We present a first principles supercell methodology for the calculation of exchange interactions of magnetic materials with arbitrary degrees of structural and chemical disorder in their high temperature paramagnetic state. It is based on a projection of the total magnetic energy of the system onto local pair clusters, allowing the interactions to vary independently as a response to their local environments. We demonstrate our method by deriving the distance dependent exchange interactions in vibrating crystalline CrN, a Ti$_{0.5}$Cr$_{0.5}$N solid solution as well as in amorphous CrN. Our method reveals strong local environment effects in all three systems. In the amorphous case we use the full set of exchange interactions in a search for the non-collinear magnetic ground state.
We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled leading to non-adiabatic effects. Those effects suppress the phonon life times at low temperature compared to an adiabatic approach. The here identified dynamic coupling provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature.
This paper presents first principles calculations of paramagnetic cubic CrN$_x$ with the aim to provide deeper insight into recently published transmission electron microscopy-based study on this material. Among several types of point defects which may result in N-deficient material, N vacancy is found to be energetically preferred to Cr interstitial and anti-sites. Electron Energy Loss Near Edge Structure of N K-edge transition is calculated for various concentrations of N vacancies in CrN$_x$, yielding the same trends as experimentally observed. Analysis of the electronic structure reveals d…
Equation of state for chromium nitride has been debated in the literature in connection with a proposed collapse of its bulk modulus following the pressure induced transition from the paramagnetic cubic phase to the antiferromagnetic orthorhombic phase [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et al., Nat Mater 9, 283 (2010)]. Experimentally the measurements are complicated due to the low transition pressure, while theoretically the simulation of magnetic disorder represent a major challenge. Here a first-principles method is suggested for the calculation of thermodynamic proper…
We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab-initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics (DLM-MD) with the temperature dependent effective potential (TDEP) method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free e…
We study propagating mode-I fracture in two dimensional amorphous materials using atomistic simulations. We used the continuous random network (CRN) model of an amorphous material, creating samples using a two dimensional analogue of the WWW (Wooten, Winer & Weaire) Monte-Carlo algorithm. For modeling fracture, molecular-dynamics simulations were run on the resulting samples. The results of our simulations reproduce the main experimental features. In addition to achieving a steady-state crack under a constant driving displacement (which had not yet been achieved by other atomistic models for a…
We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using recently introduced method: symmetry imposed force constant temperature dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstr…
CrN is observed to undergo a paramagnetic to antiferromagnetic transition accompanied by a shear distortion from cubic NaCl-type to orthorhombic structure. Our first-principle plane wave and ultrasoft pseudopotential calculations confirm that the distorted antiferromagnetic phase with spin configuration arranged in double ferromagnetic sheets along [110] is the most stable. Antiferromagnetic ordering leads to a large depletion of states around Fermi level, but it does not open a gap. Simultaneous occurence of structural distortion and antiferromagnetic order is analyzed.
Simulations of defects in paramagnetic materials at high temperature constitute a formidable challenge to solid state theory due to the interaction of magnetic disorder, vibrations, and structural relaxations. CrN is a material where these effects are particularly large due to a strong magneto-lattice coupling and a tendency for deviations from the nominal 1:1 stoichiometry. In this work we present a first-principles study of nitrogen vacancies and nitrogen interstitals in CrN at elevated temperature. We report on formation energetics, the geometry of interstital nitrogen dimers, and impact on…
The dynamical and thermodynamic phase stabilities of the stoichiometric compound CrN including different structural and magnetic configurations are comprehensively investigated using a first-principles density-functional-theory (DFT) plus U approach in conjunction with experimental measurements of the thermal expansion. Comparing DFT and DFT+U results with experimental data reveals that the treatment of electron correlations using methods beyond standard DFT is crucial. The non-magnetic face-centered cubic B1-CrN phase is both, elastically and dynamically unstable, even under high pressure, wh…
We report a combined study by optical absorption (OA) and electron paramagnetic resonance (EPR) spectroscopy on the E'-alpha point defect in amorphous silicon dioxide (a-SiO2). This defect has been studied in beta-ray irradiated and thermally treated oxygen-deficient a-SiO2 materials. Our results have pointed out that the E'-alpha center is responsible for an OA Gaussian band peaked at 5.8 eV and having a full width at half maximum (FWHM) of 0.6 eV. The estimated oscillator strength of the related electronic transition is 0.14. Furthermore, we have found that this OA band is quite similar to t…
Understanding the structural origins of the properties of amorphous materials remains one of the most important challenges in structural science. In this study we demonstrate that local 'structural simplicity', embodied by the degree to which atomic environments within a material are similar to each other, is powerful concept for rationalising the structure of canonical amorphous material amorphous silicon (a-Si). We show, by restraining a reverse Monte Carlo refinement against pair distribution function (PDF) data to be simpler, that the simplest model consistent with the PDF is a continuous …
Diffusion limited reactions of point defects were investigated in amorphous SiO2 exposed to UV laser light. Electron spin resonance and in situ absorption measurements at room temperature evidenced the annealing of E' centers and the growth of H(II) centers both occurring in the post-irradiation stage and lasting a few hours. These transients are caused by reactions involving molecular hydrogen H2, made available by dimerization of radiolytic H0.
Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this Letter we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic…
In the present Ph.D. Thesis we report an experimental investigation on the effects of gamma- and beta-ray irradiation and of subsequent thermal treatment on many types of a-SiO2 materials, differing in the production methods, OH- and Al-content, and oxygen deficiencies. Our main objective is to gain further insight on the microscopic structures of the E'_gamma, E'_delta, E'_alpha and triplet paramagnetic centers, which are among the most important and studied class of radiation induced intrinsic point defects in a-SiO2. To pursue this objective, we use prevalently the EPR spectroscopy. In part…
In this letter we report a direct observation of a magnetic proximity effect in an amorphous thin film exchange-spring magnet by the use of neutron reflectometry. The exchange-spring magnet is a trilayer consisting of two ferromagnetic layers with high $T_c$'s separated by a ferromagnetic layer, which is engineered to have a significantly lower $T_c$ than the embedding layers. This enables us to measure magnetization depth profiles at which the low $T_c$ material is in a ferromagnetic or paramagnetic state, while the embedding layers are ferromagnetic. A clear proximity effect is observed 7 K …
A magneto-structural phase transition is investigated in single crystal CrN thin films grown by rf plasma molecular beam epitaxy on MgO(001) substrates. While still within the vacuum environment following MBE growth, $\it in-situ$ low-temperature scanning tunneling microscopy, and $\it in-situ$ variable low-temperature reflection high energy electron diffraction are applied, revealing an atomically smooth and metallic CrN(001) surface, and an $\it in-plane$ structural transition from 1$\times$1 (primitive CrN unit cell) to $\mathrm{\sqrt{2}\times\sqrt{2}-R45^\circ}$ with a transition temperatu…
Magnetism in palladium has been the subject of much work and speculation. Bulk crystalline palladium is paramagnetic with a high magnetic susceptibility. Palladium under pressure and palladium nanoclusters have generated interest to scrutinize its magnetic properties. Here we report another possibility: Palladium may become an itinerant ferromagnet in the amorphous bulk phase at atmospheric pressure. Atomic palladium is a d$^{10}$ element, whereas bulk crystalline Pd is a d$^{10-x}$(sp)$^{x}$ material; this, together with the possible presence of 'unsaturated bonds' in amorphous materials, may…
We study the ferromagnetism of La0.67Ca0.33MnO3 in bulk polycrystalline, nanocrystalline and amorphous phase. The structural change from crystalline phase to amorphous phase exhibited a systematic decrease of TC(paramagnetic to ferromagnetic transition temperature) and spontaneous magnetization (MS). The experimental results suggested few more features, e.g., appearance of large magnetic irreversibility in the temperature dependence of magnetization, lack of magnetic saturation at high magnetic field, blocking of magnetization below TB, and enhancement of coercivity. In addition, the magnetic …
We propose a model for magnetic noise based on spin-flips (not electron-trapping) of paramagnetic dangling-bonds at the amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times is derived from the single-phonon cross-relaxation mechanism for a dangling-bond interacting with the tunneling two-level systems of the amorphous interface. The temperature and frequency dependence is sensitive to three energy scales: The dangling-bond spin Zeeman energy delta, as well as the minimum (E_min) and maximum (E_max) values for the energy splittings of the tunneling two-level systems. …
Ordinary multi-component silicate glasses belong to a class of amorphous insulators normally displaying no special form of magnetism, save for the Larmor dominant diamagnetism from the constituent atoms' core electrons and the extrinsic Langevin paramagnetism due to the ubiquitous Fe-group dilute paramagnetic impurities. Here we show that the macroscopic magnetisation of three case-study glass types measured in a SQUID-magnetometer cannot be explained solely by means of the Larmor-Langevin contributions. In particular, we reveal a novel {\em intrinsic} contribution to the bulk magnetisation du…
Fe/Au nanoparticles have been chemically synthesized through a reverse micelle reaction and investigated by both conventional and synchrotron based x-ray diffraction and by magnetic and Moessbauer spectral studies. The powder x-ray diffraction patterns reveal both the presence of crystalline alpha-iron and gold and the absence of any crystalline iron oxides or other crystalline products. First-order reversal curves, along with the major hysteresis loops of the Fe/Au nanoparticles have been measured as a function of time in order to investigate the evolution of their magnetic properties. The ir…
A simple route to prepare Gd$_{0.7}$Sr$_{0.3}$MnO$_3$ nanoparticles by ultrasonication of their bulk powder materials is presented in this article. For comparison, Gd$_{0.7}$Sr$_{0.3}$MnO$_3$ nanoparticles are also prepared by ball milling. The prepared samples are characterized by X-ray diffraction (XRD),field emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscope (XPS), and Superconducting Quantum Interference Device (SQUID) magnetometer. XRD Rietveld analysis is carried out extensively for the determination of crystallographic paramete…
