Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Electroresistive and magnetoresistive properties of Nd0.7Sr0.3MnO3 after quenching under pressure of 9 GPa

Polycrystalline Nd_0.7Sr_0.3MnO₃ was quenched from 1300 K to 300 K and 80 K after it had been subjected to a high quasihydrostatic pressure of 9 GPa. Such high pressure and high temperature treatment (HPT) results in significant changes of the crystallochemical parameters—Mn-O lengths and Mn-O-Mn angles within unchanged lattice symmetry of the Pnma-type. A strong increase of the resistivity and a large decrease of the FM-PI transition temperature were detected for the Nd_0.7Sr_0.3MnO₃ HPT treated samples. The intrinsic characteristic T_MI(T_C) (T_MI is the metal-insulator and T_C is the ferromagnetic-paramagnetic transition temperature) correlates with the change of the Mn-O(1)-Mn angle, which is consistent with the double exchange model of the ferromagnetic metallic state in manganites. Remarkable electroresistive (ER) and magnetoresistive (MR) effects appear after HPT treatment, which were not present in the starting Nd_0.7Sr_0.3MnO₃ sample. The structure sensitive properties such as resistivity, MR and ER effects correlate with the change of the nanograin sizes after HPT treatment. Nonlinear current-voltage characteristics showing a hysteresis appear for HPT treated samples at low temperatures. The transport in granular Nd_0.7Sr_0.3MnO₃ samples is likely defined by spin-dependent scattering of charge carriers inside the ferromagnetic metallic grains with embedded small charged isolating islands and by jumping over charged insulating barriers at the intergrain boundaries, which can be strongly affected by the external electric and magnetic fields.     

Enhancement of magnetoresistances at room temperature in YSZ doping La0.67Sr0.33MnO3 system

The temperature dependence of the resistance of composite samples (1−x)La_0.67Sr_0.33MnO₃+xYSZ with different YSZ doping level x was investigated at magnetic fields 0-3 T, where YSZ represents yttria-stabilized zirconia. Results show that the YSZ dopant does not only adjust the metal-insulator transition temperature, but also increases the magnetoresistance effect. With increase of YSZ doping level for the range of x<2%, the metal-insulator transition temperature values T_P of the composites decrease, but T_P increases with increase of x further for the range of x>2%. Meanwhile, in the YSZ-doped composites, a broad metal-insulator transition temperature region was found at zero and low magnetic field, which results in an obvious enhanced magnetoresistance in the temperature range 10-350 K. Specially, a larger magnetoresistance value was observed at room temperature at 3 T, which is encouraging with regard to the potential application of magnetoresistance materials.     

Preparation of the La0.8Sr0.2MnO3 films on STO and LAO substrates by excimer laser-assisted metal organic deposition using the KrF laser

La_0.8Sr_0.2MnO₃ films were prepared on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates using excimer laser-assisted metal organic deposition (ELAMOD). For the LAO substrate, no epitaxial La_0.8Sr_0.2MnO₃ film was obtained by laser irradiation in the fluence range from 60 to 110 mJ/cm² with heating at 500 °C. On the other hand, an epitaxial La_0.8Sr_0.2MnO₃ film on the STO substrate was formed by laser irradiation in the fluence range from 60 to 100 mJ/cm² with heating at 500 °C. To optimize the electrical properties for an IR sensor, the effects of the laser fluence, the irradiation time and the film thickness on the temperature dependence of the resistance and temperature coefficient of resistance (TCR: defined as 1/R·(dR/dT)) of the LSMO films were investigated. An LSMO film on the STO substrate that showed the maximum TCR of 3.9% at 265 K was obtained by the ELAMOD process using the KrF laser.     

The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Preparation and low field magnetoresistance of double layered manganite La1.4Ca1.6Mn2O7

Double layered manganite of La_1.4Ca_1.6Mn₂O₇ (DLCMO) was prepared using solid state reaction method and had a metal-insulator transition temperature (T_MI) of 125 K. The short range 2D-feerromagnetic ordering (T_C2) starts growing when T<168 K and it gets converted into 3D-ferromagnetic ordering (T_C1) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). For DLCMO, in the temperature range between T_C1 and T_C2, the MR showed a gradual increase with the magnetic field. The observed MR and R-T behaviour of double layered manganite for T_C1<T<T_C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM-PM mixture.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.     

Origin of the non-linear pressure effects in perovskite manganites: Buckling of Mn-O-Mn bonds and Jahn-Teller distortion of the MnO6 octahedra induced by pressure

High-pressure resistivity and X-ray diffraction measurements were conducted on La_0.85MnO_3−δ at ∼6 and ∼7 GPa, respectively. At low pressures the metal-insulator transition temperature (T_MI) increases linearly up to a critical pressure, P^* ∼3.4 GPa, followed by reduction in T_MI at higher pressure. Analysis of the bond distances and bond angles reveals that a bandwidth increase drives the increase in T_MI below P^*. The reduction in T_MI at higher pressures is found to result from Jahn-Teller distortions of the MnO₆ octahedra. The role of anharmonic interatomic potentials is discussed.     

Magnetocaloric effect in (La0.47Gd0.2)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, nanosized particles of (La_0.47Gd_0.2)Sr_0.33MnO₃ perovskite-type oxides were successfully synthesized at a relatively low calcinated temperature at 800 °C for 10 h using amorphous molecular alloy as precursor. X-ray diffraction (XRD) and electron diffraction (ED) revealed that the resulting product is of pure single-phase rhombohedral structure. The Curie temperature T_C and magnetic entropy change (MCE) in (La_0.47Gd_0.2)Sr_0.33MnO₃ polycrystalline nanoparticles are determined and compared to those of similar systems prepared by the conventional solid-state reaction method. The Curie temperature T_C is shifted to 298 k, and a relatively large MCE with a broad peak around Curie temperature is observed in (La_0.47Gd_0.2)Sr_0.33MnO₃ polycrystalline particles. These results suggested that this material is a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Low-field transport properties of (1−x)La0.7Ca0.2 Sr0.1MnO3+x(ZnO) composites

Composite samples (1−x)La_0.7Ca_0.2Sr_0.1MnO₃(LCSMO)+x(ZnO) with different ZnO doping levels x have been investigated systematically. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and ZnO grains, and that ZnO segregates mostly at the grain boundaries of LCSMO. The magnetic properties reveal that the ferromagnetic order of LCSMO is weakened by addition of ZnO. The results also show that ZnO has a direct effect on the resistance of LCSMO/ZnO composites, especially on the low-temperature resistance. With increase of the ZnO doping level, T_P shifts to a lower temperature and the resistance increases. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 3 kOe. The maximum MR appears at x=0.1. The low field magnetoresistance (LFMR) results from spin-polarized tunneling. However, around room temperature, the enhanced MR of the composites is caused by magnetic disorder.     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

Temperature dependence of electroresistance for La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistance of La_0.67Ba_0.33MnO₃ bulk sample is reported. A decrease in the resistance (electroresistance) on the application of higher bias current is observed. The electroresistance is maximum at metal-insulator transition temperature (T_MI) and decreases when the temperature is either increased or decreased from T_MI. A two-phase model is proposed to explain the occurrence of electroresistance. The higher bias current leads to an increase in alignment of spins and thus, in turn, leads to an increase in spin stiffness coefficient and decrease in the resistance at T_MI.     

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.     

Study on the phase separation of La0.7Sr0.3MnO3 nanoparticles by electron magnetic resonance

In this work, the technique of electron magnetic resonance (EMR) is used to measure the magnetic resonant spectra of La_0.7Sr_0.3MnO₃ nanoparticles synthesized by sol–gel routes with three different gelation agents (S1: Urea+citric acid; S2: citric acid, and S3: Urea+tri-sodium citrate). The purpose of this study is to investigate the influence of synthesis conditions on the magnetic properties of nanoparticles. Our ESR results show that Curie temperatures of La_0.7Sr_0.3MnO₃ nanoparticles with different gelation agents are slightly different (T_c∼340 to 360 K) and possess both paramagnetic (PM) and ferromagnetic (FM) phases in the temperature below T_c. Besides, a sharp FM–PM transition indicates that the combined agent of Urea+tri-sodium citrate creates a better quality in CMR nanomagnets.     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

Electrical conductivity and low field magnetoresistance in polycrystalline La1−xKxMnO3 pellets prepared by pyrophoric method

Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature (T_C) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature (T>T_MI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50<T<T_MI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below T_C is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated.     

Elastic anomalies of La0.67Sr0.33−xBaxMnO3 in the vicinity of TC

With a view to understand the elastic behaviour of a material system, La_0.67Sr_0.33−xBa_xMnO₃ (where x=0, 0.1, 0.2, and 0.33) especially in the vicinity of their magnetic transition temperature T_C, a systematic investigation of ultrasonic velocity over a temperature range 300-400 K has been carried out. The materials prepared by citrate gel route, were characterized structurally by XRD and on analyzing the XRD patterns, it has been concluded that all the samples are having rhombohedral structure with space group of R3?c. The magnetic (T_C) transition temperatures determined by AC susceptibility measurements are found to decrease continuously with increasing barium concentration. Finally, the ultrasonic longitudinal velocities of all the samples are found to exhibit considerable softening in the vicinity of their magnetic transition temperatures, T_C and the observed behaviour is explained using mean field theory and Jahn-Teller theorem.     

Hydrothermal synthesis and Magnetocaloric effect of La0.5Ca0.3Sr0.2MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.5Ca_0.3Sr_0.2MnO₃ have been studied. The La_0.5Ca_0.3Sr_0.2MnO₃ can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔS_M for the as-prepared sample is 0.88 J kg⁻¹ K⁻¹ at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg⁻¹ K⁻¹, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application.     

Electronic transport and magnetic studies of La1−xCax−0.08Sr0.04Ba0.04MnO3

Lanthanum based mixed valence manganite system La_1−xCa_x−0.08Sr_0.04Ba_0.04MnO₃ (LCSBMO; x=0.15, 0.24 and 0.33) synthesized through the sol-gel route is systematically investigated in this paper. The electronic transport and magnetic susceptibility properties are analyzed and compared, apart from the study of unit cell structure, microstructure and composition. Second order phase transition is observed in all the samples and significant difference is observed between the insulator to metal transition temperature (T_MI) and paramagnetic (PM) to ferromagnetic (FM) transition temperature (T_C). In contrast to the insulating FM behaviour usually observed in La_1−xCa_xMnO₃ (LCMO) for x=0.15, a clear insulator to metal transition is observed for LCSBMO for the same percentage of lanthanum. The temperature dependent resistivity of polycrystalline pellets, when obeying the well studied law ρ=ρ_o+ρ₂T² for T<T_MI, is observed to differ significantly in the values of ρ_o and ρ₂, with the electrical conductivity increasing with x. The variable range hopping model has been found to fit resistivity data better than the small polaron model for T>T_MI. AC magnetic susceptibility study of the polycrystalline powders of the manganite system shows the highest PM to FM transition of 285 K for x=0.33.