J. L. Martinez

Low-temperature magnetoresistance in polycrystalline manganites: connectivity versus grain size

We have performed magnetic and transport measurements on La0.7Ca0.3MnO3 polycrystalline pellets and thin films. We propose a model that reproduces very well the low-field and high-field magnetoresistance at low temperature. The decrease of the resistance at high fields is modeled by the opening of new conduction channels in parallel due to the ordering of Mn spins blocked at the grain surface. We found that high-field magnetoresistance is independent of grain size, at least down to 12 nm, but depends strongly on the connectivity between grains.

Origin of neutron magnetic scattering in antisite-disordered Sr 2 FeMoO 6 double perovskites

Antisite disordering in Sr2FeMoO6 double perovskites ~containing Mo atoms at Fe positions, and vice versa! has recently been shown to have a dramatic influence in their magnetic and magnetotransport properties. In the present paper, two polycrystalline Sr 2FeMoO6 samples showing different degrees of antisite disorder ~a nominally ‘‘ordered’’ sample with ;70% of cationic ordering and a nominally ‘‘disordered’’ sample with ;18% of cationic ordering! have been examined by magnetic measurements and neutron powder diffraction techniques in the 15‐500 K temperature range. Our main finding is that the ‘‘disordered’’ sample exhibits a strong magnetic scattering ~noticeable even at 500 K!, comparable to that displayed by the ‘‘ordered’’ one below TC5415 K. For the ‘‘disordered’’ sample, the magnetic scattering exhibited on low-angle Bragg positions, is not to be ascribed to a ~nonexistent! ferrimagnetic ordering: our results suggest that it originates upon naturally occurring groups of Fe cations in which strong antiferromagnetic ~AFM! Fe-O-Fe superexchange interactions are promoted, similar to those existing in the LaFeO3 perovskite. These Fe groups are not magnetically isolated, but coupled by virtue of Fe-O-Mo AFM interactions, which maintain the long-range coherence of this AFM structure. Susceptibility measurements confirm the presence of AFM interactions below 770 K.

LaCaMnO manganites thin films on silicon

Abstract We report on the processing of La 0.67 Ca 0.33 MnO 3 films by DC-sputtering on Silicon. The films are fully characterised by X-ray diffraction, magnetisation and resistivity. Thermal protocols are optimised to improve the film properties. We have also studied the effect of the film thickness on the low-field magnetoresistance of these systems. Our results evidence the hardening as the thickness of the film decreases.

Structural and magnetoresistance study of LaxMnyO3±z☆

We study the system LaxMnO3±z in order to produce proper self-doping (Mn3+Mn4+ ratio) by La vacancies only, in place of divalent substitution. The system is stable in the range 0.8 < x < 1.0, −0.05 < z < 0.05, with a rhombohedric structure, close to the orthorhombic one, but a mixed phase of both could also be present. We study the Ca substitution, as well as the Ni doping, in order to fix the Mn valence state. The system is ferromagnetic with Tc spanning from 200 to 300 K depending on the doping level, with a saturation value ≈2.7 μB/Mn atom. LaxMnO3±z present a metallic-insulator transition, and a magneto-resistance effect close to 75% at 200 K under an applied magnetic field of 9 T, with RT (300 K) value close to 50%.

Anomalous ground state in U0.9Th0.1Be13

Abstract By studying the transformation of transport, calorimetric and magnetic properties of UxTh1−xBe13 (0.64 ⩽ x ⩽ 1) solid solutions, we have found that U0.9Th0.1Be13 displays a non-Fermi liquid ground state which corresponds to the two channel Kondo effect (TCKE). For the first time, together with the anomalous temperature dependences of the thermodynamic properties, we have observed that the resistivity changes as the square root of temperature, as had been predicted theoretically for TCKE.

Influence of the microstructure on the magnetoresistance of manganite thin films

We present magnetic and electric transport studies on several series of La 0.7Ca0.3MnO3 polycrystalline pellets and thin films. The thin films were obtained by DC magnetron sputtering on several substrates and by sol-gel deposition technique on Si (100). The diffraction patterns show the polycrystalline character of films grown on Si by both techniques. The morphology was studied by AFM and the mean grain sizes were obtained. The steady decrease of the low temperature magnetoresistance (MR) at high fields is explained by the increase of the effective section for conduction by the orientation of pinned Mn magnetic moments at the surface of the grains and, therefore, depend strongly on the connectivity between grains. On the contrary, the low field MR is related to the magnetic behavior of the ferromagnetic domains of the grains. Low and high field MR characteristi cs of thin films deposited on different substrates are correlated to their microstructure.

Raman phonons in orthorhombic manganites

Abstract We have investigated the Raman phonons of several series of manganites: La x Mn y O 3 , La 1 − x Ca x MnO 3 , RMnO 3 (R = rare earth) with the (Pbmn) orthorhombic structure. Raman spectra of non-doped samples, RMnO 3 , correspond quite well with group theory analysis predictions but present anomalies in the width and intensity of several modes. The spectra of Ca-doped and non-stoichiometric samples are explained taking into account structural effects (both the disorder induced by doping and the reduction of the mean orthorhombic distortion) and the coupling of the phonons to the more or less delocalized e g electrons, related to polarons.

Spin-glass-like static and dynamic properties of mechanically alloyed Fe–Re–Cr

We report the observation of spin-glass-like features in a Fe61Re30Cr9 sample obtained after high energy milling for 460 h. X-ray diffraction showed a broad nanocrystalline peak. The zero-field-cooled and field-cooled magnetization curves exhibited a clear irreversibility, the former showing a relatively sharp maximum for low fields at Tmax≈60u200aK. In addition, the temperature dependence of the magnetic AC susceptibility (χac) also showed a peak at approximately the same temperature. The frequency sensitivity of such maximum (p=ΔTmax/TmaxΔu200alogu200aw≈0.02) is small compared to that of typical superparamagnets. However, we discuss the insufficiency of this set of phenomena to assert the existence of a true spin-glass transition and propose the blocking of interacting ferromagnetic nanoparticles as the origin of the observed behavior. The effect of annealing, rendered into nanocrystallite growth, on the χac maximum is studied in an attempt to test this explanation.

Magnetism of RCaCrO4 (R: Pr, Nd, Sm and Eu) oxides

Abstract SmCaCrO 4 and EuCaCrO 4 oxides have been characterized by means of X-ray powder diffraction and magnetization measurements. Both oxides present a distorted K 2 NiF 4 -type structure with orthorhombic symmetry, space group Bmab . The orthorhombic strain in the RCaCrO 4 (R: Pr–Eu) oxides increases with decreasing R 3+ ion size, taking the S o parameter values of 4.0×10 −3 and 13.3×10 −3 for praseodymium and europium compounds, respectively. Samarium and europium compounds show antiferromagnetic behavior on the chromium sublattice with an estimated T N of 150 and 160 K, respectively. The observed T Cr 3+ N variation in the RCaCrO 4 (R: Pr–Eu) oxides has been explained taking into account the competition between intra and interlayer magnetic interactions that take place in the CrO 2 layers of the structure. Concerning the rare earth sublattice, the Sm 3+ ions present antiferromagnetic order below 3 K while the Eu 3+ ions remain magnetically disordered because of their non-magnetic ground state.

Ground state properties of U1−xThxBe13 (0⩽x⩽0.15)

By studying the temperature (2<T<15 K) and magnetic field (H<12 T) dependences of the Hall effect in U1−xThxBe13 with 0⩽x⩽0.15 we have observed the transition from a magnetic to a nonmagnetic Kondo ground state. The two-channel Kondo scaling analysis of the nonmagnetic phase (x⩾0.1) indicates that the quadrupolar Kondo temperature increases with Th content.