Direction of the magnetization of thin films and sandwiches as a function of temperature

Spin reorientation transition in CoFeB/MgO/CoFeB tunnel junction enabled by ultrafast laser-induced suppression of perpendicular magnetic anisotropy

Magnetic tunnel junction (MTJ) is a leading contender for next generation high-density nonvolatile memory technology. Fast and efficient switching of MTJs between different resistance states is a challenging problem, which can be tackled by using an unconventional stimulus-a femtosecond laser pulse. Herein, we report an experimental study of the laser-induced magnetization dynamics in a Co₂₀Fe₆₀B₂₀/MgO/Co₂₀Fe₆₀B₂₀ (CoFeB/MgO/CoFeB) MTJ with ultrathin CoFeB electrodes possessing perpendicular magnetic anisotropy (PMA). In addition to ultrafast demagnetization, a femtosecond laser pulse gives rise to a decaying magnetization precession in the thinner CoFeB layer subjected to an in-plane magnetic field, while the magnetization of the thicker CoFeB layer remains aligned with the applied field. Remarkably, the precession frequency demonstrates a strong and nonlinear rise with increasing pump fluence, which stems from the complete laser-induced suppression of PMA in the 1.2 nm-thick CoFeB electrode reached at a moderate fluence of about 1.8 mJ cm^-2 at room temperature. This important feature signifies that the laser excitation of such an electrode can enable an ultrafast transition from a perpendicular-to-plane to an in-plane magnetization orientation in the absence of a magnetic field and reveals the feasibility of the laser-driven switching of MTJ between different states. The revealed gradual quenching of PMA with increasing fluence is explained by the laser-induced heating of the MTJ, which affects the interfacial magnetic anisotropy stronger than the shape anisotropy. Interestingly, at low fluences, the values of interfacial anisotropy and saturation magnetization altered by the laser excitation scale with each other as expected for the two-site anisotropic exchange interaction, but the scaling exponent increases significantly at moderate fluences, which enables the realization of a laser-induced spin reorientation transition.

Broadband spectroscopy of thermodynamic magnetization fluctuations through a ferromagnetic spin-reorientation transition

We use scanning optical magnetometry to study the broadband frequency spectra of spontaneous magnetization fluctuations, or "magnetization noise", in an archetypal ferromagnetic film that can be smoothly tuned through a spin reorientation transition (SRT). The SRT is achieved by laterally varying the magnetic anisotropy across an ultrathin Pt/Co/Pt trilayer, from the perpendicular to in-plane direction, via graded Ar⁺ irradiation. In regions exhibiting perpendicular anisotropy, the power spectrum of the magnetization noise, S(ν), exhibits a remarkably robust ν ^-3/2 power law over frequencies ν from 1 kHz to 1 MHz. As the SRT region is traversed, however, S(ν) spectra develop a steadily-increasing critical frequency, ν ₀, below which the noise power is spectrally flat, indicating an evolving low-frequency cutoff for magnetization fluctuations. The magnetization noise depends strongly on applied in- and out-of-plane magnetic fields, revealing local anisotropies and also a field-induced emergence of fluctuations in otherwise stable ferromagnetic films. Finally, we demonstrate that higher-order correlators can be computed from the noise. These results highlight broadband spectroscopy of thermodynamic fluctuations as a powerful tool to characterize the interplay between thermal and magnetic energy scales, and as a means of characterizing phase transitions in ferromagnets.

Magnetic Domain Imaging of Epitaxial thin Films by Spin-Polarized Scanning Electron Microscopy

The formation of magnetic domains in thin epitaxial Co/Au(111) films is investigated by spin-polarized scanning electron microscopy. Three-monolayer films are shown to decay into out-of-plane domains of micrometer size. The transition from out-of-plane to in-plane magnetization at a crossover thickness of 4.5 layers is followed by imaging the domains, and the transition is shown to occur as a continuous rotation of the magnetization. The domain size in field-free-grown perpendicular films depends linearly on film thickness. From high-resolution line scans across magnetization reversals we determine the resolution in magnetic imaging to be better than 40 nm.

Crystal field effect on a bilayer Bethe lattice

The influence of the crystal field on the phase diagrams of the bilayer spin-1 Ising model on the Bethe lattice is studied in terms of the intralayer coupling constants J1 and J2 of the two layers and interlayer coupling constant J3 between the layers for given values of the coordination number q by using the recursion relation scheme. The six distinct ground-state configurations of the model are obtained on the (J_{2}J_{1},J_{3}qJ_{1}) plane with J1>0 , the ferromagnetic coupling, for given values of the crystal field. Then, the phase diagram of the system is obtained on the (kTJ_{1},J_{3}J_{1}) plane for given values of the crystal field and alpha=J_{2}J_{1} with q=4 corresponding to the square lattice in real lattice systems. It was found that the system presents both first- and second-order phase transitions, therefore, tricritical points. The paramagnetic phase was also divided into two phases, P+ and P_{-} , by studying the thermal behavior of the quadrupolar moments of the two layers.