Structural, transport, and magnetic properties in the Ti-doped manganites LaMn1−xTixO3 (0≤x≤0.2)

The magnetism and transport properties of the samples LaMn_1−xTi_xO₃ (0≤x≤0.2) were investigated. All samples show a rhombohedral structure () at room temperature. The sample with x=0 undergoes the paramagnetic-ferromagnetic (PM-FM) transition accompanied by an insulator-metal (I-M) transition due to the oxygen excess. The doped samples show ferromagnetism and cluster behavior at low temperatures. Though no I-M transition associated with the PM-FM transition appears, the magnetoresistance (MR) effect was observed especially at low temperatures under the applied fields of 0.5 T. Due to the fact that the oxygen content in the Ti-doped samples is nearly stochiometry (3.01) and the Hall resistivity at room temperature is negative, the ferromagnetism in LaMn_1−xTi_xO₃ (0.05≤x≤0.2) is believed to be consistent with the Mn²⁺-O-Mn³⁺ double exchange (DE) mechanism. These results suggest that DE can be obtained by direct Mn-site doping.     

Magnetic resonance in crystalline La0.9Ca0.1MnO3: Comparative study of bulk and nanometer-sized samples

X-band electron magnetic resonance (EMR) measurements were done at 115?T?600 K on bulk and nanometer size-grain powder single-crystalline samples of La_0.9Ca_0.1MnO₃, in order to study an impact of structural inhomogeneity on magnetic ordering. For the nano-crystal sample, two superimposed EMR lines are observed below 240 K, while for bulk-crystal one, a second line emerges in narrow temperature interval below 130 K. Temperature dependences of resonance field and line width of the main and the secondary line are drastically different. EMR data and complementary magnetic measurements of bulk-crystal sample reveal mixed-magnetic phase, which agrees with the published phase diagram of bulk La_1−xCa_xMnO₃. In a marked contrast, the same analysis for nano-crystal sample shows two phases one of which is definitely ferromagnetic (FM) and other is likely such, or super paramagnetic. The data obtained are interpreted in terms of very different magnetic ground states in the two samples, that is attributed to different randomness of the indirect FM exchange interactions mediated by bound holes.     

Lanthanum deficiency effect in magnetic transition and transport property of La0.8−δCa0.2MnO3

The evolution of magnetic and electrical phases in La_0.8−δCa_0.2MnO₃ was investigated in terms of La deficiency. We found that the increase of the La deficiency tends to raise the Curie temperature (T_C) in La_0.8−δCa_0.2MnO₃. The FM clusters formed in compounds with large La deficiency provide percolation paths above T_C. With increasing the La defect, the transport property changes from insulating to metallic state, which is in association with the crossover from a second order to a first order magnetic phase transition in the vicinity of T_C.     

The correlation between structure and magnetic properties in the manganites La0.7Ca0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Ca_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show an orthorhombic structure (O′-Pbnm) at room temperature. It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase in the Te content. All samples exhibit an insulator-metal (I-M) transition and the resistivity increases with the increase in the Te-doping level. Additionally, the Curie temperature T_c decreases and the transition becomes broader with increasing Te-doping level, in contrast, the magnetization of Te-doping samples at low temperatures decrease with increasing x as x≤0.10 and then increase with further increasing x to 0.15. The results are discussed in terms of Jahn-Teller (JT) vibrational anisotropy Q₃/Q₂ and the opening of the new DE channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ ions for Ca²⁺ ions.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Structural, magnetic and transport properties in the manganites La0.7Sr0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Sr_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show a rhombohedral structure with the space group . It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of Te content. The Curie temperature T_C decreases with increasing Te-doping level, in contrast, the magnetization magnitude of Te-doping samples at low temperatures increase with increasing x as x≤0.05 and then decrease with further increasing x to 0.15. The results are discussed in terms of the combined effects of the opening of the new double exchange (DE) channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ for Sr²⁺ and the reduction of the transfer integral b due to the decrease of the Mn-O-Mn bond angle.     

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.     

Yafet-Kittel-type magnetic ordering in Ni0.35Zn0.65Fe2O4 ferrite detected by magnetosensitive microwave absorption measurements

Magnetosensitive microwave absorption measurements of polycrystalline ferrite Ni_0.35Zn_0.65Fe₂O₄ was carried out at 9.4 GHz (X-band) as a function of temperature. Temperature dependence of the total linewidth (ΔH_pp) deduced from the resonance spectra showed the passage through the Curie point (T_c~430 K). Additionally, the plot ΔH_pp vs. T also indicated the existence of another magnetic phase transition at ~240 K, which can be associated with a Yafet-Kittel-type canting of the magnetic moments. Low-field microwave absorption (LFMA) and the magnetically modulated microwave absorption spectroscopy (MAMMAS) were used to give a further knowledge on this material. For low temperature, these techniques give evidence of a Yafet-Kittel-type canting of the magnetic moments.     

Low-field AC susceptibility in Pr1−xHoxMn2Ge2

We report the structure and magnetic properties of Pr_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Pr leads to a linear decrease in the lattice constants and the unit cell volume. The samples with x=0 and x=0.8 have spin reorientation temperature. The results are collected in a phase diagram.     

Electrical and magnetic properties of non-stoichiometric (La0.8Sr0.2)1−xMnO3 perovskites

The structure, transport and magnetic properties of (La_0.8Sr_0.2)_1−xMnO₃ (0≤x≤0.30) polycrystalline perovskite manganites have been investigated. For all the samples the Curie temperatures, T_c, remain nearly unchanged (329±3 K). Resistivity versus temperature curves for the samples show a double-peak behavior. A significant magnetoresistance (MR) effect and different temperature dependences of the MR ratios of the samples are observed. The shapes of the MR-T curves of the samples can be adjusted by changing x. For the x=0.30 sample, a nearly constant MR ratio of (9.5±0.5)% is obtained over the temperature range from 205 to 328 K.     

Magnetic study of phase separation and charge ordering in La1−xSrxMnO3 near x=0.5

We have investigated the magnetic phase diagram of polycrystalline and single-crystal La_1−xSr_xMnO₃ near 0.46≤x≤0.50. It turns out that for x<0.48, the polycrystalline material is ferromagnetic (FM), but for x≥0.48, incipient charge ordering takes place along with antiferromagnetism. At x=0.48, the ferromagnetic-antiferromagnetic phase transition in ceramics occurs at less than 85 kOe but requires significantly larger field for increasing x. These observations are in contrast to what is found in the single crystals, which are all FM.     

Phase boundary between uniaxial and planar ferromagnets in layered perovskite manganite La2−2xSr1+2xMn2O7 (0.313⩽x⩽0.350)

We have examined magnetizations as a function of temperature and magnetic field in layered perovskite manganites La_2−2xSr_1+2xMn₂O₇ single crystals (x=0.313, 0.315, 0.318, 0.320 and 0.350) in order to determine the phase boundary between two ferromagnets (one is an uniaxial ferromagnet whose easy axis is parallel to the c-axis and the other is a planar ferromagnet whose easy axis is within the ab-plane) and following results are obtained: (i) all the present manganites exhibit magnetic transitions from a ferromagnet to a paramagnet at 76, 107, 116, 120 and 125 K for x=0.313, 0.315, 0.318, 0.320 and 0.350, respectively; (ii) for x=0.318, 0.320 and 0.350, the magnetic structure is a planar ferromagnet below Curie temperature; (iii) for x=0.313 and 0.315, the magnetic structure changes from an uniaxial to a planar ferromagnet at 66 and 85 K, respectively. From the results described above we have constructed the magnetic phase diagram of layered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (0.313?x?0.350).     

The role of oxygen vacancies in magnetism of CoxTi1−xO2−δ films on Si(001) prepared by sol-gel method

Co_xTi_1−xO_2−δ films have been prepared on Si(001) substrates by sol-gel method. When heat treated in air, Co_xTi_1−xO_2−δ films are non-ferromagnetic at room temperature. However, after further vacuum annealing or hydrogenation, Co_xTi_1−xO_2−δ films show room-temperature ferromagnetism (RTFM). When the vacuum annealed Co_xTi_1−xO_2−δ films are reheated in air, the magnetic moments of the films strongly reduce. After these films are vacuum annealed once again, the magnetic moments are greatly enhanced, confirming the role of vacuum annealing in ferromagnetism of Co_xTi_1−xO_2−δ films. The x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and measurements of magnetization (M) vs temperature (T) fail to detect Co clusters in the vacuum annealed and the hydrogenated Co_xTi_1−xO_2−δ films. Oxygen vacancies are formed in Co_xTi_1−xO_2−δ films after vacuum annealing and hydrogenation, determined by XRD and XPS measurements. These results indicate that oxygen vacancies created by vacuum annealing and hydrogenation play an important role in the generation of RTFM in Co_xTi_1−xO_2−δ films.     

Effect of illumination on magnetization and microwave absorption of La1−xCaxMnO3 (x<0.2) films

Light-induced changes of the hysteresis loops of magnetization and microwave absorption are investigated in low-doped La_1−xCa_xMnO₃ (x<0.2) thin films. The width of the hysteresis loops decreases clearly under illumination with visible or near-infrared light at temperatures below 50 K. The microwave conductivity has a minimum value at magnetic fields corresponding to the magnetization reversal and is shifted towards weaker fields under illumination. These effects show complex nonexponential time evolution and dependence on strength of the magnetic field. The results can be explained by assuming that small ferromagnetic metallic regions exist within the insulating ferromagnetic phase of the sample, and that these regions are expanded by optically induced charge transfer between Jahn–Teller split e_g states of neighboring Mn³⁺ ions. Decrease of the Mn³⁺ XPS core level spectrum is observed in the samples under illumination with a HeNe laser.     

Ferromagnetic resonance studies on (Co40Fe40B20)x(SiO2)1−x granular magnetic films

Magnetic properties of granular (Co₄₀Fe₄₀B₂₀)_x(SiO₂)_1−x   thin films (x=0.37-0.53$x=0.37-0.53$) have been studied by ferromagnetic resonance (FMR) technique. Samples have been prepared by ion-beam deposition of Co–Fe–B particles and SiO₂ on sitall ceramic substrate. The FMR measurements have been done for different orientations of DC magnetic field with respect to the sample plane. It was found that the deduced value of effective magnetization from FMR data of the thin granular film is reduced by the volume-filling factor of the bulk saturation magnetization. The overall magnetization changes from 152 to 515 G depending on the ratio of the magnetic nanoparticles in the SiO₂ matrix. From angular measurements an induced in-plane uniaxial anisotropy has been obtained due to the preparation of the film conditions as well.     

Room temperature ferromagnetism and magnetotransport properties of the layered manganite system La1.2Ba1.8Mn2−xRuxO7 (0≤x≤1.0)

The dc magnetization and ac susceptibility measurements on two dimensional layered manganite La_1.2Ba_1.8Mn₂O₇ samples reveal the occurrence of ferromagnetism above room temperature with ferromagnetic (FM) to paramagnetic (PM) transitions at 338 K. The bifurcation temperatures shown by the zero-field cooled (ZFC) and field cooled (FC) dc magnetization curves at high temperatures shift towards lower temperatures as the applied field is increased from 100 to 2500 Oe. The data are suggestive of a large magnetic anisotropy due to the strong competing ferromagnetic and antiferromagnetic interactions resulting in a spin-glass-like state. Ru doping is found to enhance the ferromagnetism and metallicity of the system in a remarkable way. The magnetoresistance (MR) values obtained are very high and about 40% even at 260 K for the undoped sample.     

Low-field transport properties of (1−x)La0.7Ca0.2 Sr0.1MnO3+x(ZnO) composites

Composite samples (1−x)La_0.7Ca_0.2Sr_0.1MnO₃(LCSMO)+x(ZnO) with different ZnO doping levels x have been investigated systematically. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and ZnO grains, and that ZnO segregates mostly at the grain boundaries of LCSMO. The magnetic properties reveal that the ferromagnetic order of LCSMO is weakened by addition of ZnO. The results also show that ZnO has a direct effect on the resistance of LCSMO/ZnO composites, especially on the low-temperature resistance. With increase of the ZnO doping level, T_P shifts to a lower temperature and the resistance increases. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 3 kOe. The maximum MR appears at x=0.1. The low field magnetoresistance (LFMR) results from spin-polarized tunneling. However, around room temperature, the enhanced MR of the composites is caused by magnetic disorder.     

Study of the magnetization and transport properties of the La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 (0≤x≤0.2) system

The samples with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 La_(2+x)/3Sr_(1−x)/3Mn_1−xCr_xO₃ (0≤x≤0.20) have been prepared. The magnetic, electrical transport, and magnetoresistance properties have been investigated. Remarkable transport and colossal magnetoresistance (CMR) effect, as well as cluster glass (CG) behaviors have been clearly observed in the samples studied. It was found that the Curie temperature T_c and insulator−metal transition temperature T_p1 are strongly affected by Cr substitution. The experiment observations are discussed by taking into account the variety of tolerance factors t; the effects of A-site radius 〈r_A〉 and the A-site mismatch effect (σ²).     

Critical behavior in Co-doped manganites La0.67Pb0.33Mn1−xCoxO3 (0≤x≤0.08)

The critical properties of perovskite manganite La_0.67Pb_0.33Mn_1−xCo_xO₃ (0≤x≤0.08) around the paramagnetic-ferromagnetic phase transition are investigated through various techniques such as the modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis. Though the nature of this transition was found to be in second order, the estimated critical exponents β (0.233≤β≤0.368), γ (1.03≤γ≤1.40) and δ (4.32≤δ≤5.54) are in between the theoretically predicted values for three-dimensional Heisenberg and tricritical mean-field model. This model suggests the coexistence of the short-range and long-range ferromagnetic orders around the critical temperature. The values of the critical exponents obtained from different methods and the well-obeyed scaling behavior confirm that the calculated exponents are unambiguous and purely intrinsic to the system.     

Disorder induced ferromagnetism in the Cu-doped molybdate SrMo1−xCuxO3 (0≤x≤0.20)

The effect of Cu-doping at Mo-site on structural, magnetic, electrical transport and specific heat properties in molybdates SrMo_1−xCu_xO₃ (0≤x≤0.2) has been investigated. The Cu-doping at Mo-site does not change the space group of the samples, but decreases the structural parameter a monotonously. The magnetic properties change from Pauli-paramagnetism for x=0 to exchange-enhanced Pauli-paramagnetism for x=0.05 and 0.10, and then ferromagnetism for x=0.15 and 0.20. All samples exhibit metallic-like transport behavior in the whole temperature range studied. The magnitude of resistivity increases initially to x=0.10 and then decreases with increasing Cu-doping concentration. The results are discussed according to the electron localization due to the disorder effect induced by the random distribution of Cu at Mo site in the samples. In addition, the temperature dependence of specific heat for the Cu-doped sample has also been studied.