Session Y6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Oxide Films and Nanoparticles

Linear magnetoresistance of hetroepitaxial thin films of pyrochlore iridates Bi$_2$Ir$_2$O$_7$

We report on the discovery of linear magnetoresistance in hetroepitaxial thin films of the pyrochlore iridates Bi$_2$Ir$_2$O$_7$. The magnetoresistance of Bi$_2$Ir$_2$O$_7$ shows a highly isotropic, linear field dependence at T = 1.6K, but gradually evolves towards a quadratic field dependence as temperature increases. By interfacing the Bi$_2$Ir$_2$O$_7$ with pyrochlore spin ice compound Dy$_2$Ti$_2$O$_7$, the magnetoresistance at sub-kelvin temperatures shows pronounce anisotropy with a complex field dependence. We argued that these unusual magnetotransport behaviors cannot be explained by disorder induced quantum correction, but might be related to the magnetism of Bi$_2$Ir$_2$O$_7$.   

Reversal of lattice, electronic structure, and magnetism in epitaxial SrCoO$_{x}$ thin films

SrCoO$_{x}$ ($x =$ 2.5 -- 3.0, SCO) is an ideal material to study the role of oxygen content for electronic structure and magnetism, since SCO has two distinct topotactic phases: the antiferromagnetic insulating brownmillerite SrCoO$_{2.5}$ and the ferromagnetic metallic perovskite SrCoO$_{3}$. In this presentation, we report direct observation of a reversible lattice and electronic structure evolution in SrCoO$_{x}$ epitaxial thin films as well as different magnetic and electronic ground states between the topotactic phases.\footnote{W. S. Choi \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 097401 (2013).} By magnetization measurements, optical absorption, and transport measurements drastically different electronic and magnetic ground states are found in the epitaxially grown SrCoO$_{2.5}$ and SrCoO$_{3}$ thin films by pulsed laser epitaxy. First-principles calculations confirm substantial, which originate from the modification in the Co valence states and crystallographic structures. By real-time spectroscopic ellipsometry, the two electronically and magnetically different phases can be reversibly changed by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides.   

Local Structure in Magnetically Phase Separated Perovskite SrCoO$_{3-y}$

Magnetic phase separation has recently been found in the oxygen deficient perovskite SrCoOx (2.88$\le $x$\le $3). Samples with appropriate oxygen concentration show two component magnetic behavior while maintaining a single crystallographic phase. The two magnetic phases match those found in SrCoO$_{2.88}$ and SrCoO$_{3}$ with Tc $=$ 220 K and 280 K, respectively. Muon Spin Rotation ($\mu$ SR) has been used to explore the local spin structures and phase behavior of these cobaltates. The data reveal the possible existence of spatially separated magnetic region and two true phase transitions.   

Metal-insulator transition with ferrimagnetic order in epitaxial thin films of spinel NiCo$_{2}$O$_{4}$

Spinel NiCo$_{2}$O$_{4}$ is attractive for various technological applications but is less studied partly because of the unavailability of NiCo$_{2}$O$_{4}$ single crystal or epitaxial thin film. We have grown high-quality crystalline epitaxial NiCo$_{2}$O$_{4}$ thin films on MgAl$_{2}$O$_{4}$ (001) substrates. The systematic investigation of the films grown at various temperatures reveals a strong correlation between the structural, magnetic, and electrical transport properties. The low-temperature grown films show metallic behavior with strong ferrimagnetic ordering while the high temperature grown films are insulating with suppressed magnetic order. In addition, these films show excellent transport and magnetic properties down to 2 unit-cell thickness. Our study of temperature- and growth-condition dependent optical conductivity provides further insight in the carrier transport of these films. We observed coherent band-like transport in both low- and high temperature grown films, whereas only thermally activated hopping conductivity was reported in previous studies. The confirmation of coherent band like transport provides a basis for further improving NiCo$_{2}$O$_{4}$ for the application as transparent conducting oxide.   

Magnetic phase diagram of thin film La$_{2- x}$Sr$_ x$CuO$_4$ studied by low energy muon spin rotation

The magnetic phase diagram of La$_{2- x}$Sr$_ x$CuO$_4$ thin film samples grown on SrLaAlO$_4$ has been determined by low-energy muon spin rotation. The obtained phase diagram shows the same features as that one of the bulk, but the transition temperatures are drastically shifted. In the antiferromagnetic phase the Neel temperatures $T_{\rm N}$ are strongly reduced compared to the bulk material and no spin freezing was observed at low temperatures. In the disordered magnetic phase ($x \geq 0.02$) the transition temperature $T_{\rm g}$ is enhanced. It is concluded that the main reason for the pronounced differences between the magnetic phase diagrams of thin film and bulk samples is strain induced disorder in the thin films.   

Spin-orbital separation in the anisotropic ladder system CaCu$_2$O$_3$

Recently, resonant inelastic X-ray scattering (RIXS) on the 1D spin system Sr$_2$CuO$_3$ has revealed an unprecedented dispersion of orbital excitations [Nature 485, 82 (2012)]. This result has been interpreted as the fractionalization of spin and orbital degree of freedom from the elementary electron, hallmark of one dimensional physics as the previously observed spin-charge separation [Nature Phys. 2, 397 (2006)]. How these phenomena carry over into higher dimensions remains currently unclear. To clarify this point, we studied the spin and orbital excitations of the anisotropic ladder CaCu$_2$O$_3$, which realizes a first step towards 2D correlated electron systems. Combining high-resolution RIXS experiments with theoretical model calculations we show that spin-orbital fractionalization indeed occurs in CaCu$_2$O$_3$ and prevails beyond the strict 1D limit [arXiv:1310.8346]. We also establish that such a fractionalization is far more robust than the spin-charge separation. The main reasons behind this are the intrinsic 1D orbital dynamics and the fact that the spinons are faster than the orbitons but slower than the holons.   

Continuous wave terahertz spectroscopy of Sr$_{2}$CrReO$_{6}$ thin films at cryogenic temperatures and in high magnetic fields

Temperature and magnetic field dependent terahertz spectroscopies have proven useful in characterizing and manipulating the structural, charge, and magnet ordering in complex oxide systems. THz transmission measurements on epitaxial thins films of the double-perovskite ferrimagnet Sr$_{2}$CrReO$_{6}$ (SCRO) were performed with a novel continuous-wave terahertz transmission spectrometer operating from 5 K to 300 K and with fields up to 9 T. Temperature-dependent changes in the film conductivity manifest as strong variations in the Fabry-Perot interference patterns from the supporting substrate. Indicative of variable-range hopping transport in the films, we find the conductivity varies with the THz frequency (f) as f$^{s}$ with s$\sim $0.8 at 5 K. Depending on the handedness of the incident THz source, magnetic fields in excess of 4 T enhance or suppress the THz transmission of the SCRO films by $\sim $8{\%}.   

The Electric, Magnetic, and Optical Characterization of Permalloy Oxide Grown by Dual-Ion Beam Sputtering

Permalloy (Ni$_{80}$Fe$_{20})$ is a commonly used soft magnetic material in magnetic reading heads. Its magnetic properties do not depend on stress, a parameter difficult to control in thin film devices. Permalloy Oxide (PyO) on the other hand, has a high resistivity (\textgreater 4$\cdot$10$^{3} \Omega $ cm), is anti-ferromagnetic and has recently been shown to strongly enhance the performance of lateral spin valve devices. Historically, the oxidation of permalloy has been seen as a defect that should be avoided by appropriate encapsulation and very little is known on its electric and optical properties. We deposited thin PyO films by Dual Ion Beam Sputtering (DIBS) at room temperature on various substrates. Van der Pauw and Hall measurements were carried out from 77K to 400K and at magnetic fields up to 9T in order to determine its electronic bandgap, resistivity, free carrier concentration, and its mobility. The dielectric properties and defects were studied using a CV-setup and an impedance analyzer. Magnetic measurements were conducted on a Quantum Design PPMS VSM to determine the state of oxidation. Optical properties were measured by a M2000 Woollam variable angle spectroscopic ellipsometer. These properties were used to determine film thickness, bandgap and the optical constants of PyO.   

Rashba Spin-Orbit Anisotropy and the Electric Field Control of Magnetism

The control of the magnetism of ultra-thin ferromagnetic layers using an electric field would lead to many technologically important applications. To date, while it is usually assumed the changes in the magnetic anisotropy, leading to such a control, arises from surface charge doping of the magnetic layer, a number of key experiments cannot be understood within such a scenario. Much studied is the fact that, for non-magnetic metals or semi-conductors, a large surface electric field gives rise to a Rashba spin-orbit coupling which leads to a spin-splitting of the conduction electrons. Here we develop a simple analytic theory for the existence and electrical control of the magnetic anisotropy based upon the Rashba spin-orbit interaction and the Stoner model of magnetism. We show that the competition between the Rashba spin-orbit fields and the exchange interaction leads to a very large magnetic anisotropy arising from the internal electric fields which exist at, e.g., ferromagnetic/metal and ferromagnetic/oxide insulator interfaces but modified by the addition of an applied electric field. This different path to an electrically induced anisotropy energy can explain the electric field, thickness, and material dependence reported in many experiments.   

Magnetic and transport signatures of Rashba spin-orbit coupling on the Kondo lattice model in two dimensional clusters

Motivated by emergent phenomena at oxide surfaces and heterostructures, particularly those involving transition metal oxides with perovskite crystal structure such as LaTiO$_3$/SrTiO$_3$, we examine the Kondo lattice model in the presence of a Rashba spin-orbit coupling (RSOC). Using an array of numerical techniques, under the assumption that the electrons on localized orbitals may be treated as classical continuum spins, we compute various charge, spin and transport properties on square clusters and on ladders at zero and finite temperatures. The main goal is to determine magnetic and transport signatures due to the RSOC. The same model can be used to study at an effective level the combined effect on magnetic and transport properties of Rashba and ferromagnetic moments, such as the ones present at LMnO$_3$/SrMnO$_3$ interfaces.   

Magnetization and Hysteresis of Dilute Magnetic-Oxide Nanoparticles

Real-structure imperfections in dilute magnetic oxides tend to create small concentrations of local magnetic moments that are coupled by fairly long-range exchange interactions, mediated by p-electrons. The robustness of these interactions is caused by the strong overlap of the p orbitals, as contrasted to the much weaker interatomic exchange involving iron-series 3d electrons. The net exchange between defect moments can be positive or negative, which gives rise to spin structures with very small net moments. Similarly, the moments exhibit magnetocrystalline anisotropy, reinforced by electron hopping to and from 3d states and generally undergoing some random-anuisotropy averaging. Since the coercivity scales as 2\textit{K}$_1$/\textit{M} and \textit{M} is small, this creates pronounced and --- in thin films --- strongly anisotropic hysteresis loops. In finite systems with \textit{N} moments, both \textit{K}$_1$ and \textit{M} are reduced by a factor of order \textit{N}$^{1/2}$ due to random anisotropy and moment compensation, respectively, so that that typical coercivities are comparable to bulk magnets. Thermal activation readily randomizes the net moment of small oxide particles, so that the moment is easier to measure in compacted or aggregated particle ensembles.   

Computational Nano-materials Design of Dynamically Created New Functional Ordered Oxide Nano-superstructures by Spinodal Nano-Decomposition: Design vs. Experimental Realizations

Based on ab initio electronic structure calculation and multi-scale simulation, we discuss the design of magnetic mechanism and the self-organized Spinodal Nano-Decomposition in dilute magnetic oxides in MgO, SrO, BaO, CaO, ZnO, NiO , and Re-RAM with d0 ferromagnetism. By controlling the dimensionality (2D and 3D) of the crystal growth, crystal growth speed, substrate temperatures, and seeding in the self-organized nanostructure formation, we design the shape controlled quantum-dot (Dairiseki-Phase) and quantum nanowire (Konbu-Phase), and the new functionality such a Re-RAM, and high-blocking temperature in super-para-magnetism. We compare our recent computational nano-materials design data with the recent available experimental verifications.\\[4pt] [1] K. Sato et al., Rev. of Mod. Phys., 82, (2010) 1633.\\[0pt] [2] M. Toyoda, et al., Physica B 376, (2006) 647.\\[0pt] [3] Nguyen Dang Vu, et al., Appl. Phys. Express, 4, (2011) 015203.\\[0pt] [4] K. Kenmochi, et al., J. Phys. Soc. Jpn, 73, (2004) 2952.\\[0pt] [5] M. Seike et al., Jpn. J. Appl. Phys.50 (2011) 090204.; ibid 51 (2012) 050201.\\[0pt] [6] K. Oka et al., J. Am. Chem.Soc. 134 (2012) 2535.