Magnetism of phase-separated GayFexN:Mn in a GaN matrix

Due to their striking magnetic properties, magnetic iron nitride-based (FexN and GayFexN) compounds are attracting attention as building blocks in high-density magnetic recording write heads and media. We have recently demonstrated, that we can control the aggregation of GaxFe4-xN nanocrystals embedded in a III-nitride matrix. By tuning the growth parameters, and consequently the stoichiometry of the nanocrystals, we can dictate the magnetic response of the system to be either ferromagnetic or antiferromagnetic [1,2]. Here we report on the influence of Mn co-doping on the magnetic properties of self-assembled planar arrays of GayFexN:Mn nanocrystals (NCs) embedded in GaN. The samples have been fabricated by means of metal organic vapor phase epitaxy (MOVPE) according to a protocol already reported [1]. We have carried out an in-depth magnetic study, by SQUID magnetometer, of samples with the same nominal Fe content (Fe flow rate = 450 standard cubic centimeters per second (sccm)) and differing for the Mn nominal content (Mn flow = 0, 250 and 450 sccm; the samples have been labelled SMn0, SMn250 and SMn450). Hysteresis loops, in the 5-300 K temperature range, feature a ferromagnetic (FM) response whose intensity reduces with increasing the Mn content; moreover, a non-saturation tendency is observed, particularly in SMn450, which only in part can be ascribed to paramagnetism of pure GaN. The thermal dependence of the zero-field-cooled and field-cooled magnetization has been measured between 5 and 300 K in a field Happl = 50 Oe. Magnetic irreversibility, in all the three samples, reveals the presence of FM NCs undergoing magnetic relaxation process, similar to superparamagnetism; the distribution of the anisotropy energy barriers indicates a decrease in the NCs effective size with increasing the Mn content. Hence, in each sample, a relaxing FM component co-exists with a non-relaxing FM one. Information on the latter was gained by measuring the field-dependent isothermal and demagnetized remanence at T = 5, 100 and 250 K, allowing the construction of DM plots [3]. The results are consistent with the existence, in each sample, of three populations of magnetic entities with different magnetic hardness (the highest switching field and irreversible coercivity are found in SMn450). At T = 5 K, the negative sign of the DM parameter indicates that dipolar interactions govern the magnetic behavior of the samples at remanence. At T = 100 and 250 K, demagnetizing interactions are still dominant for H < 1000 Oe; at higher field, DM assumes small positive values, the effect being more pronounced in SMn450 and SMn250. Based on these results, different scenarios are proposed for the magnetic structures of the studied samples and for the interplay between NCs and surrounding matrix. References: [1] A. Navarro-Quezada et al. Appl. Phys. Lett. 101, 081911 (2012). [2] A. Grois et al., Nanotechnol. 25, 395704 (2014). [3] L. Del Bianco et al. Mater. Res. Express 2, 065002 (2015