Session Y32: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Oxide Films

Novel Magnetic Phenomena in Oxide Thin Films, Interfaces and Heterostructures

Oxide films, heterostructures and interfaces present wonderful opportunities for exploring novel magnetic phenomena. The idea of \textbf{cationic vacancy induced ferromagnetism} was demonstrated by observing ferromagnetism in Ta$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{2}$(x $=$ 2 - 6{\%}). Using XAS, XPS and XMCD, the magnetism was mainly located at the Ti sites and was shown to arise from Ti vacancies as opposed to Ti$^{3+}$. The substrate-film interface was crucial for observing the ferromagnetism, as the required concentration of Ti vacancies could only be maintained close to the interface. With electron transport we were able to see with increasing thickness the emerging role of Kondo scattering (mediated by Ti$^{3+})$ and at larger thickness impurity scattering. The polar LaAlO$_{3}$/non-polar SrTiO$_{3}$ interface exhibits a mixture of magnetic phases most likely arising from cationic defects and selective electron occupancy in Ti t$_{\mathrm{2g}}$ levels. Using XMCD ferromagnetism was seen at these interfaces even at room temperature. Unlike LaAlO$_{3}$, polar LaMnO$_{3}$ is an insulator exhibiting orbital order that has a smaller band gap than SrTiO$_{3}$. It is a traditional antiferromagnetic material, but when grown on SrTiO3, LaMnO$_{3}$ exhibits ferromagnetism for film thicknesses exceeding 5 unit cells. This is discussed in terms of electronic reconstruction with polar charge transfer to the LaMnO$_{3}$ side of the interface and also to the surface of the over layer. Novel magnetic coupling effects are seen in perovskite ferromagnets separated by a polar oxide layer such as LaAlO$_{3}$ or NdGaO$_{3}$, whereas non-polar oxides do not show the same effect. The coupling between the ferromagnetic layers oscillates in sign between FM and AFM, depending on the barrier thickness. Such coupling is totally unexpected in the absence of any itinerary electrons, with insulating barriers that are too thick for tunneling. The novel magnetic coupling is shown to be mediated by spin-orbit coupling and also magnetic excitation of defect levels in the polar oxide planes.   

Odd-parity magnetoresistance in pyrochlore iridate thin films with broken time-reversal symmetry

Weyl Semimetal phase has a three dimensional Dirac-like band structure, which has been recently predicted to be materialized in lanthanides iridate pyrochlore (\textit{Ln}$_{2}$Ir$_{2}$O$_{7}$), accompanied with all-in-all-out spin ordering. Nevertheless, obtaining high quality \textit{Ln}$_{2}$Ir$_{2}$O$_{7}$ single crystal has been extremely challenging even in bulk. Here, we report on fabrication and magnetotransport property of Eu$_{2}$Ir$_{2}$O$_{7}$ single crystalline thin films. Our films show clear metal insulator transition at around 100 K. We reveal that one of the two degenerate all-in-all-out domain structures, which are interchangeable with time-reversal operation, can be selectively formed by the polarity of the cooling magnetic field. This domain is robustly sustained against sweep magnetic field of 9 T at 2 K, as evidenced by an unusual odd field dependent term in the magnetoresistance and an anomalous term in the Hall resistance. Our findings pave the way for exploring novel quantum transport predicted at their surfaces/interfaces or magnetic domain walls of the pyrochlore iridates.   

The intrinsic magnetic structure and ordering of multiferoic h-LuFeO$_{3}$ Films

It has been previously reported that h-LuFeO$_{3}$ films are ferroelectric and magnetically ordered above room temperature [1]. During this talk, we discuss our recent results [2] on well characterized molecular beam epitaxy grown films. While we find the films are ferroelectric at room temperature, we find that films grown on both YSZ (111) and Al$_{2}$O$_{3}$ (0001) substrates are magnetically ordered well below room temperature. The magnetic order is the standard 120 degree structure found in these materials, with a canted moment out of the a-b plane. We find that the magnetic structure of these films is consistent with the possibility of switching this small canted moment with an electric field [3]. \\[4pt] [1] W. Wang et al, Phys. Rev. Lett. 100, 237601 (2013).\\[0pt] [2] Steven M. Disseler et al, arXiv:1411.1694 (2014).\\[0pt] [3] H. Das et al, Nat. Commun. 5, 2998 (2014).   

X-ray absorption spectroscopy study on h-LuFeO$_{3}$ and h-YbFeO$_{3}$

We have studied the unoccupied electronic band structure of the hexagonal ferrites h-LuFeO$_{3}$ and h-YbFeO$_{3}$ using the absorption spectroscopy obtained with linearly polarized soft X-ray synchrotron radiation. The shapes of the spectra have been analyzed in terms of the splitting of atomic energy levels in various crystal fields corresponding to the local symmetry of the different atomic sites. Significant hybridization between O-2p and various iron and rare earth orbitals (such as Fe-3d, Lu/Yb-5d and Yb-4f) have been observed. The spectral weight contributions to the electronic states near the bottom of the conduction band are found to consist of Fe-3d, Lu/Yb-5d, and Yb-4f as relatively narrow bands, as well as a wide O-2p band covering much of the measured energy range. The results are consistent with the density functional theory calculation including onsite-Coulomb repulsion corrections in terms of Hubbard U.   

Time-resolved x-ray diffraction study of photoinduced strains in $h-LuFeO_{3}$ thin film

We have studied the structural response of epitaxially stabilized h-LuFeO3 (0001) thin film to above-band-gap optical excitation (pump) using time-resolved x-ray diffraction (probe) at picosecond time scale. The shift in (004) Bragg peak induced by a 390 nm excitation (30 ps duration) has been studied as a function of pump fluence and pump-probe time delay. The out-of-plane photoinduced lattice strain $(\Delta c/c)$ exhibits a non-linear relation with fluence. The relaxation time is on the order of 1 ns. These observations suggest a relaxation mechanism that may be mediated by combined effects of charge recombination and phonon relaxation.   

Local control of antiferromagnetic domains in Cr$_{2}$O$_{3}$

We have used a Cr$_{2}$O$_{3}$/Pd/(Co/Pd)$_{3}$ exchange biased heterostructure to measure the spatial distribution of anti-ferromagnetic (AFM) domains in magnetoelectric AFM Cr$_{2}$O$_{3}$. The AFM Cr$_{2}$O$_{3}$ possesses a residual roughness insensitive surface magnetization below its N\'{e}el temperature (T$_{\mathrm{N}}$ 307K) This surface magnetization couples to the ferromagnetic material (Co/Pd) and results in exchange bias. Cooling the Cr$_{2}$O$_{3}$ from above its N\'{e}el temperature in different magnetization states of the ferromagnet results in the formation of AFM domains in Cr$_{2}$O$_{3}$. The AFM domains in the Cr$_{2}$O$_{3}$ were mapped by a spatial map of the exchange bias of the ferromagnet for the sample cooled in various remnant conditions of Co/Pd and at different temperatures. Local control of AFM domains was achieved by first, controlling the magnetization of Co/Pd multilayers and cooling below the Neel temperature, resulting in a single AFM domain, and then, subsequently writing a reversed AFM domain using the heat from a 6 mW focused laser beam. Scanning the laser beam allows for the writing of any domain pattern with a spatial resolution of 5 $\mu$m, limited only by the focusing optics.   

The role of spin fluctuations in the conductivity of CrO$_{2}$

Chromium dioxide is a half-metallic ferromagnet with T$_{\mathrm{C}}=$390K. Below T$_{\mathrm{C}}$, the conductivity of CrO$_{2}$ grows by two orders of magnitude and is temperature independent below about 30 K. It is believed that electron scattering by spin fluctuations is responsible for the strong temperature dependence of the conductivity. We performed time-resolved THz spectroscopy (TRTS) and time-resolved magneto-optical Kerr effect (TRMOKE) to study the role of spin fluctuations in electron conduction. A thin film CrO$_{2}$ sample was excited by an optical pump pulse. The induced conductivity changes were measured by TRTS and the induced spin response by TRMOKE. A fast and a slow component were observed in both responses. The fast component dominates the TRTS response, while the slow dominates the TRMOKE which we attribute to the spin demagnetization in CrO$_{2}$. Since the slow component contributes only a small fraction of the total conductivity change in TRTS, we conclude that spin fluctuations may not play the dominant role in the pump-induced conductivity change. We also observed that the film transmits less THz light after the pump excitation, which corresponds to it becoming more conductive. We will discuss the relationship of our observations to the electronic and optical properties of CrO$_{2}$.   

Ferromagnetic boundary magnetization properties of epitaxial Cr$_{\mathrm{(2-x)}}$Al$_{\mathrm{(x)}}$O$_{3}$ thin films

The existence of an equilibrium net magnetization at (0001) surfaces is enabled by symmetry constrains for the magnetoelectric antiferromagnet $\alpha $-Cr$_{2}$O$_{3}$. The occurrence of this boundary magnetization (BM) is furthermore roughness insensitive [1]. The BM is hereby fully coupled to the bulk antiferromagnetic order parameter and can be reversed together with it by a combination of E and H fields in bulk materials [2], or solely by magnetic means for single crystal (0001) oriented thin chromia films [3]. In order to understand whether the BM can be extended to alloys containing different oxide materials, we investigated the effect of Al$_{2}$O$_{3}$ doping onto the structural and magnetic properties of $\alpha $-Cr$_{2}$O$_{3}$. We grew, using a hybrid growth procedure, 100 nm thick high-quality epitaxial Cr$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{3}$(0001) thin films in the concentration range between x$=$0 to x$=$0.6, preserving the original corundum crystal structure and symmetry. Using SQUID magnetometry, we showed that the critical temperature T$_{\mathrm{N}}$ can be tuned by alloying with $\alpha $-Al$_{2}$O$_{3}$ using the BM as a probe to study the magnetic transition. Furthermore, we were able to evaluate the critical exponent and the absolute BM values for different samples. Both properties are consistent with the expected values, corroborating the BM nature of the observed magnetic signals. References: [1] K. D. Belashchenko, Phys. Rev. Lett. \textbf{105}, 147204 (2010); [2] X. He et al., Nat. Mater. \textbf{9},579 (2010); [3] L. Fallarino et al., Appl. Phys. Lett. \textbf{104}, 022403 (2014)   

Electronic and magnetic properties of (1 1 1)-oriented CoCr$_2$O$_4$ epitaxial thin film

We report on the fabrication of high quality (1 1 1)-oriented ferrimagnetic normal spinel CoCr$_2$O$_4$ epitaxial thin films on single crystal Al$_2$O$_3$ substrates. The structural, electronic and magnetic properties were characterized by $in$-$situ$ reflection high energy electron diffraction, atomic force microscopy, X-ray diffraction, X-ray photoemission spectroscopy, SQUID magnetometry and element resolved resonant X-ray magnetic scattering. The comprehensive characterization reveals that no disorder in the cation distribution or multivalency issue is present in the samples. As a result, Kagom\'e and triangular layers are naturally formed via this specific growth approach. These findings offer a pathway to fabricate two dimensional Kagom\'e heterostructures with novel quantum many-body phenomena by means of geometrical design.   

Fe$_{3}$O$_{4}$ thin films: from worse to better than the bulk

Magnetite Fe$_{3}$O$_{4}$ is one of the most investigated materials from the class of transition metal oxides. It shows a first-order anomaly in the temperature dependence of the electrical conductivity at T$_{V}$= 125 K, the famous Verwey transition. However, thin films of Fe$_{3}$O$_{4}$ show always a lower T$_{V}$ compared to the bulk material. The transition in films is also always much broader than in the bulk. In order to find out the reason, we have performed a systematic investigation of the transport properties in dependence of the oxygen pressure, thickness, and the choice of the substrate. The findings point us the way how to grow films that have very sharp transitions with even higher T$_{V}$ than the bulk material.   

Structural and magnetic properties of epitaxial delafossite CuFeO2 thin films grown by pulsed laser deposition

Growth of pure phase delafossite CuFeO2 thin films on Al2O3 (00.1) substrates by pulsed laser deposition was systematically investigated as function of growth temperature and oxygen pressure. X-ray diffraction, transmission electron microscopy, Raman scattering, and x-ray absorption spectroscopy confirmed the existence of the delafossite phase. Infrared reflectivity spectra determined a band edge at 1.15 eV, in agreement with the bulk delafossite data. Magnetization measurements on CuFeO2 films demonstrated a phase transition at TC = 15K, which agrees with the first antiferromagnetic transition at 14K in the bulk CuFeO2. Low temperature magnetic phase is best described by commensurate, weak ferromagnetic spin ordering along the c-axis.   

Perpendicular magnetic anisotropy in thin ferromagnetic films adjacent to high-k oxides

Perpendicular magnetic anisotropy (PMA) in thin ferromagnetic films has attracted a great deal of attention due to interesting physics and promising application in spintronic devices. The strength of PMA is often found to be strongly influenced by the adjacent heavy metal layer and oxide layer. A strong interest has emerged recently to control the PMA of these ultra-thin films by electric fields. Here we report the fabrication and characterization of perpendicularly magnetized 3d transitional metal films next to high-k oxides such as HfO2 and ZrO2. We have investigated structural, magnetic and transport properties of these films. The PMA strongly depends on the thickness of the ferromagnetic layers and the interfacial oxidation level of the bilayers. We will also discuss electric field controlled magnetic properties in these systems. This work was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.   

IR Ellipsometry on RF sputtered Permalloy Oxide thin films

Recently several electronic devices have achieved significant enhancements that have been attributed to an oxidized NiFe layer. A study on lateral spin valves, was found to have an increased magnetoresistance after leaving it exposed to air. The enhancements were attributed to the partly oxidation of a NiFe layer. Even more recently the turn on voltages of Hematite based water splitting devices was lowered to record low of .61 V with the addition of an amorphous NiFeOx layer. We investigated the optical properties of Ni80Fe20-oxide thin films that were deposited by reactive RF sputtering on quartz and Si/SiO2 substrates. Deposition was performed in an AJA Magnetron System using an Ar gas flow of 8 sccm and an oxygen gas flow of 2 sccm for different substrate temperatures (24-600 degrees Celsius). The optical properties in the visible spectrum and the film thickness were measured using a Woollam M2000 variable angle spectroscopic ellipsometer. Additional measurements were performed with a Woollam IR-VASE from 1.7 to 30 micrometer. The measurement results show the existence of a phonon peak around 382 cm$^{-1}$ slightly red-shifted from the 390 cm$^{-1}$ phonon peak of single crystalline NiO. XRD spectra did only reveal X-ray peaks of the rocksalt structure.