Scanning Magneto-optical Kerr Effect Research Articles

Femtosecond Laser Ablation-Induced Magnetic Phase Transformations in FeRh Thin Films

In this study, we present a novel investigation into the magnetic and morphological properties of equiatomic B2-ordered FeRh thin films irradiated with single high-intensity ultrashort laser pulses. The goal is to elucidate the effect of femtosecond laser ablation on the magnetic properties of FeRh. We employed Scanning Magneto-Optical Kerr Effect (S-MOKE) microscopy to examine the magnetic phase after laser processing, providing high spatial resolution and sensitivity. Our results for the first time demonstrated the appearance of a magneto-optical signal from the bottom of ablation craters, suggesting a transition from antiferromagnetic to ferromagnetic behavior. Fluence-resolved measurements clearly demonstrate that the ablation threshold coincides with the threshold of the antiferromagnet-to-ferromagnet phase transition. The existence of such a magnetic phase transition was independently confirmed by temperature-dependent S-MOKE measurements using a CW laser as a localized heat source. Whereas the initial FeRh film displayed a reversible antiferromagnet-ferromagnet phase transition, the laser-ablated structures exhibited irreversible changes in their magnetic properties. This comprehensive analysis revealed the strong correlation between the femtosecond laser ablation process and the magnetic phase transformation in FeRh thin films.

Scanning magneto-optical Kerr effect (MOKE) measurement with element-selectivity by using a soft x-ray free-electron laser and an ellipsoidal mirror

The scanning magneto-optical Kerr effect (MOKE) method was developed by using a soft x-ray free-electron laser to visualize magnetic microstructures with element-selectivity. For focusing optics of soft x-rays, an ellipsoidal mirror with an achromatic property was utilized to obtain resonant MOKE spectra for various elements with a simple experimental setup. We performed a scanning MOKE experiment at Fe M-edge with a spatial resolution of 7 μm. This system is readily applicable to ultrafast MOKE experiments combined with a synchronized optical laser for spintronics research.

Three-dimensional frequency- and phase-multiplexed magneto-optical microscopy.

We describe a new approach to scanning magneto-optical Kerr effect (MOKE) microscopy in which two opto-mechanical choppers modulate the spatial profile of a probe laser beam to separately encode all three magnetization components at different frequencies and phases in one signal. We demonstrate this multiplexed technique in two representative regimes: the equilibrium and non-equilibrium response of a magnetic vortex to a changing magnetic field. We observe the translation of the vortex state in equilibrium and the spiraling gyrotropic trajectory of the vortex position out of equilibrium. We compare the results to a traditional MOKE measurement and to micromagnetic simulations. We find that the multiplexed method presented here provides better agreement with simulation than previous methods and equal or better signal-to-noise ratio.

Surface Morphology and Magnetic Properties of Isolated Cylindrical Nickel Nanowires

In this article, the surface morphology and magnetic properties of isolated cylindrical nickel (Ni) nanowires fabricated by electrodeposition have been thoroughly investigated using scanning electron microscopy and Magneto-Optical Kerr Effect (MOKE) magnetometry, respectively. The surfaces of most nanowires were found to be homogenous, uniform, and cylindrical in shape. Some others show different diameters and surface features, including; protrusions and branches along their length. The diameter distribution of a wide range of nanowires was found to differ from their template pore diameters. These all variations are more likely due to defects exist in the internal surfaces of the pores within the template itself, or may be associated with the trapped air pockets within the pores during nanowires growth or due to the oxide formation or residual contaminants which may cover these wires. The nanowires lengths were found to differ from their actual lengths estimated during deposition growth. This was attributed to the breakage of nanowires into small sections during releasing process. The hysteresis loops obtained by applying a magnetic field at different angles with respect to the nanowires long axis showed square hysteresis loops with a sharp jump of Kerr signal during switching behaviour, as well as a high squareness ratio, indicating the dominance of shape anisotropy. These results are quite different from the measurements of high density templated nanowires reported in the literature, due to the small number of nearest neighbour nanowires, and hence no magneto-static interaction. The magnetisation reversal of such wires is well described by the non-uniform rotation of the curling model of domain reversal.

Ferromagnetic resonance investigation of magnetic anisotropy of ion-synthesized iron-silicide thin films

40 keV iron ions have been implanted into single-crystal silicon wafers at room temperature. As a result, thin iron-silicide films were synthesized in the near-surface region of substrates. Scanning magnetooptical Kerr effect studies showed that some films had the uniaxial magnetic anisotropy. The coercive force determined in the direction of the easy magnetization for films with the magnetic anisotropy increased with the increase in the implantation fluence. However, ferromagnetic samples became isotropic when the dose of the order of 2.6×1017 cm–2 was reached. Ferromagnetic resonance studies showed that the resonance linewidth in isotropic samples increased with the temperature decrease. It was found that the ferromagnetic resonance linewidth for samples with the uniaxial anisotropy was less than that for isotropic ones. The observed behavior of resonance spectra can be explained on the basis of the model taking into account the effect of thermal fluctuations of the resonance line shape in disperse ferromagnets.

Ultrafast supercontinuum fiber-laser based pump-probe scanning magneto-optical Kerr effect microscope for the investigation of electron spin dynamics in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution

We describe a two-color pump-probe scanning magneto-optical Kerr effect microscope which we have developed to investigate electron spin phenomena in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution. The key innovation of our microscope is the usage of an ultrafast "white light" supercontinuum fiber-laser source which provides access to the whole visible and near-infrared spectral range. Our Kerr microscope allows for the independent selection of the excitation and detection energy while avoiding the necessity to synchronize the pulse trains of two separate picosecond laser systems. The ability to independently tune the pump and probe wavelength enables the investigation of the influence of excitation energy on the optically induced electron spin dynamics in semiconductors. We demonstrate picosecond real-space imaging of the diffusive expansion of optically excited electron spin packets in a (110) GaAs quantum well sample to illustrate the capabilities of the instrument.

Voltage-gated modulation of domain wall creep dynamics in an ultrathin metallic ferromagnet

The influence of gate voltage, temperature, and magnetic field on domain wall (DW) creep dynamics is investigated in Pt/Co/gadolinium oxide (GdOx) films with perpendicular magnetic anisotropy and imaged by a scanning magneto-optical Kerr effect technique. The DW creep velocity can be controlled by an electric field applied to the Co/GdOx interface via a linear modulation of the activation energy barrier with gate voltage. At low speeds, the DW velocity can be changed significantly by a gate voltage, but the effect is diminished as the DW velocity increases, which limits electric field control of fast DW motion.

Scanning MOKE investigation of ion-beam-synthesized silicide films

Fe ions with an energy of 40keV were implanted into Si plates with the fluence varying in the range of (1.6–3.0)×1017ion/cm2 in the external magnetic field. Scanning magnetooptical Kerr effect (MOKE) studies have shown that all samples possess uniaxial anisotropy. Both the coercive field and the anisotropy field increase with fluence. It was suggested that induced anisotropy is caused by inverse magnetostriction.

Magnetization switching of CoFeSiB free-layered magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprising amorphous Co 70.5Fe 4.5Si 15B 10, possessing low saturation magnetization of 560 emu/cm 3, as a free layer have been investigated. The switching behaviours were confirmed for the micrometer-sized elements experimentally, using the scanning magneto-optical Kerr effect (scanning MOKE). A micromagnetic modelling study was also carried out for the submicrometer-sized elements. By using either a CoFeSiB single or a synthetic antiferromagnetic free-layer structure, the magnetization switching field became much lower than conventionally used CoFe free layered MTJs.

Domain wall trapping in controlled submicron ferromagnetic elements

The domain configuration in permalloy wires (30 nm thick, 10 μm wide, and 205 μm long) with a wide size range of a narrow central bridge (5 μm long and w μm wide; 0.5⩽w⩽10 μm) were investigated in both their demagnetized and remanent states using magnetic force microscopy and the results were confirmed by micromagnetic calculations. At the bridge region, domain walls were found to be shifted by a small external field. Scanning magneto-optical Kerr effect revealed that the coercivity in these structures are the same as that in a straight wire, suggesting that domain wall movement is the dominant process in the magnetization reversal of these structures.