Magneto-optical Kerr Effect Setup Research Articles

Fourier-space generalized magneto-optical ellipsometry

The magneto-optical Kerr effect (MOKE) is widely exploited in laboratory-based setups for the study of thin films and nanostructures, providing magnetic characterization with good spatial and temporal resolutions. Due to the complex coupling of light with a magnetic sample, conventional MOKE magnetometers normally work by selecting a small range of incident wave-vector values, focusing the incident light beam to a small spot, and recording the reflected intensity at that angular range by means of photodetectors. Using this approach, additional methodologies and measurements are required for full vectorial magnetic characterization. Here, we computationally investigate a Fourier-space MOKE setup, where a focused beam ellipsometer using high numerical aperture optics and a camera detector is employed to simultaneously map the intensity distribution for a wide range of incident and reflected wave vectors. We employ circularly incident polarized light and no analyzing optics, in combination with a fitting procedure of the light intensity maps to the analytical expression of the Kerr effect under linear approximation. In this way, we are able to retrieve the three unknown components of the magnetization vector as well as the material' s optical and magneto-optical constants with high accuracy and short acquisition times, with the possibility of single-shot measurements. Fourier MOKE is thus proposed as a powerful method to perform generalized magneto-optical ellipsometry for a wide range of magnetic materials and devices.

Shaping the spin wave spectra of planar 1D magnonic crystals by the geometrical constraints

We present experimental and numerical studies demonstrating the influence of geometrical parameters on the fundamental spin-wave mode in planar 1D magnonic crystals. The investigated magnonic crystals consist of flat stripes separated by air gaps. The adjustment of geometrical parameters allows tailoring of the spin-wave frequencies. The width of stripes and the width of gaps between them affect spin-wave frequencies in two ways. First, directly by geometrical constraints confining the spin waves inside the stripes. Second, indirectly by spin-wave pinning, freeing the spin waves to a different extent on the edges of stripes. Experimentally, the fundamental spin-wave mode frequencies are measured using an all-optical pump-probe time-resolved magneto-optical Kerr-effect setup. Our studies address the problem of spin-wave confinement and spin-wave dipolar pinning in an array of coupled stripes. We show that the frequency of fundamental mode can be tuned to a large extent by adjusting the width of the stripes and the width of gaps between them.

Experimental exploration of the vector nature of the dynamic order parameter near dynamic magnetic phase transitions

We have devised an experimental study to explore the vector nature of the dynamic order parameter $Q$ and the corresponding dynamic fluctuation ${\mathbit{\ensuremath{\sigma}}}^{Q}$ and susceptibility ${\mathbit{\ensuremath{\chi}}}^{Q}$ tensors in the vicinity of the dynamic phase transition (DPT). For this purpose, we have fabricated epitaxial Co and CoRu thin films with ($10\underline{1}0$) surface orientation and associated in-plane uniaxial anisotropy to mimic the behavior of an anisotropic Heisenberg model in the absence of magnetostatic interactions. Specifically, we measured the time-dependent magnetization component parallel to the externally applied magnetic field for different orientations of the magnetic easy axis (EA) while exploring the dynamic phase space defined by the applied oscillatory field amplitude ${H}_{0}$ and an additionally applied time-independent bias field ${H}_{b}$. The magnetization dynamics was hereby observed by utilizing an ultrasensitive transverse magneto-optical Kerr effect setup that allowed for real-time observations of the magnetic state evolution. Our experimental results demonstrate that while the position of the critical point changes upon rotating the EA away from the field axis, all qualitative aspects of the dynamic phase diagram close to the critical point are unchanged and the DPT can be universally observed. Also, metamagnetic anomalies occur for all measurement conditions. All our results are in agreement with a domain nucleation initiated magnetization reversal process and indicate that the overall dynamic state is dominated by the magnetization component parallel to the EA, so that a change in orientation in between the field axis and EA leads to a rather trivial rotation of the $Q$ vector, which simply follows the EA.

All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances.

In this paper, we report all-optical manipulation of magnetization in ferromagnetic Co/Pt thin films enhanced by plasmonic resonances. By annealing a thin Au layer, we fabricate large-area Au nanoislands on top of the Co/Pt magnetic thin films, which show plasmonic resonances around the wavelength of 606 nm. Using a customized magneto-optical Kerr effect setup, we experimentally observe an 18.5% decrease in the minimum laser power required to manipulate the magnetization, comparing the on- and off-resonance conditions. The results are in very good agreement with numerical simulations. Our research findings demonstrate the possibility to achieve an all-optical magnetic recording with low energy consumption, low cost, and high areal density by integrating plasmonic nanostructures with magnetic media.

Advanced magneto-optical Kerr effect measurements of superconductors at low temperatures

Magneto-optical Kerr-effect (MOKE) measurements of superconducting films with soft-magnetic coatings are performed at low temperatures using a laser-based MOKE set-up. An elaborate measurement scheme with internal reference allows the quantitative comparison of the temperature dependent Kerr-amplitude with the magnetic field generated by supercurrents. For this purpose, an amorphous CoFeB thin film exhibiting a large Kerr-signal is deposited directly on top of the YBCO superconductor acting as field sensing layer. It is shown that the resulting magnetic hysteresis loops of the soft-magnetic film can be used to reconstruct the electric properties of the superconductor.

Ultrafast magnetization dynamics in Nickel: impact of pump photon energy

Magnetization dynamics on a femtosecond timescale has been observed for a huge variety of magnetic structures. However, the influence of different excitation photon energies has not been studied in detail yet. In our time-resolved magneto-optical Kerr effect setup we excite a Nickel bulk system with 1.55 and 3.1 eV, respectively, leading to different remagnetization dynamics depending on the chosen photon energy. Furthermore we complement our experimental data with a theoretical approach applying appropriate Boltzmann collision integrals including the density of states of Nickel. The comparison between the experimental data and the theoretical approach indicates that photon-energy dependent transport processes play a major role in this setup.

Crystalline phase dependent spin current efficiency in sputtered Ta thin films

We report on the optical detection of the spin Hall effect (SHE) as a function of the crystalline structure of sputtered Ta thin films using a magneto-optical Kerr system. The growth rate of Ta films is found to influence the crystalline phase of Ta films. At a lower growth rate, GR the pure α-phase of Ta is formed, which changes to the pure β-phase for GR ≥ 1.44 Å/s. For an intermediate growth rate, 0.62 Å/s ≤ GR < 1.44 Å/s, an admixture of α and β phases is formed. We optically detect spin accumulation due to the spin Hall effect in Ta films by applying a square wave current and using Fourier analysis in a magneto-optical Kerr effect setup that uses spatially modulated incident light. We show that there exists a threshold current density (Jth) above which spin current can be detected via the optical technique. Jth, which is a measure of spin current efficiency, is found to be the lowest in the mixed phase of Ta and is strongly correlated with the crystalline phase of Ta films.

Strain induced exchange-spring magnetic behavior in amorphous (TbDy)Fe2 thin films

In this paper, we report a strain induced exchange-spring magnetic behavior in sputter deposited (TbDy)Fe2 amorphous thin films with phase-separated layers of (TbDy)-rich and Fe-rich at room temperature. The magnetic hysteresis loops at different strain levels were obtained with a magneto-optic Kerr effect set-up incorporating a mechanical four-point bending fixture. The unstrained film exhibits a typical ferrimagnetic hysteresis loop while the strained structure exhibits step-like hysteresis loops representative of an exchange-spring magnetic system. The mechanically strained film changes the coercivity/remanence values from positive to negative. The observed magnetic changes under strain are attributed to magnetic anisotropy modifications in the highly magnetoelastic TbDy-rich layer.

Nanorod Surface Plasmon Enhancement of Laser-Induced Ultrafast Demagnetization.

Ultrafast laser-induced magnetization dynamics in ferromagnetic thin films were measured using a femtosecond Ti:sapphire laser in a pump-probe magneto-optic Kerr effect setup. The effect of plasmon resonance on the transient magnetization was investigated by drop-coating the ferromagnetic films with dimensionally-tuned gold nanorods supporting longitudinal surface plasmon resonance near the central wavelength of the pump laser. With ~4% nanorod areal coverage, we observe a >50% increase in demagnetization signal in nanorod-coated samples at pump fluences on the order of 0.1 mJ/cm2 due to surface plasmon-mediated localized electric-field enhancement, an effect which becomes more significant at higher laser fluences. We were able to qualitatively reproduce the experimental observations using finite-difference time-domain simulations and mean-field theory. This dramatic enhancement of ultrafast laser-induced demagnetization points to possible applications of nanorod-coated thin films in heat-assisted magnetic recording.

Exploring laser-induced interlayer spin transfer by an all-optical method

We investigate the influence of spin currents during ultrafast laser-induced demagnetization of magnetic bilayer structures by a new all-optical method to measure material- and/or depth-resolved magnetization dynamics. By describing the magneto-optical response of the bilayers in the complex Kerr plane, it is shown that the material-specific magnetization dynamics of the individual layers can be measured by a marginal adjustment to any conventional time-resolved magneto-optical Kerr effect setup. We use this technique to trace superdiffusive spin currents in magnetic Ni/Fe bilayers, providing new insight on its importance to ultrafast laser-induced demagnetization.

Magnetic measurements based on magneto-optical Kerr effect on pnictide Ba(Fe1−x Cox)2As2/Fe thin film

We present characterization of Ba(Fe1−x Cox)2As2 thin film using a magneto-optical Kerr effect setup based on photoelastic modulation. The magneto optical technique has been used to analyse the thickness and the temperature dependence of the magnetic properties of epitaxial Ba(Fe1−x Cox)2As2/Fe thin films grown on MgO substrate. The first harmonics, of the reflected laser beam intensity from pnictide thin films is compared with a reference Fe thin film. The hysteretic loops of such samples at room temperature and at 5K are presented.

Magnetic Microstructure of Rolled‐Up Single‐Layer Ferromagnetic Nanomembranes

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.

Domain wall propagation in Permalloy nanowires with a thickness gradient

The magnetization reversal behavior of Permalloy nanowires has been investigated using a magneto-optic Kerr effect setup. Nanowires with various widths, w = 250 nm to 3 μm and a thickness of t = 10 nm were fabricated by electron-beam lithography and subsequent lift-off. Furthermore, similar nanowires but with a thickness gradient along the nanowire axis have been prepared to investigate the influence of the gradient on the magnetic domain wall propagation. Magnetization hysteresis loops recorded on individual nanowires without a gradient are compared to corresponding wires with a thickness gradient. The dependence of the coercive field, H c vs. t / w shows a linear behavior for wires without a gradient. However, wires with a gradient display a more complex crossover behavior. We find a plateau in the H c vs. t / w curve at values of w , where a transformation from transverse to vortex domain wall type is expected.

High resolution in situ magneto-optic Kerr effect and scanning tunneling microscopy setup with all optical components in UHV

A surface magneto-optic Kerr effect (MOKE) setup fully integrated in an ultrahigh vacuum chamber is presented. The system has been designed to combine in situ MOKE and scanning tunneling microscopy. Magnetic fields up to 0.3 T can be applied at any angle in the transverse plane allowing the study of in-plane and out-of-plane magnetization. The setup performance is demonstrated for a continuous film of 0.9 monolayers (ML) Co/Rh(111) with in-plane easy axis and for a superlattice of nanometric double layer Co islands on Au(11,12,12) with out-of-plane easy axis. For Co/Au(11,12,12) we demonstrate that the magnetic anisotropy energies deduced from thermally induced magnetization reversal and from applying a torque onto the magnetization by turning the field are the same. For the presented setup we establish a coverage detection limit of 0.5 ML for transverse and 0.1 ML for polar MOKE. For island superlattices with the density of Co/Au(11,12,12), the latter limit corresponds to islands composed of about 50 atoms. The detection limit can be further reduced when optimizing the MOKE setup for either one of the two Kerr configurations.

Dependence of exchange coupling interaction on micromagnetic constants in hard/soft magnetic bilayer systems

To elucidate the dependence of exchange coupling behavior of hard/soft magnetic bilayer systems on various micromagnetic constants, the coupling length and the nucleation field HN were systematically measured on five thin-film libraries made of soft magnetic/hard magnetic bilayers. Hysteresis loops of CoPt/FexCo1x 0x1, CoPt/Ni, CoPt/Fe, Sm2Co7/Fe,Co, or Ni, and Sm2Co7/Ni were measured using a magnetooptical Kerr effect setup. The one-dimensional model proposed by Leineweber and Kronmuller J. Magn. Magn. Mater. 176, 145 1997 and micromagnetic simulations were used to interpret the behavior of and HN obtained by the high-throughput investigation. We found that the dominant factors determining and HN are the hard layer magnetic constants and the saturation magnetization M of the soft layer. HN and display a direct correlation with the domain-wall width of the hard layer and have an anticorrelation with M of the soft layers. They were found to not be substantially dependent on exchange stiffness A and anisotropy K constants within the group of soft layer materials studied here.

Developments in surface magneto-optical Kerr effect setup for ultrahigh vacuum analysis of magnetic ultrathin films

We describe a surface magneto-optical Kerr effect (SMOKE) setup designed to study the magnetic properties of ultrathin films in ultrahigh vacuum (UHV). As in earlier systems, a crossed-poles electromagnet is used, which allows measurements in the longitudinal or polar configurations to be carried out without displacing the sample or changing the optical path. The poles are mounted in-vacuum, while the coils and yoke are mounted externally to obtain a higher magnetic field (∼2.8kOe). The width of the gap between the poles can be adjusted depending on the size of the sample. The performance of the instrument is demonstrated by measurements of the paramagnetic-ferromagnetic phase transition in a 2 ML Fe film grown on the Mo(110) surface.

Field sweep rate dynamics in magnetic tunnel junctions

The dynamical behavior of magnetic tunnel junctions (MTJs) was investigated by varying the magnetic field sweep rate from 0.01 mT/s to 10 T/s in a magneto optical Kerr effect set-up. The bias fields of the pinned and free ferromagnetic electrodes were found to drastically decrease above a field sweep rate of 1 T/s. This decrease in the bias fields coincides with a change in the magnetization reversal process from domain wall motion at low-field sweep rates to domain nucleation at high-field sweep rates. The nucleation of inverse domains in the ferromagnetic layer changes the interfacial spin structure of the antiferromagnetic layer and therefore the magnitude of the exchange bias effect. Furthermore, the nucleation of domains induces a discontinuous magnetic charge density at the tunnel barrier interfaces and this reduces the interlayer coupling between the two ferromagnetic electrodes of the MTJ.

Correlation between structural and magnetic properties of thin Fe xCo 1− x(1 1 0) films on sapphire

Fe and Fe 40Co 60 thin films (5–60 nm) with a bcc structure have been prepared on a-axis oriented sapphire substrates by molecular beam epitaxy. The structural properties have been characterized in situ by reflection high-energy electron diffraction and ex situ by X-ray diffraction and atomic force microscopy. A two-dimensional magneto-optical Kerr effect set-up has been used to determine the in-plane magnetization components and to investigate the magnetic anisotropy and the orientation dependence of the magnetization reversal process. The Fe films and Fe 40Co 60 alloy films both display a uniaxial in-plane anisotropy. They also exhibit a comparable increase of the coercive field along the easy axis with increasing thickness. We have evaluated this dependence using the results of the structural characterization, indicating that the enhancement of the coercive field is linked to the growing surface roughness and decreasing structural coherence.

Magnetization dynamics in NiFe thin films induced by short in-plane magnetic field pulses

The magnetization dynamics in a thin NiFe film was investigated by applying short in-plane magnetic field pulses while probing the response using a time-resolved magneto-optical Kerr effect setup. In-plane magnetic field pulses, with duration shorter than the relaxation of the system, were generated using a photoconductive switch and by subsequent propagation of current pulses along a waveguide. The field pulses with typical rise and decay times of 10–60 and 500–700 ps, respectively, have a maximum field strength of 9 Oe, by which Permalloy elements of 16 nm thickness and lateral dimensions of 10×20 μm were excited. The observed coherent precession of a ferromagnetic NiFe system had precession frequencies of several GHz and relaxation times on a nanosecond time scale. The dynamic properties observed agree well the Gilberts’s precession equation and the static magnetic properties of the elements

Study of in-depth defects using magneto-optical Kerr effect by measuring the magnetic hardness coefficient in magnetic thin films

A magneto-optical Kerr effect (MOKE) system was built and used for acquiring saturation magnetization curves and measuring the magnetic hardness coefficient, a of magnetic thin film samples. Co thin films were sputtered on glass substrate under various sputtering conditions. The magnetic hardness coefficient was measured using the MOKE setup. Results for /spl alpha/ (Oe) were further correlated using transmission electron microscopy and X-ray diffraction analysis. It was found that /spl alpha/ (Oe), a useful indicator for in-depth defects in Co films, increased with both working and base pressure but decreased with higher deposition temperatures. This trend is in agreement with well-known microstructure control techniques by changing sputtering parameters.