Charge-carrier density collapse in La0.67Ca0.33MnO3and La0.67Sr0.33MnO3 epitaxial thin films

We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse. Received 22 July 1999 and Received in final form 7 October 1999     

Ambipolar gate effect and low temperature magnetoresistance of ultrathin La0.8Ca0.2MnO3 films

Ultrathin La(0.8)Ca(0.2)MnO(3) films have been measured in a field-effect geometry. The gate electric field produces a significant ambipolar decrease in resistance at low temperatures. This is attributed to the development of a pseudogap in the density of states and the coupling of localized charge to strain. Within a mixed phase scenario, the gate effect and magnetoresistance are interpreted in the framework of a "general susceptibility," which describes how phase boundaries move through a hierarchical pinning landscape.     

Electrical transport properties and laser-induced voltage effect in La0.8Ca0.2MnO3 epitaxial thin films

La_0.8Ca_0.2MnO₃ (LCMO) thin films about 200 nm thickness were grown on untilted and tilted (5°, 10° and 15°) LaAlO₃ (100) single crystal substrates by pulsed laser deposition technique. Electrical properties of the epitaxial thin films were studied by conventional four-probe technique and the anisotropic thermoelectric properties of the films grown on the tilted substrates have been investigated by laser-induced voltage (LIV) measurements. X-ray diffraction analysis and atomic force microscopy results show that the prepared LCMO thin films have a single phase and high crystalline quality. The remarkably large temperature coefficient of resistance (TCR) values (above 11 %/K) are observed in the all films. TCR value reaches 18 %/K on the film grown on 10° tilted substrate. The intensity of LIV signals monotonously increases with the tilting angles, and the largest signal is 148 mV with the fast time response 229 ns for the film grown on 15° tilted substrate.     

Temperature hysteretic effect and its influence on colossal magnetoresistance of La0.33Nd0.33Ca0.33MnO3

Electrical resistance (R) measurements of a bulk La_0.33Nd_0.33Ca_0.33MnO₃ perovskite in magnetic fields up to 40 kOe have revealed anomalous temperature hysteretic effects both in 0 Oe and 20 kOe magnetic fields. The sharp peak observed in the R vs. T plot indicates the occurrence of metal-to-insulator (M-I) transition at a temperature of T _MI=110 K and 140 K, for cooling and warming paths, respectively. An applied magnetic field of 20 kOe reduces the resistance and shifts T _MI to 160 K and 185 K for cooling and warming, respectively. We have observed a much higher resistance in the cooling path than in the warming path leading to the hysteretic resistance ratio (R _cool/R _warm) of 200 at 110 K and 1.8 at 160 K for 0 Oe and 20 kOe, respectively. Record values of colossal magnetoresistance (CMR) have been achieved. The CMR value reaches nearly 99% in the temperature ranges of 90 K to 140 K and 90 K to 170 K for 20 kOe and 40 kOe magnetic fields in the cooling mode, respectively. The observed unusual behavior is attributed to the co-existence of La-rich and Nd-rich domains assumed to be distributed randomly in the compound.     

La1－xPrxMnO3(x=0.1，0.2)薄膜庞磁电阻性质的研究

一种新的庞磁电阻氧化物薄膜La_1-xPr_xMnO₃(x=0.1，0 .2)薄膜用脉冲激光沉积(PLD)方法生长在(100)SrTiO₃单晶基底上.XRD结果显示 薄膜具有很好的外延单晶取向.电输运和磁性质的研究表明薄膜具有显著的庞磁电阻效应(CM R)效应，其中磁电阻比率达95％(在5T的磁场下).X射线光电子能谱(XPS)的结果表明薄膜体 系中Pr离子的价态为+4价，因此该薄膜很可能是电子掺杂的庞磁电阻体系. 关键词： 脉冲激光沉积 1-xPr_xMnO₃')" href="#">La_1-xPr_xMnO₃ 电子 掺杂 庞磁电阻     

Nature of the low-temperature colossal magnetoresistance of La0.35Nd0.35Sr0.3MnO3 epitaxial films

The colossal negative magnetoresistance (approximately 12%) in a field of 8.4 kOe over a wide range of temperatures below the Curie point T _C ≈240 K in a single-crystal La_0.35Nd_0.35Sr_0.3MnO₃ film on a single-crystal (001)ZrO₂(Y₂O₃) wafer substrate is discussed. Isotherms of the magnetoresistance of this film reveal that its absolute value increases with the field, abruptly in the technical magnetization range and almost linearly in stronger fields. For three single-crystal films of the same composition on (001)LaAlO₃, (001)SrTiO₃, and (001)MgO substrates, colossal magnetoresistance only occurred near T _C ≈240 K and at T<T _C it increased weakly, almost linearly, with the field. In the film on a ZrO₂(Y₂O₃) substrate the electrical resistivity was almost 1.5 orders of magnitude higher than that in the other three films. It is shown that this increase is attributable to the electrical resistance of the interfaces between microregions having four types of crystallographic orientations, while the magnetoresistance in the region before technical saturation of the magnetization is attributable to tunneling of polarized carriers across these interfaces which coincide with the domain walls (in the other three films there is one type of crystallographic orientation). The reduced magnetic moment observed for all four samples, being only 46% of the pure spin value, can be attributed to the existence of magnetically disordered microregions which originate from the large thickness of the domain walls which is greater than the size of the crystallographic microregions and is of the same order as the film thickness. The colossal magnetoresistance near T _C and the low-temperature magnetoresistance in fields exceeding the technical saturation level can be attributed to the existence of strong s-d exchange which is responsible for a steep drop in the carrier mobility (holes) and their partial localization at levels near the top of the valence band. Under the action of the magnetic field the carrier mobility increases and they become delocalized from these levels.     

Field-induced insulator-metal-insulator transitions in low-energy H2- ion-irradiated epitaxial La2/3Cal/3MnO3 thin films

Epitaxial La2/3Cal/3MnO3 thin films grown on LaA103 （001） substrates were irradiated with low-energy 120-keV H＋ ions over doses ranging from 1012 ions/cm2 to 1017 ions/cm2. The irradiation suppresses the intrinsic insulator-metal （I-M） transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 x 1015 ions/cm2 is in a threshold state of the electric insulator where the I-M transition is absent. In an external field of 4 T or higher, the I-M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.     

La0.4Ca0.6MnO3系统中Mn位Fe和Cr掺杂效应的比较性研究

利用固相反应法制备了Mn位Fe³⁺和Cr³⁺掺杂的系列锰氧化物La_0.4Ca_0.6(Mn_1-x(y)B_x(y))O₃ (B=Fe³⁺ (0≤x≤0.1); Cr³⁺ (0≤y≤0.1)) 多晶样品,研究了掺杂对样品输运性质的影响.实验结果表明两种离子具有截 关键词： 锰氧化物 反铁磁 磁电阻效应     

Electrical transport in La1−x Ca x MnO3 thin films at low temperatures

We report here the low-temperature resistivity of the chemical solution deposited La_1−x Ca _x MnO₃ (x=0.2, 0.3 and 0.33) thin films on LaAlO₃ substrates. The films were post-annealed in atmosphere at 850°C. The low temperature resistivity data has been studied in order to understand the nature of low-temperature conduction processes. The data showed T ² dependence from 60 K to 120 K consistent with the single magnon scattering process. The deviation from this quadratic temperature dependence at low temperatures is attributed to the collapse of the minority spin band. The two-magnon and electron-phonon processes contribute to scattering of carriers in the temperature range above 120 K.     

Colossal magnetoresistance effect in epitaxially grown La2/3Ca1/3MnO3 perovskite-like manganite thin films

We report in this work, study on colossal magnetoresistance (CMR) effect in epitaxial La_2/3Ca_1/3 MnO₃ thin films grown on SrTiO₃ (0 0 1) substrates by pulsed laser deposition (PLD) technique. The films were grown on as-received SrTiO₃ substrates and on SrTiO₃ substrates prepared by HF etching (Koster et al., Appl. Phys. Lett. 73 (1998) 2920; V. Leca et al., Wet etching methods for perovskite substrates, University of Twente, MESA+ Research Institute, Low Temperature Division). Two of the samples were annealed in different conditions to investigate the films heat treatment effect on electric and magnetic properties. Electrical resistance was done using the four-probe method at temperatures in the range of 2–375 K without a magnetic field and in an external field of 5 T applied in the film plane. Resistance-magnetic field (R vs. H) at 77 K for the two annealed samples was done in a 5 T sweep magnetic field. The surface morphology and structural information of the films were obtained using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. Secondary ion mass spectroscopy (SIMS) analysis was performed on the annealed samples to investigate any possible chemical reaction between La_2/3Ca_1/3MnO₃ thin films and SrTiO₃ substrate.     

Ag复合对La2/3(Ca0.45Sr0.55)1/3MnO3/xAg体系电输运和室温磁电阻效应影响

文中采用甘氨酸-硝酸盐法分别制备了La2/3(Ca0.45Sr0.55)1/3MnO3/xAg纳米复合材料(x=0;0.1;0.2;0.3).通过X射线衍射、扫描电子显微镜和磁电阻效应测试,对合成产物的结构及性能进行表征.结果表明,随Ag复合量增加,样品均为正交钙钛矿结构,低场室温磁电阻效应增强,电阻率减小.     

Annealing effect on transport properties of Nd0.67Sr0.33MnO3 thin films

Annealing dependence of the lattice parameter, resistivity, magnetoresistance and thermopower have been studied on Nd_0.7-Sr_0.33MnO₃ thin films deposited on LaAlO₃ and alumina substrates by pulsed laser ablation. Upon annealing at 800°C and 1000°C the lattice constant of the LaAlO₃ film tends toward that of the bulk target due to reduction in oxygen vacancies. This results in a metal-insulator transition at temperatures which increase with progressive annealing along with a decrease in the observed low temperature MR. Using a magnon scattering model we estimate the e _g bandwidth of the film annealed at 1000°C and show that the magnon contribution to the resistivity is suppressed in a highly oxygen deficient film and gains prominence only upon subsequent annealing. We also show that upon annealing, the polaron concentration and the spin cluster size increases in the paramagnetic phase, using an adiabatic polaron hopping model which takes into account an exchange dependent activation energy above the resistivity peak.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Transmission electron microscopy study of epitaxial La0.8MnO3 thin films on SrTiO3

The structure and microstructure of La_0.8MnO₃ thin films on SrTiO₃ substrates, fabricated by pulsed laser deposition at substrate temperatures of 873?K and 1073?K, have been studied by transmission electron microscopy. In both films, columnar growth morphology has been observed. The columnar grain size is found to increase with increasing substrate temperature. In the film deposited at a substrate temperature of 1073?K, there is only one rhombohedral phase. However, two phases, a rhombohedral one and an orthorhombic one, have been observed in the film deposited at 873?K.     

Effect of substrate temperature on electrical and magnetic properties of epitaxial La1−x Pb x MnO3 films

Epitaxial La_1−x Pb _x MnO₃ (LPMO) thin films, grown on (100) SrTiO₃ substrates by laser ablation technique at different temperatures between 600 and 850°C, have been characterized for electrical and magnetic properties. The temperature dependence of resistivity showed that the metal-insulator transition temperature (T _MI) decreases with increasing substrate temperature, which has been attributed to decrease in Pb content in the filsm. The YBa₂Cu₃O _x /La_1−x MnO₃ heterostructures, exhibiting both superconductivity and ferromagnetism, have been fabricated.     

Temperature evolution of the cluster state in La0.8Ca0.2MnO3 and La0.8Ca0.2CoO3

The results of investigations of the transport and magnetic properties (ac linear and nonlinear (second- and third-order) susceptibilities) of La_0.8Ca_0.2MnO₃ and La_0.8Ca_0.2CoO₃ single crystals have been presented. It has been found that both compounds in the paramagnetic phase contain ferromagnetic clusters with close magnetic characteristics. At high temperatures, ferromagnetic clusters nucleate in preferred sites associated with chemical inhomogeneities. Cooling below a specific temperature is accompanied by homogeneous nucleation of clusters. These two stages are observed in both compounds. In the doped cobaltite, the coalescence of clusters begins to develop at the third stage, whereas in the manganite, their behavior changes due to the development of ferromagnetic ordering of the matrix. These features indicate that the cluster state in doped manganites and cobaltites has a common nature. The difference in the behavior of ferromagnetic clusters is a consequence of the magnetically active character of the matrix in the case of manganites and the neutral character of the matrix in the case of cobaltites.     

Quantifying the electrical transport characteristics of electron-doped La0.7Ce0.3MnO3 thin films through hopping energies,Mn valence,and carrier localization length

Cerium-doped LaMnO₃ is widely discussed as one of the most prospective electron-doped thin-film prototype material that complements well-established hole-doped mixed-valence manganites. Here, we investigate La_0.7Ce_0.3MnO₃ films with respect to their electrical properties and check whether they provide an effective electron doping with Mn-valences well below +3. Thin films of a variable thickness between 10 and 100 nm are characterized through resistance measurements over a broad temperature range between 90 and 300 K deducing their hopping energies, carrier localization lengths, and the Mn valence by comparing the experimental data to different transport models. While electronic transport above 300 K is well determined by the thermally activated diffusion of small polarons, we find the carrier localization by disorder to reveal a variable-range hopping-type transport for lower temperatures. From the several parameters investigated in the study, it is mainly the oxygen content and the degree of CeO₂ phase segregation that are crucial to be controlled in such electron-doped thin-film manganites.     

Out-of-plane low field anisotropic magnetoresistance in Nd0.51Sr0.49MnO3 thin films

A comparative study of the out-of-plane anisotropic magnetoresistance (AMR) in single crystalline and polycrystalline thin films of phase separated manganite Nd_0.51Sr_0.49MnO₃ has been carried out. On-axis DC magnetron sputtering was used to deposit the single crystalline films (30 and 100 nm in thickness) on single crystal (0 0 1) LaAlO₃ (LAO) and polycrystalline films (100 nm) on (1 0 0) Yttrium-stabilized ZrO₂ (YSZ) substrates. The in-plane and out-of-plane magnetotransport properties of these films differ significantly. A large low field AMR is observed in all the films. AMR shows a peak below the insulator-metal transition temperature in the single crystalline films, while the same increases monotonically in the polycrystalline film. Relatively larger low field AMR (∼20% at T=78 K and H=1.7 kOe) in the polycrystalline films suggests the dominance of the shape anisotropy.     

Isotope effect for transport and magnetic properties of La 0.35 Pr 0.35 Ca 0.3 MnO 3 thin films

The effect of ¹⁶ O → ¹⁸ O isotope substitution on electrical resistivity, magnetoresistance, and ac magnetic susceptibility was studied for La_0.35Pr_0.35Ca_0.3MnO₃ epitaxial thin films deposited onto LaAlO₃ and SrTiO₃ substrates. For the films on LaAlO₃, the isotope substitution resulted in the reversible transition from a metal-like to insulating state. The applied magnetic field ( H ≥ 2 T) transformed the sample with ¹⁸O back to the metallic state. The films on SrTiO₃ remained metallic at low temperatures for both ¹⁶O and ¹⁸O, but the shift of the resistivity peak corresponding to onset of metallic state exceeded 63 K after ¹⁶ O → ¹⁸ O substitution. The temperature dependence of both resistivity and magnetic susceptibility was characterized by hysteresis, especially pronounced in the case of the films on LaAlO₃. Such a behavior gives certain indications of the phase separation characteristic of interplay between ferromagnetism and charge ordering. Received 11 February 2000 and Received in final form 13 September 2000     

Prediction of thermomagnetic properties of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3

The dependence of magnetization on the variation of temperature for La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃ perovskite-type ceramic materials under an external magnetic field was modelled. Application of phenomenological model was performed, showing a good agreement with the experimental data. Temperature dependence of change of both magnetic entropy and specific heat under magnetic field 0.05?T was predicted for La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃. Predicted values of maximum magnetic entropy change, full-width at half-maximum, relative cooling power and maximum specific heat under 0.05 T were evaluated.