Vibrational anisotropy of MnO6 octahedron and phase separation in La0.67−yPryCa0.33MnO3 manganites

The role of vibrational anisotropy of Mn³⁺O₆ octahedron in the phase separation behavior of La_0.67−yPr_yCa_0.33MnO₃ (x=0, 0.15, 0.25 and 0.30) has been investigated by means of magnetization M, internal friction Q⁻¹, Young's modulus E along with the X-ray powder diffraction measurements. For the samples with y=0 and 0.15, the Q⁻¹ exhibits three peaks in the ferromagnetic region, which are attributed to the intrinsic inhomogeneity of ferromagnetic phase, i.e. the electronic phase separation with the coexistence of insulating and conducting phases. However, both the samples with y=0.25 and 0.30 undergo a magnetic phase separation with the coexistence of the antiferromagnetic and ferromagnetic phases, and the Q⁻¹ peaks related to the electronic phase separation have not been observed. In addition, the Q⁻¹ exhibits a peak in the paramagnetic region for all samples, which may result from the formation of magnetic clusters. We observed that the evolution from electronic to magnetic phase separation is close related to the rapid increase in the ratio of two kinds of Jahn-Teller distortion modes Q₃ and Q₂, i.e. Q₃/Q₂. A schematic phase diagram is given in the text, and it is suggested that the enhancement of vibrational anisotropy of Mn³⁺O₆ octahedron plays a key role in the evolution from electronic to magnetic phase separation.     

The influence of the O–O isotope substitution on spin dynamics of (La0.25Pr0.75)0.7Ca0.3MnO3: La NMR data

The ¹³⁹La NMR spectra and spin–spin relaxation times have been measured for the ¹⁶O and ¹⁸O isotope-substituted manganite (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ in the external magnetic field of 5 T. The NMR signal wipe-out has been observed in the ¹⁸O-enriched sample in the charge-ordered state. This phenomenon is connected with a sharp increase in the spin–spin relaxation rate. The great isotope-effect observed provides a clear evidence of an essential role of oxygen motion in controlling the long-range magnetic order in manganites.     

Emission Mössbauer studies of the magnetoresistive compound, La0.7Sr0.3MnO3

The magnetic behavior of the Sr_0.3 manganite is studied using a local microprobe, ⁵⁷Co. In contrast with Ca substituted manganites, a much larger fraction of the material exhibits short-range order with superparamagnetic-like behavior even at 80 K. The differences in behavior are attributed to the large mismatch between the ionic radii of La⁺³ and the divalent substituent Sr⁺², which introduces anharmonicity in local vibrations. In common with all other compounds exhibiting negative bulk magnetoresistivity, the Sr_0.3 compound also exhibits very marked softening of lattice as one approaches T_c from below. Application of an external magnetic field results in coalescing of nanosized magnetic clusters to form larger ones with better alignment of spins.     

Magnetization reversal in Gd0.67Ca0.33MnO3: Comparison between epitaxial thin films and bulk

Rare-earth-based manganites ABO₃ may present interesting properties when the lanthanide (A-site) and/or the manganese (B-site) are partially substituted by divalent elements. Heavy lanthanides are particularly appealing because of the expected interplay between the intrinsic magnetic properties of the rare-earth element (Ln) and those of the ferromagnetic manganese sublattice. As such, a spin reorientation has been observed during magnetization-versus-temperature cycles due to a negative exchange interaction between Mn and Ln. We present herein high-quality epitaxial thin films (∼200 nm thick) of Gd_0.67Ca_0.33MnO₃ deposited onto (1 0 0) SrTiO₃ substrates by pulsed-laser deposition. Enhanced properties were observed in comparison with bulk samples. The magnetic transition temperature T_c of the as-grown films is much higher than the corresponding bulk values. Most interesting, magnetization measurements performed under small applied fields, exhibit magnetization reversals below T_c, no matter whether the film is field-cooled (FC) or zero-field-cooled (ZFC). The reversal mechanism is discussed in terms of a negative exchange f-d interaction and magnetic anisotropy, this latter enhanced by strain effects induced by the lattice mismatch between the film and the substrate.     

Synthesis of magnetoresistive La0.7Sr0.3MnO3 nanoparticles by an improved chemical coprecipitation method

La_0.7Sr_0.3MnO₃ nanoparticles were prepared by a simple chemical coprecipitation route. Structural, magnetoresistance (MR), and magnetic properties were investigated. Rietveld refinement of X-ray powder diffraction result shows that the sample is single-phase with the space group of R3¯C. The result of field-emission scanning electronic microscopy shows that most of the grain sizes are distributed from 50 to 200 nm. The composition determined by energy-dispersive spectroscopy is the stoichiometry of La_0.7Sr_0.3MnO₃. The ferromagnetic to paramagnetic transition is sharp with Curie temperature T_C=367 K, which further confirms that the sample is single-phase. The steep change in MR at low fields is attributed to the alignment of the magnetization, while the high-field MR is due to the grain boundary effect.     

Investigation of FIB irradiation damage in La0.7Sr0.3MnO3 thin films

In this work we analyse systematically how morphological and magnetotransport properties of manganite thin films are affected by the damage induced by focused ion beam (FIB) irradiation. We irradiate different areas of the same sample with doses ranging from 5×10¹² to 3×10¹⁷ ions/cm² and we find that the film becomes swollen for doses up to 10¹⁶ ions/cm² and is eventually eroded by ion milling for further irradiation. On the other hand, transport properties are much more sensitive to FIB irradiation: the metal–insulator transition temperature is found to decrease monotonically with increasing doses up to 1.8×10¹³ ions/cm². At doses higher than 5.6×10¹³ ions/cm² the metallic state is completely suppressed and likely, also ferromagnetism.     

Polarized SSXANES study of spin ordering in ferromagnetic and paramagnetic phases of La1.2Sr1.65Ca0.15Mn2O7

Spin-selected polarized X-ray absorption near-edge structures (SSXANES) at the Mn K-edge from a bilayer La_1.2Sr_1.65Ca_0.15Mn₂O₇ single crystal have been studied with high resolution, both in the ferromagnetic (15 K) as well as paramagnetic phase (300 K). The orientation-dependent SSXANES spectra show unmistakable temperature dependence as the system makes the ferromagnetic to paramagnetic phase transition. The pre-edge structures are too intense to be ascribed to weak quadrupole transitions and are interpreted in terms of hybridization of Mn 3d orbitals with O2p and Mn 4p orbitals over and above similar onsite hybridization. The results also indicate possible existence of a small local (time-frozen) ferromagnetic ordering in the macroscopically disordered state. Need for further experimental and theoretical work on the SSXANES spectra from the bilayer system is emphasized.     

Magnetic ordering in La0.75Sr0.25CrO3: A neutron diffraction study

Low-temperature neutron diffraction measurements were carried out on a powder sample of the compound La_0.75Sr_0.25CrO₃ in order to elucidate its magnetic structure. Rietveld analysis of the neutron diffraction data, as a function of temperature, showed that it possesses a G-type antiferromagnetic alignment of Cr spins at all temperatures below 300 K. Down to the lowest achievable temperature, viz. 17 K, the Cr site moments were found to be the weighted average of the 75% Cr³⁺ and 25% Cr⁴⁺ spin-only ionic moments. At 17 K, the Cr site moment was 2.71(5) μ_B/Cr ion. There is no observable change in the Cr–O bond lengths as a function of temperature. The tilt angles of the CrO₆ octahedra marginally increase with decreasing temperature.     

Magnetic structure determination using polarised resonant X-ray scattering

Full polarisation analysis of resonant X-ray magnetic scattering (RXMS) is shown to advance the determination of magnetic moment directions of complex magnetic structures within single crystals. Key features of this novel method are the variation of the incident beam polarisation through the use of an X-ray phase plate, and the measurement of the scattered beam polarisation in terms of Poincaré-Stokes parameters. Contrary to the established method, no azimuthal rotation is required. Thus, the major sources of systematic error are eliminated, and the method is compatible with exotic and complex sample environments. The technique is demonstrated with the example of TbMn₂O₅. The RXMS theory briefly outlined in this paper was fitted to the data, and the local and delocalized, band-specific moment directions associated with the magnetic order of the resonating species was refined with a high degree of accuracy.     

Effects of nonstoichiometry on the properties of electron-doped manganite Pr0.1Ca0.9MnO3

Mn⁴⁺-rich perovskite manganites (nominal composition: Pr_0.1Ca_0.9MnO₃) were synthesized by using a citric acid method and sintered at different temperatures (800, 900, 1000, and 1300 °C) to adjust the concentration of charge carrier. All the samples are found to be in the cluster glass state at low temperature, as indicated by dc and ac magnetization. With the increase of sintering temperature, the ferromagnetic component in samples increases at first, and attains a maximum at 1000 °C, then decreases again at 1300 °C, which could be well interpreted by the change of charge carrier concentration induced by nonstoichiometry, consistent with the trend on the phase diagram. However, nonstoichiometry breaks long-range Mn-O-Mn interaction, resulting in the variance from the phase diagram.     

Neutron diffraction studies of Tb2Rh3Si5 compound

In this work neutron diffraction studies of Tb₂Rh₃Si₅ compound are reported. The compound crystallizes in the monoclinic crystal structure of Lu₂Co₃Si₅-type. At 1.5 K an antiferromagnetic ordering with a propagation vector k=(1/2;1/2;1/2) was observed. The Tb magnetic moments of 9.8(2) μ_B form a non-collinear magnetic structure. In the vicinity of Néel temperature of 8 K a change of the magnetic ordering is evidenced. The change seems to be connected with phase transition from commensurate to incommensurate sine-wave modulation of the Tb magnetic moments.     

Electron-doped Sm1−xSrxMnO3 perovskite manganites: Crystal and magnetic structures and physical properties

We present the results of a study of electron-doped Sm_1−xSr_xMnO₃ (x>0.5) perovskite manganites by combining high-resolution neutron powder diffraction with measurements of resistivity, magnetization and magnetic susceptibility. Although investigated Sm_0.45Sr_0.55MnO₃ and Sm_0.37Sr_0.63MnO₃ compounds belonging to the same phase diagram area differ significantly in the strontium content, they are homogeneous antiferromagnetic (AF) insulators and do not exhibit CMR. They have different crystallographic symmetries (orthorhombic Pbnm and tetragonal I4/mcm, respectively) in the entire temperature range under study (1.5-288 K), differ in the type of spin ordering at low temperatures (AF-A and AF-C), are characterized by different orbital polarizations (d_x²_−y² and d_3z²_−r²), and possess two- and one-dimensional magnetic properties, respectively. The lack of magnetoresistance for these compositions is explained by the lack of coexisting magnetic phases involving double exchange ferromagnetism, in contrast to what is observed for the magnetoresistive Sm_1−xSr_xMnO₃ compounds, that is with x?0.52.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

On the coexistence of spin and lattice polarons in the La0.67−xEuxCa0.33MnO3 CMR system

The electrical and magnetic transport properties of the La_0.67−xEu_xCa_0.33MnO₃ system exhibit lowering of insulator to metal and paramagnetic to ferromagnetic transition temperature (T_C) with the increase of Eu concentration in addition to possessing CMR property. The temperature variation of electrical resistivity and magnetic susceptibility for x=0.21 is found to have two distinct regions in the paramagnetic state for T>T_P; one with the localization of lattice polaron in the high-temperature region (T>1.5T_P) satisfying the dynamics of variable range hopping (VRH) model and the other being the combination of the spin and lattice polarons in the region T_P<T<1.5T_P. The resistivity variation with temperature and magnetic field, the cusp in the resistivity peak and CMR phenomenon are interpreted in terms of coexistence of spin and lattice small polarons in the intermediate region (T_P<T<1.5T_P). The spin polaron energy in the La_0.46Eu_0.21Ca_0.33MnO₃ system is estimated to be 106.73±0.90 meV and this energy decreases with the increase of external magnetic field. The MR ratio is maximal with a value of 99.99% around the transition temperature and this maximum persists till T→0 K, at the field of 8 T.     

Magnetic and transport properties of double distorted perovskites CaCuMn6O12 and CaCu2Mn5O12

Magnetic and transport properties of double distorted perovskites CaCuMn₆O₁₂ and CaCu₂Mn₅O₁₂ are studied in a range 2–300 K. The leading role in magnetism of these compounds belongs to antiferromagnetic exchange interaction of Cu²⁺ in square coordination with Mn³⁺/Mn⁴⁺ in octahedral coordination. The values of saturation magnetization indicate that Mn³⁺ ions in square coordination are coupled ferromagnetically with Mn³⁺/Mn⁴⁺ in octahedral coordination. The colossal magnetoresistance in the pellet samples is due assumingly to intergranular spin-polarized tunneling of current carriers.     

La0.7Ce0.3MnO3 epitaxial films fabricated by a pulsed laser deposition method

La_0.7Ce_0.3MnO₃ epitaxial films were successfully fabricated via a pulsed laser deposition method by controlling the experimental conditions. A series of experiments with varying the oxygen pressure and the substrate temperature demonstrated that the use of appropriate conditions is crucial for fabricating the epitaxial thin films. The existence of such suitable conditions was thermodynamically interpreted in terms of the stability of Mn²⁺ ion. Both XRD and EPMA measurements indicated that La_0.7Ce_0.3MnO₃ thin films fabricated herein form single phases, although it was difficult to present the direct experimental evidence to prove that Ce ion can really exist within the perovskite structure. The resultant films with oxygen annealing showed a metal-insulator transition and ferromagnetic property with Curie temperature of 275 K.     

Magnetic properties and charge ordering in Pr0.75Na0.25MnO3 manganite

The structural and magnetic properties of Pr_0.75Na_0.25MnO₃ have been investigated experimentally. At room temperature, the compound shows paramagnetic characteristic. Along with decreasing temperature, a peak appears in the magnetization versus temperature curve around 220 K. To clarify whether this peak is associated with the ordering arrangement of Mn³⁺ and Mn⁴⁺ ions, electron diffraction experiments were carried out below and above 220 K respectively. Only basic Brag diffraction spots can be observed at high temperatures, however, superlattice diffraction appears below 220 K. This provides direct evidence for the existence of charge ordering in Pr_0.75Na_0.25MnO₃. We find the Mn³⁺ and Mn⁴⁺ cations form zigzag chains in a-c plane by analyzing the diffraction patterns. Combining with the magnetization measurements and the results of electron spin resonance, we confirm the antiferromagnetic phase and ferromagnetic component coexist in Pr_0.75Na_0.25MnO₃ below 120 K.     

Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3

The structural, transport and electron spin resonance properties of bulk and nanosized La_0.875Sr_0.125MnO₃ prepared by a sol-gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T_C. In addition to a sharp peak in the vicinity of T_C, another sharp peak close to is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.     

Intrinsic inhomogeneity in the electron-doped manganite Sr0.95Ce0.05MnO3

Systematic studies of structural, magnetic, electronic, and elastic properties have been performed for the electron-doped manganite Sr_0.95Ce_0.05MnO₃. The results show that light doping with Ce in place of Sr in SrMnO₃ could stabilize the perovskite-type structure. The electronic transport and magnetism measurements show that the sample exhibits a charge ordering (CO) state below , accompanied by softening of Young’s modulus due to a strong electron-phonon coupling. Cluster-glass behavior and the magnetoresistance (MR) effect are observed at low temperatures, resulting from the induced double-exchange (DE) ferromagnetic (FM) clusters embedded in the CO antiferromagnetic (AFM) matrix. Above , the high temperature range appears to be dominated by local FM fluctuations, which is further supported by internal friction measurements. Our results indicate the existence of intrinsic magnetic inhomogeneity in electron-doped Sr_0.95Ce_0.05MnO₃.     

Crystal and magnetic structure of Ho2NiGe6

The crystal and magnetic structure of Ho₂NiGe₆ was studied by powder neutron diffraction. The paramagnetic neutron diffraction data confirmed the Ce₂CuGe₆-type crystal structure reported earlier for this compound. Below the Néel temperature equal to 11 K the Ho magnetic moments form a uniaxial antiferromagnetic ordering. The Ho magnetic moments equal to 8.16(7)μ_B at 1.5 K are parallel to the b-axis. The data are compared with those published for HoNi_0.46(6)Ge₂.