Showing papers by "M. R. Ibarra published in 2005"

Hydrostatic pressure control of the magnetostructural phase transition in Gd5Si2Ge2 single crystals

Abstract: Magnetic and structural properties of single crystalline Gd5Si2Ge2 under hydrostatic pressure have been characterized by using magnetization, linear thermal expansion, and compressibility measurements. A strong dependence of Curie temperature on pressure, dTC∕dP=+4.8 K∕kbar, is observed in contrast with the smaller values of about 3 K∕kbar found in polycrystalline specimens. This difference reflects the role the microstructure may play in pressure-induced magnetic-crystallographic phase changes, likely related to stress relaxation at the grain boundaries, domain pinning and/or nucleation of defects. The pressure dependence of the critical magnetic field, d(dHC∕dT)∕dP, drops at the rate −0.122(5)kOe∕K kbar, which points to an enhancement of the magnetoelastic coupling with pressure. The latter affects the magnetocaloric behavior of the material at the rate d(ΔSM)∕dP≅1.8 J∕kg K kbar. The linear thermal expansion confirms the strongly anisotropic change of the lattice parameters through the orthorhombic to monoclinic crystallographic transformation with Δa∕a=+0.94%, Δb∕b=−0.13%, and Δc∕c=−0.22%. The structural transition temperature varies with pressure synchronously with the Curie temperature, and the size and shape of the strain anomalies remain nearly unaffected by the hydrostatic pressure, indicating, respectively, that the structural and magnetic transformations remain coupled, and the anisotropic behavior of the lattice is preserved as pressure increases. The room temperature linear compressibility data show that the magnetostructural transformation can be triggered isothermally at ∼6 kbar and that the compressibility is anisotropic.

Investigation of the high Curie temperature in Sr{sub 2}CrReO{sub 6}

Abstract: A detailed neutron diffraction study of ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{Re}{\mathrm{O}}_{6}$ half-metallic double perovskites has been carried out With increasing Cr content, the temperature for the cubic-tetragonal transition decreases (${T}_{t}\ensuremath{\approx}490\phantom{\rule{03em}{0ex}}\mathrm{K}$ for $x=0$ and $\ensuremath{\approx}260\phantom{\rule{03em}{0ex}}\mathrm{K}$ for $x=1$) and the cell volume shrinks more than 2% while the Curie temperature increases substantially (${T}_{C}\ensuremath{\approx}420\phantom{\rule{03em}{0ex}}\mathrm{K}$ for $x=0$ and $\ensuremath{\approx}620\phantom{\rule{03em}{0ex}}\mathrm{K}$ for $x=1$) We discuss the origin of the anomalously high ${T}_{C}$ of ${\mathrm{Sr}}_{2}\mathrm{Cr}\mathrm{Re}{\mathrm{O}}_{6}$ in terms of structural and band hybridization effects

Grain-boundary magnetoresistance up to 42 T in cold-pressed Fe3O4 nanopowders

Abstract: The magnetoresistance (MR) in cold-pressed magnetite nanopowders has been studied using pulsed magnetic field up to 42 T and steady field up to 12 T. Ball milling in air produces pure and stoichiometric Fe3O4 grains of nanometric size coated by a thin layer of Fe2O3, which electrically isolates the magnetite and acts as a tunnel barrier. Therefore, the intergrain magnetoresistance of magnetite grain boundaries can be analyzed regardless of the bulk transport properties. At high fields and high temperature, the MR depends linearly on the field, whereas at lower fields a direct tunneling contribution governed by the surface magnetization appears. Below the Verwey transition (T<120K) the linear high-field MR disappears. We interpret these results in terms of the grain-boundary properties.

Possible Quantum Critical Point in La2=3Ca1=3Mn1 xGaxO3

Abstract: We study the magnetic ground state in La2=3Ca1=3Mn1� xGaxO3 manganites, where a quantum critical point (QCP) has been theoretically predicted. The metallic ferromagnetic ground state for low Ga doping breaks down for x � 0:11, an insulating state being established at low temperatures. Long-range ferromagnetism coexists with short-range magnetic correlations in the concentration range 0:11 � x � 0:145 while only the short-range correlations survive for x � 0:16. We discuss the implications of such a QCP to the physics of manganites and compare to other QCP systems.

Direct evidence of the orbital contribution to the magnetic moment in AA'FeReO6 double perovskites

Abstract: Spin and orbital magnetic moments of Re in AA'FeReO6 double perovskites (A,A' = Ba, Sr, Ca) have been directly probed employing XMCD spectroscopy at the Re L2,3-edges. A considerable orbital magnetic moment is observed in all the studied compounds despite octahedral coordination. Relative orbital to spin contribution per Re atom rises with lattice distortion from mL/mS = -0.28 to -0.34 for AA'=Ba2FeReO6 and Ca2FeReO6, respectively. A preliminary XMCD measurements at the Fe L2,3-edges reveals also a significant orbital moment of iron in Ca2FeReO6. The relation of the results to the magnetic properties of the compounds is discussed.

From magnetoelectronic to biomedical applications based on the nanoscale properties of advanced magnetic materials

Abstract: In this paper, we review some results obtained in our group in the framework of research projects concerning magnetoelectronic and biomedical applications with advanced magnetic materials. First, we focus on the type of materials used, and then we describe specific magnetoelectronic devices based on spin-dependent tunnelling and magnetic nanoparticles for biomedical applications. Special attention is drawn to the phenomena occurring at the nanometric scale, which in most cases completely determine the observed macroscopic properties.

Transport and magnetic study of the spin reorientation transition in the Tb5(Si0.5Ge0.5)4 magnetocaloric compound

Abstract: Detailed measurements of the electrical resistivity ?(T), thermopower S(T) and magnetization of Tb5(Si0.5Ge0.5)4 in the vicinity of the spin reorientation transitions observed in this compound are reported. Our results indicate a complex spin reorientation process associated with three different lattice sites occupied by the Tb ions. We identify two critical transition temperatures: one at TSR1 = 57?K, as previously reported, and a new one at TSR2 = 40?K. A simple model based on an approximate magnetic anisotropy energy is presented; it gives a satisfactory qualitative description of the main features of the reorientation processes.

Multi-step and anomalous reproducible behaviour of the electrical resistivity near the first-order magnetostructural transition of Gd5(Si0.1Ge0.9)4

Abstract: Very detailed measurements of the electrical resistivity of Gd5(Si0.1Ge0.9)4 are here reported, with special emphasis on the vicinity of the first-order (magnetostructural) martensitic transition which occurs at K. The data cover more than fifty thermal cycles spanning the temperature ranges of 300–10 K (long cycles) and 105–10 K (short cycles). In the initial 10–300 K cycles the martensitic transition takes place in three closely-spaced steps, with associated resistance (R) discontinuities and large thermal hysteresis. In a subsequent series of short cycles (10–105 K) a unique transition occurs, exhibiting a common and quite reproducible R(T) behaviour within a small temperature range ( K) below TS, either in heating or cooling runs. Remarkably, this 'local reproducibility' (within ΔT) remains in spite of the significant resistance changes which occur outside the ΔT-range under thermal cycling. In particular the residual resistance systematically increases under thermal cycling, but the corresponding effect is absent in the ΔT temperature range. This excludes microcracking as a dominant resistive mechanism in our results, pointing to an intrinsic character of the reproducible behaviour just below TS. We also analyse the R(T) behaviour when changing from long to short thermal cycles, and the R(T) evolution towards a reversible final behaviour, after extended thermal cycling.

Carbon coated magnetic nanoparticles for local drug delivery using magnetic implants

Abstract: Bioferrofluids obtained from carbon coated iron nanoparticles are promising candidates for magnetic drug delivery. The carbon cages render the particles biocompatible, and provide a good support for drug adsorption. We propose a method in which gold plated permanent magnets are implanted directly in the affected organ, close to the tumour, by endoscopic techniques. The bioferrofluid charged with the chemotherapeutic agent is injected and the particles attracted to the magnet, then desorption of the drug takes place at the tumoral region. This method seems to be more promising, costless and effective than that based on the application of external magnetic fields. Preliminary results of drug adsorption and a preclinical experimental animal model are described.

Nuclear magnetic resonance study of insulator-metal transitions in (Pr,Ca) MnO 3

Abstract: An NMR study of polycrystalline ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$ ($x=0$ and 0.03) and ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ at 3 K is presented. Zero-field spin-echo spectra of the Ga-doped compound consist of an overlapping $^{69,71}\mathrm{Ga}$ signal at 74 MHz (hyperfine field of 5.3 T) and a $^{55}\mathrm{Mn}$ double-exchange (DE) line at 375 MHz (35.5 T). Measurements in an applied field show a steplike increase in the DE line intensity, which corresponds to an increase of the amount of the ferromagnetic metallic phase. This coincides with a steplike feature in the bulk magnetization measurements. The effect is similar to that in the previous field-dependent $^{55}\mathrm{Mn}$ NMR measurements of ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$. The insulator - metal transition for ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$, at ambient pressure, occurs at fields greater than 5 T; however, at 1.1 GPa, the DE line corresponding to the ferromagnetic metallic phase is already present at zero field.

Pressure effect on magnetic and magnetotransport properties of intermetallic and colossal magnetoresistance oxide compounds

Abstract: The joint power of neutron diffraction and pressure techniques allows us to characterize under unique conditions th en ature and different role of basic interactions in solids. We have covere dab road phenomenology in archetypical compounds: intermetallics and magnetic oxides. We have selected compounds in which the effect of moderate pressure is able to modify the electronic structure and bond angles that in turn are in the bases of magnetic and structural transitions. Complex magnetic and structural phase diagrams are reported for compounds with magnetic (Tb1−X YX Mn2) and structural (RE5Si4−X GeX ) instabilities. Pressure-induced change of the magnetic structure in (R2Fe17) intermetallics and the effect on the colossal magnetoresistance manganites are described.

Huge Ballistic Magnetoresistance in Multiple Nanocontacts Devices

Abstract: In this paper we report an exhaustive experimental work on magnetoresistance effects found in a system in which a large number of nanocontacts are produced between oxidized Fe fine particles. We have obtained the following performances: i) Huge low field room temperature magnetoresistance (over 1000%). ii) Non-linear I-V at different applied fields and temperatures. iii) Large thermal stability and reproducible resistance value under thermal cycles from room temperature down to 5 K. iv) Easy to fabricate with an almost 100% success. v) Heavy duty and transportable samples with reproducibility tested in several laboratories. We realized that the extraordinary effect found is related to the oxygen content at the particles surface