Low-field Magnetoresistance Value Research Articles

Adjusting the K-doping of La1-xKxMnO3 (0.1 ≤ x ≤ 0.35) films to obtain high TCR and LFMR at room-temperature

Compared with conventional thermal and magnetic sensing materials, perovskite manganese oxide films with a rapid shift from ferromagnetic metals to paramagnetic insulators can be more suitable for the fabrication of uncooled infrared bolometers and magnetic sensors. However, the main challenge is to obtain perovskite manganese oxide films with high temperature coefficient of resistivity (TCR) and low field magnetoresistance (LFMR) at room temperature. In this study, La1-xKxMnO3 (LKMO, 0.10 ≤ x ≤ 0.35) films with different K-dopant amounts were prepared on SrTiO3 substrates using sol–gel spin coating method. A comprehensive analysis of their structure, morphology, and ionic valence enabled one to conclude that varying K-dopant content from 0.10 to 0.30 caused a decrease in Mn-O bond lengths and an increase in Mn-O-Mn bond angles, as well as caused the grain growth and the rise in Mn4+ content in LKMO films. All these changes resulted in a significant alteration in the resistivity of LKMO films, thereby providing the larger TCR and LFMR values at room temperature. According to the results, La0.7K0.3MnO3 with 13.13 % K−1TCR and La0.85K0.15MnO3 with 46.39% LFMR (under an external magnetic field of 1 T) are promising materials for the manufacturing of uncooled infrared bolometers and magnetic sensors, respectively.

Large temperature coefficient of resistivity (TCR) of La1-xCaxMnO3 films prepared by spin-coating method

La1-xAxMnO3 (A = alkaline-earth or alkali metal) perovskites with large temperature coefficient of resistivity (TCR) offer immense opportunity for the application in high-performance infrared bolometer. However, numerous studies have revealed that the manganite films with large TCR values are not easily obtained by various fabrication technologies. Herein, a facile sol-gel spin-coating technique was used to prepare La1-xCaxMnO3 (LCMO, 0.280 ≤ x ≤ 0.375) films. All the samples were highly oriented along the out-of-plane direction of La0.3Sr0.7Al0.65Ta0.35O3 (001) substrates. With the rise in Ca doping amount, the resistivity of the films first decreased, and then, increased. This change can be attributed to the competition between the carrier concentration and lattice distortion. The LCMO films displayed large TCR and low-field magnetoresistance (MR) values, which exceeded those of previously reported thin films. The maximum TCR and low-field MR values were observed at x = 0.330, reaching 30.13% K−1 (266.43 K) and 68.6% (270.08 K), respectively. The La0.67Ca0.33MnO3 film with large TCR and MR values presents potential for the applications in high-sensitivity infrared bolometers and magnetic sensors.

Electrical and magnetic properties of La0.7Ca0.3MnO3 nanoparticle prepared by reactive milling method

La0.7Ca0.3MnO3 nanoparticle in an orthorhombic structure with space group of Pnma was prepared by a combination of the reactive milling method and annealing at 900 °C for 1 h in air. The mean crystal size determined through the powder x-ray diffraction data is 18 ± 2 nm. Based on the dependences of the resistivity and the magnetisation on temperature and applied magnetic field, the electrical-magnetic properties, the magnetoresistance and the magnetocaloric effects have been investigated. It shows La0.7Ca0.3MnO3 nanoparticle undergoes a metal-insulator and a ferromagnetic-paramagnetic phase transitions at T MI = 252 K and T C = 253 K, respectively. The temperature dependences of resistivity of material in the absence and presence of 4 kOe in a range of 30–300 K were fitted to the phase segregation model. The magnetoresistance and the magnetocaloric effects related to the double-exchange interactions taking place around T MI and T C were observed, corresponding to the values of the magnetoresistance MR = 9.29% and the maximum magnetic entropy change ∣ΔS max∣ = 0.18 J/kg.K under an applied magnetic field of 4 kOe. Besides, the low field magnetoresistance value at low temperature region and the change of the specific heat of La0.7Ca0.3MnO3 nanoparticle have also been concerned.

Vertically aligned nanostructure control and tunable low-field magnetoresistance in La0.5Ca0.5MnO3 single-phase thin films manipulated by a high magnetic field

Vertically aligned nanostructured (VAN) epitaxial La0.5Ca0.5MnO3 (LCMO) single-phase thin films have been achieved on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) [LSAT (001)] substrates under high magnetic fields applied in pulsed laser deposition processing. Low-field magnetoresistance (LFMR) in the LCMO VAN films can be effectively manipulated through varying the high magnetic field strength. The tunability of VAN on the electrical transport properties is dependent on control of the high magnetic field on the microstructures, including the geometrical arrangement, vertical interfaces, and vertical grain boundaries (GBs). An LFMR value as high as 45% at 150 K and 1 T has been achieved in an LCMO VAN film grown at 10 T, and its LFMR values are larger than 25% at 127–200 K and 1 T. The tunable and enhanced LFMR in the LCMO VAN films over a wide temperature range can be attributed to the increase in vertical interfaces and GB density with the increasing high magnetic field, which are highly related to the spin-polarized tunneling effect. Applying a high magnetic field in film deposition to control the microstructures of VAN single-phase films is a feasible route to achieve tunable and desirable physical properties.

Influence of BaTiO3 on Magnetic and Transport Properties of La0.7Sr0.3MnO3-BaTiO3 Nanocomposite

La0.7Sr0.3MnO3 (LSMO)-BaTiO3 (BTO) nanocomposites have been synthesized by adding different amounts of BTO using the sol-gel method. The structural analysis confirms that both phases coexist in the composite samples with the rhombohedral and tetragonal structure for LSMO and BTO, respectively. The magnetization measurement shows a decrease in magnetization with the increase in BTO concentration and a signature of the spin glass-like characteristics of the nanocomposite. The dielectric behavior indicates the existence of the magneto-electric coupling between LSMO and BTO phase. The measurement of transport properties shows that resistivity increases with BTO concentration and metal-insulator (M-I) transition temperature shifts towards lower temperature. The low-temperature (T < 50 K) resistivity behavior of the samples is analyzed by using the Kondo-like transport model. The results show the spin-disordered scattering increases with the increase in the amount of BTO in LSMO phase. The low-field magnetoresistance (LFMR) of the nanocomposite is improved as compared to the pure LSMO. The highest value of LFMR achieved for LSMO-BTO composite at 10 K is 35.5% with a 1 T magnetic field.

Magneto-transport properties of La0.7Sr0.3MnO3–manganese oxide composite films on YSZ substrates prepared using aerosol deposition

We report the magneto-transport properties of pure La0.7Sr0.3MnO3 (LSMO) and 40mol% LSMO–Mn3O4 (LSMM) composite films on YSZ (yttrium stabilized zirconia) substrates prepared using aerosol deposition (AD). As-deposited films via AD were post-annealed at 1100°C and 1200°C for 2h in air. While the as-deposited films commonly exhibited a poor low-field magnetoresistance (LFMR) property with reduced Curie temperatures (Tc), post-annealed films exhibited a very good LFMR property with high Tc values ranging from 355 to 370K. The highest LFMR value of 1.2% in 0.5kOe and an unprecedentedly high (dMR/dH)max value of 158.8%kOe−1 were obtainable at 300K from the LSMM composite film annealed at 1200°C. Compared with a pure LSMO film, the LSMM composite films possess higher LFMR values with higher (dMR/dH)max values, which is primarily attributed to a greatly reduced magnetically disordered region near the LSMO grain boundary because of the chemically compatible Mn3O4 phase.

Large, Temperature‐Tunable Low‐Field Magnetoresistance in La0.7Sr0.3MnO3:NiO Nanocomposite Films Modulated by Microstructures

Magnetic properties and low‐field magnetoresistance (LFMR) in La0.7Sr0.3MnO3 (LSMO):NiO nanocomposite films grown on SrTiO3 (001) substrates, which are shown to be tunable with different microstructures, are investigated. The LSMO:NiO nanocomposite films with NiO volume ratio of 50% have a checkerboard‐like structure and show a large LFMR in a temperature range from 200 to 300 K (≈17% at 250 K with a magnetic field of 1 T). As the NiO volume ratio is increased to 70%, a nano‐columnar structure formed in the films. Their LFMR is significantly enhanced at a wide temperature range of 10–210 K. The highest value of LFMR with 41% is achieved at 10 K in a magnetic field of 1 T. The enhanced LFMR can be considered to result from the electron scattering at the ferromagnetic LSMO/NiO interfaces and magnetic tunnel junctions (MTJs) of LSMO/NiO/LSMO at the nanometer scale. These results demonstrate that large and tunable LFMR from low temperature to room temperature can be realized by controlling the microstructures in the epitaxial La0.7Sr0.3MnO3:NiO nano­composite thin films, which will be expected to be applied in the devices using for a wide temperature range.

Study of ferromagnetic-metal type Sr2FeMoO6 + xAg (x = 0–10 wt%) composites

Ferromagnetic-metal type Sr2FeMoO6+xAg (x=0, 1, 2, 3, 5 and 10wt%) composites were synthesized by standard solid-state reaction method and studied their structural, magnetic and magneto-transport properties. The X-ray diffraction (XRD) pattern shows the XRD peaks corresponding to both Sr2FeMoO6 (SFMO) and metallic Ag phases, which indicates the successful formation of composites. High resolution field emission scanning electron microscope (FESEM) with energy dispersive X-ray spectroscopy (EDS) analysis also reveals the coexistence of both the phases in composite. It is also clear from FESEM images that the smaller size Ag particles reside over the grain surface/boundaries of SFMO grains. Dramatic decrease in electrical resistivity with the addition of Ag in the composites has been observed in the entire measured temperature range from 300–20K. Moreover, a systematic decrease in low field magnetoresistance (LFMR) is also observed as we increased the Ag content. This decrement in LFMR values of these composites is mainly due to the presence of metallic Ag at the grain surface/boundaries of SFMO, which suppresses the spin polarized tunneling and finally reduces the magnetoresistance.

The effect of MgO doping on the structural, magnetic, and magnetotransport properties of La0.8Sr0.2MnO3 manganite

Abstract The effect of MgO doping on the structural, magnetic, and magnetotransport properties of La0.8Sr0.2MnO3 (LSMO)/x MgO has been investigated. All samples were prepared by a solid-state reaction method. Alternating current susceptibility measurements for LSMO/x MgO samples show that the Curie transition temperature (T c) and magnetization decrease with the increase of MgO concentration. The rate of the decrease of T c at higher doping level is very fast. Also, samples with low doping level (x ≤ 2) show insulator-metal transition, but the transition temperature decreases and resistivity increases with the amount of MgO. It is observed that there is no insulator-metal transition at higher MgO doping level (x ≥ 3). Also, the results show that the value of low-field magnetoresistance decreases with the increase of MgO doping level. It seems that due to the higher sintering temperature and almost the same ionic radii of Mg2+ and Mn3+, Mg2+ mostly replaced Mn3+ and weakened the double-exchange interaction, and consequently, T c and magnetoresistance decrease and resistivity increases with MgO doping.

Influence of Fe segregation at grain boundaries on the magnetoresistance of Sr2Fe1+δMoO6 polycrystals

In this paper, we report the influence of Fe impurities on the magnetoresistance (MR) of double perovskite Sr2Fe1+δMoO6 (0.00 ≤ δ ≤ 0.20) system. The significant Fe impurities have been created by two different approaches: one by adding the extra amount of Fe2O3 at starting of the synthesis and other by sintering the pristine Sr2FeMoO6 sample at high temperature (1300 °C) in high reducing environment (∼8% H2 + 92% Ar). A remarkable 11% magnetoresistance at room temperature under the presence of low magnetic field (0.72 T) has been observed in the pristine sample. The achieved high low field magnetoresistance value in the sample may be due to the optimized synthesis conditions to get better inter-granular tunneling through grain boundaries. However, the presence of Fe impurity results into the sharp reduction in magnetoresistance because of reduced spin polarized tunneling. The loss of insulating nature of the grain boundaries and the inelastic scattering of the charge carriers through the metallic impurities of iron at grain boundaries may be the reason of reduced spin polarized tunneling.

Enhanced low field magnetoresistance in La0.7Sr0.3MnO3–La2O3 composites

A significant enhancement of low-field magnetoresistance (LFMR) was obtainable from the polycrystalline composites between La0.7Sr0.3MnO3 (LSMO) and La2O3 phases. For this study, the magnetic and magnetotransport properties of samples with various compositions of (1−x)LSMO–xLa2O3 (x=0, 0.025, 0.05, 0.075, 0.1, 0.2, 0.3) were systematically investigated. Compared with pure LSMO (TC=362K), all composites exhibited a slightly depressed TC values of 355–358K, implying that the magnetic ordering of LSMO is insignificantly affected by chemically compatible La2O3. While the LFMR value at 300K in 500Oe was monotonously increased from 1.36 to 1.67% with increasing x from 0 to 0.075, it was abruptly increased to the maximum value of 2.22% for x=0.1 with only one order increase in its resistivity relative to x=0.075, and further increasing x from 0.1 to 0.3 resulted in a linear decrease to 1.8% with almost no variation in resistivity. This peculiar behavior, different from normal metallic percolation threshold effect, is closely related to the microstructure variation. Remarkably improved LFMR properties of the LSMO–La2O3 composite samples are attributed to high grain boundary areal density and sharpening of a disordered LSMO grain boundary region acting as more effective spin-dependent scattering centers.

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 (La0.75Sr0.25)1.05Mn0.95O3(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 (Tan) and film thickness. High Tan 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 Tan 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.

Tilted Aligned Epitaxial La0.7Sr0.3MnO3 Nanocolumnar Films with Enhanced Low-Field Magnetoresistance by Pulsed Laser Oblique-Angle Deposition

Single-phase epitaxial La0.7Sr0.3MnO3 (LSMO) thin films with significantly enhanced low-field magnetoresistance (LFMR) properties (23% at 20 K) are demonstrated in this work. The LSMO films on SrTiO3 (001) substrates exhibit tilted and well-aligned nanocolumn structure achieved by pulsed laser oblique-angle deposition (PLOAD) followed by subsequent postannealing. The tilted aligned nanocolumnar (TAN) arrays have been achieved at relative high deposition angles (≥30o) and low deposition temperatures (≤450 °C). More attractively, the tilted grain boundaries (GBs) can be systematically manipulated by the postannealing process and so can the LFMR values of the LSMO TAN films. These results demonstrate that the tilted nanocolumnar films achieved by PLOAD and the GB tailoring by postannealing may provide a new approach to control and manipulate the magnetotransport properties of single-phase manganite perovskite films for device applications that require large LFMR effects, high epitaxial quality, and low resistivity.

Increased low field magnetoresistance in electron doped system Sr0.4Ba1.6−xLaxFeMoO6

Magnetotransport properties of electron doped polycrystalline system Sr0.4Ba1.6−xLaxFeMoO6 are presented in this paper. We have observed increased low field magnetoresistance values with significant Curie temperature increase in the electron doped system Sr0.4Ba1.6−xLaxFeMoO6. The low field magnetoresistance value (at 1000 Oe) in 20% La3+ doped sample is observed to be 2.17% at 300 K. At 0.64 T and 80 K, the magnetoresistance change measured in this sample is 21.4%. This sample also showed 50 K increase in Curie temperature over the pristine sample. The increased low field magnetoresistance values are associated with modified grain boundary barriers of the system. The results confirm the fact that the modification of grain boundary barriers has enough potential to possess high low field magnetoresistance values even in the systems with lower spin polarization values. The role of grain to grain connectivity is observed to be dominantly determining the low field magnetoresistance values in grain to grain tunnel type magnetoresistance.

Effect of Divalent Ions (A = Ca, Ba and Sr) Substitution in La-A-Mn-O Manganite on Structural, Magnetic and Electrical Transport Properties

Problem statement: Microstructure of the grain will influence the properties of a polycrystalline manganites when different dopant is introduced. In this work, an effort had been made to investigate the influence of Ca, Ba and Sr substitution in La site. Approach: Polycrystalline manganites compound of La0.67A0.33MnO3 where A = Ba, Sr and Ca had been prepared via conventional solid-state reaction method. The structure, microstructure, magnetic and electrical properties had been investigated using XRD, SEM, VSM and four-point probe techniques. Results: XRD spectrums showed that LBMO and LCMO were in single-phase orthorhombic structure whereas LSMO was rhombohedral structure. Scanning electron micrographs showed that LSMO had smaller average grain size as compared to LBMO. However, for LCMO, there was no clear grain boundary that can be observed and all the grains were well connected. The difference in the microstructure image might be due to the variance A-site cation that differs in grain growth. The Curie temperature, Tc of LBMO and LSMO was 343.0 and 363.5 K, respectively. But the Tc for LCMO was lower then 300 K. The Tp for LCMO was 215 K while it was near or above 300K for LBMO and LSMO. All samples showed extrinsic Magnetoresistance (MR) effect. Higher MR was observed well below Tc or Tp where MR value increased monotonically with the decrease of temperature. This was associated with the different grain and grain boundary formation when different cation was substituted. LCMO displayed the highest low-field MR value (-15.82% at 0.2 T, 90 K) and high-field MR (-25.60% at 1 T, 300 K). Conclusion: Hence, electrical and magnetic transition temperature (Tp and Tc) were affected by the average A-site cation radius.

Structural, magnetic and magnetotransport properties of La0.8Sr0.2MnO3/xLaMnO3 composites

Abstract The effect of LMO doping on the structure, magnetic and magnetotransport properties of La 0.8 Sr 0.2 MnO 3 (LSMO)/ x LaMnO 3 (LMO) has been investigated. Two types of LSMO/ x LMO composites, named as SL x (low temperature sintered samples) and SH x (high temperature sintered samples) samples, were prepared by different sintering temperature and solid-state reaction method. The presence of LMO at the grain boundaries increases the disordered states at the surface of the grains and therefore the magnetization and transition temperature decrease by increasing the amount of LMO doping level. Results show that the rate of decreasing of transition temperature is much more for high temperaure sintered samples. Also the resistivity of samples increases by the increase of LMO doping level. Results also show that the LMO doping has an effect on a low field magnetoresistance (LFMR). The value of LFMR increases for low doping level of 0 ≤ x ≤ 15, for SL x samples and 0 ≤ x ≤ 10 for SH x samples. Also LFMR decreases at high doping level. The spin dependent tunneling and scattering at the interfaces of the grain boundaries are responsible for the increase of LFMR at low doping level, while reduction of LFMR at high doping level may result from the grain boundary becoming too thick for electron tunneling.

Fabrication of polycrystalline La0.67(Sr1−x Cd x )0.33MnO3 thin films on Si(100) substrates by sol–gel process

Polycrystalline La0.67 (Sr1-xCdx)0.33MnO3 (LSCMO x = 0, 1/8, 2/8, 3/8, 4/8) films were fabricated on Si(100) single crystal substrates by the sol–gel and the rapid annealing process. According to the field-emission scan electron microscopy (FE-SEM) observation and X-ray (XRD) diffraction studies, all the samples are single phase. We also prepared a series of samples by fixing the ratio of La at 0.67. It is shown that with increase of the doping amount x in La0.67(Sr1-xCdx)0.33MnO3, the average A-site cation radius 〈A〉 decreases, the temperature of metal-insulator transition (TMI) decreases monotonically, ρ and magnetoresistance (MR) values increase dramatically, which can be explained by the lattice effects. The enhanced MR effect at low temperature for the LSCMO film is attributed to the suppression of the spin dependent scattering of polarized electrons at the grain boundaries. The low-field magnetoresistance values under 0.3 T are observed to be 2.48% (x = 0) at 300 K and 37.5% (x = 4/8) at 85 K, respectively.

Enhanced magnetoresistance of La2∕3Ca1∕3MnO3∕CeO2 nanocystalline composites synthesized by polymer-network gel method

Using a dispersing particle polymer-network gel method, we prepared xLa2∕3Ca1∕3MnO3∕(1−x)CeO2 nanocystalline composites, where the average grain sizes of La2∕3Ca1∕3MnO3 and CeO2 are about 30nm and less than 10nm, respectively. The magnetoresistance of the sintered composites is explored as a function of the ratio of metal/insulator, temperature, and magnetic field. The system exhibits metallic percolation threshold at x=xp∼35%, around which the high field magnetoresistance has a maximum (∼70%) at 50K and 5T, and the low field magnetoresistance value is close to 16% at 5K and 0.05T.

Tunneling characteristics and resistivity behavior ofLa0.6Pb0.4MnO3grain boundaries

The temperature dependences of resistivity, magnetoresistance (MR), and current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of grain boundaries (GB) in ${\mathrm{La}}_{0.6}{\mathrm{Pb}}_{0.4}{\mathrm{MnO}}_{3}$ thin films prepared using bicrystal ${\mathrm{SrTiO}}_{3}$ substrates have been studied. Comparison of the resistances of the bridges, with and without GB, shows a contribution from disordered region near the grain boundary. This has been taken into account for determination of the GB characteristics. Analysis of the $I\text{\ensuremath{-}}V$ characteristics shows that the transport through the grain boundary is dominated by multistep inelastic tunneling as described by the Glazman and Matveev model. In addition a small contribution of elastic tunneling in agreement with the Simmons model has been observed. Methodology for direct determination of the contribution responsible for magnetoresistance has been developed and the results suggest that only elastic tunneling contributes to magnetoresistance. Small value of elastic tunneling contribution to total current is found to be responsible for small value of low field magnetoresistance.

Low temperature synthesis, and magnetic and magnetotransport properties of a(La1−xLux)0.67Ca0.33MnO3 (0≤ x ≤ 0.12) system

We have been able to synthesizeLu3+ substituted La0.67Ca0.33MnO3 (LCMO) by an auto-combustion method. Synthesis of this compound isnot successful by conventional ceramic or other chemical methods. Themagnetic and electrical transport properties of the Lu substituted LCMO[(La1−xLux)0.67Ca0.33MnO3 ()] system have been investigated and compared with those of theY3+,Pr3+,Dy3+ andTb3+ substitutedLCMO systems. All the compounds show a ferromagnetic metal to paramagnetic insulator transition atTC. The tolerance factorreduces from 0.917 for x = 0 to 0.909 for x = 0.12, and for this range all are ferromagnetic metals, indicating the dominance of the couplingbetween spins due to double exchange over the antiferromagnetic superexchange interaction.The transition temperatures and magnetization decrease as the Lu concentration increases.This is satisfactorily accounted for on the basis of a transition from ferromagnetic atx = 0 to cantedspin order for x>0. All the samples show a higher magnitude of MR compared to that in pure LCMO at80 kOe field in the temperature range 5–320 K. A fairly high value of low fieldmagnetoresistance (LFMR) of about 30% is obtained in all the samples at a field less than5 kOe.