Perpendicular magnetic anisotropy in amorphous NdxCo1−x thin films studied by x-ray magnetic circular dichroism

Abstract

The origin of perpendicular magnetic anisotropy (PMA) in amorphous Nd x Co 1−x thin films is investigated using x-ray magnetic circular dichroism (XMCD) spectroscopy at the Co L 2,3 and Nd M 4,5 edges. The magnetic orbital and spin moments of the 3d cobalt and 4f neodymium electrons were measured as a function of the magnetic field orientation, neodymium concentration, and temperature. In all the studied samples, the magnetic anisotropy of the neodymium subnetwork is always oriented perpendicular to the plane, whereas the anisotropy of the orbital moment of cobalt is in the basal plane. The ratio L z /S z of the neodymium 4f orbitals changes with the sample orientation angle, being higher and closer to the atomic expected value at normal orientation and smaller at grazing angles. This result is well explained by assuming that the 4f orbital is distorted by the effect of an anisotropic crystal field when it is magnetized along its hard axis, clearly indicating that the 4f states are not rotationally invariant. The magnetic anisotropy energy associated to the neodymium subnetwork should be proportional to this distortion, which we demonstrate is accessible by applying the XMCD sum rules for the spin and intensity at the Nd M 4,5 edges. The analysis unveils a significant portion of neodymium atoms magnetically uncoupled to cobalt, i.e., paramagnetic, confirming the inhomogeneity of the films and the presence of a highly disordered neodymium rich phase already detected by extended x-ray-absorption fine structure (EXAFS) spectroscopy. The presence of these inhomogeneities is inherent to the evaporation preparation method when the chosen concentration in the alloy is far from its eutectic concentrations. An interesting consequence of the particular way in which cobalt and neodymium segregates in this system is the enhancement of the cobalt spin moment which reaches 1.95 μ B in the sample with the largest segregation.

Deviations from bulk behavior in TbFe(Co) thin films: Interfaces contribution in the biased composition

“…The general features are in agreement with former results on GdCo films [22] showing Gd surfactant and oxidation effects. This also supports a thickness dependence of the magnetic properties of the samples, as demonstrated in the following but in old and recent papers as well [23][24][25][26][27][28]. We next discuss how this bias in the effective composition impacts on magnetic properties.…”

Magnetic moment orientation and in depth distribution of dysprosium near the surface of DyCo$_{4.6}$ thin films determined by the analysis of their X ray circularly polarized absorption Dy $M_{5}$ spectra

“…A similar effect, although much more pronounced than in the present case, has been observed in NdCo films [21]. The effect was demonstrated to be caused by a portion of paramagnetic Nd.…”

“…Such effects should be stronger at the surface whose assumed larger RE concentration is driven by RE-RE bonding preference. Previous XMCD studies in NdCo alloys conducted by us using TEY detection [21] reported a substantial proportion of RE atoms that behaved as a paramagnetic, indicating that not all the RE atoms probed might have the same exchange interaction strength with the TM and, therefore, the same magnetic orientation distribution. Although those experiments were sensitive to the surface, their exact distribution was not demonstrated.…”

“…The magnetic moments of cobalt were deduced from their L 2,3 spectra. The method used to extract the cobalt absorption coefficient to correctly apply the XMCD sum rules [27,28], which considers saturation effects, is fully described in [21]. The number of holes was calculated comparing their unpolarized absorption spectra with that of a pure cobalt reference sample deposited in similar conditions than the rest of the films.…”

Ferromagnetic Resonance Studies in Magnetic Nanosystems

“…On the one hand, varying the Co concentration (x = 5, 7.5, and 9) in the NdCo x film allows the modification of the strength of its PMA. A maximum has been found for x = 5, whereas higher or lower Co concentrations lead to a gradually weaker PMA, respectively [29,30]. On the other hand, by adjusting the Al spacer thickness (t = 0 nm, 2.5 nm, 5 nm, and 10 nm), the type of coupling between the two magnetic layers can be set to either direct exchange coupling (t ≤ 1.5 nm) or stray field coupling (t ≥ 2.5 nm).…”