The possibility of exciting microwave oscillations in a nanomagnet by a spin-polarized current, as predicted by Slonczewski and Berger, has recently been demonstrated. This observation opens important prospects of applications in radiofrequency components. However, some unresolved inconsistencies are found when interpreting the magnetization dynamics within the coherent spin-torque model. In some cases, the telegraph noise caused by spin-currents could not be quantitatively described by that model. This has led to controversy about the need for an effective magnetic temperature model. Here we interpret the experimental results of Kiselev et al. using micromagnetic simulations. We point out the key role played by incoherent spin-wave excitation due to spin-transfer torque. The incoherence is caused by spatial inhomogeneities in local fields generating distributions of local precession frequencies. We observe telegraph noise with gigahertz frequencies at zero temperature. This is a consequence of the chaotic dynamics and is associated with transitions between attraction wells in phase space.

Magnetic random access memories (MRAMs) are a new non-volatile memory technology trying establish itself as a mainstream technology. MRAM cell operation using a thermally assisted writing scheme (TA-MRAM) is described in this review as well as its main design challenges. This approach is compared to conventional MRAM, highlighting the improvements in write selectivity, power consumption and thermal stability. The TA-MRAM writing was tested and validated in the dynamic regime down to 500 ps write pulses. The heating process was investigated for the influence of the voltage pulse width, junction area and lead volume looking at the required write power density. The possibilities to control and reduce the write power density are described. The most promising solution to optimize the heating process and reduce the power consumption is to insert two thermal barrier layers at both ends of the MTJ layer stack, between the junction and the electrical leads, using low thermal conductivity materials. This minimizes the heating losses and improves the heating efficiency.

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.

The composition in the surface region of the Heusler alloy NiMnSb has been studied using angle-resolved X-ray photoemission spectroscopy, low-energy electron diffraction and spin-polarized inverse photoemission. The results are consistent with a generally Mn-Sb terminal layer, though the surface composition is very sensitive to preparation. The surface composition has a critical influence on the polarization near the Fermi level in spin-polarized inverse photoemission. Under some conditions, the polarization at the Fermi level for normal incidence inverse photoemission can reach values very close to 100% above background.

Ristoiu, Delia; Hordequin, Ch.; Nozie`res, J. P.; Pierre, J.; Stadler, Shane; and Idzerda, Y. U., "Evidence for temperature dependent moments ordering in ferromagnetic NiMnSb(100)" (2001). Peter Dowben Publications. 23.

A quantitative analysis of the surface composition of the Heusler alloy NiMnSb has been undertaken using angle-resolved x-ray photoemission spectroscopy and the surface composition characterized. With sufficient annealing cycles, the stoichiometric surface evolves to a surface that is manganese rich. This indicates that the surface enthalpy is different from the bulk for the Heusler alloy NiMnSb.

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