Ac magnetotransport in La0.7Sr0.3Mn0.95Fe0.05O3 at low dc magnetic fields

We report the ac electrical response of La_0.7Sr_0.3Mn_1−xFe_xO₃(x=0.05) as a function of temperature, magnetic field (H) and frequency of radio frequency (rf) current (). The ac impedance (Z) was measured while rf current directly passes through the sample as well as in a coil surrounding the sample. It is found that with increasing frequency of the rf current, Z(T) shows an abrupt increase accompanied by a peak at the ferromagnetic Curie temperature. The peak decreases in magnitude and shifts down with increasing value of H. We find a magnetoimpedance of for at around room temperature when the rf current flows directly through the sample and when the rf current flows through a coil surrounding the sample. It is suggested that the magnetoimpedance observed is a consequence of suppression of transverse permeability which enhances skin depth for current flow. Our results indicate that the magnetic field control of high frequency impedance of manganites is more useful than direct current magnetoresistance for low-field applications.     

Magnetocaloric effect in polycrystalline La0.65Ba0.3M0.05MnO3 (M=Na, Ag, K) manganites

The effects of monovalent doping on the crystallographic, magnetic and magnetocaloric properties of La_0.65Ba_0.3M_0.05MnO₃ (M=Na, Ag, K) powder samples, elaborated using the solid state reaction method at high temperature, have been investigated. In our three samples the Mn⁴⁺ amount remains constant equal to 40%. The Rietveld refinement of the X-ray powder diffraction shows that all our synthesized samples are single phase and crystallize in the distorted rhombohedral system with R3¯c space group. All our studied samples undergo a paramagnetic–ferromagnetic transition with decreasing temperature. Using the Arrott plot, the second-order transition Curie temperature T_C for M=Na, Ag and K is found to be 310, 300 and 290 K, respectively. The magnetic entropy change, deduced from isothermal magnetization curves, exhibits a maximum |ΔS_M^Max| of about 2.65, 2.82 and 2.66 J/kg K for M=Na, Ag and K, respectively, in a magnetic applied field change of 5 T. Although these values are modest, the magnetocaloric effect extends over a large temperature range leading to an important value of the relative cooling power (RCP). The RCP values exhibit a nearly linear dependence with the magnetic applied field. The refrigeration capacity in a magnetic applied field of 1 T is found to be 28.8, 27.8 and 25.6 J/kg for M=Na, Ag and K compounds.     

Electrical behavior of nano-polycrystalline (La1−yKy)0.7Ba0.3MnO3 manganites

We present a study of the structural and electrical behavior of nano-polycrystalline mixed barium and alkali substituted lanthanum-based manganite, (La_1−yK_y)_0.7Ba_0.3MnO₃ with y=0.0-0.3. The samples were synthesized by the polymerization complex sol-gel method. The powder X-ray diffraction (XRD) data of the samples show a single-phase character with space group. The magnetic and electrical transport properties of the nano-polycrystalline samples have been investigated in the temperature range 50-300 K and a magnetic field up to 10 kOe. The metal-insulator transition temperature T_p of all the samples decreased with potassium doping, and also, it increased slightly with the application of magnetic field. The low field magnetoresistance, which is absent in the single-crystalline perovskite, was observed and increased with decreasing temperature. Comparing the experimental resistivity data with the theoretical models shows that the high temperature electrical behavior of these samples is in accordance with the adiabatic small polaron-hopping model. In the metal-ferromagnetic region the resistivity is found to be quite well described by ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5.     

Tuning the magnetocaloric properties of La0.7Ca0.3MnO3 manganites through Ni-doping

The effect of Ni²⁺ doping on the magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ manganites synthesized via the auto-combustion method is reported. The aim of studying Ni²⁺-substituted La_0.7Ca_0.3Mn_1 ? xNi_xO₃ ($x=0,0.02,0.07$, and 0.1) manganites was to explore the possibility of increasing the operating temperature range for the magnetocaloric effect through tuning of the magnetic transition temperature. X-ray diffraction analysis confirmed the phase purity of the synthesized samples. The substitution of Mn³⁺ ions by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice was also corroborated through this technique. The dependence of the magnetization on the temperature reveals that all the compositions exhibit a well-defined ferromagnetic to paramagnetic transition near the Curie temperature. A systematic decrease in the values of the Curie temperature is clearly observed upon Ni²⁺ doping. Probably the replacement of Mn³⁺ by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice weakens the Mn³⁺–O–Mn⁴⁺ double exchange interaction, which leads to a decrease in the transition temperature and the magnetic moment in the samples. By using Arrott plots, it was found that the phase transition from ferromagnetic to paramagnetic is second order. The maximum magnetic entropy changes observed for the $x=0,0.02,0.07$, and 0.1 composites was 0.85, 0.77, 0.63, and 0.59 J/kg?K, respectively, under a magnetic field of 1.5 T. In general, it was verified that the magnetic entropy change achieved for La_0.7Ca_0.3Mn_1 ? xNi_xO₃ manganites synthesized via the auto-combustion method is higher than those reported for other manganites with comparable Ni²⁺-doping levels synthesized via standard solid state reaction. The addition of Ni²⁺ increases the value of the relative cooling power as compared to that of the parent compound. The highest value of this parameter (～60 J/kg) is found for a Ni-doping level of 2% around 230 K in a field of 1.5 T.     

Magnetic properties of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite at low temperatures

Mössbauer spectra and magnetic measurement of Ni_0.7Mn_0.3Gd_0.1Fe_1.9O₄ ferrite were investigated by Oxford MS-500 Mössbauer spectrometer and superconducting quantum interference device (SQUID) magnetometer with a field 5 T. Ni_0.7Mn_0.3Gd_0.1Fe_1.9O₄ nanoparticles have a considerable coercivity of 1040 Oe when the test temperature is reduced to 2 K. Mössbauer spectra show that Ni_0.7Mn_0.3Gd_0.1Fe_1.9O₄ nanoparticles exhibit superparamagnetism at room temperature and ferrimagnetism at 77 K.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ^′(T) and imaginary χ^″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at , a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below , as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ^′(T) is well described by Vogel-Fulcher law, yielding a relaxation time and an interaction parameter . Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel-Brown model and the power law yields an unphysical large value of τ₀ (∼6×10⁻⁶⁹ and 1.2×10⁻²² s respectively).     

Dynamical effects in magnetic and transport properties of phase separated La0.5Ca0.5Mn0.95Fe0.05O3

We have measured transport and magnetic properties of polycrystalline La_0.5Ca_0.5Mn_0.95Fe_0.05O₃, a phase separated manganite with ferromagnetic ground state. Cooling rate dependences and time relaxation were found; the coexistence of ferromagnetic and charge ordered regions determines a dynamics which influences physical properties. We show that a dynamical contribution to the resistivity can account for the observed cooling rate dependence and ageing effects on this phase separated manganite.     

Magnetic properties of CoFe1.9RE0.1O4 nanoparticles (RE=La,Ce, Nd,Sm, Eu,Gd, Tb,Ho) prepared in polyol

Highly crystalline CoFe_1.9RE_0.1O₄ ferrite nanoparticles, where RE=La, Ce, Nd, Sm, Eu, Gd, Tb, and Ho, have been synthesized by forced hydrolysis in polyol. X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), ⁵⁷Fe Mössbauer spectrometry, Co K-edge X-ray absorption spectroscopy and magnetic measurements using a SQUID magnetometer were employed to investigate the effect of the substitution RE³⁺ ions for Fe³⁺ ones on the structure, the microstructure, the chemical homogeneity, and the magnetic properties of the cobalt ferrite system. All the produced particles are superparamagnetic at room temperature. Nevertheless, the substitution causes reduction of the blocking temperature which is mainly ascribed to partial cation exchange among the spinel-like sublattices of CoFe₂O₄ induced by the insertion of the relatively large RE³⁺ ions. The low-temperature saturation magnetization and coercivity appear to be greatly affected by the nature of RE³⁺ ions—maxima values were found for Gd³⁺ and Eu³⁺, respectively.     

Crystal structure and magnetic ordering of manganites La0.7Ca0.3Mn1 ? y Fe y O3

The influence of the replacement of Mn ions in the La_0.7Ca_0.3Mn_{1 − y} Fe _y O₃ compounds (0 ≤ y ≤ 0.09) by another transitional metal, Fe, was studied. The radii of both ions are almost identical, which makes the effect of the transitional metal on the physical properties of manganites more transparent. The crystal structure of three samples (with y = 0, 0.03, 0.09) is studied in the temperature range T = 1.5–300 K by neutron powder diffraction. All investigated samples belong to the orthorhombic space group Pnma (62). It is confirmed that Fe ions occupy the Mn positions in the unit cell. As the Fe concentration increases, the saturation value of the spontaneous magnetic moment and the Curie temperature decrease, but the ground state remains ferromagnetic for 0 ≤ y ≤ 0.1. The saturation values of the magnetic moments at T = 1.5 K are m _FM = 3.72(3), 3.40(3), and 3.27(3) μ _B /Mn for the samples with an Fe concentration y = 0, 0.03, and 0.09, respectively. Original Russian Text ? A.I. Kurbakov, V.S. Zakhvalinskii, R. Laiho, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 4, pp. 691–695.     

The influence of the sintering temperature on the structural and the magnetic properties of doped manganites: La0.95Ag0.05MnO3 and La0.75Ag0.25MnO3

La_1−xAg_xMnO₃ samples were synthesized by standard sol-gel method with Ag concentrations of x=0.05 and 0.25. The samples from each concentration were pressed and sintered at 1000, 1200 and 1400 °C for 24 h in air for a systematic study. They were examined structurally by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) and magnetically by Magnetic Properties Measurements System (MPMS). AFM and SEM analyses show that surface morphology changes with Ag concentration and sintering temperature (T_S). It was observed that high temperature sintering leads Ag to leave material as determined from EDS analyses. XRD spectra exhibited that the crystal structure changes with Ag concentration while showing pronounced change with the sintering temperature. From the magnetic measurements, the Curie temperatures (T_C) and the isothermal magnetic entropy changes (−ΔSM) were calculated. It was observed that T_C increases with Ag concentration and decreases with T_S. The maximum −ΔSM was calculated to be 7.2 J/kg K under the field change of 5 T for the sample sintered at 1000 °C with x=0.25.     

Effects of annealing temperature on structure and magnetic properties of CoAl0.2Fe1.8O4/SiO2 nanocomposites

CoAl_0.2Fe_1.8O₄/SiO₂ nanocomposites were prepared by sol–gel method. The effects of annealing temperature on the structure and magnetic properties of the samples were studied by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and Mössbauer spectroscopy. The results show that the CoAl_0.2Fe_1.8O₄ in the samples exhibits a spinel structure after being annealed. As annealing temperature increases from 800 to 1200 °C, the average grain size of CoAl_0.2Fe_1.8O₄ in the nanocomposites increases from 5 to 41 nm while the lattice constant decreases from 0.8397 to 0.8391 nm, the saturation magnetization increases from 21.96 to 41.53 emu/g. Coercivity reaches a maximum of 1082 Oe for the sample annealed at 1100 °C, and thereafter decreases with further increasing annealing temperature. Mössbauer spectra show that the isomer shift decreases, hyperfine field increases and the samples transfer from mixed state of superparamagnetic and magnetic order to the completely magnetic order with annealing temperature increasing from 800 to 1200 °C.     

Mössbauer studies and magnetic studies of Ni doped orthoferrites PrFe1−xNixO3 (x≤0.3)

The magnetic properties of polycrystalline PrFe_1−xNi_xO₃ (x≤0.3) system were studied using Mössbauer spectroscopy and magnetization measurements. The Mossbauer spectra exhibit six line spectra which loses its sharpness as the Ni substitution increases within the system. As the Ni concentration in the system increases, the hyperfine field and isomer shift shows decrease, which is vivid from the sluggish nature of the sextets. The small value of quadrupole splitting confirms the octahedral environment of the Fe⁺³ ions. The magnetization curves show the reversible behavior and represent the fall in negative molecular field leading to AFM frustration. From these results, we conclude that sagging in the spectra reveals the change from antiferromagnetic state to ferromagnetic state, which can be attributed to mixed state of Fe⁺³ ions i.e. high spin (HS) and low spin (LS) which is a consequence of progressive collapse of Hund’s rule due to HS→LS transition. These results confirm the weak ferromagnetic component due to canted-AFM spin arrangement of Fe³⁺ magnetic moments.     

Synthesis, structural, magnetic and optical properties of nanocrystalline ZnFe2O4

Nanocrystalline zinc ferrite (ZnFe₂O₄) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mössbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 Å and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mössbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.     

Magnetic hysteresis loop shift in NiFe2O4 nanocrystalline powder with large grain boundary fraction

Magnetic properties of nanocrystalline NiFe₂O₄ spinel mechanically processed for 350 h have been studied using temperature dependent from both zero-field and in-field ⁵⁷Fe Mössbauer spectrometry and magnetization measurements. The hyperfine structure allows us to distinguish two main magnetic contributions: one attributed to the crystalline grain core, which has magnetic properties similar to the NiFe₂O₄ spinel-like structure (n-NiFe₂O₄) and the other one due to the disordered grain boundary region, which presents topological and chemical disorder features (d-NiFe₂O₄). Mössbauer spectrometry determines a large fraction for the d-NiFe₂O₄ region (62% of total area) and also suggests a speromagnet-like structure for it. Under applied magnetic field, the n-NiFe₂O₄ spins are canted with angle dependent on the applied field magnitude. Mossbauer data also show that even under 120 kOe no magnetic saturation is observed for the two magnetic phases. In addition, the hysteresis loops, recorded for scan field of 50 kOe, are shifted in both field and magnetization axes, for temperatures below about 50 K. The hysteresis loop shifts may be due to two main contributions: the exchange bias field at the d-NiFe₂O₄/n-NiFe₂O₄ interfaces and the minor loop effect caused by a high magnetic anisotropy of the d-NiFe₂O₄ phase. It has also been shown that the spin configuration of the spin-glass like phase is modified by the consecutive field cycles, consequently the n-NiFe₂O₄/d-NiFe₂O₄ magnetic interaction is also affected in this process.     

Spectroscopic and magnetic properties of Fe3Fe4(AsO4)6

The infrared (IR) and ⁵⁷Fe-Mössbauer spectra of Fe₃^IIFe₄^III(AsO₄)₆ were recorded and analyzed on the basis of its structural characteristics. The IR spectrum presents a high complexity, showing an important number of bands and splittings, as a consequence of the presence of three structurally independent AsO₄³⁻ groups. The analysis of the four quadrupole signals shown by the Mössbauer spectrum allowed to attain a detailed insight into the cation distribution over the available crystallographic sites. The alternating current susceptibility measurements indicate a paramagnetic to ferrimagnetic transition in the material at about 59 K.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

A magnetic, magnetostrictive and Mössbauer study of Tb0.3Dy0.7−xPrx(Fe0.9Al0.1)1.95 alloys

The effect of Pr substitution for Dy on the magnetization, magnetostriction, anisotropy and spin reorientation of a series of Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys (x=0, 0.1, 0.20, 0.25, 0.30, 0.35) at room temperature has been investigated. It was found that the magnetization and magnetostriction of the homogenized Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys decreases drastically with increasing x and the magnetostrictive effect disappears for x>0.2, but the spontaneous magnetostriction λ₁₁₁ increases approximately linearly with increasing x. Moreover, the magnetostriction exhibits slightly bigger value at x=0.1 than the free alloys and is saturated more easily with the magnetic field H, showing that a small amount of Pr substitution is beneficial to a decrease in the magnetocrystalline anisotropy. The analysis of the Mössbauer spectra indicated that the easy magnetization direction in the {1 1 0} plane deviates slightly from the main axis of symmetry with Pr concentration x, namely spin reorientation. Comparing with the Al substitution, the effect of Pr substitution for Dy on the spin reorientation is smaller.     

Thermochemical studies on RE2O2CO3 (RE=Gd, Nd) decomposition

Thermochemistry in the decomposition of gadolinium di-oxycarbonate, Gd₂O₂CO₃(s) and neodymium di-oxycarbonate, Nd₂O₂CO₃(s) was studied over the temperature region of 774-952 K and 775-1105 K, respectively. The equilibrium properties of the decomposition reactions were obtained by tensimetric measurement of the CO₂(g) pressure over the biphasic mixture of RE₂O₂CO₃(s) and RE₂O₃(s) at different temperatures (RE=Gd, Nd) and also by thermogravimetric analysis of the decomposition temperature at different CO₂ pressures. The temperature dependence of the equilibrium pressure of CO₂ thus measured could be given by

ln p_CO2/Pa (±0.13)=−22599.1/T+35.21 (774≤T (K)≤952) for Gd₂O₂CO₃ decomposition and

ln p_CO2/Pa (±0.19)=−23824.7/T+33.14 (775≤T (K)≤1105) for Nd₂O₂CO₃ decomposition.

From the above vapor pressure expressions, the median enthalpy and entropy of the decomposition of the oxycarbonates were calculated by the second law analysis and their thermodynamic stabilities were derived. The results are discussed in the light of available thermochemical data of the compounds.     

Magnetocaloric effect at room temperature in manganese perovskite La0.65Nd0.05Pb0.3MnO3 with double resistivity peaks

Series of polycrystalline manganese perovskite oxides La_0.7−xNd_xPb_0.3MnO₃ (x=0, 0.05, and 0.1) are prepared by the sol-gel technique, La_0.65Nd_0.05Pb_0.3MnO₃ were representatively investigated because the peculiar double resistivity peaks were found; the maximum magnetic entropy change ΔS_H=−2.03 J/kg K and its good refrigerant capacity 71.05 J/kg around room temperature were obtained under 9 kOe magnetic field variation. The expected double peaks of magnetocaloric effect had not occurred since magnetic entropy change originated from the differential coefficient of magnetic moment to temperature; the relatively well refrigerant capacity possibly results from the faint magnetic inhomogeneity mixed in the double exchange strong magnetic signal.