Magnetic ordering and magnetoresistive effect in La1−x Srx(Mn1−y Mey)O3 perovskites (Me = Nb, Mg)

Perovskites of composition La_1?x Sr_x(Mn_1?x/2Nb _x/2)O₃ and La_0.49Sr_0.51(Mn_1?y Nb_y)O₃ have been synthesized and investigated. The substitution of nonmagnetic niobium ions for manganese was shown to lead to a transition from the metallic into the insulating state due to a decrease in the number of dissimilar (different-valence) manganese atoms in the lattice. In spite of the high resistivity, the niobium-containing perovskites exhibit a large magnetoresistive effect and ferromagnetic ordering. Small additions of Nb⁵⁺ to La_0.49Sr_0.51MnO₃ stimulate the transition from the antiferromagnetic into the ferromagnetic state, whereas the substitution of Mg²⁺ for Mn stabilizes the antiferromagnetic state. It is supposed that the ferromagnetism in the insulating perovskites at hand is due to the positive superexchange of the Mn³⁺-O-Mn³⁺ type, and the magnetoresistive effect owes to the intergranular transfer of spin-polarized charge carriers and the suppression of magnetic nonuniformities by an applied magnetic field near T _C.     

Magnetic dilution and electronic nature in La0.7Pb0.3Mn1−xZnxO3

We have studied the magnetic dilution and electronic nature of Zn doping on the Mn site in the colossal magnetoresistant material La_0.7Pb_0.3MnO₃ (x≤0.3). Small non-magnetic Zn²⁺ doping tends to separate the system into ferromagnetic clusters to weaken the long-range ferromagnetic order and to reduce the Curie temperature. The spin polarizability of the x=0–0.3 samples is estimated to be 0.97–1.00, indicating that the x=0–0.3 samples are the spin polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x≥0.1) induces the metal–insulator transition and destroys the metallic state with long-range ferromagnetic order.     

Electrical and magnetic properties of La0.67Ba0.33Mn1−x (Me) x O3 perovskite manganites: case of manganese substituted by trivalent (Me = Cr) and tetravalent (Me = Ti) elements

The effects of non-magnetic Ti⁴⁺ substitution on the structural, electrical and magnetic properties of La_0.67Ba_0.33Mn_1?x Ti _x O₃ (0≤x≤0.1) are investigated and compared to those existing in La_0.67Ba_0.33Mn_1?x Cr _x O₃ (magnetic Cr³⁺). The structural refinement by the Rietveld method revealed that Ti-doped samples crystallize in the cubic lattice with space group $\mathrm{Pm}\bar{3}\mathrm{m}$ , while samples with Cr crystallize in the hexagonal setting of the rhombohedral $\mathrm{R}\bar{3}\mathrm{C}$ space group for identical contents of dopant. The most relevant structural features are an increase of the lattice parameters, of the cell volume and of the inter-ionic distances with increasing Ti doping level. Both series of samples show a decrease of the paramagnetic–ferromagnetic transition temperature when the amount of chromium or titanium increases. Transport measurements show that when increasing the metal doping, the resistivity increases whereas the metallic behavior of the parent compound La_0.67Ba_0.33MnO₃ is destroyed. For a substitution higher than 5 at.% of Ti and 10 at.% of Cr, the samples exhibit a semiconducting behavior in the whole range of temperature, for which the electronic transport can be explained by variable range hopping and/or small polaron hopping models.     

La0.3Ca0.7Mn1-xWxO3(x=0.04,0.08,0.12)体系的电子自旋共振谱研究

用固相反应法制备了La0.3Ca0.7Mn1-xWxO3(x=0.04,0.08,0.12)多晶材料,测量了样品的p～T曲线及ESR谱,对ESR谱的归一化强度和线宽进行了研究.结果表明:当x≤0.08时,ρ～T曲线出现弯折点,体系出现电荷有序(CO)相,体系随温度降低发生顺磁(PM)-电荷有序(CO)-反铁磁(AFM)变化;当x=0.12时,ρ～T曲线没有出现弯折点,CO相融化,自旋序主要是顺磁(PM),但存在CO相的残留.x≤0.08样品的线宽在高于Tco的温区几乎不随温度变化,体系处在顺磁态,铁磁关联较弱;在低于Tco的温区,电子的局域化程度增强,线宽随温度的降低而增加.     

Dielectric and piezoelectric properties of (1−x)Ba0.7Sr0.3TiO3−xBa0.7Ca0.3TiO3 perovskites

Dielectric and piezoelectric properties of (1?x)Ba_0.7Sr_0.3TiO₃?xBa_0.7Ca_0.3TiO₃ (BST?xBCT) (x=0.2–0.9) perovskite ceramics have been investigated. BCT has fully incorporated into BST lattice, forming a complete perovskite solid solution, whose lattice constant χ decreases almost linearly with increase in x from 0.2 to 0.4, while showing an anomalous expansion at 0.4<x≤0.6. This, together with the deviation of tetragonal–orthorhombic phase transition temperature (T_O–T) from the linear relation T_O–T (K)=?103.7x+239.3 at x=0.5, suggests that a small amount of Ca²⁺ has substituted for Ti⁴⁺. Curie temperature T_C increases linearly with increase in x from 0.2 to 0.9, which is mainly contributed to the increase of the Ba/Sr ratio. The calculated degree of relaxation (γ) is in the range of 1.41–1.53, indicating that the BST–xBCT ceramics are ferroelectric materials with diffuse phase transition. Strain and piezoelectric constant (d₃₃) decrease with increasing x, whereas planar electromechanical coefficient (k_p) reaches a maximum (17.0%) at x=0.6.     

降温速率对La0.7Ca0.3Mn1-xCrxO3(x=0.00,0.08)电磁性质的影响

我们在不同的降温速率下得到了若干La0．7Ca0．3Mn1-xCrxO3-δ(x=0．00,0．08)多晶样品,对其进行了XRD、电阻．温度关系和磁化强度．温度关系的测量．实验发现,随降温速率的增大,LCM系列样品的电阻变大,电阻峰温度移向低温端,磁化强度值变小,金属．绝缘体转变依然很陡峭,而LGMC系列样品也兼有这些现象,前两的程度更大些,后较为平缓,另外还出现了电阻的双峰．归结原因,我们认为在制备过程中随降温速率的不同,样品中会有不同程度的氧空位存在,利用双交换的减弱和样品中氧的分布不均匀产生的相分离解释了有关现象．通过本实验可知,改变降温速率也是调节样品中氧含量的一种有效的方法．     

Mössbauer characterization of Nd0.7Sr0.3(Mn1−x57Fex)O3 (x=0.02–0.15)

The magnetic structure of colossal magneto-resistive Nd_0.7Sr_0.3(Mn_1−x⁵⁷Fe_x)O₃ (x=0.02, 0.05, 0.10, and 0.15) has been studied by Mössbauer spectroscopy in magnetic fields up to 60 kG. ⁵⁷Fe ions appear to spread over two different magnetic phases. One of them remains ferromagnetic for all values of x, but the other one changes from canted to anti-ferromagnetic with increasing x. For x=0.02 and 0.05, the latter magnetic phase is comprised of small clusters, which are very sensitive temperature and strongly influenced by the applied fields.     

La0.7Ca0.3Mn1-xCrxO3的激光超声研究

运用激光超声的方法测定了五种不同掺杂的巨磁锰氧化物La0.7Ca0.3Mn1-xCrxO3(0.01≤x≤0.60)室温(300 K)下的超声纵波声速.结合热导率的数据,对声速随掺杂浓度的变化原因进行了深入的分析.得到在顺磁绝缘态Cr3 离子的掺入造成了晶格局部涨落的变化,低掺杂使得晶格局部涨落增强,声子U散射弛豫时间减小.     

Structural transformations in La1 − x Ba x Mn0.98 57Fe0.02O3 + δ (x = 0.05−0.20)

Physics of the Solid State - A systematic investigation of the structural transformations in La1 ? x Ba x Mn0.98 57Fe0.02O3 + δ (x = 0.05?0.20) at different Ba concentrations and...     

Massive, thick-film, and thin-film La0.6Sr0.3Mn1.1−x Fe x O3±δ magnetoresistive ceramics: Structure and properties

Magnetoresistive ceramic and thick-and thin-film La_0.6Sr_0.3Mn_1.1?x Fe _x O_3±δ(x=0,0.04) samples are studied by X-ray diffraction, ⁵⁵Mn nuclear magnetic resonance (NMR), resistivity measurements, and magnetic measurements. Their rhombohedrally distorted ( $R\overline 3 c$ ) perovskite structure is found to contain anion and cation vacancies and nanocluster defects. Their broad asymmetric ⁵⁵Mn NMR spectra support high-frequency electron-hole exchange between Mn³⁺ and Mn⁴⁺ and the fact that their environments are different due to a high defect concentration and high structural inhomogeneity. Iron doping and an increase in the annealing temperature result in a decrease in the temperatures of the metal-semiconductor and ferromagnet-paramagnet phase transitions and an increase in the magnetoresistive effect (MRE). The low-field MRE in the low-temperature region (～100 K) in the ceramics and thick film is explained by tunneling in crystallite boundaries. An analysis of the effect of iron and the annealing temperature on the activation energy confirms the conclusion regarding a defect system of the perovskite structure and the presence of several mechanisms of activation processes.     

Giant magnetoresistance of MexMn1−x S (Me=Fe, Cr) sulfides

The structural, electrical, and magnetic properties, as well as the magnetoresistance of polycrystalline Me_xMn_1?x S (Me=Fe and Cr) sulfides were investigated in longitudinal magnetic fields of up to 50 kOe over the temperature range 4.2–300 K. The ferromagnetic compound Fe_xMn_1?x S (x=0.29) exhibits the giant magnetoresistance (GMR) effect with magnitude δ_H=?450% in a field of 30 kOe at 50 K. Antiferromagnetic Cr_xMn_1?x S (x=0.5) sulfide undergoes a transition to the GMR state δ_H～?25% in a field of 30 kOe at 4.2 K) in the region of antiferromagnet-ferromagnet transition (T _c ～66 K). A mechanism of the GMR in these compounds is discussed.     

A Structural,Magnetic and Mössbauer Study of Tb0.3Dy0.7(Fe1−x Al x )1.95 Alloys

A structural, magnetic and Mössbauer study of a series of Tb_0.3Dy_0.7(Fe_1–x Al _x )_1.95 alloys (x=0, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35) at room temperature is presented. It was found that the primary phase of the Tb_0.3Dy_0.7(Fe_1–x Al _x )_1.95 alloys is the MgCu₂-type cubic Laves phase structure when x<0.3 and a small amount of a second phase, RFe₃ (R: rare-earth element), is present when 0.3x0.35. The lattice constant of the Tb_0.3Dy_0.7(Fe_1–x Al _x )_1.95 alloys increases approximately monotonically with increasing x. The substitution of Al increases slightly the magnetostriction in a low magnetic field (H500 Oe). However, the magnetostriction decreases sharply, but is saturated more easily with increasing x in a higher applied field. The analysis of the Mössbauer spectra allows the determination of the easy axes of magnetization in these alloys. Moreover, the dependence of the hyperfine-field, isomer shift and quadrupole splitting on the Al concentration, x, for the Tb_0.3Dy_0.7(Fe_1–x Al _x )_1.95 alloys are reported and discussed.     

Synthesis and physicochemical properties of Li2 Me x Zr1 − x O3 − δ (Me = Nb,Ti; x = 0.05, 0.1) solid solutions

Complex lithium metallates Li₂ Me _x Zr_{1 ? x} O_{3 ? δ} (Me = Nb, Ti, x = 0.05, 0.1) with iso-and heterovalent substitutions for Zr⁴⁺ ions in lithium zirconate are synthesized for the first time using a citrate technique. The inclusion of Ti⁴⁺ and Nb⁵⁺ ions in the crystal structure of Li₂ZrO₃ is confirmed by means of X-ray diffraction and NMR. It is shown that in the temperature range of 750–820 K, Li₂Ti_0.1Zr_0.9O₃ solid solution has higher conductivity than phases of undoped lithium zirconate.     

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − x Pr x Sr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La_{0.6 ? x} Pr _x Sr_0.3Mn_1.1O₃ (x = 0?0.6) have been studied using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R $\bar 3$ c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature T _mi and the ferromagnetic-paramagnetic phase transition temperature T _c with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn³⁺ ? Mn⁴⁺. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn²⁺ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn³⁺ Mn⁴⁺ in the B-positions and a local heterogeneity of their surrounding by other ions (La²⁺, Pr³⁺, Sr²⁺) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures T _mi and T _c , and magnetoresistive properties.     

Superparamagnetic properties of La1 − x Sr x Mn0.925Zn0.075O3 (x = 0.075, 0.095, and 0.115) lanthanum manganites

Lanthanum-strontium manganites doped with zinc are studied by the method of electron magnetic resonance. Nano-objects with ferromagnetically correlated spins, which behave themselves like superparamagnetic particles in the magnetic resonance spectrum, have been found in the paramagnetic phase. The temperature dependences of the resonance magnetic field and magnetic resonance linewidth for La_{1 ? x} Sr _x Mn_0.925Zn_0.075O₃ ceramic samples at temperatures ranging from 100 to 340 K have been analyzed on the basis of the Raikher-Stepanov theory of superparamagnetic particles. The magnetic moment, anisotropy field, and characteristic size of the regions of the ferromagnetically correlated spins have been determined.     

Spin Glass Behaviour and Spin-Dependent Scattering in La0.7Ca0.3Mn0.9Cr0.1O3 Perovskites

The magnetic, electrical and thermal transport properties of the perovskite La0.7Ca0.3Mn0.9 Cr0.1O3 have been investigated by measuring dc magnetization, ac susceptibility, the magnetoresistance and thermal conductivity in the temperature range of 5-300 K. The spin glass behaviour with a spin freezing temperature of 70 K has been well confirmed for this compound, which demonstrates the coexistence and competition between ferromagnetic and antiferromagnetic clusters by the introduction of Cr. Colossal magnetoresistance has been observed over the temperature range investigated. The introduction of Cr causes the ““double-bump““ feature in electrical resistivity ρ(T). Anomalies on the susceptibility and the thermal conductivity associated with the double-bumps in ρ(T)are observed simultaneously. The imaginary part of ac susceptibility shows a sharp peak at the temperature of insulating-metallic transition where the first resistivity bump was observed, but it is a deep-set valley near the temperature where the second bump in ρ(T) emerges. The thermal conductivity shows an increase below the temperature of the insulating-metallic transition, but the phonon scattering is enhanced accompanying the appearance of the second peak of double-bumps in ρ(T). We relate those observed in magnetic and transport properties of La0.7Ca0.3Mn0.9Cr0.1O3 to the spin-dependent scattering. The results reveal that the spin-phonon interaction may be of more significance than the electron (charge)-phonon interaction in the mixed perovskite system.     

钙钛矿锰氧化物La0.7Sr0.3MxMn1-xO3(M=Cr,Fe)的巨磁电阻效应与磁性

研究了溶胶 -凝胶法制备氧化物巨磁电阻材料的工艺 ,制备了La0 .7Sr0 .3 CrxMn1-xO3 (x =0 ,0 .10 ,0 .15 )和La0 .7Sr0 .3 FexMn1-xO3 (x =0 .0 5 ,0 .10 ,0 .16 )两系列的单相钙钛矿锰氧化物多晶样品 ,并研究了Cr ,Fe替代La0 .7Sr0 .3 MnO3 中部分Mn后对其结构、磁性和巨磁电阻性质的影响 .观察到La0 .7Sr0 .3 Cr0 .15Mn0 .85O3 和La0 .7Sr0 .3 Fe0 .0 5Mn0 .95O3 两个样品的电阻 温度曲线都出现了双峰 .定性讨论了可能产生双峰的机制 .随Cr(或Fe)替代量的增加 ,材料的居里温度很快下降 ,铁磁性减弱 ,导电性降低 ,巨磁电阻效应增强 .但与Fe掺杂相比 ,相同数量的Cr掺杂对材料的影响要小 .