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 Ag addition on electrical transport and magnetic properties of La0.67Ba0.33Mn0.88Cr0.12O3

We have prepared a series of polycrystalline manganites with the nominal compositions, La_0.67Ba_0.33Mn_0.88Cr_0.12O₃/Ag_x (LBMCO/Ag_x) (x   is the mole fraction) with x=0$x=0$, 0.05, 0.1, 0.15, 0.2, 0.23, 0.27, 0.3, 0.35. The X-ray diffraction patterns show that the samples with x>0.05$x>0.05$ are two-phase composites. The Ag addition in LBMCO improves the properties of grain surfaces/boundaries and reduces the resistivity of the composites. For x=0.30$x=0.30$ sample, a minimum resistivity is obtained and a maximum room temperature magnetoresistance up to −54.5% was observed at 288 K, 1 T field. The room temperature T_C and the reduced resistivity are responsible for the enhancement of room temperature MR.     

钙钛矿La0.8Ca0.2MnO3多晶样品的制备及低温磁学特性

我们利用固相反应法制备了粉末状La0.8Ca0.2MnO3样品,其空间群为pnma,样品中存在超顺磁性粒子,其截止态和超顺磁态的转变温度为185K,从dσ/dT的负峰得到铁磁-顺磁相变居里温度为210K,在顺磁领域线性拟合而得到顺磁居里温度为216.4K,分子式有效磁矩为6.25μB,低温领域磁化曲线服从布洛赫T32定律,其自旋波劲度系数为49.2meV2.     

Magnetic and electrical transport properties in the self-doped manganite La0.9Mn0.9M0.1O3 (M=Mn,Zn and Ti)

The magnetic and electrical transport properties of La_0.9Mn_0.9M_0.1O₃ (M=Mn, Zn and Ti) were investigated. The temperature and magnetic field dependence of electrical resistivity (ρ) and dc magnetization were studied. All the compounds are found in rhombohedral structure. The excess oxygen in all three compounds was detected through iodometric titration. A modification in resistivity is observed when M=Mn is replaced by M=Zn and Ti. The high temperature resistivity above T_C follow variable range hopping model for both Zn and Ti compounds. For Zn doping, the observation of large field-cool effect and decrease in resistivity at room temperature and is assumed to be due to the implant of Mn⁴⁺ in Mn³⁺ matrix, which favor Mn³⁺/Mn⁴⁺ double exchange. The ferromagnetic behavior below T_C for the compound with M=Ti is correlated to the excess oxygen in it, which implants Mn⁴⁺ and thus incorporates ferromagnetic interactions. The substitutions lead to a reduction of T_c and magnetization.     

Density-functional studies of electronic and magnetic structures for the perovskite oxides La2/3−yNdySr1/3MnO3

The effects of Nd-doping on the transport and magnetic properties of La_2/3−yNd_ySr_1/3MnO₃ (y=0, 1/3, 2/3) are studied theoretically by using the generalized-gradient-corrected full-potential method. In order to investigate the coupling between Nd and Mn, the electronic structures of La_2/3−yNd_ySr_1/3MnO₃ with ferromagnetic (FM) and antiferromagnetic (AFM) arrangements of Nd and Mn sublattices are calculated. The calculation for FM La_2/3−yNd_ySr_1/3MnO₃ yields a half-metallic band structure, while the ferrimagnetic (FiM) system is found to have a metallic character. Hybridization of Nd 4f, Mn 3d, and O 2p bands around Fermi level (E_F) is observed, suggesting the coupling between Nd and Mn is mediated by O 2p carriers. The qualitative features of transport and magnetic properties of such a two-spin system can be interpreted in terms of half-metallic FM domains being mixed up with metallic FiM domains. The proportion of FM domains varying with Nd-doping concentration has strong influences on the magnetoresistance.     

Anomalies of ultrasonic velocities, attenuation and elastic moduli in Nd1−xSrxMnO3 perovskite manganite materials

Nd_1−xSr_xMnO₃ perovskite manganite material with different compositions (x=0.31, 0.35, 0.37, 0.39 and 0.41) have been prepared employing solid-state reaction technique. The ultrasonic velocities and attenuation of the above samples have been measured employing through transmission method operated at a fundamental frequency of 5 MHz over wide range of temperatures. The temperature dependence of ultrasonic velocities, attenuation, relative percentage variation in velocities and elastic constants show an interesting anomaly in all compositions. The observed anomalies in ultrasonic parameters at T_c in all compositions have been revealed in terms of existence of ferromagnetic (FM) state. Similarly, the anomalies at T_co show the transition from FM to charge-ordered antiferromagnetic (AFM) state. The observed results have been used to explore the competitions between FM and AFM.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7

The effect of Cr doping on magnetic and electrical properties in the bilayer manganites La_1.4Sr_1.6(Mn_1−yCr_y)₂O₇ (y=0-0.1) has been investigated. When y≤0.025, Cr doping enhances the three-dimensional magnetic transition temperature T_C and the insulator-metal transition temperature T_IM as well as decreases the peak resistivity at T_IM, and the saturated magnetization decreases slightly. When y≥0.035, T_IM decreases gradually accompanied by the increase of peak resistivity, but T_C remains nearly constant, and the saturated magnetization decreases heavily. In the whole doping region, the two-dimensional magnetic transition temperature T^? monotonously decreases with an increasing of Cr doping level. These results can be explained by considering different magnetic (including ferromagnetic and antiferromagnetic) interactions between Mn ions and Cr ions.     

Critical behavior and magnetic entropy change in the La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3 perovskite

Critical behavior in the La_0.6Sr_0.4Mn_0.8Fe_0.1Cr_0.1O₃ ceramics was studied using magnetization methods. Results show that the paramagnetic–ferromagnetic transition is of second order. Based on the critical behavior analysis using the Banerjee criterion and the Kouvel–Fisher method, we find the critical exponents: β=0.395±0.010, γ=1.402±0.010, and δ=5.208±0.007, for which the magnetic interaction is satisfied within the three-dimensional Heisenberg model. Results indicate the presence of short-range interactions. The magnetic entropy change (−ΔS_M) reached maximum values of 1.75, 1.45, 1.15, 0.8 and 0.43 J Kg⁻¹ K⁻¹ under a magnetic field variation of 5, 4, 3, 2 and 1 T, respectively. Nevertheless, these (−ΔS_M) values are much low for any potential application at this moment. The nature of this phenomenon is discussed in relation to the characteristics of the magnetic phase transition and critical exponents.     

钙钛矿锰氧化物La_(2/3)Sr_(1/3)Fe_xMn_(1-x)O_3的结构与磁性研究

本文利用溶胶-凝胶法制备了名义成分为La_(2/3)Sr_(1/3)Fe_xMn_(1-x)O_3(x=0.0,0.1,0.2,0.3,0.5)的系列样品,样品先后经过773,873,1073 K热处理,热处理时采用缓慢升温方式,X射线衍射分析表明,该系列样品均为单相钙钛矿结构,空间群为R3c,利用X'Pert HighScore Plus软件计算了样品的晶粒尺寸、晶格常数、晶胞体积及键长、键角,利用物理性能测量系统测量了样品的磁性,发现样品在10K的磁矩随掺杂量的增加而减小,但存在两个明显不同的变化区域:从x=0到x=0.2时,平均每个分子的磁矩从2.72μB迅速下降到0.33μB,居里温度从327 K下降到95 K,下降了232 K;而从x=0.2到x=0.5时,平均每个分子的磁矩从0.33μB缓慢下降到0.05μB,居里温度从95K下降到46K,只下降了49K,我们认为Fe与Mn离子磁矩反平行是样品磁矩随Fe掺杂量增加而下降的原因之一。     

Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3的磁性和输运性质

利用固相反应法制备了Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3(x=0～0.06)的多晶样品,探讨了Ru掺杂对体系结构,输运性质以及磁电阻的影响.多晶X射线衍射证实所有样品均保持简单立方钙钛矿结构.通过零场冷却(ZFC)和加场冷却(FC)下的磁化曲线的测量发现随温度降低样品发生了顺磁(PM)到铁磁(FM)的相变,且样品的居里温度(Tc)随Ru掺杂发生了显著的变化,从x=0.00时的306.7K,下降到x=0.02时的294.3K,紧接着又上升到x=0.04时的302.4K.测得居里温度明显高于La0.7Ca0.2Ba0.1Mn1-xMoxO3体系,而且其磁性也大为增强.由零场和外加磁场H=1T测量得到样品的ρ～T曲线表明随温度降低样品同时发生了从绝缘体到金属的转变,绝缘体-金属转变温度低于相应的居里温度.适量的Ru掺杂降低了样品的电阻率,增强了低温时的磁电阻.     

Critical behavior in Ga-doped manganites La0.75(Sr,Ca)0.25Mn1−xGaxO3 (0≤x≤0.1)

The critical properties of perovskite manganite AMn_1−xGa_xO₃ (A=La_0.75Ca_0.08Sr_0.17, x=0, 0.05 and 0.1) at the ferromagnetic–paramagnetic transition have been analyzed. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M(T_C, H). The critical exponent values for the undoped compound were found to match well with those predicted for the three-dimensional Heisenberg model (β=355±0.007, γ=1.326±0.002, δ=4.90±0.01). While on non-magnetic Ga substitution it tends towards mean-field with long-range interaction. The mean-field model might be due to the random dilution of the Mn sublattice by non-magnetic ion Ga³⁺ and/or the development of the physical size of the clusters which enhance the dipole–dipole interaction.     

Temperature dependence of electroresistance for La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistance of La_0.67Ba_0.33MnO₃ bulk sample is reported. A decrease in the resistance (electroresistance) on the application of higher bias current is observed. The electroresistance is maximum at metal-insulator transition temperature (T_MI) and decreases when the temperature is either increased or decreased from T_MI. A two-phase model is proposed to explain the occurrence of electroresistance. The higher bias current leads to an increase in alignment of spins and thus, in turn, leads to an increase in spin stiffness coefficient and decrease in the resistance at T_MI.     

Mn的价态对La0.8Ba0.2MnO3电磁性能的影响

在2—390K温度之间研究了La_0.8Ba_0.2MnO₃的 磁矩、磁电阻与温度的关系.发 现以不同价态的Mn元素引入得到的La_0.8Ba_0．2MnO₃ ，性能虽然都存在金属 —绝缘体转变，以及在磁场作用下居里温度附近电阻率变化非常显著的特点，但是价态对磁 性转变温度T_C，金属—绝缘体转变温度T_mi，以及磁电阻极大 值温度T_MR的影 响都非常显著.三种价态相比较，使用二价Mn的电阻率最低以及磁性转变温度更接近室温.认 为影响材料性能的主要因素是材料制备时引进的Mn元素的价态，由于原料价态的不同而形成 的氧空位浓度变化，进而影响了Mn⁴⁺/Mn³⁺的比. 关键词： 价态 磁电阻 居里温度 金属—绝缘体转变     

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.     

Magnetocaloric effect in La0.75Sr0.25MnO3 manganite

The polycrystalline manganite La_0.75Sr_0.25MnO₃ prepared by an alternative carbonate precipitation route reveals the rhombohedral perovskite structure. Magnetization isotherms measured up to 2 T are used to determine Curie temperature of 332 K by means of Arrott plot. Maximum of magnetic entropy change is found at Curie temperature. The relative cooling power equal to 64 J/kg for 1.5 T magnetic field, is superior as compared to the manganite with the same chemical composition from the sol-gel method.     

Spontaneous magnetization and resistivity jumps in bilayered manganite （La0.8Eu0.2）4/3Sr5/3Mn2O7 single crystals

Owing to the inhomogeneous state resulting from the doping of a small number of Eu ions into Laa/3Sr5/3Mn2O7, from the resulting single crystal （La0.8Eu0.2）a/3Sr5/3Mn2O7 we have observed the magnetization jump, the resistivity jump, as well as the relaxation phenomena. For （Lao.sEuo.2）a/3Sr5/3Mn2O7, it has a very delicate ground state due to the interplays among spin, charge, orbital, lattice degrees of freedom. Consequently, the magnetization state is sensitive to temperature, magnetic field, as well as time. Meanwhile, the evolution of the magnetization with time shows a spontaneous jump when both the temperature and the magnetic field are constant. Similar step-like behaviours are also observed in resistivity. All these results suggest that Eu doping can greatly modulate the physical properties of Laa/3Sr5/3Mn2O7 and cause such interesting behaviours.     

Preparation and low field magnetoresistance of double layered manganite La1.4Ca1.6Mn2O7

Double layered manganite of La_1.4Ca_1.6Mn₂O₇ (DLCMO) was prepared using solid state reaction method and had a metal-insulator transition temperature (T_MI) of 125 K. The short range 2D-feerromagnetic ordering (T_C2) starts growing when T<168 K and it gets converted into 3D-ferromagnetic ordering (T_C1) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). For DLCMO, in the temperature range between T_C1 and T_C2, the MR showed a gradual increase with the magnetic field. The observed MR and R-T behaviour of double layered manganite for T_C1<T<T_C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM-PM mixture.     

Electrical,structural, magnetic and transport properties of Zn2BaFe16O27 doped with Cu

Series of polycrystalline samples of Zn_2−xCu_xBaFe₁₆O₂₇ were prepared by usual ceramic methods, where x=0.0, 0.4, 0.6, 0.8, 1.0, 1.4. X-ray analysis done at room temperature using CoK_α with λ=1.790 Å confirms the presence of W-type hexaferrite phase structure. Saturation magnetization and hysteresis loops curves measurements at room temperature were studied as a function of Cu²⁺ substitution. It can be seen that the Cu²⁺ content slightly decreases the saturation magnetization from 25 to 20 emu g⁻¹; all hysteresis loops are closed, which indicates low anisotropy field and low saturation magnetization field. The dc conductivity and thermoelectric power were measured in a range from room temperature up to T=750 K for all samples. The thermoelectric power decreases on increasing Cu²⁺ content, and the conductivity increases with temperature. The value of the charge-carrier concentration increases by increasing the temperature and Cu²⁺ content.     

Effects of time on the magnetic properties of terbium-doped LaMnO3

The magnetic properties of the perovskite form of LaMnO₃ have been shown strong interest in recent years due to its high potential for use in magnetic devices. In this paper, the magnetic properties of a 30% terbium-doped LaMnO₃ (LMTO) perovskite manganite synthesized by a conventional solid-state reaction were investigated. Data on these properties was recorded periodically via SQUID and VSM to reveal it to be best described magnetically as a spin glass system. Thus, the time effect must be taken into consideration in instantaneously determining this material’s spin glass state as well as the overall magnetic properties in the absence of a magnetic field. The results of this paper point to a more in-depth understanding of the change in magnetic properties associated with doped LaMnO₃.