Perpendicular Magnetocrystalline Anisotropy in Tetragonally Distorted Fe-Co Alloys

Abstract

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...

Magnetic properties of(Fe1−xCox)2Balloys and the effect of doping byEdström1, Werwiński2, Iuşan3 et al. 2015Phys. Rev. B

“…The highest uniaxial anisotropy, with MAE = 1.5 MJ/m 3 , occurs for the (Fe 0.6 Co 0.4 ) 2 B alloy. Quantitative overestimation of the MAE for the whole concentration range brings reminiscence to the case of tetragonally strained Fe/Co alloys, where VCA treatment of substitutional disorder [11] also led to an overestimation of MAE compared to experiments [13] or more sophisticated treatments of disorder [16,17]. Therefore, in the next subsection we will also present results based on a more advanced description of disorder, the coherent potential approximation (CPA).…”

“…Recently, however, the availability (and thus price) of the rare-earth elements became rather volatile, calling for development of replacement materials which would use less or none of * Corresponding author: jan.rusz@physics.uu.se the rare-earth elements. Intense research efforts have started worldwide, revisiting previously known materials, such as Fe 2 P [5][6][7], FeNi [8], or Fe 16 N 2 [9], doing computational data mining among the large family of Heusler alloys [10], exploring the effects of strain [11][12][13][14][15][16][17] and doping by interstitial elements [18,19], multilayers such as Fe/W-Re [20] or, as a limiting case of multilayers, the L1 0 family of compounds [21], or promising Mn-based systems [22][23][24][25][26][27], among others.…”

Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain

“…Calculations 2,3 based on Density Functional Theory (DFT) predicted remarkably high MCAs if the unit cell would be tetragonally distorted with c/a between 1.2 and 1.25. The most common approach to strain the unit cell experimentally [8][9][10][11] is coherent epitaxial growth of thin films on suitable substrates that provide appropriate in-plane lattice parameters a. Assuming constant volume of the Fe-Co unit cell, a strain parallel to the film normal is expected when a is smaller than the equilibrium lattice parameter of Fe-Co. Up to now, tetragonally distorted Fe-Co films with perpendicular easy axis of magnetization could not be produced with film thicknesses exceeding 15 monolayers (ML) [9][10][11] .…”

Tailoring magnetic anisotropy at the ferromagnetic/ferroelectric interface

“…In thin films and multilayers, however, the reduced symmetry of surfaces and interfaces yields an additional anisotropy effect. 3 As a result, the magnetocrystalline anisotropy can be enhanced by orders of magnitude in thin films, 4 interfaces, 5 and superlattices, 6 overcoming the magnetostatic shape anisotropy which has a tendency to align the magnetization in the plane of a film or a multilayer. Therefore, high anisotropy materials, especially the thin ferromagnetic films, are widely utilized in modern perpendicular magnetic recording technology, which can achieve a storage density of 1 Tbits/ in.…”