Polymer magnetic microactuators fabricated with hot embossing and layer-by-layer nano self-assembly

Polymer-based magnetic microactuators have been fabricated with hot embossing technique and layer-by-layer (LbL) nano self-assembly. Silicon molds are fabricated with conventional UV lithography and wet etching techniques. Hot embossing is used to transfer the patterns from silicon molds to polymethylmethacrylate (PMMA) sheets. The overall processing time for the pattern transfer is less than 20 min. Low-cost devices with massive and rapid replication can be fabricated. Six layers of magnetic iron oxide (Fe2O3) nanoparticles are LbL self-assembled on the PMMA surface as the magnetically sensitive material. Positive photoresist PR1813 is used as the sacrificial layer to protect the gold electrode on the back side of the membrane. LbL nano self-assembly technique provides a simple method to obtain the magnetic film with low cost, short processing time, simple fabrication steps at room temperature. The volume of the magnetic material can be precisely controlled by the number of nano-assembled iron oxide layers. The mechanical, electrical, and magnetic properties of the microactuator are characterized by a laser interferometer. The natural frequency of the actuator is approximately 151 Hz; and the maximum deflection amplitude is about 34 nm. At all frequencies, the increase of the magnetic field increases the deflection amplitude which is in agreement with the theoretical equation.     

Graphene-based bimorph microactuators

A novel graphene-on-organic film fabrication method that is compatible with a batch microfabrication process was developed and used for electromechanically driven microactuators. A very thin layer of graphene sheets was monolithically integrated and the unique material characteristics of graphene including negative thermal expansion and high electrical conductivity were exploited to produce a bimorph actuation. A large displacement with rapid response was observed while maintaining the low power consumption. This enabled the successful demonstration of transparent graphene-based organic microactuators.     

Ferroelectric control of magnetic anisotropy

We demonstrate unambiguous evidence of the electric field control of magnetic anisotropy in a wedge-shaped Co film of varying thickness. A copolymer ferroelectric of 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE) overlays the Co wedge, providing a large switchable electric field. As the ferroelectric polarization is switched from up to down, the magnetic anisotropy of the Co films changes by as much as 50%. At the lowest Co thickness the magnetic anisotropy switches from out-of-plane to in-plane as the ferroelectric polarization changes from up to down, enabling us to rotate the magnetization through a large angle at constant magnetic field merely by switching the ferroelectric polarization. The large mismatch in the stiffness coefficients between the polymer ferroelectric and metallic ferromagnet excludes typical magnetoelectric strain coupling; rather, the magnetic changes arise from the large electric field at the ferroelectric/ferromagnet interface.     

Induced in-plane magnetic anisotropy in YIG films

We have studied the symmetry properties of the in-plane induced magnetic anisotropy in {100} and {110} disks of single crystal YIG epitaxially grown on gadolinium gallium garnet (GGG) substrates. The ferromagnetic resonance (FMR) technique was used to determine the values of the cubic and induced magnetic anisotropies. The induced anisotropy is interpreted in terms of a magnetostrictive magnetic potential energy. We find that the effective field induced in the plane of the film is isotropic for {100} disks but for {110} disks the induced anisotropy is uniaxial within the plane.     

Rotatable magnetic anisotropy in epitaxial ferrite layers

Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg0.9Mn0.3Fe1.8O4ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K1= - 2.2 × 10⁴ergs/cc, KN= 2.3 × 10⁵ergs/cc.     

Perpendicular anisotropy of evaporated magnetic films

Evidence of the existence of origins of perpendicular anisotropy other than the ordinary magnetostrictive effect is presented with a brief review of other studies. As one of these origins, the effect of the grain boundaries or the microscopic shape effect is discussed. The amount of perpendicular anisotropy observed in the stripped films does not seem capable of explanation by this effect only.     

Annealing behavior of magnetic anisotropy in CoNbZr films

The effect of magnetic and nonmagnetic annealing on the magnetic anisotropy in CoNbZr films, formed by a DC opposing-targets sputtering method, was investigated. It was revealed that the origin of the magnetic anisotropy is the directional ordering of the magnetic atoms. The anisotropy fields and the direction of the easy axis obtained when the films are annealed in zero magnetic field are almost the same as those for the magnetic field parallel to the easy axis of the as-deposited films. When the films are annealed in a magnetic field perpendicular to the easy axis, the anisotropy field induced in parallel with the magnetic field, H_k(t), is well represented by the following formula: ln {1-H_k(t )/H_k(∞)∝-√Tt, where H_k(∞) is the thermal equilibrium value of the anisotropy field and D is the diffusion constant. The activation energy of the as-deposited film is 0.86 eV. Annealing the film increases the activation energy which is 2.1 eV when the film is annealed at a temperature of 450°C for 2 h     

Induced magnetic anisotropy and strain in permalloy films deposited under magnetic field

Field induced magnetic-anisotropy is a very important but poorly understood property. There have been many hypotheses on the origin of the phenomenon, e.g. strain, atomic pair ordering, etc., but little experimental evidence exist. This study prepares 100 nm thick Permalloy films having the field-induced-magnetic-anisotropy and carefully measure strains, i.e. interplanar distance of crystallographic (111) planes in various directions, using high power synchrotron radiation and precise Grazing Incidence X-Ray Diffraction method. The result delineates that the field-induced-magnetic-anisotropy has a strong correlation with the strain-anisotropy in the film.     

Probing magnetic anisotropy and spin polarization in spintronic materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.     

In-plane magnetic anisotropy in RF sputtered Fe---N thin films

We have fabricated Fe(N) thin films with varied N₂ partial pressure and studied the microstructure, morphology, magnetic properties and resistivity by using X-ray diffraction, atomic force microscopy, transmission electron microscopy, vibrating-sample magnetometer and angle-resolved M-H hysteresis Loop tracer and standard four-point probe method. In the presence of low N₂ partial pressure, Fe(N) films showed a basic bcc -Fe structure with a preferred (110) texture. A variation of in-plane magnetic anisotropy of the Fe(N) films was observed with the changing of N component. The evolution of in-plane anisotropy in the films was attributed to the directional order mechanism. Nitrogen atoms play an important role in refining the -Fe grains and inducing uniaxial anisotropy.     

Calculation of the magnetic anisotropy energy in small clusters

The magnetic anisotropy of small Fe_N clusters is studied in the framework of a d-electron tight-binding Hamiltonian including hopping, Coulomb and spin-orbit interactions on the same electronic level. Results for the magnetic anisotropy energy (MAE), the orbital angular moment and the spin magnetic moment are given as a function of cluster size and bond length. In particular we show that the MAE may be qualitatively related to the projections of the orbital angular moment along the magnetization directions, and that the in plane anisotropy can be of the same order of magnitude as the perpendicular anisotropy.     

用环形试样测定软磁材料磁各向异性的研究

采用趋近饱和定律测定了纳米晶合金环形试样有效磁各向异性常数＜K＞.为了对比测量的准确度,同时测试了传统的晶态坡莫合金环形试样的磁晶各向异性.结果表明,用环形试样可以完成对低矫顽力的软磁材料进行磁各向异性的测定.     

Control of magnetic domains in Co/Pd multilayered nanowires with perpendicular magnetic anisotropy

Magnetic domain wall (DW) motion induced by spin transfer torque in magnetic nanowires is of emerging technological interest for its possible applications in spintronic memory or logic devices. Co/Pd multilayered magnetic nanowires with perpendicular magnetic anisotropy were fabricated on the surfaces of Si wafers by ion-beam sputtering. The nanowires had different sized widths and pinning sites formed by an anodic oxidation method via scanning probe microscopy (SPM) with an MFM tip. The magnetic domain structure was changed by an anodic oxidation method. To discover the current-induced DW motion in the Co/Pd nanowires, we employed micromagnetic modeling based on the Landau-Lifschitz-Gilbert (LLG) equation. The split DW motions and configurations due to the edge effects of pinning site and nanowire appeared.     

Effect of magnetic anisotropy on the mobility of the boundaries in thin magnetic films

This paper discusses the effect of magnetic crystallographic anisotropy on the mobility of the domain walls in thin magnetic films with an easy axis in the plane of the film. It shows that the stable configuration of a domain wall is a single-vortex Bloch domain wall. Besides this, there are two metastable states of the domain wall—a Néel domain wall and a domain wall with two magnetic vortices along the normal to the plane of the film. It is also shown that the mobilities of the single-vortex and Néel domain walls and the domain wall with two vortices decrease as the anisotropy constant increases and tend to the same value. Pis’ma Zh. Tekh. Fiz. 24, 42–46 (January 26, 1998)     

Electrodeposited amorphous Co-P alloys with variable magnetic anisotropy

A method is indicated for obtaining electrodeposited amorphous Co-P alloys with differing anisotropies, employing an alternating current density of very low frequency. The method allows samples of a high degree of permeability to be obtained with the result that, in the case of a current density at 5 Hz with amplitude between +0.6 and -0.2 A/cm², the magnetic properties are similar to those found by Dietz and coworkers, in samples deposited on rotating cylindrical substrates. In the case of samples without perpendicular anisotropy, there can be observed a distribution of the magnetization in the plane, with 180° domains.     

Perpendicular magnetic recording with a composite anisotropy film

For a recently proposed perpendicular recording system, a composite anisotropy medium has been developed to improve the recording sensitivity of the perpendicular recording head. The medium is composed of a Fe-Ni soft magnetic film and a Co-Cr perpendicular anisotropy film, which are successively deposited on a base by an r. f. sputtering. By using the new double layer medium, an extremely high recording sensitivity could be obtained, compared with the single layer Co-Cr medium. The recording current needed to saturate the double layer film decreased to one-tenth of that for the single layer Co-Cr film. Although the Fe-Ni layer was soft magnetic material, neither deterioration of the frequency response nor peak shift was observed for the double layer film. The reproduction with a perpendicular head was also investigated, and a high output voltage and a high signal-to-noise ratio were obtained.     

Thermal stability and magnetic anisotropy of nickel nanoplates

The thermal stability and magnetic anisotropies of nickel nanoplates with {111} planes as the exposure plane are studied. The melting point of Ni nanoplates drastically drops as compared to that of the bulk one due to the significant increase in the surface free energy. For the large aspect ratio, these nanoplates tend to lie flat on silicon wafer and form a thin Ni {111} plane film. Both the coercivity and the remnant magnetization of the Ni film deeply depend on the applied field direction. As the angle between the film plane and the applied field direction varies from zero to 45° and to 90°, the coercivity measured at 5 K increases from 335 Oe to 373 and to 410 Oe. Correspondingly, the remnant magnetization decreases from 18.1 to 15.8 to be 10.4 emu/g.     

Oblique-field annealing effect for in-plane magnetic anisotropy ofsoft magnetic Co-Nb-Zr thin films

The effect of annealing in a magnetic field applied obliquely to the surface of soft magnetic thin films has been investigated. This annealing method was found to be extremely effective to control in-plane magnetic anisotropy without a change of annealing temperature and to suppress local anisotropy dispersion. For sputtered amorphous Co_85.5Nb_8.9Zr_5.6 thin films, it has been found that the in-plane uniaxial anisotropy energy was varied from 250 J/m³ to near zero with the coercive force H_c less than 6.5 A/m by changing the oblique-field annealing angle α. Experimental values of in-plane anisotropy energy agreed well with calculated ones predicted from α and intrinsic anisotropy induced by quasi-directional ordering. These films showed good high frequency characteristics for applying to miniaturized inductive devices