Vectorial Magneto-optical Kerr Effect Research Articles

Structural and Magnetic Specificities of Fe-B Thin Films Obtained by Thermionic Vacuum Arc and Magnetron Sputtering

Fe-B based compounds are of high interest due to their special properties and the wide range of involved applications. While B is the element that facilitates the increase in the hardness and the degree of wear resistance, it is also an effective glass former, controlling the formation of a much-desired amorphous structure with specific magnetic properties. Major difficulties related to the proper engineering of Fe-B thin films lay especially in their preparation under well-defined compositions, which in turn, should be accurately determined. The present study closely analyzes the morpho-structural and magnetic properties of thin coatings of Fe-B of approximately 100 nm thickness and with the nominal B content ranging from 5 at. % to 50 at. %. The comparison between films obtained by two preparation methods, namely, the thermionic vacuum arc and the magnetron sputtering is envisaged. Morpho-structural properties were highlighted using X-ray diffraction supplemented with X-ray reflectometry and scanning electron microscopy, whereas the elemental investigations were performed by X-ray dispersive spectroscopy and Rutherford back-scattering spectroscopy. The magnetic properties of the Fe-B layers were carefully investigated by the vectorial magneto-optic Kerr effect and conversion electron Mössbauer spectroscopy. The high capability of Mössbauer Spectroscopy to provide the phase composition and the B content in the formed Fe-B intermetallic films was proven, in correlation to Rutherford back-scattering techniques, and to explain their magnetic properties, including the magnetic texture of interest in many applications, in correlation with longitudinal magneto-optic-Kerr-effect-based techniques.

Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe3O4 film on SrTiO3:Nb by v-MOKE and ferromagnetic resonance.

The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe3O4) thin film grown by infrared pulsed-laser deposition on SrTiO3(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, TV, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below TV by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > TV and 〈110〉 for T < TV. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe3O4 thin films by proper control on the growth parameters and substrate choice.

Preferential weakening of rotational magnetic anisotropy by keV-He ion bombardment in polycrystalline exchange bias layer systems

The consequences of keV light-ion bombardment of polycrystalline exchange bias layer systems on individual magnetic anisotropies composing the effective unidirectional magnetic anisotropy are investigated by vectorial magneto-optic Kerr effect measurements. Preferential reduction of rotatable magnetic anisotropy as compared to the other magnetic anisotropies is observable, disproving the intuitive assumption of an equable weakening of all magnetic anisotropies due to the bombardment. It is concluded that light-ion bombardment has a stronger impact on the magnetic characteristics of smaller grains in a polycrystalline antiferromagnet as compared to those of larger ones.

Azimuthal angular dependent hysteresis loops of Fe50Mn50/Ni81Fe19 bilayers grown under a magnetic field

The azimuthal angular dependence of the vectorial hysteresis loops in the Fe50Mn50(AF)/Ni81Fe19(F) bilayer grown under a magnetic field was investigated using a combination of vectorial magneto-optic Kerr effect and model calculation. From a comparison of the experimental and calculation results, it is found that the AF easy axis is not parallel with but rotated by about 20° away from the applied magnetic field during the sample growth. Moreover, the transverse loop at the AF easy axis does not vanish but displays an open full circle (i.e., magnetization changes sign between decreasing and increasing field branches for the full hysteresis measurement). Our model calculation reveals that they are reminiscent of the non-collinear uniaxial and unidirectional anisotropies. Specifically, the angular dependence of the transverse hysteresis is well reproduced with our model calculation taking non-collinear magnetic anisotropies into account. Coercivity determined from the longitudinal loops, on the other hand, is found to be nonzero and comparatively large at all azimuthal angles. This is in stark contrast with previous results regarding FeMn/NiFe bilayers field-cooled after sample growth. Neither domain wall nor incoherent magnetic rotation in the F layer is likely to be responsible for this coercivity discrepancy between theory and experiments. Apart from the uniaxial F and unidirectional AF-F anisotropies, we suggest that the F rotatable anisotropy equivalent of 40% to 60% of the interfacial coupling energy should be taken into account to properly address the coercivity enhancement in the FeMn/NiFe bilayer grown under a magnetic field.

Role of the substrate on the magnetic anisotropy of magnetite thin films grown by ion-assisted deposition

Magnetite (Fe3O4) thin films were deposited on MgO (001), SrTiO3 (001), LaAlO3 (001) single crystal substrates as well on as silicon and amorphous glass in order to study the effect of the substrate on their magnetic properties, mainly the magnetic anisotropy. We have performed a structural, morphological and compositional characterization by X-ray diffraction, atomic force microscopy and Rutherford backscattering ion channeling in oxygen resonance mode. The magnetic anisotropy has been investigated by vectorial magneto-optical Kerr effect. The results indicate that the magnetic anisotropy is especially influenced by the substrate-induced microstructure. In-plane isotropy and uniaxial anisotropy behavior have been observed on silicon and glass substrates, respectively. The transition between both behaviors depends on grain size. For LaAlO3 substrates, in which the lattice mismatch between the Fe3O4 films and the substrate is significant, a weak in-plane fourfold magnetic anisotropy is induced. However when magnetite is deposited on MgO (001) and SrTiO3 (001) substrates, a well-defined fourfold in-plane magnetic anisotropy is observed with easy axes along [100] and [010] directions. The magnetic properties on these two latter substrates are similar in terms of magnetic anisotropy and coercive fields.

Vectorial Kerr magnetometer for simultaneous and quantitative measurements of the in-plane magnetization components.

A vectorial magneto-optic Kerr effect (v-MOKE) setup with simultaneous and quantitative determination of the two in-plane magnetization components is described. The setup provides both polarization rotations and reflectivity changes at the same time for a given sample orientation with respect to a variable external magnetic field, as well as allowing full angular studies. A classical description based on the Jones formalism is used to calculate the setup's properties. The use of different incoming light polarizations and/or MOKE geometries, as well as the errors due to misalignment and solutions are discussed. To illustrate the capabilities of the setup a detailed study of a model four-fold anisotropy system is presented. Among others, the setup allows to study the angular dependence of the hysteresis phenomena, remanences, critical fields, and magnetization reversal processes, as well as the accurate determination of the easy and hard magnetization directions, domain wall orientations, and magnetic anisotropies.

Cross-over from coherent rotation to inhomogeneous reversal mode in interacting ferromagnetic nanowires

We have investigated the magnetization reversal mechanism and the effects of magnetostatic interaction in arrays of lithographically defined Permalloy nanowires with a fixed width of 185 nm, spacing of 35 nm and film thicknesses from 10 to 120 nm. The magnetization reversal has been investigated with vectorial Magneto-Optical Kerr Effect magnetometry. The vectorial hysteresis loops (in-plane magnetization components parallel and perpendicular to the applied field) recorded with the external field applied perpendicular to the length of the wires (hard magnetization direction), show a transition from coherent rotation to inhomogeneous reversal mode for wires’ thickness above 80 nm. The effects of dipolar interactions are evidenced by the variation of the saturation field in the hard-axis hysteresis loops as a function of wires thickness. In detail, the analysis of the saturation field vs. wires thickness shows that the dipolar interactions start to play a role for wires thickness ⩾20 nm.

Magnetization reversal process in elliptical Permalloy nanodots

Using the Vectorial Magneto-Optic Kerr Effect (V-MOKE) and numerical simulations, we have investigated the magnetization reversal process in arrays of 15-nm-thick Permalloy nanometer-scale dots, having elliptical shape and eccentricity, varying from 1 to 2.5. V-MOKE hysteresis loops revealed that the magnetization reversal is incoherent for elements with eccentricity of 1 and 1.5, while it becomes an almost perfect coherent magnetization rotation for elements with eccentricity equal to 2 and 2.5. In the latter case, the V-MOKE loops agree well with those predicted by the Stoner–Wohlfarth model for particles with uniaxial magnetic anisotropy. We were able to reproduce the V-MOKE results with micromagnetic simulations, gaining a deeper insight into the magnetic configurations that develop during the reversal process.

Magnetostatic interaction in arrays of nanometric permalloy wires: A magneto-optic Kerr effect and a Brillouin light scattering study

Two arrays of permalloy parallel wires, $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick, having the same width of $175\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and different spacing of 35 and $175\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were prepared by means of deep ultraviolet lithography and lift-off process. The effect of magnetostatic interaction on both the static and dynamic magnetic properties of arrays of wires has been investigated by means of magneto-optic and Brillouin light scattering techniques, respectively. In particular, the magnetization switching of the samples, measured by vectorial magneto-optical Kerr effect magnetometry and microscopy shows the effects of dipolar interaction in the case of $35\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ spaced wires, while in the other sample the measurements show that the wires are substantially noninteracting. The Brillouin light scattering measurements showed that for the sample with interwire spacing of $35\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, dipolar coupling between magnetic wires leads to the formation of a collective mode which has a continuous spectrum and exists in a range of frequencies, while for the $175\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ spaced wires the spin modes are dispersionless. To quantify the investigated effects, a theory developed earlier for an isolated wire has been extended to the case of a one-dimensional array of ferromagnetic wires.

New technique to measure magnetic anisotropy using the vectorial magneto-optic Kerr effect

A new method of interpreting the data produced by vectorial magnetometry using the magneto-optic Kerr effect (MOKE) is described. The procedure allows the angle between the magnetisation and the applied field to be measured independently of the applied field. Rotation of the sample generates a curve similar to that produced by a torque magnetometer. The technique is applied to sputtered Si/Co/Cr/Co trilayers, and the curves are analysed using a fitting method to measure the ratio of the magnetisation to the uniaxial anisotropy constant, providing an insight into the effects of thermal annealing on the samples.