GenX is a versatile program using the differential evolution algorithm for fitting X‐ray and neutron reflectivity data. It utilizes the Parratt recursion formula for simulating specular reflectivity. The program is easily extensible, allowing users to incorporate their own models into the program. This can be useful for fitting data from other scattering experiments, or for any other minimization problem which has a large number of input parameters and/or contains many local minima, where the differential evolution algorithm is suitable. In addition, GenX manages to fit an arbitrary number of data sets simultaneously. The program is released under the GNU General Public License.

We report on the experimental realization of tetragonal Fe-Co alloys as a constituent of Fe 0:36 Co 0:64 =Pt superlattices with huge perpendicular magnetocrystalline anisotropy energy, reaching 210 eV=atom, and a saturation magnetization of 2:5 B =atom at 40 K, in qualitative agreement with theoretical predictions. At room temperature the corresponding values 150 eV=atom and 2:2 B =atom are achieved. This suggests that Fe-Co alloys with carefully chosen combinations of composition and distortion are good candidates for high-density perpendicular storage materials. DOI: 10.1103/PhysRevLett.96.037205 PACS numbers: 75.30.Gw, 75.50.Bb, 75.50.Ss The enormous increase in the recording density of hard disk drives, by more than 6 orders of magnitude during the past 50 years, has mainly been achieved by simply scaling the dimensions of the bits recorded in the storage layer [1]. However, this traditional scaling is limited by the onset of superparamagnetism. This occurs when the grain volume V in the recording medium is reduced so that the ratio of the magnetic energy per grain to the thermal energy, K u V=k B T, becomes sufficiently small to cause the recorded data to be erased by thermal fluctuations in an intolerably short time [1,2]. K u is the uniaxial magnetocrystalline anisotropy energy (MAE), i.e., the energy required for rotating the magnetization direction from an easy axis to the hard axis. Thus, high-K u materials [3] are needed to further increase the recording density. The maximum practical MAE, however, is limited by the required write field H w K u =M s , which has to be delivered by the writing head. Thus, a large value of M s , the saturation magnetization of the recording medium, will be beneficial both through decreasing H w as well as by increasing the field available in the readback process. Hence, large values of K u and M s are indispensable properties of future high-density magnetic recording materials.Recently, based on first-principles calculations, tetragonal Fe-Co alloys were proposed as promising materials that combine the desired large values of K u and M s [4]. The advantages of the suggested alloys, as compared to other materials considered for magnetic storage [3], are their about 50% larger saturation magnetization, the huge perpendicular MAE, and the possibility to tailor the MAE by changing the alloy concentration. In addition, Fe-Co alloys do not require as high deposition temperatures as, e.g., chemically ordered L1 0 FePt [5], which has received considerable attention recently. From the calculations it was found that, for certain values of the ratio c=a, between the lengths of the body-centered tetragonal (bct) crystal's c and a axes, and for specific alloy concentrations, very high values of K u 800 eV=atom can be expected. This MAE, which is larger by 3 orders of magnitude than for bcc Fe, occurs theoretically for a composition of about Fe 0:4 Co 0:6 and c=a 1:20-1:25. Also, the predicted easy axis of magnetization for the tetragonal alloy is along the c axis, which facilitat...

The interlayer ordering between ferromagnetic Fe layers in Fe͞V (001) superlattices is switched from initially parallel to antiparallel, as well as antiparallel to parallel, upon introducing hydrogen to the V layers. This process is reversible upon removal of the hydrogen. The results unambiguously prove that the major cause of the interlayer coupling transitions is not the hydrogen-induced changes of the thickness of the V layers, but most likely the distortion of the Fermi surface in the V layers.[S0031-9007(97)03652-1] PACS numbers: 75.70. Cn, 68.55.Ln, 75.50.Bb Since the discovery of the oscillatory magnetic ordering in metallic multilayers [1], substantial work has been devoted to the exploration of the underlying mechanism of the interlayer exchange coupling. For transition metal superlattice systems, current interpretations attribute the basis of the coupling to extremal values of the Fermi wave vector and the discrete lattice spacing of the constituents [2][3][4]. In these models, the coupling between ferromagnetic layers oscillates between parallel and antiparallel alignments as a function of the thickness of an intervening spacer layer. Although a conceptually attractive description of the nature of the exchange coupling is obtained, the complexity of the Fermi surface makes the interpretation far from trivial for the transition metals. The detailed topography of the Fermi surface is expected to play a vital role in the exchange coupling [5]. Experimental exploration of this fundamentally important issue is difficult, since the means to continuously alter the lattice spacing and/or the Fermi energy are limited. One of the first attempts to systematically investigate these effects was on (110) Fe͞Cr 12x V x multilayers [6]. The experimental results were successfully reproduced theoretically, but no experimental data on antiferromagnetic (AFM) coupled Fe͞V (001) multilayers were presented.AFM ordered Fe͞V (001) superlattices have only recently been produced and characterized [7,8]. Samples with three monolayers of Fe were found to be magnetically isotropic in the plane, independent of the V layer thickness [8]. If the electronic states and/or the thickness of the V layers of these samples could be altered continuously, an attractive way of exploring the details of the magnetic interlayer coupling between the ferromagnetic Fe slabs across the nonmagnetic V would be obtained. We will show that this can be done by loading the V layers with H.The hydrogen uptake of Fe͞V superlattices is well known, and the H is found to reside exclusively in the V lattice [9]. The total thickness of the V layers can be changed reversibly by as much as 10% at moderate

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The hydrogen-induced lattice expansion of the vanadium layers in a Fe/V ͑001͒ single crystal superlattice (L Fe /L V ϭ1.8/1.6 nm) was investigated by x-ray diffraction. The expansion coefficient (k s ) was found to be strongly dependent on the hydrogen concentration. At concentrations below 0.1 in average H/V ͑atomic ratio͒ it was determined to k s ϭ0.35(1), which is almost twice the coefficient for bulk vanadium. The corresponding lattice parameter changes are six times those in the bulk material. This gigantic lattice expansion is inferred to be partially caused by the immediate population of the octahedral z sites in the superlattice structure. At intermediate concentrations ͑0.1-0.3͒ the expansion coefficient is approximately 0.10, and at the highest concentrations it is even further reduced. The nonlinear behavior is associated with the long range of the host-mediated elastic interaction and the local ordering of the dipolelike elastic distortion of the host lattice.