Maity et al. reply to the comment on “Superspin glass mediated giant spontaneous exchange bias in a nanocomposite of BiFeO3−Bi2Fe4O9”, A. Harres, J. Geshev, and V. Skumryev, Physical Review Letters, 114, 099703 (2015)

In this article we reply to the concerns raised by Harres et al. [Phys. Rev. Lett. 114, 099703 (2015)] about some of the results reported in our original paper [T. Maity et al. Phys. Rev. Lett. 110, 107201 (2013)]. We show that the magnetic hysteresis loops are not minor and both path dependency of exchange bias and presence of superspin glass phase in the nanocomposite are indisputable

Superspin Glass Mediated Giant Spontaneous Exchange Bias in a Nanocomposite of BiFeO3_3-Bi2_2Fe4_4O9_9

We observe an enormous $\textit{spontaneous}$ exchange bias ($\sim$300-600 Oe) - measured in an unmagnetized state following zero-field cooling - in a nanocomposite of BiFeO$_3$ ($\sim$94%)-Bi$_2$Fe$_4$O$_9$ ($\sim$6%) over a temperature range 5-300 K. Depending on the path followed in tracing the hysteresis loop - positive (p) or negative (n) - as well as the maximum field applied, the exchange bias ($H_E$) varies significantly with $\mid-H_{Ep}\mid$ $>$ $\mid H_{En}\mid$. The temperature dependence of $H_E$ is nonmonotonic. It increases, initially, till $\sim$150 K and then decreases as the blocking temperature $T_B$ is approched. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by "superinteraction bias coupling" between ferromagnetic core of Bi$_2$Fe$_4$O$_9$ (of average size $\sim$19 nm) and canted antiferromagnetic structure of BiFeO$_3$ (of average size $\sim$112 nm) via superspin glass moments at the shell.Comment: 5 pages with 4 figures; published in Phys. Rev. Let

Origin of the asymmetric exchange bias in BiFeO3/Bi2Fe4O9 nanocomposite

We show from detailed magnetometry across 2-300 K that the BiFeO3-Bi2Fe4O9 nanocomposite offers a unique spin morphology where superspin glass (SSG) and dilute antiferromagnet in a field (DAFF) coexist at the interface between ferromagnetic Bi2Fe4O9 and antiferromagnetic BiFeO3. The coexisting SSG and DAFF combine to form a local spin texture, which gives rise to a path- dependent exchange bias below the spin freezing temperature (similar to 29 K). The exchange bias varies depending on the protocol or path followed in tracing the hysteresis loop. The exchange bias has been observed below the blocking temperature (T-B) 60 K of the superparamagnetic Bi2Fe4O9. The conventional exchange bias (CEB) increases nonmonotonically as temperature decreases. The magnitude of both exchange bias (H-E) and coercivity (H-C) increase with decrease in temperature and are found to be asymmetric below 20 K depending on the path followed in tracing the hysteresis loop and bias field. The local spin texture at the interface between ferromagnetic and antiferromagnetic particles generates a nonswitchable unidirectional anisotropy along the negative direction of the applied field. The influence of this texture also shows up in " asymmetric" jumps in the hysteresis loop at 2 K, which smears off at higher temperature. The role of the interface spin texture in yielding the path dependency of exchange bias is thus clearly delineated

Improving Geologic and Engineering Models of Midcontinent Fracture and Karst-Modified Reservoirs Using New 3-D Seismic Attributes

Our project goal was to develop innovative seismic-based workflows for the incremental recovery of oil from karst-modified reservoirs within the onshore continental United States. Specific project objectives were: (1) to calibrate new multi-trace seismic attributes (volumetric curvature, in particular) for improved imaging of karst-modified reservoirs, (2) to develop attribute-based, cost-effective workflows to better characterize karst-modified carbonate reservoirs and fracture systems, and (3) to improve accuracy and predictiveness of resulting geomodels and reservoir simulations. In order to develop our workflows and validate our techniques, we conducted integrated studies of five karst-modified reservoirs in west Texas, Colorado, and Kansas. Our studies show that 3-D seismic volumetric curvature attributes have the ability to re-veal previously unknown features or provide enhanced visibility of karst and fracture features compared with other seismic analysis methods. Using these attributes, we recognize collapse features, solution-enlarged fractures, and geomorphologies that appear to be related to mature, cockpit landscapes. In four of our reservoir studies, volumetric curvature attributes appear to delineate reservoir compartment boundaries that impact production. The presence of these compartment boundaries was corroborated by reservoir simulations in two of the study areas. Based on our study results, we conclude that volumetric curvature attributes are valuable tools for mapping compartment boundaries in fracture- and karst-modified reservoirs, and we propose a best practices workflow for incorporating these attributes into reservoir characterization. When properly calibrated with geological and production data, these attributes can be used to predict the locations and sizes of undrained reservoir compartments. Technology transfer of our project work has been accomplished through presentations at professional society meetings, peer-reviewed publications, Kansas Geological Survey Open-file reports, Master's theses, and postings on the project website: http://www.kgs.ku.edu/SEISKARST

Large magnetoelectric coupling in nanoscale BiFeO3_3 from direct electrical measurements

We report the results of direct measurement of remanent hysteresis loops on nanochains of BiFeO$_3$ at room temperature under zero and $\sim$20 kOe magnetic field. We noticed a suppression of remanent polarization by nearly $\sim$40\% under the magnetic field. The powder neutron diffraction data reveal significant ion displacements under a magnetic field which seems to be the origin of the suppression of polarization. The isolated nanoparticles, comprising the chains, exhibit evolution of ferroelectric domains under dc electric field and complete 180$^o$ switching in switching-spectroscopy piezoresponse force microscopy. They also exhibit stronger ferromagnetism with nearly an order of magnitude higher saturation magnetization than that of the bulk sample. These results show that the nanoscale BiFeO$_3$ exhibits coexistence of ferroelectric and ferromagnetic order and a strong magnetoelectric multiferroic coupling at room temperature comparable to what some of the type-II multiferroics show at a very low temperature.Comment: 7 pages with 5 figures, published in Phys. Rev.

Field Demonstration of Horizontal Infill Drilling Using Cost-effective Integrated Reservoir Modeling--Mississippian Carbonates, Central Kansas

Mississippian carbonate reservoirs have produced in excess of 1 billion barrels of oil in Kansas accounting for over 16% of the state's production. With declining production from other age reservoirs, the contribution of Mississippian reservoirs to Kansas's oil production has risen to 43% as of 2004. However, solution-enhanced features such as vertical shale intervals extending from the karst erosional surface at the top introduce complexities/compartmentalizations in Mississippian carbonate reservoirs. Coupled with this, strong water drives charge many of these reservoirs resulting in limited drainage from vertical wells due to high water cuts after an initial period of low water production. Moreover, most of these fields are operated by small independent operators without access to the knowledge bank of modern research in field characterization and exploitation/development practices. Thus, despite increasing importance of Mississippian fields to Kansas production, these fields are beset with low recovery factors and high abandonment rates leaving significant resources in the ground. Worldwide, horizontal infill wells have been successful in draining compartmentalized reservoirs with limited pressure depletion. The intent of this project was to demonstrate the application of horizontal wells to successfully exploit the remaining potential in mature Mississippian fields of the mid-continent. However, it is of critical importance that for horizontal wells to be economically successful, they must be selectively targeted. This project demonstrated the application of initial and secondary screening methods, based on publicly available data, to quickly shortlist fields in a target area for detailed studies to evaluate their potential to infill horizontal well applications. Advanced decline curve analyses were used to estimate missing well-level production data and to verify if the well produced under unchanging bottom-hole conditions--two commonly occurring data constraints afflicting mature Mississippian fields. A publicly accessible databank of representative petrophysical properties and relationships was developed to overcome the paucity of such data that is critical to modeling the storage and flow in these reservoirs. Studies in 3 Mississippian fields demonstrated that traditional reservoir models built by integrating log, core, DST, and production data from existing wells on 40-acre spacings are unable to delineate karst-induced compartments, thus making 3D-seismic data critical to characterize these fields. Special attribute analyses on 3D data were shown to delineate reservoir compartments and predict those with pay porosities. Further testing of these techniques is required to validate their applicability in other Mississippian reservoirs. This study shows that detailed reservoir characterization and simulation on geomodels developed by integrating wireline log, core, petrophysical, production and pressure, and 3D-seismic data enables better evaluation of a candidate field for horizontal infill applications. In addition to reservoir compartmentalization, two factors were found to control the economic viability of a horizontal infill well in a mature Mississippian field: (a) adequate reservoir pressure support, and (b) an average well spacing greater than 40-acres

Maity et al. reply to the comment on “Superspin glass mediated giant spontaneous exchange bias in a nanocomposite of BiFeO3−Bi2Fe4O9”, A. Harres, J. Geshev, and V. Skumryev, Physical Review Letters, 114, 099703 (2015)

In this article we reply to the concerns raised by Harres et al. [Phys. Rev. Lett. 114, 099703 (2015)] about some of the results reported in our original paper [T. Maity et al. Phys. Rev. Lett. 110, 107201 (2013)]. We show that the magnetic hysteresis loops are not minor and both path dependency of exchange bias and presence of superspin glass phase in the nanocomposite are indisputable

Origin of the asymmetric exchange bias in BiFeO3/Bi2Fe4O9 nanocomposite

We show from detailed magnetometry across 2-300 K that the BiFeO3-Bi2Fe4O9 nanocomposite offers a unique spin morphology where superspin glass (SSG) and dilute antiferromagnet in a field (DAFF) coexist at the interface between ferromagnetic Bi2Fe4O9 and antiferromagnetic BiFeO3. The coexisting SSG and DAFF combine to form a local spin texture, which gives rise to a path- dependent exchange bias below the spin freezing temperature (similar to 29 K). The exchange bias varies depending on the protocol or path followed in tracing the hysteresis loop. The exchange bias has been observed below the blocking temperature (T-B) 60 K of the superparamagnetic Bi2Fe4O9. The conventional exchange bias (CEB) increases nonmonotonically as temperature decreases. The magnitude of both exchange bias (H-E) and coercivity (H-C) increase with decrease in temperature and are found to be asymmetric below 20 K depending on the path followed in tracing the hysteresis loop and bias field. The local spin texture at the interface between ferromagnetic and antiferromagnetic particles generates a nonswitchable unidirectional anisotropy along the negative direction of the applied field. The influence of this texture also shows up in " asymmetric" jumps in the hysteresis loop at 2 K, which smears off at higher temperature. The role of the interface spin texture in yielding the path dependency of exchange bias is thus clearly delineated