Electron spin mediated distortion in metallic systems (original) (raw)
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Acta Materialia, 2021
Abstract Complex concentrated alloys (CCAs) are of growing interest due to their outstanding mechanical properties that exceed the property limits of conventional alloys. Whereas the superior properties are often attributed to severe lattice distortion, to date it is not clear what controls the lattice distortion and how it affects the mechanical properties of CCAs. In this work, we study the element-resolved local lattice distortion (ELLD) in CCAs of 3d transition-metal elements (3d CCAs) by the extended X-ray absorption fine structure experiment and the density-functional theory calculations. We show that ELLD is primarily dependent upon charge transfer among elements and affects the properties through atomic-level pressure and orbital transition. The ELLD provides a qualitative measure of the effective atomic size for explaining element-specific properties and macroscopic properties.
On the Nature of Anomalous Electronic and Lattice Properties of Dilute CR-Based Alloys
Journal De Physique, 1988
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Physical Review B, 2001
Recently, we outlined a scheme to investigate the effects of both short-ranged and long-ranged compositional order on the magnetocrystalline anisotropy of alloys from a first-principles electronic structure point of view ͓Phys. Rev. Lett. 82, 5369 ͑1999͔͒ and showed that in the Co 0.5 Pt 0.5 alloy compositional order enhances the magnitude of magnetocrystalline anisotropy energy ͑MAE͒ by some two orders of magnitude. Here we describe our scheme in detail and study some more transition metal alloys. In the Co 0.25 Pt 0.75 alloy we find the perfect L1 2 structure to be magnetically soft whereas imposition of directional order greatly enhances its MAE. We also present the effect of lattice distortion ͑tetragonalization͒ on MAE on the same footing and find that in the Co 0.5 Pt 0.5 alloy it accounts for only about 20% of the observed enhancement, thus confirming that compositional order is the major player in this effect. Tetragonalization of the lattice has also a modest effect on the MAE of the Fe 0.5 Co 0.5 alloy. We also examine the electronic effects which underpin the directional chemical order that is produced by magnetic annealing of permalloy which we study within the same framework.
Competition between magnetic and structural transitions in CrN
Physical Review B, 1999
CrN is observed to undergo a paramagnetic to antiferromagnetic transition with Néel temperature TN ∼ 280 K, accompanied by a shear distortion from cubic NaCl-type to orthorhombic Pnma structure. Our first-principles plane wave calculations, based on ultrasoft pseudopotentials, confirm that the distorted antiferromagnetic phase with spin configuration arranged in double ferromagnetic sheets along the [110] direction is energetically much more stable than the paramagnetic phase, and also slightly favored over the ferromagnetic and other examined antiferromagnetic phases. The energy gain from polarization is much larger than that from distortion; nevertheless, the distortion is decisive in resolving the competition among the antiferromagnetic phases: the anisotropy in the (100) plane arising from the ferromagnetic double-sheet magnetic order allows a small but important energy gain upon the cubic-to-orthorhombic transition. Although antiferromagnetic order leads to a large depletion of states around Fermi level, it does not open a gap. The system is metallic with occupied hole and electron pockets of Fermi surface containing ∼ 0.025 carriers of each sign per formula unit. The simultaneous occurrence of structural distortion and antiferromagnetic order, as well as the competition between antiferromagnetism and ferromagnetism, is analyzed. 71., 75., 71.15.Hx, 75.50.Ee
Recently, we outlined a scheme to investigate the effects of compositional order on the magnetocrystalline anisotropy of alloys from a first-principles electronic structure point of view {Phys. Rev. Lett. 83, 5369 (1999)} and showed that compositional order enhances the magnitude of magnetocrystalline anisotropy energy (MAE) of Co0.5Pt0.5 alloy by some two orders of magnitude as well as affecting the equilibrium magnetization direction. Here we describe our scheme in detail and present an in-depth study of the effect by demonstrating its Fermi surface origin. In Co0.25Pt0.75 alloy we find that the perfect L12 structure has a very small MAE whereas imposition of directional order enhances the MAE by two orders of magnitude. We also present the effect of lattice distortion (tetragonalization) on the MAE on the same footing and find that in the Co0.5Pt0.5 alloy it accounts for only about 20% of the observed MAE, thus confirming that compositional order is the major player in the enhancement of MAE. We also examine the directional chemical order that can be produced by magnetic annealing within the same framework. We extract a Fermi surface mechanism for the effect in an explicit study of permalloy. Finally, we propose that the Fermi surface plays a major role in the strong coupling between magnetocrystalline anisotropy and compositional order in many magnetic alloys.
arxiv.org/abs/2004.09086, 2020
The presence, nature, and impact of chemical short-range order in the multi-principal element alloy CrCoNi are all topics of current interest and debate. On the basis of first-principles calculations, we present a theory of exchange interaction-driven atomic ordering in this system centered on the elimination of like-spin Cr-Cr neighbors, with significant contributions from certain magnetically aligned Co-Cr and Cr-Cr atoms. Together, these effects can explain anomalous magnetic measurements across a range of compositions and provide implications for related high-entropy alloys.
Ferromagnetism in tetragonally distorted chromium
Physical Review B, 2010
We report on the experimental observation of ferromagnetic ordering in the tetragonally distorted ␣ phase of Cr, based on the anomalous Hall effect hysteresis and corroborated by magnetization hysteresis. The ferromagnetic ordering is reasonably ascribed to an increase in the out-of-plane lattice parameter of the ␣-Cr phase due to the low-dimensional connection to the ␦-Cr crystal-lattice matrix, which suffers a large compressive lattice-mismatch strain from the substrate under two-dimensional growth. The low proportion of the ␣ phase in the Cr film results in a weak macroscopic-scale ferromagnetic moment.
Magnetic properties of point defect interaction with impurity atoms in Fe–Cr alloys
Journal of Nuclear Materials, 2009
An integrated ab initio and statistical Monte Carlo investigation has been recently carried out to model the thermodynamic and kinetic properties of Fe-Cr alloys. We found that the conventional Fe-Cr phase diagram is not adequate at low temperature region where the magnetic contribution to the free energy plays an important role in the prediction of an ordered Fe 15 Cr phase and its negative enthalpy of formation. The origin of the anomalous thermodynamic and magnetic properties of Fe-Cr alloys can be understood using a tight-binding Stoner model combined with the charge neutrality condition. We investigate the environmental dependence of magnetic moment distributions for various self-interstitial atom h1 1 0i dumbbells configurations using spin density maps found using density functional theory calculations. The mixed dumbbell Fe-Cr and Fe-Mn binding energies are found to be positive due to magnetic interactions. Finally, we discuss the relationship between the migration energy of vacancy in Fe-Cr alloys and magnetism at the saddle point configuration.
Magnetic structures and their stability inMn3Rhordered and disordered alloys
Physical Review B, 2002
The electronic and magnetic structure of Mn 3 Rh, including noncollinear structures, in the ordered and disordered states have been investigated by the tight-binding linear-muffin-tin orbital method. A magnetic long-range order of the triangular (T1) structure in the ordered state is reflected in a dip around the Fermi level in the density of states. This characteristic feature can be retained in the multiple-Q spin-density-wave ͑SDW͒ structures in the disordered alloy, implying that a stable antiferromagnetic order also remains in the disordered alloy. The most stable structure in the disordered alloy is suggested to be the 3Q-SDW structure. From the effective exchange constant, the Néel temperature in the disordered alloy is estimated to be about 680 K, close to the experimental value of about 700 K. Compared to the Néel temperature of pure ␥-Mn metal, such a high value is pertinent to the change in the electronic structure caused by the addition of Rh.
Magnetic Stress as a Driving Force of Structural Distortions: The Case of CrN
Physical Review Letters, 2000
We show that the observed transition from rocksalt to orthorhombic Pnma symmetry in CrN can be understood in terms of stress anisotropy. Using local spin density functional theory, we find that the imbalance between stress stored in spin-paired and spin-unpaired Cr nearest neighbors causes the rocksalt structure to be unstable against distortions and justifies the observed antiferromagnetic ordering. This stress has a purely magnetic origin, and may be important in any system where the coupling between spin ordering and structure is strong. 71.15.Mb,75.25.+z,75.40.Mg,75.80.+q