The control on the distribution of magnetic ions into a semiconducting host is crucial for the functionality of magnetically doped semiconductors. Through a structural analysis at the nanoscale, we give experimental evidence that the aggregation of Fe ions in (Ga,Fe)N and consequently the magnetic response of the material are affected by the growth rate and doping with shallow impurities.

We report on the metalorganic chemical vapor deposition of $\mathrm{GaN}:\mathrm{Fe}$ and $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ layers on $c$-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction, transmission electron microscopy (TEM), spatially resolved energy dispersive x-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably ${\mathrm{Fe}}_x\mathrm{N}$ with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. According to Hall effect measurements, electrons from residual donors are trapped by midgap Fe acceptor states in the limit of low iron content $x\lesssim 0.4\%$, indicating that the concentration of ${\mathrm{Fe}}^{2+}$ ions increases at the expense of Fe ions in the $3+$ charge state. This effect is witnessed by PL measurements as changes in the intensity of the ${\mathrm{Fe}}^{3+}$-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of ${\mathrm{Fe}}^{3+}$ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, ${\mathrm{Fe}}^{2+}$ does not produce any EPR response. However, the presence of Fe ions in the $2+$ charge state may account for a temperature-independent Van Vleck–type paramagnetic signal that we observe by SQUID magnetometry. Surprisingly, at higher Fe concentrations, the electron density is found to increase substantially with the Fe content. The coexistence of electrons in the conduction band and Fe in the $3+$ charge state is linked to the gradient in the Fe concentration. In layers with iron content $x\gtrsim 0.4\%$ the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. A set of precautions has been undertaken in order to rule out possible sources of spurious ferromagnetic contributions. Under these conditions, a ferromagneticlike response is shown to arise from the $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ epilayers, it increases with the iron concentration, it persists up to room temperature, and it is anisotropic—i.e., the saturation value of the magnetization is higher for in-plane magnetic field. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.

Systematic investigations of the structural and magnetic properties of single crystal ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}$ films grown by metal organic vapor phase epitaxy are presented. High-resolution transmission electron microscopy, synchrotron x-ray diffraction, and extended x-ray absorption fine structure studies do not reveal any crystallographic phase separation and indicate that Mn occupies Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic properties as a function of temperature, magnetic field and its orientation with respect to the $c$ axis of the wurtzite structure can be quantitatively described by the paramagnetic theory of an ensemble of noninteracting ${\text{Mn}}^{3+}$ ions in the relevant crystal field, a conclusion consistent with the x-ray absorption near edge structure analysis. A negligible contribution of Mn in the $2+$ charge state points to a low concentration of residual donors in the studied films. Studies on modulation-doped $p$-type ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}/\left(\text{Ga},\text{Al}\right)\text{N}:\text{Mg}$ heterostructures do not reproduce the high-temperature robust ferromagnetism reported recently for this system. DOI: http://dx.doi.org/10.1103/PhysRevB.81.235210 © 2010 The American Physical Society

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with $x\lesssim 3$%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infrared optics; and electron spin resonance. The results of high-resolution magnetic measurements and their quantitative interpretation have allowed us to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+. DOI: http://dx.doi.org/10.1103/PhysRevB.84.035206 ©2011 American Physical Society

By using highly sensitive millikelvin superconducting quantum interference device magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga1$-${}xMnxN. The interpretation of the results in the frame of tight-binding theory and of Monte Carlo simulations allows us to assign the spin interaction to ferromagnetic superexchange and to point out the limited accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators.