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1.
The magnetic and transport properties of La1−xCaxMnO3 (0≤x<0.4) have been systematically studied. The magnetoresistance (MR) maximum appears at x=0.2-0.25 and the temperature dependence of MR for x>0.25 shows a much broader profile than that of samples for x=0.2-0.25. Based on a scenario in which there is a short-range charge ordering (CO) state coexisting in the ferromagnetic state matrix for x>0.25, and the least or even no short-range CO state exists in samples for x=0.2-0.25, the above observations can be understood.  相似文献   

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3.
We report the detailed results of magnetization and magnetoresistance measurements in the Ru doped layered manganite system La1.2Sr1.8Mn2−xRuxO7 (x=0, 0.1, 0.5, 1.0). High-resolution measurements of magnetization and magnetoresistance were carried out as functions of temperature, magnetic field and time. We find evidence for the existence of competing ferromagnetic and antiferromagnetic interactions resulting in the formation of a frustrated spin-glass-like state at low temperatures. The time dependent magnetization follows the relation very well. We find that Ru doping enhances the coercive field and drives the system towards a magnetically mixed phase at low temperatures. Large negative magnetoresistance values are observed in all samples and at low temperatures the magnetoresistance varies as the square root of the applied magnetic field.  相似文献   

4.
The author of the comment objects to the characterization and the interpretation of magnetoresistance (MR) effects observed by us in La1−xCaxMnO3 (0≤x<0.4) samples. In this reply, arguments are used to show that the samples' characterization and explanation of the MR by considering the role of the short-charge ordering (CO) regions and magnetic domains are reasonable and acceptable.  相似文献   

5.
The electrical and magnetic transport properties of the La0.67−xEuxCa0.33MnO3 system exhibit lowering of insulator to metal and paramagnetic to ferromagnetic transition temperature (TC) with the increase of Eu concentration in addition to possessing CMR property. The temperature variation of electrical resistivity and magnetic susceptibility for x=0.21 is found to have two distinct regions in the paramagnetic state for T>TP; one with the localization of lattice polaron in the high-temperature region (T>1.5TP) satisfying the dynamics of variable range hopping (VRH) model and the other being the combination of the spin and lattice polarons in the region TP<T<1.5TP. The resistivity variation with temperature and magnetic field, the cusp in the resistivity peak and CMR phenomenon are interpreted in terms of coexistence of spin and lattice small polarons in the intermediate region (TP<T<1.5TP). The spin polaron energy in the La0.46Eu0.21Ca0.33MnO3 system is estimated to be 106.73±0.90 meV and this energy decreases with the increase of external magnetic field. The MR ratio is maximal with a value of 99.99% around the transition temperature and this maximum persists till T→0 K, at the field of 8 T.  相似文献   

6.
Magnetoresistance (MR) and magnetization (dc and ac) measurements have been carried out on the manganites, (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05≤x≤0.15), in the temperature range of 5-320 K. At 5 K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 T. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained in the light of various possibilities such as phase segregation, cluster spin-glass behavior, etc.  相似文献   

7.
The samples with the Mn3+/Mn4+ ratio fixed at 2:1 La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 (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 Tc and insulator−metal transition temperature Tp1 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 〈rA〉 and the A-site mismatch effect (σ2).  相似文献   

8.
We have investigated the magnetic phase diagram of polycrystalline and single-crystal La1−xSrxMnO3 near 0.46≤x≤0.50. It turns out that for x<0.48, the polycrystalline material is ferromagnetic (FM), but for x≥0.48, incipient charge ordering takes place along with antiferromagnetism. At x=0.48, the ferromagnetic-antiferromagnetic phase transition in ceramics occurs at less than 85 kOe but requires significantly larger field for increasing x. These observations are in contrast to what is found in the single crystals, which are all FM.  相似文献   

9.
The dc magnetization and ac susceptibility measurements on two dimensional layered manganite La1.2Ba1.8Mn2O7 samples reveal the occurrence of ferromagnetism above room temperature with ferromagnetic (FM) to paramagnetic (PM) transitions at 338 K. The bifurcation temperatures shown by the zero-field cooled (ZFC) and field cooled (FC) dc magnetization curves at high temperatures shift towards lower temperatures as the applied field is increased from 100 to 2500 Oe. The data are suggestive of a large magnetic anisotropy due to the strong competing ferromagnetic and antiferromagnetic interactions resulting in a spin-glass-like state. Ru doping is found to enhance the ferromagnetism and metallicity of the system in a remarkable way. The magnetoresistance (MR) values obtained are very high and about 40% even at 260 K for the undoped sample.  相似文献   

10.
The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La0.9Te0.1Mn1−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 TC 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.  相似文献   

11.
Cr-doped manganites Sr0.9Ce0.1Mn1−yCryO3 (y=0, 0.05, and 0.10) have been systematically investigated by X-ray, magnetic, transport, and elastic properties measurements. For parent compound Sr0.9Ce0.1MnO3, it undergoes a metal-insulator (M-I) transition at 318 K, which is suggested to originate from a first-order structural transition accompanied by Jahn-Teller (JT) transition. With increasing Cr doping content, the JT transition temperature decreases. The Cr doping suppresses the antiferromagnetic (AFM) state and makes the system spin-glass (SG) behavior at low temperatures. In the vicinity of JT transition temperatures, the softening of Young's modulus originating from the coupling of the orbital (quadrupolar) moment of the eg orbital of Mn3+ ion to the elastic strain has been observed. The anomalous Young's modulus properties imply the electron-phonon coupling due to the JT effect may play an important role in the system.  相似文献   

12.
The magnetic property of double doped manganite Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 with a fixed ratio of Mn3+:Mn4+=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr3+ for Mn3+ ions causes the increase of magnetization and the rise of Tc. As the Cr3+ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn3+ and Cr3+ ions causes a magnetic upturn, namely, the second FM phase transition.  相似文献   

13.
We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2) prepared by sol-gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at Tc=136 K. Both for y=0.1 and 0.2, Tc increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at Tf<Tc which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to Tc and Tf. The value of MR decreases for the Cr substituted samples.  相似文献   

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15.
Nanocrystalline La1−xBaxMnO3 (0.0≤x≤0.3) manganites have been prepared by a simple and instantaneous solution combustion method, which is a low temperature initiated synthetic route to obtain fine-grained powders with relatively high surface area. The phase purity and crystal structure of the combustion products are carried out by powder X-ray diffraction. The as-made nanopowders are in cubic phase. On calcination to 900 °C, barium doped manganites retain cubic phase, whereas barium free manganite transformed to rhombohedral phase. The scanning electron microscope (SEM) results revealed that the combustion-derived compounds are agglomerated with fine primary particles. The doped manganites have surface area in the range 24-44 m2/g. The surface area of the manganites increases with barium content, whereas it decreases on calcination. Both undoped and doped lanthanum manganites show two active IR vibrational modes at 400 and 600 cm−1. The low temperature resistivity measurements have been carried out by four-probe method down to 77 K. All the samples exhibit metal-insulator behaviour and metal-insulator transition temperature (TM-I) in the range 184-228 K and it is interesting to note that, as the barium content increases the TM-I shifts to lower temperature side. The maximum TM-I of 228 K is observed for La0.9Ba0.1MnO3 sample.  相似文献   

16.
We report the growth of single phase, c-axis aligned thin films of La1.2Ca1.8Mn2O7 on SrTiO3 (001) substrates using a controlled pulsed laser deposition method. In this method, constraint of epitaxy is utilized to stabilize the Ruddlesdon-Popper (RP) phase of La1.2Ca1.8Mn2O7. Oxygen ambient pressure and the rate of deposition play a very important role in influencing the epitaxial growth as well as maintaining phase purity of the material. The oxygen pressure inside the deposition chamber was very precisely controlled and varied during the layer-by-layer growth of the film. Films, prepared by our method, show excellent electrical and magnetic characteristics with a sharp metal-insulator transition at TM-I=90 K, closely followed by a magnetic transition at TC=91 K.  相似文献   

17.
Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La1.4Ca1.6Mn2O7 (DLCMO) are reported and compared with the infinite layered manganite La0.7Ca0.3MnO3 (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (TM-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (SV) shows 1/fα type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), SV/V2 shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures TC2 and TC1. In temperature range between TC1 and TC2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].  相似文献   

18.
The magnetism and transport properties of the samples LaMn1−xTixO3 (0≤x≤0.2) were investigated. All samples show a rhombohedral structure () at room temperature. The sample with x=0 undergoes the paramagnetic-ferromagnetic (PM-FM) transition accompanied by an insulator-metal (I-M) transition due to the oxygen excess. The doped samples show ferromagnetism and cluster behavior at low temperatures. Though no I-M transition associated with the PM-FM transition appears, the magnetoresistance (MR) effect was observed especially at low temperatures under the applied fields of 0.5 T. Due to the fact that the oxygen content in the Ti-doped samples is nearly stochiometry (3.01) and the Hall resistivity at room temperature is negative, the ferromagnetism in LaMn1−xTixO3 (0.05≤x≤0.2) is believed to be consistent with the Mn2+-O-Mn3+ double exchange (DE) mechanism. These results suggest that DE can be obtained by direct Mn-site doping.  相似文献   

19.
We present results on the Hall coefficient RH in the normal state for a GdBa2Cu3O7−δ/La0.75Sr0.25MnO3 bilayer and a La0.75Sr0.25MnO3 film grown by dc magnetron sputtering on (1 0 0) SrTiO3. We find that the electric transport on the bilayer can be qualitatively described using a simple parallel layers model. The GdBa2Cu3O7−δ layer presents a carrier density approximately equal to that reported for 7 − δ = 6.85 oxygen doping. Also we observe an unexpected presence of two Hall resistivity regimes, effects that may be associated with the internal magnetic field induced on the superconducting layer by the ferromagnetic layer.  相似文献   

20.
A comprehensive study of the effect of Fe doping on CaMnO3 is carried out by means of experiments on the structural, transport conduction, and magnetic properties of CaMn1−xFexO3 (0≤x≤0.35). With a sol-gel process for sample preparation, Fe is substituted for Mn up to x=0.35. This substitution substantially brings out the lattice expansion and gradually suppresses the antiferromagnetism. For x=0.08 and 0.10 in particular, the magnetization curves with a field-cooled mode under the field of 1 kOe behave as those of a ferrimagnetic-like system and present low-temperature negative magnetization. For x≥0.15, the negative magnetization phenomenon disappears, and a ferromagnetic component coexists with an antiferromagnetic one, but the antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show insulating behavior for all compositions. Fe doping, even at a level as low as x=0.02, can cause a marked resistivity increase in the temperature range studied. Further increasing the Fe content causes the resistivity to gradually decrease due to the increasing carrier presence.  相似文献   

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