The effect of Co ion implantation on Ge1−xMnx films

Ge_1−xMn_x (x = 0, 0.013, 0.0226, 0.0339, 0.0565, 0.0678, 0.0904, 0.113) films prepared by magnetron sputtering at 773 K had a Ge cubic structure except for x = 0.1130. Co ion implantation into these films can effectively prevent the formation of a second phase. Both single-doped and co-doped samples were ferromagnetic at room temperature. The d-d exchange interaction between the interstitial Mn (MnT) and the substituted Mn (Mn_Ge) resulted in ferromagnetism in the sputtered films. Since Co ion implantation destroyed the Mn_T-Mn_Ge-Mn_T complex, the saturated magnetization decreased. Hall measurements revealed that the Co ion implanted films were n-type semiconductors, and the anomalous Hall Effect (AHE) suggested the ferromagnetism was carrier-mediated in the implanted films.     

Magnetic disorder induced enhanced magneto-resistance in La0.5Sr0.5Co1−xRuxO3

We observe a sharp increase in negative magneto-resistance ratio up to 40% for x=0.1, in La_0.5Sr_0.5Co_1−xRu_xO₃ which is due to the magnetic disorder induced by an anti-ferromagnetic interaction between Co and Ru ions. We also observe a metal to insulator and a ferromagnetic to anti-ferromagnetic transition for 0≤x≤0.3. Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co³⁺:t_2g⁵e_g¹), forcing a double exchange mediated ferromagnetic state to an anti-ferromagnetic spin state for x≥0.2.     

Magnetic properties of transition metal substituted La0.85Ag0.15Mn1−yMyO3 compounds (M=Co, Cr and Al)

In this paper we report a systematic study of Mn-site substitution by M=Co, Cr and Al in La_0.85Ag_0.15MnO₃ series to understand the magnetic interactions between Mn and other transition metals. The long-range ferromagnetic (FM) ordering of the parent compound was significantly affected by Mn-site substitution. The measured magnetic properties of Co-doped samples have been explained on the basis of FM interactions in Mn³⁺-O-Mn⁴⁺, Co²⁺-O-Mn⁴⁺, Co³⁺-O-Mn⁴⁺ networks and simultaneous antiferromagnetic (AFM) interactions in Mn⁴⁺-O-Mn⁴⁺, Co²⁺-O-Mn³⁺ networks. The magnetic properties of Cr-doped compounds could be understood on the basis of double exchange FM interactions in Mn³⁺-O²⁻-Mn⁴⁺ networks and competing AFM in Cr³⁺-O-Mn⁴⁺, Mn⁴⁺-O-Mn⁴⁺, Cr³⁺-O-Mn³⁺ networks. However, it is found that the doping of Al ions play a role of magnetic dilution, without contributing any other competing magnetic interaction. The field variations of magnetization of all the above three series could be analysed by fitting to Brillouin function model and the effective spin contribution for FM has been determined. The measured saturation magnetization has been explained quantitatively.     

Comparative study of heterogeneous magnetic state above TC in La0.82Sr0.18CoO3 cobaltite and La0.83Sr0.17MnO3 manganite

The magnetic, transport and structural properties are studied for La_0.83Sr_0.17MnO₃ and La_0.82Sr_0.18CoO₃ single crystals with nearly the same doping and the metallic ground state. Their comparisons have shown that ferromagnetic clusters originate in the paramagnetic matrix below Т^?>T_C in both samples and exhibit similar properties. This suggests the possible universality of such phenomena in doped mixed-valence oxides of transition metals with the perovskite-type structure. The cluster density increases on cooling and plays an important role on the physical properties of these systems. The differences in cluster evolutions and scenarios of their insulator–metal transitions are related to different magnetic behaviors of the matrixes in these crystals that is mainly due to distinct spin states of the Mn³⁺ and Co³⁺ ions.     

Magnetic phases in lanthanum-strontium manganite-cobaltite La1.25Sr0.75MnCoO6

Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La_1.25Sr_0.75MnCoO₆, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at T_C=217 K and a frequency-dependent transition at T_g<100 K. The high frequency dependence of T_g is indicative of the cluster-glass behavior of La_1.25Sr_0.75MnCoO₆ (7 5 0) at T<T_C within the ferromagnetic state.     

Co和Cr掺杂对层状钙钛矿锰氧化物La1.2Sr1.8Mn2O7磁性的影响

研究了层状钙钛矿锰氧化物La_1.2Sr_1.8Mn₂O₇中Mn被Co和Cr替代对磁性能的影响.Co替代Mn后,长程铁磁序被破坏,铁磁性减弱,出现团簇玻璃态和自旋玻璃态,表明Co离子和Mn离子之间不存在双交换作用.而Mn被Cr替代后长程铁磁序仍然保持,证实Cr³⁺和Mn³⁺之间存在铁磁性交换作用. 关键词：     

多自旋态离子Co替代诱导La2/3Ca1/3MnO3体系的输运反常

利用具有多自旋态的Co离子进行Mn位替代,制备了La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) 系列样品并研究了体系的结构和输运特性.结果表明,在替代范围内,样品呈现很好的单相结构,各晶格参数随替代量的增大而减小;Co替代导致体系出现电输运反常,具体表现为在居里温度T_C以下电阻-温度曲线的二次金属-绝缘转 关键词： Mn位替代 双峰现象 自旋结构 磁电阻效应     

Structural and magnetic phase diagram and room temperature CMR effect of La1−xAgxMnO3

The study of the structural and magnetic phase diagram of the manganites La_1−xAg_xMnO₃ shows similarity with the La_1−x′Sr_x′MnO₃ series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn³⁺:Mn⁴⁺=2) with T_max=300 K (maximum of the ρ(T) curve in zero field).     

The structural and multiferroic properties of (Bi1−xLax)(Fe0.95Co0.05)O3 ceramics

La and Co co-doped BiFeO₃ ((Bi_1−xLa_x)(Fe_0.95Co_0.05)O₃ (x=0, 0.10, 0.20, 0.30)) ceramics were prepared by tartaric acid modified sol–gel method. The X-ray diffraction patterns indicate a transition from rhombohedral structure to tetragonal structure at x=0.20, which has been confirmed by the Raman measurements. The band gap increases with increasing x to 0.20, and then decreases with further increasing x to 0.30. The structural transition has significant effects on the multiferroic properties. The remnant magnetization and saturate ferromagnetic magnetization decrease abruptly with increasing x to 0.10, and then gradually increase with further increasing x up to 0.30. The coercivity is significantly reduced with increasing La doping concentration. The ferroelectricity has been improved by La doping, and the polarization increases with increasing x to 0.10, then decreases with further increasing x up to 0.30. The simultaneous coexistence of soft ferromagnetism and ferroelectricity at room temperature in tetragonal Bi_0.70La_0.30Fe_0.95Co_0.05O₃ indicates the potential multiferroic applications.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Structure and magnetic order in La0.7Ca0.3Mn0.5Co0.5O3 and La0.8Sr0.2Mn0.5Co0.5O3 perovskites

In this paper we report the study of the perovskites La_0.7Ca_0.3Mn_0.5Co_0.5O₃ and La_0.8Sr_0.2Mn_0.5Co_0.5O₃ by neutron powder diffraction at various temperatures and magnetization measurements in zero applied field and at low cooling regimes. The replacement of half Mn by Co in La_0.7Ca_0.3MnO₃ and La_0.8Sr_0.2MnO₃ destroys their long-range ferromagnetism exhibiting a cluster glass ferromagnetic order similar to the one observed in many cobaltites.     

Effect of Mo doping on magnetic and transport properties of La0.5Sr0.5CoO3

The substitutional effect of Mo on the magnetic and transport properties of double exchange ferromagnets, La_0.5Sr_0.5Co_1−x Mo_xO₃ (0?x?0.2) has been investigated. Substitution of 10% Mo at the Co-site of La_0.5Sr_0.5CoO₃ decreases the Curie temperature by ∼60 K than that of the parent compound and the long-range ferromagnetic ordering disappears for x?0.2. The Mo-doped samples, however, undergo a transition from the parent metallic state to the insulating state below T_c. The insulating state is found to obey Mott's variable range hopping of conduction. The effect of Mo substitution is attributed to the factors namely, (i) the dilution of magnetic Co sublattice, (ii) the reduction of Co⁴⁺/Co³⁺ ratio resulting in a reduced carrier concentration and (iii) disruption of the intermediate spin structure of Co, namely Co³⁺: t_2g⁵e_g¹.     

Observation of mixed-phase behavior in the Mn-doped cobaltite La0.7Sr0.3Co1−xMnxO3 (x=0-0.5)

Studies on La_0.7Sr_0.3Co_1−xMn_xO₃ (x=0-0.5) compounds evidence that the interaction between Mn and Co ions in this system is antiferromagnetic super-exchange and not ferromagnetic (FM) double-exchange (DE). As a result, antiferromagnetism and magnetic glassiness develop steadily with increasing Mn content and the system becomes a spin glass at x∼0.1. Analyses of high-field magnetization data indicate that the system consists of two major phases: a metallic FM phase which magnetically saturates in rather low field, and an insulating non-FM phase which has a linear dependence of magnetization on magnetic field. In the low doping regime, the fraction of the non-FM component expands with temperature at the expense of the FM phase and becomes maximal at T_C. Ferromagnetism reappears in highly doped (x≥0.2) compounds due to the presence of DE interaction between the Mn ions. The small volume fraction of the FM phase derived from the M(H) data in high-field region supports the coexistence of insulating and FM behaviors in the highly doped samples.     

Room temperature magnetoresistance in Ln2/3A1/3MnO3 manganites

The investigation of the manganites La_2/3−xPr_xSr_1/3MnO₃, La_2/3Sr_1/3−xCa_xMnO₃ and La_2/3+xCa_1/3−2xAg_xMnO₃, which all exhibit Mn³⁺:Mn⁴⁺=2, shows that it is possible to reach high magnetoresistance at room temperature, up to 21% under 1.2 T. These materials are compared to La_5/6Ag_1/6MnO₃ which corresponds to the same Mn³⁺:Mn⁴⁺ ratio and exhibits a magnetoresistance of 25% in this field. An interesting feature deals with the value of the insulator-metal transition temperature T_IM, often higher than T_C, especially for Ag-based compounds. It is suggested that the latter results either from a better oxygenation of the surface of the grains or from a migration of silver toward the surface.     

Magnetic and transport properties of Dy substituted layered perovskite La1.3Sr1.7Mn2O7

The effect of Dy substitution for La site in layered manganese oxides La1.3-x Dyx Sr1.7 Mn2O7 on the magnetic and electrical properties has been investigated.With the La 3+ substituting by Dy3+,the long range three-dimensional ferromagnetism transition and the insulator-metal transition disappear.These effects are attributed to the lattice distortion due to the substitution of the smaller Dy3+.Addtionally,the small Dy3+ is inclined to occupy the R site which is in the rock-salt layer,then the distribution of La,Sr,Dy ions in Dy-doped sample should be more orderly than that in La1.3 Sr1.7 Mn2O7,so there is only one insulator-metal transition in the ρ-T curve of the sample with x = 0.05 and x = 0.1.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶，主要研究了其磁电特性.结果表明，样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低，其磁性经历了一个很复杂的转变过程.当x=0时，在T^*=231K出现二维短程铁磁有序，在<     

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.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶,主要研究了其磁电特性.结果表明,样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低,其磁性经历了一个很复杂的转变过程.当x=0时,在T^*=231K出现二维短程铁磁有序,在< 关键词： 层状钙钛矿 磁性 电特性