# Two-Component Anomalous Hall and Nernst Effects in Anisotropic Fe4– x Ge x N Thin Films

## TL;DR

This study explores how adding germanium to iron nitride thin films affects their magnetic and electrical properties, revealing new behaviors in Hall and Nernst effects.

## Contribution

The paper identifies a two-component behavior in Hall and Nernst effects due to crystallographic orientation and magnetocrystalline anisotropy in Fe4–xGexN films.

## Key findings

- Ge occupies the 4b site in tetragonal Fe4–xGexN films for x > 0.35.
- Tetragonal samples show two-component hysteresis loops in Hall and Nernst effects due to coexisting crystallographic orientations.
- The maximum ANE is 0.9 μV/K for x = 0 at room temperature and −0.85 μV/K for x = 1 at 50 K.

## Abstract

A series of thin
films Fe4–x
Ge
x
N (x = 0 –
1) were fabricated onto MgO substrates by magnetron sputtering with
the aim of studying the possible enhancement of the anomalous Nernst
effect (ANE), envisaged based on density functional theory (DFT) calculations.
The Nernst and Hall effects of the series were systematically analyzed,
complemented with resistivity, magnetic, electron microscopy, and
Mössbauer experiments, and DFT calculations including elastic
properties. The Fe4N phase crystallizes in the cubic symmetry
with Pm3̅m space group, whereas
a small tetragonal distortion is realized in Fe4–x
Ge
x
N films for x > 0.35. From the comparison of the experimental isomer
shift with DFT calculations, we conclude that Ge occupies the 4b site in the tetragonal I4/mcm structure. The ferromagnetic Curie temperature decreases rapidly
from ∼750 K for x = 0 to ∼100 K for x = 1. The tetragonal samples with x =
0.8 and 1 display two-component behavior in the Hall and Nernst effects
hysteresis loops, which can be analyzed as a sum of positive and negative
loops with different saturation fields. This unusual behavior is a
product of a combination of several factors: (1) coexistence of two
different crystallographic orientations in the tetragonal thin film,
namely with the majority of c-axis and minority of a-axis normal to the film surface; (2) opposite sign of
the anomalous Hall and Nernst effects for the direction of magnetization
along the a- and c-axes revealed
by DFT calculation; and (3) the magnetocrystalline anisotropy characterized
by an easy ab-plane, which is responsible for the
different saturation fields for a- and c-axes. The maximum ANE was determined to be 0.9 μV/K for x = 0 at room temperature and −0.85 μV/K for x = 1 at
T
= 50 K. The rapid increase
of ANE of Fe3GeN from low temperatures indicates that,
were it not for its low Curie temperature, it could surpass the ANE
of Fe4N. This observation is consistent with our theoretical
assumptions and motivates further research of doped Fe4N for which ANE enhancement is predicted by DFT calculations.