A. M. L. Lopes

Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the crystal structure and multiferroic properties of the BiFeO3 perovskite

In this work, we studied the effect of heterovalent Ca, Sr, Pb, and Ba substitution on the crystal structure, dielectric, local ferroelectric, and magnetic properties of the BiFeO3 multiferroic perovskite. Ceramic solid solutions with the general formula Bi0.7A0.3FeO3 (A is a doping element) were prepared and characterized by x-ray diffraction, dielectric, piezoresponse force microscopy (PFM), and magnetic measurements. It is shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence was confirmed by the PFM data. Magnetic properties of the solid solutions are determined by the ionic radius of the substituting element. Experimental results suggest that the increase in the radius of the A-site ion leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization.

Intrinsic nature of the magnetization enhancement in heterovalently doped Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba) multiferroics

The mechanism of the formation of heterovalent-substitution-induced defects as well as their influence on the magnetic properties of BiFeO3-based multiferroics has been studied. It has been shown that heterovalent A2+ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) compounds, thus indicating the intrinsic (i.e. not related to defects themselves) mechanism of doping-induced enhancement of magnetization. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has been found. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO3.

Tunnel magnetoresistance and magnetic ordering in ion-beam sputtered Co80Fe20/Al2O3 discontinuous multilayers

Discontinuous multilayered Co80Fe20(t)/Al2O3(30 A) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18 A. At t 13 A was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in o...

Room temperature magnetocaloric effect and refrigerant capacitance in La0.7Sr0.3MnO3 nanotube arrays

High aspect ratio La0.7Sr0.3MnO3 nanotube (NT) arrays have been synthesized using nitrates based sol-gel precursor by nanoporous anodized aluminum oxide template assisted method. Their phase purity and microstructures were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Magnetocaloric effect (MCE) of as prepared NTs was investigated by means of field dependence magnetization measurements. Significant magnetic entropy change, −△SM = 1.6 J/kg K, and the refrigerant capacitance, RC = 69 J/kg, were achieved near the transition temperature at 315 K for 5 T. For comparison, a bulk sample was also prepared using the same precursor solution which gives a value of −△SM = 4.2 J/kg K and a RC = 165 J/kg. Though the bulk sample exhibits higher △SM value, the NTs present an expanded temperature dependence of −△SM curves that spread over a broad temperature range and assured to be appropriate for active magnetic refrigeration. The diminutive MCE observed in mangani...

Transport properties of discontinuous cCo/sub 80/Fe/sub 20//Al/sub 2/O/sub 3/ multilayers prepared by ion beam sputtering

Ion beam sputtered Co/sub 80/Fe/sub 20/(t)/Al/sub 2/O/sub 3/ (30 /spl Aring/) multilayers were obtained. The Co/sub 80/Fe/sub 20/ layers become discontinuous for nominal thickness t/spl les/18 /spl Aring/. Tunnel magnetoresistance was measured in CIP and CPP geometries, reaching up to 6.5% at room temperature and 11% at 15 K, for as-deposited films in CIP geometry. The temperature dependence of MR was found quite different for the two geometries: fairly strong in the CIP case and almost absent in the CPP geometry. A model is proposed to explain these large differences in behavior.

Electrical Characterization of a Magnetic Tunnel Junction Current Sensor for Industrial Applications

The objective of the work was the design of a Wheatstone bridge current sensor using MTJ as magnetoresistive elements. Each one of the four resistances of the bridge consists on 360 MTJ single elements connected in series for improved electrical robustness. A printed circuit board (PCB) was designed with a U-shaped copper trace placed under the PCB maintaining a 1.1 mm separation distance between sensor and trace. A 160% of tunnel magnetoresistance effect in the single junction and a 120% in its corresponding series elements connection has been achieved with a sensitivity of 9.2 Ω/Oe in a 65 Oe linear range. The DC sensor sensitivity in response to an external DC current sweeps of ±10, ±20, and ±30 A gave an average of 9.8 mV/A. The measured AC sensor response in all the tested cases corresponded to a - 3 dB frequency close to 200 kHz. The sensor was submitted to a DC current excursion under different temperatures showing a TC(S) sensitivity temperature coefficient of 0.031%/° C rather lower compared with the spin-valve technology. The work shows that MTJ sensor technology provides a promising tool in the R+D areas of power management and energy consumption like electric vehicles or energy metering.

Influence of O and C co-implantation on the lattice site of Er in GaN

The lattice location of low-dose implanted Er in GaN, GaN:O, and GaN:C was investigated using the emission channeling technique. The conversion electrons emitted by the probe isotope 167mEr give direct evidence that the majority (≈90%) of Er atoms are located on substitutional Ga sites for all samples. Annealing up to 900 °C does not change these fractions, although it reduces the Er root-mean-square (rms) displacements. The only visible effect of oxygen or carbon doping is a small increase in the rms displacements with respect to the undoped sample.

Magnetocaloric effect in La0.7Ca0.3MnO3 nanotube arrays with broad working temperature span

We have studied the magnetic entropy change of highly ordered La0.7Ca0.3MnO3 nanotube arrays synthesized by template assisted sol-gel method in temperatures ranging from 179 to 293 K and in magnetic fields up to 5 T. From the measurements of isothermal magnetization, we have calculated the maximum isothermal magnetic entropy change of −△SM = 1.9 J/kg K around the Curie temperature at 236 K for a field of 5 T. The nanotubes present lower magnetic entropy change compared with their bulk counterpart (−△SM = 4.8 J/kg K) which was prepared by the same sol-gel route. Such diminished magnetic entropy change observed in nanotubes is explained by the disordered magnetic states which are created on the surface sites of nanograins due to the larger surface to volume ratio. However, the nanotubes present an expanded magnetic transition that extends over a wide temperature range and suggest that such manganite nanotubes could be used for magnetic refrigeration with broad working temperature span.

Local symmetry lowering in CdMn2O4 spinel

This work presents an atomic scale study of the electric field gradient (EFG) in the tetragonally distorted CdMn2O4 spinel manganite. The EFG temperature dependence at the Cd and Mn sites was followed via perturbed angular correlation measurements with the 111In and 111mCd probes, from 873 down to 12 K at Isolde-CERN. The results show that in the 12–600 K temperature range, a single Jahn-Teller distorted local phase exists. However above 100 K, a dynamic lattice distortion, evidenced by time dependent EFG fluctuations, sets in suggesting a structural instability. Above 600 K, a local MnO6 octahedra with relaxed Jahn-Teller distortions emerge and grow in the low temperature matrix, although no macroscopic tetragonal to cubic phase transition was observed.

Hyperfine local probe study of alkaline-earth manganites SrMnO3 and BaMnO3

We report perturbed angular correlation measurements with (111m)Cd/(111)Cd and (111)In/(111)Cd probes, at the ISOLDE-CERN facility, in the manganite compounds BaMnO3, with the 6H and 15R polymorphs, and SrMnO3, with the 4H polymorph. The electric field gradient (EFG) is measured, and found approximately constant in a large temperature range for all the compounds. The EFG is also calculated from first principles with density functional theory, and compared with experimental results by considering diluted substitutional Cd impurities. Based on the results, we assign as sites for the probes the Ba (for BaMnO3-6H, 15R) and Sr (for SrMnO3-4H) sites, apart from fractions of undetermined origin in the case of BaMnO3-6H. We predict the hyperfine parameters in the recently synthesized multiferroic manganite Sr(0.5)Ba(0.5)MnO3, and its variation with the structure and electric polarization, which is found to be very small.