11 research outputs found
Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3
The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of
thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated.
The structural transition of the SrTiO3 substrate induces a magnetic transition
in the manganite films due to magnetoelastic coupling. Below the temperature
of the structural transition additional steps in the magnetization reversal
characteristics appear characterized by clearly defined coercive fields. These
additional coercive fields depend on the cooling history of the sample and
are related to the formation of structural domains in the La0.7Sr0.3MnO3 films
induced by the substrate
Identification of a possible superconducting transition above room temperature in natural graphite crystals
Measuring with high precision the electrical resistance of highly ordered
natural graphite samples from a Brazil mine, we have identified a transition at
350~K with 40~K transition width. The step-like change in
temperature of the resistance, its magnetic irreversibility and time dependence
after a field change, consistent with trapped flux and flux creep, and the
partial magnetic flux expulsion obtained by magnetization measurements, suggest
the existence of granular superconductivity below 350~K. The zero-field virgin
state can only be reached again after zero field cooling the sample from above
the transition. Paradoxically, the extraordinarily high transition temperature
we found for this and several other graphite samples is the reason why this
transition remained undetected so far. The existence of well ordered
rhombohedral graphite phase in all measured samples has been proved by x-rays
diffraction measurements, suggesting its interfaces with the Bernal phase as a
possible origin for the high-temperature superconductivity, as theoretical
studies predicted. The localization of granular superconductivity at these two
dimensional interfaces prevents the observation of a zero resistance state or
of a full Meissner state.Comment: 14 pages with 21 figure
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Weak electron irradiation suppresses the anomalous magnetization of N-doped diamond crystals
Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of âČ80âppm, the so-called C or P1 centers, are irradiated with electrons at 10âMeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30âK, after a decrease in âČ40âppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered
Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3
The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of
thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated.
The structural transition of the SrTiO3 substrate induces a magnetic transition
in the manganite films due to magnetoelastic coupling. Below the temperature
of the structural transition additional steps in the magnetization reversal
characteristics appear characterized by clearly defined coercive fields. These
additional coercive fields depend on the cooling history of the sample and
are related to the formation of structural domains in the La0.7Sr0.3MnO3 films
induced by the substrate
Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3
The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of
thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated.
The structural transition of the SrTiO3 substrate induces a magnetic transition
in the manganite films due to magnetoelastic coupling. Below the temperature
of the structural transition additional steps in the magnetization reversal
characteristics appear characterized by clearly defined coercive fields. These
additional coercive fields depend on the cooling history of the sample and
are related to the formation of structural domains in the La0.7Sr0.3MnO3 films
induced by the substrate
Existence of a magnetically ordered hole gas at the La
The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form new states of matter. Here La0.7Sr0.3MnO3/SrRuO3 superlattices with coherently grown interfaces and layer thicknesses down to one unit cell were fabricated by pulsed laser deposition. The superlattices were studied by X-ray, HRTEM, magnetization and magnetotransport measurements. For such small thicknesses La0.7Sr0.3MnO3 films are antiferromagnetic and insulating. Despite the small layer thickness, the LaSrMnO layers in the superlattices were ferromagnetic with Curie temperatures close to room temperature. Whereas the resistivity of the superlattices showed metallic behaviour and was dominated by the conducting SrRuO3 layers, the off-diagonal resistivity showed an anomalous Hall effect with ferromagnetic loop shape even far above the Curie temperature of the SrRuO3 layers as well as a positive high field slope. This indicates the presence of a highly conducting, ferromagnetically ordered hole gas at the interfaces that might be formed by a charge-transfer process. This result opens up an alternative route for the fabrication of quasi-two-dimensional systems
Nanometers-Thick Ferromagnetic Surface Produced by Laser Cutting of Diamond
In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a âČ20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface