Session A36: Focus Session: Bulk Properties of Complex Oxides -- Perovskite Manganites

Magnetism in complex oxides probed by transverse susceptibility and magnetocaloric effect

Magnetic oxides exhibit rich complexity in their fundamental physical properties determined by the intricate interplay between structural, electronic and magnetic degrees of freedom. The common theme that is often present in many systems is the strong magnetostructural coupling and possible spin frustration induced by lattice geometry. In this talk, we will demonstrate the relatively unconventional experimental methods of RF transverse susceptibility (TS) and magnetocaloric effect (MCE) as being powerful probes of multiple magnetic transitions, glassy phenomena and ground state magnetic properties in three classes of oxides including Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$, LuFe$_{2}$O$_{4}$ and mixed phase manganite (La,Pr,Ca)MnO$_{3}$. The TS experiments reveal a coupled structural/magnetocrystalline anisotropy transition in Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ driven by Pr-O hybridization and the nature of this transition is also clarified in MCE experiments. These results point to the existence of an entirely new class of phenomena in the cobaltites due to the unique interplay between structure and magnetic anisotropy. In LuFe$_{2}$O$_{4}$, our experiments show the emergence of a complex phase diagram with ferrimagnetic clusters undergoing two glass transitions followed by kinetic arrest at low temperature. Finally, in LPCMO, we will discuss the subtle balance between coexistence of ferromagnetic metal (FMM), charge-ordered insulator (COI) and paramagnetic insulator (PMI) phases that are highly sensitive to strain and dimensionality.   

Oxygen-isotope-substitution-induced nanoscale crystallinity and origin of metal-insulator transition in Sm$_{0.5}$Sr$_{0.5}$MnO$_{3}$

Competing effects of $^{16}$O$\rightarrow^{18}$O isotope substitution and magnetic field on the phase separation and structural property in Sm$_{0.5}$Sr$_{0.5}$MnO$_{3}$ have been studied by using high-resolution high-energy X-ray diffraction under magnetic fields up to 7 T. A close correlation between physical properties and the crystallinity of the compounds is observed. The oxygen-isotope-substitution-induced metal-insulator transition is caused by nanoscale crystallite formation in the compound with $^{18}$O below a phase-separation temperature T$_{ps} \sim 100$ K. Different lattice effects on the magnetoresistance are identified, among which the largest one is due to magnetic field enhanced crystallization of the nanoscale crystallites.   

The magnetic and magneto-transport properties of double perovskite Sr$_{2}$Fe$_{1-x}$Mn$_{x}$MoO$_{6}$ system

We have investigated the electrical and magnetic properties of Sr$_{2}$Fe$_{1-x}$Mn$_{x}$MoO$_{6}$ with 0$\le $x$\le $0.5. In the range of 0$<$x$\le $0.5, X-ray photoemission spectroscopy indicates the valences of Mn ions are mainly 3+ instead of 2+. B-site irons become more ordered with increasing x such that the area of antiphase boundary decreases, so as the low field magnetoresistance. For x$\ge $0.3, the magnetization under low field drops greatly below 37K. Our results suggest that this interesting behavior should be attributed to the enhancement of the magnetic anisotropy and domain wall pinning due to the substitution of Mn for Fe. Detailed analysis of isothermal magnetization data suggests that the magnetic moments of Mn ions couple in antiparallel with those of Mo ions nearby in Sr$_{2}$Fe$_{1-x}$Mn$_{x}$MoO$_{6}$ with 0$<$x$\le $0.5.   

Giant increase of magnetic transition temperature in La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3 }$under pressure

We have measured the magnetization and resistivity of La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3 }$polycrystalline samples under hydrostatic pressure up to 12 kbar, and from 300 K to 4 K. We find that there is a giant increase of the ferromagnetic phase transition temperature of more than 100 K within this modest pressure range. The origin of this giant increase is explained under the context of electronic phase separation and critical point. The device application from this effect is also discussed.   

Competing-fluctuation-induced anomalous magnetocaloric effects in perovskite manganites

A magnetocaloric (MC) effect refers to the isothermal entropy change induced by applying (or removing) a magnetic field to the materials, which is a performance index of the magnetic refrigeration technology. In this study, the variation of MC effects has been systematically investigated for colossal magnetoresistive manganites $R_{0.6}$Sr$_{0.4}$MnO$_{3}$ ($R$=La-Gd) by controlling the $R$-dependent one-electron bandwidth. With decreasing the bandwidth, the temperature profile of entropy change exhibits a larger peak at the ferromagnetic transition temperature and a steeper drop below it, due to the first-order nature of the transition promoted by a competing charge-orbital ordering instability. For the smallest-bandwidth systems adjacent to the metal- insulator phase boundary, a rectangular-shaped profile for the entropy change emerges with an anomalously wide temperature range. Model calculations have indicated that the bicritical fluctuation enhanced in the phase-competing region has a strong impact on such MC features [1]. \newline [1] H. Sakai {\it et al.}, J. Phys. Soc. Jpn. {\bf 78}, 113708 (2009).   

Current tunable electroresistance and magnetothermal cooling with a phase separated manganite

We investigate the effect of dc current on the field dependence of the magnetoresistance in the phase separated manganites Nd$_{0.5}$Ca$_{0.5}$Mn$_{0.93}$Ni$_{0.07}$O$_{3. }$ We show that temperature of the sample in presence of a magnetic field (H) and dc current (I = 20 mA) decreases abruptly as much as $\Delta $T = 45 K (7 K) accompanied by a step like decrease in the magnetoresistance at a critical value of H when the base temperature is 40 K (100 K). The magnitude of $\Delta $T and the position of magnetoresistance step decrease towards lower H with decreasing amplitude of the current. In addition, the applied current significantly modifies the irreversible behavior of magnetoresistance observed at lower currents (I $<$ 1 mA). We discuss possible origins of the current and magnetic -field driven temperature change which may find applications in magnetothermal refrigeration besides magnetocaloric effect.   

Intrinsic Local Distortions and charge carrier behavior in CMR manganites and cobaltites

We compare and contrast the local structure and electronic configurations in two oxide systems La$_{1-x}$Sr$_x$CoO$_3$ (LSCO) and La$_{1-y}$Ca$_y$MnO$_3$ (LCMO). Although these oxides may appear quite similar they have rather different properties. At x=0, LaCoO$_3$ (LCO) has unusual magnetic properties - diamagnetic at low T but developing a moment near 100K. The Sr doped LSCO materials show ferromagnetism for x $>$ 0.2. For LCO, one of the possible spin state configurations called the intermediate spin (IS) state (S=1), should be Jahn-Teller (JT) active, while the low spin (S=0) and high spin (S=2) states have no JT distortion. Early local structure measurements suggested a JT distortion was present in LCO and therefore supported an IS spin model. However we find no evidence for any significant JT distortion (and hence no support for the IS model) for a range of bulk and nanoparticle cobaltites La$_{1-x}$Sr$_x$CoO$_3$, x = 0 - 0.35. In contrast there are large JT distortions in the manganites LCMO, 0.2 $<$ x $<$ 0.5 (Mn-O bonds), for which CMR behavior is observed. We have shown that the JT distortions in the manganites depend on both temperature T and magnetic field B, and from the B-field dependence, propose the size and nature of the polarons in LCMO. We also present Co K-edge XANES data that shown no significant shift of the edge for the cobaltites as the Sr concentration increases from x =0 to 0.35 indicating essentially no change in the electronic configuration about Co; consequently, the holes introduced via Sr doping appear to go primarily into the O bands. In contrast there is a large shift of the Mn K-edge with Ca doping indicating a change in the average Mn valence, and a corresponding change in the Mn electronic configuration. We briefly discuss some possible models.   

Magnetocaloric Effect and Refrigerant Capacity in Charge-Ordered Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$

The influence of first- and second-order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity (RC) of charge-ordered Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ has been investigated. The system undergoes a ferromagnetic transition at T$_{c}\sim $255 K followed by a ferromagnetic charge-disordered to antiferromagnetic charge-ordered transition at T$_{co}\sim $165 K. The first-order magnetic transition (FOMT) at $T_{CO}$ induces a larger MCE but is limited to a narrower temperature range resulting in a smaller RC, versus that of the second-order magnetic transition at $T_{C}$. Large magnetic and thermal hysteretic losses associated with the FOMT below $T_{CO}$ are also detrimental to an efficient magnetic RC. Magnetic measurements were performed using a commercial Physical Property Measurement system from Quantum Design. MCE was numerically calculated from the magnetization isotherms using the thermodynamic Maxwell relation. Overall, we show that PSMO is an excellent system that provides an interesting possibility of comparing the magnetic entropy across a FOMT and SOMT in the same material.   

The finite size effects on transport and magnetic properties of colossal magnetoresistant manganites

The presence of electronic phase separation in strongly correlated materials has been linked to many types of exotic behavior, such as colossal magnetoresistant effects. If the spatial dimension of these materials is artificially reduced to the scale of the phase separation, significant changes in the transport and magnetic properties of manganits could be expected. Hot press technology provides the possibility to prepare bulk materials with nanosized grain which can be used to carefully study the transport properties of nanostructure of manganites. In this presentation, we carefully prepared a series of mangnaites ceramics with different grain size by hot pressure and the significant effects of finite size on physical properties of manganites were clarified. An enhanced magnetoresistant effects and collapse of charge-ordered state were identified in nanostructure manganites. The possible origin is attributed to the mesoscopic phase-separated domains inherent in the materials and the development of ferromagnetic correlations at the surface of these small systems.