Nonequilibrium sub-10 nm spin-wave soliton formation in FePt nanoparticles.

Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub-10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films.

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO2 amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu2 Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Epitaxy Induced Highly Ordered Sm2Co17-SmCo5 Nanoscale Thin-Film Magnets.

Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm2Co17 and SmCo5 phases is stabilized. While typically in the prototype Sm(Co, Fe, Cu, Zr)7.5-8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm-Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm2Co17 and SmCo5 phases with 65% Sm2Co17 and 35% SmCo5. The SmCo5 phase is stabilized directly on an Al2O3 substrate up to a thickness of 4 nm followed by a matrix of Sm2Co17 intermixed with SmCo5. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm2Co17 and SmCo5 phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm2Co17 and SmCo5 phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets.

Magneto-optical design of anomalous Nernst thermopile.

The introduction of spin caloritronics into thermoelectric conversion has paved a new path for versatile energy harvesting and heat sensing technologies. In particular, thermoelectric generation based on the anomalous Nernst effect (ANE) is an appealing approach as it shows considerable potential to realize efficient, large-area, and flexible use of heat energy. To make ANE applications viable, not only the improvement of thermoelectric performance but also the simplification of device structures is essential. Here, we demonstrate the construction of an anomalous Nernst thermopile with a substantially enhanced thermoelectric output and simple structure comprising a single ferromagnetic material. These improvements are achieved by combining the ANE with the magneto-optical recording technique called all-optical helicity-dependent switching of magnetization. Our thermopile consists only of Co/Pt multilayer wires arranged in a zigzag configuration, which simplifies microfabrication processes. When the out-of-plane magnetization of the neighboring wires is reversed alternately by local illumination with circularly polarized light, the ANE-induced voltage in the thermopile shows an order of magnitude enhancement, confirming the concept of a magneto-optically designed anomalous Nernst thermopile. The sign of the enhanced ANE-induced voltage can be controlled reversibly by changing the light polarization. The engineering concept demonstrated here promotes effective utilization of the characteristics of the ANE and will contribute to realizing its thermoelectric applications.

Dependence of the Growth Mode in Epitaxial FePt Films on Surface Free Energy.

Epitaxial thin films of L10-ordered FePt alloys are one of the most important materials in magnetic recording and spintronics applications due to their large perpendicular magnetic anisotropy (PMA). The key to the production of these required superior properties lies in the control of the growth mode of the films. Further, it is necessary to distinguish between the effect of lattice mismatch and surface free energy on the growth mode because of their strong correlation. In this study, the effect of surface free energy on the growth mode of FePt epitaxial films was investigated using MgO, NiO, and MgON surfaces with almost the same lattice constant to exclude the effect of lattice mismatch. It was found that the growth mode can be tuned from a three-dimensional (3D) island mode on MgO to a more two-dimensional (2D)-like mode on MgON and NiO. Contact angle measurements revealed that MgON and NiO show larger surface free energy than MgO, indicating that the difference in the growth mode is due to their larger surface free energy. In addition, MgON was found to induce not only a flat surface as FePt grown on SrTiO3 (STO), which has a small lattice mismatch, but also a larger PMA than that of STO/FePt. As larger lattice mismatch is favored to induce a higher PMA into the FePt films, MgO substrates are exclusively used, but 3D island growth is indispensable. This work demonstrates that tuning the surface free energy enables us to achieve a large PMA and flat film surface in FePt epitaxial films on MgO. The results also indicate that modifying the surface free energy is pertinent for the flexible functional design of thin films.

Magneto-optical painting of heat current.

Active control of heat flow is crucial for the thermal management of increasingly complex electronic and spintronic devices. In addition to conventional heat transport engineering, spin caloritronics has received extensive attention as a heat control principle owing to its high controllability and unique thermal energy conversion symmetry. Here we demonstrate that the direction of heat currents generated by spin-caloritronic phenomena can be changed simply by illuminating magnetic materials with visible light. The optical control of heat currents is realized through a combination of the spin-driven thermoelectric conversion called an anomalous Ettingshausen effect and all-optical helicity-dependent switching of magnetization. This approach enables not only pinpoint manipulation and flexible design of the heat current distribution by patterning the illuminating light but also on/off control of the resulting temperature modulation by tuning the light polarization. These versatile heat control functionalities will open up a pathway for nanoscale thermal energy engineering.

Magnetic Switching in Granular FePt Layers Promoted by Near-Field Laser Enhancement.

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle X-ray scattering at an X-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, 1 order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

Nd(2)Fe(14)B/FeCo anisotropic nanocomposite films with a large maximum energy product.

Anisotropic Nd(2) Fe(14) B/Fe(67) Co(33) nanocomposite thin films are successfully fabricated. The multilayer composite films comprise Nd-rich shell-enveloped Nd(2) Fe(14) B grains and a Fe(67) Co(33) phase. The strong (001) texture of the Nd(2) Fe(14) B grains and the presence of exchange-coupled Fe(67) Co(33) lead to a high remanence and the presence of the Nd-rich shell gives rise to a high coercivity. The unique nanocomposite microstructure provides hints for developing rare-earth-lean high-performance magnets.

Time-domain observation of the spinmotive force in permalloy nanowires.

The spinmotive force associated with a moving domain wall is observed directly in Permalloy nanowires using real time voltage measurements with proper subtraction of the electromotive force. Whereas the wall velocity exhibits nonlinear dependence on magnetic field, the generated voltage increases linearly with the field. We show that the sign of the voltage reverses when the wall propagation direction is altered. Numerical simulations explain quantitatively these features of spinmotive force and indicate that it scales with the field even in a field range where the wall motion is no longer associated with periodic angular rotation of the wall magnetization.