Magneto-optical Kerr Effect System Research Articles

Investigation of Laser Direct Writing as a Novel Method of Permalloy Patterning

The abilities of laser direct writing have been explored on thin films of permalloy (Ni81Fe19) for range of film thicknesses with two types of substrates for creating micro-scale magnetic structures. The thin films of Permalloy were deposited on both silicon and glass substrates using thermal evaporator with ranging from 5 to 100 nm. The permalloy films were successively patterned using a laser system containing of a pico-second pulsed laser with an 800 ps pulse width and wavelength of 532 nm. A series of magnetic wires were patterned then characterised by Magneto-Optic Kerr Effect system and Scanning Electron Microscopy. The patterned magnetic wires showed good responses to an applied magnetic field. The corresponding coercivities of the patterned magnetic wires were affected by their observed quality. These results can improve the understanding of laser direct writing technique to fabricate the micromagnetic structures for future application as easy, low cost and high throughput technique.

Magnetization switching behavior of exchange-coupled bilayer nanodots characterized by magneto-optical Kerr effect

We investigated the magnetization switching behavior of exchange-coupled bilayer nanodots consisting of a soft magnetic Ni81Fe19 (permalloy; Py) layer and a perpendicularly magnetized L10-FePt layer by measuring the polar magneto-optical Kerr effect (MOKE). A MOKE system having a 2-μm-diameter laser spot was used, which detected the signal from around 20 nanodots of 200 nm in diameter. The measured polar MOKE loops exhibited a two-step behavior attributable to each magnetization reversal process of the Py and L10-FePt layers, allowing us to evaluate the switching field (Hsw) of L10-FePt from the MOKE loop. Under the application of both a static magnetic field and an rf magnetic field (hrf), the nanodots showed a clear Hsw reduction with characteristic hrf frequency dependence. This Hsw reduction is well interpreted as a behavior of spin wave-assisted magnetization switching. We also discuss the influence of the Py layer thickness on spin wave-assisted Hsw reduction.

Preparation and characterisation of compositionally graded SmCo films

A compositionally graded SmCo film has been prepared by magnetron sputtering using a Co target partially covered by a Sm foil. The film was deposited onto a 100 mm thermally oxidised Si substrate and then annealed ex-situ. The SmCo film has been used as a test sample to validate an in-house developed scanning MOKE (Magneto-Optic Kerr effect) system incorporating a pulsed magnetic field source capable of producing fields as high as 10 T. A 2D array of hysteresis loops was measured across the entire wafer. The evolution in coercivity measured along a selected 1D strip of the sample is correlated with changes in composition and crystallographic structures measured using Energy Dispersive Spectroscopy and X-ray diffraction. The high field Scanning MOKE system holds much potential for optimizing the extrinsic properties of known hard magnetic phases as well as in the search for new hard magnetic phases.

Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures

Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops.

Ferroelectric control of magnetism in P(VDF–TrFE)/Co heterostructure

Multiferroic magnetoelectric composite systems such as ferroelectric and ferromagnetic heterostructures have attracted an interest recently and have the potential applications such as four state memories, magnetic sensor and spintronics. In this study, we focus on the ferroelectric copolymer/ferromagnetic Co multiferroic film. The ferroelectric copolymer poly(vinylidene fluoride–trifluoroethylene) was fabricated by Langmuir–Blodgett deposition technique. Large magnetization changes of the Co films emerge in response to ferroelectric switching of poly(vinylidene fluoride–trifluoroethylene) controlled by applied electric field. The magnetization of the Co films was detected by the magneto-optical Kerr effect system. Although ferroelectric polymer poly(vinylidene fluoride–trifluoroethylene) have the lower piezoelectric coefficient about 30 pC/N than inorganic ferroelectrics, interface strain coupling is also the primary mechanism altering the induced magnetic anisotropy in the poly(vinylidene fluoride–trifluoroethylene)/Co heterostructure.

Voltage-controlled Kerr response in Ni/Pb(Zr0.52Ti0.48)O3 heterostructures

Voltage-modified magneto-optical Kerr effect (MOKE) is widely used to describe the converse magnetoelectric (ME) effect in the ferroelectric/ferromagnetic (FE/FM) heterostructures. However, the applied voltage can possibly give rise to electro-optical effect of the FE layer, which would also affect the Kerr signals in the MOKE system. Here, we used an AC voltage to modulate the magnetization in the Ni/Pb(Zr0.52Ti0.48)O3 (PZT) heterostructures with different pre-polarization states of the PZT layers to investigate the complexity of the Kerr signals. The results suggested that the voltage control of Kerr signal contained several origins; however, the strain-induced ME effect dominated in the ME effect in the heterostructures.

Variable angle magnetometry for exchange-coupled multilayers with in-plane and perpendicular anisotropy

Co/Pd multilayers, including exchange coupled structures with thin layers of Co with different exchange coupling strengths, were investigated using variable angle magnetometry. Hysteresis loops of perpendicular and in-plane components of magnetization were measured using two-axis vibrating sample magnetometer and variable angle magneto-optical Kerr effect system. Co/Pd multilayer structures have high perpendicular remanence and the magnetization reversal mechanism is mainly domain wall motion. For the systems with Co and Co/Pd multilayers, the magnetization reversals of high coercivity (Co/Pd) multilayer and low coercivity Co single layer were found to be independent, if the soft layer (Co) is thicker than 2 nm. In the samples with a high degree of exchange coupling, the magnetization of the multilayer has two components, normal and in-plane. The proportion between them varies with field strength and orientation of the sample. Taking this into account, it was possible to determine the anisotropy energy of the (Co/Pd) multilayer, which is in the range of 400–420 kJ/m3.

Superparamagnetic Behavior of Pt/CoFe/Pt Nanowires With Decreasing Wire Width

We present experimental demonstration that the thermal unstability grows with reducing the width of Pt/CoFe/Pt nanowires on the way to the superparamagnetic transition. The magnetic hysteresis loops measured at room temperature using a magneto-optical Kerr effect system reveal that the coercive field decreases drastically when the nanowire width is reduced smaller than 160 nm. To understand the origin of the coercive field reduction, the coercive field is measured with varying the sweep rate of the external magnetic field and then, analyzed with a thermally-activated magnetization reversal model. It is revealed that the reduction of the coercive field is mainly ascribed to the decrement of the thermal stability parameter, which in turn induces the abrupt reduction of the retention time down to a few seconds.

Methodology for investigating the magnetization Process of the storage Layer in double-Layered perpendicular magnetic recording media using the anomalous Hall effect

The Hall effect is a useful phenomenon for evaluating the magnetization processes of the storage and back layers in double-layered perpendicular magnetic recording media. Although the Hall voltage in double-layered films has two components (an anomalous Hall term arising from the perpendicular component of magnetization and a planar Hall term originating from the in-plane component of magnetization), it is possible to separate the two contributions by symmetry arguments. However, in the case where the saturation field of the recording layer (RL) is comparable to that of the soft underlayer (SUL), or when the SUL dominates the signal because of its inherently high magnetic moment, it is difficult to separate the anomalous Hall effect (AHE) contributions from the RL and SUL individually. Here we propose a methodology that overcomes this limitation and allows the extraction of the magnetization process of the RL. We assume that an average perpendicular magnetization contributes to the anomalous Hall voltage in the double-layered perpendicular media. We compare the magnetization process of the RL obtained by using this approach to that measured with a magnetooptical Kerr effect (MOKE) system. The proposed technique may be useful in those situations in which alternative tools such as the MOKE system are not suitable for measuring the magnetization process.

Relation between exchange coupling and enhanced coercivity in the free layer of a patterned magnetic tunnel junction

A magneto-optical Kerr effect system with a spatial resolution of 2μm was used to measure the local M-H loops for the free layer of a magnetic tunnel junction with a structure of Ta∕Cu∕Ta∕NiFe∕Cu∕Mn75Ir25∕Co70Fe30∕Al2O3∕Co70Fe30∕Ta to investigate the exchange bias field HE and the coercivity HC for the free layer. The HE and HC measured along the direction of the free layer varied symmetrically with respect to the junction center. The measurements indicate that the enhanced HC correlated with HE, and HE could be reasonably explained by using an “orange-peel-type” coupling based on variations in the thickness of the pinned layer along the direction of the free layer. The variation in HE along the pinned-layer’s direction could be ascribed to that of the free-layer’s thickness, and the increase in HE at the junction edge along the pinned layer was due to a decrease in the thickness of the free layer near the edge. However, the nearly constant HC along the pinned layer indicates that the thickness of the free layer can be excluded from the mechanism for enhancing HC, which is a unique difference in the parameters involved in HE and HC, and in the mechanism for enhancing HC.

Distribution of interlayer-exchange coupling on MTJ multilayer

Local M–H loops have been measured on the free layer of a magnetic tunnelling junction using a magneto-optical Kerr effect system, with 2-μm spatial resolution, to investigate the distribution of the interlayer exchange-coupling field (HE) and the coercivity (Hc) on the free layer. HE and Hc increase gradually from a junction edge, reach a maximum at the centre and then fall off gradually at the other edge. Annealing enhances HE and smooths it out at the junction centre but Hc decreases after annealing. Two-dimensional plots of HE and Hc show symmetric saddle shapes with their axes aligned with the pinned layer due to the shadow-mask effect during deposition. HE measured along the pinned layer in this case shows a ridge curve with peaks at junction edges but Hc is nearly constant. Atomic force microscopy measurements show that the roughness variations over the junction are similar to HE with greater roughness at the centre, indicating mutual correlation and confirming the origin of HE to be Néel's orange-peel coupling based on surface roughness. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic domain structures and switching properties in submicron size synthetic antiferromagnets

Magnetic domain structures and spin switching properties in submicron size synthetic antiferromagnets (SyAFs), Co 90 Fe 10 /Ru/Co 90 Fe 10 with aspect ratio ( k ) of 1, have been studied by using magnetic force microscope (MFM) image under an external magnetic field and by a focused magneto-optic Kerr effect system. It is found that SyAF with k =1 shows the single domain structure during magnetization reversal, and exhibits size- and shape-independent spin switching field.

Magnetic distribution of the free layer on patterned MTJ multilayers

Local M–H loops have been measured on the free layer of a magnetic tunnel junction multilayers using the magneto-optical Kerr effect system, with 2 μm spatial resolution. The relatively irregular variations of coercivity H C (∼17.5 Oe) and exchange coupling field H E (∼7.5 Oe) are revealed in as-deposited sample prepared by the DC magnetron sputtering. After 200°C annealing, H C decreases to 15 Oe, while H E increases to 20 Oe with smooth local variation. Two-dimensional plots of H C and H E show the symmetric saddle shapes with their axes aligned with the pinned layer, where both H C and H E variations along free-layer axis are larger than those along pinned-layer axis. The increasing slope per micrometer of normalized H C and H E are the same near the junction edge along free-layer direction irrespective of junction size, giving the relatively uniform H C and H E for wider junction size.

Thin single layer materials for device application

Thermally evaporated permalloy (Ni 81Fe 19)and Co 95Fe 5 magnetic films and copper, gold, silver and aluminium non-magnetic films in the sub-15 nm range have been analysed. Silicon, glass, GaAs and MgO were all investigated as substrates. Surface roughness was determined using atomic force microscopy, while magnetic characterisation of coercivity and anisotropy field was performed by a magneto-optical Kerr effect system. Clear variation in coercivity and anisotropy is seen dependent on the substrate and film thickness. Significantly, in the permalloy system, the change in magnetic and structural properties with increasing film thickness depends strongly on the substrate used: for films deposited on glass, roughness increases with increasing film thickness whereas for films deposited on silicon, roughness remains largely unchanged with increasing film thickness.

Three-configurational surface magneto-optical Kerr effect measurement system for an ultrahigh vacuum in situ study of ultrathin magnetic films

We have constructed a three-configurational surface magneto-optical Kerr effect system, which provides the simultaneous measurements of the “polar,” “longitudinal,” and “transverse” Kerr hysteresis loops at the position where deposition is carried out in an ultrahigh vacuum growth chamber. The present system enables in situ three-dimensional vectorial studies of ultrathin film magnetism with a submonolayer sensitivity. We present three-configurational hysteresis loops measured during the growth of Co films on Pd(111), glass, and Pd/glass substrates.

Transverse Biased Initial Susceptibility Measurements of Magnetization Reversal in Fe/MgO(100) Thin Films

Transverse biased initial susceptibility (TBIS) measurements were performed on Fe(100)/MgO(100) thin films, using a magnetooptical Kerr effect system. A peak detected in the TBIS curve when the magnetic field is applied along one of the easy axes, corresponding to a small step in the hysteresis loop, is associated with the uniaxial in-plane anisotropy superimposed on the crystalline biaxial. We proved that TBIS is a suitable and sensitive technique for the study of this phenomenon, and provides an accurate description of the magnetization reversal process.

Magnetic and magneto-optical properties of MnSb films on various substrates

Magnetic and magneto-optical properties of MnSb films with different crystalline orientations on various semiconductors of GaAs(1 0 0), GaAs(1 1 1)A, B, and sapphire(0 0 0 1) have been measured by a vibrating sample magnetometer (VSM) and a home-made magneto-optical Kerr effect (MOKE) system. All these samples have their easy axes in the plane and show ferromagnetic properties. Among these samples, the film on GaAs(1 1 1)B has the lowest coercive force H c and the largest squareness (SQ) value, whereas the film on GaAs(1 0 0) shows the largest H c and the lowest SQ value. A large Kerr rotation angle of about 0.3° was observed at a wavelength of λ=632.8 nm for the film on sapphire in the field applied both parallel and perpendicular to the film plane. However, the MnSn films on other substrates do not have an observable Kerr rotation. The dynamic effect of the hysteresis was also measured using our MOKE system. As the frequency of applied magnetic field increases, the loop rounds off at the corners and the loop area increases.

Computerized magneto-optical Kerr effect system for dynamic domain structure observations

A computer-controlled set-up for investigations of the magnetic domain structure in soft magnetic materials is described. In this set-up, a special electronic shutter allows observations of changes in the domain structure while the sample is magnetized. This is realized by means of the synchronous capture of the ‘frozen’ images. Digital image processing is used to improve the magnetic contrast to such a degree that observations of domain structure on the non-polished surfaces of rapidly solidified materials are possible.

Magnetic properties of molecular-beam epitaxial grown (100) iron films

The magnetic properties of epitaxial iron films have been studied. The iron films were grown on {100} GaAs and GaInAs substrates. Both the magnitude of the anisotropy energies and direction of the easy axes were studied using a SQUID magnetometer. In particular, measurements were made of the magnetization in the plane of the film along the 〈100〉 and 〈110〉 directions. From these measurements the magnitude of the anisotropy energy and the direction of the easy axis were determined. Also measurements of the coercivity of the films were made using a magneto-optic Kerr effect system. These results are compared with those determined previously for iron films grown on GaAs.