W. S. Tan

MAGNETIC AND TRANSPORT PROPERTIES OF Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 TRILAYERS

Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 (PSMO/LCMO/PSMO) trilayers were deposited on (001)-oriented single crystal MgO by pulsed laser deposition. The thickness of both PSMO layers was 36 nm while the thickness of LCMO layer varied from 6 to 36 nm. High resolution X-ray diffraction patterns indicated that trilayers were well (001)-oriented grown with high crystalline quality, and that PSMO layers were fully-strain-relaxed while LCMO spacer was partially strained. Studies on transport and magnetic properties of trilayers indicated that metal-insulator transition temperature T-MI increased from 200 K to 260 K and the saturation magnetization was suppressed with decreasing thickness of LCMO spacer from 36 to 6 nm. Transport properties of trilayers are associated with enhancement of volume fraction of ferromagnetic clusters in charge ordered and magnetic phase separated LCMO spacer. Interestingly, exchange bias (EB) was not observed in PSMO/LCMO/PSMO trilayers. It was believed that preferential distribution of metallic ferromagnetic clusters in LCMO layer may result in disappearance of EB.

Enhanced magnetoresistance at wide temperature range in [Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3]20 superlattice on (001) MgO

Abstract[Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3]20 superlattices were epitaxially fabricated on (001) MgO substrates with 24xa0nm La0.5Ca0.5MnO3 (LCMO) buffer layer by pulsed laser deposition. As the thickness of Pr0.7Sr0.3MnO3 (PSMO) layer and LCMO layer is not identical, non-uniformity in the superlattice period thickness will occur and result in lower metal–insulator transition temperature and larger magnetoresistance (MR) at a wide low-temperature range. Furthermore, the percolation model was used to quantitatively understand the transport mechanism of superlattice. Our research results demonstrate that inhomogeneous layer thickness accompanied with charge ordering phase separation can strongly influence the transport properties of superlattice. Enhanced magnetoresistance at a wide temperature range provides possibilities for practical application in manganite-based film devices.

EFFECT OF HEAT TREATMENT ON MICROSTRUCTURE AND MAGNETOSTRICTIVE PROPERTY OF MELT-SPUN Fe85Ga15 RIBBONS

In order to study the microstructure of Fe-Ga alloy, Fe85Ga15 ribbons prepared with different wheel velocity were studied by high resolution X-ray diffraction (HRXRD) and extend X-ray absorption fine structure (EXAFS). HRXRD patterns showed that only disordered A(2) phase was observed in as-cast Fe85Ga15 alloy. A modified-DO3 phase was detected in all of the melt spun samples. The HRXRD associated with EXAFS results indicated that Ga atoms were located as second-nearest neighbor along [100] orientation. A little DO3 phase was found in ribbons annealed at 1000 degrees C under 0.06 MPa Ar atmosphere. The result of magnetostriction measurement revealed that in the ribbon prepared with higher wheel velocity, more modified-DO3 phase will enhance the magnetostriction. DO3 phase in the annealed sample will deteriorate the magnetostrictive properties of Fe-Ga ribbons.

Thickness dependence of morphology in La2/3Ca1/3MnO3 thin films

La2/3Ca1/3MnO3 (LCMO) thin films with the thickness from 4 nm to 50 nm were fabricated by the off-axis magnetron sputtering technique. Single crystal (001) SrTiO3 (STO), (0001) sapphire (ALO), and (001) yttrium-stabilized ZrO2 (YSZ) were used as substrates. The surface morphology of thin films was characterized by atomic force microscope (AFM). The surface morphology of the thin films depends on the thickness. Thin films on STO substrates with the thickness less than 20 nm show a wave-like morphology, which indicates a two-dimension growth mode. With the thickness increases, some cracks and grains appear on the surface, which indicates the release of the lattice strain due to the lattice mismatch between the substrate and the films. The morphology became smoother with the thickness more than 30 nm, which indicates the full relaxation in strain for the film. The surface roughness and the average grain size increase with the thickness. The morphology for LCMO grows on ALO and YSZ show a rougher surface than...

Exchange bias induced by surface inhomogeneities in epitaxial Pr 0.7 Sr 0.3 MnO 3 / La 0.5 Ca 0.5 MnO 3 bilayer film

The epitaxial growth of bilayer film made of ferromagnetic metal material Pr0.7Sr0.3MnO3 and charge-ordering insulating material La0.5Ca0.5MnO3 on single crystalline SrTiO3 (001) substrate. High resolution X-ray diffraction and atomic force microscopy analyses show the single crystalline nature of the film. Further atomic force microscopy measurement shows the presence of non-stoichiometric large particulates at the film surface, imparting an overall inhomogeneous composition to the film. This implied that the variation of crystalline structure occurred due to inhomogeneous composition. Magnetic measurement results indicate that the inhomogeneous composition could cause inhomogeneity of magnetic phase which in turn reduce ferromagnetism and enhance exchange bias effect.

Effect of interlayer-induced asymmetrical stress on magnetotransport properties of epitaxial [Pr 0.7 Sr 0.3 MnO 3 /La 0.5 Ca 0.5 MnO 3 ] 20 superlattice

Perovskite manganite R_1-xA_xMnO₃ (R=rare earth, A=alkaline earth) has been attracting great research interesting due to its magnetoelectronic phenomena such as colossal magnetoresistance (CMR), charge-orbital ordering, and metal-insulator transitions, etc.[1,2] In manganite superlattice, interfacial effects including cation intermixing, charge transfer, exchange coupling, strain and defect formation are crucial in determining the magnetic and transport properties of superlattice. [3-5] In this work, all-manganite [Pr_0.7Sr_0.3MnO₃/La_0.5Ca_0.5MnO₃]₂₀ superlattices were epitaxially fabricated on (001)-oriented single crystal MgO substrates with 24 nm La_0.5Ca_0.5MnO₃ buffer layer by using plused laser deposition. All the as-grown superlattices are of single phase and single orientation (see Fig.1). We obtained various stress at interface by varying relative layer thickness. As the thickness of Pr_0.7Sr_0.3MnO₃ layer and La_0.5Ca_0.5MnO₃ layer is not identical, interlayer-induced asymmetrical stress will occur and result in lower metal-insulator (MI) transition temperature and larger magnetoresistance (MR) in a wider temperature range. This result indicated that [Pr_0.7Sr_0.3MnO₃/La_0.5Ca_0.5MnO₃]₂₀ has opportunities for practical application in magnetic sensor and magnetic memory material. Furthermore, the percolation model has been used to quantitatively understand the transport mechanism of superlattice (see Fig.2). Our results demonstrated that inter-layer-induced asymmetrical stress accompanied with phase separation and charge ordering could strongly influence the magnetotransport properties of superlattice.

Magnetostriction and microstructure of melt-spun Fe77Ga23 ribbons prepared with different wheel velocities

A set of stacked ribbons with the composition of Fe77Ga23 were prepared with different wheel velocities of 7 m/s, 12.5 m/s and 25 m/s (named as S-7, S-12.5 and S-25, respectively). High resolution X-ray diffraction patterns of these ribbons show that all the ribbons present the disordered A(2) structure, whereas an additional modified-DO3 phase is detected in S-12.5 and S-25. S-25 has stronger (100) texture than other two samples. Ga K-edge extended X-ray absorption fine structure results indicate that both bond distance and the number of Ga atoms in the second neighbor shell around Ga decrease with increasing wheel velocity. No Ga cluster is detected in the studied ribbons. A short-range ordering Ga-rich phase and large local strain have no obvious influence on magnetostriction of S-7. It is believed that both the (100) texture and the additional modified-DO3 phase play a positive role in magnetostrictive properties of Fe77Ga23 ribbons.

Possible abnormal mechanism for lattice strain relaxation in La0.7Ca0.3MnO3 thin film

La0.7Ca0.3MnO3(LCMO) thin films with the thickness of 50 nm were deposited on (001)-oriented single crystal SrTiO3(STO), MgO and α-Al2O3(ALO) by 90° off-axis radio frequency magnetron sputtering. Grazing incidence X-ray diffraction technique, associated with normal X-ray diffraction, was performed to measure the in-plane lattice parameter and investigate the lattice strain and strain relaxation in LCMO films. The results indicated that critical thickness of strain relaxation is very small, which may be related to large mismatch between film and substrate. The mechanism for strain relaxation in LCMO film is perhaps different from that for tetragonal distortion.