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Register a new userEngineering the polar magneto-optical Kerr effect in strongly strained L10-MnAl films
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We report the engineering of the polar magnetooptical (MO) Kerr effect in perpendicularly magnetized L10-MnAl epitaxial films with remarkably tuned magnetization, strain, and structural disorder by varying substrate temperature (Ts) during molecular-beam epitaxy growth. The Kerr rotation was enhanced by a factor of up to 5 with Ts increasing from 150 to 350 oC as a direct consequence of the improvement of the magnetization. A similar remarkable tuning effect was also observed on the Kerr ellipticity and the magnitude of the complex Kerr angle, while the phase of the complex Kerr angle appears to be independent of the magnetization. The combination of the good semiconductor compatibility, the moderate coercivity of 0.3-8.2 kOe, the tunable polar MO Kerr effect of up to ~0.034o, and giant spin procession frequencies of up to ~180 GHz makes L10-MnAl films a very interesting MO material. Our results give insights on both the microscopic mechanisms of the MO Kerr effect in L10-MnAl alloys and their scientific and technological application potential in the emerging spintronics and ultrafast MO modulators.
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Magneto-optical spectroscopy based on the transverse magneto-optical Kerr effect (TMOKE) is a sensitive method for investigation of magnetically-ordered media. However, in magnetic materials the optical transitions are usually characterized by spectrally broad resonances with widths considerably exceeding the Zeeman splitting in the magnetic field. Here we investigate experimentally and theoretically the TMOKE in the vicinity of relatively narrow optical resonances provided by confined quantum systems. For experimental demonstration we use the exciton resonance in a (Cd,Mn)Te diluted magnetic semiconductor quantum well, where the strong exchange interaction with magnetic ions enables the giant Zeeman splitting of exciton spin states $Delta$ in magnetic fields of a few Tesla. In the weak coupling regime, when the splitting $Delta$ is smaller than the spectral broadening of the optical transitions $Gamma$, the TMOKE magnitude grows linearly with the increase of the Zeeman splitting and its spectrum has an S-shape, which remains virtually unchanged in this range. In the strong coupling regime ($Delta>Gamma$) the TMOKE magnitude saturates, while its spectrum is strongly modified resulting in the appearance of two separate peaks. The TMOKE is sensitive not only to the sample surface but can be used to probe the confined electronic states in depth if the upper layer is sufficiently transparent. Our results demonstrate that TMOKE of spectrally narrow resonances serves as a versatile tool for probing the charge and spin structure of electronic states in various confined quantum systems and can be used for spin tomography in combination with the conventional polar Kerr effect.
We have performed a systematic magneto-optical Kerr spectroscopy study of GaMnAs with varying Mn densities as a function of temperature, magnetic field, and photon energy. Unlike previous studies, the magnetization easy axis was perpendicular to the sample surface, allowing us to take remanent polar Kerr spectra in the absence of an external magnetic field. The remanent Kerr angle strongly depended on the photon energy, exhibiting a large positive peak at $sim1.7$ eV. This peak increased in intensity and blue-shifted with Mn doping and further blue-shifted with annealing. Using a 30-band ${bf kcdot p}$ model with antiferromagnetic $s,p$-$d$ exchange interaction, we calculated the dielectric tensor of GaMnAs in the interband transition region, assuming that our samples are in the metallic regime and the impurity band has merged with the valence band. We successfully reproduced the observed spectra without emph{ad hoc} introduction of the optical transitions originated from impurity states in the band gap. These results lead us to conclude that above-bandgap magneto-optical Kerr rotation in ferromagnetic GaMnAs is predominantly determined by interband transitions between the conduction and valence bands.
Using density functional theory calculations, we have found an enhanced magneto-optical Kerr effect in Fe/insulator interfaces. The results of our study indicate that interfacial Fe atoms in the Fe films have a low-dimensional nature, which causes the following two effects: (i) The diagonal component $sigma_{xx}$ of the optical conductivity decreases dramatically because the hopping integral for electrons between Fe atoms is suppressed by the low dimensionality. (ii) The off-diagonal component $sigma_{xy}$ of the optical conductivity does not change at low photon energies, but it is enhanced at photon energies around 2 eV, where we obtain enhanced orbital magnetic moments and spin-orbit correlations for the interfacial Fe atoms. A large Kerr angle develops in proportion to the ratio $sigma_{xy}/sigma_{xx}$. Our findings indicate an efficient way to enhance the effect of spin-orbit coupling at metal/insulator interfaces without using heavy elements.
Antiferromagnetic metals attract tremendous interest for memory applications due to their expected fast response dynamics in the terahertz frequency regime. Reading from and writing information into these materials is not easily achievable using magnetic fields, due to weak high-order magneto-optical signals and robustness of the magnetic structure against external magnetic fields. Polarized electromagnetic radiation is a promising alternative for probing their response, however, when ideal antiferromagnetic symmetry is present, this response vanishes. Hence, in this work we combine first-principles simulations with measurements of the polar magneto-optical Kerr effect under external magnetic fields, to study magneto-optical response of antiferromagnetic M$_2$As (M=Cr, Mn, and Fe). We devise a computational scheme to compute the magnetic susceptibility from total-energy changes using constraints on magnetic-moment tilting. Our predictions of the spectral dependence of polar magneto-optical Kerr rotation and ellipticity allow us to attribute these effects to breaking of the magnetic symmetry. We show that tilting affects the exchange interaction, while the spin-orbit interaction remains unaffected as the tilting angle changes. Our work provides understanding of the polar magneto-optical Kerr effect on a band structure level and underscores the importance of the magnetic susceptibility when searching for materials with large magneto-optical response.
We examined the photo-induced dynamics of ferromagnetic Co/Pt thin films demonstrating perpendicular magnetic anisotropy with element specificity using resonant polar magneto-optical Kerr effect measurements at Pt~N${}_{6,7}$ and Co~M${}_{2,3}$ edges with an x-ray free electron laser. The obtained results showed a clear element dependence of photo-induced demagnetization time scales: $tau_textrm{demag.}^textrm{Co}=80pm60~textrm{fs}$ and $tau_textrm{demag.}^textrm{Pt}=640pm140~textrm{fs}$. This dependence is explained by the induced moment of the Pt atom by current flow from the Co layer through the interfaces. The observed magnetization dynamics of Co and Pt can be attributed to the characteristics of photo-induced Co/Pt thin film phenomena including all-optical switching.
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