Stress and defect induced enhanced low field magnetoresistance and dielectric constant in La0.7Sr0.3MnO3 thin films

Colossal magnetoresistive manganite La_0.7Sr_0.3MnO₃ (LSMO) films were prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. Characterizations of their surface morphologies, structural, magnetic and magneto-transport properties show that films on MgO single crystal substrates contain higher amount of structural defects compared to those on SrTiO₃ (STO) and NdGaO₃ (NGO) substrates. Low deposition rate and thicker films give rise to polycrystallinity and grain boundaries. The films on MgO substrate showed a broad paramagnetic (PM) to ferromagnetic (FM) transition accompanied with metal-insulator transition (MIT) much below their Curie temperature (T_C) indicating growth of strained structures due to large lattice mismatch (9%) between the substrate and the film. The deposited films on STO and NGO show least effect of substrate induced strain exhibiting sharper PM-FM transition and metallic behavior below T_C. The magnetoresistance (MR) measured with 300 mT field clearly shows two contributions, one due to grain boundary tunneling and the other due to colossal MR effect. The highest low field MR effect of 17% was achieved for the film on MgO with the highest thickness and surface roughness indicating the presence of grain boundary related defects. Also a high dielectric constant was observed for the same film at room temperature up to 100 kHz frequency. Coexistence of defect induced large low-field MR and abnormally high dielectric constant can give rise to different exciting applications.     

Effects of ferrites on magnetic transport properties in La2/3Sr1/3MnO3/ferrites composites

Effects of soft-magnetic MnZn ferrite (Mn0.5Zn0.5Fe2O4, MZF) and hard-magnetic Ba ferrite (BaO·6Fe2O3, BaM) on the structure and magnetic transport properties of [La2/3Sr1/3MnO3] (LSMO)/(x) [ferrites] (ferrites=MZF, BaM) composites have been investigated. It was found that the inclusion of MZF phase reduces magnetization and ferromagnetic-paramagnetic transition temperature (Tc) of the composites. With increasing the content of the dopants, the high-temperature magnetoresistance (MR) decreases, whereas low-...     

Synthesis of Nd0.7Sr0.3MnO3 ceramics by high-energy ball milling

Strontium doped perovskite-type manganite Nd_0.7Sr_0.3MnO₃ is synthesized by high-energy ball milling. Nd_0.7Sr_0.3MnO₃ single phase with perovskite structure is formed completely after milling 4 h and the perovskite phase decreases gradually and changes into amorphous state with increasing ball milling time up to 12 h. The interesting point in this work is that the Nd_0.7Sr_0.3MnO₃ perovskite can be formed again from the amorphous with subsequent 6 h ball milling. In addition, the re-obtained perovskite seems to be more stable than Nd_0.7Sr_0.3MnO₃ compound prepared using solid-state reaction method.     

Giant negative magnetoresistance in the vicinity of the ferromagnetic phase transition TC in Sm0.55Sr0.45MnO3

The temperature dependence of the resistance in Sm_0.55Sr_0.45MnO₃ at zero magnetic field and its first derivative served for the determination of T_C which turned out to be equal to 129.1 K. Then, the resistance measurements were carried out as functions of the stationary magnetic fields up to 14 T at the following temperatures: 118.6, 124.6, 133.6 and 139.7 K. Two of them are below and two of them are above T_C. It turned out that at all the four temperatures the resistance reveals a hysteresis. It turned also out that the closer the temperature to T_C the greater this hysteresis. At all the four temperatures the magnetoresistance is negative, its absolute value depends on the magnetic field and reaches 100% at 12 T. At lower fields the absolute value of the negative magnetoresistance increases monotonically with the decrease of temperature starting from 139.7 K. On the other hand, a maximum observed on all the primary and secondary curves above T_C moves with the decrease of temperature towards the lower magnetic fields and disappears on all the curves below T_C. On the secondary curves above T_C this maximum is flat.     

A-site-disorder-dependent magnetocaloric properties in the mono-valent-metal doped La0.7Ca0.3MnO3 manganites

The influence of mono-valence-metal (Li, Na, and K) doping effect on the structural, resistivity, magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ polycrystalline samples is studied for a fixed (5% at Ca site) dopant concentration. All the samples crystallize in orthorhombic structure and the lattice parameters increase continuously as the dopant atoms changes from Li to Na and then K. Paramagnetic-ferromagnetic phase transition at T_C and insulator-metal phase transition at T_p are observed for all studied samples. The transition temperature decreases as Ca atoms is replaced by Li, while the transition temperature shifts to higher values as Ca is substituted by Na or K. In addition, the maximum magnetic entropy change of the K-doped sample is much smaller than that of the free- and Na-doped samples. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of A-site average ionic radius 〈r_A〉 and A-site-cation mismatch σ².     

Magnetic and electrical transport properties of nanostructured La0.67Ca0.33MnO3 networks

Contiguously nanostructured networks of La_0.67Ca_0.33MnO₃ (LCMO) are fabricated successfully by pulsed electron deposition (PED) onto the surface of porous anodic alumina oxide (AAO). The Curie temperature (T_C) is about 250 K. The metal–insulator transition temperature (T_p) is about 145 K without the magnetic field. The magnetoresistance can reach 84% near the peak temperature, which suggesting a strong magnetic correlated electronic transportation process. Zero field cooling (ZFC) and field cooling (FC) are split below the paramagnetic–ferromagnetic transition. The ZFC curves exhibit a typical blocking process. The observed spin glass and weak localization phenomena are due to the size effect, and it is found that the resistance dependent temperature curve above metal–insulator transition temperature (T_p) is more suitable to small polaron hopping (SPH) model.     

On the synthesis and characterization of a new manganite type CMR material: La0.7Hg0.3MnO3

A new manganite type CMR material, La_0.7Hg_0.3MnO₃ has been successfully synthesized and has been found to exhibit magnetoresistance (≈9%) at low fields (≈1.5 kG). The synthesis has been carried out through a solid state reaction route consisting of the formation of La_0.7MnO₃ followed by diffusion of Hg leading to La_0.7Hg_0.3MnO₃. The as grown samples are polycrystalline and correspond to an orthorhombic unit cell with the lattice parameters; a=5.5183 Å, b=5.6383 Å and c=7.5368 Å. The typical grain size as revealed by scanning electron microscopy is in the range of 0.5–2 μm. The ρ–T behaviour shows a peak at T_IM=227 K. The ρ–T behaviour above this temperature corresponds to that of an insulator and below this to that of a metal. The ρ–T behaviour remains unaltered when a magnetic field (H_dc=1.5 kG) is applied. However, with this magnetic field a drop in the resistivity is observed up to 77 K. At room temperature the magnetoresistance ratio (MRR) is too small but it steadily increases as the temperature is decreased. Thus, MRRs at 227.13 and 77 K are 3.41 and 9.05%, respectively, in an applied field of H_dc=1.5 kG. At a given temperature the variation in MRR with field H_dc is rapid at lower field values (H_dc<1.2 kG) and scales linearly for higher field values (H_dc>1.2 kG). It may be mentioned that the present work on the synthesis and magnetoresistance behaviour of La_0.7Hg_0.3MnO₃, is the first of its type.     

Coexistence of superconductivity and ferromagnetism in La1.85Sr0.15CuO4-La2/3Sr1/3MnO3 matrix composites

In order to clarify the interaction between superconductivity and magnetism, a series of (La_1.85Sr_0.15CuO₄)_1−x (La_2/3Sr_1/3MnO₃)_x matrix composites (x = 0-0.2, mole fraction) was successfully prepared by solid-state reaction method. Based on the electrical transport measurements, it is found that the superconductivity is gradually suppressed as increasing the content of La_2/3Sr_1/3MnO₃ (LSMO) manganites and that the superconductivity still exists in the composites even though plenty of LSMO is introduced into La_1.85Sr_0.15CuO₄ (LSCO) superconducting cuprate. At the same time, the results of the magnetic measurements also demonstrate the coexistence between superconductivity and ferromagnetism when the CuO₂ planes are intact as follows from the results of X-ray diffraction (XRD). In the whole, the present experiments show that the ferromagnetism in the microscale does not destroy superconductivity for LSCO cuprate in this kind of the matrix composites, and the intercalation of LSMO may lead to an electronic phase separation in LSCO with the hole rich and/or hole poor regions.     

Effect of frozen spin on the magnetocaloric property of La0.7Ca0.3CoO3 polycrystalline and single crystal samples

Polycrystalline (PC) and single crystalline (SC) samples of La_0.7Ca_0.3CoO₃ (LCCO) with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We conducted isothermal magnetization measurements of the PC and SC samples at temperatures from 2.8 K to 140 K, and evaluated the magnetic entropy change (−ΔS_M) under zero field cooling (ZFC) and field cooling (FC) to 2.8 K. An interesting result has been obtained, where the −ΔS_M-T curves in the low temperature range show totally different features between the ZFC and FC cooling procedures. The −ΔS_M shows a large inverse irreversibility value for the ZFC process, while the −ΔS_M also shows a normal positive value, but one that is slightly larger at 2.8 K for the FC process for both samples. We also present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on DC magnetization, AC susceptibility, and exchange-bias. These findings suggest that complex structural phases, including ferromagnetic and spin-glass/cluster-spin-glass (SG/CSG)) states and their transitions, exist in PC samples, while there is a much simpler magnetic phase regime in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect.     

Post annealing effect on transport properties of La0.67Ca0.33MnO3 films grown on vicinal cut substrates

La_0.67Ca_0.33MnO₃ thin films have been grown on 10°, 15°, and 20° vicinal cut SrTiO₃ (1 0 0) substrates by pulse laser deposition. The single phase and the least textured growth have been studied by X-ray diffraction analysis. The post annealing effect with high temperature and high oxygen pressure on the transport properties of films has been investigated by resistance versus temperature measurements. Films with post annealing show large enhancement of metal-insulator transition temperature T_p about 20-30 K towards higher temperature and obvious decrease of resistance, which is attributed to the refilling of oxygen, the change of Mn-O-Mn angle and the improvement of crystallinity by the post annealing effect. Specially, film on 20° vicinal cut substrate exhibits the biggest range gap of peak resistance drop, which may originate from more defects caused by steps at this tilt angle and many of these defects are removed after post annealing.     

Effect of potassium doping on the structural, magnetic and magnetocaloric properties of La0.7Sr0.3−xKxMnO3 perovskite manganites

We investigate the effect of potassium doping on the structural, magnetic and magnetocaloric properties of La_0.7Sr_0.3−xK_xMnO₃ (x = 0.05, 0.1, 0.15 and 0.2) powder samples. Our polycrystalline compounds were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted rhombohedral system with space group. With increasing potassium content, the unit cell volume exhibits a broad maximum around x = 0.15. Magnetization measurements versus temperature showed that all our samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to decrease from 365 K for x = 0 to 328 K for x = 0.2 as well as the saturated magnetization M_sp which shifts from 3.68 μ_B/Mn for x = 0 to 3.05 μ_B/Mn for x = 0.2. The critical exponent γ defined as M_sp (T) = M_sp(0)[1−(T/T_C)]^γ is found to remain almost constant and equal to 0.33 for all our samples. The maximum of magnetic entropy changes |ΔS_max| of La_0.7Sr_0.3−xK_xMnO₃ for x = 0.05 and 0.15 is found to be respectively, 1.37 and 1.2 J kg⁻¹ K⁻¹ under a magnetic field change of 1 T.     

Influence of A-site cation size-disorder on structural, magnetic and magnetocaloric properties of La0.7Ca0.3−xKxMnO3 compounds

The structural and magnetic properties of perovskite oxides La_0.7Ca_0.3−xK_xMnO₃ (0 ≤ x ≤ 0.15) have been investigated to explore the influence of the A-site cation size-disorder (σ²). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x ≤ 0.075, the samples crystallize in the orthorhombic structure, while for x ≥ 0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ₀H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ₀H values. When increasing μ₀H, the structural distortion decreases and for a high applied magnetic field, the M (μ₀H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|ΔS_M|  5 J/kg K) is obtained in all samples at Curie temperatures between 270 and 280 K for an applied magnetic field of 3 T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.     

Deterioration and film forming abilities of the slurries used for preparing La0.8Sr0.2MnO3 films through composite sol-gel method

The slurries, used for preparing La_1−xSr_xMnO₃ (LSM) films through composite sol-gel method with citric acid, were made up of LSM particles and precursor sol with different pH values. The reasons and key factors for the deterioration of the acidic slurry were analyzed by XRD, SEM and TG/DSC. Furthermore, the LSM films were fabricated with different slurries for studying their film forming abilities. It has been found that the deterioration of the acidic slurry is determined by the abundant H⁺ ions, generated from ionization of the citric acid. Discoloration of the alkaline sol and slurry is considered as a result of complicated complexes of Mn²⁺ ions, which have a good effect on the film forming ability of the alkaline slurry, together with the complexes of La³⁺ and Sr²⁺ ions. The alkaline slurry has better long-term stability than the acidic slurry, and the films made from it are more homogeneous, thicker and have lower sheet resistance.     

Critical behavior and the absence of glass state in ferromagnetic La0.7Ca0.3CoO3 nanowires

We report a systematic study of the magnetic and thermodynamic properties of uniform La_0.7Ca_0.3CoO₃ nanowires prepared by a hydrothermal method, and compare them with the bulk counterpart. The nanowires, ∼59 nm in diameter, are single crystalline as revealed by electron microscopy. With decreasing temperature, the bulk La_0.7Ca_0.3CoO₃ shows a ferromagnetism followed by a reentrant glass state, whereas the nanowires show only a purely ferromagnetic ground state. The thermopower and specific heat results indicate that there exist enhanced spin and/or orbital fluctuations in the nanowires that could be responsible for the absence of the glass state. Critical behavior analysis shows that the nanowires fall into a three-dimensional Heisenberg ferromagnet class.     

High tunability of pulsed laser deposited Ba0.7Sr0.3TiO3 thin films on perovskite oxide electrode

Ferroelectric thin films such as BST, PZT and PLZT are extensively being studied for the fabrication of DRAMS since they have high dielectric constant. The large and reversible remnant polarization of these materials makes it attractive for nonvolatile ferroelectric RAM application. In this paper we report the characterization of Ba_0.7Sr_0.3TiO₃ (BST) thin films grown by pulsed laser ablation on oxide electrodes. The structural and electrical properties of the fabricated devices were studied. Growth of crystalline BST films was observed on La_0.5Sr_0.5CoO₃ (LSCO) thin film electrodes at relatively low substrate temperature compared to BST grown on PtSi substrates. Electrical characterization was carried out by fabricating PtSi/LSCO/BST/LSCO heterostructures. The leakage current of the heterostructure is studied and a band structure is modeled based on the transport properties of the heterostructure. The dielectric constant of the BST film is found to be 630 at 100 kHz with a loss tangent of 0.04. The capacitance voltage characteristics show high tunability for BST thin films.     

Fabrication and characterization of ceramic thin films by RF magnetron sputtering using Zn-enriched (Ba0.3Sr0.7)(Zn1/3Nb2/3)O3 as target

Dielectric ceramic thin film was fabricated on SiO₂ (1 1 0) substrates by radio frequency (RF) magnetron sputtering method using a Zn-enriched (Ba_0.3Sr_0.7)(Zn_1/3Nb_2/3)O₃ target. The microstructure, components, and morphological properties of the thin films were characterized thoroughly. The results reveal that the main phases of the thin films are Ba_xSr_1−xNb₂O₆, which are of different compositions from that of the ceramic target due to Zn loss. The thin films are polycrystalline and of dense structure with uniform grain sizes and well-defined grain boundaries.     

Low field magnetoresistance properties of (La0.75Sr0.25)1.05Mn0.95O3 polycrystalline thin films on a-SiO2/Si substrates prepared by ex-situ solid phase crystallization

We report the low field magnetoresistance (LFMR) properties of (La_0.75Sr_0.25)_1.05Mn_0.95O₃(LSMO) films on a-SiO₂/Si substrates, prepared by ex-situ solid phase crystallization of amorphous films deposited by dc-magnetron sputtering at room temperature. The average grain size of the LSMO films was gradually increased with increasing annealing temperature (T _an ) and film thickness. High T _an also caused the growth of an amorphous inter-diffusion layer between a-SiO₂ and LSMO. The highest LFMR values of 16 and 1.0 % were achieved at 100 K, 1.2 kOe and 300 K, 0.5 kOe, respectively, from an LSMO film of 200 nm thickness annealed at 900 °C. In accordance with a modified brick layer model, grain boundary areal resistance gradually increased with increasing T _an and decreasing film thickness due to the penetration of the amorphous inter-diffused phase into the LSMO grain boundary. Improved LFMR values are attributable to modification of the LSMO grain boundary into a more effective spin-dependent scattering center.     

Thickness-dependence of the B2-B19 martensitic transformation in nanoscale shape memory alloy thin films: Zero-hysteresis in 75 nm thick Ti51Ni38Cu11 thin films

The influence of film thickness on the B2-B19 martensitic transformation properties of nanoscale Ti₅₁Ni₃₈Cu₁₁ thin films with thicknesses ranging from 750 to 50 nm is reported. For these films an unexpected behavior of the phase transformation temperatures was observed: A_f and O_s initially decrease with decreasing film thickness but increase sharply again for thicknesses <100 nm. The phase transformation temperatures and thermal hysteresis width range from 58 to 35 °C (A_f) and 14 to ∼0 K, respectively. For the first time we can show that substrate-attached Ti-Ni-Cu thin films as thin as 50 nm show reversible B2-B19 phase transformations. Furthermore, it is shown that with decreasing film thickness a change in the tetragonality of the B19 martensite phase occurs. This leads to fulfilling the so-called λ₂ criterion, causing a vanishing hysteresis for a film thickness of 75 nm.