Transition In La0 Research Articles

Electron beam-induced brownmillerite–perovskite phase transition in La0.6Sr0.4CoO3− δ

The electron beam, during high-resolution transmission electron microscopy, was employed to induce a phase transition in La0.6Sr0.4CoO2.5 (LSC) from a brownmillerite ordering to an oxygen deficient perovskite structure. Prior to irradiation, a strongly alternating out-of-plane lattice parameter was observed, reflecting electrostatic interactions between AO and BO/BO2 planes in the brownmillerite ordering. During electron beam irradiation for one hour, the oxygen vacancy ordering vanished gradually, and a uniform cubic perovskite structure prevailed. To exclude beam-induced heating effects, in situ heating experiments were performed, revealing a stable brownmillerite ordering in the relevant temperature range (up to at least 500 °C). Thus, we conclude that the phase transition is caused by knock-on processes that affect oxygen vacancies in terms of a transition from structural vacancies toward extremely high concentrations of randomly distributed point defects in the ABO3 structure.

Ferro- to Paramagnetic Phase Transition in La0.90Pr0.10Mn0.8Co0.2O3 Perovskite

Ferro- to Paramagnetic Phase Transition in La0.90Pr0.10Mn0.8Co0.2O3 Perovskite

Atomic-scale observation of strain-dependent reversible topotactic transition in La0.7Sr0.3MnOx films under an ultra-high vacuum environment

Reversible topotactic phase transition between perovskite (PV) and oxygen-vacancy-ordered brownmillerite (BM) structures provides an effective platform for realizing the control of physical properties in complex transition metal oxides. However, such reversibility always requires extreme external conditions, that is, a high temperature and vacuum environment during the PV-BM transition, while an oxidizing atmosphere and relatively low temperature vice versa. Here, we experimentally observe the reversible process in strained La0.7Sr0.3MnOx films at atomic scale by using in-situ heating aberration-corrected scanning transmission electron microscopy (STEM). Apart from the conventional reduction reaction of creating oxygen-vacancy-ordered frameworks after heating in the TEM, the inverse process of BM-to-PV transition is unexpectedly discovered under such an ultra-high vacuum atmosphere (∼10−9 Torr) at room temperature. Moreover, this abnormal behavior is strain-dependent. The large compressive strain is found to be detrimental to the inverse phase transition. The density functional theory (DFT) calculations show that the high oxygen affinity of La0.7Sr0.3MnO2.5 is responsible for the reversible transitions. Our findings provide a new insight into the redox reactions of manganite and might be further utilized for potential applications in solid fuel cells, oxygen sensors or resistive switching memories.

Migration Kinetics of Surface Ions in Oxygen-Deficient Perovskite During Topotactic Transitions.

Oxygen diffusivity and surface exchange kinetics underpin the ionic, electronic, and catalytic functionalities of complex multivalent oxides. Towards understanding and controlling the kinetics of oxygen transport in emerging technologies, it is highly desirable to reveal the underlying lattice dynamics and ionic activities related to oxygen variation. In this study, the evolution of oxygen content is identified in real-time during the progress of a topotactic phase transition in La0.7 Sr0.3 MnO3-δ epitaxial thin films, both at the surface and throughout the bulk. Using polarized neutron reflectometry, a quantitative depth profile of the oxygen content gradient is achieved, which, alongside atomic-resolution scanning transmission electron microscopy, uniquely reveals the formation of a novel structural phase near the surface. Surface-sensitive X-ray spectroscopies further confirm a significant change of the electronic structure accompanying the transition. The anisotropic features of this novel phase enable a distinct oxygen diffusion pathway in contrast to conventional observation of oxygen motion at moderate temperatures. The results provide insights furthering the design of solid oxygen ion conductors within the framework of topotactic phase transitions.

Staircase effect in field-induced metamagnetic transitions in La0·9Ce0·1Fe12B6

We report on peculiar magnetic-field-induced metamagnetic transitions in the antiferromagnetic itinerant-electron compound La0·9Ce0·1Fe12B6. Below 8 K, the first magnetization curves display multiple steep jumps generating a staircase-like shape. Whereas the magnetization of La0·9Ce0·1Fe12B6 varies smoothly with the external applied magnetic field at T ≥ 8 K. We further show that this staircase effect is extremely sensitive to the thermomagnetic history of the sample.

Study on magnetocaloric effect and phase transition in La0.7(La–Ce)0.3FexAl11.5−xSi1.5 alloys

Magnetocaloric materials with high performance are crucial to the achievement of magnetic refrigeration which is a promising cooling technology to replace traditional gaseous compression refrigeration. In this work, the magnetocaloric effect and phase transition of La0.7(La–Ce)0.3FexAl11.5−xSi1.5 alloys have been investigated. The structural and magnetic properties were characterized by means of X-ray diffraction and magnetization measurements. Belov-Arrott plots and the exponent n from field dependence of isothermal entropy change were applied to determine the nature of the phase transition. The XRD patterns indicate that the alloys crystallize mainly in NaZn13-type phase with small amounts of α-Fe and LaFeSi as secondary phases. With an increase of x from 11.0 to 11.4, the Curie temperature decreases from 211.2 K to 185.4 K while the magnetocaloric effect increases from 4.9 J kg−1 ⋅K−1 to 23 J kg−1 ⋅K−1 for a field change of 2 T. The alloys show refrigeration capacity between 111 J ⋅kg−1 and 152 J ⋅kg−1 for a magnetic field of 2 T. Analyses of the Belov-Arrott plot and the exponent of n reveal that the first-order itinerant-electron metamagnetic phase transition in the alloys is weakened by decreasing x, and the alloys with x = 11.2, 11.3 and 11.4 undergo a second-order phase transition.

Understanding the metal-to-insulator transition in La1\u2212xSrxCoO3\u2212\u03b4 and its applications for neuromorphic computing

Transition metal oxides that exhibit a metal-to-insulator transition (MIT) as a function of oxygen vacancy concentration are promising systems to realize energy-efficient platforms for neuromorphic computing. However, the current lack of understanding of the microscopic mechanism driving the MIT hinders the realization of effective and stable devices. Here we investigate defective cobaltites and we unravel the structural, electronic, and magnetic changes responsible for the MIT when oxygen vacancies are introduced in the material. We show that, contrary to accepted views, cooperative structural distortions instead of local bonding changes are responsible for the MIT, and we describe the subtle interdependence of structural and magnetic transitions. Finally, we present a model, based on first principles, to predict the required electric bias to drive the transition, showing good agreement with available measurements and providing a paradigm to establish design rules for low-energy cost devices.

Co Doping and High Pressure Studies of the Iron Arsenide La0.4Na0.6Fe2As2

We investigate the effects of Co doping and high pressure on 122-type La0.4Na0.6Fe2As2 by electrical resistivity measurement. It is found that the antiferromagnetic (AFM) transition in La0.4Na0.6Fe...

Critical behavior near the ferromagnetic–paramagnetic phase transition in La0.5Pr0.3Ba0.2Mn1−xTixO3 (x = 0.0 and 0.1)

The $${\text{La}}_{0.5 } { \Pr }_{0.3} {\text{Ba}}_{0.2} {\text{Mn}}_{1 - x} {\text{Ti}}_{x} {\text{O}}_{3}$$ (x = 0.0 and 0.1) compounds were elaborated by the solid-state reaction method. The X-ray diffraction characterization has indicated that these samples crystallized in the rhombohedral symmetry with $$R\bar{3}C$$ space group. The magnetic results which we obtain indicated that both samples show a paramagnetic–ferromagnetic transition with decreasing Curie temperature ($$T_{\text{C}}$$) from 170 K (x = 0.0) to 110 K (x = 0.1). The zero-field cold curve for x = 0.1 compound exhibit a clear cusp at low temperature (T = 35 K) that is generally related to a spin-glass or a cluster-glass state. Furthermore, the critical exponents’ values were founded by different techniques. Therefore, the average values of the critical exponent and the critical temperature obtained by the various methods are ($$\beta_{\text{moy}}$$ = 0.296; $$\gamma_{\text{moy}}$$ = 1.126; $$\delta_{\text{moy}}$$ = 4.875; $$T_{\text{C}}$$ = 177.63) and ($$\beta_{\text{moy}}$$ = 0.26; $$\gamma_{\text{moy}}$$ = 1.081; $$\delta_{\text{moy}}$$ = 5.155; $$T_{\text{C}}$$ = 109.99 K) for x = 0.0 and 0.1, respectively. The obtained critical exponents are in agreement with those forecasted out by the 3D-Ising model and the tricritical mean-field model for x = 0.0 and 0.1, respectively.

Magnetic phase transition in La0.8Sr0.2Mn0.9Sb0.1O3 manganite under pressure

The high-pressure evolution of structural and magnetic properties of the single valence Mn3+ manganite La0.8Sr0.2Mn0.9Sb0.1O3 was studied by means of neutron powder diffraction up to 4.5 GPa in the temperature range of 10–300 K. At ambient pressure, a ferromagnetic ground state with the magnetic ordering temperature TC ≈ 175 K is stabilized. Application of high pressure suppresses the initial FM phase and leads to the appearance of a new A-type antiferromagnetic AFM phase with TN ~ 130 K at P = 2.2 GPa. Under compression, both TC and TN almost linearly increases with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. It is assumed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to the egdx2-z2 orbital polarization at high pressures.

Phase Transition of La0.62Sr0.38MnO3 Perovskite Manganites

Bulk and nano La0.62Sr0.38MnO3 perovskite manganite samples were prepared using solid state and sonochemical reaction respectively. The ultrasonic velocities measurement was made on prepared samples using ultrasonic through transmission method, at a fundamental frequency of 5 MHz over wide range of temperatures. The temperature dependence of the ultrasonic parameters shows an interesting anomaly in bulk and nano perovskite samples. The observed dramatic softening and hardening in sound velocities are related to phase transitions. Further, a decrease in grain size in the nanostructured sample leads to a shift in the ferromagnetic transition temperature (TC) from 375 to 370 K.

Ginzburg–Landau theory for the magnetic and structural transitions in La1−y(Ca1−xSrx)yMnO3

We present a phenomenological theory for the ferromagnetic transition temperature, the magnetic susceptibility at high temperatures, and the structural distortion in the La(CaSr)MnO system. We construct a Ginzburg–Landau free energy that describes the magnetic and the structural transitions, and a competition between them. The parameters of the magnetic part of the free energy are derived from a mean-field solution of the magnetic interaction for arbitrary angular momentum. The theory provides a qualitative description of the observed magnetic and structural phase transitions as functions of Sr-doping level () for .

Adiabatic temperature change, magnetic entropy change and critical behavior near the ferromagnetic–paramagnetic phase transition in La0.7(Ca,Sr)0.3MnO3 perovskite

ABSTRACTA systematic study of the magnetocaloric effect in La0.7Ca0.15Sr0.15MnO3 perovskite has been performed by both direct and indirect measurement techniques. In particular, the adiabatic temperature change has been measured by a purpose-built probe suitable for laboratory-scale samples, while magnetic measurements were carried out by standard magnetometric techniques. The observation of the effective behavior of the studied systems in adiabatic conditions and the significant value of the isothermal entropy variation indicate that La0.7Ca0.15Sr0.15MnO3 could be considered as a potential candidate for application in magnetic refrigeration around room temperature. Moreover, the critical exponents (, and ) determined using the Kouvel–Fisher method, the modified Arrott plot and the critical isotherm analysis are in good agreement with the theoretical prediction of the 3D-Ising model, indicating that the magnetic interactions are of short-range nature.

Structural, magnetic and on magnetocaloric properties near the paramagnetic to ferromagnetic phase transition in La0.5□0.1Ca0.4MnO3 oxide

Structural, magnetic and magnetocaloric properties of La0.5□0.1Ca0.4MnO3 manganite prepared by conventional solid state reaction route have been carried out. The X-ray diffraction characterization has revealed that this sample crystallized in the orthorhombic symmetry, with Pnma space group. In order to check if this sample can be good candidate for magnetic refrigeration, relative cooling power was determined depicting that the sample can be useful for magnetic refrigeration at with about 232.20 J/kg under a magnetic field of 5 T. The magnetocaloric data has been well analyzed by using the Landau theory. The theoretical and experimental plots seem nearly similar. Based on these amazing properties, our compound is suggested as an effective candidate for various potential applications such as refrigeration application.

Strain-induced magnetization control in an oxide multiferroic heterostructure

Controlling magnetism by using electric fields is a goal of research towards novel spintronic devices and future nano-electronics. For this reason, multiferroic heterostructures attract much interest. Here we provide experimental evidence, and supporting DFT analysis, of a transition in La0.65Sr0.35MnO3 (LSMO) thin film to a stable ferromagnetic phase, that is induced by the structural and strain properties of the ferroelectric BaTiO3 (BTO) substrate, which can be modified by applying external electric fields. X-ray Magnetic Circular Dichroism (XMCD) measurements on Mn L edges with a synchrotron radiation show, in fact two magnetic transitions as a function of temperature that correspond to structural changes of the BTO substrate. We also show that ferromagnetism, absent in the pristine condition at room temperature, can be established by electrically switching the BTO ferroelectric domains in the out-of-plane direction. The present results confirm that electrically induced strain can be exploited to control magnetism in multiferroic oxide heterostructures.

Multiple magnetization steps and plateaus across the antiferromagnetic to ferromagnetic transition in La1−xCexFe12B6 : Time delay of the metamagnetic transitions

Multiple magnetization steps and plateaus across the antiferromagnetic to ferromagnetic transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi mathvariant="normal">L</mml:mi><mml:msub><mml:mi mathvariant="normal">a</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">e</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:msub><mml:mi mathvariant="normal">e</mml:mi><mml:mn>12</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">B</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math> : Time delay of the metamagnetic transitions

A Coexistence of Short- and Long-Range Ferromagnetic Interactions in La1–xKxMnO3 Compounds

In this paper, we present a detailed study on the critical behavior around the ferromagnetic (FM)–paramagnetic phase transition in La1− x K x MnO3 (LKMO) compounds with $x = 0.05 - 0.20$ . Our results pointed out the easiness in controlling the value of $T_{C}$ from 212 to 306 K by increasing K-doping concentration (x) from 0.05 to 0.20. Analyzing temperature and magnetic field dependences of magnetization M(T, H) at temperatures around $T_{C}$ reveals all the samples undergoing the second-order phase transition. Using the modified Arrott plots, the Kouvel–Fisher, and the critical isotherm analysis methods, we have found that the values of exponent $\beta $ in the range of 0.435–0.493, which deviate from the mean-field theory (MFT) for the long-range FM interactions, indicating a coexistence of short- and long-range FM orders. We also pointed out that the critical exponents ( $\beta $ and $\delta)$ depend strongly on K-concentration, shifting from values approaching those of the MFT ( $\beta = 0.5$ and $\delta = 3$ ) to values approaching those of the 3-D-Heisenberg model ( $\beta = 0.365$ and $\delta = 4.80$ ). In contrast, the value of the exponent $\gamma (=1.222 - 1.249)$ is almost independent of K-concentration. It suggests that K-doping favors establishing a short-range FM order in LKMO compounds.

Effect of Partial Substitution of La by Ce on the Nature of Phase Transition in Magnetocaloric La1−x Ce x Fe11.2Co0.7Si1.1 Alloys

The influence of Ce addition on the nature of phase transition in the La1−x Ce x Fe11.2Co0.7Si1.1 (x = 0, 0.1) alloy system has been investigated. The substitution of Ce for La leads to a decrease of Curie temperature from 264 K to 254 K and to an increase of the maximum value of magnetic entropy change from 19.3 J/(kg K) to 21.5 J/(kg K). A detailed analysis of phase transition using Arrott plots, the Landau theory for order parameter and the construction of a universal scaling curve revealed that replacing 10 at.% La with Ce drives the magnetic transition from second-order to first-order and enhances a field-induced itinerant electron metamagnetic transition above T C.

Improvement of magnetocaloric properties over a large temperature range in 0.5La0.7Ca0.2Sr0.1MnO3/0.5La0.7Ca0.15Sr0.15MnO3 composite

In the present research work, we investigate the magnetocaloric properties of a composite system by experiments and numerical calculations. The ability to adjust the magnetic entropy change in a large temperature range revealed to be possible by the mixture of both phases La0.7Ca0.2Sr0.1MnO3 and La0.7Ca0.15Sr0.15MnO3. Our composite 0.5La0.7Ca0.2Sr0.1MnO3/0.5La0.7Ca0.15Sr0.15MnO3 was synthesized using the solid–state reaction. Crystallographic structure and magnetocaloric properties were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). Rietveld refinements of the X-ray diffraction patterns show that both samples are single phase and we have noticed a structural transition from orthorhombic structure with Pnma space group for La0.7Ca0.2Sr0.1MnO3 to rhombohedral structure with R-3C space group for La0.7Ca0.15Sr0.15MnO3. A second-order magnetic phase transition from ferromagnetic to paramagnetic state is observed and confirmed in terms of Landau theory. Therefore, two successive magnetic transitions in La0.7Ca0.2Sr0.1MnO3 and La0.7Ca0.15Sr0.15MnO3 resulting a nearly constant “table-like” magnetic-entropy change. Both peaks are partially overlapped with each other and induced an enhancement of the RCP in the composite when comparing with that of individual components. These results make that the proposed composite has substantial advantages for magnetic refrigeration.

Critical Behavior Near the Paramagnetic-Ferromagnetic Phase Transition in La1−x Sr x CoO3 (x = 0.3 and x = 0.5) Cobaltites

In this work, we present the study of structural, magnetic and critical behavior of La1−x Sr x CoO3 (x = 0.3 and 0.5) cobaltites. X-ray diffraction analysis using the Rietveld refinement shows that the sample La0.7Sr0.3CoO3 crystallizes in a rhombohedral distorted perovskite structure with $R\overline {3} c$ space group, while La0.5Sr0.5CoO3 exhibits both phases rhombohedric and cubic, with $R\overline {3} c$ and $Pm\overline {3} m$ space groups. Our samples exhibit thermo-magnetic irreversibility behavior in the magnetization data under a magnetic applied field of 0.05 T. The temperature dependence of magnetic susceptibility at higher temperatures for La0.7Sr0.3CoO3 sample reveals the presence of the Griffiths’ phase above the Curie temperature, while it follows the Curie–Weiss law perfectly for La0.5Sr0.5CoO3. Different techniques including the modified Arrott plot, the Kouvel-Fisher method and the critical isotherm analysis have been used to analyze the paramagnetic-ferromagnetic phase transition. The values of the critical exponents associated to this transition were close to those expected in the mean-field model. The validity of the calculated critical exponents has been confirmed by the scaling equation of state. The determined critical exponents using various methods are found to follow the scaling equation with the magnetization data scaled into two different curves below and above the critical temperature.