Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single-crystal films (invited)

Abstract

Single-crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X-ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic moment of 3.2μB per Fe atom and 3.5μB, respectively. These values of the magnetic moment of Fe atoms are literally giant, far beyond the Slater–Pauling curves. The origin of the giant magnetic moment has been discussed based on the calculation carried out by Sakuma. However, there was a significant disagreement between experimental values and calculated ones, so the origin remained to be clarified. Also, magneto-crystalline anisotropy of Fe16N2 films has been investigated.

Effects of alloying and strain on the magnetic properties of Fe16N2

“…For example, Sugita et al 3 obtained an in-plane MAE, while Takahashi 18 found a large uniaxial MAE. The only available theoretical calculations of MAE used an empirical tight binding (TB) approximation.…”

“…2 This result was not independently confirmed until twenty years later. 3 Owing to the rapid development of the magnetic recording technologies, this confirmation inspired numerous studies of thin-film samples. However, the existence of the "giant" Fe moment remains controversial as many researchers did not reproduce these findings, while others confirmed them.…”

“…In particular, measurements of T C are hampered by the decomposition of the metastable Fe 16 N 2 into Fe 4 N and Fe, which was reported to occur above 200 • C, 6 in the 230-300 • C range, 2 or at 400 • C. 3 Sugita et al extrapolated their data to estimate T C at 540 • C. 3 Thermal stability of Fe 16 N 2 was reported to increase with addition of Co and Ti 16,17 (up to 700 • C in the Ti case). However, no experimental information is presently available about the T C of Co or Ti-doped Fe 16 N 2 , or of any other Fe 16 N 2 samples stabilized at high temperatures.…”

Theoretical investigation into the possibility of very large moments in Fe16N2

“…It was then, 20 years later, that Kim and Takahashi 2 reported polycrystalline, mixed-phase Fe-N films with a saturation magnetization exceeding that of both α-Fe and Co 35 Fe 65 (∼ 2 × 10 6 A/m). However, it took another 20 years for the result to be reproduced (and, in fact, surpassed) by Sugita et al 3,4 Throughout the 1980s and '90s, other measurements of Fe 16 N 2 thin films were reported that generally did not find this large magnetic moment. [5][6][7][8] Concurrently, density-functional theory (DFT) electronic structure calculations were performed, 9-14 finding the moment per Fe ion to be modestly increased with respect to bulk bcc Fe but far short of the 3.5 µ B reported by Sugita et al It was shown 15 that LSDA+U 18 calculations could yield an average moment comparable to that of some experiments (∼ 2.8µ B per Fe), but the parameters (U ≈ 3.94, 1.0, and 1.34 eV on the 4d, 4e, and 8h sites, respectively, with J = U/10) were obtained via an embedded-cluster method with a small screening constant and were not calculated from first principles.…”

“…Mössbauer spectroscopy has been performed in many previous works 3,4,8,[38][39][40][41] , and the hyperfine field has been calculated 9,12,13,42 from DFT. Our calculated B hf (found along with our calculated Fe moments in Table I) agrees well with these past results; we find that the Fe sites with N nearest-neighbors exhibit approximately the same field (-23 and -22 T on the 4e and 8h sites), while B hf = −31 T for the 4d sites.…”