Session F17: Focus Session: Magnetic Spinel and Perovskite Heterostructures

Growth and Properties of Magnetic Spinel Ferrite Thin Films and Heterostructures

There is considerable interest in the growth of single crystal spinel ferrites films because of their numerous technological applications in areas such as microwave integrated devices, magnetoelectric coupling heterostructures, and potentially as an active barrier material in an emerging class of spintronic devices called spin filters. Unlike perovskites, the study of spinel ferrite films is quite limited in part due to the complex crystal structure with a large unit cell consisting of many interstitial sites and that the transition metal cations can adopt various oxidation states. We have grown high-quality, atomically smooth epitaxial ferrite (NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ and LiFe$_{\mathrm{5}}$O$_{\mathrm{8}})$ films using chemical vapor deposition and pulsed laser deposition techniques and carried out detailed studies of their structural, magnetic and optical properties. Of particular interest are systematic studies on the formation of antiphase boundaries in epitaxial NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ films grown on different substrates and the accurate determination of the band gap of this material using optical spectroscopy and first principles calculations. Additionally, we have grown ferrite films on piezoelectric substrates and observed large shifts in the ferromagnetic resonance profile due to magnetoelectric coupling resulting from electrostatic field-induced changes in the magnetic anisotropy field. Work done in collaboration with N. Z. Bao, W. H. Butler, R. Datta, B. S. Holinsworth, M. Iliev, S. Kanuri, S. V. Karthik, G. Kim, T. M. Klein, N. Li, M. Liu, P. R. LeClair, J. X. Ma, D. Mazumdar, T. Mewes, D. V. B. Murthy, J. L. Musfeldt, K. R. O'Neal, N. Pachauri, V. M. Petrov, H. Sato, S. Sch\"{a}fer, L. Shen, H. Sims, G. Srinivasan, N. X. Sun, Q. -C. Sun, and Z. Zhou.   

Temperature-dependent time-domain THz spectroscopic study of spinel NiCo$_{2}$O$_{4}$ thin films

The unique combination of electrical conductivity, infrared transparency, electro catalytic activity, and ferrimagnetic order makes the spinel NiCo$_{2}$O$_{4}$ an attractive material for various technological applications. Our previous study showed that high quality epitaxial spinel NiCo$_{2}$O$_{4}$ films on MgAl$_{2}$O$_{4}$ (001) substrate exhibit metallic behavior accompanied by ferrimagnetic order. The electrical properties of these films can be tuned from metallic to insulating by changing the growth temperature. The comprehensive understanding of the microscopic details of carrier transport in these films requires the study of frequency-dependent optical properties. Terahertz time-domain spectroscopy (THz TDS) can determine the frequency dependent complex dielectric constant, refractive index, and optical conductivity. We used THz TDS to measure the optical properties of NiCo$_{2}$O$_{4}$ in the 0.2 -- 2.7 THz spectral region. The complex conductivities display a Drude-type frequency response. The extrapolated DC conductivity is consistent with our previous work. The temperature- and growth-condition dependent Drude parameters provide further insight in the metal-insulator transition in these materials.   

Theory of the magnetic and metal-insulator transitions in RNiO3 bulk and layered

A slave rotor-Hartree Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within an unified formulation, the significant aspects of the rare earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO$_3$ and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and un-strained thin-film form. For ultrathin films, epitaxial strain and charge-transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.   

Perovskite BaCrO$_{3}$: completing a materials system with an anomalous Mott transition

Perovskite BaCrO$_{3}$ cannot be stabilized in bulk but we have synthesized this compound as a film. BaCrO$_{3}$ films have a substantially larger lattice constant than other chromates, are insulating, and exhibit weak ferromagnetism likely associated with canted antiferromagnetism. Comparison with the sister compounds CaCrO$_{3}$ and SrCrO$_{3}$ suggests an anomalous Mott transition caused by lattice expansion where magnetism is independent of whether the compound is metallic or insulating.   

Above room temperature ferroelectricity and weak ferromagnetism in LaFeO$_3$/LnFeO$_3$ digital superlattices

We have studied from first principles the structural, ferroelectric, and magnetic properties of the (LaFeO$_3$)$_1$/ (LnFeO$_3$)$_1$ digital superlattices, with Ln = lanthanide (or Y). We show that in this class of artificial materials constructed from Pnma perovskites, which are highly amenable to advanced oxide thin film growth techniques, octahedral rotations induce a spontaneous electrical polarization (consistent with the recently developed rules of Rondinelli and Fennie). Furthermore, this rotation pattern is shown to induce linear magnetoelectricity and weak-ferromagnetism, much like the recently discussed `327' manganite Ruddlesden-Popper. In these ferrite superlattices, however, it is clear that both the ferroelectric and magnetic ordering temperatures should occur above room temperature. Finally we discuss how the `La/Ln' cation radius mismatch controls the magnitudes of the induced polarization and magnetization, as well as the barrier to switch the polarization.   

Polarization in epitaxial LaFeO$_{3}$/SrFeO$_{3}$ superlattice thin films

Controlling ferroelectricity in perovskite thin-films requires an understanding of the many factors that are known to affect their polar behavior, such as octahedral rotations, cation ordering, oxygen vacancies, and the surface and interface terminations. Here, we report a study using a combination of aberration-corrected scanning transmission electron microscopy, electron-energy loss spectroscopy (EELS), and density functional calculations (DFT) to demonstrate how these factors work in concert to give rise to polarization in LaFeO$_{3}$/SrFeO$_{3}$ superlattices grown on SrTiO$_{3}$ substrates. Although both LaFeO$_{3}$ and SrFeO$_{3}$ are non-polar in the bulk, microscopy results show Fe-displacements in the superlattices indicating a dipole-like electric field. The magnitude of the observed displacements peaks in the interior of the films and goes to zero towards the substrate and the surface. O K EELS results show variation in intensities within the films, suggesting that oxygen vacancies may play a role. DFT results explaining the origin of the observed polar displacements within the superlattices and the effect of the abovementioned factors will be presented.   

Tuning the electronic structure of (SrTiO$_{3}$)$_{n}$/(SrFeO$_{3-x}$)$_{m}$ superlattices

SrTiO$_{3}$ and other $d^{0}$ perovskite-derived compounds are of interest as possible solar water-splitting catalysts, due to their band-edge energies and stability in water. To optimize their ability to absorb and convert solar energy, it is desirable to understand how to tune the electronic structure and band gap of these compounds. One controllable way to experimentally tune the crystal structure, and consequently the electronic structure, of these compounds is through the growth of epitaxially layered superlattices. Past computational work has studied the interleaving of SrTiO$_{3}$ and SrFeO$_{3}$, in which the $d^{4}$ Fe$^{4+}$ atoms result in metallic electronic structure. However, the synthesis of related compounds suggests that oxygen vacancies in these superlattices would likely reduce some or all of the Fe$^{4+}$ to Fe$^{3+}$ ($d^{5}$), which could once again open a tunable band gap. We use density functional theory and beyond to study the energetics of oxygen vacancy patterns in (SrTiO$_{3}$)$_{n}$/(SrFeO$_{3-x}$)$_{m}$, and the possibility of favorably tuning the electronic structure and band gap of these superlattices via changes in layering, oxygen vacancy concentration, and biaxial strain. Our results are thoroughly discussed in the context of recent experiments.   

Electronic and Magnetic Reconstruction at Manganite Interfaces

We investigate interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. In FM/AFI heterostructures the magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent FM and AFI manganites have the same electron density n. For n=0.6, the induced ferromagnetic moment in the AFI layers sandwiched between FM manganites decreases monotonically with increasing layer width. For n=0.5 instead, the induced ferromagnetic moment varies non-monotonously with the layer width. These differences for n=0.6 and n=0.5 originate from different charge-transfer profiles and magnetic reconstructions at the FM/AFI interfaces. The width of the AFI layers furthermore controls the magnitude of the magnetoresistance and the metal to insulator transition of the FM/AFI heterostructure. These results are discussed in the context of recent experiments on LSMO/PCMO [1] and LCMO/PCMO superlattices [2].\\[4pt] [1] D. Niebieskikwiat {\it et al.}, Phys. Rev. Lett. 99, 247207 (2007).\\[0pt] [2] H. Li {\it et al.} Appl. Phys. Lett. 80, 628 (2002).   

DFT investigation of structural effects on perovskites exhibiting metal-insulator phase transitions

The rich variety of electronic, magnetic, etc. properties available in perovskite materials are closely linked to octahedral tilting and other details of the arrangements of atoms within these materials. Furthermore, it has been demonstrated that the tilts and structural details are altered by the growth of films of these materials on substrates that provide strain and changed by the creation of new layered or superlattice structures from these materials. We employ density functional methods to investigate the relationship between tilting and charge ordering in a variety of strained-layered perovskite materials. We present these DFT results along with model calculations that aid in interpreting the complex connections between atomic structure and electronic properties, \emph{e.g.}\ structural control of metal-insulator phase transitions.   

The effect of interfacial octahedral behavior on magnetic properties in ultrathin manganite films

In \textit{AB}O$_{3}$ perovskites, the rotation and distortions of $B$O$_{6}$ octahedra lead to crystal symmetric variants of the basic perovskite structure. The rotation angles play a role in magnetic exchange with previous work demonstrating a clear relationship between bond angles and ordering temperatures. Recent work has shown that heteroepitaxial oxide films can be stabilized with non-equilibrium crystal structures due to structural coupling of octahedral behavior across the substrate/film interface. However, it is not yet apparent how the crystal symmetry across a heteroepitaxial oxide interface contributes to magnetic properties. Here, we report on the effect of crystal symmetry in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO), a canonical magnetic oxide, grown using molecular beam epitaxy on different symmetric substrates with similar lattice parameters. For this study, we have used x-ray magnetic circular dichroism, transport, and magnetoresistance measurements to explore the magnetic properties of ultrathin LSMO films for a direct comparison of magnetic behavior in isocompositional perovskites with different octahedral behavior.   

Electronic and Magnetic Tunability of Sr$_{2}$CrReO$_{6}$ Thin Films by Growth-mediated Oxygen Modulation and Template Variation

Highly ordered epitaxial films of ferrimagnetic semiconductor Sr$_{2}$CrReO$_{6}$ have been fabricated by off-axis magnetron sputtering, and characterized as a function of the oxygen partial pressure at optimal growth conditions. In this letter, we report 18,000{\%} modulation in electrical resistivity at T$=$ 7K (60{\%} at room temperature) from a 1{\%} modulation in the oxygen partial pressure during film growth. The growth window was chosen to center around the condition for peak saturation magnetization, which drops due to both increasing and decreasing oxygen growth pressure. The results suggest that n-type doping from oxygen vacancies in the film likely play the dominant role in the electrical properties and modulation of Sr$_{2}$CrReO$_{6}$ thin films. We also explore the effects of substrate templates on the structural, electrical, and magnetic properties of Sr$_{2}$CrReO$_{6}$. Sr$_{2}$CrReO$_{6}$ films fabricated on double perovskite substrates or buffer layers exhibit increased resistivities at low temperatures.   

High-resolution terahertz spectroscopy of Sr$_{2}$CrReO$_{6}$ at cryogenic temperatures and high magnetic fields

Temperature and magnetic field dependent terahertz spectroscopies have proven useful in elucidating the interplay between structure charge, and magnetism in complex oxide systems. To this end, we are developing a turn-key, continuous-wave (CW) terahertz transmission spectrometer operating from 6 K to 300 K and in fields up to 9 T. Fiber-coupled photoconductive switches operate from 200 GHz to 1.8 THz in the cryogenic and high-field sample environment -- eliminating the need to align a THz beam through multiple cryostat windows. In this work we compare CW-THz measurements on epitaxial thins films of Sr$_{2}$CrReO$_{6}$, a double-perovskite ferrimagnet, with conventional THz time-domain spectroscopy.   

Indications of spin-polarized transport in thin film double perovskites Sr2FeMoO6 and Ba2FeMoO6

Double perovskite oxides Sr2FeMoO6 and Ba2FeMoO6 have attracted attention for spintronic device development due to their predicted highly spin-polarized transport characteristics. However, most published experimental results are from bulk material, not thin films that are more relevant for realistic device development. We will present our results of the growth as well as structural, magnetic and transport properties of thin films of double perovskites Sr2FeMoO6 and Ba2FeMoO6 produced by pulsed laser deposition. We have produced highly crystalline, near-epitaxial thin films of each material. We will survey the magnetic and magnetotransport properties, including the magnetoresistance, planar and anomalous Hall effects, which provide evidence for the presence of spin-polarized charge carriers well above room temperature and the potential for developing high sensitivity magnetic sensors.