Session B43: Magnetic Spinel and Binary Oxide Films

Magnetic field tunability of spin polarized excitations in a high temperature magnet

Magnetic semiconductors are at the heart of modern device physics because they naturally provide a non-zero magnetic moment below the ordering temperature, spin-dependent band gap, and spin polarization that originates from exchange-coupled magnetization or an applied field creating a spin-split band structure. Strongly correlated spinel ferrites are amongst the most noteworthy contenders for semiconductor spintronics. NiFe$_2$O$_4$, in particular, displays spin-filtering, linear magnetoresistance, and wide application in the microwave regime. To unravel the spin-charge interaction in NiFe$_2$O$_4$, we bring together magnetic circular dichroism, photoconductivity, and prior optical absorption with complementary first principles calculations. Analysis uncovers a metamagnetic transition modifying electronic structure in the minority channel below the majority channel gap, exchange splittings emerging from spin-split bands, anisotropy of excitons surrounding the indirect gap, and magnetic-field dependent photoconductivity. These findings open the door for the creation and control of spin-polarized excitations from minority channel charge charge transfer in NiFe$_2$O$_4$ and other members of the spinel ferrite family.   

Low Damping in Spinel Ferrite Thin Films Enabled by Chemical Substitution

Spinel ferrites are versatile magnetic insulators whose properties can be tuned by chemical substitution of their constituent elements. However, it has been a challenge to realize spinel ferrite thin films with sufficiently low damping for emerging spintronic applications. We achieve low damping in spinel ferrite films by substituting a large fraction of Fe with Al. Films of thickness $\approx $25 nm and nominal compositions Ni$_{\mathrm{0.65}}$Zn$_{\mathrm{0.35}}$Al$_{\mathrm{x}}$Fe$_{\mathrm{2-x}}$O$_{\mathrm{4}}$ are grown on single-crystal MgAl$_{\mathrm{2}}$O$_{\mathrm{4}}$(001) substrates by pulsed laser deposition. Fully coherent growth of Ni-Zn ferrite (x$=$0) on MgAl$_{\mathrm{2}}$O$_{\mathrm{4}}$ is elusive due to the large substrate-film lattice mismatch of \textgreater 3{\%}. High concentrations of Al (x$\ge $0.5) decrease the lattice constant of the ferrite such that the films are fully strained to the substrate and highly crystalline. As the Al concentration is increased from x$=$0.5 to 1.0, the Gilbert damping parameter $\alpha $ is reduced from $\approx $0.02 to $\approx $0.005, lower than $\alpha $ of permalloy. These low-damping spinel ferrite thin films facilitate the development of new spintronic devices based on insulating oxides.   

A joint neutron scattering and micromagnetic simulation approach to understanding magnetoelectric coupling in a mesoscale multiferroic

Heterogeneous structures consisting of two materials of different ferroic order with a coherent interface provide a natural path to magnetoelectric coupling. Recently we have been studying nanopillars of magnetostrictive CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ in a matrix of piezoelectric BaTiO$_{\mathrm{3}}$ as a system for strain-mediated electric field control of the magnetic order parameter. Using the newly developed polarization capabilities of GP-SANS at ORNL to perform polarized beam small angle neutron scattering, we have demonstrated an electric field dependence of the CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ magnetization at different magnetic fields. In combination with the neutron data, micromagnetics simulations using the Object Oriented MicroMagnetic Framework (OOMMF) have been employed to probe the spatial dependence of the magnetization. These simulations have established that a shell with large uniaxial anisotropy is required to replicate magnetometry and SANS data, which gives an indication of the depth of strain propagation across the interface.   

Study of strain-modulated effects on CoFe$_2$O$_4$ epitaxial films

Due to the improvement of thin film growth technique, epitaxial films directly grown on flexible substrates became possible recently. These kinds of flexible systems not only have the advantage of easy integration for device applications, but also provide a template to purify strain effects for physical mechanism study. Here, we investigate the evolution of the spinel CoFe$_2$O$_4$ (CFO) epitaxial film on a muscovite substrate with variable curvatures. CFO possesses superior magnetic properties with high Curie temperature and large magnetostrictive anisotropy. The CFO film on muscovite is (111) oriented, corresponding to a magnetic hard-axis along the out-of-plane direction. Under the in-plane asymmetric strain, based on the frequency shift of Raman A1g phonon, the unit cell volume of CFO increases with the tensile strain while decreases with the compressive strain. The tunable volume ratio is about 0.7$\%$. The A1g and T2g phonon evolutions also show the Co/Fe cation migration temperature decreases both under tensile and compressive strain with the degradation up to 40 K. Moreover, when out-of-plane magnetic field is applied, the magnetostriction constant increases with the compressive strain, which implies the tunable orientation of magnetic easy axis in this flexible system.   

Tuning magnetic proximity effect inside CoFe2O4/Pt bilayers by controlling the interface structure

Magnetic proximity effect (MPE) allows magnetic order to be introduced into an intrinsically non-magnetic system by placing it adjacent to a ferromagnet. Thus, using MPE from a ferromagnetic insulator is a promising approach to manipulate spin currents inside adjacent 2D materials. In this study, we demonstrate the influence of interface structure on MPE inside ferrimagnetic insulator CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$/non-magnet Pt bilayers. Molecular beam epitaxy was used to grow high quality CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ thin films and control the termination layer which came into contact with Pt. Subsequent resistivity and Hall measurements show that the strength of MPE inside Pt depends on which if the two possible termination layers of CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ comes into contact with the Pt atoms.   

Shaping the Magnetic Easy Axis in CoFe2O4 through Strain Doping

Structural engineering of the lattice through epitaxy and/or isovalent substitutions are widely used for investigating and controlling the fundamental coupling effects inherent in strongly correlated materials. However, these techniques do not allow for continuous and fine control over structural properties post-growth which hampers our ability to systematically study structure-function relationships. We present recent studies on epitaxial CoFe2O4 films which demonstrate how low energy helium ion implantation can be used to bypass these limitations. We show that imposing single axis lattice expansion provides continuous crystal symmetry control which can be used to finely manipulate spin texture through magnetostrictive effects. This allows for full control over the spin reorientation transition. Further, we demonstrate that the flexibility of the strain doping process allows these properties to be written locally into a single crystal and across many length scales, which provides a never before possible means of dictating local susceptibilities. This work was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.   

Electronic, magnetic, and optical properties of Semiconducting Spinel Fe$_{\mathrm{2}}$CrO$_{\mathrm{4}}$

Transition metal oxides offer significant flexibility in tailoring functional properties by virtue of the high degree of solid solubility of different cations within the host lattice. For instance, the electronic properties of magnetite (Fe$_{\mathrm{3}}$O$_{\mathrm{4}})$, a ferrimagnetic half metal, can be substantially changed by substituting one third of the Fe cations with Mn, Ni, Co, Zn or Mg. The actual magnetic properties of any given ferrite depend critically on whether the dopant occupies the tetrahedral (A) or octahedral (B) sites, or a mix of the two. Doping magnetite to produce a ferromagnetic semiconductor would be of considerable interest for spintronics and photocatalysis, particularly if the bandgap remains small. The detailed functional properties depend on the local structure, which is dictated in large measure by the cation sublattice(s) the dopants occupy, the valence(s) they exhibit, and the relative energy scales of competing effects, including short-range disorder, that determine the overall electronic structure. We have investigated Cr as the dopant in Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ by carrying out epitaxial film growth by molecular beam epitaxy and characterization, along with first principles modeling to explore new model materials. We find that replacing 1/3 of the Fe atoms with Cr atoms results in a low-gap, thermally robust ferrimagnetic semiconductor that is photoconductive in the visible, whereas replacing 2/3 of the Fe with Cr produces an insulator with no net magnetization.   

Spin-polarized two-dimensional electron gas at the EuO/SrTiO$_3$ interface

The development of novel nano-oxide spintronic devices could benefit greatly from interfacing with emergent phenomena at oxide interfaces. However, due to complicating factors such as thermodynamics and band alignment, the formation of such an interface is nontrivial. In this paper, we integrate highly spin-split semiconductor EuO on SrTiO$_3$ (001) by molecular beam epitaxy. Using density functional theory, we predict a spin-polarized two dimensional electron gas at the EuO/SrTiO$_3$ interface. Even at room temperature, out-diffusion of oxygen from SrTiO$_3$ during EuO epitaxy creates a highly conductive layer of oxygen vacancies on the SrTiO$_3$ side of the interface. The films are ferromagnetic with a Curie temperature of 70 K and display giant magnetoresistance below the transition temperature. Leveraging this approach to offers an as-yet unexplored route to seamlessly integrate ferromagnetism and the oxide two-dimensional electron gas.   

Low temperature magnetic characterization of EuO$_{1-x}$

EuO is a widely studied magnetic semiconductor. It is an ideal case of a Heisenberg ferromagnet as well as a model magnetic polaron system. The interesting aspect of this material is the existance of magnetic polarons in the low temperature region. We study the properties of oxygen deficient EuO prepared by pulsed laser deposition. Besides normal ferromagnetic transitions near 70K and 140K, we observe a different transition at 16K. We also observe a shift in the coercivity for field cooling versus zero field cooling. Possible mechanisms driving these behaviors will be discussed.   

Surface spin characterization of Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ films epitaxially grown on (001) TiO$_{\mathrm{2}}$ and (0001) Al$_{\mathrm{2}}$O$_{\mathrm{3}}$

Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ is an interesting antiferromagnetic material, which has been widely investigated because of its high Neel temperature and the electric field control of its exchange bias at room temperature. We have epitaxially grown the Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ films on (001) TiO$_{\mathrm{2}}$ and (0001) Al$_{\mathrm{2}}$O$_{\mathrm{3}}$. For the growth on TiO$_{\mathrm{2}}$, we find that the Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ film is (10-10) oriented, confirmed by XRD and high resolution TEM. As the spins of the Cr atoms are parallel to the $c$ axis, the surface spins of the (10-10) Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ film are manipulated to align in-plane, as indicated from the exchange bias measurement of the Py/(10-10) Cr$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$interface. Furthermore, we observe a positive exchange bias that depends on the direction of the cooling and measuring magnetic fields. On the other hand, (0001) Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ film is epitaxially grown on (0001) Al$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$substrate. And the crystal structure is confirmed by XRD. For this (0001) film, the exchange bias is found to be perpendicular to the plane, indicating that the spins of the (0001) Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ film are out-of-plane. Our results demonstrate crystal structure manipulations of the exchange bias and the collinear exchange coupling between the surface spins of the Cr atoms and the adjacent FM layer.   

Fabrication and magnetic properties of a magnetite/hematite epitaxial bilayer generated with ion bombardment

The combination of several oxides in the same material may give rise to novel phenomena not displayed by either of the constituent oxides alone. Thus, the improvement or development of methods for the synthesis of heterostructures is mandatory. We present in this work a novel approach to obtain a bilayer formed by a sub-oxide on top of the corresponding oxide: Low Energy Ion Bombardment (LEIB). We have already successfully applied this technique to the TiO(001)/TiO$_{\mathrm{2}}$(110) system. Now we show its application in iron oxide thin films: we fabricate a magnetite/hematite (Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/$\alpha $-Fe$_{\mathrm{2}}$O$_{\mathrm{3}})$ single-crystalline epitaxial bilayer. As far as we know, this system has not been synthesized yet in the form of a thin bilayer. This novel fabrication method involves the transformation of the upper layers of a single-crystalline hematite film to single-crystalline magnetite, a process driven by the preferential sputtering of oxygen atoms. We show the reversibility of the transformation between hematite and magnetite, always keeping the epitaxial and single-crystalline character of the films. The magnetic characterization of the bilayer grown with this method shows that the magnetic response is mainly determined by the magnetite thin film.   

Temperature dependent behavior of epsilon-Fe2O3 embedded in SiO2 thin films by micro-Raman and XAS.

Epsilon-Fe2O3 nanoparticles show very attractive properties, among them, a giant coercive field (around 2T at room temperature), magnetoresistance or millimeter wave ferromagnetic resonance (FMR). [1] We have recently discovered that the epsilon-phase is present in sol-gel films from 350C and remains stable up to 900C. [2] This is an important advance regarding its growth since the typical temperature range used in the literature for obtaining this elusive phase is comprised from 900C to 1100C. In the present work, we analyse the evolution of Raman and XAS spectra as a function of the temperature and results are compared for each synthetic path. Nevertheless, other phenomena are also studied, such as the structural transition from epsilon-phase to alpha-phase varying the output laser power, or the behavior when the material undergoes a Neel transition at 500 K [3]. References [1] L. Machala et al., Chem. Mater., 23, 2011, 3255 [2] J. Lopez-Sanchez et al., Chem. Mater. 28, 2016, 511 [3] J. Lopez-Sanchez et al., RSC Adv., 2016,6, 46380   

Magnetic properties of partially oxidized Fe films

Hybrid magnetic nanostructures exhibit appealing properties due to interface and proximity effects. A simple and interesting system of hybrid magnetic nanomaterials are partially oxidized ferromagnetic films. We have fabricated Fe films by thermal evaporation and performed a partial oxidation to magnetite (Fe$_{\mathrm{3}}$O$_{\mathrm{4}})$ by annealing in air at different times and temperatures. The magnetic properties of the films evolve from those of pure metallic iron to pure magnetite, showing intermediate states where the proximity effects control the magnetic behavior. At some stages, the magnetization curves obtained by SQUID and MOKE magnetometry exhibit important differences due to the dissimilar contribution of both phases to the magneto-optical response of the system   

Pressure tuning of coercivity states in Ni-V2O3 magnetic heterostructures

Control over coercivity of magnetic materials (e.g. Fe, Ni, Co) in bulk or thin films can be achieved by using compounds which undergo a structural phase transition[1,2]. The transition temperatures of these materials are sensitive to externally applied pressure. We show that this translates well to the coercivity change of the magnetic materials in close proximity which renders pressure an additional tuning parameter for the coercivity control. We recorded hysteresis loops as a function of temperature and pressure with a standard magnetometer and a pressure cell for magnetic measurements to study systematically the role of pressures up to 1.3 GPa in Ni/V2O3 thin films. Funding: The oxide research at UCSD is supported by an AFOSR grant FA9550-14-1-0202. The magnetism aspects at UCSD are funded Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy's Office of Basic Energy Science, DMR under grant DE FG02 87ER-45332. One of us (I.K.S.) acknowledges a US DoD Vannevar Bush Fellowship. REF.: 1. J. de la Venta, S. Wang, J. G. Ramirez and I. K. Schuller, Appl. Phys. Lett. 102, 122404 (2013). 2. C. Urban, A. Quesada, T. Saerbeck, M. A. Garcia, M. A. de la Rubia, I. Valmianski, J. F. Fernandez, and Ivan K. Schuller, Appl. Phys. Lett\textbf{.} 109, 112401 (2016)   

A new bonding insight of Ti adatom on MgO/Ag system

Atomic scale magnetism attracts attention due to both possible applications in nanoscale spintronic devices and interest in the basic quantum mechanical interactions. First principle simulations of these type of systems are challenging, especially where transition metal – oxygen bonding plays key role. In our talk we will show a comparison of two sets of calculations i) DFT+Hubbard U and ii) DFT+U+spin orbit coupling (SOC), for a system of a Ti adatom (which is magnetic in this environment) on a monolayer of MgO, on bulk Ag. In our work we calculated U using linear response theory, for the specific adatom site. Scanning tunneling microscopy data shows more resemblance to DFT+U+SOC results. Though our system has a 3d Ti, where relativistic effects are not expected to play a significant role, we nonetheless found inclusion of SOC drastically changes the electronic and magnetic structure of Ti-O bonding compared to DFT+U results. Our findings reinforce the fact that magnetic transition metal -oxygen bonding in a complex surface system demands extra care and can show surprises compared to similar bulk systems.