In order to further increase the recording density in hard disk drives, new media materials are required. Two essential parameters of future recording media are a large uniaxial magnetic anisotropy energy (MAE) K(u) and a large saturation magnetization M(s). Based on first-principles theory, we predict that very specific structural distortions of FeCo alloys possess these desired properties. The discovered alloy has a saturation magnetization that is about 50% larger than that of FePt--a compound that has received considerable attention lately-with a uniaxial MAE that can easily be tailored reaching a maximum value that is 50% larger than that of FePt.

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 uniaxial magnetic anisotropy energy ͑MAE͒ of L1 0 FePt and Fe 1−x Mn x Pt, x = 0 − 0.25, was studied from first principles using two fully relativistic computational methods, the full-potential linear muffin-tin orbitals method and the exact muffin-tin orbitals method. It was found that the large MAE of 2.8 meV/f.u. is caused by a delicate interaction between the Fe and Pt atoms, where the large spin-orbit coupling of the Pt site and the hybridization between Fe 3d and Pt 5d states is crucial. The effect of random order on the MAE was modeled by mutual alloying of the sublattices within the coherent potential approximation ͑CPA͒, and a strong dependence of the MAE on the degree of chemical long-range order was found. The alloying of FePt with Mn was investigated with the virtual crystal approximation and the CPA as well as supercell calculations. The MAE increases up to 33% within the concentration range studied here, an effect that is attributed to band filling. Furthermore, the dependence of the MAE on the structural properties was studied.

The internal friction Q-1 and the sound velocity deltav/v of vitreous silica were measured at very low temperatures using mechanical double paddle resonators operated at frequencies ranging from 0.33 to 14 kHz. Below approximately 40 mK the internal friction showed an unexpected temperature and frequency dependence, with absolute values of Q-1 clearly exceeding those predicted by the standard tunneling model. Even though the most plausible origin of the observed excess internal friction appears to be the mutual interaction between tunneling states, the results are difficult to reconcile quantitatively with present theories taking into account this interaction.

Using density functional calculations, we have studied the magnetic properties of nanocomposites composed of rare-earth-metal elements in contact with 3d transition metals (Fe and Cr). We demonstrate the possibility to obtain huge magnetic moments in such nanocomposites, of order 10mu(B)/rare-earth-metal atom, with a potential to reach the maximum magnetic moment of Fe-Co alloys at the top of the so-called Slater-Pauling curve. A first experimental proof of concept is given by thin-film synthesis of Fe/Gd and Fe/Cr/Gd nanocomposites, in combination with x-ray magnetic circular dichroism.