P. K. J. Wong

Spin and orbital moments of nanoscale Fe3O4 epitaxial thin film on MgO/GaAs(100)

Nanoscale Fe3O4 epitaxial thin film has been synthesized on MgO/GaAs(100) spintronic heterostructure, and studied with X-ray magnetic circular dichroism. We have observed a total magnetic moment (ml+s) of (3.32 ± 0.1)μB/f.u., retaining 83% of the bulk value. Unquenched orbital moment (ml) of (0.47 ± 0.05)μB/f.u. has been confirmed by carefully applying the sum rule. The results offer direct experimental evidence of the bulk-like total magnetic moment and a large orbital moment in the nanoscale fully epitaxial Fe3O4/MgO/GaAs(100) heterostructure, which is significant for spintronics applications.

Hybrid Spintronic Structures With Magnetic Oxides and Heusler Alloys

Hybrid spintronic structures, integrating half-metallic magnetic oxides and Heusler alloys with their predicted high spin polarization, are important for the development of second-generation spintronics with high-efficient spin injection. We have synthesized epitaxial magnetic oxide Fe 3O 4 on GaAs(100) and the unit cell of the Fe3O4 was found to be rotated by 45deg to match the gallium arsenide GaAs. The films were found to have a bulk-like moment down to 3-4 nm and a low coercivity indicating a high-quality magnetic interface. The magnetization hysteresis loops of the ultrathin films are controlled by uniaxial magnetic anisotropy. The dynamic response of the sample shows a heavily damped precessional response to the applied field pulses. In the Heusler alloy system of Co2MnGa on GaAs, we found that the magnetic moment was reduced for thicknesses down to 10 nm, which may account for the lower than expected spin-injection efficiency from the spin-light-emitting diode structures. Using the element-specific technique of X-ray magnetic circular dichroism (XMCD), the reduced spin moments were found to originate from the Mn rather than the Co atoms, and the improvement of the interface is thus needed to increase the spin injection efficiency in this system. Further studies of the I-V characteristics of Fe3O4/GaAs(100) and Fe3O4/MgO/GaAs(100) show that the Fe3 O4 -based spintronic structures have a well-defined Schottky or tunneling barrier of moderate barrier height, which is encouraging for high-efficient spin injection.

The interface effect of the magnetic anisotropy in ultrathin epitaxial Fe3O4 film

The magnetic anisotropy of epitaxial layered structures of Fe3O4(tFe=4nm)∕GaAs(100), MgO(3nm)∕Fe3O4(tFe=4nm)∕GaAs(100), and Fe3O4(tFe=4nm)∕MgO(3nm)∕Fe3O4(tFe=4nm)∕GaAs(100) was studied by ferromagnetic resonance. It was shown that a predominant in-plane uniaxial magnetic anisotropy and a small fourfold cubic magnetocrystalline anisotropy existed. The in-plane uniaxial anisotropy constant decreased when the MgO layer was covered on Fe3O4∕GaAs, while the cubic anisotropy increased. In the sandwich structures, two resonance peaks were observed. One is similar to that in Fe3O4∕GaAs layer, while another corresponding to Fe3O4 on MgO showed more remarkable fourfold anisotropy and lower uniaxial anisotropy due to smaller mismatch between Fe3O4 and MgO. The interface the Fe3O4 layer is deposited on has dominant effect.

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

The influence of Nd dopants on spin and orbital moments in Nd-doped permalloy thin films

Magnetic properties of NdX-Ni80Fe20(1−X) thin films have been investigated using x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at room temperature. With the Nd concentration increasing, the ratio of orbital-to-spin moment of Ni and Fe increases significantly, indicating that the spin-orbit coupling in permalloy thin films is enhanced due to the Nd impurities. The spin and orbital moments have been obtained by the sum rules analysis, which shows that the Nd impurities lead to a strong dispersion of spin moments of Fe and Ni while have no effect on orbital moments in Nd-doped permalloy thin films. Element-specific XMCD hysteresis loops suggest an antiferromagnetic coupling between the magnetic moments of Nd and permalloy at room temperature. The static magnetic properties have been studied by vibrating sample magnetometer for comparison, which shows a nice agreement with the XMCD results.

Engineering Gilbert damping by dilute Gd doping in soft magnetic Fe thin films

By analyzing the ferromagnetic resonance linewidth, we show that the Gilbert damping constant in soft magnetic Fe thin films can be enhanced by ∼6 times with Gd doping of up to 20%. At the same time, the magnetic easy axis remains in the film plane while the coercivity is strongly reduced after Gd inclusion. X-ray magnetic circular dichroism measurements reveal a strong increase in the orbital-to-spin moment ratio of Fe with increasing Gd concentration, in full agreement with the increase in the Gilbert damping obtained for these thin films. Combined with x-ray diffraction and vibrating sample magnetometry, the results demonstrate that the FeGd thin films with dilute Gd doping of up to 20% are promising candidates for spin-transfer-torque applications in soft magnetic devices, in which an enhanced damping is required.

Influence of Capping Layers on Magnetic Anisotropy in Fe/MgO/GaAs(100) Ultrathin Films

Magnetic anisotropies of Fe/MgO/GaAs(100) hybrid structure with two different nonmagnetic capping materials, Au and MgO have been studied by ferromagnetic resonance (FMR). A uniaxial anisotropy, unexpected from the crystal structure was observed in the ultrathin films for both capping materials. Its global easy axis is along [0-11] direction while two (010) directions are equally magnetic hard regardless of the overlayer material. The in-plane uniaxial anisotropy (in-plane cubic anisotropy) of the Au capped samples is stronger (weaker) than that of the MgO capped ones within the range of t Fe = 7.1 to 28 ML. This suggests that the MgO overlayer suppresses the uniaxial anisotropy faster than the Au overlayer.

Spin-injection device prospects for half-metallic Fe3O4:Al0.1Ga0.9As interfaces

Electrical spin-injection across the Fe3O4:Al0.1Ga0.9As interface has been measured. We quantify this effect in an In0.2Ga0.8As:GaAs spin-light emitting diode optical device. The optical polarization signal is maintained from 4.2 up to 200 K without influence of the metal–insulator Verwey transition in the bulk of the Fe3O4 film. An incomplete oxidation at the interface may be detrimental for this device, as it has a similar spin-injection efficiency to that of Fe:Al0.1Ga0.9As. Ambient temperature operation of this device may be possible although the present polarization levels remain too low for practical spintronic applications. We demonstrate the first step in the integration of molecular beam epitaxy-grown magnetic oxides into III–V semiconductor devices.

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.

Observation of current-driven oscillatory domain wall motion in Ni80Fe20/Co bilayer nanowire

Direct observation of current-driven oscillatory domain wall motion above the Walker breakdown by x-ray magnetic circular dichroism in photoemission electron microscopy is reported in Ni80Fe20/Co nanowire, showing micrometer-scale displacement at ∼13 MHz. We identify two key factors that enhance the oscillatory motion: (i) increase of the hard-axis magnetic anisotropy field value |H⊥| and (ii) increase of the ratio between non-adiabatic spin-transfer parameter to the Gilbert damping coefficient, β/α, which is required to be larger than 1. These findings point to an important route to tune the long-scale oscillatory domain wall motion using appropriate geometry and materials.