Paramagnetic Domains Research Articles

Building in biologically appropriate multifunctionality in aqueous copper indium selenide-based quantum dots.

Advanced nanoplatforms equipped with different functional moieties for theranostics hold appealing promise for reshaping precision medicine. The reliable construction of an individual nanomaterial with intrinsic near-infrared (NIR) photofunction and magnetic domains is much desired but largely unexplored in a direct aqueous synthesis system. Herein, we develop an aqueous phase synthetic strategy for Mn2+ doping of ZnS shell grown on Zn-Cu-In-Se core quantum dots (ZCISe@ZnS:Mn QDs), providing the optimal NIR fluorescence quantum efficiency of up to 18.9% and meanwhile efficiently introducing paramagnetic domains. The relaxometric properties of the water-soluble Mn-doped QDs make them desirable for both the longitudinal and transverse (T1 and T2) magnetic resonance (MR) contrast enhancement due to the shell lattice-doped Mn2+ ions with slow tumbling rates and favoured spin-proton dipolar interactions with surrounding water molecules. Surprisingly, the incorporation of Mn2+ ions into the shell is found to significantly enhance the production of reactive oxygen species (ROS) by combining both the chemodynamic and photodynamic processes upon NIR light irradiation, showing great potential for efficient photo-assisted ablation of cancer cells. Furthermore, a broad-spectrum excitation range beneficial for bright NIR fluorescence imaging of breast cancer has been proven and offers high flexibility in the choice of incident light sources. Multiparametric MR imaging of the brain has also been successfully demonstrated in vivo.

Characterization of Spanish River Carbonatite (SRC) for agricultural fertilizer

Carbonatite is an igneous rock that is composed mainly of carbonate minerals and silicates. It is recently applied as fertilizer due to its high content of calcite and dolomite. This study presents a physical and chemical characterization of the Spanish River Carbonatite (SRC) and its application as fertilizer is tested. The chemical composition obtained by X-ray fluorescence and Energy Dispersive X-ray analyses indicates that Ca, Fe and Si are the main contained elements. Calcite and SiO2 are the principal mineralogical phases as detected by X-ray diffraction. Fourier transform infrared spectroscopy confirms the presence of calcite, silicates and also organic material and water. It also suggests the presence of vermiculite. Thermal analysis reveals only a one-step reaction that corresponds to the decomposition of calcite. Mössbauer spectroscopy performed at room temperature do not show magnetic order between the iron moments instead it shows Fe2+ and Fe3+ doublets in M1 and M2 octahedral sites belonging to vermiculite. At 50 K, we observe that the isomer shift increases their values due to the second order Doppler shift. SQUID magnetometry measurements reveal that most of the sample contains paramagnetic domains assigned to the silicates and calcite components. However, under low applied fields, a tiny signal for Verwey transition appears at 124 K (suggesting the presence of magnetite) although screened by the strong paramagnetic signal of the main components. The SRC has been tested as potential fertilizer on sweet cucumber (Solanum muricatum) which responded positively to the application to carbonatite as an amendment, expressing itself in high foliar biomass.

Anomalous Thermoelectric Transport Properties of Fe‐Rich Magnetic FeTe

The interplay between magnetism and quantum effects has motivated several thermoelectric studies on iron‐telluride yet with little insight on the anomalous features in transport properties near magnetostructural transition temperature (≈70 K). A detailed investigation is carried out on Fe1.1Te by characterizing magnetic, heat capacity, galvanomagnetic, and thermoelectric transport properties to understand the electronic, magnetic, and structural origin of those anomalies. The magnetic susceptibility indicates a bicollinear stripe and short‐range ordering in the antiferromagnetic and paramagnetic domains, respectively. Hall conductivity and transverse magnetoresistance reveal a multicarrier transport impacted by spin fluctuations and magnons. Contributions from phonon‐drag and magnon‐drag are evaluated to understand the origin of the broad peak in antiferromagnetic thermopower. The peak at ≈50 K and the insignificant entropy contribution from the magnonic heat capacity support the phonon‐drag as the origin. The field‐dependent enhancement of thermal conductivity must be associated with field‐dependent spin‐phonon coupling modification. The field‐induced thermopower reduction can be attributed to the suppression of magnons or paramagnons, as evidenced by the magnetic susceptibility data. Above 70 K, the thermal conductivity drops sharply due to the structural change modifying phonon modes. Understanding these properties originated from the spin, and quantum effects are instrumental for designing high‐performance spin‐driven thermoelectrics.

Structural disorder and antiferromagnetism in LaNi1–xPtxO3

We report on the B-site substitution of Pt for Ni in the system LaNi1–xPtxO3. The system can only be synthesized between x ​≤ ​0.50, with LaNiO3 (x ​= ​0.00) and the stoichiometric double perovskite La2NiPtO6 (x ​= ​0.50) as the end members. Higher Pt-contents (x ​> ​0.50) are unachievable due to the preference of Pt to being in oxidation state ​+ ​IV in octahedral coordination. Upon introducing Pt into LaNiO3, a phase transformation from rhombohedral (R-3c) to monoclinic (P21/n) symmetry is observed for 0.075 ​≤ ​x ​≤ ​0.125, where all monoclinic samples are B-site ordered, and Pt show a strong preference for the Pt-site (2c-site) for all compositions (x ​≥ ​0.20). Powder X-ray diffraction analysis reveal disorder of the Pt-distribution in several of the non-equimolar samples (0.20 ​≤ ​x ​≤ ​0.40), which point toward cluster formation with domains of high and low Pt-content within each sample. La2NiPtO6 further show an antiferromagnetic transition at ∼ 40 ​K. A similar transition is observed for all monoclinic samples (x ​≥ ​0.20), however, the transition is less pronounced for lower x. This is explained in terms of the structural disorder, i.e. by the coexistence of antiferromagnetic domains with long range order and paramagnetic domains dominated by short range antiferromagnetic interactions.

Composition dependent room temperature structure, electric and magnetic properties in magnetoelectric Pb(Fe1/2Nb1/2)O3[sbnd]Pb(Fe2/3W1/3)O3 solid-solutions

We report on the studies of room temperature (RT) crystal structure, electric and magnetic properties of (1−x) Pb(Fe1/2Nb1/2)O3 – x Pb(Fe2/3W1/3)O3 (PFN1−x – PFWx) (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) solid solutions through the measurements of X-ray diffraction, FTIR, scanning electron microscopy (SEM), Neutron diffraction, Raman, Magnetic, Mössbauer and ferroelectric measurements. FTIR spectra showed two main perovskite related transmission bands. The SEM analysis shows an average grain size of 2 μm for all the solid solutions. Rietveld refinement was performed on RT X-ray diffraction (XRD) and neutron diffraction (ND), which reveals, the monoclinic phase for x = 0.0 with space group Cm and Cubic phase for x = 1.0 with space group Pm-3m. In other words, increasing x, the samples exhibit a gradual phase transition from monoclinic to cubic. In addition, the Raman spectroscopy corroborates the change in structural symmetry from monoclinic (Cm) to cubic (Pm-3m) on varying x. The coexistence of both monoclinic and cubic symmetries was observed between x = 0.2–0.8. Magnetic measurements shows that, the magnetic phase transition from paramagnetic to antiferromagnetic (AFM) was observed at or above RT for x = 0.6 and above. The magnetic structure was refined using the propagation vector k = (½, ½, ½) and structure was found to be G-type antiferromagnetic. Magnetic properties (M-H loops) shows, a weak ferromagnetic behaviour with antiferromagnetic ordering at RT. At RT, x = 0.0–0.6 the samples exhibits disordered paramagnetic property but weakly coupled with antiferromagnetic domains. But, x = 0.8 and 1.0 samples show antiferromagnetic and they are weakly coupled with paramagnetic domains. The temperature dependent magnetization (M(T)) confirms, the augmentation of Néel temperature (TN) from 155 K to 350 K on increasing x. Mössbauer spectroscopy confirms superparamagnetic nature with the presence of Fe in 3+ state and on increasing x, the spectra changes from doublet to sextet. The ferroelectric (P-E) study confirms the existence of ferroelectric ordering with leaky behaviour. The reasonable ferroelectric loops with antiferromagnetic properties indicate samples with x = 0.2–0.6 show good magnetoelectric characteristics and may find applications in multiferroics.

Fe–Ni solid solutions in nano-size dimensions: Effect of hydrogen annealing

Nanoparticles of Ni0.50Fe0.50 and Ni0.75Fe0.25 alloys were prepared by chemical reduction in ethylene glycol medium. XRD and 57Fe Mössbauer studies have confirmed the formation of Fe–Ni solid solution in nano-size dimensions with FCC structure. These samples consist of both ferromagnetic and paramagnetic domains which have been attributed to the coexistence of large and small particles as confirmed by atomic force microscopic (AFM) and 57Fe Mössbauer spectroscopic studies. Improved extent of Fe–Fe exchange interaction existing in Ni0.50Fe0.50 alloy compared to Ni0.75Fe0.25 alloy explains the observed increase in the relative extent of ferromagnetic domains compared to paramagnetic domains in the former sample. Increase in the relative extent of ferromagnetic domains for hydrogenated alloys is due to increase in particle size brought about by the high temperature activation prior to hydrogenation.

Electron Spin Resonance Study on Room-Temperature Ferromagnetic La<sub>0.9</sub>Sr<sub>0.1</sub>MnO<sub>3</sub> Nanoparticles

La 1−x Sr x MnO 3 half-metal manganites attract much attentions due to the colossal magnetoresistance (CMR) effect1 related to the Griffiths singularity2 with a critical percolation threshold (x=∼0.08). For a ferromagnetic (FM) insulating homogeneous phase of bulk La 0.9 Sr 0.1 MnO 3 (T C ∼ 150 K), the Griffiths phase which contains both FM and paramagnetic (PM) domains could be induced by intrinsic inhomogeneity at a transition temperature T G (275K) above T C .2 Such Griffiths phase could also be affected by the size of sample when the sample dimension is downscaled to nanometers. Previous studies reported many interesting phenomena related to size effects including the size-induced structural transition, core-shell phase separation and the enhancement of magnetization and exchange bias. However, the studies on nano-size effects are mostly focused on the doped metallic phase. Recently, room temperature ferromagnetism is reported in nano-sized insulating La 0.9 Sr 0.1 MnO 3 3 which remains to be understood. In this work, temperature dependent magnetization (M-T) and electron spin resonance (ESR) are investigated in La 0.9 Sr 0.1 MnO 3 nano-particles. ESR is a very sensitive probe to detect the core-shell phase separation and FM signal in Griffiths phase. The field dependent M-T results show the coexistence of multiple magnetic phases which is similar to Griffiths phase in bulk samples. Multiple lines appear in ESR spectra. By fitting with Lorenz/Gaussian functions, the multiple phases are identified as FM and PM states for 270 C and T G are enhanced in nano-sized La 0.9 Sr 0.1 MnO 3 comparing with bulk one. Based on our analysis, the size effect on the crystal structure of nanoparticle could be the cause for room-temperature ferromagnetism.

Observation of the structural, optical and magnetic properties during the transformation from hexagonal NiS nano-compounds to cubic NiO nanostructures due to thermal oxidation

Single phase α-NiS nano-compounds with uniformly distributed hierarchical networks were synthesized by a microwave-assisted hydrothermal technique. The materials were evaluated for thermal stability under an oxidative environment and at temperatures between 150°C and 600°C. NiS materials showed stability at 300°C and NiO formation was observed from 350°C to 600°C. The annealing effect on the crystalline size and IR absorption of the annealed samples is reported by XRD and FTIR studied. The EPR properties of the annealed materials were studied and compared to the oxidized materials. The transition temperature of 350°C for the formation of NiO from NiS oxidation was confirmed by calculating the number of spins obtained from the EPR data. Vibrating sample magnetometry (VSM) shows that this pure NiS has both ferromagnetic ordering and paramagnetic domains. Further, the transition temperature of −9°C of the pure α-NiS nano-compounds was confirmed by performing electrical measurements on the as-synthesized material.

Use of Laser Heat Treatment for Creating Gradient Materials Based on the Fe – Cr – Ni System

The possibility of the use of laser radiation heating for making materials with specified distribution of ferro-and paramagnetic domains is studied. It is shown that application of plastic deformation with a high degree or reduction combined with subsequent laser heating in a specified temperature range to austenite-martensite alloys based on the Fe – Cr – Ni system produces materials with required distribution of regions of ferromagnetic martensite and paramagnetic austenite. The modes of laser treatment providing maximum strength of the formed austenite are determined.

Homogeneous coexistence and phase segregation in 1111 iron-based pnictides studied via NMR

We performed 75As and 51Co nuclear magnetic resonance (NMR) measurements on LaFeAsO1−xFx (La1111), a prototype of iron-based superconductors, and on Ca(Fe1−xCox)AsF (Ca1111), which has a small overlap between the antiferromagnetic (AF) and superconducting (SC) domes in the electronic phase diagram. We found in the Ca1111 series that AF and SC states coexist homogeneously in the overlapped region, while in the La1111 series, we found that paramagnetic (PM) and SC domains coexist around the AF-SC phase boundary. The coexistence of these domains indicates that the AF and SC domes segregate from each other in the electronic phase diagram.

Homogeneous Coexistence in CaFe1−x Co x AsF and Phase Segregation in LaFeAsO1−x F x Studied via NMR

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of 75As NMR measurements in LaFeAsO1−xFx (x=0.026) (La1111), and CaFe1−xCoxAsF (x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(Fe1−xCox)2As2 and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below Tc, suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S±-wave superconductivity, whereas separation of the AF and SC phases gives credence to S++-wave superconductivity.

Magnetic properties of Bi-doped [formula omitted

Structure and magnetic properties of the La 0.5− x Bi x Ca 0.5MnO 3 ( x = 0 , 1/16, 1/8, 1/4, 3/8 and 1/2) system have been experimentally investigated. The substitution of Bi 3+ ion for La 3+ ion systematically enhances the charge-ordering (CO) temperature. Electron spin resonance (ESR) and isothermal magnetization measurements reveal the coexistence of ferromagnetic (FM), antiferromagnetic (AFM) domains and paramagnetic (PM) matrix below T C O in 0 ≤ x ≤ 3 / 8 . Kinetic hindrance to the first order charge ordering transition plays an important role in this coexisting phases. Such a coexistence of PM, FM and AFM domains leads to a compressive strain effect, resulting in a counterclockwise thermal hysteresis in M ( T ) curves. The hysteresis weakens with increasing Bi content, suggesting that the inner compressive strain becomes increasingly weaker. For x = 1 / 2 , only AFM state coexist with PM matrix below T C O and FM state only coexists with AFM state below ∼50 K. Therefore, almost no thermal hysteresis in M ( T ) curves is observed around T C O for x = 1 / 2 .

Phase Separation of Electrons Strongly Coupled with Phonons in Cuprates and Manganites

Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron–electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron–phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott–Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Frohlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high- and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.

Near-interface magnetotransport in La0.75Sr0.25MnO3 epitaxial films on SrTiO3 substrate

The magnetotransport properties of La0.75Sr0.25MnO3 (LSMO) films epitaxially grown on SrTiO3 (0 0 1) substrate were studied. The Curie temperature (Tc) of LSMO films decreases from 300 to 105 K with a reduction in the film thickness to 5 nm. A clear metal–insulator transition is observed at a temperature consistent with Tc for films thicker than 10 nm. However, the 5 nm thick film displays an insulative characteristic along with a high magnetoresistence effect in a wide range of temperatures below and above Tc. A phase-separation phenomenon in the 5 nm thick film was demonstrated by thermally activated hopping transport between ferromagnetic metallic domains embedded in an insulative matrix at temperatures below Tc, but between paramagnetic metallic domains at temperatures above Tc. It was also confirmed by conductive atomic force microscopy images.

Postgrowth annealing effects on heteroepitaxial MnAs thin films grown on GaAs(001) and Si(001)

We have carried out postgrowth annealing studies on heteroepitaxial MnAs thin films deposited on GaAs(001) and Si(001) by molecular-beam epitaxy. Before annealing, a paramagnetic behavior is exhibited for MnAs∕Si(001), whereas a ferromagnetic behavior is observed for MnAs∕GaAs(001). The paramagnetic β-MnAs phase domains of the as-grown MnAs∕GaAs(001) sample disappear completely after postgrowth annealing. Surprisingly, after postgrowth annealing, the crystal orientation of the MnAs∕Si(001) film changes, and is accompanied by a shift from paramagnetic to ferromagnetic behavior. We attribute these observations to relaxation of the biaxial strain associated with the substrates.

Transport study of percolation modes in the mixed-phase manganites

Based on the phase separation scenario, cluster percolation mode is introduced to simulate the percolative conduction of ${\mathrm{La}}_{0.9}{\mathrm{Te}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films with the abrupt metal-to-insulator transition of resistivity, while standard percolation mode, which seems to work well for ${\mathrm{La}}_{0.9}{\mathrm{Te}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3}$ bulk with smooth transition, fails in it. The agreement between the simulation and the experimental data reveals that the sizes of the paramagnetic domains in the films increases, but the number of those domains does not change with the increased temperature. This process is very different from the case in the bulk, where the domain sizes almost remain unchanged and the domain number increases with the increased temperature. The present results also further verify that phase separation plays a crucial role in the transport process and the size of coexisting phase may be related to the size of the crystal grains.

Simulation of the temperature‐dependent resistivity of La1–xTexMnO3

AbstractThe resistivity ρ of La1–xTexMnO3 (x = 0.1, 0.14) is studied using a random resistor network, based on phase separation between ferromagnetic (FM) and paramagnetic (PM) domains. By considering the irregular shape of the domains, a revised method, which is used to find the shortest paths across the sample, leads to a good agreement between the simulated results and experiment data. Moreover, it is found that FM components increase with Te doping, leading to a reduction of the resistivity and a shift of the transition temperature. This method is proved to be valid and the phase separation scenario is shown theoretically to be good enough in describing the electrical conductivity of the doped manganese perovskites. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Percolation Model of the Temperature Dependence of ExoticMagnetic Field in Doped Manganese Perovskites

We investigate the magnetic transitions in a(La1−xBx)2/3Ca1/3MnO3 system, whichconsists of paramagnetic and ferromagnetic domains, based on a magnetictheoretical percolation model. In the mean-field approximation, theresistance as a function of temperature and magnetic field has beenderived analytically and simulated numerically. It is found that thedependence of the critical temperature on magnetic field is linear whenapplied magnetic field is not too strong. Our theoretical predicationsare in good agreement with recent experimental observations.

Ultrafast Optical and Magneto-Optical Dynamics in Colossal-Magnetoresistance Manganites

Transient pump-probe reflectance measurements on La0.67Ca0.33MnO3/MgO and La0.67Ca0.33MnO3/SrTiO3 thin films reveal a hitherto unknown magnetization-dependent energy gap that determines the relaxation process of the optically excited charge carriers on the picosecond time scale. In the case of La0.67Ca0.33MnO3/SrTiO3, the subsequent reflectance dynamics at low temperatures is determined to be substrate-induced strain that drives the sample into a metastable state characterized by an increased number of paramagnetic domains.

Magnetic and electronic properties of charge ordered Nd0.8Na0.2MnO3

Nd0.8Na0.2MnO3 polycrystalline ceramic is prepared by sol–gel method and its magnetic properties, electron spin resonance spectra and electronic transport properties have been investigated experimentally. Above 155K, the compound is a paramagnetic insulator. As the compound is cooled from room temperature, a charge-ordered state first develops below 155K. Above 110K, the resistivity data follow the variable-range hopping model, and an abrupt reduction in the localization length is observed around charge ordering temperature. Below 110K, the compound tends to be intrinsically inhomogeneous, i.e., ferromagnetic metal components and paramagnetic insulator domains coexist in this system at zero field. Below 40K, the fractions of ferromagnetic metal components exceed the percolation threshold value and the compound shows a metallic behavior. When a magnetic field with magnitude of 140kOe is applied, the charge-ordered state is melted and only a paramagnetic insulator to ferromagnetic metal transition is observed over the whole measured temperature range.