Showing papers by "M. R. Ibarra published in 1999"

Structural Instability of the Charge Ordered Compound Nd 0.5 Sr 0.5 MnO 3 under a Magnetic Field

Abstract: We report an anomalous magnetostriction behavior of the charge ordered compound Nd0.5Sr0.5MnO3. We have found that the applied magnetic field not only gives rise to a large negative magnetoresistance but also produces a huge positive magnetovolume effect. This unusual effect is explained considering that the applied magnetic field induces a structural transition at which the volume drastically increases. This effect is also seen in the anisotropic magnetostriction which shows clear anomalies at the field induced transition.

Magnetic structure of gdcu through the martensitic structural transformation : a neutron-diffraction study

Abstract: We report investigations on the magnetic structure through the martensitic structural transformation in the GdCu system obtained by means of neutron-diffraction experiments. At room temperature, the as-cast bulk samples adopt a CsCl-type crystallographic structure, but when the temperature is lowered a martensitic structural transformation $\mathrm{CsC}\stackrel{\ensuremath{\rightarrow}}{l}\mathrm{FeB}$ takes place at around 250 K propagating down to 120 K. After a thermal cycle through the forward and the reverse transformation, at room temperature the percentage of both phases is found to be \ensuremath{\sim}25% for the CsCl-type structure and \ensuremath{\sim}75% for the FeB-type one. In contrast, in powdered samples the CsCl-type phase is stable at any temperature. A comparative neutron thermodiffractometric study in both types of samples allows us to separate and investigate the magnetic behavior of these phases. The magnetic structure of the CsCl-type phase below ${T}_{N}^{\mathrm{CsCl}}=150\mathrm{K}$ is most consistent with a simple antiferromagnetic one with a propagation vector ${\mathbf{Q}}^{\mathbf{CsCl}}=(\frac{1}{2},\frac{1}{2},0),$ the magnetic moments lying along the c direction. However, for the FeB-type structure below ${T}_{N}^{\mathrm{FeB}}=45\mathrm{K},$ the situation is more complex: a helimagnetic structure with a propagation vector ${\mathbf{Q}}^{\mathbf{FeB}}=(0,\frac{1}{4},\frac{1}{4})$ is proposed. Furthermore, it is concluded that while $R\mathrm{Cu}$ cubic magnetic structures could be understood within a simple isotropic free-electron Ruderman-Kittel-Kasuya-Yosida model, an exchange anisotropy is needed in the orthorhombic ${\mathrm{GdNi}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ compounds to account for the evolution of the magnetic structures. Finally, an insight into the mechanism of the martensitic transformation is also discussed.

A 55Mn nuclear magnetic resonance study of mixed-valence manganites

Abstract: The temperature dependence of the 55Mn nuclear magnetic resonance of some manganese perovskites of the form [La1-xAx]MnO3 (A = Ca,Sr) for x of 1/3 is reported. The 55Mn spin-echo spectra were measured in zero field and in an applied magnetic field up to 6 T for polycrystalline samples of [La0.67-yTbyCa0.33]MnO3 (y = 0,0.1,0.22), [La0.6Y0.07Ca0.33]MnO3 and [La0.7Sr0.3]MnO3. The single resonance line observed for all compounds, for temperatures between 4.2 K and 293 K, is attributed to a fast carrier motion (<10-9 s) between the Mn3+ and Mn4+ sites resulting from the double-exchange mechanism. For some of the samples the zero-field nuclear magnetic resonance signal has been detected at temperatures far above the magnetic ordering temperature, providing direct evidence of magnetic correlations whose lifetime is greater than 10-5 s. The correlations can possibly be related to magnetic polarons.

X-ray magnetic circular dichroism probe of the Rh magnetic moment instability in Fe 1-x Rh x alloys near the equiatomic concentration

Abstract: X-ray magnetic circular dichroism (XMCD) measurements performed at the Rh ${L}_{2,3}$ edges and Pd ${L}_{3}$ edge in ${\mathrm{Fe}}_{0.51}{\mathrm{Rh}}_{0.49}$ and ${\mathrm{Fe}}_{1.005}({\mathrm{Rh}}_{0.85}{\mathrm{Pd}}_{0.15}{)}_{0.995}$ evidence the instability of the Rh magnetic moment $({\ensuremath{\mu}}_{\mathrm{Rh}})$ through the ferromagnetic-antiferromagnetic transition and demonstrate the induced nature of both ${\ensuremath{\mu}}_{\mathrm{Rh}}$ and ${\ensuremath{\mu}}_{\mathrm{Pd}}.$ In the case of ${\mathrm{Fe}}_{0.51}{\mathrm{Rh}}_{0.49}$ these experiments have also revealed the existence of a ferromagnetic ground state at low temperatures in which ${\ensuremath{\mu}}_{\mathrm{Rh}}\ensuremath{\sim}0.7{\ensuremath{\mu}}_{B}.$

Magnetization and magnetocrystalline anisotropy of R2Fe17 intermetallics under pressure

Abstract: The measurements of magnetization and magnetic anisotropy of R2Fe17 (R=Y, Er, Ho, and Gd) single crystals were performed under hydrostatic pressure up to 1 GPa, in temperature range 5–300 K and at magnetic fields up to 5 T, using the SQUID magnetometer with a miniature pressure CuBe cell A pronounced decrease of spontaneous magnetization M of the R2Fe17 intermetallics has been observed under pressure The values of dM/dp vary in an interval from −032μB/fu per kbar (Y2Fe17) up to −01μB/fu per kbar (Ho2Fe17) The R elements in the R2Fe17 intermetallics have a strong influence on the d(ln mFe)/dp values at 5 K A remarkable decrease of the anisotropy constant with pressure, d ln K1/dp=−17×10−2 kbar−1 at 5 K, was observed on the Y2Fe17 compound In the Gd2Fe17 compound, |K1(T)| increases with increasing pressure below 150 K and decreases with increasing pressure above this temperature value The critical field HC(T) of the FOMP in the Er2Fe17 compound exhibits a similar behavior, it increases under pr

Correlation of the magnetic and magnetotransport properties in a Nd 1.5 Sr 1.5 Mn 2 O 7 single crystal

Abstract: A detailed study of the anisotropic magnetic and magnetotransport behavior has been performed on a single crystal of the layered perovskite ${\mathrm{Nd}}_{1.5}{\mathrm{Sr}}_{1.5}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}.$ The magnetic properties are consistent with an antiferromagnetic (AFM) ordering below ${T}_{N}=190\mathrm{K}$ of the manganese moments within the ab plane and a moment reorientation around \ensuremath{\approx}125 K towards the c direction. A spin-glass behavior develops for $Tl50\mathrm{K}$ attributed to the freezing of the magnetic moment of the Nd(1) ions placed within the perovskite blocks, whereas the Nd(2) ions in the rocksalt layer contribute to the observed AFM order. An additional ferromagnetic contribution is observed below 275 K, ascribed to the existence of intergrowths. Low-field magnetoresistance (MR) is observed for $H\ensuremath{\parallel}c$ in both ${\ensuremath{\rho}}_{c}$ and ${\ensuremath{\rho}}_{\mathrm{ab}},$ reaching values up to 30% at $T=10\mathrm{K}.$ It is associated with the reduction of the spin disorder of the Mn moments aligned parallel to the c axis. For higher-field values the sharp decrease in MR is correlated with the magnetic ``flipping'' transitions. Within the whole temperature range the magnetoresistence is anisotropic, depending on both the direction of the applied field and current. Evidence is provided against the existence of spin-polarized tunneling between the layers in this compound.