D. X. Niu

Magnetic Domain Wall Formation in Ferromagnetic Wires With a Nanoconstriction

The magnetization distribution and the formation of magnetic domain wall (DW) in ferromagnetic metal wires with a nanoscale constriction have been investigated in details using the micromagnetic simulation. It is found that the angle of the nanoconstriction plays an important role in controlling the formation of the magnetic DW. For different ferromagnetic metals, NiFe, Ni, Fe and Co, the domain structures and formation of the DW are also found to be distinctly different. In the NiFe wires, the optimum constriction angle for a well defined head-to-head/tail-to-tail DW is around 10deg while in Ni, it is around 14deg. For large constriction angles in Fe and Co wires, the magnetizations across the nanocontact tend to align along the same direction without a DW. However, Fe and Co wires tend to form complex vortex magnetic domains or single domains in the wires and across the nanocontacts in sharp contrast with the NiFe and Ni wires of the same shape and size

Anisotropic magnetization reversal in 30 nm triangular FeNi dots

We present the room temperature magnetization hysteresis loop measurements of a triangularly shaped Fe64Ni36 dot array with the feature size down to 30 nm using magneto-optical Kerr effect. An in-plane anisotropic magnetization reversal and an enhanced coercivity have been observed in these magnetic dots. In combining with micromagnetic calculations, we found that the magnetization process follows two steps, the rotation of the top corner and the switching of the bottom base, respectively, controlled by the nanometer scale local magnetic shape anisotropy, and the thermal activation is negligible even in this length scale.

Reduction of In-Plane Uniaxial Magnetic Anisotropy in Patterned Single-Crystal Fe Dot Arrays

Single-crystal Fe dot arrays with the lateral size varying from 400 nm to 50 nm are fabricated by focused ion beam (FIB) direct writing from a single-crystalline 10 monolayer (ML) Fe (100) continuous thin film grown on GaAs substrate. The Kerr hysteresis loops of both dot arrays and continuous thin film are measured by focused magnetooptical Kerr effect (MOKE) measurements along four major crystal directions: [0-11] [010] [011] and [001]. It is found that the in-plane uniaxial anisotropy has been greatly reduced down to zero in the dot arrays when the size is less than 150 nm. The micromagnetic simulations confirm the reduction of this intrinsic in-plane uniaxial anisotropy in the patterned dots by separating the effect of the shape anisotropy. The experimental and simulation results further indicate an additional magnetic uniaxial anisotropy along the [010] direction.

Ultrafast optically induced spin dynamics in patterned single crystal Fe dot arrays

Optically induced ultrafast spin dynamics in nanoscale single crystal Fe dot arrays have been investigated using time-resolved magneto-optical Kerr effect (MOKE) pump-probe measurements. An ultrafast demagnetization process has been found to occur in the first 400fs after the excitation with the femtosecond laser pulses for both the patterned dot arrays and the continuous film. While in the continuous Fe film the magnetization recovers monotonically with no precession, an optically induced spin precession has been observed in the patterned dot array with the precession frequency and the damping dependent on bias field strength. This result demonstrates that the modified magnetic anisotropies, as shown by the static focused MOKE measurements, in the patterned nanostructures can give rise to the optically induced spin precession.

Probing the Buried Magnetic Interfaces.

Understanding magnetism in ferromagnetic metal/semiconductor (FM/SC) heterostructures is important to the development of the new-generation spin field-effect transistor. Here, we report an element-specific X-ray magnetic circular dichroism study of the interfacial magnetic moments for two FM/SC model systems, namely, Co/GaAs and Ni/GaAs, which was enabled using a specially designed FM1/FM2/SC superstructure. We observed a robust room temperature magnetization of the interfacial Co, while that of the interfacial Ni was strongly diminished down to 5 K because of hybridization of the Ni d(eg) and GaAs sp(3) states. The validity of the selected method was confirmed by first-principles calculations, showing only small deviations (<0.02 and <0.07 μB/atom for Co/GaAs and Ni/GaAs, respectively) compared to the real FM/SC interfaces. Our work proved that the electronic structure and magnetic ground state of the interfacial FM2 is not altered when the topmost FM2 is replaced by FM1 and that this model is applicable generally for probing the buried magnetic interfaces in the advanced spintronic materials..

Discontinuous properties of current-induced magnetic domain wall depinning

The current-induced motion of magnetic domain walls (DWs) confined to nanostructures is of great interest for fundamental studies as well as for technological applications in spintronic devices. Here, we present magnetic images showing the depinning properties of pulse-current-driven domain walls in well-shaped Permalloy nanowires obtained using photoemission electron microscopy combined with x-ray magnetic circular dichroism. In the vicinity of the threshold current density (Jth = 4.2 × 1011 A.m−2) for the DW motion, discontinuous DW depinning and motion have been observed as a sequence of “Barkhausen jumps”. A one-dimensional analytical model with a piecewise parabolic pinning potential has been introduced to reproduce the DW hopping between two nearest neighbour sites, which reveals the dynamical nature of the current-driven DW motion in the depinning regime.

Investigations on magnetic properties in ultrathin single crystal Fe rectangular arrays patterned by selective wet-etching

The magnetic properties of the rectangular element arrays of the ultrathin epitaxial Fe films on GaAs (100) have been studied with focused magneto-optical Kerr effect and ferromagnetic resonance. The coercivity increases upon decreasing aspect ratio of the elements. When the magnetic field is along the [0–11] direction of the GaAs substrate, which is the long edge of the element, a high remanent ratio of all the hysteresis loops is shown, indicating a global magnetic easy axis and a single domain remanent state, while along the [001] direction the breakage of the single domain remanent state occurs for the element with the aspect ratio lower than 2. These results are attributed to the competition among the intrinsic anisotropies, the demagnetizing field, and interelement dipole coupling as determined by the ferromagnetic resonance measurements.

Focused MOKE Study of Self-Assembly Nanoscale Fe

Using nanosphere lithography (NSL) technique, we have fabricated well defined triangular shaped Fe64Ni36 dot arrays with feature size down to 30 nm on silicon substrate. In-plane focused magneto-optical Kerr effect (MOKE) measurements show that these nanoscale dot arrays have an enhanced coercivity of around 200 Oe compared with that of the continuous thin film of 50 Oe. In combining with micromagnetic calculations, we found that the magnetization process follows two steps, the rotation of the top corner and then a switching of the bottom base, which controls the coercivity of the dots.

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A technique has been developed to pattern single crystal ultrathin Fe films by selective chemical wet etching of the Au capping layer and then simultaneous oxidization of the ferromagnetic Fe layer underneath. The focused magneto-optical Kerr effect and ferromagnetic resonance measurements demonstrate that the intrinsic magnetic anisotropy has not been changed in the patterned elements, showing that the chemical bonding at the metal–semiconductor interface remains the same. Further x-ray energy dispersive spectroscopy measurements show that this selective wet-etching technique is suitable for the patterning of thin Fe films with thicknesses less than around 25 ML.

Ni

Patterned magnetic thin films of NiFeCo(3nm)∕Cu(3nm)∕NiFeCo(6nm) spin valve structure with arrays of rectangular elements of micron and submicron sizes have been studied by ferromagnetic resonance. All the rectangular elements have the same aspect ratio of 12 but with different sizes. A set of evenly spaced peaks was observed when the magnetic field was applied near the film normal. The resonant fields of these peaks are in linear proportion with the peak number, and the separation of resonant fields between peaks are approximately inversely proportional to the width of the elements, which suggests that spin waves are excited in the film plane and along the short edge of the elements.Patterned magnetic thin films of NiFeCo(3nm)∕Cu(3nm)∕NiFeCo(6nm) spin valve structure with arrays of rectangular elements of micron and submicron sizes have been studied by ferromagnetic resonance. All the rectangular elements have the same aspect ratio of 12 but with different sizes. A set of evenly spaced peaks was observed when the magnetic field was applied near the film normal. The resonant fields of these peaks are in linear proportion with the peak number, and the separation of resonant fields between peaks are approximately inversely proportional to the width of the elements, which suggests that spin waves are excited in the film plane and along the short edge of the elements.