04 Mar 2021-Physical Chemistry Chemical Physics (The Royal Society of Chemistry)-Vol. 23, Iss: 8, pp 4912-4918

Abstract: High-quality epitaxial LaCoO3 (LCO) thin films have been deposited on SrTiO3 (STO) substrates with pulsed laser deposition (PLD). We find that the LCO films undergo a typical ferromagnetic–paramagnetic (FM–PM) phase transition at ∼80 K. To understand the nature of magnetic phase transition, various methods, including the modified Arrott plot and critical isotherm analysis, were used to determine the critical exponents, which are β = 0.754(1) with a Curie temperature TC = 79.8(8) K and γ = 1.52(2) with TC = 79.9(2) K. The reliability of these critical exponents was confirmed using the Widom scaling relation and the scaling hypothesis. Further analysis revealed that the spin coupling within the LCO films exhibits two-dimensional (2D) long-range magnetic interaction and the magnetic exchange distance decays as J(r) ∼ r−(3.46). Our critical behavior analysis may shed new light on the further understanding of the origin of FM and the relatively fixed TC in LCO thin films.

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Topics: Critical exponent (55%), Arrott plot (55%), Curie temperature (54%) ... show more

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6 results found

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Abstract: The structure, surface topography and magnetic properties of LaCoO3 (LCO) thin films deposited epitaxially on the SrTiO3 substrates have been investigated in detail. The LCO thin films show a typical ferromagnetic-paramagnetic (FM-PM) phase transition at 78 K. Based on the measurements of isothermal magnetization around the Curie point, the magnetic entropy change (ΔSM) of LCO thin films under various applied magnetic fields were obtained. By utilizing scaling theory, all of the ΔSM curves can be re-scaled, confirming that the FM-PM phase transition is second-order magnetic phase transition. Moreover, the magnetic entropy change -ΔSM features a maximum around TC, whereas, power law fitting of - Δ S M max with H gives n = 0.9704, which is obviously deviating from the standard value n = 2/3 for the Landau mean field model. This indicates that the mean field theory cannot be used to explain the critical behavior of LCO thin films which implies that the critical behavior of LCO thin films may involve complex magnetic interactions. These interactions are correlated with a long-term puzzling finding in this system why its Curie point is always around 80 K regardless of the different fabrication methods and conditions used in the film growth.

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Topics: Curie temperature (58%), Phase transition (56%), Ferromagnetism (53%) ... show more

2 Citations

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Abstract: The critical behavior of single phase Y2Fe17 melt-spun ribbons with the Th2Fe17-type crystal structure has been studied around the transition temperature (TC). This alloy undergoes a second-order ferromagnetic (FM)–paramagnetic (PM) phase transition at TC = 301 K. Various techniques, such as the modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis, were used to determine the critical exponents that were found to be β = 0.226(3), γ = 1.296(2), and δ = 6.804(5). The universality class of the critical phenomenon in the Y2Fe17 ribbons can be explained with the help of the renormalization group theory approach, in which the magnetic properties show a feature changing from three (3D)- to two-dimensional (2D) Ising model. The first-principles calculations based on density functional theory qualitatively explain the experimental results, confirming the strong correlation between lattice atoms and critical behavior in magnetic intermetallic Y2Fe17.

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Topics: Critical exponent (63%), Ising model (60%), Phase transition (58%) ... show more

1 Citations

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Abstract: In this work, we successfully prepared epitaxial La1-xYxCoO3 (x = 0, 0.05, 0.1, 0.15) films on (100) SrTiO3 by polymer assisted deposition. The synergistic effect of biaxially tensile strain and doping yttrium on the structural distortion and photoconductive process of the films has been investigated. Due to lattice mismatch, the epitaxial strain along c-axis as well as the strain-induced tetragonal distortion of La1-xYxCoO3 films increase with the content of doping yttrium. As-prepared films have higher degree of CoO6 octahedral distortion, i.e., Jahn-Teller-like tetragonal distortion, which result in lower crystal field splitting energy and narrower band gap energy, thereby elevating the charge transition. Additionally, increase of epitaxial strain leads to lower adsorption free energy in La1-xYxCoO3 films, and results in the increase of chemisorbed oxygen, which is beneficial to electron transport in the process of light response. The introduction of new oxygen vacancy defect sites by doping Y3+ ions and the proper amount of oxygen vacancy can effectively inhibit the recombination of the photo-induced electron-hole pairs, thus improving the activity for photoconductive response. The results reveal that the photoconductive properties of the as-prepared films are largely related to the synergistic effect of biaxially tensile-strained tetragonal distortion of CoO6 and doping yttrium.

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Topics: Tetragonal crystal system (54%), Band gap (51%), Yttrium (51%) ... show more

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Abstract: Epitaxial strained LaCoO3 thin films with (001) orientation were prepared on various substrates, i.e., LaAlO3, SrTiO3 and LaSrAlO4, respectively, by polymer assisted deposition. Owing to lattice mismatch, different lattice strain and tetragonal distortion of the oxygen octahedra exist in the obtained LaCoO3 films. These films exhibit excellent visible-light photoconductive performance. It reveals that the tetragonal distortion of CoO6 octahedra results in a decrease of ΔCF as well as Eg, enhancing the photoconductive performance of LaCoO3 thin films. A relatively high degree of the tetragonal distortion corresponds to higher photocurrent of the films, and the tetragonal distortion as well as photoconductive properties can be “adjusted” by epitaxial strain introduced in the LaCoO3 thin films. The adjusting method by epitaxial strain developed in this study provides a viable way for developing high-performance perovskite photoconductive semiconductor and can be extended to other optoelectronic systems.

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Topics: Tetragonal crystal system (55%), Thin film (53%), Perovskite (structure) (51%)

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44 results found

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Abstract: The theory of critical phenomena in systems at equilibrium is reviewed at an introductory level with special emphasis on the values of the critical point exponents α, β, γ,..., and their interrelations. The experimental observations are surveyed and the analogies between different physical systems - fluids, magnets, superfluids, binary alloys, etc. - are developed phenomenologically. An exact theoretical basis for the analogies follows from the equivalence between classical and quantal `lattice gases' and the Ising and Heisenberg-Ising magnetic models. General rigorous inequalities for critical exponents at and below Tc are derived. The nature and validity of the `classical' (phenomenological and mean field) theories are discussed, their predictions being contrasted with the exact results for plane Ising models, which are summarized concisely. Pade approximant and ratio techniques applied to appropriate series expansions lead to precise critical-point estimates for the three-dimensional Heisenberg and Ising models (tables of data are presented). With this background a critique is presented of recent theoretical ideas: namely, the `droplet' picture of the critical point and the `homogeneity' and `scaling' hypotheses. These lead to a `law of corresponding states' near a critical point and to relations between the various exponents which suggest that perhaps only two or three exponents might be algebraically independent for any system.

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Topics: Critical phenomena (62%), Percolation critical exponents (60%), Critical exponent (59%) ... show more

1,693 Citations

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1,628 Citations

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Abstract: A criterion is proposed for determining the onset of ferromagnetism in a material as its temperature is lowered from a region in which the linearity of its magnetic moment versus field isotherm gives an indication of paramagnetism. Within the limits of validity of a molecular field treatment, the Curie temperature is shown to be in general indicated by the third power of the magnetization being proportional to the internal magnetic field. The method has been employed to redetermine the Curie point of nickel from the data of Weiss and Forrer, of ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ from the data of Smith and of some alloys from the data of Kaufmann and his collaborators and the author.

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Topics: Curie–Weiss law (59%), Arrott plot (58%), Curie's law (58%) ... show more

969 Citations

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Abstract: A specific form is proposed for the equation of state of a fluid near its critical point. A function Φ(x, y) is introduced, with x a measure of the temperature and y of the density. Fluids obeying an equation of state of van der Waals type (``classical'' fluids) are characterized by Φ being a constant. It is suggested that in a real fluid Φ(x, y) is a homogeneous function of x and y, with a positive degree of homogeneity (Sec. 2). This leads to a nonclassical compressibility, the behavior of which is determined by the degree of homogeneity of Φ (Sec. 3). A previously derived relation connecting the degree of the critical isotherm, the degree of the coexistence curve, and the compressibility index, again follows, this time without the restrictive assumption of effective isochore linearity (Sec. 4). The locus in the temperature—density plane of the points of inflection in the pressure—density isotherms, as determined experimentally by Habgood and Schneider, is accounted for (Sec. 5). It is shown that if a certain combination of the compressibility and coexistence curve indices is an integer, then the constant‐volume specific heat on the critical isochore has a logarithmic singularity at the critical temperature with, in general, a superimposed finite discontinuity (Sec. 6).

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Topics: Theorem of corresponding states (60%), Critical point (thermodynamics) (58%), Compressibility (55%) ... show more

920 Citations

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Abstract: Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

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Topics: Ferromagnetism (57%), Magnetocrystalline anisotropy (53%), van der Waals force (53%) ... show more

845 Citations