N. A. Babushkina

Metal-insulator transition induced by oxygen isotope exchange in the magnetoresistive perovskite manganites

Perovskite manganites derived from LaMnO3 have recently become the subject of intensive study following the discovery of ‘colossal’ magnetoresistance (a magnetically induced change in electrical resistance of up to several orders of magnitude) in several members of this family of compounds. The manganites exhibit a broad range of electronic and magnetic phases, ranging from low-resistance ferromagnetic metals to high-resistance insulators, which are extremely sensitive to variation of composition, temperature and pressure. A recent study showed that such sensitivity also extends to oxygen isotope exchange: replacing 16O with 18O in La0.8Ca0.2MnO3 produces an unusually large change in the magnetic properties (a 21-kelvin decrease in the Curie temperature). The magnitude of this isotope shift is evidence for the essential role played by electron–phonon coupling in determining the transport properties of these materials. Here we show that this sensitivity to oxygen isotope exchange can be even more extreme. In its normal state, the compound La0.175Pr0.525Ca0.3MnO3 undergoes an insulator-to-metal transition as it is cooled below ∼95 K. But we find that, after substituting 18O for 16O, the compound remains an insulator down to 4.2 K, so providing a vivid demonstration of the importance of lattice vibrations in these materials.

Perovskite rare-earth nickelates in the thin-film epitaxial state

We have succeeded in the preparation of thin films of rare-earth nickelates RNiO3 (R=Pr, Nd, Sm, and Gd) under reduced oxygen pressure <0.02 bar by metalorganic chemical-vapor deposition owing to their epitaxial stabilization on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3 phases. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The epitaxial films of nickelates were strained and consisted of 90° domains with the orthorhombic Pnma structure. The transport properties of the strained films on LaAlO3 were similar to those of the bulk material of the same composition under applied pressure of 9 kbar but they were different from the properties of the bulk material under ambient pressure. The result implies that transport properties of RNiO3 films with sharp metal-to-insulator transition can be effectively tuned by the control of the lattice strain.

Quenched‐Disorder‐Induced Magnetization Jumps in (Sm,Sr)MnO3

Magnetic field induced steplike changes in magnetization and resistivity of Sm1-xSrxMnO3 manganites were studied. A strong dependence of these features on the cooling rate was observed. Magnetostriction, however, does not show the presence of large strain in our samples. From all these features we can rule out the conventional explanation of magnetization jumps as a consequence of martensitic transition. We propose instead that quenched by fast cooling disorder leads to the formation of an inhomogeneous metastable state and to subsequent magnetization jumps.

METAL-INSULATOR TRANSITION INDUCED BY 16O-18O OXYGEN ISOTOPE EXCHANGE IN COLOSSAL NEGATIVE MAGNETORESISTANCE MANGANITES

The effect of 16O→18O isotope exchange on the electric resistivity was studied for (La1−yPry)0.7Ca0.3MnO3 ceramic samples. It was found that at y=0.75, the substitution of 16O by 18O results in the reversible transition from a ferromagnetic metal (FM) to charge ordered (CO) insulator at zero magnetic field. The applied magnetic field (H⩾2 T) transformed the sample with 18O again to the metallic state and caused the increase in the FM transition temperature TC of the 16O sample. As a result, the isotope shift of TC at H=2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal–insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.

Magnetocaloric properties of La0.7Ca0.3Mn16O3 and La0.7Ca0.3Mn18O3 manganites and their “sandwich”

The effect of 16О → 18О isotope substitution on specific heat and magnetocaloric effect of polycrystalline La0.7Ca0.3MnO3 manganite is studied. Mainly the effect of isotope substitution for the specific heat and magnetocaloric effect is only the reduction of temperatures of anomalies. ΔTad values at magnetic field change ΔH = 18 kOe are equal to ΔTad = 2.41 K and 2.60 K for La0.7Ca0.3Mn16O3 (LCMO16) and La0.7Ca0.3Mn18O3 (LCMO18), respectively. The sandwich of the LCMO16 and LCMO18 samples was produced for direct measurement of ΔTad. The use of sandwich from materials with near similar magnetocaloric properties increases the relative cooling power by about 20%.

Giant magnetoresistive thin films of (La,Pr)0,7(Ca,Sr)0,3MnO3 prepared by aerosol MOCVD

It has been demonstrated that high quality thin films of Ln 1-x M x MnO 3 can be successfully prepared by aerosol MOCVD at 750 °C from volatile thd complexes. Subsequent annealing in oxygen at 750 °C is necessary to stabilize the oxygen content of the films. XRD patterns of the films showed them to be pseudocubic with an apparent lattice parameter linear in the average ionic radius of Ln and M. The evolution of Ln 1-x M x MnO 3 film morphology with increasing the film thickness has also been studied. The morphological instability of the MOCVD process results in the formation of a hillocky surface with thickness >2000 A. The electrical properties of La 1-x Ca x MnO 3 and La 1-x Sr x MnO 3 correlate with those reported for bulk and thin film materials. The substitution of Pr for La in La 1-x Sr x MnO 3 reduces the maximum resistivity temperature, T p , non-linearly. La 1-x Ca x MnO 3 thin films reveal a shift of T p downwards in the case of substrate materials with a positive lattice mismatch with the films. La 0.35 Pr 0.35 Ca 0.3 MnO 3 /LaAlO 3 demonstrates a very complex temperature dependence of the resistivity, which is described using a conceptual phase diagram of Ln 1-x M x MnO 3 . A marked GMR effect was observed for La 0.35 Pr 0.35 Ca 0.3 MnO 3 /LaAlO 3 below 21 K (ca. 10 10 %) and at ca. 70 K even in a field of 1 T.

Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution

The effect of O-16-->O-18 isotope substitution on electrical resistivity and magnetic susceptibility of Sm1-xSrxMnO3 manganites is analyzed. It is shown that the oxygen isotope substitution drastically affects the phase diagram at the crossover region between the ferromagnetic metal state and that of antiferromagnetic insulator (0.4

The effect of oxygen isotope substitution on magnetic properties of (La1-yPry)0.7Ca0.3MnO3 manganites

The effect of oxygen isotope substitution on the alternating-current magnetic susceptibility for (La1-yPry)0.7Ca0.3MnO3 ceramics (y = 0, 0.25, 0.5, 0.75, 1) was studied in the temperature range 4.2-300 K. At y < 0.75, the low-temperature behaviour of these manganites is typical for a ferromagnetic metal, whereas at y close to 1, the system behaves as an antiferromagnetic charge-ordered insulator. The 16O18O isotope substitution significantly shifts the Curie point TC, but has a smaller effect on the Neel temperature TN. At the crossover concentration y = 0.75, the exchange of 16O with 18O gives rise to a metal-insulator transition. The temperature dependence of the magnetic susceptibility is characterized by a pronounced hysteresis at y0 or 1, and the width of the hysteresis loops is strongly affected by the 16O18O exchange. The behaviour of the susceptibility in the paramagnetic phase gives an indication of the phase separation over quite a wide temperature range near the transition point. The phase diagram in the T-y plane is obtained and its modification with isotope substitution is discussed.

The effect of oxygen isotope substitution on the phase diagram of nearly half-doped R1-xSrxMnO3 manganites (R = Sm, NdTb, NdEu)

The effect of isotope substitution on the electrical resistance, magnetic susceptibility and neutron diffraction patterns of nearly half-doped R1?xSrxMnO3 manganites was studied for ceramic samples with R = Sm (x = 0.425?0.525), Nd/Tb (x = 0.45) and Nd/Eu (x = 0.45). In Nd/Tb and Nd/Eu manganites, the composition was chosen to yield the same mean ionic radius as in the Sm compound, but the samples differed in the cation disorder parameter ?. The oxygen isotope substitution leads to fundamental changes in the phase diagram of the manganites studied, favouring enhanced inhomogeneity, stabilizing the insulating antiferromagnetic (AFM) state and even causing a metal?insulator transition (MIT). The MIT induced by the oxygen isotope substitution was found here for the first time in manganites with A-type AFM; earlier this unusual phenomenon was observed only for compounds with the CE-type AFM structure. This suggests that such a MIT is closely related to the ferro?AFM crossover independently on the specific nature of the AFM phase and related orbital order.

Phase Separation in La-Pr Manganites and Its Evolution in a Magnetic Field

The effect of the magnetic field H on the electrical resistivity ρ and magnetic moment M of (La0.25Pr0.75)0.7Ca0.3MnO3 ceramic samples with various concentrations of 16O and 18O isotopes is studied. The behavior of ρ(H) and M(H) is characterized by an unusual shifted hysteresis loop and slow relaxation at some of its sections. The effects observed are interpreted within the framework of a model that takes into account the coexistence of regions of a ferromagnetic metal and an antiferromagnetic insulator and their evolution in an external magnetic field.