Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Magnetic domains and anisotropy in single crystals of Er(Co,Mn)O3

We have previously reported the magnetic properties of the Er(Co,Mn)O₃ solid solution, in which Mn atoms have been partially substituted by Co. Several magnetic entities coexist in this system (Er³⁺, Mn³⁺, Mn⁴⁺, Co²⁺, Co³⁺), resulting into a subtle interplay between different interactions and leading to unusual phenomena such as: a spin reversal due to antiferromagnetic exchange between erbium and transition-metal spins, a step-like transition due to reorientation of magnetic domains, a relaxation mechanism connected to the rotation energy of the domains, the intersection of the increasing and decreasing branches in the magnetization loops, etc. These results have been obtained in highly dense ceramic samples for compositions near ErCo_0.50Mn_0.50O₃. In order to better understand the different mechanisms, we have started a program on crystal growth. High-quality single crystals of orthorhombic structure (S.G. Pbnm) of Er(Co,Mn)O₃ manganite were grown in a floating-zone oven under oxygen atmosphere. Although the zero-field-cooled/field cooled (ZFC/FC) curves confirm the spin reversal observed in bulk samples, the remarkable effects seen on hysteresis loops for bulk samples were not present in single crystals, probably due to oxygenation conditions. On the other hand, a very large anisotropy was observed between two perpendicular orientations of the single crystal.     

The effects of vacancies in oxygen sites on physical properties of the lanthanum manganites

Abstract

The non-stoichiometric solution Ln_1?xSr_xMnO_3-δ□_δ was prepared by the classic ceramic method. The physical properties as crystallographic, magnetic and electrical properties were studied. A structural phase transition from rhombohedral to orthorhombic was observed at a concentration of Mn⁴⁺ between 10% and 15% per Mn atom in the unit formula. The magnetic properties are very sensitive to the presence of vacancies at the oxygen sites. The non-stoichiometric, samples change from metallic to insulating behaviour depending on their vacancy concentration. In the semiconductor phase, the activation energy value changes with the structural phase, increasing in the rhombohedral phase and decreasing in the orthorhombic phase.     

Spin-dependent transport in trilayer junctions of doped manganites

Trilayer junctions of doped manganites provide a model system for the study of spin-polarized transport across a half metallic interface. Novel phenomena observed in these systems include a large low-field magnetoresistance (MR), with nearly an order of magnitude change in junction resistance in 100 Oe at 14 K, and a strong temperature and bias dependence of the junction MR. They also enabled the first demonstration of a spin-angular momentum transfer-induced reversal of a magnetic moment. Systems containing both manganite and transition metal ferromagnetic electrodes demonstrate a complex interface structure between the perovskite oxide such as SrTiO₃ and the transition metal electrode such as Co or Fe. It reveals the important effect this interface has on the magnetotransport property of the junction.     

Critical behavior studies in Ti-substituted lanthanum bismuth perovskite manganites

Perovskite manganite La_0.4Bi_0.6Mn_1−xTi_xO₃ (x = 0.05 and 0.1) synthesized using conventional solid state route method give rise to critical phenomenon in their magnetic interactions due to the substitution of non magnetic Ti ions. The critical behavior is observed near paramagnetic–ferromagnetic transition and is studied by magnetization measurements. Various techniques like Modified Arrott plot, Kouvel–Fisher method, scaling equation of state analysis and the critical magnetization isotherm were used to analyze the magnetization data on magnetic phase transition. The values of critical exponents β and γ obtained using different techniques are in good agreement. The obtained critical exponents are found to follow scaling equation with the magnetization data scaled into two different curves below and above the transition temperature, T_C. This confirms that the critical exponents and T_C are reasonably accurate. The obtained critical exponents for both the samples deviates from mean-field model and do not completely follow the static long range ferromagnetic ordering. This behavior is consistent with non magnetic nature of Ti substituted at Mn site and can be associated with Griffiths phase like phenomenon.     

Synthesis of lanthanum–strontium manganites by oxalate-precursor co-precipitation methods in solution and in reverse micellar microemulsion

Nanostructured lanthanum–strontium manganites were synthesized using two different co-precipitation approaches, one in bulk solution, and the other in reverse micelles of CTAB/1-hexanol/water microemulsion. In both cases, precursor cations were precipitated by using oxalic acid. The properties of the materials synthesized by using these two methods were compared in order to reveal potential advantages of the microemulsion-assisted approach. The influence of the annealing conditions on the properties of synthesized manganites was investigated by using X-ray diffraction, transmission electron microscopy, differential thermal analysis, thermogravimetric analysis and magnetic measurements.     

Magnetic inhomogeneity in La-deficient manganate crystals

We present here, the results of an investigation of the intrinsic magnetic properties of high-quality lanthanum manganate crystals. Ferromagnetic resonance experiments show that the linewidth is small (∼100 Oe) and independent of temperature, key indications of magnetic homogeneity. The magnetization shows full alignment of the manganese spins; however, the magnetocrystalline anisotropy is not monotonic and displays an abrupt change near the ordering temperature. The results are discussed in terms of a two-phase model.     

Ferromagnetic lanthanum manganites

The short overview of the physical properties of the ferromagnetic manganites exhibiting colossal magnetoresistance is given.     

Magnetic and high-pressure magnetotransport properties of cesium-substituted lanthanum calcium manganites

The nature of the double-exchange (DE) interaction in lanthanum manganites is studied through chemical substitutions, Cs for La, and high-pressure measurements. Static and high-frequency magnetic measurements and high-pressure electrical transport studies were carried out on bulk polycrystalline and radio-frequency sputtered thin films of La_0.7-xCs_xCa_0.3MnO₃ for x=0-0.1. The samples are found to be cubic. Curie temperature T_c measurements provide evidence for bond-length-related weakening of DE as x is increased from 0 to 0.03. For higher x, the bond-angle-related changes lead to an increase in the strength of DE. High-pressure mangetoresistance data indicate both bond length and bond-angle-related increase of 10–20 K/GPa in T_c with pressure, with the largest increase measured for x=0.03. The rate of increase in the Curie temperature with pressure decreases with increasing T_c. Anomalies are observed in the magnetic parameters for x=0.03. The Cs-concentration dependence of the low-temperature saturation magnetization shows a minimum close to x=0.03. Ferromagnetic resonance studies at x-band reveal a 5% decrease in the g-value for x=0.03 relative to the end members (x=0 and 0.1). The low-field magnetostriction for x=0.03 indicates a relatively strong electron–phonon spin coupling compared to neighboring compositions. Received: 15 May 2000 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Elastic properties of lanthanum manganites

We give a brief overview of the data on elastic properties of the CMR manganites published during last decade. The main emphasis is put on the results obtained for single crystals.     

Magnetic properties of UFe5Sn single crystals

Millimetre-size UFe₅Sn single crystals were grown by the top seed solution growth method and characterized by magnetization, ⁵⁷Fe Mössbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower temperature is associated with a reorientation towards b. Mössbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding γ(0 K)140 mJ/(mol K²) and θ290 K for UFe₅Sn.     

Adaptive segmentation of magnetic resonance images with intensity inhomogeneity using level set method

It is a big challenge to segment magnetic resonance (MR) images with intensity inhomogeneity. The widely used segmentation algorithms are region based, which mostly rely on the intensity homogeneity, and could bring inaccurate results. In this paper, we propose a novel region-based active contour model in a variational level set formulation. Based on the fact that intensities in a relatively small local region are separable, a local intensity clustering criterion function is defined. Then, the local function is integrated around the neighborhood center to formulate a global intensity criterion function, which defines the energy term to drive the evolution of the active contour locally. Simultaneously, an intensity fitting term that drives the motion of the active contour globally is added to the energy. In order to segment the image fast and accurately, we utilize a coefficient to make the segmentation adaptive. Finally, the energy is incorporated into a level set formulation with a level set regularization term, and the energy minimization is conducted by a level set evolution process. Experiments on synthetic and real MR images show the effectiveness of our method.     

磁力显微镜对相分离锰氧化物薄膜中反铁磁绝缘相融化的成像

用自制磁力显微镜研究了一个受各向异性应变的锰氧化物薄膜中的相分离以及由磁场导致的从反铁磁绝缘相到铁磁金属相的转变．磁力显微镜图片显示,在0 T这两种竞争的相就已经共存,且两种相 的畴呈非常明显的各向异性的条状分布,这可以定性解释输运上的各向异性．在2.1 T 以上,反铁磁绝缘相逐渐转变为铁磁金属相,并在3.2 T时结束．当去掉磁场时铁磁金属相能够保持．     

Giant thermal hysteresis in lanthanum manganites

The giant thermal hysteresis of the longitudinal sound velocity and internal friction turns out to be a specific feature of the orthorhombic-to-rhombohedral structural transition in La_1−xSr_xMnO₃ and La_1−xBa_xMnO₃ manganites. The mixture of the orthorhombic and rhombohedral phases can exist far beyond the thermal hysteresis loop as it is observed in magnetization or resistivity experiment. This means that in the CMR manganites, the boundary between the crystal phases is fuzzy.     

Magnetic properties and Griffiths singularity in La0.45Sr0.55 Mn1−xCoxO3

The magnetic properties and the Griffiths singularity were investigated in Mn-site doped manganites of La_0.45Sr_0.55Mn_1−xCo_xO₃ (x=0, 0.05, 0.10 and 0.15) in this work. The parent sample La_0.45Sr_0.55MnO₃ undergoes a paramagnetic-ferromagnetic transition at T_C=290 K and a ferromagnetic-antiferromagnetic transition at T_N=191 K. The doping of Co ions enhances the ferromagnetism and suppresses the antiferromagnetism. The enhanced ferromagnetism results from the fact that the Co doping enhances the Mn³⁺-Mn⁴⁺ double-exchange interaction and induces the Co²⁺-Mn⁴⁺ ferromagnetic superexchange interaction. Detailed investigation on the magnetic behavior above T_C exhibits that the Griffiths singularity takes place in this series of Mn-site doped compounds. The correlated disorder induced by the Co ionic doping, together with the phase competition from the ferromagnetic and the antiferromagnetic interactions among Mn ions, is responsible for the Griffiths singularity.     

Magnetoresistance of lanthanum manganites with activation-type conductivity

The temperature dependence of the resistivity and magnetic moment of La_0.85Ba_0.15MnO₃ and La_0.85Sr_0.15MnO₃ manganite single crystals in magnetic fields up to 90 kOe is investigated. Analysis of the experimental results shows that the magnetoresistance of lanthanum manganites far from the Curie temperature T _C can be described quantitatively by the s-d model normally used for ferromagnets and taking into account only the exchange interaction between the spins of charge carriers and magnetic moments. These data also show that the features of lanthanum manganites responsible for colossal magnetoresistance (CMR) are manifested in a narrow temperature interval δT ≈ 20 K near T _C. Our results suggest a CMR mechanism analogous to the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr-type multilayers with nanometer layer thickness. The nanostratification observed in lanthanum manganites and required for GMR can be described taking into account the spread in T _C in the CMR range δT.     

Structure and magnetocaloric properties of La1−xKxMnO3 manganites

A technology of obtaining the single-phase ceramic samples of La_1−xK_xMnO₃ manganites and the dependence of their structural parameters on the content of potassium has been described. Magnetocaloric effect (MCE) in the obtained samples has been measured by two independent methods: classical direct methodic and a method of magnetic field modulation. The values of MCE obtained by both methods substantially differ. The explanation of the observed divergences is given. The correlation between the level of doping and MCE value has been defined. The value of T_C determined by the MCE maximum conforms with the literature data obtained by other methods.     

Intrinsic magnetic inhomogeneity of Eu substituted La0.7Pb0.3Mno3 single crystals

The study of magnetic and magnetotransport properties of the crystals of (La_1−yEu_y)_0.7Pb_0.3MnO₃ system has been carried out. Eu ions enter the crystals being in trivalent nonmagnetic state. Europium ions possessing of smaller ionic radius in comparison with La ions, induce local distortions of Mn–O–Mn bonds in the system that cause random distribution of magnetic exchange interactions in magnitude and, probably, in sign. The competition of magnetic interactions leads to the appearance of the inhomogeneous magnetic state in the crystals. The enhancement of concentration of Eu ions results in decrease of the Curie temperature and broadening of the inhomogeneous magnetic state area. At y=0–0.4 the coexistence of the paramagnetic phase with conductivity of the polaronic type and the ferromagnetic metallic phase is observed in a bounded temperature interval both above and below T_C. Below T_C the increasing of y up to 0.6 induces the magnetic state representing the coexistence of two different FM phases. These phases are spatially separated due to frustration of FM and AFM exchange interactions on phase boundaries. Above T_C, up to 1.6T_C ferromagnetic clusters exist in a paramagnetic matrix similar to the case of samples with y=0–0.4. Concerning electric properties, the samples with y=0–0.4 reveal the metal–insulator transition at temperature that practically coincides with T_C. The sample with y=0.6 has conductivity of insulator character up to the lowest temperatures. For all investigated compositions y=0–0.6 the CMR effect is observed in the area where the inhomogeneous magnetic state exists. The effect is determined by different conductivity of the coexisting phases, as well as by sensitivity of the inhomogeneous state to external magnetic field.