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.     

Synthesis and magnetic properties of InM2+CrS4(M2+ = Mn,Fe,Co, Ni)

The crystallographic and magnetic properties of the compounds InM²⁺CrS₄(M = Mn, Fe, Co, Ni) were studied. Three of these compounds except MnInCrS₄ were antiferromagnetic. The Néel termperature and paramagnetic Curie temperature were determined to be 20 and 82K for InFeCrS₄, 36 and 115K for InCoCrS₄ and 22 and 26K for InNiCrS₄, respectively.     

Imperfection of the clustered perovskite structure, phase transitions, and magnetoresistive properties of ceramic La0.6Sr0.2Mn1.2 ? x Ni x O3 ± δ (x = 0–0.3)

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Ni _x O_{3 ± ??} (0 ?? x ?? 0.3) have been investigated using the X-ray diffraction, magnetic (??_ac), ⁵⁵Mn NMR, resistive, and magnetoresistive methods. The specific features of the influence of the composition on the structure and properties of nonstoichiometric manganite perovskites have been established. It has been found that the rhombohedrally (R $\bar 3$ c) distorted perovskite structure contains cation and anion vacancies, as well as nanostructured clusters with Mn²⁺ ions in the A-positions. The substitution of Ni³⁺ ions (r = 0.74 ?) for Mn³⁺ ions (r = 0.785 ?) leads to a decrease in the lattice parameter a, the ferromagnetic-paramagnetic phase transition temperature T _C, and the metal-semiconductor phase transition temperature T _ms due to the disturbance of the superexchange interactions between heterovalent manganese ions Mn³⁺ and Mn⁴⁺. The observed anomalous magnetic hysteresis at 77 K has been explained by the antiferromagnetic effect of the unidirectional exchange anisotropy of the ferromagnetic matrix structure on the magnetic moments of the superstoichiometric manganese Mn²⁺ ions located in nanostructured planar clusters. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic superexchange of the ions Mn³⁺ ? O^2? ? Mn⁴⁺; a local heterogeneity of their surrounding by other ions, vacancies, and clusters; and a partial localization of Mn⁴⁺ ions. The local hyperfine interaction fields on ⁵⁵Mn nuclei have been determined. The concentration dependences of the activation energy and charge hopping frequency have confirmed that the Ni ions decrease the electrical conductivity due to the weakening of the electronic superexchange Mn³⁺ ? O^2? ? Mn⁴⁺. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between magnetic and electrical properties in rare-earth manganites.     

Spin polarons at high temperatures in nonstoichiometric La1 − x Mn1 − y O3 manganites

The magnetic properties of the La_{1 ? x} Mn_{1 ? y} O₃ family of nonstoichiometric lanthanum manganites are studied in the 80 K < T < 640 K temperature range. The Curie temperature varies nonmonotonically with the growth in the density of Mn⁴⁺ ions. In the existence range for the paramagnetic phase, magnetic polarons appear in both the orthorhombic and rhombohedral phases. In the range of paramagnetism, the temperature dependence of the magnetic susceptibility is nonlinear and can be fitted by the Curie law with the temperature-dependent Curie constant.     

Nuclear and magnetic order in Y(Ni,Mn)O3 manganites by neutron powder diffraction

Neutron powder diffraction experiments performed on two selected compositions of the yttrium-based solid solution YNi_xMn_1−xO₃ clearly reveal a nuclear order between the Ni²⁺ and Mn⁴⁺ ions in the half-substituted compound YNi_0.50Mn_0.50O₃, so that the crystal structure is no longer described in the conventional orthorhombic Pbnm space group, but in the monoclinic P2₁/n, all over the investigated temperature range (1.5-300 K). However, both X-rays diagrams and neutron patterns of the YNi_0.25Mn_0.75O₃ phase are indexed in the Pbnm orthorhombic-like symmetry, indicating that the Mn and Ni ions are randomly distributed on the octahedral sites.In addition, neutron diffraction points out that the nature of the magnetic ordering is strongly connected to the structural properties. Whereas no long-range 3D-magnetic ordering was detected for the Pbnm YNi_0.25Mn_0.75O₃ phase, the YNi_0.50Mn_0.50O₃ compound exhibits a magnetic transition at The magnetic structure consists of two collinear Mn⁴⁺ and Ni²⁺ ferromagnetic layers (F_x0F_z magnetic configurations) with saturated magnetic moment values of 2.25(2) and 1.57(2) μ_B for Mn⁴⁺ and Ni²⁺, respectively, at 1.5 K.     

Structure,magnetic properties and magnetocaloric effects of Fe50Mn15−x Co x Ni35 alloys

Fe50Mn15-xCoxNi35(x=0,1,3,5,7)alloys were prepared by arc melting under purified argon atmosphere.The ingots were homogenized at 930°C for 90h followed by water quenching.The crystal structure,magnetic properties and magnetocaloric effects of the alloys were studied by X-ray diffraction(XRD)and MPMS-7-type SQUID.The results show that all samples still maintained a single-(Fe,Ni)-type phase structure.With the increase of the content of Co,the Curie temperatures of these alloys increased and exhibited a second-order magnetic transition from ferromagnetic(FM)to paramagnetic(PM)state near Curie temperature.The maximum magnetic entropy change and the relative cooling power of Fe50Mn10Co5Ni35alloy was 2.55 J/kg·K and 181 J/kg,respectively,for an external field change of 5T.Compared with rare earth metal Gd,Fe50Mn15-xCoxNi35 series of alloys have obvious advantage in resource price;their Curie temperatures can be tuned to near room temperature,maintain a relatively large magnetic entropy change at the same time and they are a type of potential magnetic refrigeration materials near room temperature.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.     

Preparation and investigation of magnetic properties of MnNiTi-substituted strontium hexaferrite nanoparticles

A series of M-type strontium hexaferrite powders with substitution of Mn²⁺, Ni²⁺ and Ti⁴⁺ ions for Fe³⁺ ions according to the formula SrFe₉(Mn_0.5−xNi_xTi_0.5)₃O₁₉, where x ranges from 0 to 0.5 with a step of 0.1, has been prepared via the conventional ceramic method. In order to get nanoparticles, the obtained powders were milled in a high energy SPEX mill for 1 h. XRD investigations of the unmilled and milled powders show that the prepared samples are all single phase hexaferrite. Lattice parameters and mean crystallite sizes of the powders were determined from the XRD data and Scherrer’s formula. Transmission electron microscope (TEM) was used to analyze their structures. Room temperature magnetizations and coercivities of the samples in a magnetic field of 15 kOe have been determined from the hysteresis loops. It was found that magnetizations of the milled samples were smaller than the magnetization of the unmilled samples. This decrease, based on core-shell model, has been attributed to the presence of a magnetically dead layer on the particles’ surface of the milled powders. In addition, the magnetizations of the milled samples decrease with the increase in x value. This decrease has been discussed according to site occupation of the substituted cations on the sublattices. The discussion also supports the increase of lattice parameters and the decrease of Curie temperature as x increases.     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

Localized magnon gap modes in the 2-d Ising antiferromagnets K2CoF4 and Rb2CoF4

The defect (Mn²⁺,Ni²⁺,Fe²⁺) induced magnon gap modes in the layered antiferromagnets K₂CoF₄ and Rb₂CoF₄ were investigated with the methods of FIR absorption-and IR emission spectroscopy. The anisotropic exchange-parameters describing the strongly localized Mn²⁺ spin excitations far below the host lattice magnon band and the Ni²⁺ excitations in the vacinity of this band are presented. In the diluted system K₂Co_1-cMn_cF₄ localized Mn²⁺ cluster modes up to about C≈0.1 were observed. The excitation energy of these modes can only be explained by assuming an anisotropic Mn²⁺-Mn²⁺ exchange which is in contrast to the pure isomorphous system K₂MnF₄. In the spin mismatch system K₂CoF₄: Fe the magnetic moments of the isolated Fe²⁺ impurities are pulled from the plane perpendicular to the c-axis and aligned parallel to the easy axis of the magnetic crystal.     

Er2AlFe16－xMnx化合物的磁性及反常热膨胀

通过X射线衍射及磁测量手段研究了Er₂AlFe_16-xMn_x(x=1，2，3，4，6，8)化合物的结构和磁性. 研究结果表明Er₂AlFe_16-xMn_x化合物具有六角相的Th₂Ni₁₇型结构. 采用X射线热膨胀测定法在103—654K的温度范围内测量了Er₂AlFe_16-xMn_x(x=1，2，3，4)化合物的热膨胀性质，发现这些化合物在低温下存在热膨胀反常现象，在居里点附近具有负膨胀性质. 对自发磁致伸缩的研究结果表明Er₂AlFe_16-xMn_x化合物中存在着较强的各向异性的自发磁致伸缩，低温下自旋重取向的出现使得化合物的自发体磁致伸缩有所增强. 磁测量结果表明Mn的替代导致Er₂AlFe_16-xMn_x化合物的居里温度及自发磁化强度急剧下降，并且使得化合物的磁晶各向异性发生显著改变. 关键词： 2AlFe_16-xMn_x化合物')" href="#">Er₂AlFe_16-xMn_x化合物 反常热膨胀 自发磁致伸缩     

Pressure dependence of temperature variation of AC susceptibility for Fe50Ni37Mn13 and comparison to Fe68.1Ni31.9

Fe₅₀Ni₃₇Mn₁₃, which is a ferromagnetic alloy with FCC crystal structure, has been reported to show the Invar effect below the Curie temperature; however, this alloy shows a typical anti-Invar effect above the Curie temperature. In this paper, we discuss the pressure dependence of the temperature variation of the alternative current (AC) susceptibility at a frequency of 1 kHz for Fe₅₀Ni₃₇Mn₁₃ at various pressures up to 7.5 GPa above 77 K; we then compare the results with those for Fe_68.1Ni_31.9, which were obtained in a previous study. Fe₅₀Ni₃₇Mn₁₃ was in a ferromagnetic state throughout the entire pressure range measured. T_C decreases in inverse proportion to the increasing pressure; dT_C/dp and dln T_C/dp for Fe₅₀Ni₃₇Mn₁₃ are −26 K GPa⁻¹ and −0.07 GPa⁻¹, respectively. Further, the temperature variation of the shape of the χ′-T curve for different pressures indicates continuous combining of magnetic interactions occurs at high pressures. These results are similar to those obtained for Fe_68.1Ni_31.9.     

Magnetic properties and DC electrical resistivity studies on cadmium substituted nickel-zinc ferrite system

Polycrystalline Ni_0.65−xCd_xZn_0.35Fe₂O₄ ferrites with x varying from 0.00 to 0.20 in steps of 0.04 have been prepared by conventional ceramic route. Calcination and sintering of samples were performed at 950 and 1250 °C for 4 and 2 h, respectively. The prepared samples were characterized by powder X-ray diffraction. The observed modifications in structure and increase in lattice constant are attributed to the difference in ionic radius of substituted Cd²⁺ ion and displaced Ni²⁺ ion. The room temperature specific saturation magnetization and Curie temperature are observed to decrease continuously with decrease in cadmium content and are attributed to the decline of A-B exchange interaction. The monotonic increase in initial permeability and decrease in magnetic loss are observed with cadmium concentration. An increase in dc electrical resistivity is observed up to x=0.12 of cadmium followed by a continuous decrease. The variation of electrical resistivity with temperature was measured in the temperature range of RT-140 °C and the corresponding activation energies for conduction obtained from the log ρ vs 1/T graphs.     

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).     

Ferromagnetism and antiferromagnetism in Ni2+x+yMn1−xAl1−y alloys

The magnetic behavior of Ni_2+xMn_1−xAl alloys around the stoichiometric 2:1:1 composition was investigated with several experimental techniques. The results of low-temperature magnetization measurements indicate that a competition mechanism between ferromagnetism and antiferromagnetism is expected in off-stoichiometric alloys. Although the Curie temperature is strongly dependent on the composition, the saturation magnetization has an unsystematic variation for deviations from the stoichiometric Ni₂MnAl alloy. A reentrant-spin-glass behavior is observed below 50 K.     

Magnetic properties of complex oxides Ba6Ni5 ? xMnxO15 (x = 2, 3, 4) of the A3n + 3mA′nB3m + nO9m + 6n family

The magnetic interactions in the quasi-one-dimensional complex oxides Ba₆NiMn₄O₁₅, Ba₆Ni₂Mn₃O₁₅, and Ba₆Ni₃Mn₂O₁₅ have been investigated using electron paramagnetic resonance and magnetic susceptibility measurements. It has been revealed that, for the Ba₆Ni₂Mn₃O₁₅ and Ba₆Ni₃Mn₂O₁₅ compounds, the magnetic moments are considerably smaller than the theoretical values and depend on the temperature. The results obtained have been interpreted under the assumption that the structure contains Ni³⁺ ions in which the energy level structure can lead to the above anomalies.     

Effect of hydrostatic pressure on the Curie temperature of the Heusler alloys Ni2MnZ(Z = Al,Ga, In,Sn and Sb)

The pressure derivative of the Curie temperature dT_c/dp of the Heusler alloys Ni₂MnZ(Z = Al, Ga, In, Sn and Sb) has been obtained from the results of temperature dependence of initial permeability under pressure up to about 6 kbar. For all alloys the Curie temperatures increase linearly with increasing pressure at the rate of dT_c/d_p: +0.7 K/kbar for Ni₂MnAl, +1.0 K/kbar for Ni₂MnGa, +0.9 K/kbar for Ni₂MnIn, +1.4 K/kbar for Ni₂MnSn and +4.1 K/kbar for Ni₂MnSb. On the basis of these results, the interatomic dependence of the exchange interaction for Heusler alloys is discussed. The magnetic susceptibilities of those alloys are also reported.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.