RESUMEN
The cubic perovskite SrMoO3 with a paramagnetic ground state and remarkably low room-temperature resistivity has been considered as a suitable candidate for the new-era oxide-based technology. However, the difficulty of preparing single-phase SrMoO3 thin films by hydrogen-free sputtering has hindered their practical use, especially due to the formation of thermodynamically favorable SrMoO4 impurity. In this work, we developed a radio frequency sputtering technology enabling the reduction reaction and achieved conductive epitaxial SrMoO3 films with pure phase from a SrMoO4 target in a hydrogen-free, pure argon environment. We demonstrated the significance of controlling the target-to-substrate distance (TSD) on the synthesis of SrMoO3; the film resistivity drastically changes from 1.46 × 105 µΩ·cm to 250 µΩ·cm by adjusting the TSD. Cross-sectional microstructural analyses demonstrated that films with the lowest resistivity, deposited for TSD = 2.5 cm, possess a single-phase SrMoO3 with an epitaxial perovskite structure. The formation mechanism of the conductive single-phase SrMoO3 films can be attributed to the plasma-assisted growth process by tuning the TSD. Temperature-dependent resistivity and Hall effect studies revealed metal-like conducting properties for low-resistive SrMoO3 films, while the high-resistive ones displayed semiconductor-like behavior. Our approach makes hydrogen-free, reliable and cost-efficient scalable deposition of SrMoO3 films possible, which may open up promising prospects for a wide range of future applications of oxide materials.
For the first time, we developed a plasma-assisted RF sputtering technology enabling the reduction reaction for the synthesis of single-phase conductive SrMoO3 epitaxial films from insulating SrMoO4 in pure-argon atmosphere.
RESUMEN
We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe1-xRux)2O4by means of detaileddc-magnetization andac-susceptibility studies combined with the heat capacity analysis. Two specific compositions (x = 0.5, 0.75) have been investigated in detail along with the substitution of Jahn-Teller (JT) active spin-1/2 Cu2+ions at B-sites. Measurements based on the frequency and temperature dependence ofac-susceptibility (χac(f,T)) and the subsequent analysis using the empirical scaling laws such as: (a) Vogel-Fulcher law and (b) Power law reveal the presence of cluster SG state below the characteristic freezing temperatureTSG(17.77 K (x = 0.5) and 14 K (x = 0.75)). Relaxation dynamics of both the compositions follow the non-mean field de Almeida-Thouless (AT)-line approach(TSG(H)=TSG(0)(1-AH2/Ï)), with an ideal value ofφ = 3. Nevertheless, the analysis of temperature dependent high fielddc-susceptibility,χhf(2kOe ⩽ HDC ⩽ 20kOe,T) provides evidence for Gabay-Toulouse type mixed-phase (coexistence of SG and ferrimagnetic (FiM)) behaviour. Further, in the case of Cu0.2Zn0.8FeRuO4system, slowly fluctuating magnetic clusters persist even above the short-range FiM ordering temperature (TFiM) and their volume fraction vanishes completely across â¼6TFiM. This particular feature of the dynamics has been very well supported by the time decay of the thermoremanent magnetization and heat-capacity studies. We employed the high temperature series expansion technique to determine the symmetric exchange coupling (JS) between the spins which yieldsJS=-3.02×10-5 eV for Cu0.2Zn0.8FeRuO4representing the dominant intra-sublattice ferromagnetic interactions due to the dilute incorporation of the JT active Cu2+ions. However, the antiferromagnetic coupling is predominant in ZnFeRuO4and Cu0.2Zn0.8Fe0.5Ru1.5O4systems. Finally, we deduced the magnetic phase diagram in theHDC-Tplane using the characteristic parameters obtained from the field variations of bothac- anddc-magnetization measurements.
RESUMEN
The nature of magnetism in the doubly-diluted spinel ZnTiCoO4= (Zn2+)A[Ti4+Co2+]BO4is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (χ), ac susceptibilities (χ' andχâ³), and heat capacity (Cp) measurements. Whereas antiferromagnetic (AFM) Néel temperatureTN= 13.9 K is determined from the peak in the ∂(χT)/∂TvsTplot, the fit of the relaxation timeτ(determined from the peak in theχâ³ vsTdata at different frequencies) to the Power law:τ=τ0[(T-TSG)/TSG]-zνyields the spin glass freezing temperatureTSG= 12.9 K,zνâ¼ 11.75, andτ0â¼ 10-12s. Since the magnitudes ofτ0andzνdepend on the magnitude ofTSG, a procedure is developed to find the optimum value ofTSG= 12.9 K. A similar procedure is used to determine the optimumT0= 10.9 K in the Vogel-Fulcher law:τ=τ0 exp[Ea/kB(T-T0)] yieldingEa/kB= 95 K, andτ0= 1.6 × 10-13s. It is argued that the comparatively large magnitude of the Mydosh parameter Ω = 0.026 andkBT0/Ea= 0.115 (âª1) suggests cluster spin-glass state in ZnTiCoO4below TSG. In theCpvsTdata from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence ofλ-type anomaly nearTNorTSGcombined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence ofTSGshows slight departure (Ïâ¼ 4.0) from the non-mean-field Almeida-Thouless lineTSG(H) =TSG(0) (1 -AH2/Ï). Strong temperature dependence of magnetic viscositySand coercivityHCwithout exchange bias, both tending to zero on approach toTSGfrom below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn2+and Ti4+ions leading to magnetic frustration. Magnetic phase diagram in theH-Tplane is established using the high-field magnetization dataM(H,T) forT
RESUMEN
Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H-T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6sample verified its phase purity. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 5.97 K determined from the peak in the computed ∂(χT)/∂TvsTplot, in agreement with theTN= 6.00 K determined from the peak in theCPvsTdata. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.10(0.13) forT>TN(T
RESUMEN
This work presents the magnetic field-temperature (H-T) phase diagram, exchange constants, specific heat (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6polycrystals. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 4.33 K determined from the peak in the computed ∂(χT)/∂TvsTplot in agreement with the transition in theCPvsTdata atTN= 4.36 K. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.12 (0.15) forT>TN(T
RESUMEN
We report the electronic structure and magnetic properties of Co2Ti1-xGexO4(0 ⩽x⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (Ueff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a≠ 1) due to the non-degeneratedz2anddx2-y2states (egorbitals) of the B-site Co. We observe large exchange splitting (ΔEXâ¼ 9 eV) between the up and down spin bands oft2gandegstates, respectively, of tetrahedral and octahedral Co2+(4A2(g)(F)) and moderate crystal-field splitting (ΔCFâ¼ 4 eV) and the Jahn-Teller distortion (ΔJTâ¼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽Eg⩽ 3.3 eV). The exchange interaction (JBBâ¼ -4.8 meV, for (x,y) = (0.25, 0)) between the Co2+dominates the overall antiferromagnetic behaviour of the system for all 'x' as compared toJAA(â¼-2.2 meV, for (x,y) = (0.25, 0)) andJAB(â¼-1.8 meV, for (x,y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δµ) remains constant, however, increases progressively with increasingxdue to the imbalance of Co spins between the A- and B-sites.
RESUMEN
Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+)[Formula: see text] are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature range T = 2 K-400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at T N â¼ 15.2 K, along with a spin-glass state at slightly lower temperature (T SG) near 14.2 K. It is argued that T N is mainly governed by the ordering of the spins of Co2+ ions occupying the A-site, whereas the exchange interaction between the Co2+ ions on the A-site and randomly distributed Ru3+ on the B-site triggers the spin-glass phase, Co3+ ions on the B-site being in the low-spin non-magnetic state. Analysis of measurements of M (H, T) for T < T N are used to construct the H-T phase diagram showing that T SG shifts to lower T varying as H2/3.2 expected for spin-glass state whereas T N is nearly H-independent. For T > T N, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie-Weiss law, χ = χ 0 + C/(T + θ), with χ 0 = 0.0015 emu mol-1Oe-1 yielding θ = 53 K and C = 2.16 emu-K mol-1Oe-1, the later yielding an effective magnetic moment µ eff = 4.16 µ B comparable to the expected value of µ eff = 4.24 µ B per Co2RuO4. Using T N, θ and high temperature series for χ, dominant exchange constant J 1/k B â¼ 6 K between the Co2+ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near T SG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ 0 â¼ 1.16 × 10-10 sec characteristic of cluster spin-glasses and the observed time-dependence of M(t) is supportive of the spin-glass state. Large M-H loop asymmetry at low temperatures with giant exchange bias effect (H EB â¼ 1.8 kOe) and coercivity (H C â¼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below T N is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for T > T N to inverse MCE for T < T N observed in Co2RuO4 (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.
RESUMEN
Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping ([Formula: see text]) on the electronic structure and magnetic properties of the spinel [Formula: see text]O4. A detailed comparison is given for the [Formula: see text] and [Formula: see text] cases for both the bulk-like samples and nanoparticles. The electronic structure determined from x-ray photoelectron spectroscopy and Rietveld analysis of x-ray diffraction patterns shows the structure to be: ([Formula: see text])A [Formula: see text] [Formula: see text] [Formula: see text]]B [Formula: see text] i.e. [Formula: see text] substitutes for [Formula: see text] on the octahedral B-sites. For the bulk samples, the ferrimagnetic [Formula: see text] K for [Formula: see text] is lowered to [Formula: see text] K for the [Formula: see text] sample, this decrease being due to the effect of Cu doping. For the nanosize [Formula: see text] ([Formula: see text]) sample, the lower [Formula: see text] K ([Formula: see text] K) is observed using [Formula: see text] analysis, this lowering being due to finite size effects. For [Formula: see text], fits of dc paramagnetic susceptibility data of [Formula: see text] versus T in nanosize samples to the Néel expression are used to determine the exchange interactions between the A and B sites with exchange constants: [Formula: see text] K (4.1 K), [Formula: see text] K (16.3 K) and [Formula: see text] K (13.8 K) for [Formula: see text]. The temperature dependence of ac susceptibilities [Formula: see text] and [Formula: see text] at different frequencies shows that in bulk samples of [Formula: see text] and [Formula: see text], the transition at T C is the normal second order transition. But for the nanosize [Formula: see text] and 0.2 samples, analysis of the ac susceptibilities shows that the ferrimagnetic transition at T C is followed by a re-entrant spin-glass transition at lower temperatures [Formula: see text] K (138 K) for [Formula: see text] ([Formula: see text]). Analysis of the ac susceptibilities, [Formula: see text] and [Formula: see text], versus T data is done in terms of two scaling laws: (i) Vogel-Fulcher law [Formula: see text] [Formula: see text]; and (ii) power law of critical slowing-down [Formula: see text]. These fits confirm the existence of glassy behavior below T SG with the parameters [Formula: see text] (8.91), [Formula: see text] (9.6 × 10[Formula: see text]) and [Formula: see text] K (â¼138 K) for the samples [Formula: see text] (0.2), with similar results obtained for other samples. The linear behavior of the peak maximum in [Formula: see text] versus [Formula: see text] (AT-line) further supports the existence of glassy states in nanosize samples. For [Formula: see text], the temperature and composition dependence of the hysteresis loop parameters are investigated; all the samples with x ⩾ 0.1 have the coercivity H C and remanence [Formula: see text]. Since the results reported here in these nanostructures are significantly different from those in bulk [Formula: see text] [Formula: see text], further investigations of their magnetic structures using neutron diffraction are warranted.
RESUMEN
Peptide based nano-assemblies with their self-organizing ability has shown lot of promise due to their high degree of thermal and chemical stability, for biomaterial fabrication. Developing an effective way to control the organization of these structures is important for fabricating application-oriented materials at the molecular level. The present study reports the impact of electric and magnetic field-mediated perturbation of the self-assembly phenomenon, upon the chemical and structural properties of diphenylalanine assembly. Our studies show that, electric field effectively arrests aggregation and self-assembly formation, while the molecule is allowed to anneal in the presence of applied electric fields of varying magnitudes, both AC and DC. The electric field exposure also modulated the morphology of the self-assembled structures without affecting the overall chemical constitution of the material. Our results on the modulatory effect of the electric field are in good agreement with theoretical studies based on molecular dynamics reported earlier on amyloid forming molecular systems. Furthermore, we demonstrate that the self-assemblies formed post electric-field exposure, showed difference in their crystal habit. Modulation of nano-level architecture of peptide based model systems with external stimulus, points to a potentially rewarding strategy to re-work proven nano-materials to expand their application spectrum.
Asunto(s)
Campos Magnéticos , Nanoestructuras/química , Péptidos/química , Nanoestructuras/ultraestructura , Nanotubos/química , Nanotubos/ultraestructura , Espectrometría Raman , Difracción de Rayos XRESUMEN
An attempt has been made to synthesize nickel (up to 5 at%) containing ZnO diluted magnetic semiconductors by sol-gel process using zinc acetate dihydrate, nickel acetate tetrahydrate and oxalic acid as precursors and ethanol as solvent. The process comprises of gel formation, digestion at 80 degrees C for 24 h to obtain oxalate and calcination at 400 degrees C or above for 2 h for decomposition to oxide. The resulting powders correspond to wurtzite hexagonal structure similar to that of pure ZnO and contain nanoparticles of average diameter decreasing with (i) decrease of calcination temperature and (ii) rise in nickel content (range being 13-29 nm). Evidence has been advanced to suggest occupancy of Ni2+ ions in Zn2+ sites (i.e., in the centre of O2- tetrahedra) without any secondary phase. Further, the first nearest neighbor average distance of Ni2+ ions vary inversely as cube root of nickel content ('x'). Optical absorption peaks are attributed to transitions from 3T1 (F) to 3A2(F), 3T1(P) and 3T2(F) states of Ni2+ ion (3d8 configuration). The bifurcation in magnetization versus temperature curves under zero field cooled (ZFC) and field cooled (FC) conditions indicates the presence of magnetic nanoparticles with the blocking temperature (TB) decreasing with increase in nickel content; typical values being 70 and 30 K for 1 and 5 at% of Ni2+ containing ZnO, respectively. The observations of (i) hysteresis loops both at 10 and 298 K and (ii) shift in electron spin resonance (ESR) spectra from the free electron position enunciate ferromagnetic nature of Zn(1-x)Ni(x)O nanoparticles at room temperature.
RESUMEN
Nickel oxide thin films have been prepared by spin coating on glass, silicon(1ll) and quartz substrate using a solution of nickel acetate tetrahydrate in 2-methoxyethanol and mono-ethanolamine and subsequent annealing at 300-600 degrees C for 2 h in air. These films have been characterized with regard to phase, microstructure and optical absorption using X-ray diffraction, atomic force microscopy and UV-visible spectrophotometry, respectively. It is shown that NiO thin films have polycrystalline nature and possess an f.c.c. (NaCl-type) structure with lattice parameter varying with annealing temperature (T) and solution molarity; typical value being a = 4.186 A with T = 500 degrees C and molarity 0.5 M. The microstructure clearly reveals the formation of ellipsoids with average projected major and minor diameters as approximately 58 and approximately 38 nm, respectively and nano-rods of average diameter approximately 12 nm with aspect ratio of approximately 5.2. On the other hand, thin films formed by dip coating with the same solution contain spherical particles of average diameter approximately 28 nm. NiO thin films exhibit (i) high optical transmittance (80-95%) in the wave length range of 400-800 nm, (ii) sharp absorption in the interval 300-400 nm like that of semiconductor/insulator, (iii) decrease of energy band gap, E(g) (value lies in the range 3.66-3.83 eV; bulk value being 3.55 eV) with increase of annealing temperature and molarity both. The higher values of band gap have been attributed to the reduced average size of the crystallites.