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Magnetization and magnetoresistance (MR) measurements are reported on superlattices consisting of alternate layers of 20 unit cell (u.c.) thick La0.7Ca0.3MnO3 (LCMO) and n u.c. LaNiO 3 (LNO), where n varies from 1 to 10. The saturation field (H s) and remnant magnetization (Mr) show antiferromagnetic (AF) interlayer coupling for n ≤ 2 followed by an overdamped oscillatory behavior with a period of ≈ 4 u.c. with the increasing n. The low-temperature MR of the AF-coupled superlattices is characterized by a steep increase up to a critical field Hs* (≈ Hs) followed by a ∼ Hα (α &lt; 1.0) dependence, whereas in the ferromagnetically coupled samples the MR rises monotonically as ∼ Hα with no Hs*. We attribute the overdamped oscillations in Mr and Hs, and the dual nature of MR to disorder modified quantum interference in the spacer layers.

Department Of Physics

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Europhysics Letters

Overdamped interlayer exchange coupling and disorder-dominated magnetoresistance in La0.7Ca0.3MnO3/LaNiO 3 superlattices

The temperature- and magnetic-field-dependent resistivity of La0.7Ca0.3MnO3/LaNiO3 superlattices grown on (001)-oriented SrTiO3 and LaAlO3 substrates is compared as a function of LaNiO3 spacer layer thickness. The electron transport in superlattices with the spacer layer thickness ≤4 unit cells is characterized by a thermally activated resistivity p(T) between 4.2 and 300 K, and a large (∼90%) magnetoresistance at low temperatures. A parallel resistor model, which explicitly takes into account the interfacial disorder in each unit of the superlattice, correctly reproduces the broad features of the p(T) curves. We attribute the disordered interfaces to lattice-mismatch-induced strain and a magnetic roughness, which results from truncation of the three-dimensional coordination of magnetic ions at the interfaces.

Physical Review B - Condensed Matter and Materials Physics

Interfacial disorder-driven metal-insulator transition and enhanced low-temperature magnetoresistance in La0.7Ca0.3MnO3/LaNiO3 superlattices

Electric- and magnetic-field-dependent resistivity, and magnetization are studied in epitaxial films of (formula presented) between 4.2 and 300 K. Attention is focused on how the substitution of Sr at the Ca sites of the parent compound (formula presented) affects the electrical and magnetic states of this canonical charge-ordered (CO) insulator. The resistivity (ρ) of the parent compound is characterized by a gradual increase on cooling below 300 until 205 K, where it shows a steplike enhancement. We identify this step as the onset temperature (formula presented) of the CO state. Below 205 K, a well-defined Arrhenius-type of resistivity with activation energy of 0.13 eV suggests excitation of holes across the CO gap as the mechanism of charge transport in the parent compound. In the films with (formula presented) and 0.07, this band-to-band excitation process gives way to a Mott-type, spin-dependent hopping transport from (formula presented) to a crossover temperature (formula presented) Over a narrow temperature range below (formula presented) and a second crossover temperature (formula presented) the films show a metallic character followed by the onset of a second insulating state, which persists down to the lowest temperature of measurement (4.2 K). In the regime of temperature between (formula presented) and 4.2 K, the transport in films with (formula presented) and 0.07 is highly nonlinear in electric field, and displays hysteretic and history effects. In this regime of temperature, the resistivity also shows a large drop on application of a magnetic field. In samples with (formula presented) while the large magnetoresistance in the vicinity of (formula presented) and the minimum in ρ at (formula presented) persist, the transport remains Ohmic. Our magnetization measurements show the onset of ferromagnetic ordering in the vicinity of (formula presented) in all Sr-substituted films. However, for (formula presented) a low value of the field-cooled moment and a spin-glass type of behavior seen at temperatures below (formula presented) suggest formation of ferromagnetic clusters whose moment is gradually blocked with decreasing temperature. We argue that the nonlinear and hysteretic effects seen in samples with (formula presented) are a result of classical percolation and quantum transport in a topologically inhomogeneous medium. © 2001 The American Physical Society.

Electric- and magnetic-field-driven nonlinear charge transport and magnetic ordering in epitaxial films of (formula presented)

Electric- and magnetic-field-dependent resistivity, and magnetization are studied in epitaxial films of Pr0.7Ca0.3-xSrxMnO3 between 4.2 and 300 K. Attention is focused on how the substitution of Sr at the Ca sites of the parent compound Pr0.7Ca0.3MnO3 affects the electrical and magnetic states of this canonical charge-ordered (CO) insulator. The resistivity (ρ) of the parent compound is characterized by a gradual increase on cooling below 300 until 205 K, where it shows a steplike enhancement. We identify this step as the onset temperature (TCO) of the CO state. Below 205 K, a well-defined Arrhenius-type of resistivity with activation energy of 0.13 eV suggests excitation of holes across the CO gap as the mechanism of charge transport in the parent compound. In the films with x=0.03 and 0.07, this band-to-band excitation process gives way to a Mott-type, spin-dependent hopping transport from TCO to a crossover temperature T2 (&lt; TCO). Over a narrow temperature range below T2 and a second crossover temperature T3, the films show a metallic character followed by the onset of a second insulating state, which persists down to the lowest temperature of measurement (4.2 K). In the regime of temperature between T2 and 4.2 K, the transport in films with x=0.03 and 0.07 is highly nonlinear in electric field, and displays hysteretic and history effects. In this regime of temperature, the resistivity also shows a large drop on application of a magnetic field. In samples with x≥0.1, while the large magnetoresistance in the vicinity of T2 and the minimum in ρ at T3 persist, the transport remains Ohmic. Our magnetization measurements show the onset of ferromagnetic ordering in the vicinity of T2 in all Sr-substituted films. However, for x&lt;0.1, a low value of the field-cooled moment and a spin-glass type of behavior seen at temperatures below T3 suggest formation of ferromagnetic clusters whose moment is gradually blocked with decreasing temperature. We argue that the nonlinear and hysteretic effects seen in samples with x≤0.1 are a result of classical percolation and quantum transport in a topologically inhomogeneous medium.

Electric- and magnetic-field-driven nonlinear charge transport and magnetic ordering in epitaxial films of Pr0.7Ca0.3-xSrxMnO3

We report measurements of non-linear charge transport in epitaxial (La1-x Prx)0.7Ca0.3MnO3 thin films fabricated on (100) oriented SrTiO3 single crystals by pulsed laser deposition. The end members of this series, namely Pr0.7Ca0.3MnO3 and La0.7Ca0.3MnO3 are canonical charge-ordered (CO) and ferromagnetic manganites, respectively. The onset of the CO state in Pr0.7Ca0.3MnO3 is manifested by a pronounced insulating behavior below ∼ 200 K. The CO state remains stable even when a large (∼ 2 × 105 V/cm) electric field is applied across the thin film samples. However, on substitution of Pr with La, a crossover from the highly resistive CO state to a state of metallic character is observed at relatively low electric fields. The current-voltage characteristics of the samples at low temperatures show hysteretic and history dependent effects. The electric field driven charge transport in the system is modelled on the basis of an inhomogeneous medium consisting of ferromagnetic metallic clusters dispersed in a CO background.

Pramana - Journal of Physics

Percolative transport in the vicinity of charge-order ferromagnetic transition in a hole-doped manganite

Measurements of electric- and magnetic-field-induced changes in electrical conduction indicate the coexistence of charge-ordered and ferromagnetically ordered regions in the Sr-substituted Pr0.7Ca0.3MnO3 epitaxial films. Percolative, metal-like channels open up in this topologically inhomogeneous medium on the application of these fields, and a technologically promising orders-of-magnitude drop in resistance ensues. Measurements of current-voltage characteristics and the temperature dependence of resistivity show hysteretic and history effects, which are intimately linked to the relative stability of the two phases.

Current switching effects induced by electric and magnetic fields in Sr-substituted Pr0.7Ca0.3MnO3 films

Magnetic and electric field dependent charge transport in epitaxial films of La0.7Ca0.3MnO3 deposited on (001) SrTiO3 and LaAlO3 substrates is compared as a function of the film thickness. A hysteretic and history dependent insulating behaviour and a strongly field dependent resistivity observed at small thicknesses suggest coexistence of charge ordered and ferromagnetically ordered regions in these films. This inhomogeneous film growth is presumably due to the lattice mismatch induced strain and atomic scale roughness of the substrates. It is shown that on SrTiO3 substrates treated to obtain atomically flat TiO2 terminated surfaces, films retain the metallic character of bulk La0.7Ca0.3MnO3 down to 60 angstroms thickness.

Solid State Communications

Transition from a double exchange ferromagnetic metal to hysteretic insulator mimicking charge ordering effects in ultra-thin epitaxial films of a perovskite manganite

Strong anisotropy is observed in the magnetization and magnetoresistance (MR) of a series of La0.7Sr0.3MnO3 (LSMO) films grown on (001) oriented Si using pulsed sputtering plasma. Magnetic anisotropy (MA) of LSMO films is in the order of 106 erg/cm3, comparable to or larger than that of the other heterostructures. Analysis using the Stoner-Wohlfarth model and the density functional theory (DFT) indicates that the crystalline and induced anisotropies are almost compensating with each other, and observed MA is of the same order of shape anisotropy. The rise in MA with the LSMO thickness is attributed to the shape anisotropy and interfacial spin reordering. These LSMO films exhibit positive and negative MR, even though the LSMO is well known for colossal negative MR. The 12.4 nm thick LSMO film shows a maximum ∼7 % in-plane low-field positive MR, which increases to ∼12 % for 20.0 nm thick LSMO, but the variation of out-of-plane positive MR is ∼ ± 0.8 %. The maximum in-plane positive MR occurs at ∼ ± 0.2 kG (switching field), which corresponds to the coercive field. In contrast, the out-of-plane positive MR switching field is relatively higher, ∼ ± 5 kG, 25 times larger than the in-plane switching field. The anisotropy in positive MR and its switching field is due to the strong MA, modulated by the charge transfer-induced interfacial exchange coupling strength, confirmed using the DFT. The MA and anisotropy in MR with the dual sign of MR are intriguing phenomena; their tunability could pave the way for advanced technology in magnetic devices.

Journal of Alloys and Compounds

Enhanced low-field positive magnetoresistance and magnetic anisotropy in La0.7Sr0.3MnO3 films grown on (001) Si