Research Theme T1

 

(T1)  Magnetic, electrical, thermal and ferroelectric properties of the oxygen-isotope substituted half-doped manganites

CSULA Faculty Participants: Guo-meng Zhao, Oscar Bernal, Jose Rodriguez, Radi Jishi

Penn State Faculty Participants: Roman Engel-Herbert, Susan Trolier-McKinstry

Oxygen-isotope effect on the Curie-temperature in the A-site disordered half-doped manganites

We have performed the oxygen-isotope exchange for the A-site disordered samples and run magnetic measurements on them. Figure 1 shows the oxygen-isotope effects on the Curie temperature TC for A-site disordered Pr0.5Ba0.5MnO3 (Left Panel) and La0.5Ba0.5MnO3 (Center Panel). The oxygen-isotope shift of TC for the Pr sample is 10 K while that for the La one is about 4 K. The substantial oxygen-isotope effect on TC suggests that polarons still play a role in the ferromagnetic ordering in half-doped manganites where the electron-phonon interaction should be strongly reduced due to maximum hole concentration and screening.

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Figure 1. Left panel: Temperature dependence of the field-cooled (FC) susceptibility for the 16O and 18O samples of Pr0.5Ba0.5MnO3; Center Panel: La0.5Ba0.5MnO3; Right Panel: Oxygen-isotope exponent as a function of A-site ionic radius.

 

The strong correlation between the oxygen-isotope exponent (aO=-dlnTC/dlnMO) with the A-site ionic radius (Fig. 1, Right Panel) demonstrates that the polaronic effect decreases with increasing A-site ionic radius. The fact that the TC increases along with A-site ionic radius suggests that the stronger the polaronic effect, the lower TC. These results indicate electron-phonon coupling plays a role in the microscopic mechanisms of the ferro- and antiferro-magnetism, charge-ordering, and orbital ordering.

 

Oxygen-isotope effects on the electric transport properties in the epitaxial thin films of La1-xCaxMnO3 with x = 0.20, 0.30, 0.40. We have performed electrical resistivity and Hall coefficient measurements on the oxygen-isotope exchanged thin films of La1-xCaxMnO3 with x = 0.20, 0.30, 0.40. The substrate and thickness of the films are LaAlO3 and about 150 nm, respectively.  The Van der Pauw method was used to measure the absolute resistivity and Hall coefficient in the epitaxial thin films with the same thickness over the entire area.

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Figure 2. Temperature dependence of the resistivity for the 16O and 18O films of La1-xCaxMnO3 with x = 0.20 (left panel) and x = 0.40 (right panel).

 

The oxygen-isotope shift of TC (corresponding to the peak position of the resistivity curve in Figure 2) for x = 0.20 is about 30 K while the shift is reduced to about 10 K for x = 0.40. The giant oxygen-isotope shift of TC for the x = 0.20 film suggests that the polaronic effect is very strong at lower hole doping levels. The polaronic effect is reduced but still substantial at the high doping level (x = 0.40). The results clearly demonstrate that the polaronic effect plays a very important role in the ferromagnetic ordering in these metallic ferromagnets.

The temperature dependence of the resistivity at low temperatures can be quantitatively explained in terms of small-polaron conduction (see fitted curves in Figure 3, left panel).  The magnitude of the low temperature resistivity is also strongly dependent on the oxygen-isotope mass: the residual resistivity of the 18O sample is 81% higher than the 16O sample. The huge oxygen-isotope effect on the residual resistivity can be only explained by the strong polaronic effect even at the ferromagnetic state of the x = 0.20 sample. For the x = 0.40 sample, the oxygen-isotope effect on the residual resistivity is significantly reduced (about 10%), indicating a much weaker polaronic effect.

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Figure 3. Temperature dependence of the resistivity for the 16O and 18O films of La0.80Ca0.20MnO3 at low temperatures (left panel) and at high temperatures just above TC (right panel).

 

The temperature dependence of the resistivity at high temperatures (above TC can be quantitatively explained in terms of a model where small-polaron and small-bipolarons coexist (see fitted curves in Figure 3, right panel). The magnitude of the resistivity is also strongly dependent on the oxygen-isotope mass, which is consistent with the fact that the 18O sample has a narrower polaronic bandwidth than the 16O sample by about 20%.

 

Magnetoresistance effect in epitaxial multilayer thin films of Ca-doped ferromagnetic manganites (LCMO) and Zr/Hf doped ferroelectric barium titanates (BZHTO): We have performed electrical resistivity and Hall coefficient measurements on epitaxial multilayer thin films of LCMO and BZHTO with different configurations. Figure 5 shows temperature dependence of the resistance in different magnetic fields for a double-layer configuration (left panel) and a triple-layer configuration (right panel). For the double-layer configuration where LCMO is on the top of BZHTO, the magnetoresistance (MR) effect is similar to that for single layer LCMO. This suggests that BZHTO does not have significant influence on the MR effect when BZHTO is electrically un-polarized. The work is underway to investigate the MR effect when BZHTO is electrically polarized.

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Figure 4. Temperature dependence of the resistances in different magnetic fields for a double-layer configuration (left panel) and a triple-layer configuration (center panel). The magnetoresistance effects at 9 T for the two configurations (right panel). The MR effect for the triple-layer configuration is about 2 times as large as that for the double-layer configuration.

On the other hand, the magnetoresistance effect for the triple-layer configuration is significantly changed: there is a large MR effect at 300 K, which is important for applications. Figure 4 compares 9T MR effects for the two configurations. The MR effect for the triple-layer configuration is about 2 times as large as that for the double-layer configuration. The reason for such an unusual enhancement of the MR in the triple-layer configuration has not been understood yet.

 

19F-NMR and Susceptibility in LaO0.5F0.5BiS2 potential for spintronic applications: We have concentrated on analyzing the 19F-NMR data, which shows unexpected magnetic effects in both the static (Knight shift K and linewidth FWHM) and dynamic (spin-lattice relaxation 1/T1) parameters as functions of temperature T and applied field. Complementary measurements of the magnetic susceptibility, which shows contributions from magnetic impurities were taken. Chemical analysis found Ce atoms in the range of 200 ppm. These account for about 20% of the contributions measured both by magnetization and NMR. The 19F-T1 data can be fitted to two independent relaxation mechanisms; one of which can be attributed to the Ce impurities. The larger effect on T1, (defined as t) however, follows a ½-power exponential decay law, implying that it comes from a source of inhomogeneous fluctuations. These could be associated with disorder in extended spin sources, which in turn would be consistent with spin textures induced by spin-orbit coupling due to lack of inversion symmetry in the crystal structure.

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Figure 5. Left Panel: Intrinsic relaxation rate 1/(t T) vs. Intrinsic Magnetization. Right Panel: the rate after subtraction of the linear part evident above 12 K.

 

Recently, a set of ARPES experiments performed by others demonstrated that a single crystal of LaO0.55F0.45BiS2 has a double band structure for valence and conduction bands coming from the splitting of levels corresponding to spin +1/2 and -1/2 in the bands. Figure 5 (left panel) shows that the intrinsic relaxation rate t divided by T behaves anomalously (linear below 50 K and strongly enhanced below about 12 K) with respect to the magnetization. The excess relaxation below 12 K develops as a kind of order parameter, indicating a change of microscopic state below this temperature. The coincidence of the relaxation enhancement with the temperature at which some samples display superconductivity might indicate that the new state sampled here is that of superconducting fluctuations.