xSrxMnO3 Manganites Research Articles

Magnetocaloric effect in manganites in alternating magnetic fields

Direct measurements of the magnetocaloric effect in La0.7Ca0.3MnO3, La1−xAgyMnO3, and Sm1−xSrxMnO3 manganites in alternating magnetic fields at frequencies up to 20 Hz and an amplitude of 6.2 kOe are carried out. A decrease of the MCE with the field frequency reaching 20%, 55% and 70% in La0.7Ca0.3MnO3, Sm1−xSrxMnO3 and La1−xAgyMnO3 is discovered in all studied manganites. The value estimates of the cooling power depending on the frequency of the alternating magnetic field are carried out. The largest value among the studied materials is achieved for La0.7Ca0.3MnO3 manganite, which is the most promising to be used in magnetic refrigeration technology. The data are compared with those for Gd in the same conditions.

Structural characterization and ZFC/FC magnetization study of La0.6Ca0.4−xSrxMnO3 nanoparticle compounds

Structural and magnetic properties of La0.6Ca0.4−xSrxMnO3 manganites have been investigated for x = 0.0, 0.1 and 0.4. The Rietveld analysis of X-ray diffraction data indicates that, with increasing Sr content, a structural phase transition from rhombohedral to orthorhombic is occurred. Structural parameters are calculated and found to be dependent upon the composition. The nanometric size of our samples estimated using Debye–Scherrer equation is confirmed by SEM images. Magnetic measurements according to the zero-field cooled and field cooled procedures are performed. The observed irreversibility between the two modes is assigned to the presence of magnetic anisotropy which has been explained as arising from the surface effects. The blocking temperature (TB) is estimated and found to increase as the particle size increases. Above TB, the materials show a superparamagnetic (SPM) behavior where the coercivity and remanence magnetizations are almost zero that favors their application in the biomedical field. Studying the thermal variation of dc susceptibility confirms the SPM nature of the studied compounds. The good fit of the magnetization vs. magnetic field curves with Langevin function supports our findings. From this simulation, the saturation magnetization is calculated and found to increase as particle size increases which is explained in terms of Core–shell model. With the reduction of particle size to the nanometric scale, single domain structure and SPM regime structure are detected.

Structural and Hysteretic Properties of La0.7Ca0.3−xSrxMnO3 Manganites Using the Hydrothermal Route

Manganitesof La0.7Ca0.3−xSrxMnO3 have been produced using the hydrothermal method. The influence of the concentration of La, Ca, and Sr on the structural, morphological, and magnetic properties was studied and analyzed. The x-ray diffraction studies showed the occurrence of orthorombic and trigonal symmetries in the crystalline structure, depending on the stoichiometry. X-ray photoelectron spectroscopy studies were performed, showing the presence of the elements La3d, Ca2p, Sr3d, Mn2p, and O1s. In all samples, a deficiency of oxygen was observed and ascribed to hydroxide formation. The FTIR spectra shows the Mn–O–Mn bonds in the stretching vibration mode; on the other hand, a great relative amount of Mn4+ ions that are not included in the manganite phase was observed. Furthermore, morphology analyses developed using scanning electron microscopy showed cauliflower-type microstructures in all samples. The dependence of magnetization on magnetic field was also studied where a low coercivity and a high-saturation magnetization were evidenced. Values of the squareness ratio coefficient ranged between 0.0587 and 0.0867, indicating that the individual particles exhibit a strong random anisotropy.

Electronic structure and chemical bonding in La1−xSrxMnO3 perovskite ceramics

This study reports on the synthesis of La1−xSrxMnO3 (x = 0.3, 0.4 and 0.5) manganites by high temperature solid state reaction method using lanthanum oxide, strontium carbonate and manganese oxide as starting materials. The synthesized samples were characterized by XRD, UV-vis, SEM/EDS and VSM. Structural characterization shows that all the prepared samples have the perovskite rhombohedral structure. Influence of Sr doping on electron density distributions in the lattice structure of LaMnO3 were analyzed through maximum entropy method (MEM). Cell parameters are found to be decreasing with the addition of Sr content. The qualitative and quantitative analysis by MEM reveals that, incorporation of Sr into LaMnO3 lattice enhances the ionic nature between La and O ions and decreases the covalent nature between Mn and O ions. Optical band gap values are determined from the UV–visible absorption spectra. Particles with polygonal form are observed from the SEM micrographs. The elemental compositions of the synthesized samples are confirmed by EDS. The magnetic properties studied from the M-H plot taken at room temperature indicated that, the prepared samples are exhibited ferromagnetic behavior.

Magnetic and XMCD studies of Pr1−xSrxMnO3 manganite films

Magnetic properties of the Pr0.8Sr0.2MnO3 and Pr0.6Sr0.4MnO3 polycrystalline films have been studied using temperature and magnetic field dependences of the static magnetization and X-ray magnetic circular dichroism (XMCD) spectroscopy. For the both compositions, the difference between the temperature dependences of magnetization obtained in the zero fields cooling (ZFC) and field cooling (FC) modes has been revealed. The ZFC curves demonstrate a pronounced maximum at temperature Tm. It is shown that the Tm value dependence on the magnetic field follows the Almeida-Thouless line typical for the classic spin glass, what allows us to assume the possible spin-glass behavior of the films. Effect of the disorder in a direction of the crystallites easy-axis on the difference between FC and ZFC curves has been discussed also. Magnetic field dependences of the sample magnetization are presented by the hysteresis loops with the shape changing upon temperature variation. This behavior has been attributed to the effect of crystallographic anisotropy and Pr ions. The spectra and magnetic field dependences of XMCD at the Pr L2- and Mn K-edges have been studied at 90K. The magnetic field dependences of the XMCD at the Pr L2-edge had shown Van Vleck paramagnetism from Pr3+ ions.

Spin-lattice coupling, Jahn-Teller effect and the influence of the measurement rate in La0.7Ca0.3−xSrxMnO3 manganites

This work presents a study of how the magnetic and temperature variables, as well as the measurement rate, affect magnetic and structural phase transitions for La0.7Ca0.3−xSrxMnO3 (x=0.15, 0.08, 0.07 and 0.06) manganites. A single phase rhombohedral crystal structure for Sr0.15 compound and a single phase orthorhombic crystal structure for Sr0.08, Sr0.07, and Sr0.06 compounds were identified by X ray diffraction. The Curie temperature adjustment to room temperature was reached for Sr0.07 sample, in which a Jahn-Teller transition was observed. This Jahn-Teller transition showed a susceptible response to 0.5, 1, 2, 3, 5 and 7 K/min measurement rates, and was revealed without measurement sequence at 234, 257, 196, 233, 245 and 238 K respectively in thermomagnetic curves (10mT). Also, Curie temperature increases sequentially around 2 K for the same measurement rates. To avoid magnetic field effect, a differential scanning calorimetry for Sr0.07 sample was obtained in the same temperature region, for 2, 3, 5, 7 K/min showing the Jahn-Teller phase transition at 225 K and a vibrational like behavior in the heat flux below 225 K, for all the temperature rates. The latter, is related to the static to dynamic Jahn-Teller transition. Furthermore, Curie temperature appears around 284 K.

Magnetocaloric and phase coexistence in La0.5Ca0.5–xSrxMnO3 manganites

Structural, magnetic, and magnetocaloric properties of the phase separated La0.5Ca0.5–xSrxMnO3 (x = 0, 0.2, 0.3, 0.4, and 0.5) manganites have been studied. The results show that the phase coexistence state can be investigated by magnetocaloric studies (especially the field dependence of magnetic entropy change at constant temperature). Magnetic entropy change (△SM) shows positive (negative) peak at the vicinity of TN (TC). However, in the intermediate temperatures between TN and TC, both the sign and magnitude of △SM are strongly dependent on temperature and magnetic field, manifesting the competition of ferromagnetic (FM) (negative △SM) and non-FM (positive △SM) phases. This behavior is more pronounced in the parent compound, La0.5Ca0.5MnO3, in which the intermediate phase separation ranges between 160 and 225 K. The substitution of Ca by Sr enhances the ferromagnetic state, weakens the phase separation, and thus narrows the temperature range in which the field related effects (such as the sign change of △SM) are observed.

Percolative nature of A-site disordered La0.75Ca0.25−xSrxMnO3 manganites

Magnetic, resistive, and magnetoresistance measurements were used to investigate the percolative nature of A-site disordered La0.75Ca0.25−xSrxMnO3(x = 0, 0.10) manganites. La0.75Ca0.15Sr0.10MnO3 has an orthorhombic structure and second order magnetic phase transition indicates the presence of two prominent downturns T* and Tferro above the Curie temperature (TC) in the derivative of the inverse susceptibility measurements. These observations are in agreement with the percolation model and the results are discussed in the light of phase separation happening in small polarons present in the insulating phase.

Magnetocaloric Effect Near Room Temperature of La0.67Ca0.33−x Sr x MnO3 (x = 0.06, 0.07, 0.08) Manganites

Structural and magnetic properties of La0.67 Ca0.33−xSrxMnO3 (x = 0.06, 0.07, 0.08) manganites synthesized using the solid-state reaction method were investigated. Rietveld refinement of the X-ray diffraction patterns shows that all our samples crystallize in the orthorhombic structure with Pbnm space group. The temperature dependence of magnetization reveals that all the samples exhibit a ferromagnetic (FM) to paramagnetic (PM) transition at temperatures of 279 K for La0.67 Ca0.27Sr0.06MnO3, 286 K for La0.67Ca0.26Sr0.07MnO3, and 293 K for La0.67Ca0.25 Sr0.08MnO3. Using Arrot plots, the phase transition from FM to PM is found to be of second order. The maximum of themagnetic entropy change (−ΔSM) is found to be 3.37, 2.74, and 2.60 J kg−1 K−1 for x = 0.06, 0.07, and 0.08 under a magnetic field change of 3 T. The relative cooling power (RCP-1) for La0.67Ca0.33−xSrxMnO3 sample is 152, 162, and 164 J kg−1 for x = 0.06, 0.07, and 0.08. According to these results, these kinds of samples are promising materials for magnetic refrigerants in the room-temperature region.

Structural and electrical properties of Pr1−xSrxMnO3 (x=0.25, 0.3, 0.35 and 0.4) manganites

In this work, we have studied the structural features and electrical conductivity of the polycrystalline Pr1−xSrxMnO3 (x=0.25, 0.3, 0.35, 0.4) manganites. All the compounds have orthorhombic structure with space group Pbnm. The electrical resistivity of all the samples exhibits metal–insulator transition TMI for both H=0T and H=8T. At H=8T, ρ (T) drops down considerably which indicates the existence of the CMR effect. The electron–phonon, electron–electron and electron–spin fluctuation interactions are effective to describe the resistivity behavior for temperature less than the metal–insulator transition (TMI). For temperatures, T>TMI, the insulating nature is discussed with a small polaron conduction (SPC) model. The resistivity of all the samples shows a field-dependent minimum at low temperature. The above is due to the Coulomb interaction between carriers strongly enhanced by disorder and Kondo-like spin dependent scattering.

Current density and intrinsic electroresistance of the Sm1−xSrxMnO3 manganite

We experimentally investigate the role of geometry on the current and current density dependencies of the intrinsic electroresistance of Sm1−xSrxMnO3 of two compositions (x=0.40 and x=0.45). It is found that for each composition, the plot of the intrinsic electroresistance versus current density for samples with different dimensions and resistances coincide whereas this does not happen in the case of the electroresistance versus the magnitude of the current. These results confirm that the current density is indeed the relevant “universal” parameter for controlling the intrinsic electroresistance of these manganites.

Influence of the Jahn-Teller effect on the structural, magnetic, and electrical properties of lightly doped manganites

Features of the structural, magnetic and electric properties in lightly doped La1−x Sr x MnO3 (0.12 < x < 0.18) manganites are studied. Agreement between the types of orbital ordering and the local cooperative Jahn-Teller distortion octahedra is established. The interaction between the Jahn-Teller effect and magnetic ordering is described.

Pulsed laser deposition and characterization of La1−xSrxMnO3

La1−xSrxMnO3 manganite films with x=0.15, 0.33 and 0.4 were deposited onto silicon substrates by pulsed laser deposition in an 80/20 Ar/O2 atmosphere at room temperature. After being deposited, the films were annealed at 900°C in air in order to obtain the desired crystalline phase. Structural characterizations using X-Ray diffraction showed polycrystalline compounds having the predominant peaks corresponding to the perovskite structure. Morphological studies carried out by scanning electron microscopy revealed a very rough surface and led to deducing the nature of the growth process. Spectral ellipsometry was performed between 1.5 and 5eV range at room temperature, showing electronic transitions near to 2.2 and 4.7eV in a range of thicknesses between 94 and 107nm.

The dielectric response of La0.5Ca0.5−xSrxMnO3 (0.1≤x≤0.4) manganites with different magnetic ground states

The dielectric behavior of half doped manganites La0.5Ca0.5−xSrxMnO3 (0.1≤x≤0.4) with varying magnetic ground states has been studied. The real part of relative permittivity as a function of temperature ε′(T), exhibits a maximum around the ferromagnetic (TC) and charge ordering transition (TCO) temperatures accompanied with high dielectric losses. The activation energies obtained for x=0.1 and 0.3 samples below TCO are the same ∼0.12 eV, whereas the relaxation time constant varies in the range 2.8×10−9 s–6.03×10−11 s. In contrast to samples having x≤0.3, for x=0.4 doping, the dielectric permittivity exhibits a strong temperature dependence in the vicinity of magnetic phase transitions. This behavior may be correlated with the presence of competing magnetic interactions (magnetic polarons) close to the magnetic transitions.

Electronic, structural and magnetic phase diagram of Sm1-xSrxMnO3 manganites

The electronic, structural and magnetic phase diagram of the colossal magnetoresistive Sm1−xSrxMnO3 (0.16≤х≤0.67) manganites is constructed on the basis of their systematic studies by high-resolution neutron powder diffraction. It is shown the tendency of researched system to formation of the phase-separated states on crystallographic as well as, in the even greater extent, on magnetic level. A clear correlation between fine specific features and temperature evolution of crystal structures, spin ordering of the manganese ions, and the physical properties of the samarium–strontium series of manganites obtained from temperature magnetic and transport measurements is demonstrated. It was found that ground state of Sm–Sr manganites shows very various physical properties depending first of all on the doping level х and, for given х, determined by two distortion parameters, namely, the average A-cation size <rA> and the local A-cation size mismatch σ2.

Enhanced transport properties in LaxMnO3−δ thin films epitaxially grown on SrTiO3 substrates: The profound impact of the oxygen content

We report on structural, magnetic, and transport properties of LaxMnO3−δ (LMO) thin films, epitaxially grown on SrTiO3 substrates by molecular beam epitaxy. Transport properties were investigated as a function of the La/Mn stoichiometry and the oxygen content. Optimal oxygenated LMO films (with stoichiometry ratio La/Mn=0.88) show a metal-insulator transitions temperature TMI∼387 K and magnetotransport similar to those found in strontium-doped La1−xSrxMnO3 manganites compounds. Also in presence of a La/Mn slight unbalance (namely, x=0.98), LMO films show a very high TMI∼370 K. However, LMO samples with La/Mn ratio larger than one, show severely depressed conducting properties, being insulating for x&amp;gt;1.10. All these findings clearly demonstrate that the lanthanum deficiency is a very efficient way to dope manganites, as in the case of bivalent cation-substitution.

Study of the electron structure of Mn, Sr, La, Ce, and Sm cations in Ln1 − x Sr x MnO3 (Ln = La, Ce, Sm) manganites by the X-ray line shift method

The X-RayKα line shifts of all cations in Ln1 − xSrxMnO3 (Ln = La, Ce, Sm) manganites have been measured using crystal diffraction spectrometers. The Mn Kαline shifts have been found in the Sm1 − xSrxMnO3 system correspond to an increase in the valence from Mn3+ to Mn4+. In the Ce1 − xSrxMnO3 system, in addition to this, the valence of cerium have been found also increases to almost 4. It is shown that the dependence of the valence of manganese on the content of strontium does not correspond to a simple model of the local mixture of Mn3+ and Mn4+ ions, which is usually applied to these systems. It is also demonstrated that the bonds of Sr, La, and Sm in the manganites studied exhibit a significant covalent character. In cation-deficient Sm1 − yMn1 − zO3, an increase in the valence of Mn is found accompanied by a decrease in the covalence of Sm.

Penetration of high-frequency electromagnetic field through bulk samples of La1−x Sr x MnO3 (0.33 ≤ x ≤ 0.60) manganites

Penetration of radio-frequency electromagnetic field through bulk samples of La1−xSrxMnO3 (0.33 ≤ x ≤ 0.60) manganites has been studied in the frequency range from 20 kHz to 30 MHz at temperatures from 300 to 400 K, which includes the temperature of the magnetic phase transition of some investigated compositions. It has been shown that, in spite of the differences in the static properties, the dynamic properties of the investigated compositions of these manganites are similar; the differences are mainly pronounced at frequencies higher than 1 MHz. In the ferromagnetic range, significant changes in the transmission coefficient have been observed for all compositions upon magnetization. The magnitudes of the initial dynamic magnetic permeability have been estimated. The changes in the transmission coefficient in the ferromagnetic region are mainly caused by changes in the dynamic magnetic permeability rather than by magnetoresistance. It is shown that in the paramagnetic temperature range the magnitude of the dynamic reversible permeability exceeds unity for all compositions, which supports the assumption on the retention of local magnetic ordering.

Charge ordering in crystals of Pr1−x Sr x MnO3 (x = 0.22, 0.24) ferromagnetic manganites

The structural and magnetic states of Pr1−xSrxMnO3 (x = 0.22, 0.24) manganite crystals were studied over a wide temperature range. Measurements of the magnetic susceptibility and electrical resistivity demonstrated that these manganites belong to the class of ferromagnetic semiconductors. Thermal neutron elastic scattering patterns revealed that, in the temperature range 4.2–350 K, the manganites have an orthorhombic structure (space group Pnma) with a well-pronounced cooperative Jahn-Teller effect. Major emphasis was placed on the nuclear magnetic superstructure with a wave vector q = (2π/2a, 0, 2π/2c). It was shown that this superstructure suggests 1/4-type charge ordering in the manganites under investigation.

Structural, magnetic and magneto-transport studies on the (La2/3Sm1/3)0.67Ba0.33−xSrxMnO3 (x = 0, 0.1, 0.2 and 0.33) manganites

The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of the (La2/3Sm1/3)0.67Ba0.33MnO3 system has been investigated. The samples of (La2/3Sm1/3)0.67Ba0.33−xSrxMnO3 (x = 0.0, 0.1, 0.2 and 0.33) are synthesized by the citrate gel route and crystallize in an orthorhombic structure. The unit cell volume decreases while the metal–insulator transition temperature (TMI) increases with increasing Sr content. The atomic disorder, which is represented by the size variance, increases with Ba content. The small anomaly in ρ(T) due to the atomic disorder which leads to the different magnetic ordering followed by a broad metal–insulator transition is observed in Ba-rich systems. The peak at T* (small anomaly) is presumed to be due to spin-dependent interfacial tunnelling that causes the difference in magnetic order between the surface and the core. The suppression in the resistivity values with increasing Sr content could be due to the more symmetric MnO6 octahedra network. The paramagnetic phase coexists with the magnetically ordered phase between T* (∼TC) and TMI. The suppression of the metastable magnetic phase leads to the high magnetoresistance in Ba-rich samples. The transport data has been analysed using a small polaron correlated model. The magnetic behaviour may be understood by the Ising model. The overall magnetic behaviour could be accounted for by the competing antiferromagnetic and ferromagnetic interactions.