Ralph Skomski PublicationsCopyright (c) 2018 University of Nebraska - Lincoln All rights reserved.
https://digitalcommons.unl.edu/physicsskomski
Recent documents in Ralph Skomski Publicationsen-usThu, 01 Feb 2018 11:20:22 PST3600Large energy product enhancement in perpendicularly coupled MnBi/CoFe magnetic bilayers
https://digitalcommons.unl.edu/physicsskomski/97
https://digitalcommons.unl.edu/physicsskomski/97Thu, 06 Apr 2017 10:51:27 PDT
We demonstrate substantial enhancement in the energy product of MnBi-based magnets by forming robust ferromagnetic exchange coupling between a MnBi layer and a thin CoFe layer in a unique perpendicular coupling configuration, which provides increased resistance to magnetization reversal. The measured nominal energy product of 172 kJ/m^{3} at room temperature is the largest value experimentally attained for permanent magnets free of expensive raw materials. Our finding shows that exchange-coupled MnBi/CoFe magnets are a viable option for pursuing rare-earth-free magnets with energy products approaching those containing rare-earth elements.
]]>
T. R. Gao et al.On the Ising character of the quantum-phase
transition in LiHoF<sub>4</sub>
https://digitalcommons.unl.edu/physicsskomski/96
https://digitalcommons.unl.edu/physicsskomski/96Mon, 13 Jun 2016 08:05:32 PDT
It is investigated how a transverse magnetic field affects the quantum-mechanical character of LiHoF_{4}, a system generally considered as a textbook example for an Ising-like quantum-phase transition. In small magnetic fields, the low-temperature behavior of the ions is Ising-like, involving the nearly degenerate low-lying J_{z} = ± 8 doublet. However, as the transverse field increases, there is a substantial admixture of states having | J_{z} | < 8. Near the quantum-phase-transition field, the system is distinctively non-Ising like, and all J_{z} eigenstates yield ground-state contributions of comparable magnitude. A classical analog to this mechanism is the micromagnetic single point in magnets with uniaxial anisotropy. Since Ho^{3}^{+} has J = 8, the ion’s behavior is reminiscent of the classical limit (J = ∞), but quantum corrections remain clearly visible.
]]>
Ralph SkomskiIntrinsic magnetic properties of L10 FeNi obtained from meteorite NWA 6259
https://digitalcommons.unl.edu/physicsskomski/95
https://digitalcommons.unl.edu/physicsskomski/95Thu, 16 Jul 2015 08:54:25 PDT
FeNi having the tetragonal L10 crystal structure is a promising new rare-earth-free permanent magnet material. Laboratory synthesis is challenging, however, tetragonal L10 FeNi—the mineral “tetrataenite”—has been characterized using specimens found in nickel-iron meteorites. Most notably, the meteorite NWA 6259 recovered from Northwest Africa is 95 vol.% tetrataenite with a composition of 43 at.% Ni. Hysteresis loops were measured as a function of sample orientation on a specimen cut from NWA 6259 in order to rigorously deduce the intrinsic hard magnetic properties of its L10 phase. Electron backscatter diffraction showed that NWA 6259 is strongly textured, containing L10 grains oriented along any one of the three equivalent cubic directions of the parent fcc structure. The magnetic structure was modeled as a superposition of the three orthonormal uniaxial variants. By simultaneously fitting first-quadrant magnetization data for 13 different orientations of the sample with respect to the applied field direction, the intrinsic magnetic properties were estimated to be saturation magnetization 4pMs¼14.7 kG and anisotropy field Ha¼14.4 kOe. The anisotropy constant K¼0.84 MJ/m3 is somewhat smaller than the value K¼1.3 MJ/m3 obtained by earlier researchers from nominally equiatomic FeNi prepared by neutron irradiation accompanied by annealing in a magnetic field, suggesting that higher Ni content (fewer Fe antisite defects) may improve the anisotropy. The fit also indicated that NWA 6259 contains one dominant variant (62% by volume), the remainder of the sample being a second variant, and the third variant being absent altogether.
]]>
Eric Poirier et al.Correlations in rare-earth transition-metal permanent magnets
https://digitalcommons.unl.edu/physicsskomski/94
https://digitalcommons.unl.edu/physicsskomski/94Thu, 16 Jul 2015 08:47:08 PDT
It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition- metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independentelectron level, the use of a single Slater determinant with broken spin symmetry introduces Hund’s rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDAþU approximations to DFT. By contrast, rareearth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe’s crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.
]]>
Ralph Skomski et al.Spin-modified catalysis
https://digitalcommons.unl.edu/physicsskomski/93
https://digitalcommons.unl.edu/physicsskomski/93Thu, 16 Jul 2015 08:45:08 PDT
First-principle calculations are used to explore the use of magnetic degrees of freedom in catalysis. We use the Vienna Ab-Initio Simulation Package to investigate both L10-ordered FePt and CoPt bulk materials and perform supercell calculations for FePt nanoclusters containing 43 atoms. As the catalytic activity of transition-metal elements and alloys involves individual d levels, magnetic alloying strongly affects the catalytic performance, because it leads to shifts in the local densities of states and to additional peaks due to magnetic-moment formation. The peak shift persists in nanoparticles but is surface-site specific and therefore depends on cluster size. Our research indicates that small modifications in stoichiometry and cluster size are a useful tool in the search for new catalysts.
]]>
R Choudhary et al.Magnetic properties of Fe-doped MnAl
https://digitalcommons.unl.edu/physicsskomski/92
https://digitalcommons.unl.edu/physicsskomski/92Mon, 07 Jul 2014 10:54:54 PDT
Electronic and magnetic properties of L1_{0}-ordered Fe_{x}Mn_{1−x}Al alloys (x = 0, 0.0625, 0.125, 0.1875, 0.5) are investigated by first-principle supercell calculations. Pristine MnAl exhibits robust ferromagnetism involving the dense-packed Mn atoms in (001) planes of the tetragonal structure. Iron substitution for Mn significantly deteriorates the magnetization of the alloy. The reduction is a dilution effect, caused by the relatively small Fe moment of about 1.9 μ_{B} per atom, as compared to the Mn moment, which exceeds 2.4 μ_{B}. By contrast, 50% Fe substituted for Mn (x = 0.5) yields a magnetocrystalline anisotropy enhancement of about 40%. Both the magnetization reduction and the anisotropy enhancement mainly arise due to the change in d_{xy} and d_{zx} minority density of states (DOS) at the Fermi level.
]]>
Priyanka Manchanda et al.Are there superspin glasses?
https://digitalcommons.unl.edu/physicsskomski/91
https://digitalcommons.unl.edu/physicsskomski/91Mon, 31 Mar 2014 09:30:20 PDT
The effect of magnetostatic and exchange interactions on the spin structure of interacting nanoparticles and granular nanomagnets is investigated by model calculations. Effective exchange stiffnesses for inhomogeneous media are defined and determined for some geometries and interactions, and it is argued that typical ensembles of interacting small nanoparticles are micromagnetic systems rather than superspin glasses or superferromagnets. The spin structures of granular magnets often have the character of interaction domains, with far-reaching implications for magnetic phenomena such as hysteresis-loop overskewing.
]]>
Ralph SkomskiInteratomic exchange in Mn-doped III–V semiconductors
https://digitalcommons.unl.edu/physicsskomski/90
https://digitalcommons.unl.edu/physicsskomski/90Thu, 20 Mar 2014 12:17:13 PDT
Density-functional calculations are used to determine the electronic structure and magnetic properties of dilute magnetic semiconductors with the composition X_{1−x}MnxN (X = Al, Ga, In, x = 6.25% and 12.5%). Emphasis is on the interatomic exchange as a function of the Mn–Mn distance. Our superlattice calculations show that the Mn dopants are spin-polarized with a half-metallic band gap and a magnetic moment of 4 μB per Mn atom at x = 6.25 and 12.5%. The Mn (3d) bands lie in the band gap but partially hybridize with valence band or N 2p electrons, depending on the group-III element and on the spin direction. To calculate the exchange interaction parameters J_{ij}, we have used a Green-function approach. The interaction between Mn atoms extends over several interatomic interactions and is mediated by nitrogen (2p) electrons. The exchange is always ferromagnetic and largest for the first nearest neighbors, but substantial ferromagnetic interactions persist over Mn–Mn distances up to sixth nearest neighbors in the considered supercell.
]]>
Vinit Sharma et al.Magnetism of Less Common Cobalt-Rich Alloys
https://digitalcommons.unl.edu/physicsskomski/89
https://digitalcommons.unl.edu/physicsskomski/89Thu, 19 Dec 2013 11:38:10 PST
Alternative permanent-magnet materials with intermediate performance, such as K_{1} ≥ 1 MJ/m^{3}, are investigated experimentally. Our focus is on the structural and magnetic properties of bulk YCo_{5-x}Fe_{x} and on rare-earth-free transition-metal alloys such as TM_{x}Co_{100-x} (TM = Zr, Hf). YCo_{5-x}Fe_{x} alloys (x = 0 to 0.75), which crystallize in the hexagonal CaCu_{5}-type structure, exhibit an improved of anisotropy and magnetization as x increases from 0 to 0.75, but the anisotropy increase is much less pronounced than recent theoretical predictions. Zr_{x}Co_{100-x} and Hf_{x}Co_{100-x} exhibit reasonable room-temperature hard-magnetic properties, such as coercivities of 0.10 to 0.28 T and magnetizations of 0.60 to 0.94 T.
]]>
Bhaskar Das et al.Intrinsic Magnetic Properties of L10-Based Mn-Al and Fe-Co-Pt Alloys
https://digitalcommons.unl.edu/physicsskomski/88
https://digitalcommons.unl.edu/physicsskomski/88Thu, 19 Dec 2013 11:36:19 PST
Density functional theory is used to investigate how atomic substitutions modify the magnetization and anisotropy of L10-ordered ferromagnets. Our VASP supercell calculations focus on two classes of materials: Mn-Al-(Fe) and Fe-Co-Pt. We find that the Mn and Al moments in pure MnAl are 2.420 μ_{B} and -0.61 μ_{B} per atom, respectively. The calculated zero temperature anisotropy is 1.77 MJ/m^{3}. Replacing 50% of Mn by Fe enhances the anisotropy from 1.77 MJ/m^{3} to 2.50 MJ/m^{3} but reduces the magnetization. We have also calculated the magnetic moments of L1_{0}-ordered Fe_{1-x}Co_{x}Pt with various degrees of Fe-Co disorder. Configurational supercell averaging shows that the net moment decreases systematically with Co concentration, but the individual Fe and Co moments depend on the number of Fe-Co bonds.
]]>
Priyanka Manchanda et al.Spin correlations and Kondo effect in a strong ferromagnet
https://digitalcommons.unl.edu/physicsskomski/87
https://digitalcommons.unl.edu/physicsskomski/87Thu, 19 Dec 2013 11:31:20 PST
The spin structure and electron-transport behavior of Pt-substituted MnBi thin films have been investigated. The electrical resistivity of these ferromagnetic films shows an unusual low-temperature resistance minimum or Kondo effect accompanied by positive magnetoresistance. First-principles calculations show that Mn atoms displaced to the bipyramidal interstitial sites are antiferromagnetically coupled to the Mn atoms on their regular lattice sites. We explain the observed Kondo effect and the positive magnetoresistance as the consequences of local spin correlations involving Mn atoms displaced to interstitial sites by Pt doping.
]]>
Parashu Kharel et al.Overcoming the spin-multiplicity limit of entropy by means of lattice degrees of freedom:
A minimal model
https://digitalcommons.unl.edu/physicsskomski/86
https://digitalcommons.unl.edu/physicsskomski/86Thu, 19 Dec 2013 11:24:21 PST
The discovery of the giant magnetocaloric effect with isothermal field-induced entropy change beyond the spin-multiplicity limit gave rise to some indistinctness in the literature regarding the applicability of fundamental thermodynamics in data analysis. Those misleading interpretations concerning, for instance, the rigorousness of phenomenological thermodynamics are clarified here. Specifically, it is shown that the Maxwell relation incorporates contributions from the spin degrees of freedom and potential lattice degrees of freedom into the isothermal entropy change. A minimalist model involving pairs of exchange-coupled, mobile Ising spins is investigated. It is explicitly shown that lattice degrees of freedom can be activated via applied magnetic fields and the integrated Maxwell relation contains this lattice contribution. A simple and intuitive analytic expression for the isothermal entropy change in the presence of field-activated lattice degrees of freedom is provided.
]]>
Tathagata Mukherjee et al.Magnetocaloric properties of Co/Cr superlattices
https://digitalcommons.unl.edu/physicsskomski/85
https://digitalcommons.unl.edu/physicsskomski/85Thu, 19 Dec 2013 11:16:10 PST
Nanostructured materials for refrigeration applications are experimentally realized by molecular beam epitaxial growth of Co/Cr superlattices using mean-field theoretical concepts as guiding principles. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. More generally, the potential of artificial antiferromagnets for near room-temperature refrigeration is explored. The effects of intraplane and interplane exchange interactions on the magnetic phase diagram in Ising-type model systems are revisited in mean-field considerations with special emphasis on tailoring magnetocaloric properties. The experimental results are discussed in light of our theoretical findings, and extrapolations for future improved nanostructures are provided.
]]>
Tathagata Mukherjee et al.Experimental and theoretical studies of
hydroxyl-induced magnetism in TiO nanoclusters
https://digitalcommons.unl.edu/physicsskomski/84
https://digitalcommons.unl.edu/physicsskomski/84Thu, 19 Dec 2013 11:10:59 PST
A main challenge in understanding the defect ferromagnetism in dilute magnetic oxides is the direct experimental verification of the presence of a particular kind of defect and distinguishing its magnetic contributions from other defects. The magnetic effect of hydroxyls on TiO nanoclusters has been studied by measuring the evolution of the magnetic moment as a function of moisture exposure time, which increases the hydroxyl concentration. Our combined experiment and density-functional theory (DFT) calculations show that as dissociative water adsorption transforms oxygen vacancies into hydroxyls, the magnetic moment shows a significant increase. DFT calculations show that the magnetic moment created by hydroxyls arises from 3d orbitals of neighboring Ti sites predominantly from the top and second monolayers. The two nonequivalent hydroxyls contribute differently to the magnetic moment, which decreases as the separation of hydroxyls increases. This work illustrates the essential interplay among defect structure, local structural relaxation, charge redistribution, and magnetism. The microscopic differentiation and clarification of the specific roles of each kind of intrinsic defect is critical for the future applications of dilute magnetic oxides in spintronic or other multifunctional materials.
]]>
Xiao-Hui Wei et al.Magnetism and electron transport of MnyGa (1 < y < 2) nanostructures
https://digitalcommons.unl.edu/physicsskomski/83
https://digitalcommons.unl.edu/physicsskomski/83Thu, 19 Dec 2013 11:07:46 PST
Nanostructured Mn_{y}Ga ribbons with varying Mn concentrations including Mn_{1.2}Ga, Mn_{1.4}Ga, Mn_{1.6}Ga, and Mn_{1.9}Ga were prepared using arc-melting and melt-spinning followed by a heat treatment. Our experimental investigation of the nanostructured ribbons shows that the material with y = 1.2, 1.4, and 1.6 prefers the tetragonal L1_{0} structure and that with y = 1.9 prefers the D0_{22} structure. We have found a maximum saturation magnetization of 621 emu/cm^{3} in Mn_{1.2}Ga which decreases monotonically to 300 emu/cm^{3} as y reaches 1.9. Although both the L1_{0}- and D0_{22}-Mn_{y}Ga samples show a high Curie temperature (T_{c}) well above room temperature, the value of T_{c} decreases almost linearly from 702K for Mn_{1.9}Ga to 551K for Mn_{1.2}Ga. All the ribbons are metallic between 2K and 300K but the Mn_{1.2}Ga also shows a resistance minimum near 15K. The observed magnetic properties of the Mn_{y}Ga ribbons are consistent with the competing ferromagnetic coupling between Mn moments in the regular L1_{0}-MnGa lattice sites and antiferromagnetic coupling with excess Mn moments occupying Ga sites.
]]>
Yung Huh et al.Ab-initio study of free standing TiO2 clusters: Stability and magnetism
https://digitalcommons.unl.edu/physicsskomski/82
https://digitalcommons.unl.edu/physicsskomski/82Thu, 19 Dec 2013 11:05:10 PST
We report the structural behavior of nanoscale Titanium Dioxide (TiO_{2}) clusters as well as their magnetic properties by varying the cluster size with the help of ground state geometries. The clusters of atomic scale rutile (TiO_{2})_{n}, where n = 1-11, have been considered and geometrically stabilized through the Density Functional Theory as implemented in Vienna ab-initio Simulation Package. It is being observed that as the size of cluster increases from n = 2 to 11, the total energy decreases. The results of formation energy reveal the fact that as the cluster grows, it moves towards the stability and it is observed that n = 11 is the most stable structure. The stabilized clusters are different in geometries and co-ordination numbers. Finally, all the clusters have been investigated with self-consistent treatment of spin orbit coupling for magnetism studies. The magnetic properties of free clusters depict oscillatory behavior for magnetic moment with respect to the cluster size.
]]>
T.H. Rana et al.Predicting the Future of Permanent-Magnet Materials
https://digitalcommons.unl.edu/physicsskomski/81
https://digitalcommons.unl.edu/physicsskomski/81Thu, 19 Dec 2013 11:00:38 PST
There are two main thrusts towards new permanent-magnet materials: improving extrinsic properties by nanostructuring and intrinsic properties by atomic structuring. Theory—both numerical and analytical—plays an important role in this ambitious research. Our analysis of aligned hard-soft nanostructures shows that soft-in-hard geometries are better than hard-in-soft geometries and that embedded soft spheres are better than sandwiched soft layers. Concerning the choice of the hard phase, both a high magnetization and a high anisotropy are necessary. As an example of first-principle research, we consider interatomic Mn exchange in MnAl and find strongly ferromagnetic intralayer exchange, in spite of the small Mn-Mn distances.
]]>
Ralph Skomski et al.Magnetism of Rapidly Quenched Sm<sub>1-x</sub>Zr<sub>x</sub>Co<sub>5</sub> Nanocrystalline Materials
https://digitalcommons.unl.edu/physicsskomski/80
https://digitalcommons.unl.edu/physicsskomski/80Thu, 19 Dec 2013 10:54:13 PST
The effect of Zr addition on nanostructure and magnetic properties in nanocrystalline Sm_{1-x}Zr_{x} Co_{5} (x = 0 – 0.6) has been investigated. (Sm, Zr)Co_{5} with the CaCu_{5} structure was synthesized by melt spinning. The lattice parameters a and b decrease with x, whereas c increases. Thus, the unit cell volume of (Sm, Zr)Co_{5} shrinks because the smaller Zr atoms occupy the sites of the larger Sm atoms. Zr addition decreases the grain size and induces the formation of planar defects. The coercivity decreases with x, due to weakening of magnetocrystalline anisotropy energy and effective intergrain exchange coupling. A very high coercivity of 39 kOe and energy product of 13.9 MGOe are obtained for x = 0. The remanence of (Sm, Zr)Co_{5} increases with x. For ≤ 0.4, the energy product slightly decreases with x. The results show that 40% of the Sm can be replaced by the less expensive Zr, with an energy-product reduction of only 10%. In addition, the planar defects are responsible for the change of coercivity mechanism from the nucleation-type of reverse domain for the x = 0 to the pinning-type of domain wall for the x = 0.4.
]]>
Wenyong Zhang et al.Magnetism of MnBi-Based Nanomaterials
https://digitalcommons.unl.edu/physicsskomski/79
https://digitalcommons.unl.edu/physicsskomski/79Thu, 19 Dec 2013 10:40:28 PST
Nanostructured MnBi ribbons doped with impurity elements including B, C, Fe, Hf, Sm and Tb were prepared using the arc melting and melt-spinning techniques. The melt-spun ribbons were annealed in vacuum furnace at 350°C to obtain the intended hexagonal structure. The external impurity doping made a significant change in the magnetic properties of the nanostructured MnBi ribbons including a decrease in saturation magnetization (M_{s}) and anisotropy energy (K) and an increase in coercivity (H_{c}). However, Hf andC co-doping showed the opposite effect with a small increase in both (M_{s}) and K. Interestingly, the anisotropy energy of the boron doped sample increased by about 15% irrespective of the small decrease in magnetization. A significant increase in (H_{c}) of MnBi ribbons was found due to Hf, Tb and Sm doping. (H_{c}) as high as 13 kOe was achieved in Hf-doped sample after the sample was aligned in a magnetic field. A thermal hysteresis was observed at the structural phase transition of MnBi, which shifts by about 5 K towards higher temperatures due to impurity doping. The observed magnetic properties of the impurity doped MnBi ribbons are explained as the consequences of the disorder and the competing ferromagnetic and antiferromagnetic interactions.
]]>
Parashu Kharel et al.Magnetic and Structural Properties of Rapidly Quenched
Tetragonal Mn3-x Ga Nanostructures
https://digitalcommons.unl.edu/physicsskomski/78
https://digitalcommons.unl.edu/physicsskomski/78Thu, 19 Dec 2013 10:34:03 PST
Nanostructured Mn_{3-x} Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D0_{19} and cubic L2_{1} Heusler crystal structures based on the concentration of Mn in Mn_{3-x} Ga. Upon vacuum-annealing the samples at 450°C for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D0_{22} structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn_{1.9} Ga and Mn_{2.1} Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn_{2.6 }Ga and Mn_{3.0 }Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn_{3-x} Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn_{2.6 }Ga and Mn_{3.0} Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn_{3-x}Ga. The maximum magnetization of 57 emu/g (300 emu/cm^{3}) and the coercivity of 6.5 kOe were measured in the Mn_{1.9} Ga and Mn_{3.0} Ga ribbons, respectively.
]]>
Yung Huh et al.