Magnetic and Mössbauer studies of single-crystal Fe16N2 and Fe-N martensite films epitaxially grown by molecular beam epitaxy (invited)

Abstract

Single-phase, single-crystal Fe16N2(001) films and Fe-11 at. %N martensite films of 200–900 Å thickness have been epitaxially grown on In0.2Ga0.8As(001) substrates by evaporating Fe in an atmosphere of mixed gas of N2 and NH3, followed by annealing. The saturation magnetizations 4πMs’s for Fe16N2 and Fe-N martensite films have been measured to be around 29 and 24 kG at room temperature, respectively, and almost constant in the above thickness range by using a vibrating sample magnetometer. 4πMs for Fe-N martensite films has been increased with ordering of N atoms caused by annealing and finally reached around 29 kG for Fe16N2. Mössbauer spectra have been measured for those films. The spectrum for Fe-N martensite films was a superposed one with hyperfine fields of 360, 310, and 250 kOe, similar to those previously reported for martensite. While the spectrum became simpler with ordering, finally reaching a single hyperfine field of 330 kOe for Fe16N2. 4πMs of 29 kG for Fe16N2 (3.2 μB/Fe atom) and 4πMs of 24 kG for martensite (2.6 μB/Fe atom) has not been explained based on the conventional band theory of 3d metal magnetism. Behaviors of Mössbauer spectra could not be understood based on the conventional concept either. Thus a new physical concept is likely to be needed for clarification of giant magnetic moments and Mössbauer spectra for Fe16N2 and Fe-N martensites.

Magnetite/maghemite mixture prepared in benzyl alcohol for the preparation of α″-Fe16N2 with α-Fe

“…Large magnetization value has been expected for α"-Fe 16 N 2 thin films [1][2][3] showed the saturation magnetization of 162 emu•g -1 , which was higher than 151 emu•g -1 of α-Fe powder before the nitridation, but lower than the previously reported values. The lower value was expected to be due to the effects of non-magnetic surface amorphous oxide and also Al 2 O 3 added to improve the durability of in the starting γ-Fe 2 O 3 .…”

“…The yield of α"-Fe 16 N 2 was smaller in the ammonia nitrided product of α-Fe with the larger particle size. In our previous study, Fe 3 O 4 nanoparticle with a particle size of 40 nm prepared from aqueous solution was used as a reference of the starting powder in low temperature nitridation.…”

“…In the present study, low temperature nitridation reaction was investigated on the -Fe obtained from reduction of iron oxide fine powder prepared in benzyl alcohol changing the amount of iron acetylacetonate, hereafter denoted as Fe(acac) 3 , dissolved as the starting materials.…”

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.…”