Néel's theory of magnetostatic coupling between two magnetic layers with inplane magnetization separated by a non-magnetic spacer has been extended to the case of multilayers with perpendicular anisotropy. It is shown that the presence of a correlated roughness between the successive interfaces induces an interlayer coupling through the spacer analogous to the well-known orange peel coupling. However, depending on the parameters describing the interfacial roughness, the magnetic anisotropy and the exchange stiffness constant, this coupling can favor either parallel or an antiparallel alignment of the magnetization in the two ferromagnetic layers. This model was used to quantitatively interpret the variation of interlayer coupling vs. thickness of Pt spacer layer in out-of-plane magnetized exchange-biased spin-valves comprising (Pt/Co) multilayers as free and pinned layers. It is shown that the net coupling can be interpreted by the coexistence of perpendicular orange peel and oscillatory RKKY couplings. Interestingly, since these two couplings have different thickness dependence, in certain range of Pt thickness, the coupling changes sign during growth, being antiferromagnetic at the early stage of the growth of the top (Co/Pt) multilayer but ferromagnetic once the growth is completed.

International audienc

In (Pt/Co)n/FeMn multilayers, the magnitude of exchange bias, HE, can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer (ML) and the antiferromagnetic (AFM) layer. The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased. This bias enhancement is accompanied by an increase of coercivity, HC. This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic (FM)/AFM spin projection, thus leading to the HE and HC enhancements. The decrease of HE and HC for thicker spacer layers is due to the limited range of the FM–AFM proximity effect.

The use of microwave-assisted synthesis (in water) of α-Fe 2 O 3 nanomaterials followed by their transformation onto iron oxide Fe 3 O 4 -γ-Fe 2 O 3 hollow nanoparticles encoding well-defined sizes and shapes [nanorings (NRs) and nanotubes (NTs)] is henceforth described. The impact of experimental variables such as concentration of reactants, volume of solvent employed, and reaction times/temperatures during the shape-controlled synthesis revealed that the key factor that gated generation of morphologically diverse nanoparticles was associated to the initial concentration of phosphate anions employed in the reactant mixture. All the nanomaterials presented were fully characterized by powder X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared, Mossbauer spectroscopy, and superconducting quantum interference device (SQUID). The hollow nanoparticles that expressed the most promising magnetic responses, NTs and NRs, were further tested in terms of efficiencies in controlling the magnetic hyperthermia, in view of their possible use for biomedical applications, supported by their excellent viability as screened by in vitro cytotoxicity tests. These systems NTs and NRs expressed very good magneto-hyperthermia properties, results that were further validated by micromagnetic simulations. The observed specific absorption rate (SAR) and intrinsic loss power of the NRs and NTs peaked the values of 340 W/g and 2.45 nH m 2 kg −1 (NRs) and 465 W/g and 3.3 nH m 2 kg −1 (NTs), respectively, at the maximum clinical field 450 Oe and under a frequency of 107 kHz and are the highest values among those reported so far in the hollow iron-oxide family. The higher SAR in NTs accounts the importance of magnetic shape anisotropy, which is well-predicted by the modified dynamic hysteresis (β-MDH) theoretical model.

The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

We report on the study of magnetic properties of the La 1.5 Ca 0.5 CoIrO 6 double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature, the material exhibits exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of Ir 4+ ions located between the antiferromagnetically coupled Co ions.

The zero-field-cooled exchange bias (ZEB) effect is a remarkable phenomenon recently reported for some reentrant spin glass-like compounds. In this work, the time-evolution of magnetization is thoroughly investigated for two ZEB materials in order to figure out the role played by the spin glasslike phase on such effect. La1.5Sr0.5CoMnO6 and La1.5Ca0.5CoMnO6 were chosen as representative samples of ZEB systems, since the former compound presents the largest ZEB reported so far, while the second has a much smaller effect, despite being structurally/chemically similar. Comprehensive magnetic measurements were carried on both samples, and the results are discussed in terms of the amount and time-evolution of the spin glass-like phase under the influence of a varying field. We also propose a phenomenological model, based on the pinning of spin glass-like moments and on the dynamics of their magnetic relaxation, to explain the asymmetry observed in the hysteresis loops. The good agreement between the simulated and experimental results confirms our hypothesis that the spin glass-like phase is key to the ZEB effect.

We report the observation of exchange bias in (Pt/Co0.90Fe0.10)n/FeMn multilayers, with perpendicular magnetic anisotropy. We analyze the behavior of the exchange-bias field Heb as a function of temperature, FeMn thickness, number of (Pt/Co0.90Fe0.10) bilayers, and annealing conditions. Measurements carried out with field applied perpendicular to the plane of the samples indicate that the hysteresis loops present 100% of remanent magnetization, with Heb reaching 200 Oe and a tunable coercive field, depending on Co and Pt thickness ratio and on Pt buffer thickness. Furthermore, spin-valves of the form (Pt/Co0.90Fe0.10)n/NM/(Co0.90Fe0.10/Pt)m/Co0.90Fe0.10/FeMn with NM=Cu or Pt have been prepared. They exhibit two well separated hysteresis loops when the field is applied perpendicular to the plane.