LCMO Films Research Articles

All Oxide-Based Magnetic Tunnel Junctions Fabricated by Focused Ion Beam Nanomachining Technique

This study offers a comprehensive investigation into the fabrication of all-oxide-based magnetic tunnel junctions (MTJs) utilizing the focused ion beam (FIB) nanomachining method. The oxide thin films of SrTiO3 (STO), La[Formula: see text]CaxMnO3 (LCMO) and La[Formula: see text]SrxMnO3 (LSMO) were grown using pulsed laser deposition (PLD). Magnetization measurements at 5K revealed a notable difference in coercivity between LCMO and LSMO films, highlighting their suitability as electrodes in MTJ device fabrication. The deposited trilayer structure of LCMO/STO/LSMO exhibited heteroepitaxial growth, enabling the independent magnetic switching of electrode layers in the MTJ with the STO barrier layer. Conventional photolithography and ion milling were used to create a series of 4[Formula: see text][Formula: see text]m Hall bar tracks in the deposited films. Nanopillar devices, sized 250[Formula: see text]nm[Formula: see text]×[Formula: see text]250[Formula: see text]nm, were successfully fabricated on these 4[Formula: see text][Formula: see text]m tracks by optimizing the FIB nanomachining technique. The fabricated MTJs demonstrated a high tunnel magnetoresistance (TMR) response, leading to the conclusion that the FIB nanomachining technique holds significant promise for the development of high-performance oxide-based spintronic devices.

Escalated Phase Separation Driven Enhanced Magnetoresistance in Manganite/Iridate Epitaxial Heterostructures

AbstractPhase separation in manganites leads to unique magnetic and electronic properties. 50% Ca‐doped LaMnO3 (LCMO), at the boundary of ferromagnetic (FM) and antiferromagnetic (AFM) states in La1‐xCaxMnO3 (0 ≤ x ≤ 1), is an ideal system to study phase separation behavior. The investigation reveals the effect of a 5d‐metal perovskite SrIrO3 (SIO) on the phase separation, magnetic, and magnetoresistance (MR) properties of LCMO. Single‐layer and bilayer LCMO films, both appear purely ferromagnetic along the in‐plane (IP) magnetic field direction, but show the tendency of temperature‐dependent ferromagnetic and antiferromagnetic or charge‐ordered (CO) phase separation with the out‐of‐plane (OOP) applied field. The MR, and colossal magnetoresistance (CMR), observed in LCMO/SIO bilayers are two orders and an order of magnitude (in %) larger, respectively than that in the single‐layer film. The coexistence of FM and AFM/CO phases is responsible for the CMR and MR enhancement in the LCMO/SIO bilayer, pointing toward the importance of the phase separation and competition of both the individual materials in enhancing their magnetic and electronic properties.

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.

Off-axis electron holography of manganite-based heterojunctions: Interface potential and charge distribution**Project supported by the National Natural Science Foundation of China (Grant No. 10974105), the High-end Foreign Experts Recruitment Programs, China (Grant Nos. GDW20173500154 and GDW20163500110), the Taishan Scholar Program of Shandong Province, Shandong Province “Double–Hundred Talent Plan” on 100 Foreign Experts and 100 Foreign Expert Teams Introduction Project, the Top-notch

The interfacial electrical potentials and charge distributions of two manganite-based heterojunctions, i.e., La0.67Ca0.33MnO3/SrTiO3:0.05 wt% Nb (LCMO/STON) and La0.67Ca0.33MnO3/LaMnO3/SrTiO3:0.05 wt% Nb (simplified as LCMO/LMO/STON), are studied by means of off-axis electron holography in a transmission electron microscope. The influences of buffer layer on the microstructure and magnetic properties of the LCMO films are explored. The results show that when a buffer layer of LaMnO3 is introduced, the tensile strain between the STON substrate and LCMO film reduces, misfit dislocation density decreases near the interfaces of the heterojunctions, and a positive magnetoresistance is observed. For the LCMO/STON junction, positive and negative charges accumulate near the interface between the substrate and the film. For the LCMO/LMO/STON junction, a complex charge distribution takes place across the interface, where notable negative charges accumulate. The difference between the charge distributions near the interface may shed light on the observed generation of positive magnetoresistance in the junction with a buffer layer.

Charge transport in chemically grown manganite based heterostructure

Abstract In this communication, we report the results of the structural and transport studies performed on ZnO/La0.7Ca0.3MnO3/LaAlO3 (ZnO//LAO) heterostructure grown using simple, low cost, vacuum free and environment friendly chemical solution deposition (CSD) method. ZnO is found to be polycrystalline while LCMO film orientations suggest its single crystalline growth, parallel with the crystallographic orientations of LAO substrate. Double peak transition behavior, observed in the R – T, has been discussed on the basis of phase coexistence scenario. Charge transport behavior and charge conduction mechanisms have been investigated by performing current–voltage (I–V) characteristics at different temperatures. Various electronic and thermal processes have been proposed as sources of backward diode like characteristics observed across ZnO/LCMO interface. Complex variations in electroresistance (ER) have been discussed on the basis of charge injection, depletion region modifications, tunneling, breakdown and thermal processes.

Static and dynamic signatures of anisotropic electronic phase separation in La2/3Ca1/3MnO3 thin films under anisotropic strain

The electronic phase separation (EPS) of optimally doped ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films under various degrees of anisotropic strain is investigated by static magnetotransport and dynamic relaxation measurements. Three LCMO films were grown simultaneously on (001) $\mathrm{NdGa}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition, and then postgrowth annealed at $780{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ in ${\mathrm{O}}_{2}$ for different durations of time. With increasing annealing time, the films developed significant strains of opposite signs along the two orthogonal in-plane directions. The static temperature-dependent resistivity, $\ensuremath{\rho}(T)$, was measured simultaneously along the two orthogonal directions. With increasing annealing time, both zero-field-cooled and field-cooled $\ensuremath{\rho}(T)$ show significant increases, suggesting strain-triggered EPS and appearance of antiferromagnetic insulating (AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, $\ensuremath{\rho}(T)$ along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100]. The enhanced resistivity anisotropy indicates that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100], possibly due to the cooperative deformation and rotation/tilting of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra under the enhanced anisotropic strain. The anisotropic EPS can also be tuned by an external magnetic field. During a field cycle at several fixed temperatures, the AFI phases are melted at high fields and recovered at low fields, resulting in sharp resistance changes of ratio as high as ${10}^{4}$. Furthermore, the resistivity was found to exhibit glasslike behavior, relaxing logarithmically in the phase-separated states. Fitting the data to a phenomenological model, the resulting resistive viscosity and characteristic relaxation time are found to evolve with temperature, showing a close correlation with the static measurements in the EPS states.

Enhancement of temperature coefficient of resistance (TCR) and Magneto-resistance (MR) in La1–x Ca x MnO3:Ag0.2 polycrystalline composites

La1−x Ca x MnO3 (x ranges from 0.28 to 0.34) ceramics with Ag addition were synthesized by sol–gel method using methanol as solvent. The EDS elemental mapping shows that Ag element was detected dominantly on the sample surface. The sample of x = 0.28 has almost no pore and bigger grain size, which indicated that it has better crystallization and high density. Temperature dependence of resistivity shows that the samples for all Ca content exhibit sharp metal–insulator transition, and the corresponding metal–insulator temperature shift toward higher temperature with increasing Ca content. The temperature coefficient of resistance value for x = 0.28 reaches its the highest value, 71.8%·K−1. This temperature coefficient of resistance value is even higher than the previously reported for LCMO films and single crystals, and it shows a very promising application for the infrared and bolometric detectors. The high magnetoresitance for x = 0.28 reaches up to 69.3% in magnetic field of 1 T near room temperature. It was concluded that it improved the properties of LCMO:Ag composites are attributed to its improved crystallization by Ag addition, homogeneity and Mn4+/Mn3+ ratio. Different theoretical models are employed to analyze the resistance behaviors in different temperature regions, which give good agreements with experimental results.

Strain-induced perpendicular magnetic anisotropy inLa2CoMnO6−ɛthin films and its dependence on film thickness

Ferromagnetic insulating La$_2$CoMnO$_{6-\epsilon}$ (LCMO) epitaxial thin films grown on top of SrTiO$_3$ (001) substrates presents a strong magnetic anisotropy favoring the out of plane orientation of the magnetization with a strong anisotropy field ($\sim 70$ kOe for film thickness of about 15 nm) and with a coercive field of about 10 kOe. The anisotropy can be tuned by modifying the oxygen content of the film which indirectly has two effects on the unit cell: i) change of the orientation of the LCMO crystallographic axis over the substrate (from c in-plane to c out-of-plane) and ii) shrinkage of the out of plane cell parameter, which implies increasing tensile strain of the films. In contrast, LCMO films grown on (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ and LaAlO$_3$ substrates (with a larger out-of-plane lattice parameter and compressive stress) display in-plane magnetic anisotropy. Thus, we link the strong magnetic anisotropy observed in La$_2$CoMnO$_{6-\epsilon}$ to the relation between in-plane and out-of-plane parameters and so to the film stress.

Enhancing the orthorhombicity and antiferromagnetic-insulating state in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films by inserting a SmFeO3 buffer layer

Structural and magnetotransport properties of epitaxial La0.67Ca0.33MnO3(30 nm)/NdGaO3(001) [LCMO/NGO(001)] films are tuned by inserting an insulating SmFeO3 (SFO) buffer layer at various thicknesses (t). All the layers and the NGO substrates have the same Pbnm symmetry with the octahedra tilting about the b-axis, but different orthorhombicity (d). We found that as t increases, the fully strained (≤15 nm) or partially relaxed (30–60 nm) SFO layers can produce different d in the upper LCMO films. Correspondingly, the induced antiferromagnetic-insulating (AFI) state in LCMO is greatly enhanced with TAFI shifted from ∼250 K for t ≤ 15 nm to ∼263 K for t = 30–60 nm. We also show that the strain relaxation for t ≥ 30 nm is remarkably anisotropic, with a stable lattice constant a as that of the NGO substrates but increasing b of both SFO and LCMO layers. This indicates the octahedral coupling across the interfaces, leaving the strain along the a-axis accommodated by the octahedral tilts, while along the b-axis most probably by the octahedral deformations. The AFI state in the LCMO layer could be ascribed to the enhanced orthorhombicity with cooperatively increased Jahn-Teller-like distortions and tilting of the MnO6 octahedra. The results strongly suggest that the interfacial octahedral coupling plays a crucial role in epitaxial growth and in tuning functionalities of the perovskite oxide films.

Magnetoresistance and Resistance Relaxation of Nanostructured La-Ca-MnO Films in Pulsed Magnetic Fields

The results of magnetoresistance (MR) and resistance relaxation of nanostructured La1− x Ca x MnO3 films, with different composition x grown by metal–organic chemical vapor deposition technique, are presented and compared with the La0.83Sr0.17MnO3 films. The MR was investigated in pulsed magnetic fields up to 60 T in the temperature range 1.5–294 K while the relaxation processes were studied in pulsed fields up to 10 T and temperatures in the range of 80–300 K. It was demonstrated that at low temperatures the MR has higher values in the LCMO films in comparison with the LSMO ones, while at room temperatures, the highest MR values are obtained for the LSMO films. The fast ( $\boldsymbol \sim 100~\boldsymbol {\mu } $ s) and slow ( $\boldsymbol \sim $ ms) resistance relaxation processes were observed after the magnetic field pulse was switched off. It was shown that the fast process could be analyzed using the Kolmogorov–Avrami–Fatuzzo model, considering the reorientation of magnetic domains into their equilibrium state, while the slow process—by the Kohlrausch–Williams–Watts model considering the interaction of the magnetic moments in disordered grain boundaries having spin-glass properties. It was concluded that La1− x Ca x MnO3 films having a higher sensitivity and lower memory effects and should be favored for the development of fast pulsed magnetic field sensors operating at low temperatures.

Large low field magneto-resistance and temperature coefficient of resistance in La0.8Ca0.2MnO3 epitaxial thin film

An epitaxial La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film was fabricated using the pulsed laser deposition (PLD) technique to evaluate the magnetoresistance (MR) and temperature coefficient of resistance (TCR). The LCMO film was about 200nm in thickness and appeared to have a strong out-of-plane texture. A giant value of MR 73% and 57% for both the ab-plane and in the c-directions respectively are obtained at 1.5T applied field. The values of TCR are about 14.2% K−1 and 11.5% K−1 for both the ab-plane and in the c-directions respectively in a wide temperature range. These results proved that La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film is a promising candidate of perovskites for novel electronic applications.

High-TC ferromagnetic order in CaRuO3/La2/3Ca1/3MnO3 superlattices

Ferromagnetic-metallic ground state with high Curie temperature (TC) of 200–258 K has been observed in CaRuO3/La2/3Ca1/3MnO3 (CRO/LCMO) superlattices with the ultrathin LCMO layer of 0.8–3.2 nm thick. This contradicts the antiferromagnetic or low-TC insulating ground state observed in single-layer LCMO thin-films. TC and the saturated magnetization of the superlattices are determined dominantly by the LCMO layer thicknesses, indicating no direct magnetic contribution from the CRO layers or the interfaces. Also, they are less sensitive to the growth oxygen pressure as compared to the pure LCMO films. We ascribe the stabilized, bulklike ferromagnetism in the ultrathin LCMO layer to charge transfer from CRO at the interfaces, which could enhance the double-exchange and meanwhile suppress the phase separation, contrary to the case for LCMO thin-films. This interface engineering that can greatly depress the notorious “dead layer” in manganites might be significant in designing the correlated spintronic devices.

Morphology and Structure of Epitaxial Pb(Zr<sub>0.52</sub>Ti<sub>0.48</sub>)O<sub>3</sub>/La<sub>0.8</sub>Ca<sub>0.2</sub>MnO<sub>3</sub> Heterostructures Grown on Sr TiO<sub>3</sub> Substrates

Pb(Zr0.52Ti0.48)O3/La0.8Ca0.2MnO3 (PZT/LCMO) thin films were deposited onto SrTiO3 (STO) single crystalline substrates by pulse laser deposition (PLD). The AFM morphology results show that the LCMO ( PZT) layer is very smooth and homogenous with very low values of RMS roughness. The XRD results show that only the (00l) peaks are observed for both LCMO and PZT layers which are single crystalline and have an epitaxial relationship with STO substrates: PZT (001)//LCMO(001)//STO (001). The FWHM values of the (001) omega-rocking curves of the LCMO and PZT film were 0.276o and 0.565o, respectively, which suggests high crystalline quality of epitaxial PZT/LSMO LSMO heterostructures were grown on STO substrates.

Electric field modulation of magnetism and electric properties in La-Ca-MnO3/Pb(Zr0.52Ti0.48)O3 magnetoelectric laminate

We investigated the effects of electric field on the magnetization and electric transport properties in La0.6Ca0.4MnO3/Pb(Zr0.52Ti0.48)O3 (LCMO/PZT) thin film prepared by sol-gel method and simple spin coating technology. Obvious converse magnetoelectric effect (the relative change in magnetization, ΔM ∼ 15%), as well as large electroresistance effect (the relative change in resistance, ER ∼ 61%), has been obtained in LCMO thin film with applying electric field on the PZT ceramic. The efficient mechanical coupling, which originated from ferroelectric polarization and inverse piezoelectric effect in PZT substrate, leads to large changes in the magnetism and resistance of LCMO thin film. The experimental results can be interpreted by the changes of both spin exchange coupling and electron-lattice coupling strengths in LCMO film, which result from the strain at the ferromagnetic/ferroelectric interface.

X-ray photoelectron diffraction study of dopant effects in La0.7X0.3MnO3 (X = La, Sr, Ca, Ce) thin films

We present and discuss element-specific x-ray photoelectron diffraction (XPD) patterns of La, Mn, O and the dopant ions Ca, Sr and Ce of various La0.7X0.3MnO (LXMO) films grown epitaxially on SrTiO3(001) substrates. The recorded XPD polar scans are explained in the framework of multiple-scattering cluster calculations, where in general a very good agreement between experimental and theoretical data has been found. Results for all thin films are compatible with a tetragonally distorted cubic perovskite structure with similar MnO6 network. Strong evidence for Mn-site termination was found in all thin films. Dopant locations on A-type sites were clearly confirmed for Sr in LSMO and Ca in LCMO films by means of XPD polar scans. The absence of surface-sensitive Ce3d diffraction features for Ce in LCeMO points to non-equivalent Ce sites and related near-surface disorder.

Correlation between structural parameters and the magnetocaloric effect in epitaxial La0.8Ca0.2MnO3/LaAlO3 thin film

An epitaxial La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film was fabricated using the pulsed laser deposition technique to evaluate the correlation between the crystal structure and the magnetocaloric effect. In our study, the LCMO film was 200 nm in thickness and appeared to have a strong out-of plane texture. We found that each column in the LCMO thin film layer is a domain which has a different ordering direction. These microscopic feature results in anisotropic properties of magnetization, entropy, and relative cooling power. The film exhibited a paramagnetic-to-ferromagnetic second order phase transition at 249 K. The lack of any hysteresis loss also confirmed that the material is intrinsically reversible. In addition, the large magnetization of the thin film results in an entropy change larger than those of all other perovskite type materials. Consequently, the relative cooling power is significantly enhanced.

Large, Controllable Spikes of Magnetoresistance in La2/3Ca1/3MnO3/SrTiO3 Superlattices

We have investigated superlattices consisting of up to 30 epitaxial nanomultilayers (3-7 nm thick) of ferromagnetic La(2/3)Ca(1/3)MnO(3) (LCMO) and insulating SrTiO(3) (STO) hybrids. The superlattices demonstrate dramatic shifts of Curie temperature, indicating the possibility of its tunability. The metal-insulator transition (MIT) has been observed around 140 K. Below the MIT temperature, the superlattices have shown sharp drops of resistivity, facilitating the largest and sharpest magnetoresistance peaks (>2000%) ever observed in LCMO films and superlattices at low temperatures. The observed experimental results can be explained in the frame of the phase separation model in manganites with well-organized structures. The results of magnetic and transport measurements of such hybrid structures are discussed, indicating a magnetodielectric effect in STO interlayers. The magnetic and transport properties of the superlattices are shown to be technology-dependent, experiencing dimensional transitions, which enables the creation of structures with prescribed magnetoresistance characteristics for a broad range of applications.

Variations in the electrical resistivity of La0.67Ca0.33MnO3 films and induced interconversions of ferromagnetic and nonferromagnetic inclusions in their bulk

A significant (similar to 1.8%) positive unit between the parameters of the crystal lattice is the reason of tetragonal distortion (a (aSyen)/a (aEuro-) a parts per thousand 1.04) and reduction in the volume of the unit cell of La(0.67)Ca(0.33)MnO(3) films (15 nm) quasicoherently grown on the (001) surface of a LaAlO(3) substrate. The films consist of single-crystal blocks with the lateral size of 30-50 nm. The atomically smooth LaAlO(3)-La(0.67)Ca(0.33)MnO(3) interphase boundary has no misfit dislocations. At T = 4.2 K, the transformation of nonferromagnetic phase inclusions into ferromagnetic ones in a constant magnetic field H is accompanied by a stable reduction in the electrical resistivity rho of manganite films with time, so that the curve rho(t) is well approximated by the relationship rho(t) similar to rho(1)(t - t (0))(1/2), (where t (0) is the time for establishment of the specified value (mu(0) H = 5 T) of the magnetic field and rho(1) is a coefficient independent of H). The magnetocrystalline anisotropy due to the elastic deformation of films by the substrate and stratification of electronic phases are the reasons of the distinct hysteresis in the dependences rho(mu(0) H, T 0 -> -5 T -> 0 -> 5 T. At T = 50 K and mu(0) H = 0.4 T, the magnetoresistance MR = 100% [rho(mu(0) H) - rho(mu(0) H = 0)]/rho(mu(0) H = 0) of LCMO films attains 150%.

Scanning tunneling spectroscopy study ofc-axis proximity effect in epitaxial bilayer manganite/cuprate thin films

Recent experimental studies have indicated novel superconducting proximity effects in thin-film heterostructures comprising ferromagnetic manganites and superconducting cuprates. To look for such effects microscopically, we performed scanning tunneling spectroscopy on La${}_{2/3}$Ca${}_{1/3}$MnO${}_{3}$/YBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$ (LCMO/YBCO) bilayer thin films. $c$-axis-oriented films of varying thickness were grown on SrTiO${}_{3}$ substrates using pulsed laser-ablated deposition. Heteroepitaxiality of the films was confirmed by cross-sectional transmission electron microscopy. Tunneling spectra were measured at 4.2 K and analyzed for signatures of a pairing gap on the LCMO layer. For bilayer samples with LCMO thickness down to 5 nm, asymmetric conductance spectra characteristic of single-layer LCMO films were observed, showing no clear gap structures. These observations are consistent with a very short-range proximity effect involving spin-singlet pairs, and difficult to reconcile with longer-range proximity scenarios involving spin-triplet pairs.

Interfacial effects in manganite thin films with different capping layers of interest for spintronic applications

We report on the use of surface-sensitive techniques (x-ray absorption spectroscopy and x-ray magnetic circular dichroism) to investigate interfacial effects in sputtered manganite thin films, La${}_{2/3}$Ca${}_{1/3}$MnO${}_{3}$ (LCMO) and La${}_{2/3}$Sr${}_{1/3}$MnO${}_{3}$ (LSMO), with different capping layers [MgO, LaAlO${}_{3}$ (LAO), SrTiO${}_{3}$ (STO), NdGaO${}_{3}$ (NGO), and Au]. To ensure surface sensitivity, data were acquired by using the total electron yield detection mode. It is found that LSMO and LCMO films exhibit similar behavior when capped with oxide layers but dissimilar effects when a metallic capping is used. Almost bulklike Mn valence at the interface was observed in the case of LAO capping. However, a notorious increase of the Mn oxidation state was detected for both MgO and NGO capping layers. In contrast, metallic Au and STO capping promotes a reduction of the Mn oxidation state. These results are correlated with the x-ray magnetic circular dichroism data, showing a concomitant decrease of the saturation magnetization at the interface in those cases where a modification of the Mn oxidation state is observed.