518 research outputs found
An efficient control of Curie temperature in Ni-Mn-Ga alloys
We have studied the influence of alloying with a fourth element on the
temperature of ferromagnetic ordering in Ni-Mn-Ga Heusler alloys. It is
found that increases or decreases, depending on the substitution. The
increase of is observed when Ni is substituted by either Fe or Co. On the
contrary, the substitution of Mn for V or Ga for In strongly reduces .Comment: presented at ICM-200
Thermal collapse of spin-polarization in half-metallic ferromagnets
The temperature dependence of the magnetization and spin-polarization at the
Fermi level is investigated for half-metallic ferromagnets. We reveal a new
mechanism, where the hybridization of states forming the half-metallic gap
depends on thermal spin fluctuations and the polarization can drop abruptly at
temperatures much lower than the Curie point. We verify this for NiMnSb by
ab-initio calculations. The thermal properties are studied by mapping ab-initio
results to an extended Heisenberg model which includes longitudinal
fluctuations and is solved by a Monte Carlo method
Kohn-Sham potential with discontinuity for band gap materials
We model a Kohn-Sham potential with a discontinuity at integer particle
numbers derived from the GLLB approximation of Gritsenko et al. We evaluate the
Kohn-Sham gap and the discontinuity to obtain the quasiparticle gap. This
allows us to compare the Kohn-Sham gaps to those obtained by accurate many-body
perturbation theory based optimized potential methods. In addition, the
resulting quasiparticle band gap is compared to experimental gaps. In the GLLB
model potential, the exchange-correlation hole is modeled using a GGA energy
density and the response of the hole to density variations is evaluated by
using the common-denominator approximation and homogeneous electron gas based
assumptions. In our modification, we have chosen the PBEsol potential as the
GGA to model the exchange hole, and add a consistent correlation potential. The
method is implemented in the GPAW code, which allows efficient parallelization
to study large systems. A fair agreement for Kohn-Sham and the quasiparticle
band gaps with semiconductors and other band gap materials is obtained with a
potential which is as fast as GGA to calculate.Comment: submitted to Physical Review
Spin-polarized tunneling between an antiferromagnet and a ferromagnet: First-principles calculations and transport theory
By combining first-principles calculations with transport theory we investigate the origin of the magnetoresistance of a magnetic tunnel junction consisting of a ferromagnetic and an antiferromagnetic lead. The (001) oriented Fe/vacuum/Cr planar junction serves as model junction. Even though the conduction electrons of antiferromagnetic Cr are spin-degenerate, it is possible to observe magnetoresistance due to two mechanisms: Firstly, the surface magnetism of Cr creates a spin-dependent potential barrier, and secondly, exchange-split surface states and resonances result in a tunneling conductance which depends on the relative orientation of the Fe and Cr magnetizations. Spin-dependent tunneling between a ferromagnet and an antiferromagnet happens frequently in tunneling setups such as in spin-polarized scanning tunneling microscopy or magnetic tunnel junctions for magnetic random access memory
Coexistence of ferro- and antiferromagnetic order in Mn-doped NiMnGa
Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy
showing large magnetic field induced strains. We present here results for the
magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and
theory. Experimental trends for the composition dependence of the magnetization
are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up
to several tesla and at low temperatures. The saturation magnetization has a
maximum near the stoichiometric composition and it decreases with increasing Mn
content. This unexpected behaviour is interpreted via first-principles
calculations within the density-functional theory. We show that extra Mn atoms
are antiferromagnetically aligned to the other moments, which explains the
dependence of the magnetization on composition. In addition, the effect of Mn
doping on the stabilization of the structural phases and on the magnetic
anisotropy energy is demonstrated.Comment: 4 pages, 3 figure
Atomistic simulations of magnetic shape memory alloys
Magnetic shape memory (MSM) alloys are novel smart materials which exhibit magnetic field induced strains of up to 10 %. As such they have potential for many technological applications. Also, the strong magneto-structural couplings of the MSM effect make the phenomenon very interesting from a scientific point of view. In this thesis, materials and properties related to the MSM effect are studied with atomistic simulations. Main interest is in the known MSM alloy Ni-Mn-Ga around the Ni2MnGa stoichiometry.
One pre-requisite for the MSM effect is the existence of a structural transformation in a magnetic material, and therefore some candidate materials are investigated from this perpective. Here, Ni2MnAl is found to have potential for further studies. The magnetic moment is seen to originate mainly from Mn in the Mn-containing alloys and the existence of different structural phases is ascribed to a band Jahn-Teller effect in the Ni band. This picture is confirmed by comparisons between theoretical and experimental neutron diffraction results. In Ni2MnGa the structural phase transformations are found to be driven by vibrational entropy at finite temperatures.
The magnetic key property in the MSM effect is the magnetic anisotropy energy which is studied in Ni2MnGa. The tetragonal structure with c/a = 0.94 is magnetically uniaxial characterized by the first anisotropy constant, but in the presence of several twin variants only the second anisotropy constant may be observed in the measurements. Analysis of the microscopic origins of the magnetic anisotropy shows that Ni has the largest contribution to the magnetic anisotropy energy. Investigations of other structures show that in Ni2MnGa the shortest crystal axis is always the easy axis of magnetization. From other magnetic properties, the Curie temperatures of Ni2MnGa and Ni2MnAl are estimated on the basis of total energy calculations of spin spirals. Ni is found to have an important effect also on the Curie temperatures despite its smaller magnetic moment when compared to Mn.
Non-stoichiometric compositions of Ni-Mn-Ga are studied within the rigid band approximation and with supercell calculations. In some cases the rigid band approximation describes the correct trends, but more insight into the alloying effects can be obtained from the supercell calculations. The most important result of these investigations is that in Mn-rich compositions the extra Mn atoms couple antiferromagnetically to the neighbouring Mn atoms. This result implies a decrease of the total magnetic moment with Mn-doping. Also, all the experimentally observed martensite phases are explained theoretically when the extra Mn is explicitly included.reviewe
Magnetic anisotropy in Ni2MnGa
We study here, within the density-functional theory, the magnetic anisotropy energy (MAE) in Ni2MnGa which is a prototype of a magnetic shape-memory alloy. We calculate the MAE, which is a key property for the magnetic shape-memory effect, for tetragonal structure with different ratios of the c and a lattice constants, reproducing the experimental easy axes both in compression and elongation of the structure. Good agreement between the theory and the experiments in the actual values of the MAE is also found when the nonstoichiometry of the experimental samples is modeled with a simple rigid band approximation. In addition, we estimate the magnetostriction coefficient, confirming the difference between the ordinary magnetostriction and the magnetic shape-memory effect. Equally important, we study the microscopic origin of the MAE in Ni2MnGa with the spin density and the orbital moment anisotropy and extend the analysis of the orbital moment anisotropy to the ternary compounds. These results show that the largest contribution to the MAE comes from Ni, in spite of the larger magnetic moment in the Mn sites.Peer reviewe
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