Galvanomagnetic transport properties of nitrogenated (La,Sr)MnO<Subscript>3-δ</Subscript> and LaMnO<Subscript>3-δ</Subscript>

Epitaxial thin films of nitrogenated La_0.65Sr_0.30MnO₃ were grown on MgO(100) substrates by pulsed laser deposition (PLD). The nitrogenation was achieved by a continuous nitrogen flow in the PLD chamber with pressures of up to 0.12 mbar. The chemical analysis of the samples regarding the exchange of oxygen by nitrogen was achieved by time of flight secondary ion mass spectrometry, sputtered neutral mass spectrometry (SNMS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and yielded a content of incorporated nitrogen ranging from 0.6% to 3.8%. Without nitrogenation the electrical resistivity of La_0.65Sr_0.30MnO₃ exhibited a metal–insulator (MI) transition at about 180 K. The magnetoresistance (MR) effect (ΔR/R(0)) was about -50% at the transition temperature. Our nitrogen contents affected the MI transition so as to completely disappear and resulted in a resistivity increase of more than three orders of magnitude as well. By carefully reoxidizing the samples with subsequent heat treatments in air the MI transition reappeared at lower temperatures and we found a continuously enhanced MR ratio for decreasing temperatures. MR ratios of more than -99% were observed for a magnetic field of 10 T. The results are interpreted as a percolation phenomenon of ferromagnetic–metallic domains within an antiferromagnetic–semiconducting matrix. PACS 75.47.Gk; 75.47.Lx; 74.62.Dh; 81.15.Fg     

Pressure-induced antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structures of La_0.75Ca_0.25MnO₃ manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature T _C = 240 K, a ferromagnetic state is formed in La_0.75Ca_0.25MnO₃. At high pressures P ≥ 1.5 GPa and at temperatures T < T _N ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in T _C with the slope dT _C /dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of T _C on the basis of experimental data.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

The low-temperature minimum of the resistivity of La<Subscript>0.85</Subscript>Ag<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> manganite

The low-temperature minimum of the La_0.85Ag_0.15MnO₃ resistivity has been investigated. The analysis of the experimental data shows that this minimum of resistivity in zero magnetic field and the large magnetoresistive effect, which increases with a decrease in temperature, can be explained within the model of spin-polarized tunneling of carriers through grain boundaries.     

Metal-insulator transition in a radiation-disordered La<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> manganite

The temperature dependences of the electrical resistivity ρ(T) and the ac magnetic susceptibility χ(T, H = 0) are thoroughly investigated for a perovskite-like lanthanum manganite, namely, La_0.85Sr_0.15MnO₃, which is preliminarily exposed to neutron irradiation with a fluence _F = 2 × 10¹⁹ cm^?2 and then annealed at different temperatures ranging from 200 to 1000°C. The results of the electrical resistance measurements demonstrate that neutron irradiation of the samples leads to the disappearance of the low-temperature insulating phase. As the annealing temperature increases, the insulating phase is not restored and the manganite undergoes a transformation into a metallic phase. Analysis of the magnetic properties shows that, under irradiation, the ferromagnet-paramagnet phase transition temperature T_C decreases and the magnetic susceptibility is reduced significantly. With an increase in the annealing temperature, the phase transition temperature T_C and magnetic susceptibility χ(_{T, H} = 0) increase and gradually approach values close to those for an unirradiated sample. This striking difference in the behavior of the electrical and magnetic properties of the radiation-disordered La_0.85Sr_0.15MnO₃ manganite is explained qualitatively.     

Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Phase separation in a manganite La<Subscript>0.95</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript> crystal

The structure and magnetic states of a crystal of lightly doped manganite La_0.95Ba_0.05MnO₃ were studied using thermal-neutron diffraction, magnetic measurements, and electrical resistance data in a wide temperature range. It is shown that, in terms of its magnetic properties, the orthorhombic crystal is characterized by two order parameters, namely, antiferromagnetic (T _N = 123.6 K) and ferromagnetic (T _C = 136.7 K). The results obtained differ in detail from known information on the manganites La_0.95Ca_0.05MnO₃ and La_0.94Sr_0.06MnO₃. Two models of the magnetic state of the La_0.95Ba_0.05MnO₃ crystal are discussed, one of which is a model of a canted antiferromagnetic spin system and another is associated with the phase separation of the manganite. Arguments are advanced in favor of the coexistence in this crystal of the antiferromagnetic phase (about 87%) with a Mn⁴⁺ ion concentration of 0.048 and the 1/16-type charge-ordered ferromagnetic phase (about 13%) with a Mn⁴⁺ ion concentration of 0.0625. The specific features of the manganite studied are due to self-organization of the La_0.95Ba_0.05MnO₃ crystal lattice caused by the relatively large barium ion size.     

Heat capacity of the La<Subscript>0.9</Subscript>Ag<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> manganite near the curie temperature

The heat capacity of the La_0.9Ag_0.1MnO₃ manganite is measured in the temperature range 77–350 K and studied in detail in the vicinity of the Curie temperature for the first time. The regularities of the variation in the universal critical parameters in the vicinity of the phase transition point are established. The critical exponent and the amplitude of the heat capacity are calculated to be α = ?0.127 and A ⁺/A ^? = 1.146 with due regard for the scaling corrections. These parameters correspond to the critical behavior within the three-dimensional Heisenberg model. The size of ferromagnetic droplets in the paramagnetic range at T > T _C is estimated as ξ ≈ 19 Å. The results obtained are analyzed thoroughly and compared with theoretical data for a number of model systems.     

Two-phase paramagnetic-ferromagnetic state of La<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single-crystal lanthanum manganite

Two phases, paramagnetic and ferromagnetic, were shown by the magnetic resonance method to coexist below the temperature T _C in La_0.7Pb_0.3MnO₃ single crystals exhibiting colossal magnetoresistance. The magnetic resonance spectra were studied in the frequency range 10–78 GHz. The specific features in the behavior of the spectral parameters were observed to be the strongest at the temperatures corresponding to the maximum magnetoresistance in the crystals. The concentration ratios of the paramagnetic and ferromagnetic phases in the samples were found to be sensitive to variations in temperature and external magnetic field. This behavior suggests realization of the electronic phase separation mechanism in the system under study.     

Magnetotransport characteristics of strained La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> epitaxial manganite films

The electrical and magnetic characteristics of La_0.7Sr_0.3MnO₃ (LSMO) epitaxial manganite films are investigated by different methods under conditions when the crystal structure is strongly strained as a result of mismatch between the lattice parameters of the LSMO crystal and the substrate. Substrates with lattice parameters larger and smaller than the nominal lattice parameter of the LSMO crystal are used in experiments. It is shown that the behavior of the temperature dependence of the electrical resistance for the films in the low-temperature range does not depend on the strain of the film and agrees well with the results obtained from the calculations with allowance made for the interaction of electrons with magnetic excitations in the framework of the double-exchange model for systems with strongly correlated electronic states. Investigations of the magneto- optical Kerr effect have revealed that an insignificant (0.3%) orthorhombic distortion of the cubic lattice in the plane of the NdGaO₃(110) substrate leads to uniaxial anisotropy of the magnetization of the film, with the easy-magnetization axis lying in the substrate plane. However, LSMO films on substrates (((LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7)(001)) ensuring minimum strain of the films exhibit a biaxial anisotropy typical of cubic crystals. The study of the ferromagnetic resonance lines at a frequency of 9.76 GHz confirms the results of magnetooptical investigations and indicates that the ferromagnetic phase in the LSMO films is weakly inhomogeneous.     

Multifrequency electron spin resonance study of the charge-ordered manganite Nd<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

We performed multifrequency electron spin resonance (ESR) on the antiferromagnetic (T_N = 160 K) and charge-ordered (T _co = 250 K) insulating manganite Nd_0.5Ca_0.5MnO₃. Temperature (4–300 K) and frequency (9.4–285 GHz) dependence of the linewidth, intensity and position of the ESR line were studied. In the paramagnetic state we observe a single Lorentzian absorption line. For a given frequency, the ESR line position is temperature independent (close tog = 1.99). A strong linewidth broadening is observed below T_co. This indicates that there is no magnetic order in the temperature rangeT _cos>T >T _N but strong antiferromagnetic fluctuations are present. Below T_N, due to high-frequency and high-field ESR (up to 12 T) measurements, we were able to observe unexpected lines within the antiferromagnetic gap revealing the presence of a phase separation.     

Crystal and magnetic structures and physical properties of the Sm<Subscript>0.37</Subscript>Sr<Subscript>0.63</Subscript>MnO<Subscript>3</Subscript> manganite

The ¹⁵²Sm_0.37Sr_0.63MnO₃ manganite is investigated using neutron diffraction. The parameters of the crystal and magnetic structures of the manganite are determined. The diffraction data are compared with the transport and magnetic characteristics of this compound. A comparison is performed between the ¹⁵²Sm_0.37Sr_0.63MnO₃ and ¹⁵²Sm_0.45Sr_0.55MnO₃ manganites. Although these compounds differ insignificantly in the strontium doping level, are homogeneous antiferromagnets, and do not exhibit a colossal negative magnetoresistance, they have different crystal symmetries (tetragonal I4/mcm and orthorhombic Pnma), differ in the type of spin ordering (C-type antiferromagnetic and A-type antiferromagnetic ordering), are characterized by different orbital polarizations (\(d_{3z^2 - r^2 } \) and \(d_{x^2 - y^2 } \)), and possess one-and two-dimensional magnetic and transport properties, respectively. The critical concentration range in which samarium strontium manganites undergo a concentration structural transition from the orthorhombic to tetragonal crystal symmetry with a change in the type of orbital and magnetic order is revealed.     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

The giant volume magnetostriction and colossal magnetoresistance in Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

A doped manganite with the composition Eu_0.55Sr_0.45MnO₃ exhibits giant negative magnetostriction and colossal negative magnetoresistance at temperatures in the vicinity of the magnetic phase transformation (T～41 K). In the temperature interval 4.2 K≤T ≤40 K, the isotherms of magnetization, volume magnetostriction, and resistivity exhibit jumps at the critical field strength H_c1, which decreases with increasing temperature. At 70 K ≤T ≤120 K, the jumps on the isotherms are retained, but the shapes of these curves change and the H_c1 value increases with the temperature. At H<H_c1, the magnetoresistance is positive and exhibits a maximum at 41 K; at H>H_c1, the magnetoresistance becomes negative, passes through a minimum near 41 K and then reaches a colossal value. The observed behavior is explained by the existence of three phases in Eu_0.55Sr_0.45MnO₃, including a ferromagnetic (in which the charge carriers concentrate due to a gain in the s-d exchange energy) and two antiferromagnetic phases (of the A and CE types). The volumes of these phases at low temperatures are evaluated. It is shown that the colossal magnetoresistance and the giant volume magnetostriction are related to the ferromagnetic phase formed as a result of the magnetic-field-induced transition of the CE-type antiferromagnetic phase to the ferromagnetic state.     

Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>/Nd<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript> MnO<Subscript>3</Subscript> heteroepitaxial structure: Optical and magnetotransport properties

Sm_0.55Sr_0.45MnO₃/Nd_0.55Sr_0.45MnO₃ heterostructure consisting of layers with different Curie temperatures is studied. By comparing data for IR transmission, resistivity, magnetotransmission, magnetoresistance, and Kerr effect measured on the side of the film and substrate, the Curie temperatures of the layers are determined and the contributions of the layers to the magnetoresistance and magnetotransmission are estimated. A weak temperature dependence of the magnetotransmission and magnetoresistance makes manganites with a colossal magnetoresistance and magnetotransmission candidate materials for devices without temperature stabilization.     

Effect of high pressure on the crystal and magnetic structure and on the raman spectra in Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structure and the Raman spectra in Pr_0.7Ba_0.3MnO₃ manganite have been studied by the neutron diffraction technique at pressures up to 5 GPa as well as by the X-ray diffraction and Raman spectroscopy at pressures up to 30 GPa. The pressure dependence is determined for the lattice parameters, unit cell volume, Mn-O bond lengths in the orthorhombic structure of the Imma symmetry, and bending and stretching vibration modes for oxygen octahedra. In the low-temperature range at pressure P = 1.9 GPa, the magnetic transition from the initial ferromagnetic (FM) ground state (T _C = 197 K) to the A-type antiferromagnetic (AFM) state (T _N = 153 K) has been revealed. The FM and AFM phases coexist at pressures up to 5.1 GPa and exhibit negative and positive values of the pressure coefficient for the Curie and Néel temperature, respectively (dT _C/dP = −2.3 K/GPa and dT _N/dP = 8 K/GPa). The pressure dependence of the Curie temperature in Pr_0.7Ba_0.3MnO₃ differs drastically from that observed in other manganites of nearly the same composition with the orthorhombic Pnma and rhombohedral R[`3]cR\bar 3c structures, where the FM phase is characterized by the positive values of dT _C/dP. The structural mechanisms of these phenomena are discussed.     

Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.     

Study of crystal and magnetic structures of manganite Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of manganite Pr_0.7Ba_0.3MnO₃ have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T _C = 200 K, a ferromagnetic state forms in Pr_0.7Ba_0.3MnO₃. At high pressures P ≥ 1.9 GPa and T < T _N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T _C increases with the characteristic quantity dT _C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr_0.7Ba_0.3MnO₃ at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.     

Thermoelectric properties of La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite at ultrahigh pressures up to 20 GPa

Pressure dependences of the thermopower and electrical resistivity of the La_0.75Ca_0.25MnO₃ manganite are measured in the pressure range 0–20 GPa at room temperature. The absolute value of the thermopower increases in the pressure range 0–3 GPa and decreases at higher pressures. At the same time, the electrical resistivity decreases over the entire pressure range. It is found that the competing effect of the closing of the bandgap, which is determined by the activation energy for the thermopower, and the pressure broadening of the d bands is the cause of the observed behavior of the thermoelectric properties of La_0.75Ca_0.25MnO₃, which is untypical for the majority of dielectrics and semiconductors with single-band unipolar conductivity in the absence of phase transitions and is accompanied by a change in the sign of the pressure coefficient of the thermopower. The interrelation between the magnetic and thermoelectric properties of manganites under pressure is analyzed in the framework of the double exchange model. The causes of the considerable decrease in the pressure coefficients of the insulator-metal transition and Curie temperatures under pressure experimentally observed in manganites are discussed.     

Features of acoustic wave propagation near the structural phase transition in the La<Subscript>0.875</Subscript>Sr<Subscript>0.125</Subscript>MnO<Subscript>3</Subscript> manganite

Features of the phase transition from the disordered state to the ordered orbital state in a La_0.875Sr_0.125MnO₃ single crystal, caused by the cooperative Jahn-Teller effect, have been investigated. A significant change in the acoustic wave parameters in the entire range of cooperative distortion of the structure is revealed. Application of an external magnetic field shifts the structural phase transition to low temperatures.