The superconducting quantum interference device (SQUID) magnetometer is one of the most sensitive experimental techniques to magnetically characterize samples with high sensitivity. Here we present a detailed discussion of possible artifacts and pitfalls characteristic for commercial SQUID magnetometers. This includes intrinsic artifacts which stem from the inherent design of the magnetometer as well as potential issues due to the user. We provide some guidelines how to avoid and correct these, which is of particular importance when the proper magnetization of nano-scale objects shall be established in cases where its response is dwarfed by that of the substrate it comes with, a situation frequently found in the field of nano-magnetism.

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The local chemistry, structure, and magnetism of (Ga,Fe)N nanocomposites grown by metal organic vapor phase epitaxy is studied by high resolution synchrotron x-ray diffraction and absorption, transmission electron microscopy, and superconducting quantum interference device magnetometry as a function of the growth temperature Tg. Three contributions to the magnetization are identified: i) paramagnetic -originating from dilute and non-interacting Fe 3+ ions substitutional of Ga, and dominating in layers obtained at the lowest considered Tg (800 • C); ii) superparamagnetic-like -brought about mainly by ferromagnetic nanocrystals of ε−Fe3N but also by γ ′ -Fe4N and by inclusions of elemental α-and γ-Fe, and prevalent in films obtained in the intermediate Tg range; iii) component linear in the magnetic field and associated with antiferromagnetic interactions -found to originate from highly nitridated FexN (x ≤ 2) phases, like ζ-Fe2N, and detected in samples deposited at the highest employed temperature, Tg = 950 • C. Furthermore, depending on Tg, the Fe-rich nanocrystals segregate towards the sample surface or occupy two-dimensional planes perpendicular to the growth direction.

Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and ab initio studies, that the codoping of GaN:Mn with Mg allows to control the Mnn+ charge and spin state in the range 3≤n≤5 and 2≥S≥1. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mgk, where the number of ligands k is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.

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 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; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed 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+.

We report on the fabrication of pseudomorphic wurtzite Ga1−xMnxN grown on GaN with Mn concentrations up to 10% using molecular beam epitaxy. According to Rutherford backscattering, the Mn ions are mainly at the Ga-substitutional positions, and they are homogeneously distributed according to depth-resolved Auger-electron spectroscopy and secondary-ion mass-spectroscopy measurements. A random Mn distribution is indicated by transmission electron microscopy, and no Mn-rich clusters are present for optimized growth conditions. A linear increase of the c-lattice parameter with increasing Mn concentration is found using x-ray diffraction. The ferromagnetic behavior is confirmed by superconducting quantum-interference measurements showing saturation magnetizations of up to 150 emu/cm3.

The temperature dependence of magnetic anisotropy in ͑113͒A ͑Ga,Mn͒As layers grown by molecular-beam epitaxy is studied by means of superconducting quantum interference device magnetometry as well as by ferromagnetic resonance ͑FMR͒ and magnetooptical effects. Experimental results are described considering cubic and two kinds of uniaxial magnetic anisotropy. The magnitude of cubic and uniaxial anisotropy constants is found to be proportional to the fourth and second power of saturation magnetization, respectively. Similarly to the case of ͑001͒ samples, the spin reorientation transition from uniaxial anisotropy with the easy axis along the ͓110͔ direction at high temperatures to the biaxial ͗100͘ anisotropy at low temperatures is observed around 25 K. The determined values of the anisotropy constants have been confirmed by FMR studies. As evidenced by investigations of the polar magnetooptical Kerr effect, the particular combination of magnetic anisotropies allows the out-of-plane component of magnetization to be reversed by an in-plane magnetic field. Theoretical calculations within the p-d Zener model explain the magnitude of the out-of-plane uniaxial anisotropy constant caused by epitaxial strain but do not explain satisfactorily the cubic anisotropy constant. At the same time the findings point to the presence of an additional uniaxial anisotropy of unknown origin. Similarly to the case of ͑001͒ films, this additional anisotropy can be explained by assuming the existence of a shear strain. However, in contrast to the ͑001͒ samples, this additional strain has an out of the ͑001͒ plane character.