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Structural and magnetic properties of Ni2-xPtxMnGa alloys are investigated from first principles calculations with the help of the spin-polarized relativistic Korringa-Kohn-Rostoker and Plane-Wave Self-Consistent Field methods. The atomic chemical disorder at specific site has been implemented using coherent potential approximation. Calculated equilibrium lattice parameters are in a good agreement with experimental data and other theoretical calculations. The composition dependences of the magnetic exchange couplings and the Curie temperature for cubic phase are obtained. Our calculations have shown that an increase content of Pt results to decrease of magnetic interactions between Mn atoms and to change of interaction sign from ferromagnetic type to antiferromagnetic one for composition Ni1.0Pt1.0MnGa. Calculated Curie temperatures are in an agreement with experimental data.
In this work we study the influence of supercell scaling on magnetic properties in Ni-Mn-X-Z alloys by means of ab initio calculations with the help of Quantum Espresso PWSCF package and the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) code based on DFT approximation. It is shown that the supercell calculations for the equilibrium lattice parameter are coincided with the calculations for simple primitive lattice. The exchange parameters for Ni-Mn-X alloys obtained from supercell calculations are large than calculated for simple primitive lattice.
The structural, electronic and magnetic properties of functional Ni-Mn-(Ga, In, Sn) and Pt-Ni-(Ga, Sn) alloys are studied by first-principles and Monte Carlo tools. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from the competition of ferro- and antiferromagnetic interactions for excess Mn atoms in the unit cell. We show that the resulting complex magnetic ordering is the driving mechanism of structural transformations and multifunctional properties of Heusler alloys associated with magnetic shape-memory, magnetocaloric and elastocaloric effects. The thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. Entropy and specific heat changes associated with the magnetic changes and emergence of microstructure across the magnetostructural transition are pointed out. We show how to optimize the functional properties by tuning the compositional changes, for example, a magnetic shape-memory effect of more than 14% can be achieved in Pt-Ni-Mn-Ga alloys. The theoretical studies are accompanied by experimental investigations.
Density functional theory (DFT) based on the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) method is used to investigate the magnetic properties of nonstoichiometric Fe2+xMn1-xAl Heusler alloys, where 0 ≤ x ≤ 0.9. The composition dependences of the magnetic exchange couplings and the Curie temperature for the cubic L21 phase are obtained. Our simulations have shown that the Fe-Fe nearest neighbors present a strong ferromagnetic coupling. Moreover, these exchange interactions are larger than other interactions. The substitution of Mn by Fe in Fe2+xMn1-xAl (0 ≤ x ≤ 0.9) leads to an increase in the Curie temperature. This tendency and the values of Curie temperatures are in agreement with the experimental results for Fe2+xMn1-xAl (x = 0, and 0.1). The highest Curie temperature was observed for the Fe-richer alloy.
New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles.
A simple theoretical five-state Potts model for the investigation of magnetocaloric effect in systems with competing ferromagnetic and antiferromagnetic interactions has been proposed. It is shown that this simple model can be applied to the description of the origin of the negative and positive magnetocaloric effect in systems with competing interactions, for example, Heusler alloys.
In present work we propose a theoretical model for investigation of the exchange bias effect in Ni50Mn37.5Sb12.5 alloy. In the model, we use a three-dimensional cubic lattice with periodic boundary conditions. Also we take into account the magnetic interactions between atoms in 1st, 2nd and 3rd coordination spheres and the ferromagnetic and antiferromagnetic anisotropy terms. It is shown that the obtained theoretical temperature dependence of the exchange bias field for Ni50Mn37.5Sb12.5 alloy is close to the experimental data.
Transition metals doped ZnO are possible candidates for multiferroics. Here, we have investigated the evolution of ferromagnetism due to Co dopants. The magnetic properties have been studied for Co concentrations from 0 to 100% by using ab-initio methods, i.e., KKR Green's function techniques. In order to estimate the Curie temperature we have performed Monte Carlo simulations with ab-initio calculated exchange parameters.
From our calculations the onset of ferromagnetism occurs between 5 to 20% of Co depending on the numerical details of KKR method used. For Co concentrations larger than 50% the system is dominated by antiferromagnetic coupling and no Curie temperature can be obtained.
We performed an ab initio characterization of ferro- and nonmagnetic Ni2CoGa and Ni2CoZn compounds with respect to their potential application as new ferromagnetic shape memory alloys. The calculation of structural energy differences and mixing energies in the common
X2YZ Heusler structure and the inverse (XY)XZ structure revealed, that both alloys are stable in the tetragonal distorted Heusler structure with a c/a ratio of 1.38 and show ferromagnetic ordering. The Curie temperatures are of the order of ≃ 250 K. Exchanging Ga with Zn improves the magnetic properties of the alloy without qualitative modiﬁcation of the structural energy landscape, but at the expense of a reduced mixing energy.
Two theoretical Monte Carlo models have been presented for description of the positive and negative magnetocaloric effects of Heusler Ni-Mn-X (X = Ga, In) alloys undergoing first order coupled magnetostructural phase transition. For both models all quantities such as temperature dependence of the magnetic contribution to the total specific heat, magnetization and isothermal magnetic entropy change are in qualitative agreement with the available experimental data.
An overview is given of new ferromagnetic Heusler alloys like Ni-Co-(Al, Ga, Zn), Co-Ni-(Al, Ga, Zn), Fe-Ni-(Al, Ga, Zn) and Fe-Co-(Al, Ga, Zn), which are compared with today's mostly investigated systems such as Ni-Mn-Z (Z = Al, Ga, In, Sn, Sb). The investigations are based on first-principles as well as Monte Carlo calculations. For some new systems, the simulations of atomic structure and magnetic and electronic properties allow to predict higher Curie and martensitic transformation temperatures than those of prototypical Ni-Mn-Z materials. Some of the new materials may be distinguished for devices which exploit the magnetic shape memory effect. Interestingly, in general, all off-stoichiometric alloys display competing antiferromagnetic correlations, which may be important for devices using the magnetocaloric effect. The Curie temperatures are obtained from Monte Carlo simulations using magnetic exchange parameters from ab initio calculations while the structural instability is inferred from local minima in the ab initio total energy curves as a function of the tetragonal distortion. The manifestation of phonon softening as a precursor of structural transformations is present in the austenitic phase of most of the calculated ferromagnetic shape-memory alloys. However, quite remarkably, we find that phonon softening is absent in a few systems such as Co2NiGa.
FePt nanoparticles are promising materials for high-density magnetic data storage media  and bio-medical applications such as drug-targeting and hyperthermia . To understand their magnetic properties  it is essential to get insights into the lattice structure of isolated nanoparticles which influence the magnetic behavior.
Typically, lattice fringes are observed with high-resolution transmission electron microscopy (HR-TEM). In this case delocalization effects disturb imaging of the lattice structure in particular if 2 to 6 nm small nanoparticles are involved. Therefore, FePt nanocrystals were investigated by reconstructing amplitude and phase of the scattered electron wave from a focal series of HRTEM images, which can produce delocalization free and direct images of the crystal structure . The formation of 5-fold twinned structures of 3 to 7 nm face-centered cubic FePt nanocrystals is investigated that were grown from a colloidal solution . The results are compared with abinitio density functional (DFT) calculations of FePt particles with a diameter of larger than 2 nm. Image simulations were performed with the Accelrys Cerius2 software package (Version 4.6). Good agreement between the ab-initio calculations and the experimental data is found.
Among the magnetic shape memory Heusler alloys, Ni-Mn-Ga near stoichiometry displays the largest shape change in the martensitic 5M or 7M structure with a strain of the order of 10% in an external magnetic field of less than one Tesla. In addition, the alloys exhibit a sequence of intermediate martensites with the modulated structures usually appearing at c/a < 1 while the low-temperature nonmodulated tetragonal structures have c/a > 1. Typically, the martensitic phase changes are accompanied by a shift of a peak in the electronic density arising from the non-bonding Ni states, a reconstruction of the associated Fermi surface, and, in some cases, by pronounced phonon anomalies. These appear in the cubic high-temperature austenitic and premartensitic phases but also in the modulated phases. In addition, the modulated phases have highly mobile twins which can be rearranged under the action of an external magnetic field due to the high magnetic anisotropy, which builds up in martensite and which is at the origin of the magnetic shape memory effect. First-principles calculations confirm the overall scenario.
The transport properties of Fe(001)/Cr/Fe(001) trilayers are discussed with respect to the influence of transition metal impurities in form of layers. We are able to show that the periodicity of the giant magnetoresistance is directly influenced by the interlayer exchange coupling (IEC). Furthermore, it is observed that the behavior of the IEC strongly depends on whether an impurity overlayer of Mn or V is used. It turns out that the size of the GMR is only little effected by 3d-transition metal impurities, which is in agreement with the experimental findings. The electronic and magnetic properties of the trilayers have been investigated within the fully relativistic, spin-polarized SKKR method and the LDA. The transport properties of the Fe/Cr/Fe systems have been derived from the fully relativistic spin-polarized Kubo-Greenwood equation.
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