O. Yu. Gorbenko

Magnetocaloric effect in La1?xAgyMnO3 (y ? x): direct and indirect measurements

For the first time the magnetocaloric properties of La0.9Ag0.1MnO3, La0.8Ag0.2MnO3, La0.85Ag0.15MnO3, La0.8Ag0.15MnO3 and La0.8Ag0.1MnO3 manganites have been investigated by direct and indirect measurement techniques. All samples showed almost the same relative cooling power (RCP). Temperatures of maxima of the magnetocaloric effect (MCE) are between a few degrees below freezing and the room temperature region. The compounds showed RCP values of about 100 J kg−1 at a field change of 2.6 T, which is about half the RCP of gadolinium. Because of considerable MCE and the Curie temperatures ranging from 269 to 303 K, these materials could be used as magnetic refrigerants for magnetic refrigeration in the sub-room and room temperature range.

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.

Epitaxial phase stabilisation phenomena in rare earth manganites

Abstract Using injection, powder-flash and band-flash MOCVD techniques, epitaxial thin films of Nd, Ho, Y, Tm, Dy and Lu manganites were prepared. The formation of high-pressure phase NdMn 7 O 12 as a thin surface layer on Pnma Nd 1− x MnO 3+δ ( x ∼0.15) in an Nd-deficient system and HoMnO 3 , YMnO 3 , TmMnO 3 and LuMnO 3 as perovskite phases (known in the bulk only as high-pressure phases) on a LaAlO 3 substrate were observed. The remarkable reduction in the pressure needed for formation of the phases in epitaxial films in comparison with bulk samples is a manifestation of the epitaxial stabilisation discussed in the paper. The details of the crystal and domain structure of the films as observed by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution electron microscopy (HREM) techniques are described.

Ag-doped manganite nanoparticles: New materials for temperature-controlled medical hyperthermia

The purpose of this study was to introduce newly synthesized nanomaterials as an alternative to superparamagnetic ironoxide based particles (SPIO) and thus to launch a new platform for highly controllable hyperthermia cancer therapy and imaging. The new material that forms the basis for this article is lanthanum manganite particles with silver ions inserted into the perovskite lattice: La(1-x)Ag(x)MnO(3+delta). Adjusting the silver doping level, it is possible to control the Curie temperature (T(c)) in the hyperthermia range of interest (41-44 degrees C). A new class of nanoparticles based on silver-doped manganites La(1-x)Ag(x)MnO(3+delta) is suggested. New nanoparticles are stable, and their properties were not affected by the typical ambient conditions in the living tissue. It is possible to monitor the particle uptake and retention by MRI. When these particles are placed into an alternating magnetic field, their temperature increases to the definite value near T(c) and then remains constant if the magnetic field is maintained. During the hyperthermia procedure, the temperature can be restricted, thereby preventing the necrosis of normal tissue. A new class of nanoparticles based on silver-doped manganites La(1-x)Ag(x)MnO(3+delta) was suggested. Ag-doped perovskite manganites particles clearly demonstrated the effect of adjustable Curie temperature necessary for highly controllable cellular hyperthermia. The magnetic relaxation properties of the particles are comparable with that of SPIO, and so we were able to monitor the particle movement and retention by MRI. Thus, the new material combines the MRI contrast enhancement capability with targeted hyperthermia treatment.

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.

Anisotropic magnetoresistance and anomalous Hall effect in manganite thin films

We report on anisotropic magnetoresistance (AMR) and Hall effect measurements along the and [001] directions in a (110)-oriented La2/3Ca1/3MnO3 thin film. While the electrical resistivity and the ordinary Hall coefficient are smaller for I along [001], evidencing some anisotropy of the Fermi surface, both the AMR and the anomalous Hall coefficient are larger when the current is applied along . Since these two phenomena originate from spin–orbit coupling effects, we state that our results support an anisotropy of the spin–orbit interaction in manganites. The possible origin of this anisotropy is discussed.

Kinetic effects in manganites La{sub 1-x}Ag{sub y}MnO{sub 3} (y {<=} x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La1 − xAgyMnO3 (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρFM(T) = ρ0 + AT2 + BT4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.

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.

Synthesis, crystal structure and properties of manganese(II) hexafluoroacetylacetonates Mn(hfa)2(H2O)2 and KMn(hfa)3

Abstract Two manganese(II) hexafluoroacetylacetonate complexes, dihydrate, the Mn(hfa)2(H2O)2 (1), and the bimetallic KMn(hfa)3 (2) have been synthesized and characterized by X-ray single crystal structure analysis. They have a molecular structure and a polymeric structure, respectively. The high stability of 2 allowed sublimation without decomposition. It was then used as a precursor for MOCVD of fluoroperovskite, KMnF3.