Session V9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Transport Properties

Scaling of the anomalous Hall effect in SrRuO$_{3}$

Being one of the most intriguing manifestations of a transport phenomenon that is sensitive to spin and topology, the anomalous Hall effect (AHE) is at the focus of considerable theoretical and experimental efforts. SrRuO$_{3}$ has played a pivotal role in the study of the AHE and numerous attempts have been made to elucidate its complicated behavior. By using SrRuO$_{3}$ films with a wide range of thicknesses that vary considerably in the temperature-dependence of their resistivity, we show that the AHE scales with resistivity. The scaling provides a compelling piece of evidence that resistivity, \emph{irrespective of its sources or nature (elastic or inelastic)}, determines the AHE of SrRuO$_{3}$ in the entire ferromagnetic phase. This observation strongly suggests that changes in Berry phase due to assumed temperature-dependent exchange splitting cannot explain the complicated temperature dependence of the AHE. On the other hand side jumps mechanism combined with Karplus-Luttinger (Berry phase) mechanism that takes into account effects of finite scattering time may explain the observed behavior.   

Optical spectroscopy of magnetic exchange splitting above the Curie temperature in ferromagnetic SrRuO$_{3}$ thin film

SrRuO$_{3}$ is a representative itinerant ferromagnetic material with the Curie temperature about 150K. To investigate the relation between magnetism and electronic structure, we measured temperature dependent optical spectra of SrRuO$_{3}$ thin film on SrTiO$_{3}$ substrate. With decreasing temperature, spectral weight transfer between 2.5eV and 3.2eV transition was observed. Comparing to the first-principles calculation, we attributed the weight transfer as a magnetic exchange splitting due to the Stoner transition. Interestingly, the exchange splitting remained above the Curie temperature, and destroyed near the room temperature. This observation indicates the long range magnetic ordering of SrRuO$_{3}$ is destroyed at the Curie temperature due to the transverse spin fluctuation not a Stoner transition. Moreover, using the optical sum rule of optical spectra, we systematically and quantitatively studied the ferromagnetic property of SrRuO$_{3}$ thin film.   

Electronic and magnetic properties of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ epitaxial thin films

Strongly correlated Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) has attracted much attention for its rich physical properties such as Mott metal-insulator (MI) transition, antiferromagnetism (AFM), and spin-triplet superconductivity. We have grown epitaxial CSRO thin films on LaAlO$_{3}$ (001) substrates using a pulsed laser deposition method and investigated their electronic and magnetic properties. Ca$_{2}$RuO$_{4}$ thin films show strong compressive strain leading to an itinerant ferromagnetic (FM) phase coexisting with insulating AFM phase in the ground state and a suppressed broad and gradual MI transition. This is in sharp contrast to bulk Ca$_{2}$RuO$_{4}$, which exhibits an AFM Mott-insulating ground state and sharp MI transition. While the $x$=0.1 and 0.5 CSRO films also exhibit coherent strain, the MI transition and itinerant ferromagnetism are partially suppressed in the $x$=0.1 film and fully suppressed in the $x$=0.5 film. In contrast, Sr$_{2}$RuO$_{4}$ thin films are not susceptible to strain on any perovskite substrates including LaAlO$_{3}$; superconductivity in these films is suppressed due to disorders resulting from strain relaxation.   

Optical properties of ferrimagnetic NiFe$_{2}$O$_{4}$ thin films

Magnetic insulators like NiFe$_{2}$O$_{4 }$are attracting attention due to the high Curie temperature (850 K), which is rare among oxides . We recently demonstrated the growth of high-quality NiFe$_{2}$O$_{4}$ thin films practically down to room temperature, which permits investigation of the optical properties over wide growth temperatures. Our spectroscopic work reveals that NiFe$_{2}$O$_{4}$ displays both direct and indirect band gaps. A plot of ($\alpha $E)$^{2}$ vs. energy places the 300 K direct gaps for the highest quality films at 2.77 and 2.36 eV for the majority and minority channels, respectively whereas a plot of ($\alpha $E)$^{0.5}$ vs. energy places the indirect band gap at 1.64 eV. For the indirect case, we extract a coupling phonon energy of $\sim $50 meV (400 cm$^{-1})$, which corresponds an infrared active O-Fe-O bending mode. The difference between the direct and indirect gap energies reveals an opportunity to obtain spin-polarized carriers via optical excitation. These features have strong overlap with the solar spectrum.   

Effects of Strain in Highly Ordered Sr$_{2}$CrReO$_{6}$ Epitaxial Films

Sr$_{2}$CrReO$_{6}$, a double-perovskite ferrimagnet, has attracted much attention because of its Curie temperature well above room temperature and predictions of half-metallicity. We have deposited pure-phase Sr$_{2}$CrReO$_{6}$ epitaxial films on several (001)-oriented substrates and buffer layers by ultrahigh vacuum off-axis magnetron sputtering, in order to study tensile and compressive strain effects due to lattice mismatches up to 1.5{\%}. Triple-axis x-ray diffractometry was used in tandem with direct observations via HAADF STEM to confirm film epitaxy, phase purity, Cr/Re ordering, and strain via film lattice constants. Magnetic characterization shows a marked effect on the saturation magnetization due to strain, with slight changes in the Curie temperatures. Finally, electrical and optical characterization suggest that Sr$_{2}$CrReO$_{6 }$is a gapped material under both unstrained and strained systems, and will be discussed in detail as well.   

Probing of polarization reversal and charge conduction in epitaxial (Ga,Fe)$_{2}$O$_{3}$ thin films on conducting oxide SrRuO$_{3}$

Ga$_{2-x}$Fe$_{x}$O$_{3}$ (GFO) thin films are the promising room-temperature multiferroics since their magnetic T$_{C}$ has been reported up to 370 K at x=1.4. However, most polarization hysteresis loops of the GFO thin films have been showed lossy behaviors due to the large leakage current. The origin probably lies on charge movement between Fe$^{3+}$ and Fe$^{2+}$ sites which is generated by oxygen vacancy. We report the large reduced leakage current of the GFO thin films by chemical doping to reduce Fe$^{2+}$. The doped GFO thin films were deposited by pulsed laser deposition at 750$^{\circ}$C for 15 min in oxygen partial pressure of 200 mTorr on SrRuO$_{3}$/SrTiO$_{3}$ substrates with various doping concentration. Epitaxy of b-axis orientation in out-of plane was confirmed by x-ray diffraction. The leakage current was reduced up to 5$\sim $6 order of magnitude depending on doping concentration. In order to investigate their conduction mechanism, temperature dependent macroscopic I-V curves were measured. Ferroelectric polarization and switching of the films were acquired over a wide range of temperature as well. Scanning probe microscopy has been used to measure local leakage currents as well as polarization reversal as a mode of conductive atomic force microscopy and piezoelectric microscopy, respectively. Local investigation of their electrical properties alludes to ferroelectricity in GFO.   

Manipulating magnetic phase competition in manganites

We will discuss recent investigations attempting to isolate single order parameters in the spin-charge-orbital-lattice hierarchy in order to further understand how these complex interactions govern macroscopic electronic and magnetic properties. Specifically, we will present recent findings on strongly correlated thin films of [LaCa]MnO3, [LaSr]MnO3, and [LaPrCa]MnO3. We have developed new experimental methods that allow for both spin engineering and strain engineering at an interface to be tested on these materials; this has allowed us a glimpse at the interplay driving emergent phenomena. We have found that it is possible to exert a measure of control over the electronic phase competition leading to colossal magnetoreistance and the metal-insulator transition in manganites. This work has also been coupled with novel confinement techniques that allow us to observe single domain transitions using basic resistivity measurements which has led to several new discoveries on the formation and dynamics of electronic domains. These studies offer new means to quantitatively investigate the balanced energetics that drive complex materials and promise an ability to tune critical temperatures and desired electronic/magnetic properties.   

Role of Interface Transport in the Magnetoresistance of Strained Rare Earth Manganite Thin Films

We are studying the temperature and field dependence of magnetoresistance in epitaxial thin films of rare earth manganites which are subject to lattice mismatch strain, with the goal of investigating the possible role of transport across strain-induced internal interfaces. We will present our results, comparing La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) thin films grown on by Pulsed laser deposition on different substrates with varying degrees of compressive or tensile lattice mismatch. Strained films of thickness $\sim $ 10 nm, show a large magnetoresistance in fields $<$ 1 Tesla, which continues to increase with decrease in temperature, similar to magnetoresistance associated with grain boundary transport. The temperature dependence of MR in these films does not correlate with the temperature dependence of resistivity and thus seems to originate from effects other than Mn spin alignment. Lattice mismatch strain is known to suppress the insulator-metal transition and enhance charge and orbital ordering, leading to the coexistence of insulating and ferromagnetic metallic phases. Our results suggest possible contributions to the magnetoresistance from transport across the interfaces between the different phases.   

Correlation between the transport properties, atomic structure and oxygen vacancies of perovskites thin films

We present a study of the role of the oxygen vacancies on the atomic structure and the transport properties of a 20 nm thick La0.7Ca0.3MnO3 thin film grown by the pulsed laser deposition method on a SrTiO3 (001) substrate. The results show that the manganite lattice can accommodate up to 13{\%} of oxygen vacancies maintaining high crystalline order. Under such conditions the atomic structure is characterized by the movement of the La/Ca cations to the perovskite regular position, by the absence of the MnO6 basal plane rotation, and by a cooperative tilting of the octahedra along the out-of-plane direction. The metal-to-insulator transition temperature decreases from 265K for the fully oxygenated sample to 28 K for the sample with 13{\%} of oxygen deficiency..   

The metal-insulator transition in a phase-separated manganite studied by in situ STS

Electronic phase separation (EPS) is a key feature at the heart of the wide variety of electronic and magnetic properties in complex oxides. One consequence of EPS is that electronic transport experiments in bulk materials or 2D films mostly probe the low resistivity electronic phases due to the percolative path of the current. We study oxygen deficient La$_{5/8-x}$Pr$_{x}$Ca$_{3/8}$M nO$_{3}$ (LPCMO) thin films using both \emph{in situ} scanning tunneling spectroscopy (STS) and \emph{ex situ} transport experiments. The oxygen deficiency is known to decrease the metal-insulator transition (MIT) temperature or even completely suppress the MIT in conventional transport experiments. We show that \emph{in situ} STS is able to detect the MIT even in systems where conventional transport experiments do not show an MIT at zero magnetic field.   

Electroresistance and Joule heating effects in manganite thin films

Electroresistance (ER), i.e. electric field- and/or current-induced resistance switching, has attracted much attention recently because of the possibility of using it for the implementation of resistance random access memories (ReRAM). Although ER is a quite common phenomenon in transition metal oxides, that has been extensively studied both theoretically and experimentally, the precise mechanism involved is not clear yet. In this work we report on the ER measurements in patterned La$_{2/3}$Sr$_{2/3}$MnO$_{3}$ (LSMO) thin films prepared by sputtering. In order to analyze Joule heating effects we have evaporated a Pt layer on top of the LSMO path to have access to the actual temperature of the sample while measuring resistance of LSMO path or I(V) characteristic curves. I(V) curves have been measured at different temperatures and the corresponding resistance values are compared with that of the R(T) curve taking into account the actual temperature of the sample in order to clarify the role of Joule heating in the observed change of the resistance.   

Anisotropic magnetoresistance in thin films of the Mott metal CaVO$_{3 }$

Bulk CaVO$_{3}$ (CVO) is a Pauli paramagnetic metal with a singe 3d electron. Some unusual drastic changes in the magneto-resistance, magnetic susceptibility and the Hall effect have been reported in single crystal CVO. We have simultaneously synthesized epitaxial CVO films grown on three differently oriented SrTiO$_{3}$ substrates. Colossal magneto-resistance (MR) as well as large crystalline anisotropic was observed at low temperatures. The maximum MR, defined as (R(7 T)-R(0 T))/R(0 T)*100 {\%}, was over 1,0000 {\%} at 2 K and 35 Tesla (parallel magnetic field) on the CVO films deposited on a (110) SrTiO3 single crystal substrate, and didn't show any sign of saturation. When the magnetic field was perpendicular, MR was dropped to 6,000{\%}. The MR ratio was much larger than that of single crystal CVO. We have also investigated the magneto-transport behaviors of CVO films deposited on (111) and (100) STO and will discussed the dependence of MR in CVO on the crystal orientation as well as the orientation of external magnetic field.   

Magnetoelectric effects in Fe3O4 thin films

Recently, there has been great interest in voltage-induced manipulation of magnetic properties inside magnetic materials. One attractive avenue is using magnetic oxides, which could be susceptible to manipulation of magnetic properties by electric fields at the interface and/or directly altering the electronic behavior within strongly correlated systems, for example. Magnetite (Fe3O4) is quite interesting because it exhibits many interesting properties such as ferrimagnetism, ferroelectricity (at low temperatures) and a metal-insulator phase transition near 120 K known as the Verwey transition. Single crystal Fe3O4 samples are deposited through reactive molecular beam epitaxy (MBE) on MgO (001) substrates and the structural quality is confirmed through XRD, RHEED, and LEED. Magnetic properties are examined through magneto-optic Kerr effect (MOKE) measurements and 4-probe I-V / Van Der Pauw measurements were used to determine the electronic properties. Samples are incorporated into an electrically gated structure by the addition of a dielectric layer and metallic top electrode and we report our results and observations of voltage-induced manipulation of the magnetic properties inside thin films of Fe3O4.   

Dynamics of multiple phases in manganite as revealed by dielectric spectroscopy

Phase separation is a very important feature in correlated electron oxides. The coexistence and competition of multiple phases give rise to gigantic response to tiny stimuli, producing dramatic changes in magnetic, transport, and other properties in these compounds [1]. It is crucial to probe the physical properties of each phase separately for a comprehensive understanding of correlated electron oxide materials and their phase separation, but it is difficult due to their nano-scale distribution. Here we report dynamic properties of multiple phases in manganite thin films by using dielectric spectroscopy with a unique $p-N$ junction configuration. The multiple dielectric relaxations have been detected and we distinguished their corresponding multiple phases. The activation energy and dielectric properties of different phases have been deduced separately. We also elucidated their phase evolution with changing of temperature or applied magnetic field from viewpoint of dielectric response. These results provide a guideline to explore the electronic phase separation phenomena in correlated electron oxides. \\[4pt] [1] Y. Tokura et al. Rep. Prog. Phys. 69, 797 (2006).   

Interface induced giant magnetoelectric coupling in multiferroelectric superlattices

AMnO$_{3}$ (A=Ca, Sr, ...) are good candidates as building blocks for the multiferroic supperlattices, because they have several competing instabilities coupled to the magnetic ordering. The coupling between the spin, the AFD modes and the FE modes depends on the relative energetics of these instabilities. Unfortunately in bulk AMnO3, there is a strong AFD instability associated with a large oxygen octahedral rotation that suppresses the FE mode. As a result, the linear magnetoelectric (spin-ferroelectricity) coupling is usually found to be weak. We take the CaMnO$_{3}$(CMO)/BaTiO$_{3}$(BTO) SLs as our model systems. We find that the MnO$_{6}$ octahedral rotation will be strongly suppressed by the neighboring BaO layers, leading to the enormous enhancement of magnetoelectric coupling and the local electric polarizations in the CMO layers are significant, comparable to that in the BTO layers. This enhancement will be strengthened with the increasing density of interfaces and reaches its maximum at the shortest SL, (i.e., n=1) where one observes a huge change of electric polarization between the antiferromagnetic (AFM) and ferromagnetic (FM) states.