J Díaz

Asymmetric grazing incidence small angle x-ray scattering and anisotropic domain wall motion in obliquely grown nanocrystalline Co films.

Strong asymmetries have been observed in grazing incidence small angle x-ray scattering (GISAXS) in situ patterns obtained from 30 nm-thick nanocrystalline Co films prepared by oblique sputtering (15°-75° off-sample normal). These asymmetries have been qualitatively simulated by a simple model consisting of an ensemble of 8 nm-wide inclined Co nanocolumns. It is found that narrow inclined features appear in the diffuse background resembling those characteristic of faceted systems, which can be used to obtain straightforward non-destructive estimations of buried nanocolumnar grains inclination, even for oblique angles below 45°, when the stronger and broader asymmetric features of the pattern are not yet fully formed. Furthermore, using magneto-optical microscopy, a marked change in the magnetic domains nucleation and growth process has been observed in the sample prepared at 75°, with the stronger GISAXS asymmetries. Easy axis magnetization reversal starts by a random and homogeneous nucleation of small (∼μm) elongated domains aligned with the nanocolumns long axis and proceeds through the preferred propagation of head-to-head domain walls (DWs) along the applied field direction. This peculiar magnetic behavior indicates that the strongly anisotropic nanostructuring created by the oblique growth process is equivalent, from a magnetic point of view, to an array of self-assembled buried nanowires. These results show how GISAXS and magneto-optical microscopy can be combined as a powerful tool for correlating the morphology and magnetism of thin nanostructured systems.

Large negative thermal expansion of the Co subnetwork measured by EXAFS in highly disordered Nd1−xCox thin films with perpendicular magnetic anisotropy

We have measured a negative thermal expansion (NTE) of the Co subnetwork in amorphous Nd1-xCox (0.78 < x < 0.84) thin films of the order of 1% in volume using linearly polarized EXAFS spectroscopy at RT and 10 K. The expansion, which is anisotropic, is uncorrelated with the perpendicular magnetic anisotropy (PMA) observed in all the films, but correlated with the method used to deposit them. The atomic environments of the Nd atoms resulted in such a strong disorder that Nd-Nd and Nd-Co environments were invisible to EXAFS, and only Co-Co atomic environments were detected. The information on the Nd subnetwork was obtained through its magnetic moment measured by XMCD. These measurements demonstrate an increasing interaction of neodymium atoms with their particular local crystal field as the temperature decreased, suggesting possible structural modifications at their sites. Since the magnetic moment of the cobalt subnetwork remains essentially constant with the temperature, it is proposed that its detected NTE may be caused by the mechanical response of the amorphous network to structural transformations at the Nd sites. These results support that the PMA in RE-TM alloys is localized at the RE sites. The complete absence of EXAFS oscillations in the Nd L3 EXAFS spectra is remarkable: it means that the coherence length of the photoemitted electrons in disordered matter can be strongly reduced from that expected by atomic calculations to the point of being less than first neighbor distances, which is contrary to the common belief that first neighbors are always visible by EXAFS.

Microscopic origin of perpendicular magnetic anisotropy in amorphous Nd-Co homogeneous and compositionally modulated, thin films studied by XMCD

Amorphous Nd-Co films deposited by DC-magnetron sputtering presented perpendicular magnetic anisotropy (PMA) with energies, KN, of the order of 106 erg/cc at RT. To understand the origin of their PMA, we measured the orbital and spin magnetic moments in Co and Nd by XMCD at the Co L3,2 and Nd M5,4 edges in two kinds of samples of similar thickness (30 nm) and composition: one compositionally modulated Nd/Co film (CM) with strong PMA (KN ~107 erg/cc at 10 K) and a homogenous alloy (A) with not strong enough PMA to see stripe domains for such thickness. The XMCD analysis evidenced the significant role of Nd in the PMA of these films.

Structural and magnetic properties of CoxSi1−x thin films and multilayers

The structural and magnetic properties of thin films and multilayers based on CoxSi1−x magnetic layers and Si spacers have been studied by surface x-ray diffraction, Auger electron spectroscopy and magneto-optical transverse Kerr effect measurements. Surface x-ray diffraction has been used to follow the evolution of the Co lattice as Si is incorporated into the network for the low Si content range, 1.00≤x≤0.77, and a contraction of the unit cell, around 0.2% out-of-plane and 0.9% in-plane, has been found. No fingerprint of structural anisotropy in the grain size, either parallel or perpendicular to the magnetic easy axis, has been detected. Magneto-optical transverse Kerr effect hysteresis loops of single films show both an increase of coercive field and a rotation of the magnetic anisotropy axis as Si enters the lattice in the polycrystalline range, 1.00≤x≤0.85. For the multilayer case, the loops confirm the formation of antiferromagnetic ordering through the layers when the composition of the alloy is tuned to be in the amorphous range. The corresponding AES signals show asymmetric depth profiles, suggesting the formation of intermediate compounds at the broader Co-on-Si interfaces.

Transverse Kerr effect as an analytical tool in the study of the magnetism of transition metal—yttrium alloys

A theoretical model of the transverse Kerr effect (TKE), suitable for the study of the magnetism of surfaces and interfaces, has been developed. In this work, it is applied in a Co/Y bilayer with different Co thickness, where a reversal in the sign of TKE is observed at thin Co thickness.