G. J. Mankey

The growth of nanoscale structured iron films by glancing angle deposition

Chiral nanostructured thin films can be produced through precise control of the angle of incidence of a vapor flux concurrent with substrate rotation. The technique has been employed to create unique porous iron thin film structures on Si(100) with a columnar microstructure. Scanning electron microscopy images illustrate columnar iron films produced with azimuthal rotation during sample growth with the incident flux at an angle of 75° with respect to the surface normal. The columns were found to be well isolated with a narrow distribution of diameters, resulting in aspect ratios of approximately 8 to 1. Hysteresis loops reveal the columnar growth induced a large magnetic shape anisotropy relative to that observed for an iron film grown with normal incidence. The evolution of the columnar microstructure was followed from simple oblique deposition (no substrate rotation), giving a fibrous slanted microstructure, to high-speed rotation where a broad size distribution of highly faceted columnar structures was observed. The measured microstructure is related to the observed magnetic properties.Chiral nanostructured thin films can be produced through precise control of the angle of incidence of a vapor flux concurrent with substrate rotation. The technique has been employed to create unique porous iron thin film structures on Si(100) with a columnar microstructure. Scanning electron microscopy images illustrate columnar iron films produced with azimuthal rotation during sample growth with the incident flux at an angle of 75° with respect to the surface normal. The columns were found to be well isolated with a narrow distribution of diameters, resulting in aspect ratios of approximately 8 to 1. Hysteresis loops reveal the columnar growth induced a large magnetic shape anisotropy relative to that observed for an iron film grown with normal incidence. The evolution of the columnar microstructure was followed from simple oblique deposition (no substrate rotation), giving a fibrous slanted microstructure, to high-speed rotation where a broad size distribution of highly faceted columnar structures was...

Lattice symmetry and magnetization reversal in micron-size antidot arrays in Permalloy film

The magnetization reversal in four arrays of micron-size circular holes (antidots) in a Permalloy film has been studied by means of quantitative magneto-optic Kerr vector magnetometry and magnetic force microscopy. The primitive antidot meshes of the arrays investigated here can be classified as square, rectangular, hexagonal, and oblique. The vector magnetometry data show that the hole arrays induce a magnetic anisotropy completely different from that of the unpatterned film, with new hard axes along the directions connecting nearest neighboring holes. Also the coercive field is strongly affected by the pattern. The results of the vector magnetometry analysis indicate that the reversal process takes place through a collective and periodic domain nucleation and expansion process. The domain structure in the remanent state has been investigated by magnetic force microscopy imaging. The images display well-defined domain structures, which are periodic and commensurate with the holes array.

The magnetic anisotropy and domain structure of permalloy antidot arrays

The magnetic properties of antidot arrays in permalloy films were studied using magnetic force microscopy (MFM), the magneto-optical Kerr effect (MOKE), and torque magnetometry. New observations of the magnetic domain structure, magnetization reversal process, and magnetic anisotropy are presented. Magnetic domains were imaged during magnetization reversal to identify the magnetization switching processes with the field applied along the diagonal and the edge of the hole mesh. A four-fold anisotropy related to the confinement of domains by the hole mesh was observed by torque magnetometry at intermediate fields.The magnetic properties of antidot arrays in permalloy films were studied using magnetic force microscopy (MFM), the magneto-optical Kerr effect (MOKE), and torque magnetometry. New observations of the magnetic domain structure, magnetization reversal process, and magnetic anisotropy are presented. Magnetic domains were imaged during magnetization reversal to identify the magnetization switching processes with the field applied along the diagonal and the edge of the hole mesh. A four-fold anisotropy related to the confinement of domains by the hole mesh was observed by torque magnetometry at intermediate fields.

Dipolar induced, spatially localized resonance in magnetic antidot arrays

Dipole induced, spatially localized ferromagnetic resonances (at 35 GHz) are observed in micron-sized antidot arrays in permalloy films fabricated with photolithography. All square (3 μm×3 μm) and rectangular (3 μm×4, 5, and 7 μm) array samples exhibit double resonances, with each resonance possessing uniaxial in-plane anisotropy. Interestingly, the easy axes of the two resonances are orthogonal in all cases. The magnitude of the induced dipolar anisotropy decreases with increasing rectangular aspect ratio for one of the resonances, but remains essentially constant for the other. Micromagnetic simulations reveal that the two resonance peaks are the consequence of a dipole field distribution producing two areas with distinctly different demagnetizing field patterns.

Effect of interface roughness on the exchange bias for NiFe/FeMn

The effect of interface roughness on exchange bias for NiFe/FeMn bilayers is investigated for polycrystalline films and epitaxial films. Three different systems were investigated: polycrystalline Ta (10 nm)/Ni80Fe20 (10nm)/Fe50Mn50 (20 nm) films on oxygen plasma-etched Si(100) or Cu/H–Si(100) and epitaxial Ni80Fe20 (10nm)/Fe60Mn40 (20 nm) films on Cu/H–Si(110). For films grown on plasma-etched substrates, as the etching time is increased, film roughness increases up to 12 nm. For the polycrystalline films grown on ultrathin Cu underlayers, x-ray diffraction shows the fcc (111) texture is greatly reduced as the thickness is increased. The epitaxial Cu/Si(110) buffer layer induces fcc (111) epitaxial growth and modifies the interface morphology. The dependence of exchange bias on roughness for each set of samples is explained in terms of a competition between the interfacial exchange coupling and the af uniaxial anisotropy.

NONCOLLINEAR MAGNETISM IN SUBSTITUTIONALLY DISORDERED FACE-CENTERED-CUBIC FEMN

We use first principles electronic structure techniques to study the magnetic structure of γ-FeMn using the Korringa–Kohn–Rostocker multiple-scattering approach in conjunction with an extension of the single site coherent potential approximation to noncollinear magnetic structures. Our results show that the noncollinear 3Q and 2Q structures are both stable solutions with the former being slightly lower in energy. The collinear solutions could only be converged in a traditional spin-polarized calculation and are unstable in a noncollinear treatment.

Magnetic and structural properties of MnBi multilayered thin films

Magnetic and structural properties of MnBi films with thicknesses up to 50 nm were investigated. Thin films of the MnBi LTP (Low Temperature Phase) were fabricated onto silica-glass substrates by sputter-deposition of Bi/Mn multilayer, followed by a subsequent annealing at about 550 °C for 30 min. Coercivity of such thin films is higher than 15 kOe, even though the film thickness is about 10 nm. These thin films show the preferential growth of c-axis of the LTP along the film normal. Moreover, high resolution transmission electron microscopy indicates that the LTP regions of 30–50 nm in size are physically isolated by Bi. The magnetization reversal mechanism of such a LTP region is mainly governed by a coherent rotation mode based on the δM curve measurement.

Electronic states in magnetic nanostructures

Abstract The electronic states responsible for oscillatory magnetic coupling, giant magnetoresistance (GMR), and spin-polarized tunneling are explored. They occupy well-defined locations in ( E, k ) space. Their energy E has to be within a few kT of the Fermi level, a range that is now becoming accessible to high-resolution photoemission. Particular attention is paid to k -regions near the Fermi level crossings of the s, p-band, where a sizable group velocity is combined with a d-like magnetic splitting and spin-polarization. These electronic states can be tailored by quantization in structures with single-digit nanometer dimensions, such as two-dimensional quantum wells and one-dimensional arrays of stripes and dots. Such arrays can be produced by self-assembly on top of stepped silicon surfaces.

Spin injection into carbon nanotubes and a possible application in spin-resolved scanning tunnelling microscopy

We present magnetotransport measurements on multiwall nanotubes contacted by ferromagnetic electrodes that are 1 µm apart. The observed magnetoresistance of 2.2% is consistent with a spin-scattering length of at least 380 nm. We further discuss a novel concept for spin-resolved scanning tunnelling microscopy (SR-STM) with a carbon-nanotube tip. The concept relies on a long spin-scattering length as well as on spin-dependent scattering at the interfaces to the ferromagnetic electrodes. The observed magnitude of the magnetoresistance effect and the distance between the electrodes are large enough for application in a SR-STM.

Strong uniaxial magnetic anisotropy in CoFe films on obliquely sputtered Ru underlayer

Co90Fe10 films with an in-plane uniaxial magnetic anisotropy have been grown on an obliquely sputtered thin Ru underlayer. The anisotropy field can be increased up to 200 Oe. The hysteresis curves show a very high squareness in the easy axis direction and almost no hysteresis in the hard axis direction, suggesting that the induced uniaxial anisotropy is uniform throughout the films. The switching characteristics of the nanoelements fabricated from the films by e-beam lithography are also investigated. There is no degradation of the magnetic anisotropy after the annealing and lithographical process.