Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F53: Invited Session: Physics at Magnetic Interfaces, Both Engineered and Unexpected |
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Sponsoring Units: GMAG DCMP Chair: Barry Zink, University of Denver Room: Grand Ballroom C3 |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F53.00001: Kondo Physics at Interfaces in Metallic Non-Local Spin Transport Devices Invited Speaker: Chris Leighton Despite the maturity of metallic spintronics there remain large gaps in our understanding of spin transport in metals, particularly with injection of spins across ferromagnetic/non-magnetic (FM/NM) interfaces, and their subsequent diffusion and relaxation. Unresolved issues include the limits of applicability of Elliott-Yafet spin relaxation, quantification of the influence of defects, surfaces, and interfaces on spin relaxation at nanoscopic dimensions, and the importance of magnetic and spin-orbit scattering. The non-local spin-valve is an enabling device in this context as, in addition to offering potentially disruptive applications, it allows for the separation of charge and spin currents. One particularly perplexing issue in metallic non-local spin valves is the widely observed non-monotonicity in the $T$-dependent spin accumulation, where the spin signal actually \textit{decreases} at low $T$, in contrast to simple expectations. In this work, by studying an expanded range of FM/NM combinations (encompassing Ni$_{80}$Fe$_{20}$, Ni, Fe, Co, Cu, and Al), we demonstrate that this effect is not a property of a given FM or NM, but rather of the FM/NM \textit{pair}. The non-monotonicity is in fact strongly correlated with the ability of the FM to form a dilute local magnetic moment in the NM. We show that local moments, resulting in this case from the ppm-level tail of the FM/NM interdiffusion profile, suppress the injected spin polarization and diffusion length \textit{via} a novel manifestation of the Kondo effect, explaining all observations associated with the low $T$ downturn in spin accumulation [1]. We further show: (a) that this effect can be promoted by thermal annealing, at which point the conventional charge transport Kondo effect is simultaneously detected in the NM, and (b) that this suppression in spin accumulation can be quenched, even at interfaces that are highly susceptible to the effect, by insertion of a thin non-moment-supporting interlayer. Important implications for room temperature devices will be discussed. \\[4pt] Work supported by: Seagate Technology, NSF MRSEC (DMR-0819885), Marie Curie International Outgoing Fellowship, 7th European Community Framework Programme (No. 299376). Work at SNS, ORNL, supported by DOE. \\[4pt] [1] L. O'Brien, M. Erickson, D. Spivak, H. Ambaye, R. Goyette, V. Lauter, P. Crowell and C. Leighton, \textit{Nature Communications} \textbf{5}, 3927 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 9:12AM |
F53.00002: Spin-orbit torques in magnetic bilayers Invited Speaker: Paul Haney Spintronics aims to utilize the coupling between charge transport and magnetic dynamics to develop improved and novel memory and logic devices. Future progress in spintronics may be enabled by exploiting the spin-orbit coupling present at the interface between thin film ferromagnets and heavy metals. In these systems, applying an in-plane electrical current can induce magnetic dynamics in \textit{single domain} ferromagnets, or can induce rapid motion of domain wall magnetic textures. There are multiple effects responsible for these dynamics. They include spin-orbit torques and a chiral exchange interaction (the Dzyaloshinskii-Moriya interaction) in the ferromagnet. Both effects arise from the combination of ferromagnetism and spin-orbit coupling present at the interface. There is additionally a torque from the spin current flux impinging on the ferromagnet, arising from the spin hall effect in the heavy metal. Using a combination of approaches, from drift-diffusion to Boltzmann transport to first principles methods, we explore the relative contributions to the dynamics from these different effects. We additionally propose that the transverse spin current is locally enhanced over its bulk value in the vicinity of an interface which is oriented normal to the charge current direction. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:48AM |
F53.00003: Anomalous net magnetization in collinear antiferromagnets with uncompensated surfaces Invited Speaker: Frances Hellman Like ferromagnets (FM), antiferromagnets (AFM) exhibit spontaneous long-range spin order below a transition temperature. The traditional FM order parameter is the spontaneous magnetization, while that of a simple AFM is the staggered magnetization, sometimes called the Neel vector N. The net magnetization M of a perfect AFM is (seemingly) zero at all temperatures T; however, defects such as vacancies, grain boundaries, and even surfaces create an M(T) which has a non-trivial relationship to the staggered magnetization N(T), even in ideal systems. As a specific example, we consider AFM CoO, which consists of AFM-coupled FM (111) planes; (111)-oriented epitaxial films with an odd number of planes will exhibit non-zero M due to uncompensated surfaces. These uncompensated surfaces were used to produce an artificially-structured FM semiconductor using epitaxial layers of AFM CoO with a doped semiconductor Al:ZnO (AZO). Both M(T) and the anomalous Hall effect show oscillatory behavior with thickness of either CoO (odd vs even numbers of planes) or AZO ($\sim$1 nm RKKY-like oscillations related to the AZO Fermi wavevector due to electron-induced coupling between Co moments at its two CoO surfaces). Mean field theory and Monte Carlo simulations show that M(T) of collinear AFM such as CoO with uncompensated surfaces exhibits T-dependence unlike that of N(T), of the absolute value of its individual layers, or m(T) of any single atomic plane including the uncompensated surface, due to incomplete cancellations of different planes. This phenomenon is valid even in the limit of semi-infinite systems; it is a topological state due to the presence of a free surface. Modifications of surface exchange coupling (leading to ordinary or extraordinary transitions), due to electron correlations in these Mott insulators, changes in crystal fields, spin-orbit coupling, or an incomplete (rough) surface, result in compensation points and highly non-Brillouin-like M(T). [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:24AM |
F53.00004: Interface-induced magnetism and strong correlation in oxide heterostructures Invited Speaker: Susanne Stemmer Two-dimensional electron gases (2DEGs) at interfaces between two insulating oxides have attracted significant attention because they can exhibit unique properties, such as strong electron correlations, superconductivity and magnetism. In this presentation, we will discuss the emergent properties of 2DEGs in SrTiO$_{3}$ quantum wells that are interfaced with Mott insulating rare earth titanates (RTiO$_{3})$. We show that the magnetic properties of the 2DEG can be tuned to be either (incipient) ferromagnetic or (incipient) antiferromagnetic, depending on the specific RTiO$_{3}$ that interfaces it. The thickness of the quantum well is a critical tuning parameter and determines the onset of magnetism, the proximity to a quantum critical point, and the onset of non-Fermi liquid behavior for those quantum wells that are in proximity to an antiferromagnetic transition. We will also discuss the role of symmetry-lowering structural transitions in the quantum well. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 11:00AM |
F53.00005: Beller Lectureship: Magnetism at the edge; Anhysteretic, athermal magnetic response at oxide surfaces and interfaces Invited Speaker: Michael Coey Magnetism in oxides is normally associated with transition-metal ions having a partly-filled $d $(or $f)$ shell. The common hexagonal ferrite magnets BaFe$_{12}$O$_{19}$ and SrFe$_{12}$O$_{19}$ that are produced in quantities exceeding a million tons a year annum are a good example. In recent years, techniques honed to cater for the high-T$_{\mathrm{C}}$ superconductor boom have been applied to produce a range of oxide thin films and nanoparticles, which exhibit magnetic properties that are quite different to those of the bulk material. Oxide-oxide interfaces are full of surprises, especially when one of them is polar, inducing electronic reconstruction to avoid a polar catastrophe. The formation, location and magnetism of the resulting two-dimensional electron gas will be discussed. The data give lie to the common assumption of additivity of the magnetism of a thin film and its substrate. More puzzling is the elusive temperature-independent, anhysteretic magnetism of some thin films and nanoparticles, which cannot be accommodated in the current paradigm of magnetism in solids. Classical micromagnetic analysis would suggest that only a tiny fraction of the sample volume is involved ($\sim$ 10$^{-3}$ for thin films, $\sim$ 10$^{-6}$ for nanoparticles), yet there are no signs of collective magnetic order such as increasing coercivity at low temperatures or a Curie point at high temperature. The anhysteretic magnetisation curve is temperature-independent (unlike that of a superparamagnet). In CeO$_{2}$ nanoparticles there is a characteristic length scale of order 100 nm required for the appearance of the anomalous magnetic response. We propose an explanation in terms of giant orbital paramagnetism due to coupling with zero-point fluctuations of the vacuum electromagnetic field, which predicts a magnetization curve of the form $M = M_{0}$x/(1$+$x$^{2})^{1/2}$, where x $=$ \textit{CB}; the constant $C$ corresponds to the characteristic wavelength and lengthscale, 330 nm in this case. [Preview Abstract] |
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