晶界对庞磁电阻颗粒薄膜的磁学和输运性能的影响

采用脉冲电子束沉积技术，在Si(100)单晶衬底上沉积庞磁电阻La_0.67Ca_0.33MnO₃颗粒薄膜，并对它的磁学性能和电学输运性能进行了表征.研究晶界对庞磁电阻薄膜的物理性能的影响，结果表明，晶界的存在使得晶粒之间的耦合变弱，在变温磁化过程中表现出团簇玻璃态行为，金属—绝缘体转变温度(T_p)远远低于铁磁—顺磁转变温度(T_c).低温下电子输运具有弱局域化行为.在低磁场下，晶界的存在掩盖了La_0.67Ca_0.33MnO₃的本征磁电阻行为. 关键词： 脉冲电子束沉积 晶界 磁学和电学输运性能 庞磁电阻     

Structural and electrical transport properties of nanosized La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> sample synthesized by a simple low-cost novel route

We have synthesized nanosized La_0.67Ca_0.33MnO₃ by a simple low-cost novel synthesis route without calcination at high temperature. The study of these nanoparticles indicates excellent properties similar to colossal magnetoresistance (CMR) materials sintered at ∼1600°C for 20 h. The resulting particle size is in the range of 50–160 nm as determined by scanning electron microscopy. Resistivity measurement has been carried out down to 12 K. The sample shows metal-to-insulator (M–I) transition at 205 K.      

Effect of an interface boundary on the magnetooptical and magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> heterostructures

The optical, magnetooptical (Kerr effect and magnetotransmission), and magnetotransport properties of La_2/3Ca_1/3MnO₃/La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃/SrTiO₃/La_2/3Sr_1/3MnO₃ heterostructures on SrTiO₃ substrates are studied. The contribution of the interface boundary to the magnetotransmission is typical of a material with a transitional composition. It is found that a 2-nm-thick SrTiO₃ spacer does not influence the shape and position of the magnetotransmission peak in a field normal to the surface of the heterostructure but increases the contribution of the upper layer to the magnetotransmission in the Voigt geometry and also enhances the magnetoresistance that is due to the tunneling of spin-polarized carriers through the spacer. The Kerr spectra taken of the heterostructures are typical of single-layer single-crystal films.     

Magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> with different grain sizes

The magnetotransport and magnetoresistive (MR) properties of manganese-based La_0.67Ca_0.33MnO₃ perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T _IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T _IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T _IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E _F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The ρ–T ² curves are nearly linear in the metallic regime, but the ρ–T ^2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers. MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling effect.     

Response of the electrical resistivity and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films to biaxial tensile strains

The structure, electrical resistivity, and magnetoresistance of (50-nm)La_0.67Ca_0.33MnO₃ epitaxial films grown on a [(80 nm)Ba_0.25Sr_0.75TiO₃/La_0.3Sr_0.7Al_0.65Ta_0.35O₃] substrate with a substantial positive lattice misfit have been studied. The tensile biaxial strains are shown to account for the increase in the cell volume and in the relative concentration of Mn⁺³ ions in the manganite films as compared to those for the original material (33%). The peak in the temperature dependence of the resistivity ρ of La_0.67Ca_0.33MnO₃ films was shifted by 30–35 K toward lower temperatures relative to its position in the ρ(T) graph for a manganite film grown on (001)La_0.3Sr_0.7Al_0.65Ta_0.35O₃. For T < 150 K, the temperature dependences of ρ of La_0.67Ca_0.33MnO₃/Ba_0.25Sr_0.75TiO₃/La_0.3Sr_0.7Al_0.65Ta_0.35O₃ films could be well fitted by the relation ρ = ρ₀ + ρ₁T^4.5, where ρ₀ = 0.35 mΩ cm and the coefficient ρ₁ decreases linearly with increasing magnetic field. In the temperature interval 4.2–300 K, the magnetoresistance of manganite films was within the interval 15–95% (μ₀H = 5 T).     

Structure and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films grown coherently on (001)NdGaO<Subscript>3</Subscript>

40-to 120-nm-thick (001)La_0.67Ca_0.33MnO₃ films grown through laser evaporation on (001)NdGaO₃ were studied. The lattice parameters of the La_0.67Ca_0.33MnO₃ films measured in the substrate plane (a_∥=3.851 Å) and along the normal to its surface (a_⊥=3.850 Å) practically coincided with that of the pseudocubic neodymium gallate. The unit-cell volume of the La_0.67Ca_0.33MnO₃ film was slightly smaller than that of stoichiometric bulk samples. The position of the maximum in the temperature dependence of electrical resistivity did not depend on the thickness of the La_0.67Ca_0.33MnO₃ film. The negative magnetoresistance (MR≈?0.25, H=0.4 T) of La_0.67Ca_0.33MnO₃ films reached a maximum at 239–244 K.     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003     

Enhancing low-field magnetoresistance of La0.67Ca0.33MnO3 films deposited on anodized aluminium-oxide membranes

In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.     

Effect of Ta<Superscript>5+</Superscript> substitution for Mn on the ground state of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

We report the charge state modification effects at the Mn site on the ground state properties of colossal magnetoresistive manganites. Ta⁵⁺ substitution results in an appreciable increase in the lattice parameters and unit cell volume due to increased Mn³⁺ concentration. The ferromagnetic-metallic ground state modifies to a cluster glass insulator for . The reduction in the transition temperatures with increasing x is ∼39 K/at.%. Besides the modification of majority carrier concentration due to increased Mn³⁺ concentration and enhanced local structural effects, the local electrostatic potential of the substituent seems to contribute to the unusually strong reduction of the transition temperatures of the compounds. Thermo magnetic irreversibility just below Curie temperature (T_c), non-saturation of magnetization, two distinct magnetic transitions in ac susceptibility in an appropriate static field: close to T_c and other at low temperature (the spin freezing temperature (T_g)) and non-stationary dynamics with a characteristic maximum in the magnetic viscosity close to T_g confirm a cluster glass state for . These results find additional support from a linear low temperature magnetic specific heat of x = 0.10 with a characteristic broad maximum close to T_g.     

La0.67Ba0.33MnO3薄膜的激光辐照效应研究

利用脉冲激光沉积技术在LaAlO₃（00l）单晶衬底上制备了La_067Ba_033MnO₃薄膜,研究了CO₂激光辐照对La_067Ba_033MnO₃薄膜的微结构和磁电性能的影响.结果表明,经激光辐照后,La_067Ba_033MnO₃薄膜的结晶性增强,薄膜应变减小;薄膜表面形貌由“岛状”结构变为“平原"结构,且粗糙度大大降低;同时,薄膜的饱和磁化强度、铁磁居里温度、金属—绝缘态转变温度和磁电阻增大,而矫顽场和电阻率减小.根据对传统退火效应的分析和理论计算,认为激光辐照导致的表面微结构的变化以及薄膜的氧含量和均匀性的提高对La_067Ba_033MnO₃薄膜的磁电性能的改善与优化密切相关. 关键词： 庞磁电阻 激光辐照 脉冲激光溅射沉积     

Magnetotransport properties of nano-constriction array in La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> film

Nano-constriction array in La_0.67Sr_0.33MnO₃ film was fabricated by using ion beam etching masked by a monolayer of packed and ordered array of SiO₂ microspheres. Nano-constrictions of around 50 nm in width were fabricated. The low field magnetoresistance (LFMR) exhibited in the samples were observed to be current dependent and the I-V characteristics of the film were found to be nonlinear. These observations were attributed to the co-existence of the ferromagnetic regions and the nano-constricted region of weakened ferromagnetic coupling where Mn³⁺-O-Mn⁴⁺ bond were distorted due to the ion beam bombardment. The spin polarized bias current would strengthen local ferromagnetic coupling when passing through this nano-constricted regions. This current effect is relatively large comparing to the external magnetic field to the drop of resistance.     

La0.67Pb0.33MnO3的磁性及输运特性

利用固相反应烧结法制备了La067Pb033MnO3单相多晶样品.研究了其结构、磁性及输运特性.结果表明，样品呈菱面相晶体结构，空间群为R3C，居里温度TC(=353K)非常接近TMI(=360K).在居里温度附近，发生铁磁-顺磁转变，导电特性由金属特征向半导体特征过渡.磁电阻在居里点达到极大值.当H=16T时，磁电阻的极大值为145%；当H=08T时，磁电阻的极大值为9%.输运性质表明，TTMI时 关键词： 磁电阻 输运特性 磁极化子 钙钛石     

Effect of the substrate ferroelastic transition on epitaxial La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films grown on LaAlO<Subscript>3</Subscript>

Thin films of La_0.7Sr_0.3MnO₃ were grown by molecular beam epitaxy on (001)LaAlO₃ crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at T_F = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively.     

Improvement of refrigerant capacity of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> single crystal with a few percent Fe doping

Large low-field-induced magnetic entropy changes, ΔS _M, are observed in La_0.67Ca_0.33MnO₃ and La_0.67Ca_0.33Mn_0.96Fe_0.04O₃ single crystals. The peaks of ΔS _M broadened asymmetrically to high temperatures under higher magnetic fields for two materials should be attributed to the first-order magnetic phase transition at T _c. A small amount of iron doping results in an increase in the refrigerant capacity of the material though the magnetic entropy change decreases. The discovery of excellent magnetocaloric features of these single crystals in the low magnetic field can provide some ideas for exploring novel magnetic refrigerants operating under permanent magnet rather than superconducting one as magnetic field source. Supported by the State Key Project of Fundamental Research (Grant No. 2005CB724402), and the National Natural Science Foundation of China (Grant No. 50672126) Contributed by CHENG ZhaoHua     

Response of the electrical resistivity and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films to biaxial compressive mechanical (001) or (110) strains

The behavior of the electrical resistivity and magnetoresistance of 40-to 120-nm-thick La_0.67Ca_0.33MnO₃ films grown on differently oriented lanthanum aluminate substrates was studied. The cell volume in thin (40 nm) La_0.67Ca_0.33MnO₃ films grown coherently on (001)LaAlO₃ was found to be substantially smaller. Mechanical stress relaxation in biaxially strained La_0.67Ca_0.33MnO₃ films is accompanied by an increase in the cell volume. The temperatures at which the electrical resistivity and magnetoresistance in biaxially strained La_0.67Ca_0.33MnO₃ films were maximum can differ by 60–70 K from those observed in bulk single crystals.     

Iron-induced magnetic,transport and magnetoresistance behavior in Nd<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The effects of Fe doping on Mn site in the colossal magnetoresistive film, Nd_0.67Sr_0.33MnO₃ have been studied by preparing the series Nd_0.67Sr_0.33Mn_1-xFe_xO₃ (x=0,0.05 and 0.1). Upon doping, no structural changes have been found. However, the Curie temperature, the associated metal-to-insulator transition temperature and the magnetization decrease drastically with Fe doping. The resistivity in the paramagnetic regime for all the samples follows Emin–Holsteins theory of small polaron. The polaron activation energy, W_p and resistivity coefficient, A increase with Fe doping. This effect may be ascribed to the fact that upon Fe doping, the long-range ferromagnetic order is destroyed and, therefore, W_p is enhanced in the system. As compared to the La-based system, Fe doping has a stronger tendency to destabilize the long-range ferromagnetic order in the Nd-based system. Large MR (as high as 90%) observed in the epitaxial NSMFO film may be attributed to the good lattice-matching between the grown film and substrate. PACS 75.47.Gk; 75.47.Lx; 75.70.-i     

Coercive field enhancement in microstructured (La<Subscript>0.4</Subscript>Pr<Subscript>0.6</Subscript>)<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The perovskite material (La_0.4Pr_0.6)_0.67Ca_0.33MnO₃ (LPCMO) has complex electronic and magnetic behavior based on phase competition between ferromagnetic metallic (FMM) and insulating phases with similar free energies. Experimental evidence has indicated that in-plane stress anisotropy influences these phases and can affect electronic and magnetic properties. Here we investigate the roles that both stress and shape anisotropies may play in controlling the coercive field of the material. LPCMO thin films of various thicknesses (20, 25, and 30 nm) were deposited on (110) NdGaO₃ (NGO) substrates using pulsed laser deposition and the coercive fields were measured. Photolithography was then used to fabricate microstructured arrays of LPCMO on the NGO substrates for each of the films. The coercive fields of these arrays of LPCMO were compared to the behavior of the corresponding unpatterned LPCMO thin films across a range of temperatures. Microstructure arrays for the thicker (25 and 30 nm) films showed a substantial increase in the coercive field after forming the arrays, whereas a thinner film (20 nm) showed almost no change in the coercive field. Stress anisotropy continues to play a dominant role in the behavior of LPCMO thin films and dimensionality of the magnetic domains also influences the results. The films show 2D behavior when film thickness approaches the size of the critical radius for single-to-multidomain transitions. Making thicker films allows for 3D behavior and a role for shape anisotropy to influence the coercive fields.     

Hysteresis of magnetoresistance in granular La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at low temperatures

The magnetoresistance of granular La_0.7Ca_0.3MnO₃ is studied experimentally over wide ranges of temperatures and magnetic fields. The emphasis is on anomalously large hysteresis of magnetoresistance at low temperatures (T = 4.2 K). The observed ρ(H) dependence can be qualitatively explained by spin-dependent tunneling of electrons through the dielectric boundaries of conducting granules characterized by a wide spread in the magnetic-moment magnitudes.     

Elastic properties of La<Subscript>0.82</Subscript>Ca<Subscript>0.18</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La_0.82Ca_0.18MnO₃ single crystal with the Curie temperature T _C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase.     

Electrical resistivity and magnetotransport properties of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films biaxially compressed by the substrate

The structure, electrical resistivity, and magnetoresistance of La_0.67Ba_0.33MnO₃(20 nm) films grown coherently on an La_0.3Sr_0.7Al_0.65Ta_0.35O₃(001) substrate with a lattice misfit of about 1% were studied. The rigid connection of the manganite layer with the bulk substrate brought about the unit cell distortion of the substrate (a _⊥/a _‖ = 1.02) and a decrease in the unit cell volume as compared to that of the corresponding bulk crystals (a _‖ and a _⊥ are the unit cell parameters measured in the substrate plane and along the surface normal, respectively). The temperature T _M ≈ 295 K, at which the electrical resistivity ρ of the (20 nm)La_0.67Ba_0.33MnO₃ films reached a maximum, was 40–45 K lower than that for the corresponding bulk crystals. The negative magnetoresistance (MR ≈ ?0.25 for μ₀ H = 1 T) attained a peak value at T _MR ≈ 270 K. The response of ρ to a magnetic field depended substantially on the angle between the current flow in the film and the direction of the magnetic field.