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Magneto-optical (based on polar and longitudinal Kerr effects) and FMR studies were carried out on ultrathin cobalt films deposited on Au(1 1 1) and covered with either a V or Mo cover layer, and finally by an Au protective layer. Co thickness at which the magnetization reorientation transition from an easy axis into an easy plane occurs is influenced by the type of cover layer. Even 0.1 nm thick V-coverage is efficient enough to induce full reorientation. A strong decrease of domain structure size (down to the submicrometer scale) was observed while approaching reorientation phase transition (RPT). The influence of a Mo-overlayer on Co in-plane magnetic anisotropy is shown to be much weaker in comparison to the influence of Mo-buffer.

A series of recent magneto-optical studies pointed to contradicting values of the $s\text{\ensuremath{-}}d$ exchange energy ${N}_{0}\ensuremath{\alpha}$ in Mn-doped GaAs and GaN as well as in Fe-doped GaN. Here, a strong sensitivity of weak-localization phenomena to symmetry-breaking perturbations (such as spin-splitting and spin-disorder scattering) is exploited to evaluate the magnitude of ${N}_{0}\ensuremath{\alpha}$ for $n$-type wurtzite (Ga,Mn)N:Si films grown by metalorganic vapor phase epitaxy. Millikelvin magnetoresistance studies and their quantitative interpretation point to ${N}_{0}\ensuremath{\alpha}l40$ meV, a value at least 5 times smaller than the one found with similar measurements on, e.g., $n$-(Zn,Mn)O. It is shown that this striking difference in the values of the $s\text{\ensuremath{-}}d$ coupling between $n$-type III-V and II-VI dilute magnetic semiconductors can be explained by a theory that takes into account the acceptor character of Mn in III-V compounds.

We report on micromagnetic simulations of magnetization distributions in infinite and laterally limited ultrathin magnets of different geometries. We reveal the crucial role of the finite-size effect on magnetization states in nanowires and disks. In low-perpendicular-anisotropy nanomagnets, out-of-plane magnetization states can be achieved by varying geometry and/or lateral size of the sample.

In this work we report results of ferromagnetic resonance studies of a 6% 15 nm (Ga,Mn)As layer, deposited on (001)-oriented GaAs. The measurements were performed with in-plane oriented magnetic field, in the temperature range between 5K and 120K. We observe a temperature induced reorientation of the effective in-plane easy axis from [-110] to [110] direction close to the Curie temperature. The behavior of magnetization is described by anisotropy fields, H_{eff} (= 4\piM -H_{2\perp}), H_{2\parallel}, and H_{4\parallel}. In order to precisely investigate this reorientation, numerical values of anisotropy fields have been determined using powerful - but still largely unknown - interval calculations. In simulation mode this approach makes possible to find all the resonance fields for arbitrarily oriented sample, which is generally intractable analytically. In 'fitting' mode we effectively utilize full experimental information, not only those measurements performed in special, distinguished directions, to reliably estimate the values of important physical parameters as well as their uncertainties and correlations., Comment: 3 pages, 3 figures. Presented at The European Conference "Physics of Magnetism 2011" (PM'11), June 27 - July 1, 2011, Poznan, Poland

Numerous works on diluted magnetic semiconductors carried out over the last decade have aimed in examining intricate interplay of ferromagnetic and semiconducting properties [1]. Wide band gap semiconductors such as GaN and ZnO doped with magnetic ions are particularly interesting in view of their wide ranging applications. Here we present results of magnetic circular dichroism (MCD) and Magneto-optical Kerr effect (MOKE) measurements in the excitonic spectral region performed on (Ga,Fe)N layers. In contrast to previous magnetooptical studies [2], we focus here on possible magnetoptical signatures of Fe-rich nanocrystals that appear in (Ga,Fe)N films obtained under suitable growth conditions [3]. We compare magnetoptical properties of two layers with similar concentrations of paramagnetic Fe ions (310cm and 210cm, respectively), one containing Fe-rich nanocrystals, the other not, according to extensive SQUID, XRD, and HRTEM characterization. As found in SQUID measurements, the sample with Fe-rich nanocrystals reveals a ferromagnetic response persisting up to above room temperature for two orientations of the magnetic field in respect to the c-axis. Reflectivity and MOKE measurements are performed in the Faraday configuration with the c-axis parallel to the incident beam in the magnetic field up to 7 T at 2 K. The sample is illuminated by a Xe lamp. We find that the field dependent magnitudes of MCD and Kerr rotation angle in the excitonic region are well descried by the paramagnetic Brillouin function in both samples. This means that ferromagnetic features, clearly seen in SQUID magnetometry, are not contributing to the magnetooptical response. We conclude that either character of nanocrystal magnetoptical properties or the nanocrystal distribution and density are not optimized to result in sizable magnetoptical phenomena. Instead, the presence on Fe-rich nanocrystals reduces the magnitude of the paramagnetic-like magnetooptical signals. In line with this observation, we find that a large magnetooptical response can be recovered by Ar sputtering of the sample surface, the finding consistent with the location of nanocrystals near the film surface [3].

Millikelvin magnetotransport studies are carried out on heavily $n$-doped wurtzite GaN:Si films grown on semi-insulating GaN:Mn buffer layers by metal-organic vapor phase epitaxy. The dependency of the conductivity on magnetic field and temperature is interpreted in terms of theories that take into account disorder-induced quantum interference of one-electron and many-electron self-crossing trajectories. The Rashba parameter $\alpha_{\text{R}}\,=\,(4.5 \pm 1)$ meV$\AA$ is determined, and it is shown that in the previous studies of electrons adjacent to GaN/(Al,Ga)N interfaces, bulk inversion asymmetry was dominant over structural inversion asymmetry. The comparison of experimental and theoretical values of $\alpha_{\text{R}}$ across a series of wurtzite semiconductors is presented as a test of current relativistic ab initio computation schemes. It is found that electron-electron scattering with small energy transfer accounts for low temperature decoherence in these systems., Comment: the version after the revision process

International audience; We investigated the magnetic switching of MgO/Co/Pt pillars with perpendicular magnetic anisotropy, for lateral pillar sizes from 30 nm to 2 lm and for Co layer thicknesses between 1.8 and 2.6 nm. For large pillars, both the coercivity and the remanent magnetization decrease for increasing Co thickness. For all Co thicknesses, the coercivity strongly increases upon decreasing the pillar size. A comparison with micromagnetic simulations shows that the change in coercivity is determined by size-dependent demagnetizing effects. Our results show that small pillars with perpendicular magnetization and a tunable coercivity can be fabricated from continuous layers with in-plane magnetization.

The question of the correlation between magnetization, band splittings, and magnetic circular dichroism (MCD) in the fundamental gap region of dilute magnetic semiconductors is examined experimentally and through model calculations, taking the case of wurtzite Ga${}_{1\ensuremath{-}x}$Fe${}_{x}$N as an example. Magnetization and polarization-resolved reflectivity measurements have been performed down to 2 K and up to 7 T for $x=0.2$%. Optical transitions originating from all three free excitons $A$, $B$, and $C$, specific to the wurtzite structure, have been observed and their evolution with the magnetic field determined. It is demonstrated that the magnitude of the exciton splittings evaluated from reflectivity-MCD data can be overestimated by more than a factor of 2, as compared to the values obtained by describing the polarization-resolved reflectivity spectra with appropriate dielectric functions. A series of model calculations shows that the quantitative inaccuracy of MCD originates from a substantial influence of the magnetization-dependent exchange interactions not only on the spin splittings of excitons but also upon their linewidth and oscillator strength. At the same time, a method is proposed that allows us to evaluate the field and temperature dependencies of the magnetization from MCD spectra. The accurate values of the excitonic splittings and of the magnetization reported here substantiate the magnitudes of the apparent $sp\ensuremath{-}d$ exchange integrals in (Ga,Fe)N previously determined.

A series of (Zn,Co)O layers with Co contents x up to 40% grown by atomic layer deposition have been investigated. All structures deposited at 160 degrees C show magnetic properties specific to II-V ...

Molecular beam epitaxy has been employed to obtain Ga1-xMnxN films with x up to 10% and Curie temperatures T_C up to 13 K. The magnitudes of T_C and their dependence on x, T_C(x) ~ x^m, where m = 2.2 +/- 0.2 are quantitatively described by a tight binding model of superexchange interactions and Monte Carlo simulations of T_C. The critical behavior of this dilute magnetic insulator shows strong deviations from the magnetically clean case (x = 1), in particular, (i) an apparent breakdown of the Harris criterion; (ii) a non-monotonic crossover in the values of the susceptibility critical exponent gamma_eff between the high temperature and critical regimes, and (iii) a smearing of the critical region, which can be explained either by the Griffiths effects or by macroscopic inhomogeneities in the spin distribution with a variance Delta x = (0.2 +/- 0.1)%., 5 pages, 5 figures

For more than a decade ZnO doped with Mn and Co has remained as one of the most prospected diluted magnetic semiconductor for spintronic applications with conflicting outcome concerning the genuineness of its room temperature ferromagnetism. In order to clarify this issue we investigate (Zn,Co)O layers grown by atomic layer deposition at low temperatures. We employ and relay on wide range of extensive material characterization, which in combination with superconducting quantum interference device magnetometry allow us decisively exemplify the growth temperature as the key factor discriminating between paramagnetic (obtained at 160 °C) and various forms of ferromagnetic responses, seen when the grows is carried out at 200 °C and above.

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., Comment: 18 pages PRB style, 13 figures (1st and 3rd wrongly reproduced, ???)

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 $T_{\mathrm{g}}$. 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 $T_{\mathrm{g}}$ (800$^{\circ}$C); ii) superparamagnetic-like -- brought about mainly by ferromagnetic nanocrystals of $\epsilon-$Fe$_3$N but also by $\gamma'$-Fe$_4$N and by inclusions of elemental $\alpha$- and $\gamma$-Fe, and prevalent in films obtained in the intermediate $T_{\mathrm{g}}$ range; iii) component linear in the magnetic field and associated with antiferromagnetic interactions -- found to originate from highly nitridated Fe$_x$N ($x \leq$ 2) phases, like $\zeta$-Fe$_2$N, and detected in samples deposited at the highest employed temperature, $T_{\mathrm{g}}$ = 950$^{\circ}$C. Furthermore, depending on $T_{\mathrm{g}}$, the Fe-rich nanocrystals segregate towards the sample surface or occupy two-dimensional planes perpendicular to the growth direction., Comment: 12 pages, 11 figures

Systematic investigations of the structural and magnetic properties of single crystal (Ga,Mn)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 non-interacting 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 (Ga,Mn)N/(Ga,Al)N:Mg heterostructures do not reproduce the high temperature robust ferromagnetism reported recently for this system., 15 pages, 14 figures

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+., 12 pages, 14 figures; This version contains the detailed characterization of the crystal structure as well as of the Mn distribution and charge state

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.

By micromagnetic simulations and analytically we study the nonhomogeneous magnetization states of ultrathin films with perpendicular and in-plane anisotropy. Ground and metastable states are mapped onto a (K1,K2) phase diagram (where K1 and K2 are the first and second anisotropy constants, accordingly). It is shown that in the part of the phase diagram where K2