Perpendicular magnetic anisotropy and high spin-polarization ratio in epitaxial Fe-N thin films

Abstract

We have demonstrated a general framework for realizing and modulating perpendicular magnetic anisotropy in a rare-earth-element and heavy-metal -free material system. Using GaAs(001)/Fe(001) template, we have developed a synthesis scheme to produce epitaxial body center tetragonal Fe-N with (001) texture. By varying the N doping concentration, the crystal tetragonality (c/a) can be tuned in a relatively wide range. It is found that the Fe-N layer developed a strong perpendicular magnetic crystalline anisotropy (MCA) as it approaches the iron nitride interstitial solubility limit. Further annealing process significantly improves the MCA due to the formation of chemically ordered Fe 16 N 2 . In addition to realize an MCA up to 10 7 erg/cm 3 , the spin polarization ratio (P~0.52), as probed directly by a Point Contact Andreev Reflection (PCAR) method, even shows a moderate increase in comparison with normal metal Fe (P~0.45). These combined properties make this material system a promising candidate for applications in spintronic devices and also potential rare-earth-element free magnets.

Effects of alloying and strain on the magnetic properties of Fe16N2

“…Experimentally, the large tetragonality can be obtained in films, where it can be tuned by the nitrogen concentration. 37 However, according to our results above, doping bulk Fe 16 N 2 in a way that increases c/a may lead to MAE increase. The MAE and AMAE are well correlated as shown in Fig.…”

Engineering perpendicular magnetic anisotropy in Fe via interstitial nitrogenation: N choose K

“…X-ray diffraction (XRD) revealed that the films grow epitaxially with the c-axis oriented perpendicular to the film surface, and with the epitaxial relation α in reasonable agreement with previous results. 6,7 Magnetic force microscopy (MFM) measurements in demagnetized state and zero field at room temperature confirm this physical picture. A sample with small c/a value (c/a = 1.01) and low K u showed almost no magnetic signal (see Fig.…”

“…But more than that for α ′ -Fe 8 N x thin films experimentally also a switching of the magnetization from in-plane direction to out-of-plane (perpendicular) direction along the crystallographic c-axis has been observed, 7,8 but remained elusive to theoretical understanding. Tetragonal distortions alone are not enough to induce PMA in α-Fe.…”

“…A key method to obtain high perpendicular magneto-crystalline anisotropy (PMA) is to impose specific structural distortions, as, e.g., suggested for the case of high-magnetization FeCo alloys. 3 One particularly interesting material system is the body-centered tetragonal (bct) α ′ -Fe 8 N x 4 iron nitride phase, showing high magnetization as well as decent PMA, [5][6][7][8][9] with a uniaxial magneto-crystalline anisotropy constant, K u , as high as 1 · 10 7 erg/cm 3 for the ordered α ′′ -Fe 16 N 2 10 phase.…”

Effect of dopants on thermal stability and self-diffusion in iron-nitride thin films

“…Due to dominant Fe-Fe interactions, N atoms are weakly bonded to Fe atoms and preserve the structure and the magnetic properties of Fe [11]. Magnetic properties of interstitial Fe-N supersedes pure Fe, giving rise to interesting effects such as giant magnetic moment [12][13][14], soft magnetic properties [15,16], perpendicular magnetic anisotropy [14,17,18], etc. However, interstitial Fe-N with such a large amount of N are thermodynamically unfavorable [11], and unlike iron oxides, their enthalpy of formation ( H • f ) is at least an order of magnitude more, and even positive as in the case of α -Fe 16 N 2 ( H • f = 85 ± 47) [11].…”