Figure 1

(Color) Two pictures of circles with arrows inside: (a) from De Magnete, published in 1600 by William Gilbert. The image is a woodcut representation of the Earth, including a mountainous topology, in an effort to understand geomagnetism. (b) A generic modern micromagnetic simulation of a submicrometer magnetic vortex structure in Permalloy, such as is discussed by 74. It includes a central core where the spins point out of plane.Reuse & Permissions

Figure 2

(Color online) Grand challenges in nanomagnetism, emphasizing those related to strategic national goals, such as stimulating the economy, energy efficiency, homeland security, and defense. The underlying basic research needed involves creating new materials and understanding their spin-transport and spin dynamic properties.Reuse & Permissions

Figure 3

Spin injection, diffusion, and detection in an all-metallic lateral spin-valve structure, from 46. The two ferromagnetic Permalloy (Py) electrodes are fabricated with different aspect ratios to provide different coercive fields. This permits the applied magnetic field sweep to create magnetic orientations of two electrodes that are parallel at the maximum field and antiparallel in an intermediate field range. The two electrodes are separated by a length $L$ and sandwich a Au stripe. Note the nontraditional placement of the current $\left(I\right)$ and voltage $\left(V\right)$ leads. Spins are injected from the top electrode, diffuse through Au, and are detected as a voltage across the bottom electrode that senses the nonequilibrium spin splitting of the chemical potential of Au.Reuse & Permissions

Figure 4

The spin resistance of the lateral spin-valve structure shown in Fig. 3 plotted as a function of the external magnetic field that is applied along the long axis of the Py electrodes. The experiment by 46 is performed at 10 K. An $\sim 30\%$ change in spin resistance $\left(\Delta R_s\right)$ is found in comparing parallel vs antiparallel orientations of the magnetization of the two Py electrodes. The antiparallel configuration gives rise to the low-resistance state, unlike in traditional giant magnetoresistance structures.Reuse & Permissions

Figure 5

The change in spin resistance $\Delta R_s$ of lateral spin valves, from data by 46, summarized in linear and semilogarithmic (inset) plots in order to extract the spin diffusion length in Au at 10 K of $63\pm 15\mathrm{nm}$.Reuse & Permissions

Figure 6

(Color online) Arrays of Permalloy dots $(\text{thickness}=60\mathrm{nm})$ with systematically varying interdot separation in the top panel, based on 74. The dots form magnetic vortex structures in the absence of an applied magnetic field, as shown in Fig. 1b. Such structures are used to explore the magnetostatic coupling of the dots. In the bottom panel schematics of a variety of magnetic domain structures for a rectangular ferromagnetic object, taken from the renowned textbook by 54. The magnetostatic energy is most effectively reduced for closure domain cases shown as the last two examples to the right in the bottom panel. For the Permalloy dot geometry in the top panel, a magnetic vortex state is the corresponding equivalent to the closure domain.Reuse & Permissions

Figure 7

(Color) Virtual fab representation of the magnetic hysteresis loop of a submicron dot array, such as is shown in Fig. 6, based on 37. Micromagnetic simulations are used to represent the spin configurations of the Permalloy dot array along the path of a hysteresis loop. (a) Spin configuration aligned with the applied field direction; (b) at the point of nucleation of the vortex core at the right edge of the dot; (c) the symmetric vortex in zero field; (d) the vortex core, as it shifts to the left with increasing applied field, but before being annihilated at the left edge of the dot; and (e) the reverse of (a). Note that the central portion of the loop is nonhysteretic, as the vortex core shifts to the left or right, while the side lobes are hysteretic as the core nucleates or is annihilated. Magnetostatic interactions will affect the magnitude of the nucleation and annihilation fields.Reuse & Permissions

Figure 8

(Color) Virtual fab representation of the geometry, spin arrangement, and spin densities (insets) of 29 Fe atoms, from electronic structural calculations of 69 carried out via the local-density approximation. The outer shells of Fe atoms have their spin structures constrained to form a magnetic vortex, while the spins of the inner Fe atoms are self-consistently calculated to relax into the vortex structure indicated, with the spin of the central Fe atom forming the out-of-plane singularity, and the spins on the next shell of Fe atoms forming a canted structure.Reuse & Permissions

Figure 9

(Color online) A composite of magnetic images of lithographically patterned Permalloy structures is shown that exhibits size-dependent, metastable remanent states, taken from the work of 14. The images are obtained utilizing the magnetic contrast from resonant x-ray magnetic circular dichroism (XMCD) experiments that utilize photoemission electron microscopy (PEEM) at the Advanced Light Source at Lawrence Berkeley National Laboratory. The samples are subjected to rapidly pulsed magnetic fields and are then imaged in zero field. (a) Two images of single domain states of opposite orientation (with light and dark magnetic contrast, respectively), and (b) two images whose magnetic contrast is indicative of magnetic vortex states of opposite chirality, as indicated by the arrows. The corresponding vortex simulation is shown in Fig. 1b. (c) The dark-light-dark contrast indicative of a double-vortex spin configuration, as indicated by the arrows and the micromagnetic simulation in the bottom panel, and (d) two opposing magnetic domains that run along the major axis of an elliptically shaped structure. The bottom panel provides a micromagnetic simulation of a double-vortex spin configuration to compare to the image in (c).Reuse & Permissions

Figure 10

(Color) Magnetic force microscopy (MFM) images of Permalloy submicron dots, from 72, that indicate a hint of the magnetic vortex core singularity via the contrast change at the dot centers. Also shown is a schematic of the sample and the MFM tip that senses the out-of-plane magnetic flux gradient.Reuse & Permissions

Figure 11

(Color online) The exchange-spring principle is illustrated. A magnetically hard ferromagnet possessing a wide hysteresis loop, as shown to the left, exchange couples to a magnetically soft ferromagnet possessing a tall hysteresis loop, as shown to the right, forming a composite ferromagnet that has both a wide and tall hysteresis loop, as indicated below. The coupling principle can be used to create ultrastrong magnets that theoretically can outperform the strongest present-day commercial permanent magnets. The image to the right illustrates a bilayer structure that has similarly magnetically hard (bottom layer) and soft (top layer) layers. If the soft layer thickness exceeds a magnetic domain-wall thickness, a twisted magnetization structure can form in the soft layer, as shown, in response to an applied magnetic field, indicated by $H$, that is opposed to the magnetization of the composite. This illustrates why spring magnets are nanocomposites in nature, because the domain-wall thickness is of nanoscale dimensions. The twist must be suppressed in order to achieve an ultrastrong permanent magnet configuration.Reuse & Permissions

Figure 12

(Color online) Schematic surfactant-mediated chemical processing steps are shown to create core-shell nanocomposite spring magnets that then self-assemble or agglomerate into bulk structures. The TEM image at the top is of a film of self-assembled, monodispersed 9.5-nm Co nanocrystals made via surfactant-mediated processing, as reported by 84.Reuse & Permissions

Figure 13

(Color) A schematic of a potential three-step hierarchical assembly process is indicated for organizing magnetic nanoparticles, with AFM insets shown for each step, based on the work of 24. The first step is to form lithographic grooves in a substrate, which serve to assist the long-range ordering of diblock copolymers that self-assemble into stripe domains in the second step. In the third step, surfactant-mediated magnetic nanoparticles are attached selectively from solution to the hydrophobic stripes because of the chemical affinity to the organic surfactant. The breakdown of the second and third steps at the right represents the chemistries involved, while the image at the bottom right is a cartoon of a magnetic hard disk for data storage that is fabricated based on the three-step process. The image is suggestive of the role chemistry and self-assembly might play in the future quest for affordable patterned storage media with densities in the $\mathrm{Tbit}∕{\mathrm{in.}}^2$ range. The example is a hybrid strategy that would involve lithographically assisted self-assembly, with imprint lithography, polymer spin-coating, and dip-coating steps for ease of manufacture.Reuse & Permissions

Figure 14

(Color online) Schematics of the process to make artificial magnetic viruses. The rendition in the top left is of a virus showing its self-assembled protein capsid container and its attachment structure that is used to anchor it to a living cell, from 82. At the top right is a cryo TEM cross section of a capsid shell with DNA inside, from 16. At the bottom is a cartoon of biomineralization that results in Co nanoparticle growth inside the capsid shell that was hollowed out by an alkaline treatment process, based on 59.Reuse & Permissions

Figure 15

(Color online) The fabrication of magnetic viruses is documented in TEM images, from 59. The image at the left shows normal T7 bacteriophage and the image to the right shows the hollow capsid, or ghost virus, after alkaline treatment that triggers the ejection of the DNA and loss of the attachment structure. At the bottom shown are three magnetic viruses consisting of the native protein capsid filled with iron oxide after the biomineralization process.Reuse & Permissions