sample, is suppressed in the nano sample and ferromagnetism is induced instead. (C) 2014 AIP Publishing LLC.

Epitaxial Co metal thin film grown by pulsed laser deposition using oxide target

We report here the growth of epitaxial Co metal thin film on c-plane sapphire by pulsed laser deposition (RD) using Co:ZnO target utilizing the composition inhomogeneity of the corresponding plasma. Two distinct plasma composition regions have been observed using heavily alloyed Co0.6Zn0.4O target. The central and intense region of the plasma grows Co:ZnO film; the extreme tail grows only Co metal with no trace of either ZnO or Co oxide In between the two extremes, mixed phases (Co +Co-oxides +Co:ZnO) were observed. The Co metal thin film grown in this way shows room temperature ferromagnetism with large in plane magnetization similar to 1288 emu cm(-3) and a coerciviLy of similar to 230 Oe with applied field parallel to the film-substrate interface. Carrier density of the film is similar to 10(22) cm(-3). The film is epiLaxial single phase Co metal which is confirmed by both X-ray diffraction and transmission electron microscopy characierizaLions. Planar Hall Effect (PHE) and Magneto Optic Kerr Effect (MOKE) measurements confirm that the film possesses similar attributes of Co metal. The result shows that the epiLaxial Co metal thin film can be grown from its oxides in the PLD. (C) 2014 Elsevier B.V. All rights reserved.

Study of ground state phases for spin-1/2 Falicov-Kimball model on a triangular lattice

The spin dependent Falicov-Kimball model (FKM) is studied on a triangular lattice using numerical diagonalization technique and Monte-Carlo simulation algorithm. Magnetic properties have been explored for different values of parameters: on-site Coulomb correlation U, exchange interaction J and filling of electrons. We have found that the ground state configurations exhibit long range Neel order, ferromagnetism or a mixture of both as J is varied. The magnetic moments of itinerant (d) and localized U) electrons are also studied. For the one-fourth filling case we found no magnetic moment from d- and f-electrons for U less than a critical value. `.2014 Elsevier Ltd. All rights reserved.

Observation of magnetically hard grain boundaries in double-perovskite Sr2FeMoO6

Unusual low-temperature magneto-resistance (MR) of ferromagnetic Sr2FeMoO6 polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr2FeMoO6 by utilizing the different probing depths of the different detection modes of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), namely, the total electron yield (TEY) mode (probing depth similar to 5 nm) and the total fluorescence yield (TFY) mode (probing depth similar to 100 nm). At 20 K, the magnetic coercivity detected in the TEY mode (H-c,H- TEY) was several times larger than that in the TFY mode (H-c,H- TFY), indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft and H-c,H- TEY and H-c,H- TFY were nearly the same. From the line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe2+ or well-screened signals, while the non-magnetic component is dominated by Fe3+ or poorly screened signals. Copyright (C) EPLA, 2014

Comparative study of magnetic ordering in bulk and nanoparticles of Sm0.65Ca0.35MnO3: Magnetization and electron magnetic resonance measurements

To explore the effect of size reduction to nanoscale on the hole doped Sm0.65Ca0.35MnO3 compound, dc magnetic measurements and electron magnetic resonance (EMR) were done on bulk and nanoparticle samples in the temperature range 10 <= T <= 300 K. Magnetization measurement showed that the bulk sample undergoes a charge ordering transition at 240K and shows a mixed magnetic phase at low temperature. However, the nanosample underwent a ferromagnetic transition at 75 K, and the charge ordered state was destabilized on size reduction down to nanoscale. The low-temperature ferromagnetic component is found to be enhanced in nanoparticles as compared to their bulk counterpart. Interestingly around room temperature, bulk particles show higher magnetization where as at low temperature nanoparticles show higher magnetization. Ferromagnetism in the bulk is due to super exchange where as ferromagnetism in nanoparticles is due to uncompensated spins of the surface layer. Temperature variation of EMR parameters correlates well with the results of magnetic measurements. The magnetic behaviour of the nanoparticles is understood in terms of the core shell scenario. (C) 2015 AIP Publishing LLC.

Investigations on doping induced changes in structural, electronic structure and magnetic behavior of spintronic Cr-ZnS nanoparticles

Dilute magnetic semiconducting Zn1-xCrxS (x = 0.00, 0.01, 0.03, 0.05, 0.07) nanoparticles were synthesized by the co-precipitation technique using thioglycerol as the capping agent. Powder X-ray diffraction studies showed that Zn1-xCrxS nanoparticles exhibit zinc blende structure with no secondary phase, indicating that Cr ions are substituted at the Zn sites. Photoluminescence and Raman studies show the incorporation of Cr in ZnS nanoparticles. X-ray absorption studies depict that the valence of Zn remains unchanged and maintained in the divalent state, upon doping with Cr. The M-H curves at room temperature indicate the presence of weak ferromagnetism at room temperature due to structural defects. The increase in ferromagnetism with increasing Cr content up to 3%, demonstrates the possibility of tailoring the weak ferromagnetism in ZnS by appropriate Cr doping. (C) 2015 Elsevier Ltd. All rights reserved.

On the Room Temperature Ferromagnetic and Ferroelectric Properties of Pb(Fe1/2Nb1/2)O-3

We report the origin of room temperature (RT) ferromagnetic and ferroelectric properties of Pb(Fe1/2Nb1/2)O-3 (PFN) ceramic sample prepared by modified solid-state reaction synthesis by a single-step method, based on X-ray diffraction (XRD), neutron diffraction (ND), Mossbauer spectroscopy and electron paramagnetic resonance (EPR) spectroscopy results. Formation of single-phase monoclinic PFN ceramic with Cm space group was confirmed by XRD and ND at RT. The morphology studied by scanning electron microscopy (SEM) confirmed uniform microstructure of the sample with average grain size of similar to 2 mu m. The ND, Mossbauer spectroscopy, M-H loop and EPR studies were carried out to confirm the existence of weak ferromagnetism at RT. A clear opening of hysteresis (M-H) loop is evidenced as the existence of weak ferromagnetism at RT. EPR spectrum clearly shows the ferromagnetism through a good resonance signal. The symmetric EPR line shape with g = 1.9895 observed in PFN sample was identified to be due to Fe3+ ions. Mossbauer spectroscopy at RT shows superparamagnetic behaviour with presence of Fe in 3+ valence state. Ferroelectric P-E loops on PFN at RT confirm the existing ferroelectric ordering. Our observation is in agreement with literature, and it supports that the origin of ferromagnetism and ferroelectricity is isolated, i.e. from different regions in the sample. Our results do not support the multiferroic nature of PFN at RT.