Session A13: Focus Session: Magnetic Nanostructures-Patterned Nanostructures and Nanowires

High Frequency Excitation of Nanometer-Scale, Strongly Coupled FM / NM / FM Disks

There is great interest in the manipulation of magnetic domains in nanostructures from both a fundamental and applications perspective. In particular, the use of resonant frequency excitations permits a power reduction of the driving forces necessary to induce detectable motion in magnetic vortex structures. Here we present an experimental and numerical study of patterned tri-layered disk stacks which are composed of 25nm Permalloy$\vert $1nm Copper$\vert $15nm Permalloy, excited at resonance, ranging from 250-500nm in radii. In-situ Lorentz microscopy was used to acquire time averaged real space images of the vortices' gyrotropic motion and micromagnetic simulations were implemented to further understand the coupled dynamics between the ferromagnetic layers across the thin non-magnetic spacer layer. We discuss the effects of interlayer coupling on the vortex trajectories and resonant frequencies for the individual ferromagnetic layers.   

Controlling the formation process of vortex states in magnetic nanodots with asymmetric geometry

Magnetic vortex structures in nanoscale elements are currently highly attractive, since they offer exciting topological spin configurations to study on a fundamental level nanoscale spin behavior and they show great potential for applications in data storage and memory technologies. One of the primary questions is to tailor the nucleation process of vortex structures, which would open the opportunity for the control of magnetic/spin phenomena in magnetic vortices such as the resonant motion of a magnetic vortex core, etc. We have investigated the stochastical character of the formation process of vortex states in permalloy (Ni$_{80}$Fe$_{20})$ nanodots with different geometries by direct imaging of vortex structures with high resolution magnetic transmission soft X-ray microscopy (MTXM). We observe that the formation process of vortex state sensitively depends on the geometry of nanodots. Based on our experimental result, we will discuss the possibility to control the generation process of magnetic vortex states by directed modifications of the geometry of nanodots. \\[4pt] References: \\[0pt] T. Shinjo, et al., Science \textbf{289}, 930 (2000). \\[0pt] M.-W. Yoo, et al., Phys. Rev. B \textbf{82}, 174437 (2010). \\[0pt] P. Fischer, et al., Phys Rev B \textbf{83} 212402 (2011) \\[0pt] H. Jung, et al., NPG Scientific Reports 1 59 (2011) \\[0pt] M.-Y. Im, et al., Phys Rev Lett 102 147204 (2009)   

Quantum depinning of the magnetic vortex core in micron-size permalloy disks

The vortex state being characterised by an in-plane closed flux domain structure and an out-of-plane magnetization at its centre (known as the vortex core) is one of the magnetic equilibria of thin soft ferromagnetic micron-size dots. The vortex core is a mesoscopic object and so it is a suitable candidate to observe quantum tunneling of its magnetic moment between classically stable magnetic configurations. For the first time, we report experimental evidence of quantum dynamics of the vortex core of micron-size Permalloy (Fe$_{19}$Ni$_{81})$ disks induced by the application of an in-plane magnetic field. It is attributed to the quantum tunneling of the vortex core through pinning barriers, which are associated to structural defects in the dots, towards its equilibrium position. The crossover temperature from the thermal to the quantum regime is obtained within the framework given by the Caldeira-Leggett theory. Comparison between experiments and theory points to tunneling of the vortex core by steps of the order of 0.3 nm and gives estimates to the parameters characterising the pinning barriers.   

Energy barriers for vortex nucleation and annihilation in sub-100 nm magnetic dots

Understanding energy barriers involved in nucleating and annihilating magnetic vortices in nanodots is important for magnetic memories and nano-oscillators. We used a ``rigid-vortex approximation'' and micromagnetic approach to calculate the total magnetic energy of a nanodot for various magnetic configurations. This was done for 20 nm-thick iron nanodots with different diameters (30, 40, 65, and 80 nm) as a function of applied magnetic field. By analyzing the energy landscape for different magnetic configurations, we calculated the energy barrier for switching from the vortex to the single-domain state (vortex annihilation) and the converse (vortex nucleation). The applied fields required to overcome these two barriers are compared to those obtained from the simulations directly and to the experimental values.\footnote{R. K. Dumas, \textit{et. al.}, Appl. Phys. Lett. \textbf{91}, 202501 (2007).} The role of the thermal fluctuations in the temperature dependence of these critical fields will be discussed by comparison of the energy barriers with the thermal energy, kT.   

A Novel Non-lift-off Block Copolymer Nanolithography Technique for Etch-damage Susceptible Magnetic Materials

Nanolithographic techniques based on self-assembled block copolymer templates offer exceptional potential for fabrication of large-area nanostructure arrays from a wide variety of functional materials. Despite significant progress with control of the template ordering and development of pattern transfer schemes, significant issues exist with common techniques such as lift-off and etching. Here, we demonstrate successful execution of a nanolithographic process based on climate-controlled solvent annealing of easily degradable cylinder-forming poly(styrene-$b$-lactide) block copolymer films that avoids both lift-off, and some of the most challenging aspects of etching. In particular, our overfill/planarize/etch-back scheme leads to retention of robust ferromagnetism even in 24 nm diameter dots of a material (Ni$_{80}$Fe$_{20})$ that is both magnetically soft and susceptible to etch damage. The result is a large-area array of 24 $\pm $ 1.6 nm diameter magnetic nanodots with exceptional hexagonally-close-packed long range order that retain their crystallinity and $\sim $ 70 {\%} of the bulk magnetization. Extensive diffraction, microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation and fidelity. Funded by NSF MRSEC.   

Quenching of the initial ac Susceptibility in Single Domain Ni Nanobars

The ac susceptibility measurement probes the dynamic properties of a magnetic material, which is believed to consist of magnetization rotation and domain wall motion contributions. Here we report the observation of a complete quenching of the initial ac susceptibility for a single domain Ni nanobar array, when the ac field is aligned with the long axis of the bars. The vanishing of the susceptibility in one direction is a unique nanoscale phenomena, allowing an unambiguous determination of the magnetic state of the nanostructure and a clean separation of different contributions to its dynamic properties. For example, an unambiguous determination of the temperature dependent surface anisotropy energy is obtained when the field is applied perpendicular to the long axis. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy.   

Microwave assisted magnetization reversal in cylindrical antidot arrays with in-plane and perpendicular anisotropy

Porous anodic alumina is a particularly attractive self-ordered system used as template to fabricate nanostructures. The anodic film contains a self-ordered hexagonal array of parallel pores with tunable pore size and interpore distance, and whose pore locations can be templated. Deposition of magnetic films onto porous alumina leads to the formation of porous magnetic films, whose properties differ significantly from those of unpatterned films. The study of antidot arrays has both technological and fundamental importance. Although porous alumina films are typically synthesized in a planar geometry, in this work we deposited NiFe and Ti/CoCrPt magnetic films with in-plane and out-of-plane anisotropy onto cylindrical-geometry porous anodic alumina substrates to achieve cylindrical antidot arrays. The effect of both, the magnitude of the AC current and the circular magnetic field on the magnetization reversal has been studied for in-plane and perpendicular anisotropies. The level of reduction in the switching field was found to be dependent on the power, the frequency of the microwave pulses and the circular applied magnetic field. Such a reduction is associate with the competition between pumping and damping processes.   

DC and High-frequency Magnetic Properties of Patterned Ferromagnetic Nanostructures

Magnetic mesoscopic and nanostructures have promising applications such as high-density data storage, magnetic field sensors, and microwave devices. Patterned magnetic structures are especially interesting because their constitutive material, sizes and geometry are easily adjustable in fabrication. This work aims to study dc and radio frequency magnetic properties of Co and permalloy patterned structures and the effect of magnetic coupling. We use electron-beam lithography and complementary techniques to ferromagnetic nanostructures with various separations to control the strength of magnetic interaction. SQUID and complimentary MFM characterization are performed to observe the dc magnetic properties. AC susceptibility is used to investigate the low frequency response. Microstrip transmission lines are then incorporated to measure the scattering parameters between 300kHz and 6GHz. The equivalent RLCG circuit elements can be extracted to obtain the effective magnetic permeability for different ferromagnetic structures.   

Precise control of vortex chirality and polarity in ``Pac-Man''-like magnetic nanodots by in-plane magnetic field

Here we explore size-dependent magnetic states of sub-100 nm Permalloy nanomagnets of specific geometry. The geometry is suitable for independent setting and readout of vortex polarity and chirality by applying \emph{in-plane magnetic fields} only. Micromagnetic calculations show that in ``Pac-Man''-like magnetic nanodots the relaxation channels to specific chirality and polarity states from uniform magnetization state are deterministic and are not influenced by the presence of moderate out-of-plane fields. The particular geometry opens straight channel for magnetization relaxation towards stable closure-domain vortex state with specific chirality and polarity. We explore a wide geometrical phase space in search for stable and predictable remanent vortex configurations. We find that in these nanomagnets the write process is simple and the signal is easily readable.   

Direct imaging of complex domain walls and chirality sensors in magnetic nanostripes

Domain walls (DWs) in patterned ferromagnetic nanostripes are increasingly being considered for non-volatile and radiation hard data storage and logic applications. We use scanning electron microscopy with polarization analysis (SEMPA) to image the formation of complex domain walls in nanostripes and local patterned structures used for sensing DW chirality. The DWs studied have a transverse orientation, where the in-plane spin direction of a 180\r{ }DW is perpendicular to the nanostripe axis. We demonstrate a technique where two 180\r{ }DWs of alternating chirality may interact to form a stable DW with 360\r{ } rotation as opposed to DW annihilation. Higher order DWs with n$\pi $ rotation are demonstrated, where n is an integer number of interacting 180\r{ }DWs. The detection of moving 180\r{ }DWs via external fields is studied by placing patterned magnetic triangular elements above and below the nanostripe in-plane. As the DW propagates across the wire, the stray field interacts and switches the magnetization of the triangles. The chirality of the DW may be sensed by designing the triangles to respond to the inherent asymmetry of the DW's stray field.   

A high-spin atomic 1-D system with a spin-induced CDW instability

We report on low-temperature STM measurements of a Co atomic-wire system that has been realized by the technique of self-assembly on a vicinal Cu(111) substrate [1,2]. We show that for this bimetallic case, the Co-wire system undergoes a CDW instability leading to a 1-D high-spin system. This type of instability does not appear to have been previously reported for a bi-metallic system, particularly for a chain of Co atoms. Using ab initio theoretical calculations, it is deduced that the CDW instability is spin-induced by way of symmetry breaking in the spin population. This result presents a fundamental electronic-structure mechanism for CDW instability that is distinct from previously reported metal-semiconducting systems [3], in that spin clearly plays an essential role in lowering the energy of the system. Furthermore, the calculations indicate that the high-spin correlated state of the constituent Co atoms is a necessary consequence of this CDW instability. Finally, the ferromagnetic nature of this realized system raises questions with regard to substrate spin mediation, such as the possible role of Kondo and RKKY interaction. [1] N. Zaki et al, Phys. Rev. B 80, 155419 (2009) [2] N. Zaki et al, Phys. Rev. B 83, 205420 (2011) [3] P. C. Snijders and H. H. Weitering, Rev. Mod. Phys. 82, 307 (2010)   

Spin resonance in Luttinger liquid with spin-orbit interaction

The spin-orbit interaction leads to a narrow spin resonance at low temperatures, even in the absence of an external magnetic field [1]. We study the effect of electron-electron correlations on the resonance. These correlations are strong in quantum wires and cannot be neglected. We show that the electron correlations change the shape and width of the resonance and produce an additional weak resonance at the plasmon frequency. \\[4pt] [1] Ar. Abanov, V. L. Pokrovsky, W. M. Saslow, and P. Zhou, arXiv:1008.1225.   

Telegraph Noise in LSMO Nanowires

Hole-doped manganites with the perovskite structure exhibit a variety of superlative properties because of close competition among ferromagnetic metallic, paramagnetic insulating, as well as various charge, spin, and orbitally ordered phases. We have recently observed random telegraph noise (RTN) in low-temperature conductance measurements of epitaxially-grown La$_{2/3}$Sr$_{1/3}$MnO$_3$ nanowires patterned by electron-beam lithography and ion milling to widths of $\sim$ 80nm. The RTN is apparent at temperatures less than 30K. It is thought that the RTN is the result of domain fluctuations, which are more clearly observable in such narrow wires.   

Direct Imaging of Non-Adiabatic Spin Torque Effects on Vortex Core Orbits

Recently high frequency, current induced vortex motion has received a great deal of interest from a spintronic perspective, as it suggests a possible low power, high speed writing process. However, understanding the processes that govern this motion, specifically the relative contributions of adiabatic and non-adiabatic spin torque effects, has been difficult due to experimental constraints. We developed a novel TEM sample stage in which we apply high frequency currents in-situ to excite resonant motion in Permalloy disc structures (2000x2000x50nm) with high spatial resolution ($<$5nm for dynamic measurements). We have imaged the time-averaged vortex trajectory through resonance. We find that the orbital amplitudes are drastically different for clockwise and counterclockwise chiralities, indicating the presence of both Oersted fields and non-adiabatic spin torque effects, and that the orbital size scales linearly with current density varied between (7.1-10.0)x10$^{10}$ A/m$^{2}$. These results allow us to extract a value for the non-adiabatic spin torque with unprecedented precision. Additionally, we report on off-resonance effects, such as tilting and variations in the ellipticity of the orbit as it is swept through resonance, with first of their kind experimental observations.