Session E5: Magnetic Thin Films and Multilayers

Spin-orbit coupling of 3d transition metal atoms on MgO/Ag

Spin-orbit coupling is normally predominant for high Z metal atoms, but we observe Fe and Co showing significant orbital moments on a MgO/Ag surface. DFT results show that on MgO/Ag both Co and Fe prefer O top binding sites. Calculation of orbital moments using DFT is always challenging, and we compare two DFT based protocols to calculate orbital moments. Our calculations show the magnitude of orbital moments strongly dependent on a number of factors including the number of Ag layers in our unit cell and the approximation we are using. Our results exhibit significant agreement with scanning tunneling microscopy (STM) and XMCD experiments. We show that Co retains its full atomic orbital moment on the O top site of MgO whereas the orbital moment for Fe is somewhat less than its atomic orbital moment.   

Enhancement of the Co magnetic moment in bcc Co$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ on MgO

Using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (MCD), we show that the elemental Co moment for MBE grown thin films of bcc Co$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ grown on MgO(001) is enhanced by 40{\%} to a maximum value of 2.1 $\mu_{\mathrm{B}}$ at x$=$0.24. The net Mn moment is found to align parallel with Co for all concentrations and remains roughly constant until x$=$0.3, then drops steadily, up to x$=$0.7, where the total moment of the film abruptly collapses to zero. Using a low-concentration Mn moment of 3.0 $\mu _{\mathrm{B}}$, the average magnetization lies directly on the Slater-Pauling (SP) curve for concentrations up to about x$=$.25, where it reaches a maximum moment of 2.3 $\mu_{\mathrm{B}}$ /atom. This peak is slightly shifted and the slope is steeper on the high-Mn concentration side of the peak relative to the standard SP curve. This is in stark contrast to the fcc CoMn and hcp CoCr bulk behavior which shows only a rapid total moment reduction with Mn concentration.   

Cap-Induced Magnetic Anisotropy in Ultra-thin Fe/MgO(001) Films

Magnetic anisotropy plays an important role in the design of spintronic devices. Perpendicular magnetic anisotropy (PMA) is preferred for magnetic tunnel junctions because the resulting energy barrier between magnetization states can be very high and this allows enhanced device scalability suitable for magnetic random access memory applications. Interface induced anisotropy is often used to control magnetic easy axes. For example, the Fe/MgO(001) system has been predicted to exhibit PMA in the ultrathin Fe limit. We have used \textit{in-situ} magneto optic Kerr effect and \textit{ex-situ} SQUID to study the changes in anisotropy constants between bare Fe/MgO(001) films and those capped with MgO, Pt, and Ta. In some cases in-plane anisotropy terms reverse sign after capping. We also observe transitions from superparamagnetic to ferromagnetic behavior induced by capping layers. Perpendicular anisotropy is observed for Pt/Fe/MgO(001) films after annealing to 300\textdegree C. These effects are characterized and incorporated into a magnetic simulation that accurately reproduces the behavior of the films.   

Spin and orbital magnetic moments of Fe and Co in Co/Fe and Fe/Co multilayers on Si from L$_{\mathrm{2,3}}$ edge X-ray Magnetic Circular Dichroism Spectroscopy

Nanostructured FeCo thin films are interesting for magnetic recording applications due to their high saturation magnetization, high Curie temperature and low magnetocrystalline anisotropy. It is desirable to know how the magnetism is modified by the nanostructrure. We report Fe L$_{\mathrm{\thinspace 2,3}}$ edge and Co L$_{\mathrm{2,3\thinspace }}$edge x-ray magnetic circular dichroism (XMCD) investigations of element specific spin and orbital magnetism of Fe and Co in two multilayer samples: (S1) Si/SiO2/[Co 0.8 nm/Fe 1.6 nm]x32/W (2nm) and (S2) Si/SiO$_{\mathrm{2}}$/[Co 1.6 nm/Fe 0.8 nm]x32/W (2nm) thin films at room temperature. Sum rule analysis of XMCD at Fe L$_{\mathrm{2,3\thinspace }}$edge in sample S1 shows that the orbital moment of Fe is strongly enhanced and the spin moment is strongly reduced as compared to the values found in bulk Fe. Details of sum rule analysis will be presented to compare and contrast spin magnetic moments and orbital magnetic moments of Fe and Co in the two multilayer samples.   

Direct Mapping of Magnetic and Structural Profiles of Electric Field Moderated Oxygen Migration

Recent studies on metal/oxide heterostructures have demonstrated control of interfacial magnetic anisotropy and saturation magnetization in ultrathin (5 ML) Co films through electric-field controlled oxygen migration. This approach presents a promising route to realizing next-generation, ultralow power sensor and data-storage technologies. Here we demonstrate magnetoelectric coupling moderated by electrically-driven oxygen migration in much thicker AlOx(1 um)/GdOx(2 nm)/Co (15 nm) heterostructures. Using polarized neutron reflectometry, we present direct, quantitative depth profiling of the magnetization and oxygen concentration in these systems. Electro-thermal conditioning moves oxygen from AlOx and GdOx base-layers throughout the entire thickness of the 15 nm Co layer, resulting in a suppressed magnetization. Switching the electric field polarity semi-reversibly ejects oxygen preferentially from the GdOX/Co interface, partially recovering the magnetization and establishing a practical limit to this approach. First order reversal curve diagrams show that the conditioned samples exhibit two distinct magnetic phases, while the as-grown samples are single phase, suggesting that the treatments alters the Co film microstructure. X-ray spectroscopy confirms the oxidation states of the Co and Gd, and suggest that the GdOX acts to transmit oxygen but does not source or sink it.   

Magnetic localization limit in $T_{C}$ graded ferromagnetic thin films

We have recently demonstrated that the effective Curie temperature ($T_{C}$) of a ferromagnetic alloy thin film can be continuously varied as a function of depth via a corresponding compositional gradient.[1] This work showed that the effective $T_{C}$ can be made to vary continuously over tens of nm. However, over a short enough distance, the system must become localized, with exchange coupling dominating the effects of the compositional gradient. Understanding this localization limit is important for potential applications, as it dictates the length-scale below which this technique stops being a viable engineering tool (at least for itinerant ferromagnets and their thermodynamic properties). To determine the localization limit in this class of system, we have fabricated a series of Co[1-$x$]Cr[$x$] alloy alloy films, where $x$ varies sinusoidally between 0.28 (nominal $T_{C}$ $\approx$ 250 K) and 0.22 ($T_{C}$ $>$ 300 K), and have used polarized neutron reflectometry to study samples of differing oscillation wavelength. These measurements confirm the desired sinusoidal pattern was achieved, and reveal the temperature-dependence of the magnetic depth profile. Results will be presented in the context of mean-field simulations. [1] arXiv:1510.07535 [cond-mat.mtrl-sci].   

Magnetic Irreversibility in VO$_{2}$/Ni Bilayers.

The temperature dependence of the coercivity and magnetization of VO$_{2}$/Ni bilayers was studied. VO$_{2}$ exhibits a well-known Structural Phase Transition (SPT) at 330-340 K, from a low temperature monoclinic (M) to a high temperature rutile (R) structure. The SPT of VO$_{2}$ induces an inverse magnetoelastic effect that strongly modifies the coercivity and magnetization of the Ni films. In addition, the growth conditions allow tuning of the magnetic properties. Ni films deposited on top of VO$_{2}$ (M) show an irreversible change in the coercivity after the first cycle through the high temperature phase, with a corresponding change in the surface morphology of VO$_{2}$. On the other hand, the Ni films grown on top of VO$_{2}$ (R) do not show this irreversibility. These results indicate that properties of magnetic films are strongly affected by the strain induced by materials that undergo SPT and that it is possible to control the magnetic properties by tuning the growth conditions.   

Layer Resolved Imaging of Magnetic Domain Motion in Epitaxial Heterostructures

We use X-ray Excited Luminescence Microscopy (XELM) to image the elemental and layer resolved magnetic domain structure of an epitaxial Fe/Cr wedge/Co heterostructure in the presence of large magnetic fields.~ The observed magnetic domains exhibit several unique behaviors that depend on the Cr thickness (tCr) modulated interlayer exchange coupling (IEC) strength. For Cr thickness tCr?\textless ?0.34?nm and tCr?\textgreater ?1.5?nm, strongly coupled parallel Co-Fe reversal and weakly coupled layer independent reversal are observed, respectively.~ The transition between these two reversal mechanisms for 0.34?\textless ?tCr?\textless ?1.5?nm is described by a combination of IEC guided domain wall motion and stationary zig zag domain walls. We observe domain walls nucleated at switching field minima are guided by IEC spatial gradients and collapse at switching field maxima.   

Magnetic profile of a graphene wrapped ferromagnetic surface

Graphene has one of the highest electron mobilities at room temperature, making it ideal for next generation electronic devices. However, due to its small spin-orbit coupling it is not possible to manipulate spins directly in a pristine graphene monolayer. This may be overcome by proximity to a ferromagnet. Recent theoretical and experimental publications [1] indicate that on a Ni surface the graphene band structure is spin split. The authors used XMCD to measure the magnetic moment induced on the $\pi$- electrons in graphene due to the proximity effect with Ni, obtaining a value between 0.05-0.10 $\mu_B$ per C atom. We have produced a uniform graphene layer grown by a CVD process directly a Ni coated substrate (the catalyst). By varying key growth parameters (temperature & pressure) the interaction between graphene and the catalyst can be tuned to provide strong epitaxial alignment between graphene and Ni or a more weakly oriented rotated alignment. We will present results showing a magnetic enhancement at the ferromagnet - C interface extracted from recent polarized neutron reflectivity measurement on both epitaxial and rotated graphene wrapped ferromagnetic surfaces. [1] V. Karpan, et al., Phys. Rev. B 78, 195419 (2008), M. Weser,et al., Appl. Phys. Lett. 96, 012504 (2010)   

Self Exchange Bias and Bi-stable Magneto-Resistance States in Amorphous TbFeCo and TbSmFeCo Thin Films.

Amorphous ferrimagetic TbFeCo and TbSmFeCo thin films are found to exhibit strong perpendicular magnetic anisotropy. Self exchange bias effect and bi-stable magneto-resistance states are observed near compensation temperature by magnetic hysteresis loop, anomalous Hall effect and transverse magneto-resistance measurements. Atom probe tomography, scanning transmission electron microscopy, and energy dispersive spectroscopy mapping have revealed two nanoscale amorphous phases with different Tb concentration distributed within the amorphous films. The observed exchange anisotropy originates from the exchange interaction between the two nanoscale amorphous phases. Exchange bias effect is used for increasing stability in spin valves and magnetic tunneling junctions. This study opens up a new platform for using amorphous ferrimagnetic thin films that require no epitaxial growth in nanodevices..   

Angular dependence of exchange bias and magnetization reversal controlled by electric-field-induced competing anisotropies.

Combination of exchange-biased systems and FE materials$^{\mathrm{\thinspace }}$gives a new avenue to study angular dependence of exchange bias and achieve reversible electric-field-controlled magnetization reversal. We study the angular dependence of electric-field-controlled exchange bias and magnetization reversal in CoFeB/IrMn/Pb(Mg$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})_{\mathrm{0.7}}$Ti$_{\mathrm{0.3}}$O$_{\mathrm{3}}$. It is demonstrated that the ratio of the exchange-coupled unidirectional anisotropy and the uniaxial anisotropy of the FM layer, as well as their relative orientation can be dramatically and continuously tuned via electric fields. Simulations confirm that the electric-field-controlled exchange bias originates from the competition between the uniaxial anisotropy induced by the piezostrain and the exchange-coupled unidirectional anisotropy. Moreover, electric-field-controlled magnetization reversal was realized at zero magnetic field.   

Rationale for contrasting phonon confinement and interface localization effect in FeAg and FeCr multilayers

We have performed density functional theory based calculations to investigate the propensity for formation of FeAg and FeCr multilayers. A perfect lattice match between Fe and Ag layers at the FeAg interface was obtained by modeling 45\textdegree rotated Ag(100) layers epitaxially on bcc Fe(100). In comparison, the FeCr interface was modeled by epitaxial layers of bcc Fe(100) and Cr(100). In FeAg, we find the signature peak of Fe bulk phonons (35 meV) to be substantially diminished and the low energy peaks to be remarkably enhanced, in qualitative agreement with experiment [1]. In contrast, the phonon density of state in the FeCr multilayers do not show any outstanding feature except a slight decrease in the 35 meV peak for the Fe layer at the interface, as compared to that of the middle Fe layer in excellent agreement with experiment [2]. The magnetic moment of the interfacial Fe atoms is larger than those Fe atoms in other layers, as a result of charge transfer from Fe to Ag at the interface. As compared to the middle layers, more spin-up and less spin-down states are occupied at the interface in such a way that Fe donates a large number of spin-down electrons to Ag but receives only a few spin-up electrons from the latter because of the almost fully occupied Ag d-band. [1] B. Roldan Cuenya et al., to be published. [2] Roldan et al, Phys. Rev. B 77, 165410 (2008).   

Magneto-optical mapping of the domain wall pinning potential in ferromagnetic films

The propagation of domain walls in ferromagnetic films is influenced by defects which suppress and pin the motion of the domain walls. We map the nanoscale effective pinning potential in a ferromagnetic film by raster scanning a single ferromagnetic vortex domain and measuring the hysteretic displacement vs. applied magnetic field.[1] We use a differential magneto-optical microscopy technique which yields spatial sensitivity of $\sim$ 10 nm to measure the motion of the vortex domain.[2] Using a simple algorithm, we extract the effective pinning potential from the measured vortex displacement vs. applied field. The resulting effective pinning potential maps reveal different types of nanoscale pinning features which we attribute to different structural defects of the film. By comparing the pinning map to atomic force microscopy maps, we identify correlations be between pinning sites and topographic features. [1] R. Badea, and J. Berezovsky, cond-mat/1510.07059, (2015). [2] R. Badea, J. A. Frey, and J. Berezovsky, Journal of Magnetism and Magnetic Materials 381, 463 (2015).   

Kinetic Monte Carlo simulations of thermally activated magnetization reversal in dual-layer Exchange Coupled Composite recording media.

The kinetic Monte Carlo (KMC) method developed for thermally activated magnetic reversal processes in single-layer recording media [1] has been extended to study dual-layer Exchange Coupled Composition (ECC) media used in current and next generations of disc drives [2]. The attempt frequency is derived from the Langer formalism with the saddle point determined using a variant of Bellman Ford algorithm. Complication (such as stagnation) arising from coupled grains having metastable states are addressed. MH-hysteresis loops are calculated over a wide range of anisotropy ratios, sweep rates and inter-layer coupling parameter. Results are compared with standard micromagnetics at fast sweep rates and experimental results at slow sweep rates. \\ 1. T.J. Fal, J.I. Mercer, M.D. Leblanc, J.P. Whitehead, M.L. Plumer, and J. van Ek, Phys. Rev. B. 87, 064405 (2013).\\ 2. Ahmad M. Almudallal, J. I. Mercer, J. P. Whitehead, M. L. Plumer, J. van Ek, and T. J. Fal, Phys. Rev. B 92, 134418 (2015).