The structural and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferroand antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition. PACS numbers: 75.50.-y, 75.10.-b, 75.30.SgFollowing the concepts of Hume-Rothery the influence of composition on martensitic and magnetic transformation temperatures is commonly condensed as a dependency of electrons per atom (e/a-ratio) [1]. Experiment and first-principles calculations, however, reveal that the Z element in Ni-Mn-Z Heusler alloys (Z = Ga, In, Sn) also affects the transformation temperatures substantially [2]. Moreover, recent experiments on samples with identical composition but different heat treatment indicate that chemical disorder also plays an important role [3][4][5]. Here, we use first-principles calculations to identify the influence of chemical disorder on the magnetic exchange parameters and derive guidelines for a further systematic improvement of magnetocaloric materials [6].Besides the magnetocaloric effect (MCE) in Gd and other alloys at room temperature [7,8], the metamagnetic Ni-Mn based Heusler materials [9,10], have attracted much interest recently [11,12]. In these alloys the metamagnetic features are responsible for magnetic glass behavior and frustration due to chemical disorder [13][14][15] as well as unusual magnetization behavior under an external magnetic field such as a large jump of the magnetization ∆M (T m ) at the martensitic/magnetostructural transformation temperature T m [16]. This gives rise to the large inverse MCE of the materials [9,10,17,18]. The MCE can be influenced when Ni is substituted in part by Co: It is strongly enhanced in the case of In-based intermetallics [19,20] (with adiabatic temperature change ∆T ad = −6 K in 2 T field [20]) while in the case of Ga the MCE is turned from direct to inverse by decoupling T m and Curie temperature T C [21] (with ∆T ad = −1.6 K in 1.9 T field [22,23]).Chemical disorder in the Mn-rich Heusler alloys is responsible for competing magnetic interactions (ferromagnetic versus antiferromagnetic) because the extra Mn atoms occupy lattice sites of the Z-sublattice which interact antiferromagnetically with the Mn atoms on the Y-sublattice due to RKKY-type interactions. This competition of magnetic interactions leads to the characteristic drop of magnetization curves at T m , wh...

The vibrational density of states of silicon nanoparticles in the range from 2.3 to 10.3 nm is studied with the help of molecular-dynamics simulations. From these simulations the vibrational density of states and frequencies of bulklike vibrational modes at high-symmetry points of the Brillouin zone have been derived. The results show an increase of the density of states at low frequencies and a transfer of modes from the high-frequency end of the spectrum to the intermediate range. At the same time the peak of transverse optical modes is shifted to higher frequencies. These observations are in line with previous simulation studies of metallic nanoparticles and they provide an explanation for a previously observed discrepancy between experimental and theoretical data

No abstract

Magnetic Ni–Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni–Co–Mn–Sn on the right‐hand side). The cubic austenitic phase at high temperature shows long‐range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non‐modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long‐range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non‐magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape‐memory and exchange‐bias effects as well as magnetocaloric effects. The ‘ferroic cooling’ is of particular relevance since it avoids the use of ozone‐depleting and greenhouse chemicals compared with conventional fluid‐compression technology. Experimental phase diagram of Ni50−x Cox Mn39 Sn11 for 0≤x≤10. Here, TC is the Curie temperature of austenite; at TM the system transforms to paramagnetic martensite and at TS to superparamagnetic martensite (SPM) and then to superspin‐glass martensite (SSG) at TP. The possible strain‐glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field‐cooling/zero‐field‐cooling experiments.

The electronic and magnetic bulk properties of half-metallic Heusler alloys such as Co 2 FeSi, Co 2 FeAl, Co 2 MnSi, and Co 2 MnAl are investigated by means of ab initio calculations in combination with Monte Carlo simulations. The electronic structure is analyzed using the plane-wave code QUANTUM ESPRESSO and the magnetic exchange interactions are determined using the Korringa-Kohn-Rostoker (KKR) method. From the magnetic exchange interactions, the Curie temperature is obtained via Monte Carlo simulations. In addition, electronic transport properties of trilayer systems consisting of two semi-infinite platinum leads and a Heusler layer in-between are obtained from the fully relativistic screened KKR method by employing the Kubo-Greenwood formalism. The focus is on thermoelectric properties, namely, the Seebeck effect and its spin dependence. It turns out that already thin Heusler layers provide highly spin-polarized currents. This is attributed to the recovery of half-metallicity with increasing layer thickness. The absence of electronic states of spin-down electrons around the Fermi level suppresses the contribution of this spin channel to the total conductance, which strongly influences the thermoelectric properties and results in a spin polarization of thermoelectric currents.