arXiv subjects

J. W. Freeland

Publications and source records attributed to J. W. Freeland.

72 records · Page 4Linked to original sources

arXiv2011

Strain dependent transport properties of the quasi two-dimensional correlated metal, LaNiO$_{3}$

We explore the electrical transport and magneto-conductance in quasi two-dimensional strongly correlated ultrathin films of LaNiO$_{3}$ (LNO) to investigate the effect of hetero-epitaxial strain on electron-electron and electron-lattice interactions from the low to intermediate temperature range (2K$\sim$170K). The fully epitaxial 10 unit cell thick films spanning tensile strain up to $\sim4%$ are used to investigate effects of enhanced carrier localization driven by a combination of weak localization and electron-electron interactions at low temperatures. The magneto-conductance data shows the importance of the increased contribution of weak localization to low temperature quantum corrections. The obtained results demonstrate that with increasing tensile strain and reduced temperature the quantum confined LNO system gradually evolves from the Mott into the Mott-Anderson regime.

cond-mat.str-el↗

arXiv2011

Quantum confinement of Mott electrons in ultrathin LaNiO3/LaAlO3 superlattice

We investigate the electronic reconstruction in (LaNiO$_{3}$)$_n$/(LaAlO$_{3}$)$_3$ (n=3, 5 and 10) superlattices due to the quantum confinement (QC) by d.c. transport and resonant soft x-ray absorption spectroscopy. In proximity to the QC limit, a Mott-type transition from an itinerant electron behavior to a localized state is observed. The system exhibits tendency towards charge-order during the transition. ab initio cluster calculations are in good agreement with the absorption spectra, indicating that the apical ligand hole density is highly suppressed resulting in a strong modification of the electronic structure. At the dimensional crossover cellular dynamical-mean-field calculations support the emergence of a Mott insulator ground state in the heterostructured ultra-thin slab of LaNiO$_{3}$.

cond-mat.str-el↗

arXiv2010

Local Electronic and Magnetic Studies of an Artificial La2FeCrO6 Double Perovskite

Through the utilization of element-resolved polarized x-ray probes, the electronic and magnetic state of an artificial La2FeCrO6 double perovskite were explored. Applying unit-cell level control of thin film growth on SrTiO3 (111), the rock salt double perovskite structure can be created for this system, which does not have an ordered perovskite phase in the bulk. We find that the Fe and Cr are in the proper 3+ valence state, but, contrary to previous studies, the element-resolved magnetic studies find the moments in field are small and show no evidence of a sizable magnetic moment in the remanent state.

cond-mat.str-el↗

arXiv2010

Orbital Symmetry of Ba(Fe1-xCox)2As2 Superconductors Probed with X-ray Absorption Spectroscopy

The orbital symmetries of electron doped iron-arsenide superconductors Ba(Fe1-xCox)2As2 have been measured with x-ray absorption spectroscopy. The data reveal signatures of Fe d electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the Fe dxz+yz states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied s and d states are found to be in remarkably good agreement with the predictions of standard density functional theory.

cond-mat.supr-con↗

arXiv2010

Strain-mediated metal-insulator transition in epitaxial ultra-thin films of NdNiO3

We have synthesized epitaxial NdNiO$_{3}$ ultra-thin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO$_{3}$ (001) and LaAlO$_3$ (001), respectively. A combination of X-ray diffraction, temperature dependent resistivity, and soft X-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO$_{3}$, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior.

cond-mat.str-el↗

arXiv2010

Effect of polar discontinuity on the growth of LaNiO3/LaAlO3 superlattices

We have conducted a detailed microscopic investigation of [LaNiO3(1 u.c.)/LaAlO3(1 u.c.)]N superlattices grown on (001) SrTiO3 and LaAlO3 to explore the influence of polar mismatch on the resulting electronic and structural properties. Our data demonstrate that the initial growth on the non-polar SrTiO3 surface leads to a rough morphology and unusual 2+ valence of Ni in the first LaNiO3 layer, which is not observed after growth on the polar surface of LaAlO3. A newly devised model suggests that the polar mismatch can be resolved if the perovskite layers grow with an excess of LaO, which also accounts for the observed electronic, chemical, and structural effects.

cond-mat.mtrl-sci↗

arXiv2009

Electronic structure, magnetic and dielectric properties of the edge-sharing copper-oxide chain compound NaCu$_{2}$O$_{2}$

We report an experimental study of \nco, a Mott insulator containing chains of edge-sharing CuO$_4$ plaquettes, by polarized x-ray absorption spectroscopy (XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and pyroelectric current measurements. The XAS data show that the valence holes reside exclusively on the Cu$^{2+}$ sites within the copper-oxide spin chains and populate a $d$-orbital polarized within the CuO$_4$ plaquettes. The RMXS measurements confirm the presence of incommensurate magnetic order below a Néel temperature of $T_N = 11.5$ K, which was previously inferred from neutron powder diffraction and nuclear magnetic resonance data. In conjunction with the magnetic susceptibility and XAS data, they also demonstrate a new "orbital" selection rule for RMXS that is of general relevance for magnetic structure determinations by this technique. Dielectric property measurements reveal the absence of significant ferroelectric polarization below $T_N$, which is in striking contrast to corresponding observations on the isostructural compound \lco. The results are discussed in the context of current theories of multiferroicity.

cond-mat.str-el↗

arXiv2009

Electronic and Magnetic Properties of the Interface between a doped cuprate Y0.6Pr0.4Ba2Cu3O7 and a colossal-magnetoresistance manganite La2/3Ca1/3MnO3

The interfacial properties of Y0.6Pr0.4Ba2Cu3O7/La2/3Ca1/3MnO3 superlattices have been studied by resonant soft x-rays and diffuse scattered neutrons. Linearly polarized X-ray absorption at Cu L3 -edge reveals dramatic interfacial changes from the bulk including charge transfer between LCMO and YPBCO and an orbital reconstruction in the interface CuO2 plane. The similarities to the case of zero Pr-doping are due to the strongly hybridized covalent bond between Cu and Mn. However, reduced charge transfer and a more bulk-like interfacial orbital occupation are observed and related to the effect of Pr-doping. Neutron reflectometry measurements reveal a drastic increase in diffuse scattering with decreasing temperature, likely due to buckling caused by the structural phase transition of the SrTiO3 substrate. We observe no evidence that this diffuse scattering is related to the superconducting transition.

cond-mat.str-el↗

arXiv2009

Charge transport and magnetization profile at the interface between a correlated metal and an antiferromagnetic insulator

A combination of spectroscopic probes was used to develop a detailed experimental description of the transport and magnetic properties of superlattices composed of the paramagnetic metal CaRuO$_3$ and the antiferromagnetic insulator CaMnO$_3$. The charge carrier density and Ru valence state in the superlattices are not significantly different from those of bulk CaRuO$_3$. The small charge transfer across the interface implied by these observations confirms predictions derived from density functional calculations. However, a ferromagnetic polarization due to canted Mn spins penetrates 3-4 unit cells into CaMnO$_3$, far exceeding the corresponding predictions. The discrepancy may indicate the formation of magnetic polarons at the interface.

cond-mat.str-el↗

arXiv2007

Nanoscale Suppression of Magnetization at Atomically Assembled Manganite Interfaces

Using polarized X-rays, we compare the electronic and magnetic properties of a La(2/3)Sr(1/3)MnO(3)(LSMO)/SrTiO(3)(STO) and a modified LSMO/LaMnO(3)(LMO)/STO interface. Using the technique of X-ray resonant magnetic scattering (XRMS), we can probe the interfaces of complicated layered structures and quantitatively model depth-dependent magnetic profiles as a function of distance from the interface. Comparisons of the average electronic and magnetic properties at the interface are made independently using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The XAS and the XMCD demonstrate that the electronic and magnetic structure of the LMO layer at the modified interface is qualitatively equivalent to the underlying LSMO film. From the temperature dependence of the XMCD, it is found that the near surface magnetization for both interfaces falls off faster than the bulk. For all temperatures in the range of 50K - 300K, the magnetic profiles for both systems always show a ferromagnetic component at the interface with a significantly suppressed magnetization that evolves to the bulk value over a length scale of ~1.6 - 2.4 nm. The LSMO/LMO/STO interface shows a larger ferromagnetic (FM) moment than the LSMO/STO interface, however the difference is only substantial at low temperature.

cond-mat.str-el↗

arXiv2007

On Magnetic Interlayer Coupling and Proximity Effect in a La$_{0.67}$Ca$_{0.33}$MnO$_3$(10 nm)/YBa$_2$Cu$_3$O$_7$(10 nm) Superlattice

We present a study of interlayer coupling and proximity effects in a La$_{0.66}$Ca$_{0.33}$MnO$_3$(10 nm)/YBa$_2$Cu$_3$O$_7$(10 nm) superlattice. Using element-sensitive x-ray probes, the magnetic state of Mn can be probed without seeing the strong diamagnetism of the superconductor, which makes this approach ideal to study changes in the magnetic properties across the superconducting transition. By a combined experiment using {\it in situ} transport measurements during polarized soft x-ray measurements, we were able to see no noticeable influence of the superconducting state on the magnetic properties and no evidence for magnetic coupling across a 10 nm YBCO layer.

cond-mat.str-el↗

arXiv2007

Demagnetization Protocols for Frustrated Interacting Nanomagnet Arrays

We report a study of demagnetization protocols for frustrated arrays of interacting single domain permalloy nanomagnets by rotating the arrays in a changing magnetic field. The most effective demagnetization is achieved by not only stepping the field strength down while the sample is rotating, but by combining each field step with an alternation in the field direction. By contrast, linearly decreasing the field strength or stepping the field down without alternating the field direction leaves the arrays with a larger remanent magnetic moment. These results suggest that non-monotonic variations in field magnitude around and below the coercive field are important for the demagnetization process.

cond-mat.mtrl-sci↗

arXiv2006

Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites

What happens to ferromagnetism at the surfaces and interfaces of manganites? With the competition between charge, spin, and orbital degrees of freedom, it is not surprising that the surface behavior may be profoundly different than that of the bulk. Using a powerful combination of two surface probes, tunneling and polarized x-ray interactions, this paper reviews our work on the nature of the electronic and magnetic states at manganite surfaces and interfaces. The general observation is that ferromagnetism is not the lowest energy state at the surface or interface, which results in a suppression or even loss of ferromagnetic order at the surface. Two cases will be discussed ranging from the surface of the quasi-2D bilayer manganite (La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$) to the 3D Perovskite (La$_{2/3}$Sr$_{1/3}$MnO$_3$)/SrTiO$_3$ interface. For the bilayer manganite, that is, ferromagnetic and conducting in the bulk, these probes present clear evidence for an intrinsic insulating non-ferromagnetic surface layer atop adjacent subsurface layers that display the full bulk magnetization. This abrupt intrinsic magnetic interface is attributed to the weak inter-bilayer coupling native to these quasi-two-dimensional materials. This is in marked contrast to the non-layered manganite system (La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrTiO$_3$), whose magnetization near the interface is less than half the bulk value at low temperatures and decreases with increasing temperature at a faster rate than the bulk.

cond-mat.str-el↗

arXiv2005

Full Bulk Spin Polarization and Intrinsic Tunnel Barriers at the Surface of Layered Manganites

Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. At surfaces however this degree of polarization can be quenched by competing interactions. Using a combination of surface sensitive x-ray and tunneling probes, we show for the quasi-two-dimensional bilayer manganites that the outermost Mn-O bilayer, alone, is affected: it is a 1-nm thick insulator that exhibits no long-range ferromagnetic order while the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite while the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase. The creation of a well-defined surface insulator demonstrates the ability to naturally self-assemble two of the most demanding components of an ideal magnetic tunnel junction.

cond-mat.str-el↗

arXiv2004

Local Environment of Ferromagnetically Ordered Mn in Epitaxial InMnAs

The magnetic properties of the ferromagnetic semiconductor In0.98Mn0.02As were characterized by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. The Mn exhibits an atomic-like L2,3 absorption spectrum that indicates that the 3d states are highly localized. In addition, a large dichroism at the Mn L2,3 edge was observed from 5-300 K at an applied field of 2T. A calculated spectrum assuming atomic Mn2+ yields the best agreement with the experimental InMnAs spectrum. A comparison of the dichroism spectra of MnAs and InMnAs show clear differences suggesting that the ferromagnetism observed in InMnAs is not due to hexagonal MnAs clusters. The temperature dependence of the dichroism indicates the presence of two ferromagnetic species, one with a transition temperature of 30 K and another with a transition temperature in excess of 300 K. The dichroism spectra are consistent with the assignment of the low temperature species to random substitutional Mn and the high temperature species to Mn near-neighbor pairs.

cond-mat.mtrl-sci↗

arXiv2003

Domain-specific magnetization reversals on a permalloy square ring array

We present domain-specific magnetization reversals extracted from soft x-ray resonant magnetic scattering measurement on a permalloy square ring array. The extracted domain-specific hysteresis loops reveal that the magnetization of the domain parallel to the field is strongly pinned, while those of other domains rotate continuously. In comparison with the micromagnetic simulation, the hysteresis loop on the pinned domain indicates a possibility of the coexistence of the square rings with the vortex and onion states.

cond-mat.mtrl-sci↗

arXiv2003

Extraction of domain-specific magnetization reversal for nanofabricated periodic arrays using soft x-ray resonant magnetic scattering

A simple scheme to extract the magnetization reversals of characteristic magnetic domains on nanofabricated periodic arrays from soft x-ray resonant magnetic scattering (SXRMS) data is presented. The SXRMS peak intensities from a permalloy square ring array were measured with field cycling using circularly polarized soft x-rays at the Ni L$_3$ absorption edge. Various SXRMS hysteresis loops observed at different diffraction orders enabled the determination of the magnetization reversal of each magnetic domain using a simple linear algebra. The extracted domain-specific hysteresis loops reveal that the magnetization of the domain parallel to the field is strongly pinned, while that of the perpendicular domain rotates continuously.

cond-mat.mtrl-sci↗

arXiv2003

Induced Ge Spin Polarization at the Fe/Ge Interface

We report direct experimental evidence showing induced magnetic moments on Ge at the interface in an Fe/Ge system. Details of the x-ray magnetic circular dichroism and resonant magnetic scattering at the Ge L edge demonstrate the presence of spin-polarized {\it s} states at the Fermi level, as well as {\it d} character moments at higher energy, which are both oriented antiparallel to the moment of the Fe layer. Use of the sum rules enables extraction of the L/S ratio, which is zero for the {\it s} part and $\sim0.5$ for the {\it d} component. These results are consistent with layer-resolved electronic structure calculations, which estimate the {\it s} and {\it d} components of the Ge moment are anti-parallel to the Fe {\it 3d} moment and have a magnitude of $\sim0.01 μ_B$.

cond-mat.mtrl-sci↗