Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Electric Control of Magnetism in Oxides |
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Sponsoring Units: DMP GMAG Chair: Will Echtenkamp, University of Nebraska-Lincoln Room: 108 |
Monday, March 3, 2014 11:15AM - 11:51AM |
B6.00001: Full Electric Field Control of Exchange Bias Invited Speaker: Stephen Wu Exchange bias is the shift of a magnetic hysteresis curve due to interfacial magnetic coupling between a ferromagnet (FM) and an antiferromagnet (AFM). This ubiquitous effect has long been used in the electronics industry to bias the magnetization of FM layers in magnetic devices. Its continued understanding is of critical importance to advance the development of future high-density magnetic storage media and other novel magnetic devices. However, due to the technological limitations of manipulating and observing an atomically thin interface, exchange bias is not well understood. In this talk we present a multiferroic field effect device with BiFeO$_{3}$ (BFO) (antiferromagnetic-ferroelectric) as the gate dielectric and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) (ferromagnetic) as the conducting channel, which exhibits the direct, bipolar electric control of exchange bias. Here the magnetic states at the AFM/FM interface can be directly manipulated with electric fields and the results can be observed as a change in exchange bias polarity and magnitude. Control of exchange bias at this level has significant implications because it represents a form of electric field control of magnetism and may potentially offer a route toward the eventual full electric field control of magnetization. In this device, exchange bias is reversibly switched between two stable states with opposite exchange bias polarities upon ferroelectric poling of the BFO. No field cooling, temperature cycling, or additional applied magnetic or electric field beyond BFO poling is needed for this bipolar modulation effect. Detailed temperature dependent measurements and a model will be presented which will attribute this effect to the coupled antiferromagnetic-ferroelectric order in BFO along with the modulation of interfacial exchange interactions due to ionic displacement of Fe$^{3+}$ in BFO relative to Mn$^{3+/4+}$ in LSMO. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B6.00002: Voltage controlled exchange bias in an all-thin-film Cr$_{2}$O$_{3}$ based heterostructure Will Echtenkamp, Christian Binek Spintronics utilizes the electron's spin degree of freedom for an advanced generation of electronic devices with novel functionalities. Controlling magnetism by electrical means has been identified as a key challenge in the field of spintronics, and electric control of exchange bias is one of the most promising routes to address this challenge. Previously, robust isothermal electric control of exchange bias has been achieved near room temperature utilizing a bulk single crystal of Cr$_{2}$O$_{3}$ [1,2]. In this study electric control of exchange bias in an all-thin-film system is demonstrated with significant implications for device realization. In particular, voltage controlled switching of exchange bias in a Cr$_{2}$O$_{3}$ based magnetoelectric magnetic tunnel junction enables nonvolatile memory storage with virtually dissipationless writing at, or above, room temperature. Additionally, unique physical properties which arise due to the Cr$_{2}$O$_{3}$ thin film geometry are highlighted.\\[4pt] [1] Xi He, et. al, Nat. Mater. 9, 579-585 (2010)\\[0pt] [2] W. Echtenkamp, Ch. Binek, Phys. Rev. Lett. 111, 187204 (2013) [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B6.00003: Full control of magnetism in a manganite bilayer by ferroelectric polarization Shuai Dong, Elbio Dagotto An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the $n$-type interface side than at the $p$-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change $>90\%$ of the total magnetization can be achieved by switching the polarization's direction. This function is robust and provides full control of the magnetization's magnitude, not only its direction, via electrical methods. Reference: S. Dong and E. Dagotto, Phys. Rev. B 88, 140404(R) (2013). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B6.00004: \textit{In Situ} Electrical Biasing Studies of Magnetoelectric Coupling in La$_{1-x}$Sr$_{x}$MnO$_{3}$-PbZr$_{x}$Ti$_{1-x}$O$_{3}$ Thin Film Oxide Heterostructures Steven Spurgeon, Ian McDonald, Esther Huang, Rama Vasudevan, Samuel Lofland, Brian Kirby, Nagarajan Valanoor, Mitra Taheri Thin film La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) -- PbZr$_{x}$Ti$_{1-x}$O$_{3}$ (PZT) magnetoelectric heterostructures possess desirable properties for a range of spintronic applications, but a poor understanding of interfacial coupling dynamics has made them difficult to implement. Here we present a series of magnetization studies utilizing direct \textit{in situ} electrical biasing and switching of PZT polarization. We show that a piezoelectric strain effect gives rise to significant changes in the bulk saturation magnetization of LSMO. We complement these measurements with novel \textit{in situ} polarized neutron reflectometry measurements that reveal the spatial extent of induced magnetization. We then correlate these magnetic measurements with local structural and chemical probes to elucidate a structural basis for the observed magnetic properties. From these results we suggest ways to tune coupling for a particular application and we also propose ways to extend these techniques to other composites. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B6.00005: Using neutron scattering to explore new magnetoelectric phenomena in both thin films and skyrmion lattices Invited Speaker: Jonathan White Neutron scattering continues to be an invaluable tool for exploring the microscopic magnetic properties of magnetoelectric (ME) and multiferroic materials. Here I will present studies where neutron scattering techniques less commonly used for studying MEs have provided pivotal insight into new ME coupling phenomena. Firstly, we have used polarized neutron reflectometry (PNR) in a study of multiferroic and strained orthorhombic (o-) LuMnO$_{3}$ thin films [1]. Unlike bulk o-LuMnO$_{3}$ which is a commensurate antiferromagnet, the films display drastically different properties and are simultaneously incommensurately antiferromagnetic and ferromagnetic at low temperature. The pivotal PNR experiments allowed us to measure the spatial distribution of the ferromagnetic magnetization in the films, and show that the ferromagnetism is most pronounced close to the film-substrate interface which is highly strained due to the lattice mismatch. We could further show the ferromagnetism and antiferromagnetism in the film to be directly coupled, and so demonstrate the promising functional properties of these films. Secondly, we have used small-angle neutron scattering (SANS) to study the topologically protected magnetic spin vortices, or skyrmions, in the chiral-lattice ME insulator Cu$_{2}$OSeO$_{3}$. Until 2012, skyrmions had been observed only in (semi)conducting B20 compounds where it is known that they can be manipulated by conduction electrons. From our SANS experiments on Cu2OSeO3 [2], we show that applied electric \textit{fields} can control the skyrmion lattice orientation in insulators, and in an essentially lossless manner that is dependent on both the size and sign of the electric field. These results provide the first evidence for a the electric field control of topologically protected magnetism in bulk magnetoelectrics. \\[4pt] [1] J.S. White \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 037201 (2013).\\[0pt] [2] J.S. White \textit{et al.}, J. Phys.: Condens. Matter \textbf{24}, 432201 (2012). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B6.00006: Submicronic Spatial Mapping of Magnetoelectric Coupling in an Electronically Inhomogeneous System Gervasi Herranz, Ondrej Vlasin, Nico Dix, Florencio Sanchez Electric-field control of data stored in magnetic units prefigures a promising alternative to nowadays conventional electronics. A development of such technology demands a complete understanding of the dynamics and magnetoelectric response at small scales. Yet, present experimental approaches are hampered by the extreme difficulty of having simultaneous access to magnetism and ferroelectricity. Here we present an innovative approach that exploits optics to achieve a magnetoelectric coupling mapping with unprecedented resolution. More specifically, we used the effects that ferroelectricity and magnetism exert on light polarization, by electro-optic and magneto-optic effects, respectively. The analysis was performed at room temperature in a Pt(10 nm)/BaTiO$_{\mathrm{3}}$ (120 nm)/La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ (15 nm) trilayer. We uncovered a stunningly large coupling by which the magnetization was modulated by up to above 50{\%}. The magnetoelectric coupling was, however, distributed non uniformly with micron-scale inhomogeneities. The origin of such a large effect is discussed in terms of electric-field modulation of competing electronic phases in La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$. Additionally, our work emphasizes the potential of intrinsically electronically inhomogeneous systems for large magnetoelectric responses. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B6.00007: Solving magnetoelectric coupling at La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ interfaces Jinling Zhou, Vu Thanh Tra, Robbyn Trappen, Disheng Chen, Matthew Marcus, Catherine Jenkins, Charles Frye, Evan Wolfe, Srinivas Polisetty, Ying-Hao Chu, Mikel Holcomb La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 heterostructures exhibit interfacial magnetoelectric (ME) coupling. ME coupling is the coupling between the magnetic order and the electric order in a system, the understanding of which would lead to innovative device designs. In our talk, we will discuss our x-ray absorption spectroscopy and x-ray magnetic circular dichroism experimental results and disclose how solving polar catastrophe at the interface of this system unravels the coupling mechanism. Potentially, our results may lead to design of ME materials capable of stronger coupling. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B6.00008: Magnetic coercive field changes due to electric field generated anisotropy in (La$_{1-y}$Pr$_{y}$)$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films Daniel Grant, Brian Schaefer, Amlan Biswas The hole-doped manganite (La$_{1-y}$Pr$_{y}$)$_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO) shows phase competition between ferromagnetic metallic and anti-ferromagnetic charge order insulator regions due to the interplay between magnetic, electronic, and structural interactions. Of particular interest is the possibility of utilizing the phase competition to modify the magnetic properties of LPCMO using an electric field. We will present electric field dependent magnetization data on LPCMO thin films grown on (110) NdGaO$_{3}$ substrates which show shifts in the coercive magnetic fields when an in-plane electric field is applied to the sample. The electric field effect is also influenced by the in-plane magnetic anisotropy of the thin films. The proposed dielectrophoresis model offers a qualitative scenario through which we can explain these observations. This model states that application of an electric field can cause an alignment of the ferromagnetic metallic regions to create an anisotropic low resistance path, which could affect the in-plane shape anisotropy of phase separated LPCMO thin films. We will also discuss our results on LPCMO microstructures. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B6.00009: Electric field control of spin transfer torque in multiferroic tunnel junctions Artur Useinov, Alan Kalitsov, Julian Velev, Nicholas Kioussis Based on model calculations we predict that the spin transfer torque (STT) in magnetic tunnel junctions with ferroelectric barriers can be strongly influenced by the saturated polarization of the barrier. The STT in such multiferroic tunnel junctions is calculated within the non-equilibrium Keldysh formalism generalized for non-collinear transport and implemented in the framework of a single-band tight-binding (TB) model. We calculate the bias dependence of both the in-plane (T$_{\mathrm{\parallel }})$ and out-of-plane (T$_{\mathrm{\bot }})$ components of STT as a function of the ferroelectric polarization (P) in the barrier. We find that the components of STT strongly depend on both the magnitude and the direction of the polarization. In particular switching of the polarization direction can dramatically alter the value of the STT and can even lead to a change of sign of T$_{\mathrm{\parallel }}$ and the voltage-induced part of T$_{\mathrm{\bot }}$. The effect is proportional to the magnitude of the polarization. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B6.00010: Magnetic control of electric polarization in (Cu,Ni)B$_2$O$_4$ Khanh Nguyen, Nobuyuki Abe, Mitsuru Akaki, Masashi Tokunaga, Kyota Kubo, Takahiko Sasaki, Taka-hisa Arima We report the generation and control of electric polarization using an external magnetic field in a noncentrosymmetric system nickel doped copper metaborate (Cu,Ni)B$_2$O$_4$ and discuss the origin of this effect. In this material, weak ferromagnetic moment can be rotated by applying an electric field. While this implies spin-driven ferroelectricity, the previous study via examining the structure parameters and dielectric constant showed no clear evidence for this effect, which is successfully observed in this study. Applying a magnetic field along the $[110]$ or $[1-10]$ axis induces electric polarization along the $[001]$ axis. The polarization is reversed by switching the magnetic field direction between the $[110]$ and $[1-10]$ axes. The result can be well explained in the framework of spin-dependent metal-ligand hybridization. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B6.00011: Origin of magnetoelectric response induced by respective magnetic ions $R^{3+}$/Fe$^{3+}$ in a chiral antiferromagnet $R$Fe$_{3}$(BO$_{3})_{4}$ Takashi Kurumaji, Kenya Ohgushi, Yoshinori Tokura Recent discoveries of the spin-induced ferroelectricity in frustrated magnets and the strong ME correlation in noncentrosymmetric magnets have stimulated the revived interest on the ME phenomena [1]. Rare earth iron borates $R$Fe$_{3}$(BO$_{3})_{4}$, whose structures possess a noncentrosymmetric space group ($R$32 or $P$3$_{1}$21), have recently been discovered to show multiferroicity [2]. While their magnetic and ME properties were extensively investigated, the origin of the $P$, or specifically the relationship between the electricity and the respective magnetism of iron ions (Fe) and rare-earth ions ($R)$, remains elusive. We measured the $P$ under a magnetic field and observed the linear ME effect and/or the spontaneous $P$ which are ascribed to spins of Fe and/or magnetic moments of $R$. We constructed a model for the spin-induced $P$ at the Fe/$R$ sites, with which we could reproduce the observed behavior of the magnetic field dependence of $P$. Thus, we could extract the respective contributions to $P$ from Fe and $R$ magnetic ions. \\[4pt] [1] T. Arima, J. Phys. Soc. Jpn. 80 (2011) 052001 \\[0pt] [2] A. M. Kadomtseva et al. Low Temp. Phys. 36 (2010) 511 [Preview Abstract] |
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