SPIN

We present the study of the dynamic properties of CoFeB (CFB) based magnetic tunnel junctions (MTJ) by Brillouin light scattering spectroscopy and micro-strip ferromagnetic resonance technique as well as magnetoresistance measurements. Two different MTJs: single and double barrier systems have been investigated. The dynamic studies revealed mostly similar perpendicular anisotropy field for both CoFeB layers for the single barrier MTJ with the thickest CFB layers, while two sensibly different perpendicular anisotropy fields were revealed in the case of the double barrier MTJ with the thickest CFB soft and hard layers. Modeling the spin wave frequencies behavior allows us the analysis of the experimental data and the derivation of the pertinent magnetic parameters. Our results suggest an interfacial origin of the anisotropy. Therefore, by changing the thickness of the perpendicularly magnetized CFB layers, it is possible to obtain a free and pinned layer usually ensured by the use of synthetic antiferromagnetic material. The tunnel magnetoresistance has been measured in elliptical elements of double barrier MTJ of different sizes. Strong size-dependent switching fields and antiparallel state being more unstable in the smallest device are observed. Patterning effect, lowering the anisotropies around the edge areas and increasing the dipolar coupling that favors the parallel state in the case of stacked layers, are probably the origin.

The ferromagnetic resonance (FMR) and the magnetization switching fields have been analytically determined for weakly coupled Magnetic Tunnel Junction or spin valve systems. Such systems consist of an antiferromagnetic (AF) and two ferromagnetic (F) thin films separated by a nonmagnetic one. Both bilinear $J_1$ and biquadratic $J_2$ coupling parameters are used to describe the interlayer exchange coupling, while the exchange anisotropy field $H_E$ characterizes the interfacial ($F$/AF) interaction. It will be shown how $J_1$, $J_2$, $H_E$ and the in-plane anisotropy fields can be derived for the experimental measure of the FMR field and the different magnetization switching fields.

6, 10 and 20nm thick hexagonal Fe${}_{1.7}$Ge thin films have been grown on face-centered-cubic Ge(111) substrates by molecular beam epitaxy. X-ray diffraction investigations revealed an excellent epitaxy of the Fe${}_{1.7}$Ge films, with crystallographic $[11\bar{2}]$ and $[1\bar{1}0]$ axes lying in the sample plane. Ferromagnetic resonance, magneto-optical Kerr effect and transverse bias initial inverse susceptibility and torque (TBIIST) measurements reveal that the in-plane anisotropy results from the superposition of a uniaxial and a sixfold symmetry terms. The sixfold anisotropy field, which is in agreement with the crystal structure of the samples, increases with the film thickness suggesting an enhancement of the sample quality, while the small uniaxial anisotropy field does not show regular thickness dependence. Frequency and angular dependences of the FMR linewidth show two-magnon scattering and inhomogeneities contributions which depend on the film thickness. A Gilbert damping parameter as low as 0.0064 is found.

It is shown that spin waves propagating in a planar periodic array of dipolarly coupled magnetic strips or dots with in-plane static magnetization can be nonreciprocal. The conditions of the spin waves nonreciprocity are: (i) breaking of the mirror symmetry of an array with respect to its plane, and (ii) complex structure of the array lattice. These conditions can be satisfied, e.g., if an array consists of at least two groups of strips (dots) having different thicknesses (heights). The largest nonreciprocity can be achieved if the static magnetic state of an array is antiferromagnetic, and the difference in the strip thicknesses is about two times. A significant dependence of the spin wave nonreciprocity on the static magnetic state of a strip array allows one to create magnonic crystals with reconfigurable nonreciprocal properties.

The magnetic properties of an iron nitride thin film obtained by ion implantation have been investigated. N${}_2^{+}$ ions were implanted in a pristine iron layer epitaxially grown on ZnSe/GaAs(001). X-ray diffraction measurements revealed the formation of body-centered tetragonal N-martensite whose $c$-axis is perpendicular to the thin film plane and $c$-parameter is close to that of ${\alpha }^{\prime }$-Fe₈N. Magnetic measurements disclosed a weak perpendicular magnetic anisotropy (PMA) whose energy density $K_{\mathrm{\text{PMA}}}$ was assessed to about 10⁵J/m³. A sharp decline of the in-plane magnetocrystalline anisotropy (MCA) was also observed, in comparison with the body-centered cubic iron. The origin of the PMA is attributed to the MCA of N-martensite and/or stress-induced anisotropy. As a result of the PMA, weak magnetic stripe domains with a period of about 130nm aligned along the last saturating magnetic field direction were observed at remanence by magnetic force microscopy. The application of an increasing in-plane magnetic field transverse to the stripes $H_{\mathrm{\text{trans}}}$ highlighted a threshold value (${\mu }_0{H}_{\mathrm{\text{trans}}}\approx 0.1$T) above which these magnetic domains irreversibly rotated. Interestingly, below this threshold, the stripes do not rotate, leading to a zero remanent magnetization along the direction of the applied field. The interest of this system for magnetization dynamics is discussed.

We develop an advanced analytical model for calculating the broadband stripline ferromagnetic resonance (FMR) response for metallic ferromagnetic films, taking into account the exchange interaction as well as the exchange boundary conditions at the film surface. This approach leads to simple analytical expressions in the Fourier space. As a result, a numerical code which implements inverse Fourier transform of these equations is very quick. This allows us to explore a wide space of parameters as numerical examples of application of this theory. In particular, we investigate the joint effects of microwave eddy current shielding and magnetization pinning at the ferromagnetic film surfaces on the shape of the stripline FMR response of the film.