Session B32: Focus Session: Nanocontacts and Inhomogeneous Magnetic States

Fabrication of point contacts by FIB patterning

Nanoscale electrical contacts currently receive an increased amount of attention due to their capability to produce extremely high current densities needed, e.g., in experiments on current-driven precession and reversal of magnetization. Here we describe a new technique for the fabrication of such point contacts using a focused ion beam (FIB) patterning. FIB-fabricated point contacts combine the robustness and size-control of other lithographical methods with the flexibility of mechanical techniques to produce contacts to samples of arbitrary shape and composition. After sample coverage with a thin insulating layer (SiO), an FIB is used to mill a 100-nm-diameter hole through the insulator. Electrical contact to the sample is then made in-situ by filling the hole with a metal (Pt) using the ion beam assisted chemical vapor deposition capability of our FIB system. We have demonstrated the use of two such contacts (as an emitter and collector) in a transverse electron focusing (TEF) experiment. The contacts were made to a single crystal of bismuth, ballistic electrons were injected into the crystal through the emitter, and then focused onto the collector by a magnetic field. We see the expected voltage peaks at the collector as a function of the applied magnetic field.   

Electronic excitations in four structurally similar but magnetically different Nickel chain compounds

We report the low temperature zero-field and magnetic field- dependent optical spectra of four Ni$^{2+}$ chain compounds: NENP (Ni[$en$]$_2$NO$_2$ClO$_4$), NENB (Ni[en]$_2 $NO$_2$BF$_4$), NTNB (Ni[$tn$]$_2$NO$_2$BF$_4$), and NINO (Ni[tn] $_2$NO$_2$ClO$_4$). The four differ in the counterions isolating the chains and the rings coordinated to the Ni$^{2+}$ ions. Despite the structural similarities, only three of the compounds exhibit the Haldane gap typical of a spin-1 chain; the fourth, NTNB, behaves like spin glass, likely due to finite chain effects.$^1$ We focus on the near infrared spin-forbidden (SF) electronic $d-d$ transitions and the visible Ni$^{2+}$-to-NO$_2^- $ charge transfer (CT) band. The zero-field absorption spectra differ in the $en$ and $tn$ ring compounds but are nearly identical in compounds with identical rings. The SF and CT band absorption intensities depend on field in a way that reflects the magnetic ground state. In the Haldane compounds, the onset of intensity changes occurs above the crossover field, whereas in NTNB the field-dependent absorption intensities respond to any finite field.$^1$ R.D. Willett and C. J. Gomez-Garcia, EMRS2007, Strasbourg.   

Theory and application for chain formation in break junctions

We introduce a generic model for chain formation in break junctions by formulating separate criteria for the stability and producibility of suspended monoatomic chains based on total energy arguments. Using first-principles calculations [1], we apply our model to break junctions of $4d$ and $5d$ transition-metals (TMs), as well as Ag and Au, including the effects of spin-polarization and spin-orbit coupling. Thereby, we can explain the physical origin of the experimentally observed trend of increasing probability for the creation of long suspended chains in break junctions for $5d$-TMs at the end of the series [2] and suppressed chain elongation for $4d$ elements. Moreover, we clearify why the probability of chain elongation is greatly enhanced by the presence of oxygen in experiments with Au and Ag. Our model also allows us to make predictions on the ballistic transport properties of suspended chains. [1] Y.Mokrousov {\sl et al.}, Phys.\ Rev.\ B\ {\bf 72}, 045402 (2005). [2] R.H.M.Smit {\sl et al.}, Phys.\ Rev.Lett.{\bf 87}, 266102 (2001).   

Anderson model of local magnetism at a break junction nanocontact

Atoms at break junction nanocontacts in nearly magnetic heavy transition metals such as Pt and Pd may develop a nonzero magnetization. Since here the nanocontact is strongly electronically tied to the two bulk leads, it is not automatically clear what the correct physical picture of the system should be, and in particular whether the nanocontact should or should not become analogous to a Kondo impurity as in quantum dot devices. To clarify that, we consider one (or more) impurity sites inserted into a linear chain (representing the nonmagnetic leads), every site endowed with orbitally degenerate orbitals, large spin orbit coupling, and Hund's rule exchange; neighboring sites connected by electron hopping and by intersite ferromagnetic exchange. The mean-field solution when the impurity site (where Hund's rule exchange is made stronger) is locally magnetic shows a ferromagnetic polarization around it, in agreement with realistic density functional calculations for nanocontacts consisting of monatomic chain segments. Our results suggest that this type of nanocontact, regarded as an Anderson impurity, is ferromagnetically coupled to the leads, and hence that Kondo screening does not occur in this case. The physical consequences for the conductance through the chain are discussed.   

Dynamics of a pinned magnetic vortex

Disks patterned from soft ferromagnetic films typically form a single magnetic vortex for diameters on the order of a few microns or less. The vortex dynamics include both ordinary spin waves and a gyrotropic mode, in which the vortex core undergoes circular motion about its equilibrium position [1, 2]. This mode has sub-GHz frequencies which ideally depend only on the aspect ratio (diameter over thickness) of the disk [2, 3]. We have used time-resolved Kerr microscopy to investigate the gyrotropic mode as a function of the equilibrium position of the core, which can be tuned by an applied field with a sensitivity of $\sim 1$~nm/Oe. In the limit of high excitation amplitude, the gyrotropic frequency $f_G$ is independent of the vortex core position, as previously predicted and observed [1, 3]. For small amplitudes, however, we observe unexpected fluctuations in $f_G$ as a function of the applied field. The average core displacement between consecutive frequency peaks, as well as the average frequency shift, is observed to be independent of disk diameter. These observations indicate that the fluctuations are due to a distribution of nanoscale defects that pin the vortex core by lowering its energy [4]. Furthermore, they are consistent with a model in which the frequency shift for a particular fluctuation is a direct measure of the interaction energy of the vortex core with one defect. By mapping $f_G$ as a function of orthogonal in-plane static fields, we image the 2D spatial distribution of defects with nanoscale resolution. \newline [1] K. Yu Guslienko \textit{et.al.}, JAP \textbf{91}, 8037 (2002). \newline [2] J. Park \textit{et.al.}, PRB \textbf{67}, 020403(R) (2003). \newline [3] V. Novosad \textit{et.al.}, PRB \textbf{72}, 024455 (2005). \newline [4] R. L. Compton and P. A. Crowell, PRL \textbf {97}, 137202 (2006).   

Angular Dependence of Vortex Annihilation Field in Asymmetric Co Nanodots$^{\ast }$

Magnetization reversal via a vortex state is a common occurrence in sub-micron magnetic nanodots. We have investigated arrays of 500 nm polycrystalline Co dots prepared by e-beam lithography. The circular symmetry of the dots has been broken by introducing a flat edge to the dots. Magneto-optical Kerr effect (MOKE) measurements and micromagnetic simulations confirm the reversal mechanism of the dots is via the nucleation, propagation, and annihilation of a vortex core. The asymmetric dot shape favors vortex nucleation from the flat edge and therefore allows for control over the vortex chirality. Additionally, by modifying the applied field sweep, we can control which side of the dot the vortex annihilates from. We have studied the vortex annihilation field as a function of the angle between the applied field and the flat edge of the dot. At small angles, the annihilation field depends on the chirality of the vortex and annihilation is easier from the flat edge of the dot. The difference in annihilation fields for the two chiralities is strongly dependent on the angle of the applied magnetic field. This behavior is due to the complex motion of the vortex core across an asymmetric dot during reversal. *Work supported by ACS-PRF, AFOSR-MURI, and the Alfred P. Sloan Foundation.   

Influence of excitation fields on vortex core dynamics in micron-sized magnetic disks

Magnetization vortices in micron-sized magnetic disks have been of great interest because of potential applications in memory devices. Theory predicts a rich spectrum of excitations including the fundamental or gyrotropic mode. Experimentally the gyrotropic mode is observed in some experiments while in others a linear or elliptical trajectory is seen. We have imaged free vortex core motion in permalloy disks of 6 $\mu $m diameter using time-resolved x-ray photoemission electron microscopy at beamline 4-ID-C of the Advanced Photon Source with 90 ps temporal resolution. We demonstrate that the vortex core motion trajectory depends on the magnitude of the excitation field. The vortex core exhibits a gyrotropic trajectory under low excitation fields, while under high excitation fields the core shows a more linear trajectory. We find that if the initial displacement of the core is greater than 20{\%} of the disk radius, transient magnetic domains appear in the first 1 ns after removal of field. These domain states then profoundly influence the subsequent motion. The core oscillation frequencies are consistent with theoretical predictions, regardless of the excitation amplitude.   

Direct Observation of Magnetic Vortex Cores using Scanning Electron Microscopy with Polarization Analysis (SEMPA)

Magnetic singularities associated with magnetic vortex cores are a common feature in patterned magnetic nanostructures. Their small size, on the order of 10 nm, makes them technologically interesting, but also difficult to measure or image directly. We used Scanning Electron Microscopy with Polarization Analysis (SEMPA) to image magnetic vortices in a wide variety of patterned nanostructures. Since SEMPA can measure both the in-plane and the out-of-plane component of the surface magnetization, SEMPA can potentially determine both the chirality and the polarity of the vortex core, simultaneously. Samples consisted of NiFe (25nm) / Ta (3nm), and other soft magnetic films, patterned by electron beam lithography and lift-off into disks with various diameters. The films were grown on 85nm thick SiN membranes to reduce image degradation from backscattered electrons. The experimental results were compared to micromagnetic simulations and the vortex core profile showed a good correspondence with theoretical predictions, which considers only the exchange and magnetostatic energy. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement.   

Transition states of magnetization reversal in ferromagnetic nanorings

Thin ferromagnetic rings are of interest for fundamental studies of magnetization reversal, in part, because they are a rare example of a geometry for which an analytical solution for the rate of thermally induced switching has been determined [1]. The theoretical model predicts the transition state to be either a global magnetization rotation of constant azimuthal angle or a localized fluctuation, denoted the instanton saddle. Numerically we have confirmed that for a range of values of external magnetic field and ring size the instanton saddle is energetically favored [2]. The model takes the annular width to be small compared to the mean radius of the annulus; in which case the main contribution to the magnetization energy comes from the surface magnetostatic energy. We present numerical micromagnetic calculations of the activation energy for thermally induced magnetization reversal for the two different transition states for the case when the annular width is equal in magnitude to the mean radius of the ring. Results of the total and surface magnetostatic energies are compared for different ring sizes. [1] K. Martens, D.L. Stein, A.D. Kent, PRB 73, 054413 (2006) [2] G.D. Chaves-O'Flynn, K. Xiao, D.L. Stein, A. D. Kent, arXiv:0710.2546 (2007)   

Probing a SET nanomagnet with shot noise.

Although recent experiments show that single atomic spins [1] and molecular magnets [2] can be proved via transport measurements, their magnetic properties can hardly be tuned once they are fabricated. In a recent Letter [3], we have shown that a single-electron transistor (SET) based upon a II--VI semiconductor quantum dot and doped with a single-Mn ion behaves like a quantum nanomagnet with magnetic properties which can be controlled electrically. Conversely, the electrical properties of this SET depend on the quantum state of the Mn spin. Here, we extend these previous ideas and study the shot noise of this kind of nanomagnets. Our results reveal that shot noise contains much more information that the one contained in the average current. Interestingly, important quantities of the nanomagnet like the spin relaxation time and information about current-induced spin precession can be directly extracted from shot noise measurements. [1] Cyrus F. Hirjibehedin et al, Science, 317, 1199 (2007). [2] Moon-Ho Jo et al, Nanoletters, 6, 2014, (2006); H. B. Heersche et al., Phys. Rev. Lett. 96, 206801 (2006). [3] J. Fernandez-Rossier and R. Aguado, Phys. Rev. Lett. 98, 106805 (2007).