Showing papers by "M. R. Ibarra published in 1997"

Evidence for magnetic polarons in the magnetoresistive perovskites

Abstract: Manganese perovskites based on the compound LaMnO3 are attracting considerable theoretical and technological interest by virtue of their unusual magnetic and electronic properties1–4. Most notable of these properties is the extremely large change in resistivity that accompanies the application of a magnetic field, an effect known as 'colossal' magnetoresistance. The origin of this effect has been attributed5–7 to the presence of magnetic polarons—charge carriers accompanied by a localized (and magnetically polarized) distortion of the surrounding crystal lattice8,9— but their existence and properties remains a matter of speculation. Here, using a combination of volume thermal expansion (with and without an applied field), magnetic susceptibility and small-angle neutron scattering measurements, we present evidence for the existence of magnetic polarons above the ferromagnetic ordering temperature, Tc. We detect the spontaneous formation of localized ∼12-A magnetic clusters above Tc which, on application of a magnetic field, grow in size but decrease in number. We argue that the response of these magnetic polarons to an applied magnetic field underlies the pronounced magnetoresistive properties in the compounds (La1–xAx)2/3Ca1/3MnO3 (where A is Y or Tb).

Influence of oxygen content on the structural, magnetotransport, and magnetic properties of LaMnO 3 + δ

Abstract: A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}}$ samples, with $\ensuremath{\delta}=0,$ 0.025, 0.07, 0.1, and 0.15. Measurements of the ac initial magnetic susceptibility, magnetization, magnetoresistance, and neutron diffraction, including small-angle neutron scattering (SANS), were performed in the temperature range 1--320 K using high magnetic fields up to 12 T. The antiferromagnetic order found in ${\mathrm{LaMnO}}_{3}$ evolves towards a ferromagnetic order as \ensuremath{\delta} increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The ferromagnetic coupling weakens for $\ensuremath{\delta}g~0.1.$ The magnetic behavior is interpreted by taking into account two effects caused by the increase in \ensuremath{\delta}: cation vacancies and ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for $\ensuremath{\delta}g~0.1.$ The sample $\ensuremath{\delta}=0.1$ shows a structural transition from rhombohedral to orthorhombic below ${T}_{S}\ensuremath{\approx}300\mathrm{K}$ with a huge change in the cell volume. All the studied compounds were found to be insulating at low temperatures with no appreciable magnetoresistance, except for $\ensuremath{\delta}=0.15,$ in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below ${T}_{C}$ for $\ensuremath{\delta}g~0.07,$ which could explain the intriguing ferromagnetic insulator state. In the $\ensuremath{\delta}=0.07$ and $\ensuremath{\delta}=0.10$ samples we found at temperatures below ${T}_{C}$ magnetic and structural anomalies that are characteristic of charge ordering.

Structural, magnetic, and transport properties of the giant magnetoresistive perovskites La 2 / 3 Ca 1 / 3 Mn 1 − x Al x O 3 − δ

Abstract: The effect of the substitution for Mn with Al in the magnetoresistive perovskite ${\mathrm{La}}_{2\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{MnO}}_{3}$ has been studied by preparing the series ${\mathrm{La}}_{2\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{Mn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (x\ensuremath{\leqslant}02) A careful study of the magnetic, structural, and transport properties has been carried out by means of electrical resistance, magnetoresistance, ac magnetic susceptibility, x-ray-diffraction, and neutron-diffraction techniques Up to x=005 the Curie temperature (and the associated metal-insulator transition) decreases drastically with Al doping and the magnetoresistive properties do not change very much For x\ensuremath{\geqslant}01 the lattice spontaneously begins to lose oxygen atoms and for x=02, 3% of oxygen vacancies are present This fact along with the random distribution of the Al atoms makes these compounds rather disordered from a structural and magnetic point of view However, the magnetoresistance is enhanced, reaching colossal values of ${10}^{7}$% at H=12 T at low temperatures for x=02

Lattice effects, stability under a high magnetic field, and magnetotransport properties of the charge-ordered mixed-valence La 0.35 Ca 0.65 MnO 3 perovskite

Abstract: The highly ${\mathrm{Mn}}^{+4}$-doped compound ${\mathrm{La}}_{0.35}{\mathrm{Ca}}_{0.65}{\mathrm{MnO}}_{3}$ has been studied up to high temperature (700 K) by using thermal-expansion, magnetostriction, magnetoresistance, and neutron-diffraction techniques. From 700 K down to room temperature the electrical conduction takes place through thermally activated hopping of polarons with ${E}_{\mathrm{hop}}$=45 meV. At the charge-ordering (CO) transition temperature ${T}_{\mathrm{CO}}$=275 K, pronounced anomalies in the resistivity and the lattice are observed. The neutron thermodiffractogram clearly establishes that the CO state occurs in the paramagnetic (P) phase and is accompanied by a large anisotropic lattice distortion with a simultaneous large distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The antiferromagnetic (AF) phase appears at ${T}_{N}$=160\ifmmode\pm\else\textpm\fi{}3 K. At this temperature no lattice effect is observed. The CO-P and the CO-AF ground states are stable under an applied magnetic field up to 12 T.

Charge ordering at room temperature in

Abstract: has been studied by means of a.c. magnetic susceptibility, x-ray and neutron diffraction from 1.5 up to 330 K. This compound shows a structural transition at 299.7 K. Below this temperature a charge ordering state was observed by both x-ray and neutron diffraction measurements. The structure is orthorhombic at temperatures higher than 300 K, but at lower temperatures the patterns show new reflection peaks that can be accounted for in a monoclinic unit cell with stripes of and in the ac-plane. Moreover, the system orders antiferromagnetically at around 120 K giving rise to a magnetic structure of CE type. The magnetic susceptibility measurement clearly shows an anomalous behaviour at both transition temperatures. The charge ordering temperature for the compound is the highest reported in the series (RE = rare earth). This is argued to be a consequence of the larger orthorhombic distortion produced in the unit cell by the lower size of ion.

Analysis of the intrinsic magnetic properties of R 2 Fe 17 single crystals (R=Y,nDy,nHo,nEr)

Abstract: The magnetic behavior of some R{sub 2}Fe{sub 17} single crystals have been analyzed quantitatively in a wide temperature range, using a two-sublattice approximation for the magnetic structure and taking into account isotropic exchange and single-ion crystal-field interactions. The 3d sublattice behavior has been described phenomenologically, from the study of the experimental magnetization results in a Y{sub 2}Fe{sub 17} single crystal. The parameters A{sub 2}{sup 0}, A{sub 4}{sup 0}, A{sub 6}{sup 0}, A{sub 6}{sup 6}, describing the crystal-field interaction in the different R{sub 2}Fe{sub 17} compounds (R=Er, Dy, Ho) have been determined. The calculated magnetic behavior shows good agreement with experimental magnetization results in the temperature range 4.2 to 300 K, demonstrating the reliability of the determined parameters. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic structures and magnetic phase diagram of Nd{sub x}Tb{sub 1{minus}x}Mn{sub 2}Ge{sub 2}

Abstract: The various magnetic structures and magnetic phase transitions in the series Nd{sub x}Tb{sub 1{minus}x}Mn{sub 2}Ge{sub 2} have been thoroughly studied by means of macroscopic magnetic and thermal measurements (such as magnetization, ac initial magnetic susceptibility and linear thermal expansion) and microscopic neutron-diffraction experiments As a result, the magnetic phase diagram has been determined over the whole temperature range Large changes in the local Mn magnetic moments (eg, {Delta}{mu}{sub Mn}/{mu}{sub Mn}{approx}16{percent} in TbMn{sub 2}Ge{sub 2}) have been detected at the magnetic phase transitions observed at low temperatures, {approx}100{endash}140 K, in the x=0{endash}04 alloys This variation, together with the appearance of magnetic ordering in the rare-earth sublattice, has been related to the volume anomalies found (eg, {Delta}V/V{approx}03{percent} in TbMn{sub 2}Ge{sub 2}) A new magnetic structure of the Mn sublattice in the RMn{sub 2}X{sub 2} (R=rare earth, X=Si, Ge) family has been found in Nd{sub 04}Tb{sub 06}Mn{sub 2}Ge{sub 2} (140K{lt}T{lt}350 K) where two antiferromagnetic commensurate components within the (001) plane coexist with a ferromagnetic component along the c axis The peculiar layered structure of the RMn{sub 2}Ge{sub 2} compounds favors a cancellation of the molecular field at the rare-earth sites in the case of antiferromagnetic arrangements of the Mn sublattice, effectively isolating themore » R atoms and making a paramagnetic behavior of these possible despite the presence of long-range order The existence of a ferromagnetic component in the Mn sublattice has been concluded to be indispensable to allow the ordering of the rare-earth magnetic moments {copyright} {ital 1997} {ital The American Physical Society}« less

Magnetic and magnetoelastic behavior of mechanically alloyed FeRh compound

Abstract: Mechanical milling and subsequent annealing have been applied in order to obtain FeRh compound, as an alternative to bulk sample preparation methods (plasma jet- and arc-melting, induction, etc). Structural and magnetic characterization of the as-milled sample show that heat treatment at the proper temperature is necessary in order to obtain the equilibrium ordered α′-bcc phase, in which a first order ferro–antiferromagnetic transition accompanied by a large magnetovolume effect takes place. In order to study the magnetoelastic behavior of the mechanically alloyed FeRh compound, magnetostriction measurements were performed using high pulsed magnetic fields up to 14.2 T within the temperature range 4–450 K. The large magnetostriction observed in the room temperature range has been related to the field instability of the rhodium magnetic moment within the bcc ordered phase.

Magnetovolume effects and anomalous compressibility of Nd2Fe17 and Nd2(FeTi)17 compounds

Abstract: We present the results of direct measurements of the isothermal compressibility up to 13 kbar on Nd2Fe17 and Nd2(FeTi)17 compounds and the results of measurements of the linear thermal expansion on Nd2Fe17 compound under hydrostatic pressure up to 6 kbar in temperature range 200–400 K covering both ferromagnetic and paramagnetic region using a modified strain gauge method A large magnetic contribution to the compressibility κm∼015 Mbar−1 was observed in the ferromagnetic state of the Nd2Fe17 compound The pronounced lambda-shaped anomalies in the thermal expansion coefficient α (negative values of α) and the maximum of compressibility were simultaneously observed nearby the Curie temperature The corresponding thermodynamic relations are used to analyze the pressure behavior

Cubic-to-tetragonal structural phase transition in Rb 1-x Cs x CaF 3 solid solutions: Thermal expansion and EPR studies

Abstract: The influence of crystal mixing on the structural phase transitions in Rb{sub 1{minus}x}Cs{sub x}CaF{sub 3} (0{lt}x{lt}1) fluoroperovskite crystals has been studied by thermal expansion and EPR measurements of Ni{sup 2+} and Ni{sup 3+} paramagnetic probes. A cubic-to-tetragonal phase transition has been detected in crystals with x=0, 0.1, 0.21, 0.27, and 0.35. The critical temperature and the tetragonal distortion decrease as x increases. No transition was observed for x{ge}0.44. This transition shows a weak first-order component in the x=0 and 0.1 samples, which is progressively smeared out for x{gt}0.1, indicating a spatial distribution of the critical temperature in those crystals with high ionic substitution rate. In RbCaF{sub 3}, another structural phase transition was observed at 20 K with a thermal hysteresis between 20 and 40 K. This transition has not been found in any of the mixed crystals.

Thermal expansion and magnetostriction in colossal magnetoresistance mixed valence manganites (abstract)

Abstract: Structural aspects are closely related to the large spontaneous and field-induced magnetic and magnetogalvanic effects in mixed valence manganites. Local distortions are the source of an extra contribution to the thermal expansion. These were associated with a charge localization process.1 The existence of ferromagnetic order in these compounds favors the carriers mobility and, as a consequence, produces unexpected magnetovolume effects.2 We report thermal expansion measurements within the temperature range 4–800 K and magnetostriction up to 15 T in the series (R1−yRy)1−xCaxMnO3 (R=La, Nd and R′=Y, Tb). We have found completely unusual volume magnetostriction at temperatures at which the system is in the insulator state (paramagnetic, antiferromagnetic, spin-glass states). These results give a deep insight into the mechanism responsible for the colossal magnetoresistance and reveal these experimental techniques as an excellent tool to follow the spontaneous and field-induced localization and delocalizatio...

First-order valence change in CeNi1−xCoxSn (0.35 ⩽ x ⩽ 0.4)

Abstract: The first-order Ce valence change in CeNi1−xCoxSn (035 ⩽ x ⩽ 04) has been studied using neutron diffraction A dramatic change in the lattice parameters takes place at a valence transition temperature, Tv, which is strongly x dependent The observed diffraction patterns are consistent with the orthorhombic Pnma space group above and below Tv The transition occurs with coexistence of the high- and low-temperature phases across a certain range of temperature The very narrow range of concentrations (035 ⩽ x ⩽ 04) which makes this first-order valence change possible seems to indicate that a very specific Ce local environment is necessary for this transition However, a careful Rietveld refinement has not clarified this point

Magnetoelastic and pressure effects at the ferro–antiferromagnetic transition in Hf1−xTaxFe2−y alloys (abstract)

Abstract: Magnetostriction measurements using high pulsed magnetic field up to 14 T and thermal expansion measurements under hydrostatic pressure up to 10 kbar have been performed in the Hf1-xTaxFe2−y alloys with x=0.17 and 0.15 and y=0.02. These compounds have a first-order magnetic phase transition from an antiferromagnetic (AF) high temperature state to a ferromagnetic (F) low temperature state at TAF–F=245 and 290 K, respectively. This transition is accompanied by a volume expansion from the low volume AF state to the high volume F state of ΔV/V≈0.56% and 0.38% for x=0.17 and x=0.15, respectively. The hydrostatic pressure significantly decreases the temperature of the AF–F transition whereas the thermal expansion anomaly is becoming more pronounced. The dependence of TAF–F on pressure follows a quadratic behavior and TAF–F reaches the value of 106 K at 10.5 kbar for x=0.17. In the search for high magnetostrictive materials, we have found in these alloys a giant value of the volume magnetostriction (ω≈0.6% for t...