Magnetocaloric properties of (La0.67−xGdx)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, magnetic property and magnetocaloric effect (MCE) in nanoparticles perovskite manganites of the type (La_0.67−xGd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) synthesized by using an amorphous molecular alloy as precursor have been reported. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature (T_C) of the prepared samples is found to be strongly dependent on Gd content. The Curie temperature of samples is 358.4, 343.2, and 285.9 K for x=0.1, 0.15, and 0.2, respectively. A large magnetocaloric effect close to T_C has been observed with a maximum of magnetoentropy change in all the samples, ∣ΔS_M∣_max of 1.96 and 4.90 J/kg K at 2 and 5 T, respectively, for a substitution rate of 0.15. In addition, the maximum magnetic entropy change observed for samples with different concentration of Gd, exhibits a linear dependence with the applied high magnetic field. These results suggest that (La_0.67−x Gd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) compounds could be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Influence of Tb substitution for La on the structure,magnetic and magneto-transport properties of La1.2Sr1.8Mn2O7

The physical properties of Tb³⁺ ions substitution at A-site are investigated in the layered manganite La_1.2Sr_1.8Mn₂O₇. A series of La_1.2−xTb_xSr_1.8Mn₂O₇ (x=0, 0.05, 0.15, and 0.20) shows that doping with a Tb ion of smaller radius in La_1.2Sr_1.8Mn₂O₇ caused diffraction peaks to shift to high angle. Some samples have an impure diffraction at about 30°, but all samples form single-phase. Samples can be well indexed on a Sr₃Ti₂O7-type tetragonal structure with the space group I4/mmm. According to the M-T curves, when x≤0.05, the series of samples shows ferromagnetism at low temperatures. With increasing temperature, they have two magnetic transitions at different temperatures. When x≥0.15, the magnetizations dramatically decrease. The ρ–T curves of samples show the metal–insulator transition for x=0, 0.05, and the maximum MR values in magnetic field 5 T are 74% at about 73 K and 94% at about 86 K. When x≥0.15, the samples remain in the insulator state in the whole observed temperature range, and the maximum MR values of 86% and 69% appeared at 74 K and 42 K.     

Magnetic ordering above room temperature in the sigma-phase of Fe66V34

Magnetic properties of four sigma-phase Fe_100−xV_x samples with 34.4?x?55.1 were investigated by Mössbauer spectroscopy and magnetic measurements in the temperature interval 4.2-300 K. Four magnetic quantities, viz. hyperfine field, Curie temperature, magnetic moment and susceptibility, were determined. The sample containing 34.4 at% V was revealed to exhibit the largest values found up to now for the sigma-phase for average hyperfine field, 〈B〉=12.1 T, average magnetic moment per Fe atom, 〈μ〉=0.89 μ_B, and Curie temperature, T_C=315.3 K. The quantities were shown to be strongly correlated with each other. In particular, T_C is linearly correlated with 〈μ〉 with a slope of 406.5 K/μ_B, as well as 〈B〉 is so correlated with 〈μ〉, yielding 14.3 T/μ_B for the hyperfine coupling constant.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Magnetocaloric properties in the Cr-doped La0.7Sr0.3MnO3 manganites

Single-phase polycrystalline samples of La_0.7Sr_0.3Mn_1-xCr_xO₃ with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature T_C decreases with increasing x and the maximum magnetic entropy change (−ΔS_M) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

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.     

Structural,magnetic and electrical properties of Co1−xZnxFe2O4 synthesized by co-precipitation method

Nanoparticles of Co_1−xZn_xFe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.     

The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

Microstructure and some magnetic properties of bulk amorphous (Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)100−xYx (x=0, 2, 3 or 4) alloys

Microstructure, revealed by X-ray diffraction, transmission electron microscopy and Mössbauer spectroscopy, and magnetic properties such as magnetic susceptibility, its disaccommodation, core losses and approach to magnetic saturation in bulk amorphous (Fe_0.61Co_0.10Zr_0.025Hf_0.025Ti_0.02W_0.02B_0.20)_100−xY_x (x=0, 2, 3 or 4) alloys in the as-cast state and after the annealing in vacuum at 720 K for 15 min. are studied. The investigated alloys are ferromagnetic at room temperature. The average hyperfine field induction decreases with Y concentration. Due to annealing out of free volumes its value increases after the heat treatment of the samples. The magnetic susceptibility and core losses point out that the best thermal stability by the amorphous (Fe_0.61Co_0.10Zr_0.025Hf_0.025Ti_0.02W_0.02B_0.20)₉₇Y₃ alloy is exhibited. Moreover, from Mössbauer spectroscopy investigations it is shown that the mentioned above alloy is the most homogeneous. The atom packing density increases with Y concentration, which is proved by the magnetic susceptibility disaccommodation and approach to magnetic saturation studies.     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

Neutron-powder-diffraction study of structural and magnetic structure of La0.7Sr0.3Mn1−xTixO3 (x=0, 0.10, 0.20, and 0.30)

Neutron-diffraction and magnetic measurements have been used in order to study the structure and magnetic changes of La_0.7Sr_0.3Mn_1−xTi_xO₃ (x=0, 0.10, 0.20, and 0.30) perovskites. Magnetic measurements show that our samples exhibit a ferromagnetic–paramagnetic transition with decreasing temperature. Effects of Ti doping on the magnetic and nuclear structures are studied here by neutron-powder-diffraction for La_0.7Sr_0.3Mn_1−xTi_xO₃ (x=0.10, 0.20, and 0.30). Our analysis indicates that the three representative samples, with x=0.10, 0.20, and 0.30, have a hexagonal system of R3¯c space group, and that the magnetic moments order ferromagnetically into the (1 0 1) crystallographic planes, i.e. the magnetic moments of the Mn ion are in the ac plane, with no component along the b-axis. There is no crystalline structure changes when the Ti doping level increases.     

Metamagnetic transition and magnetocaloric effect in charge-ordered Pr0.68Ca0.32−xSrxMnO3 (x=0, 0.1, 0.18, 0.26 and 0.32) compounds

In this study, magnetic and magnetocaloric properties of Pr_0.68Ca_0.32−xSr_xMnO₃ (x=0, 0.1, 0.18, 0.26 and 0.32) compounds were investigated. X-ray results indicated that all the samples have a single phase of orthorhombic symmetry. The orthorhombic unit cell parameters increase with the increase in Sr content. Large negative magnetic entropy changes (−26.2 J/kg K at 38 K and 5 T for x=0 and −6.5 J/kg K at 83 K and 6 T for x=0.1) were attributed to ultrasharp metamagnetic transitions. The peak value of ΔS_m decreased from −4.1 J/kg K for x=0.18 sample to −2.4 J/kg K for x=0.32 at 1 T magnetic field.     

Monodispersed nanocrystalline Co1–xZnxFe2O4 particles by forced hydrolysis: Synthesis and characterization

Zinc-substituted cobalt ferrites, Co_1–xZn_xFe₂O₄, were for the first time successfully prepared by forced hydrolysis method. The obtained materials are single phase, monodispersed nanocrystalline with an average grain size of about 3 nm. These materials are superparamagnetic at room temperature and ferrimagnetic at temperature lower than the blocking temperature. When the zinc substitution increases from x=0 to 0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications.     

Magnetic anomalies in single crystalline ErPd2Si2

Considering certain interesting features in the previously reported ¹⁶⁶Er Mössbauer effect, and neutron diffraction data on the polycrystalline form of ErPd₂Si₂ crystallizing in the ThCr₂Si₂-type tetragonal structure, we have carried out magnetic measurements (1.8–300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in features due to magnetic ordering in the plot of magnetic susceptibility χ versus temperature T at low temperatures. The χ(T) data reveal that there is a pseudo-low-dimensional magnetic order setting in at 4.8 K, with a three-dimensional antiferromagnetic order setting in at a lower temperature (3.8 K). A new finding in the χ(T) data is that, for H∥〈1 1 0〉 but not for H∥〈0 0 1〉, there is a broad shoulder in the range 8–20 K, indicative of the existence of magnetic correlations above 5 K as well, which could be related to the previously reported slow-relaxation-dominated Mössbauer spectra. Interestingly, the temperature coefficient of electrical resistivity is found to be isotropic; no feature due to magnetic ordering could be detected in the electrical resistivity data at low temperatures, which is attributed to magnetic Brillioun-zone boundary gap effects. The results reveal the complex nature of magnetism of this compound.     

Magnetic and crystallographic study of Tb0.75Y0.25Co3B2

Tb_0.75Y_0.25Co₃B₂ was studied as a function of temperature by neutron powder diffraction, ac susceptibility and SQUID magnetization measurements. The solid solution, which is of hexagonal symmetry and is paramagnetic at 300 K, undergoes a magnetic Co–Co ordering transition at ∼150 K, and a second magnetic Tb–Tb ordering transition at ∼17 K. The latter induces a spin-reorientation transition, in which the magnetic axis rotates from the c-axis toward the basal plane. The component of the magnetic axis, which is perpendicular to c, leads to a crystal symmetry reduction from hexagonal to monoclinic. The observed magnitude of the magnetic moment of the Tb ion is 1.5 μ_B, unusually small relative to the free ion and parent compound (TbCo₃B₂) values. These magnetic and crystal properties are discussed and compared with what was previously published for the parent compound.     

Synthesis and investigation of magnetic properties of substituted ferrite nanoparticles of spinel system Mn1−xZnx[Fe2−yLy]O4

Superparamagnetic nanoparticles of the spinel ferrite four-element system Mn_1−xZn_x[Fe_2−yL_y]O₄ (where L:Gd³⁺, La³⁺, Ce³⁺, Eu³⁺, Dy³⁺, Er³⁺,Yb³⁺) were synthesized by the co-precipitation method. The magnetic moments of the 10 nm diameter nanoparticles were comparable to the ones of Fe₃O₄ nanoparticles. A comparatively low T_C (∼52–72 °C) was observed for some of the compositions. The heating mechanism of the superparamagnetic particles in the AC magnetic field at radiofrequency range is discussed and especially the absence of the hysteresis loop in the M–H curve at room temperature. One possible explanation—spontaneous particle agglomeration—was experimentally verified.     

The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2Co1−xFexO3 (x=0.15, 0.2, 0.3)

La_0.8Sr_0.2Co_1−xFe_xO₃ (x=0.15, 0.2, 0.3) samples were studied by means of AC magnetic susceptibility, magnetization, magnetoresistance and ⁵⁷Fe Mössbauer spectrometry. Iron was found to take on a high spin 3d^5−α electronic state in each of the samples, where α refers to a partly delocalized 3d electron. The compounds were found to exhibit a spin-cluster glass transition with a common transition temperature of ∼53 K. The spin-cluster glass transition is visualized in the ⁵⁷Fe Mössbauer spectra as the slowing down of magnetic relaxation below ∼70 K, thereby showing that iron takes part in the formation of the glassy magnetic phase. The paramagnetic-like phase found at higher temperatures is identified below T_c≈195 K as being composed of weakly interacting, magnetically ordered nanosized clusters of magnetic ions in part with a magnetic moment oriented opposite to the net magnetic moment of the cluster. For each of the samples a considerable low-temperature negative magnetoresistance was found, whose magnitude in the studied range decreases with increasing iron concentration. The observed results obtained on the present compounds are qualitatively explained assuming that the absolute strengths of magnetic exchange interactions are subject to the relation ∣J_Co–Co∣<∣J_Fe–Co∣<∣J_Fe–Fe∣.