Session J36: Focus Session: Nanomagnetism -- Domain Dynamics

Near-field interaction between domain walls in adjacent permalloy nanowires

A domain wall (DW) moving in a ferromagnetic nanowire may interact with the stray field from another DW travelling in an adjacent wire. This could greatly impact the operation of proposed DW based data storage schemes which rely on the controlled propagation of DWs in densely packed nanowires [1, 2]. Here we experimentally study the interaction between two DWs travelling in adjacent Permalloy nanowires [3]. We find that the interaction causes significant pinning, with measured pinning fields of up to 93 Oe ($\sim $5 times the intrinsic pinning field of an isolated wire) for the smallest separations. We present an analysis of the observed pinning field dependence on wire separation in terms of the full magnetostatic charge distribution within a DW. By considering an isolated DW, and accounting for finite temperature, it is possible to fully reproduce the experimentally observed dependence. This suggests that the DW internal structure is not appreciably perturbed by the interaction and so remains rigid, consistent with a finite sized quasi-particle description [4]. The full charge distribution must be considered in understanding these near-field interactions as other models based on simpler descriptions of the charge distribution within the DW, including a point-like distribution, cannot reproduce the observed dependence. Finally, we develop the idea of using localized stray fields to pin a DW and show how specific potential landscapes can be created by tailoring a pinning charge distribution, with the added advantage that neither DW internal structure nor nanowire geometry is appreciably perturbed. \\[4pt] [1] Allwood, Cowburn et al. Science \textbf{309}, 1688 (2005) \\[0pt] [2] S. S. Parkin, Science \textbf{320}, 190 (2008) \\[0pt] [3] O'Brien, Cowburn et al. Phys. Rev. Lett. \textbf{103}, 7, 077206 (2009) \\[0pt] [4] Saitoh, Miyajima et al. Nature \textbf{432}, 203 (2004)   

Remotely Tunable Mobile Magnetic Traps from Domain Walls in Patterned Magnetic Wires

Remote manipulation of fluid-borne magnetic particles on a surface is useful to probe, assemble and sort micro- and nano-scale objects. In this study, magnetic domain walls in a-few-micron-wide magnetic wires patterned on a surface are shown to act as effective traps for such objects. The required characteristics of the magnetic domain walls are designed with the aid of micromagnetic simulations and the magnetization profiles in the wires observed by Kerr microscopy. Using small ($\sim $50 Oe) in-plane and out-of-plane external magnetic fields, the trapping energy profile of the surface can be altered without significantly changing the magnetization of the wire, allowing for the trap to become mobile. Calculations reveal field gradients of greater than 10$^{4}$ T/m at a distance of 500 nm from the domain walls and tunable forces of 10-100's of picoNewtons applied to magnetic micro-particles in fluid environments. Several examples of micro- and nano-particle manipulation will be presented.   

Bi-directional magnetic domain wall shift register

Data storage devices based on magnetic domain walls (DWs) propagating through ferromagnetic nanowires have attracted a great deal of attention in recent years [1,2]. Here we experimentally demonstrate a shift register based on an open-ended chain of ferromagnetic NOT gates. When used in combination with a globally applied magnetic field such devices can support bi-directional data flow [3]. We have demonstrated data writing, propagation, and readout in individually addressable NiFe nanowires 90 nm wide and 10 nm thick. Up to eight data bits are electrically input to the device, stored for extended periods without power supplied to the device, and then output using either a first in first out or a last in first out mode of operation. Compared to traditional electronic transistor-based circuits, the inherent bi-directionality afforded by these DW logic gates offers a range of devices that are reversible and not limited to only one mode of operation. [1] S. S. Parkin, US Patent 6,834,005 (2004) [2] D. A. Allwod, et al., Science 309 (5741), 1688 (2005) [3] L. O'Brien, et al. accepted for publication in APL (2009)   

Dynamics of domain walls in thin films with out-of-plane magnetization

A thin magnetic film with a strong easy-axis anisotropy favoring the out-of-plane direction breaks up into mesoscopic magnetic domains separated by Bloch domain walls. Depending on magnetic history, these domains can form ordered stripes or disordered labyrinthine patterns. The physics of these domain walls is strongly influenced by dipolar interactions that mediate a long-range interaction between domain walls and make the wall tension negative [1]. Here we point out that the dominance of the gyrotropic force over the viscous one makes the dynamics of Bloch walls rather unusual. Low-frequency waves on such a wall are chiral: the speed of propagation is different for the two directions along the wall. The puzzling star-shaped trajectory of a magnetic bubble noted in [2] is a result of superposition of two waves with the same wavenumber and different frequencies running in opposite directions along the wall that surrounds the bubble. We point out a similarity to the edges of a quantum Hall state. [1] S. A. Langer, R. E. Goldstein, and D. P. Jackson, Phys. Rev. A \textbf{46,} 4894 (1992). [2] C. Moutafis, S. Komineas, and J. A. C. Bland, Phys. Rev. B \textbf{79,} 224429 (2009).   

360 $^{\circ}$ domain wall formation in ferromagnetic nanorings in an applied azimuthal field

Ferromagnetic nanorings form unique magnetic states that hold tremendous promise for maximizing storage densities. One such state is the vortex state, in which the magnetic field is completely enclosed within the ring, though it is challenging to control the chirality. We study a straightforward method to control the clockwise or counterclockwise chirality using an azimuthal field, as if from a current carrying wire passing through the center of the ring. Our simulations predict the formation of 360 $^{\circ}$ domain walls during switching of 5 nm thick rings in a variety of geometries. The number and location of the domain walls depends on the ring geometry. We explore the reason 360 $^{\circ}$ domain walls form for different widths, asymmetries, and sizes. Experimental implementation is underway to confirm computational predictions. The micromagnetic simulations are performed using OOMMF, Object Oriented Micro Magnetic Framework, a public domain program distributed by NIST to study the evolution of magnetic states with the application of a circular magnetic field.   

Temperature dependent nucleation and annihilation of individual magnetic vortices in sub-micron permalloy disks

Using micro-Hall and band-resistance magnetometry we studied the temperature dependence of the magnetization reversal in individual permalloy disks with diameters of 526-865 nm and thickness of 50 nm. We identified interesting thermal effects in the smallest and largest disks. The nucleation field exhibits a non-monotonic dependence with positive and negative slopes at low and high temperatures, respectively, while the annihilation field monotonically decreases with increasing temperature, but with distinctly different slopes at low and high temperatures. Our analysis suggests that at low temperatures vortex nucleation and annihilation proceeds via thermal activation over an energy barrier, while at high temperatures they are governed by the temperature dependence of the saturation magnetization due to thermally populated spinwaves. For intermediate-size disks we observed complex reversal behavior. Despite this, probing individual disks is much more revealing of the underlying physics than is probing arrays of disks.   

Thermally Activated Decay of Magnetic Vortices

We experimentally probe thermally activated decay of magnetic vortices, by observing annihilations within an array of Ni$_{80} $Fe$_{20}$ discs through hysteresis measurements. Specifically, the statistics of vortex annihilation are mapped as a function of the magnitude of, and the dwell time at, the peak fields applied during hysteresis scans. Magnetic vortices in micro- and nano-scale thin film ferromagnetic elements exhibit interesting and complex behavior. Demagnetization interactions make understanding processes like the annihilation of a vortex during magnetic switching challenging. Recent work has shown that the annihilation process can take place over an extended period of time\footnote{Z. Liu, R.D. Sydora and M.R. Freeman, PRB 77, 174410 (2008).} implying that there is a characteristic decay process, likely thermally governed. Through application of an Arrhenius model we extract information about the energy barrier preventing decay, and hence information about the energetic contributions of the demagnetization effects. We anticipate that this information will be useful in extending analytical models of magnetic vortices.   

Domain wall propagation due to the synchronization with circularly polarized microwaves

Finding a new control parameter for magnetic domain wall (DW) motion in magnetic nanostructures is important in general and in particular for the spintronics applications. Here, we show that a circularly polarized magnetic field (CPMF) at GHz frequency (microwave) can efficiently drive a DW to propagate along a magnetic nanowire. Two motion modes are identified: rigid-DW propagation at low frequency and oscillatory propagation at high frequency. Moreover, DW motion under a CPMF is equivalent to the DW motion under a uniform spin current in the current perpendicular to the plane magnetic configuration proposed recently by Khvalkovskiy et al. [Phys. Rev. Lett. 102, 067206 (2009)], and the CPMF frequency plays the role of the current.   

Effects of disorder on vortex gyration

The dynamics of magnetic domain wall structures driven by fields or currents is a subject of practical importance related to possible schemes for nanoscale magnetic memory devices. Experimental results are typically interpreted in comparison to ideal models that ignore the effects of extrinsic disorder and internal dynamics of domain wall structures. To understand the effect of disorder on the dynamics of vortex domain walls, we study the dynamics of vortex gyration driven by an external magnetic field pulse in the presence of extrinsic random potential. We analyze micromagnetic simulations using ideal models and observe that effective damping increases as the gyration frequency increases. We discuss the origin of the enhanced effective damping.   

Large reduction of the depinning field for a transverse domain wall under application of rf and dc currents

A new generation of proposed spintronic devices are based on domain wall (DW) motion (DW-MRAM, DW logic, racetrack memory...). However, reliable depinning of domain walls remains elusive, especially in zero field. Here, we have studied the combined effect of rf and dc currents on the depinning of transverse walls in the soft NiFe layer of a 100 nm wide Co/Cu/NiFe spin valve wire. Using the GMR effect, we ensure that the domain wall is always prepared at the same intrinsic defect and then measure the depinning field for different applied dc and rf currents. Notably, for a narrow range of rf frequencies at around 3GHz, we evidence a strong reduction in the depinning field (from $\sim$80 Oe to $\sim$30 Oe). Our results are suggestive of a very efficient resonant depinning effect in our spin valve wire which depends not only on the rf power but also on the polarity and amplitude of the accompanying dc current.   

Dynamical evidence for a domain pattern phase transition

Ultrathin 1.5ML Fe/ 2ML Ni/W(110) films have a perpendicular magnetic anisotropy, and form domain patterns, similar to many systems with attractive short-range and repulsive long-range interactions. Numerous microscopy studies have indicated that the domain pattern changes from an ordered stripe (smectic) phase to a positionally disordered (nematic or tetragonal) phase as the temperature is increased, and pattern defects proliferate. There is very little known about the dynamics of these phases. We study the domain dynamics on time scales of minutes to hours by quenching the films from high temperature (360 K), and measuring the ac magnetic susceptibility as the film is heated at a series of constant rates, from R=0.70 to 0.03 K/s. The entire susceptibility curve is observed to relax in temperature, with the peak temperature increasing as R decreases. This opposite to what is expected for relaxation of the smectic stripe density with temperature. A quantitative analysis is consistent instead with the relaxation of the pattern defects trapped by quenching as the domain pattern undergoes a transition to the low temperature stripe phase.   

Magnetic domain dynamics in Fe/Gd magnetic multilayers

We study the evolution of magnetic domains and effect of pinning centers in thin film magnetic systems as a function of magnetic field and dopants to identify the role the disorder in formation and stability of the domains in these systems. We have studied Fe/Gd multilayer exhibiting ordered stripes due to perpendicular magnetic anisotropy (PMA). Polar Kerr effect and Vibrating sample magnetometry measurements are used to characterize the sample. Magnetic Force Microscopy (MFM) measurements show out-of-plane magnetized stripe domains. We study the effects of field pinning of the local magnetic structure of these systems through their magnetization hysteresis loops both along easy and hard axis of magnetization. Using element sensitivity and depth resolution of resonant magnetic x-ray coherent scattering technique we investigate the magnetic domain structure and intermittent switching dynamics. Comparison of the magnetic speckles (in momentum space) provides information on correlation between the magnetic structures (in real space). We will present the X-ray Coherent Speckle Metrology approach to study Barkhausen noise spectrum as a function of the applied magnetic field, and will discuss extension of this study to Tb-doped Fe/Gd magnetic films.   

Giant magnetic coercivity and nano-domains in Fe$_{0.25}$TaS$_2$

We have explored giant magnetic coercivity (H$_c \sim$ 7 tesla) in the highly anisotropic ferromagnet of Fe$_{0.25}$TaS$_2$ through harnessing order of Fe$^{2+}$ ions intercalated in-between TaS$_2$ layers. Fe ions order well in annealed crystals and form a (1/2,0,0) superlattice. However, a (1/3,1/3,0) superlattice, in addition to the (1/2,0,0) superlattice, can form in quenched crystals. These coexisting superlattices with nano-size domains result in significant change of zero-field-cooled magnetic domain configurations and huge enhancement of H$_c$, probably through efficient magnetic domain wall pinning by nano-size superlattice domains.\footnote{Y.J. Choi, et al, EuroPhys. Lett., 86, 37012 (2009).}