Kerr Rotation θK Research Articles

Temperature lag with the onset of exchange bias, superparamagnetic blocking, and antiferromagnetic ordering in ultrathin ferromagnet/antiferromagnet thin film

The magnetization of a nanosized magnet, such as an ultrathin film, thermally fluctuates and can become superparamagnetic. In ferromagnetic/antiferromagnetic thin films, superparamagnetism can be suppressed in accordance with antiferromagnetic ordering. The exchange bias can also be induced at the ferromagnetic/antiferromagnetic interface, and it is nontrivial whether the superparamagnetic blocking temperature (TB_SPM) can match either the onset temperature of the exchange bias (TB_EB) or the Néel temperature (TN). In this study, we investigated the temperature dependence of parameters such as coercivity, exchange bias field, magneto-optic Kerr rotation (θK), and AC magnetization (MAC) to elucidate the matching of TB_EB, TB_SPM, and TN in a Pt/Co/Au/Cr2O3/Pt thin film. Based on the temperature dependences of MAC, TB_SPM was yielded as about 283 K. TB_EB and TN, which were determined based on the temperature dependence of θK, were 278 and 282 K, respectively. TB_SPM was almost equal to TN but TB_EB was smaller. This temperature lag was caused by the difference in the magnetic anisotropy energy required to induce the exchange bias and suppress superparamagnetism.

Layer-sensitive magneto-optical Kerr effect study of magnetization reversal in Fe/MnAs/GaAs(001)

Fe/MnAs/GaAs(001), a prototypical system for thermally assisted magnetization reversal, is studied by magneto-optical Kerr effect measurements. The results show that it is possible to recover elemental sensitivity from magneto-optical measurements when both Kerr rotation (θK) and Kerr ellipticity (ϵK) are measured under the same conditions. Both Fe and MnAs magnetic cycles can be extracted from simple linear combinations of θK and ϵK cycles. The data analysis shows that the orientation of the Fe magnetization at remanence can be controlled through the temperature of the system as a result of the peculiar temperature dependent self-organized stripes pattern in MnAs/GaAs(001).

Surface modes enhanced magneto-optical Kerr effect in Fe films underneath two-dimensional array of polystyrene spheres

For bilayers consisting of 20 nm Fe film and top two-dimensional arrays of hexagonal close-packed polystyrene spheres, fine structures are observed in the reflection R and magneto-optical polar Kerr rotation θK spectra. The θK achieves narrow peaks or dips near R minima and are both shifted with the sphere diameter. All phenomena are attributed to the excited guided waves and surface plasmonic modes. The θK peaks and dips near the R minima can be mainly explained as a result of the pure optical constant effect.

Full potential results on the magneto-optical properties of the Heusler compoundsCo2FeX (X = Al, Ga, Si and Ge)

We have calculated the magneto-optical (MO) properties ofCo2FeX (X = Al, Ga, Si and Ge) Heusler compounds using the full potential linearized augmented planewave (FPLAPW) method as implemented in the WIEN2K code using the local spin densityapproximation (LSDA) and also by using the generalized gradient approximation (GGA)for the electronic exchange and correlation. In all the compounds, Kerr rotationθK has a strong minimum near 2 eV, the value of|θK| corresponding to this minimum being almost as large as in pure Co–Fecompounds. The calculated MO spectra help to identify the features of theexperimental spectra. A comparison of the results shows that the Kerr spectrum isquite similar from both LSDA and GGA but the latter gives better agreementwith experiment. Moreover, we find that inclusion of correlation effects usingGGA+U removes the discrepancy in magnetic moment ofCo2FeX (X = Si, Ge) though it has an insignificant effect on the Kerr spectra.

Enhancement of polar Kerr effect by forming Au nanoparticles on Ni surface

Au(x)∕Ni(y)∕glass(sub) bilayered films, with x ranging from 0to310Å and y=85Å, were made by the vapor evaporation method in vacuum. The surface morphology of each bilayer was examined by an atomic force microscope. The results show that when x=xm=25 and 100Å, respectively, there were, in particular, a large number of Au nanoparticles (or nanoislands) forming on top of the Ni surface. As a result, we observed considerable enhancement in either the polar Kerr rotation θK or the extraordinary Hall coefficient RS at these two thicknesses. As is well known, the penetration depth δP of the electron transverse flow, crossing the Au∕Ni interface from the Ni to the Au layer, is of the order of the electron mean free path ℓAu, i.e., δP=(3∕8)ℓAu≈116Å, in the Au layer. Hence, the situation xm<δP is always satisfied. Then, the strong enhancement of θK is due to the surface plasma resonance effect on the Au nanoparticles (e.g., by reducing the real part of the diagonal dielectric tensor εxx so that Re[εxx(ω)]≈1, where ℏω=1.96eV). Moreover, the formation of Au nanoparticles roughens the bilayer’s surface. That means the enhancement of RS is due to the increase of the surface resistivity ρS. Finally, since the side-jump mechanism is effective, it could affect θK too (e.g., by enhancing the imaginary part of the off-diagonal conductivity Im[σxy]).

Magnetic properties of ultrathin Co/Ni/Pt(1 1 1) films

Magneto-optical (MO) and structural properties of ultrathin 1 monolayer (ML) Co/1ML Ni/Pt(111) film were investigated upon thermal annealing. The change of Kerr rotation (θK) in polar configuration is correlated to rich phase transitions of Co–Ni intermixing and alloy formations of Co–Pt and Ni–Pt. After 830K annealing, the film shows low-temperature saturation magnetization with Pt-rich Co–Pt alloys confined in surface upper layers and Ni–Pt alloys distributed in deeper layers. We observed that the magnetic order of Co–Pt was affected by Ni–Pt.

反射型磁性フォトニック結晶の特性

To investigate the potential of magnetophotonic crystal for use in optical isolators, theoretical and experimental studies were carried out. The calculations revealed that the required optical isolator performance (reflectance R 95% and Kerr rotation θK 45°) can be achieved with a reflection-type one-dimensional mgnetophotonic crystal (1D-MPC) having only one thin magnetic layer. Provided that a dielectric multilayer film and an Al film are used in combination as the reflection layer, the structure of 1D-MPC satisfying the isolator requirements becomes much simpler than that of the transmission-mode 1D-MPC. This is indeed very attractive for fabrication of actual isolators.

Magnetic and magneto-optical properties of Pd/Cr/Co multilayers

Recent studies found that the (Pt/Co/Pt) trilayers can be used as a unit in combination with nonmagnetic or magnetic layer, X (X=Pd, Ag, Cu, and Ni), to enhance the perpendicular magnetic anisotropy of the films, reduce the Curie temperature, and alter the magneto-optical properties. The effects of intercalating Cr into Pd/Co multilayers on the magnetic and magneto-optical properties are studied in this article. The perpendicular magnetic anisotropy Ku and the coercivity Hc of the system decrease rapidly with increasing the Cr thickness (XCr) up to 0.4 nm, and change slightly when Cr thickness further increases. The dependence of the coercivity Hc on the Cr thickness, which obeys the law: Hc (XCr)=XCr−2.66, indicates that the magnetization reversal is controlled by domain wall moving, mainly due to the interface roughness. Large decrease of the Kerr rotation θk of the Pd/Cr/Co multilayers compared with pure Pd/Co multilayers is also found in the wavelength ranging from 200 to 800 nm. As it is well known, the large anisotropy and Kerr rotation in Pd/Co system are mainly caused by the polarization of Pd atoms due to nearby Co atoms. As the intercalating of Cr layer between Pd and Co layer, the average polarization of Pd atoms will be reduced largely. As a matter of fact, the Cr atoms can also be polarized by nearby Co atoms, which, however, seems to take a minor effect on the anisotropy and Kerr rotation of the system.

Reversal behavior in a bimodal magneto-optical medium

Bimodal magnetic behavior, in which Kerr rotation (θK) and ellipticity (εK) each exhibit major magneto-optic hysteresis loops with two distinct field-dependent steps, was encountered in a (0.3 nm Co/1.2 nm Pt)15 multilayer film with perpendicular magnetization. A model is postulated in which there are two different interacting magnetic entities present in the layered structure of this material, each with a stable perpendicular magnetization state either parallel or antiparallel to the applied field, and which reverse quasi-independently from one another. Due to interactions between the two, the reversals occur at the intrinsic coercivities displaced by an interaction field, Hi, whose sign depends upon the magnetization of the other phase. Using major loop and first-order reversal measurements, the intrinsic coercivities, as well as Hi, were obtained.

Magnetic and magneto-optical properties of TbFeCo/(Pt, Pd) multilayers optimized for short wavelength recording

Magnetic and magneto-optical properties of TbFeCo/Pt and TbFeCo/Pd multilayer films prepared by sputter deposition are discussed in conjunction with the multilayer structure. The magnetic behaviors in TbFeCo/Pt and TbFeCo/Pd multilayers are very similar to each other, and the saturation magnetization Ms at 298 K increases with thickness t of Pt and Pd up to tPt, tPd=12 Å. In TbFeCo/Pt multilayers, the contribution of Pt to the polar Kerr spectra is significant at ultraviolet photon energies. The Kerr rotation θK and ellipticity ηK are enhanced with Pt layer thickness and become saturated at tPt=12 Å. On the other hand, little contribution from Pd atoms is found in the TbFeCo/Pd multilayer.

Magnetic aftereffect in a bimodal Co/Pt magneto-optical medium

Magneto-optical Kerr effect measurements were used to perform magnetic aftereffect studies of the reversal behavior in a bimodal magneto-optical medium with perpendicular magnetic anisotropy. This (0.3 nm Co/1.2 nm Pt)15 multilayer film has shown both the Kerr rotation (θK) and ellipticity (εK) to exhibit major hysteresis loops with two distinct field-dependent steps. However, whereas both steps in the θK loop occur in the same direction, in the εK loop they occur in opposite directions. Thus, upon traversing the descending major loop, at the first magnetic reversal, εK increases even though the magnetization decreases. Magnetic aftereffect in this material was investigated in the vicinity of each magnetization step by measuring both θK and εK over time. The data clearly show the time dependence of each of the two separate reversals, including the changes in εK which occur in opposite directions.

90° magneto-optical polar Kerr effect in layered magnetic semiconductor/metal structures

Using analytical and numerical methods, we show that the polar Kerr rotation θK of 90° may be attained in a large class of (magnetic-semiconductor)/metal layered structures just below the absorption edge. The corresponding reflectivity varies over wide limits. For example, in a EuS/Ag multilayer with a small period it is of several percent, while in a thick layer of EuS deposited on Ag it achieves 95%.

Doping Effect of PtCu/Co and PtAl/Co Multilayers

The effect of doping Cu and Al into the Pt spacer layers on the polar Kerr rotation θk in the wavelength range of 4000A∼8000A has been investigated for sputtered 8APt/3ACo multilayers. It is found that the small concentrations of the doped atoms result in a large enhancement of θK at 7700A (1.59eV) for PtCu/Co and at 6800A (1.94eV) for PtAl/Co multilayers. Moreover, the polar Kerr rotation θK at this enhanced peak shows an oscillation behavior with the increasing concentrations of the doped atoms. To interpret the experimental results, we apply the formula of Zak et al. to calculate the magneto-optical properties of 600APt1-xCux/20×(3ACo/8A Pt1-xCux) multilayers. where x is in the range of 0 to 0.35, and find that the enhancement and its oscillation are mainly determined by the variation in the dielectric tensor of Co layers, which are caused by the diffusion of the doped atoms.

Enhancement of Magneto-optical Effects through NdCo and Pt in TbFeCo

Magneto-optical effects of TbFeCo/NdCo and TbFeCo/Pt multilayer films and NdTbFeCo alloy films prepared by sputter deposition are discussed in conjunction with the multilayer structure and the composition. The contribution to the polar Kerr spectra due to NdCo and Pt layers is significantly found at ultra- violet photon energies for both cases. In TbFeCo/NdCo multilayers, it is found that the enhancements of Kerr rotation θK and ellipticity ηK increase with NdCo layer thickness and become saturated at tNdCo = 15 Å for the case of tTbFeCo = 30 Å. In TbFeCo/Pt multilayer, the θK and ηK are also enhanced with Pt layer thickness and become saturated at tpt= 12 Å.

The Magnetic and Magneto-Optic Properties of the Amorphous TbCo/Si Multilayers

The Tb0.17Co0.83/Si multilayers prepared by a rf magnetron sputtering system with different Si thickness have been investigated. X-ray diffraction, magnetic measurement and Kerr rotation have been performed. With increasing thickness of Si layer tSi, the perpendicular anisotropy constant Ku decreased rapidly. The saturation magnetization Ms and the Kerr rotation θK decreased linearly when tSi increased. It was assumed that Co2Si and Tb had been formed in the interfacial zone between TbCo and Si layers. The reduction of Ku, Ms and θK is attributed to the decrease of the effective thickness of magnetic layer, which is linear with tSi.

Concurrent variation of giant magnetoresistance and saturation Kerr rotation in NiCo/Cu multilayers

The dependences of magneto-optical effect, effective optical constants, and giant magnetoresistance (GMR) on the thickness of Cu layers were investigated in NiCo/Cu multilayers prepared by rf magnetron sputtering. A peak of saturation polar Kerr rotation θK, occurred simultaneously with that of GMR ratio when the Cu thickness is around 1.0 nm, where a clear drop of the effective optical constants n and k appeared. The peak of θK is mainly caused by the reduction of the effective optical constants, which dominate over the small drop of the effective off-diagonal elements of the dielectric tensor. The concurrent variation of GMR and θK is related to antiferromagnetic interlayer coupling, which may change the electron band structure and thus the optical and magneto-optical transitions of electrons.

Effect of Pt in TbFeCo on magnetic and magneto-optical properties

The effect of Pt additives in TbFeCo(Pt)/Pt multilayers on magnetic properties and magneto-optical Kerr effect is investigated. The perpendicular magnetic anisotropy Ku of the multilayer becomes larger (∼2×106 erg/cm3) than that of the TbFeCo(Pt) single layer. Pt layers are found to contribute significantly to the polar Kerr spectra at ultraviolet photon energies beyond 4 eV. The largest enhancement in the Kerr rotation θK is found at about 5 eV. The Kerr rotation θK and ellipticity ηK are enhanced with increasing Pt layer-thickness tPt, and become saturated at tPt=12 Å. This result implies that Pt atoms up to about 6 Å from the interfaces are contributing to the Kerr activity.

Magneto-Optical Properties of PtCu/Co Multilayers

The effect of Cu doping in Pt spacer layers on the magneto-optical properties of sputtered Pt/Co multilayers has been investigated. It was found that the polar Kerr rotation θk, ellipticity ϵk, refractive index n and absorptive index k show an oscillation behavior as a function of the Cu content x, reflective index R keeps almost constant with the decreasing wavelength λ, three θk peaks appear at x = 0.03, 0.12, 0.25, respectively. Values of conductivity tensor σ1xx, σ2xx, σ1xy, and σ2xy in the PtCu layers have been derived from experimental θk, ϵk, n, and k values at wavelength 770 nm. The oscillation of the polar Kerr rotation θk may be resulted from spin polarized quantum size effects in the PtCu layers.

The Magnetic Coupling, Optical and Magneto-Optical Properties of Oscillation Observed in Sputtered Co–Al/Cu Multilayers

We have studied the optical and magneto-Optical properties, and the behavior of interlayer coupling and magnetoresistance in sputtered [Co90Al10(1.6 nm)/Cu]30 multilayers. It is found that for Co–Al/Cu multilayers, the magnetoresistance ratio ΔR/R, the in-plane saturation field Hs and coercive force Hc oscillate with a period of about 0.9 nm, as a function of the Cu layer thickness. It is significant to find that the magneto-Optical polar Kerr rotation θK, and particularly the effective dielectric constant ε oscillate in a period of about 0.9 nm. This result can be attributed to the oscillations of the optical constants in Cu spacer layers, induced by the oscillatory interlayer coupling or quantum well effect. Numerical calculations show that the oscillations of the effective optical constants play an important role in the magneto-Optical Kerr effect oscillations.

Oscillation of polar Kerr rotation in PtCu/Co multilayers

The effect of Cu doped into Pt spacer layers on magneto-optical properties for sputtered Pt/Co multilayers has been investigated. The polar Kerr rotation θk oscillates with the content of Cu in Pt spacer layers, and an obvious enhancement of θk at the first and second peaks on its oscillation curve can be seen. The numerical calculation indicates that the components of conductivity tensor in PtCu layers make a simultaneous oscillation with the polar Kerr rotation. These behaviors may result from the spin polarization in PtCu layers.