Session V13: Focus Session: Magnetic Nanostructures-Domain Walls

Domain-wall dynamics in ferromagnetic nanowires

Current-induced domain-wall (DW) dynamics is studied in a thin ferromagnetic nanowire. We derive effective equations of motion describing the dynamics of the DW soft modes with or without topological defects. Because the DWs are topological objects with a rigid spin structure, these equations are rather universal. The DW rigidity makes the microscopic details irrelevant, and it allows us to solve the DW dynamics for a very general class of spin Hamiltonians. We show that the DW dynamics is described by simple equations with only four parameters. Based on these equations, we study DW dynamics in a ferromagnetic wire with Dzyaloshinskii-Moriya interaction (DMI). We find spin spiral DW structure and how the critical current required to move the domain wall depends on DMI. We also investigate the DW dynamics driven by time-dependent currents. We find the most efficient (with the lowest Ohmic losses) way to move the DWs by resonant current pulses. In addition, we propose a procedure to unambiguously determine the DW dynamics parameters by all-electric measurements of the time-dependent voltage induced by moving DW. Furthermore, based on the derived DW dynamics equations for the translationally non-invariant nanowires, we show how to make potential magnetic memory nanodevices much more energy efficient.   

Reduction in critical current for domain wall injection by ion irradiation of perpendicular magnetic anisotropy nanowires

One of the key problems for realization of domain wall motion devices is the reliable and energy efficient injection of domain walls (DWs) into magnetic nanowires. In this work, we explore the injection of domain walls in perpendicular magnetic anisotropy nanowires (Co/Ni multilayers) which are locally softened by ion irradiation. We observe a minimum in the domain wall injection critical current, which occurs where the anisotropy of the irradiated region transitions from out of plane to in plane anisotropy. Furthermore, we find that the irradiated site acts as a pinning site for the DWs. At the irradiation site, we are able to create localized nanosecond long pulsed magnetic fields used to inject the DWs. By performing DC resistance measurements after each injection event, we are able to probe for the existence of the domain wall, and also find the strength of the ion irradiated pinning site. Using the above technique, we have demonstrated a five fold reduction in the domain wall injection current.   

360 Degree DW formation during vortex to vortex switching in thin ferromagnetic nanorings in an applied circular field

We present simulations of the switching process between clockwise and counterclockwise vortex states in ferromagnetic nanorings in an applied circular field, relevant to potential data storage devices. This circular field can be experimentally generated by passing current through the solid metal tip of an atomic force microscope, which has achieved vortex-to-vortex switching in thicker asymmetric rings [1]. We find that in sufficiently thin rings, the vortex switching process occurs through the nucleation and annihilation of pairs of 360 degree domain walls (DW), with opposite topological indices. The DW with the same circulation as the vortex annihilates first. We can control which DW annihilates first by offsetting the center of our circular field to target a specific DW. Both exchange energy and demagnetization energy must be considered in predicting the energy barrier to DW annihilation. [1] T. Yang, N.R. Pradhan, A. Goldman, A.S. Licht, Y.Li, M. Kemei, M.T. Tuominen, K.E. Aidala. APL, 98, 242505 (2011).   

Individual domain wall manipulation in a local oersted circular field

Understanding domain wall (DW) motion in nanoscale ferromagnetic structures reveals intriguing physics, with potential applications in nanoscale devices and DW data storage. One challenge is to create and move individual DWs in arbitrary locations. We developed a technique to generate localized circular magnetic field by applying a current through the tip of the atomic force microscope (AFM) and thereby manipulating the state of ferromagnetic rings [1]. Now we extend our ability to control domain walls in various structures, such as straight wires with notches and zigzag wires. By placing the tip near a 180 DW in a vertex of a zigzag wire, we can move the 180 DW along the wire and form a stable 360 DW in nearby vertex. We can also move 360 DWs with the local magnetic field around the AFM tip. We will discuss simulations and experimental implementations. \\[4pt] [1] T. Yang, N. R Pradhan, A Goldman, A. Licht, Y. Li, M T. Tuominen and K. E. Aidala, Applied Physics Letter, http://apl.aip.org/resource/1/applab/v98/i24/p242505\_s1 98, 242505, (2011)   

Probing and manipulating magnetization at the nanoscale

Combining semiconductors with magnetism in hetero- and nano-structured geometries provides a powerful means of exploring the interplay between spin-dependent transport and nanoscale magnetism. We describe two recent studies in this context. First, we use spin-dependent transport in ferromagnetic semiconductor thin films to provide a new window into nanoscale magnetism [1]: here, we exploit the large anomalous Hall effect in a ferromagnetic semiconductor as a nanoscale probe of the reversible elastic behavior of magnetic domain walls and gain insight into regimes of domain wall behavior inaccessible to more conventional optical techniques. Next, we describe novel ways to create self-assembled hybrid semiconductor/ferromagnet core-shell nanowires [2] and show how magnetoresistance measurements in single nanowires, coupled with micromagnetic simulations, can provide detailed insights into the magnetization reversal process in nanoscale ferromagnets [3]. The work described here was carried out in collaboration with Andrew Balk, Jing Liang, Nicholas Dellas, Mark Nowakowski, David Rench, Mark Wilson, Roman Engel-Herbert, Suzanne Mohney, Peter Schiffer and David Awschalom. This work is supported by ONR, NSF and the NSF-MRSEC program.\\[4pt] [1] A. L. Balk et al., Phys. Rev.Lett. {\bf 107}, 077205 (2011).\\[0pt] [2] N. J. Dellas et al., Appl. Phys. Lett. {\bf 97}, 072505 (2010).\\[0pt] [3] J. Liang {\it et al}., in preparation.   

Beyond a compact magnetic domain wall

The generally accepted concept that limits magnetic domain wall velocity is the Walker breakdown. This is the magnetic field at which wall motion becomes oscillatory capping the performance of domain wall-based spintronic devices. To understand the limiting mechanisms, we study vortex walls in Ni$_{80}$Fe$_{20}$ wires with widths between 300 and 900 nm. We detect the walls by time-resolved magneto-optical Kerr effect in a pump-probe technique setup. The wires are fabricated by electron-beam lithography and by a nanostencil tool [1]. We find the dynamics of vortex walls to depart significantly from the current description of a compact entity evolving along the wire. Instead, the wall is composed of several substructures propagating in different dynamic regimes with very different velocities. Wire edges crucially affect this dynamics and can be influenced by variation of growth parameters. Extensive, parallelized micromagnetic simulations reveal the unusual wall structure and complement the experimental findings [2]. Possibilities how to overcome the limits imposed by the Walker breakdown will be discussed.$\\$ [1] L. Gross $\emph{et al., Nanotechnology}$ $\bf{21}$, 325301 (2010)$\\$ [2] C. Zinoni $\emph{et al., Phys. Rev. Lett.}$ $\bf{107}$, 207204 (2011)   

Domain walls in ultrathin magnetic nanowires at finite temperatures

The possibility of inducing domain-wall (DW) motion in magnetic nanowires by means of electric currents has recently renewed theoretical interest in this field. The problem is usually modelled on a micromagnetic approach, but ignoring thermal fluctuations. However, some relevant experimental facts - like the correct order of magnitude of the critical current needed for DW motion - still lack satisfactory explanations. We thus developed a one-dimensional stochastic model for DW dynamics, which allowed us to take into account both thermal fluctuations and external drifts (magnetic field and electric spin-polarized current) simultaneously. We also provided a general theoretical framework, which highlights the crucial role played by thermal fluctuations at the centre of DWs. The latter, for example, qualitatively accounts for the shrinking of magnetic domains observed in Fe films on Cu(001) with increasing temperature and the renormalisation of the critical current for DW motion in magnetic nanowires at finite temperatures.   

Antiferromagnetic Domain Wall Engineering in Chromium Films

We have engineered an antiferromagnetic domain wall by utilizing a magnetic frustration effect of a thin iron cap layer deposited on a chromium film. Through lithography and wet etching we selectively removed areas of the Fe cap layer to form a patterned ferromagnetic mask over the Cr film. Removing the Fe locally removes magnetic frustration in user-defined regions of the Cr film. We present x-ray microdiffraction results confirming the formation of an antiferromagnetic spin-density wave propagation domain wall in Cr. This domain wall nucleates at the boundary defined by our Fe mask. We have characterized the region surrounding the domain wall using x-ray microdiffraction and microfluorescence with a resolution of 1 micron.   

Formation and structure of 360 and 540 degree domain walls in thin magnetic stripes

A method is presented for forming a 360$^{\circ}$ domain wall (DW) and more complex structures such as a 540$^{\circ}$ DW in a wire attached to an injection pad by applying an alternating in-plane field perpendicular to the wire. SEMPA, MFM measurements and OOMMF micromagnetic simulations give a consistent picture of the magnetic structure and stray field distribution of the 360$^{\circ}$ DW. Equilibrium 360$^{\circ}$ DWs in wires have a well-defined structure and size, persist over a wide field range, and can be distinguished from configurations consisting of two 180$^{\circ}$ DWs pinned near each other. The formation and stability of these complex walls has implications in memory and logic devices based on field- or current-induced DW motion, where impingement of adjacent 180$^{\circ}$ DWs can produce composite DWs whose behavior and stray field distribution differ significantly from that of a 180$^{\circ} $DW, and these structures could also be used to examine intriguing resonant behavior as predicted by modeling. [Phys. Rev. B 82, 214411; Phys. Rev. B 82, 134411]   

Polarization Dependent Switching of Asymmetric Nanorings with a Circular field

We present experimental switching from the onion to vortex states in asymmetric cobalt nanorings in an applied circular field. We initialize the onion state in two polarizations, along the symmetric or asymmetric axes. We apply a circular field by passing current through a solid metal AFM tip positioned at the center of the ring [1]. The asymmetry of the ring leads to different switching fields depending on the location of the domain walls (DWs) and direction of applied field. For polarization along the asymmetric axis, the field required to move the DWs to the narrow side of the ring is smaller than moving the DWs to the larger side of the ring. The direction of the DW motion is controlled by the circular field. When polarizing the ring along the symmetric axis, establishing one DW in the narrow side and one on the wide side, the field required to switch to the vortex state is an intermediate value. We will be presenting detail of the switching field of cobalt nanoring by circular field with two different direction of polarization. \\ (1) T. Yang, N. R. Pradhan, A. Goldman, A. Licht, Y. Li, M. T. Tuominen and K. E. Aidala, \textit{Applied Physics Letter}, 98, 242505, (2011)   

Non-volatile electric field tuning of magnetic domains in permalloy thin films coupled to ferroelastic PZT bilayers

We are investigating electric field controlled magnetic domain motion in Py films on Pb(ZrxTi(1-x))O3 (PZT) bilayers. Previously, we have shown that bilayered heterostructures consisting of a tetragonal PbZr0.3Ti0.7O3 film (70 nm) above a rhombohedral PbZr0.7Ti0.3O3 film (70 nm) display large ferroelastic domains in tetragonal PZT layer. The reversible non-volatile ferroelastic domain wall motion in this layer can serve as a basis for inducing controlled strain on magnetic thin films. This results in different ferroelastic domain configurations in the tetragonal PZT layer. This in turn leads to changes in magnetic domains of Py film. We find that a Py film on the ferroelastic PZT layer exhibits sharp magnetic domain patterns usually associated with out-of-plane magnetization by MFM. SEMPA imaging reveals that the magnetic domains are indeed in-plane magnetized as expected for Pyfilms. OOMMF analysis indicates presence of unusual metastable in-plane anisotropy modulation in the Py film.