Abstract
The magnetocrystalline anisotropy (MCA) energy of the giant saturation magnetization candidate material α″–Fe16N2 was investigated using first-principles electronic-structure calculations. The plane-wave density-functional theory (DFT) code Quantum ESPRESSO was employed to study the effect of different DFT approaches on the system, particularly the influence of exchange-correlation functionals and pseudopotential methods. The MCA energies obtained this way are within the range of previous theoretical and experimental results, while exhibiting significant variation between the different approaches. The role and limitations of these approaches in the view of Fe16N2 band structure are discussed in detail.
Highlights
One of the most promising candidates for an environmentfriendly rare-earth-free magnetic material is the ordered iron nitride phase α′′–Fe16N2.1 First discovered by Jack in 1951,2 reports of “giant” saturation magnetization Ms by Kim and Takahashi in 19723 and Sugita et al in 19914 raised interest in its use in magnetic recording applications
The local-density approximation (LDA) structure has notably lower a compared to both generalized gradient approximation (GGA) and experiment, while NC and Projector-Augmented Wave (PAW) PP yield fairly similar structures. (The agreement of the latter in particular is consistent with previous research on consistency between density-functional theory (DFT) methods.34) The addition of Hubbard U expectedly leads to increased unit cell volume, for the high-M case: here, the relaxed volume is larger than experiment
We briefly consider the total magnetization: whereas most results are fairly well in line with previous LDA or GGA results,5,10,11 note that the LDA result is abnormally low, while the high-moment case exhibits a higher magnetization as expected. (For comparison, the lower range of experimental values are equivalent to around 36–42 μB/unit cell;5 with high-moment measurements as high as 51 μB/unit cell.7)
Summary
One of the most promising candidates for an environmentfriendly rare-earth-free magnetic material is the ordered iron nitride phase α′′–Fe16N2.1 First discovered by Jack in 1951,2 reports of “giant” saturation magnetization Ms (well above Fe-Co alloy) by Kim and Takahashi in 19723 and Sugita et al in 19914 raised interest in its use in magnetic recording applications. Several more advanced paradigms have been employed to bridge this gap and give insight into a potential mechanism for higher magnetization, including the addition of a Hubbard correction term U,9 hybrid functionals incorporating exact exchange terms such as the Heyd–Scuseria–Ernzerhof (HSE) functional, or the Green’s function–based GW method.. Several more advanced paradigms have been employed to bridge this gap and give insight into a potential mechanism for higher magnetization, including the addition of a Hubbard correction term U,9 hybrid functionals incorporating exact exchange terms such as the Heyd–Scuseria–Ernzerhof (HSE) functional, or the Green’s function–based GW method.10,11 Despite these efforts, the precise mechanism is still under debate, though recent Mössbauer spectra and polarized neutron diffraction experiment has provided support for the DFT calculation based on the “cluster+atom” physical model put forth by Ji et al nearly a decade ago. More conventional methods such as the local-density approximation (LDA) and generalized gradient approximation (GGA) have particular difficulty explaining a higher magnetization. Several more advanced paradigms have been employed to bridge this gap and give insight into a potential mechanism for higher magnetization, including the addition of a Hubbard correction term U,9 hybrid functionals incorporating exact exchange terms such as the Heyd–Scuseria–Ernzerhof (HSE) functional, or the Green’s function–based GW method. Despite these efforts, the precise mechanism is still under debate, though recent Mössbauer spectra and polarized neutron diffraction experiment has provided support for the DFT calculation based on the “cluster+atom” physical model put forth by Ji et al nearly a decade ago.
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