Session E19: Magnetic Oxide Thin Films and Heterostructures: Novel Interfacial Phenomena

Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling

Ultimate miniaturization of magnetic random access memory (MRAM) devices is expected by the utilization of spin-transfer torques, because they present an efficient means to switch elements with a very high magnetic anisotropy. To overcome the low switching speed in current collinearly magnetized devices, new routes are being explored to realize magnetic tunnel junction stacks with non-collinear magnetization between two magnetic electrodes. Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network would provide a promising direction for non-collinear magnetization in correlated oxide magnetic tunneling junctions. Here, we demonstrate how to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result we possess control of the lateral magnetic and electronic anisotropies by atomic scale design of the oxygen octahedral rotation.   

Interfacial Control of Magnetic Properties at LaMnO$_{\mathrm{3}}$/LaNiO$_{\mathrm{3}}$ Interfaces

The functional properties of oxide heterostructures ultimately rely on how the electronic and structural mismatches occurring at interfaces are accommodated by the chosen materials combination. We discuss here LaMnO$_{\mathrm{3}}$/LaNiO$_{\mathrm{3}}$ heterostructures, which display an intrinsic interface structural asymmetry depending on the growth sequence with the LaMnO$_{\mathrm{3}}$-on-LaNiO$_{\mathrm{3}}$ interface being sharper than the LaNiO$_{\mathrm{3}}$-on-LaMnO$_{\mathrm{3}}$ one, which exhibits 2-3 unit cells intermixing [1]. Using a variety of synchrotron-based techniques, we show that the degree of intermixing at the monolayer scale allows interface-driven properties such as charge transfer and the induced magnetic moment in the nickelate layer to be controlled. Further, our results demonstrate that the magnetic state of strained LaMnO$_{\mathrm{3}}$ thin films dramatically depends on interface reconstructions. [1] Gibert \textit{et al., }NanoLetters in press.   

Understanding the Origin of Ferromagnetism in LaNiO$_{3}$/CaMnO$_{3}$ Superlattices

Interfacial ferromagnetism (FM) in transition metal oxide heterostructures is a promising route for engineering new low-dimensional devices. In 2001, FM was discovered in CaRuO$_{3}$/CaMnO$_{3}$ superlattices (SLs), which is attributed to an itinerant electron-mediated Mn-Mn double-exchange (DE). Since then we have discovered interfacial FM in (LaNiO$_{3}$)$_{N}$/(CaMnO$_{3}$)$_{8}$ SLs that is consistent with this DE interaction\footnote{A.J. Grutter et al., \textit{Phys. Rev. Lett.} \textbf{111}, 087202 (2013)}. Now we have explored even further reduced dimensionality by fabricating [(LNO)$_{n=2-7}$/(CMO)$_{4}$]$_{10}$ SLs. Transport measurements confirmed a thickness dependent metal-insulator transition, with insulating films for N$<$4. Bulk magnetometry measurements reveal interfacial FM in insulating and conducting SLs. Since there are no itinerant electrons in the insulating SLs, this FM must arise from a different source. Using x-ray absorption spectroscopy and magnetic circular dichroism, we have identified the coexistence of Ni$^{2+}$ and Ni$^{3+}$ and Ni magnetism. We therefore speculate that the FM in insulating SLs originates from a Mn-Ni superexchange interaction. We discuss the role of these interactions in interfacial FM and methods for controlling them.   

Exploring interfacial ferromagnetism in manganite-based superlattices

Heterointerface of complex oxides provides a rich playground to explore the emergent phenomena that are not found in bulk. In particular, emergent interfacial ferromagnetism has been successfully demonstrated in heterostructures composed of materials which are paramagnetic and antiferromagnetic in bulk. In our previous work, leakage of itinerant electrons from a paramagnetic metal to an antiferromagnetic insulator has been shown to give rise to interfacial ferromagnetism in CaMnO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} based superlattices. However interfacial ferromagnetism in insulating superlattices suggests a more complicated scenario. Therefore a thorough investigation of coupling between charge, lattice and spin degrees of freedom is necessary. In this talk, we focus on the NdNiO{\$}\textunderscore \textbraceleft 3\textbraceright {\$}/CaMnO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} system. By choosing a paramagnetic layer that undergoes a metal-insulator transition, we can explore the role of electron itinerancy in interfacial ferromagnetism in the same sample to eliminate the inconsistencies that may originate from the deposition of multiple samples. We demonstrate that NdNiO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} exhibits a metal-insulator transition as a function of temperature, which can be tuned as a function of film thickness. We have also grown NdNiO{\$}\textunderscore \textbraceleft 3\textbraceright {\$}/CaMnO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} heterostructures with excellent crystallinity. Preliminary transport measurements indicate that the presence of an adjacent CaMnO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} layer also affects the transport in NdNiO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} so that charge transfer from the itinerant layer into the adjacent antiferromagnetic insulating CaMnO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} is likely not the only contribution to interfacial ferromagnetism.   

Magnetotransport in LaNiO$_3$/La$_{2/3}$Sr$_{1/3}$MnO$_3$ superlattices with non-collinear magnetic ordering

Non-collinear magnetic textures can give rise to exotic charge and spin transport behaviors, and may allow for the control of magnetism using small electric currents. While these textures have been observed in a number of bulk materials and in thin films, realizing non-collinear magnetism in heterostructures presents new avenues to tune their properties using tailored interfaces and gate electric fields. We have previously used polarized neutron reflectometry (PNR) to demonstrate that superlattices of paramagnetic LaNiO$_3$ (LNO) and ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO) adopt a non-collinear magnetic structure. In this work, we characterize the non-collinearity as a function of temperature and magnetic field using anisotropic magnetoresistance (AMR) and Nernst effect measurements. We observe rotational hysteresis at low fields, while the magnitude of the AMR is found to vary non-monotonically with applied field. To understand this behavior, we develop a simple free-energy model that includes contributions from biaxial anisotropy, Zeeman energy, and exchange coupling between the LSMO and the LNO layers. From this analysis, we are able to extract the orientation of the magnetization of the individual LSMO layers, which agrees well with the values measured using PNR.   

Influence of quantum confinement and strain on orbital polarization of strained four-layer LaNiO$_3$ superlattices: a DFT+DMFT study

Here we use the combination of density functional theory and dynamical mean field theory to study Ni d orbital polarization in strained LaNiO$_3$/LaAlO$_3$ superlattices consisting of four layers of nominally metallic NiO$_2$ and four layers of insulating AlO$_2$ separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analysed in terms of structural, quantum confinement, and correlation effects. The overall dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent X-ray absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found depending on the sign of strain. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk LaNiO$_3$ and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk LaNiO$_3$. The quantum confinement effect is as important as the strain effect and more stronger for tensile strain.   

Interfacial Symmetry Control of Emergent Ferromagnetism

Atomically precise complex oxide heterostructures provide model systems for the discovery of new emergent phenomena since their magnetism, structure and electronic properties are strongly coupled. Octahedral tilts and rotations have been shown to alter the magnetic properties of complex oxide heterostructures, but typically induce small, gradual magnetic changes. Here, we demonstrate sharp switching between ferromagnetic and antiferromagnetic order at the emergent ferromagnetic interfaces of CaRuO$_3$/CaMnO$_3$ superlattices. Through synchrotron X-ray diffraction and neutron reflectometry, we show that octahedral distortions in superlattices with an odd number of CaMnO$_3$ unit cells in each layer are symmetry mismatched across the interface. In this case, the rotation symmetry switches across the interface, reducing orbital overlap, suppressing charge transfer from Ru to Mn, and disrupting the interfacial double exchange. This disruption switches half of the interfaces from ferromagnetic to antiferromagnetic and lowers the saturation magnetic of the superlattice from 1.0 to 0.5 $\mu_B$/interfacial Mn. By targeting a purely interfacial emergent magnetic system, we achieve drastic alterations to the magnetic ground state with extremely small changes in layer thickness.   

4d electron Ruthenate systems: their unique and new magnetic properties

The Ruddlesden-Popper series (PR) of Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$ has attract much interest of their unique physical properties. Among them, SrRuO$_3$ (n = $\infty$) (SRO) is the only ferromagnetic metallic oxide especially in Ru 4\textit{d} transition metal oxides. Bulk SRO has orthorhombic structure showing the Curie temperature (T$_{C}$) $\sim$ 160 K. It is well known that RuO$_6$ octahedral distortion plays critical roles in its mangetic properties. In film systems, such RuO$_6$ octahedra can be easily controlled by strain-engineering. In this talk, with high quality SRO films fully strained (-1.7\%-1\%) using various substrates, we systematically studied their structural changes and associated magnetic properties. Compared to theoretical predictions, the structural changes can be explained, while the magnetic property changes cannot be understood. Surprisingly, when SRO113 is grown on its PR series of Sr$_2$RuO$_4$ (n=1) (SRO214) single crystal, the exact substrate of SRO214 magnetization results in strongly enhanced magnetization (M > 3 $\mu$$_{B}$/Ru, T$_{C}$ $\sim$ 160 K), which has never found SRO113 (001) since the low-spin configuration of SRO113 prevent M never exceed 2 $\mu_{B}$/Ru. The mystery of M in SRO113 (especially SRO113/SRO214) will be further discussed.   

Chemical Ordering Modulated Electronic Phase Separation and Macroscopic Properties in Colossal Magnetoresistance Manganites

Using unit cell by unit cell superlattice growth technique, we determine the role of chemical ordering of the Pr dopant in a colossal magnetoresistance (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO) system, which has been well known for its large length scale electronic phase separation (EPS) phenomena. Our experimental results show that the chemical ordering of Pr leads to dramatic reduction of the length scale of EPS. Moreover, compared to the conventional Pr-disordered LPCMO system, the Pr-ordered LPCMO system has \textasciitilde 100 K higher metal-insulator transition temperature. We have further investigated the n-dependence of the physical properties of the (LCMO)$_{2n}$/(PCMO)$_{n}$ superlattices. Magnetic and transport measurements indicate that the physical properties change nonmonotonically with increasing n, reaching a minimum for both the Curie temperature and the meta-insulator transition temperature. The crossover thickness thus reflects the characteristic correlation length scale along the vertical direction of the superlattice. For superlattices with n smaller than the correlation length, we combine MFM studies and model calculations to explain the weakened ferromagnetism and metallicity with increasing n.   

Polarized Neutron Reflectometry Study of Tunable Metal-insulator Superlattices

Superlattices composed of equal thickness of ferromagnetic (FM) metal La$_{0.5}$Sr$_{0.5}$MnO$_{3}$ (LSMO) and charge-orbital ordered (COO) insulator Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (PCMO) on (011)-oriented (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ substrate have been investigated using polarized neutron reflectometry. In a 200-Oe magnetic field, the magnetization depth profile shows strong temperature dependence. Between the FM transition temperature of LSMO and the COO transition temperature of PCMO, a uniform magnetization throughout the superlattices was obtained. Below the COO transition temperature of PCMO, the magnetization depth profile shows a strong contrast between the LSMO and PCMO regions. At 5000 Oe, both LSMO and PCMO show magnetizations close to their bulk saturation value at low temperature. Our result demonstrates the tunability of the PCMO/LSMO superlattices' magnetic structure with field and temperature and the behavior of this system could be explained as the result of coexistence of the FM and COO phases and their competition.   

Superparamagnetism at oxide interfaces revealed by scanning SQUID-on-tip microscopy

Our novel scanning SQUID-on-tip technique[1] is used to study nanoscale magnetism present in systems such as atomically sharp oxide heterostructures. Here we report a new emergent phenomenon at the LaMnO$_{3}$/SrTiO$_{3}$ interface in which an antiferromagnetic insulator abruptly transforms into a magnetic state that exhibits unexpected nanoscale superparamagnetic dynamics. Upon increasing the thickness of LaMnO$_{3}$ above five unit cells, our scanning nanoSQUID-on-tip microscopy shows spontaneous formation of isolated magnetic islands of 10 to 50 nm diameter, which display random moment reversals by thermal activation or in response to an in-plane magnetic field[2]. Our charge reconstruction model of the polar LaMnO$_{3}$/SrTiO$_{3}$ heterostructure describes the sharp emergence of thermodynamic phase separation leading to nucleation of metallic ferromagnetic islands in an insulating antiferromagnetic matrix. The model further suggests that the nearby superparamagnetic-ferromagnetic transition can be gate tuned, holding potential for applications in magnetic storage and spintronics. [1] D. Vasyukov \textit{et al}, Nature Nanotech. 8, 639 (2013) [2] Y. Anahory \textit{et al}, arXiv:1509.01895   

Giant structural modulation {\&} abnormal ferromagnetism in ferroelectric {\&} ultrathin ferromagnetic digital superlattices

The nature of magnetoelectric coupling in oxide heterostructure remains interesting but illusive, largely because the complex nature of interface intermixing and diffusion. In this work, we present our ability to fabricate superlattices consist of ferroelectric BTO {\&} ferromagnetic LSMO, with minimum interfacial intermixing confined within half a unit cell. Such high quality superlattices with sharp interfaces allow us to explore magnetoelectric coupling effect into ultrathin region (reduced dimensionality) and observe ferroelectric induced abnormal magnetic behavior. A detailed STEM study reveals that the traditional electron/hole carrier doping scenario does not play a major role. Instead, distinct modulation of lattice displacement and octahedron tilting is responsible for the coupling effect and abnormal magnetic behavior. Our study highlights the importance of structural-property relationship in oxide heterostructures.