arXiv subjects

J. B. Staunton

Publications and source records attributed to J. B. Staunton.

31 records · Page 2Linked to original sources

arXiv2006

An ab-initio theory for the temperature dependence of magnetic anisotropy

We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T, in metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced `local moment' magnetic fluctuations are described within the relativistic generalisation of the `disordered local moment' (R-DLM) theory from which the T dependence of the magnetisation, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry, L1_0-ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m squared for a broad range of values of m. This is the same trend that we have previously found in L1_0-ordered FePt and which agrees with experiment. This account, however, differs qualitatively from that extracted from a single ion anisotropy model. We also study the magnetically soft cubic magnet, the Fe(50)Pt(50) solid solution, and find that its small magnetic anisotropy constant K_1 rapidly diminishes from 8 micro-eV to zero. K evolves from being proportional to the seventh power of m at low T to the fourth power near the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2004

Temperature dependent magnetic anisotropy in metallic magnets from an ab-initio electronic structure theory: L1_0-ordered FePt

On the basis of a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy, K, with magnetisation, M, in metallic ferromagnets. We apply the theory to the high magnetic anisotropy material, L1_0-ordered FePt, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pt layering for all M and to be proportional to M^2 for a broad range of values of M. For small M, near the Curie temperature, the calculations pick out the easy axis for the onset of magnetic order. Our results are in good agreement with recent experimental measurements on this important magnetic material.

cond-mat.mtrl-sci↗

arXiv2004

An ab-initio theoretical investigation of the soft-magnetic properties of permalloys

We study Ni80Fe20-based permalloys with the relativistic spin-polarized Korringa-Kohn-Rostoker electronic structure method. Treating the compositional disorder with the coherent potential approximation, we investigate how the magnetocrystalline anisotropy, K, and magnetostriction, lambda, of Ni-rich Ni-Fe alloys vary with the addition of small amounts of non-magnetic transition metals, Cu and Mo. From our calculations we follow the trends in K and lambda and find the compositions of Ni-Fe-Cu and Ni-Fe-Mo where both are near zero. These high permeability compositions of Ni-Fe-Cu and Ni-Fe-Mo match well with those discovered experimentally. We monitor the connection of the magnetic anisotropy with the number of minority spin electrons, Nmin. By raising Nmin via artificially increasing the band-filling of Ni80Fe20, we are able to reproduce the key features that underpin the magnetic softening we find in the ternary alloys. The effect of band-filling on the dependence of magnetocrystalline anisotropy on atomic short-range order in Ni80Fe20 is also studied. Our calculations, based on a static concentration wave theory, indicate that the susceptibility of the high permeability of the Ni-Fe-Cu and Ni-Fe-Mo alloys to their annealing conditions is also strongly dependent on the alloys' compositions. An ideal soft magnet appears from these calculations.

cond-mat.mtrl-sci↗

arXiv2003

Mangetic properties of Ising thin-films with cubic lattices

We have used Monte Carlo simulations to observe the magnetic behaviour of Ising thin-films with cubic lattice structures as a function of temperature and thickness especially in the critical region. The fourth order Binder cumulant is used to extract critical temperatures, and an extension of finite size scaling theory for reduced geometry is derived to calculate the critical exponents. Magnetisation and magnetic susceptibility per spin in each layer are also investigated. In addition, mean-field calculations are also performed for comparison. We find that the magnetic behaviour changes from two dimensional to three dimensional character with increasing thickness of the film. The crossover of the critical temperature from a two dimensional to a bulk value is also observed with both the Monte Carlo simulations and the mean-field analysis. Nevertheless, the simulations have shown that the critical exponents only vary a little from their two dimensional values. In particular, the results for films with up to eight layers provide a strong indication of two dimensional universality.

cond-mat.stat-mech↗

arXiv2003

The easy axis of the magnetic response of the conduction electrons of yttrium and rare earths

We describe a scheme for first-principles calculations of the static, paramagnetic, spin susceptibility of metals with relativistic effects such as spin-orbit coupling included. This gives the direction for an applied modulated magnetic field to maximise its response. This easy axis depends on the modulation's wave-vector q. For h.c.p. yttrium we find the peak response at a q= (0,0,0.57)pi/c, coincident with a Fermi surface nesting vector, to have an easy axis perpendicular to q. This is consistent with the {\it helical} anti-ferromagnetic order found in many dilute rare earth-Yalloys. Conversely, the easy axis for the response to a uniform magnetic field lies along the c-axis. The conduction electrons' role in the canting of magnetic moments in Gd-Y alloys and other rare earth materials is mooted.

cond-mat.mtrl-sci↗

arXiv2003

A density functional study of pressure induced superconductivity in P and its implication for spintronics

The stability of high-pressure phases of P has been studied using density functional theory and the local density approximation. Using a linear response technique, we have calculated the phonon spectrum and electron-phonon interaction for bcc P and predict it to be superconducting with $T_c$ of 19 K. We propose that this phase might be realized in epitaxial thin films using templates such as V(100), Fe(100) or Cr(100) relevant to spintronics applications.

cond-mat.supr-con↗

arXiv2003

The Korringa-Kohn-Rostoker Non-Local Coherent Potential Approximation (KKR-NLCPA)

We introduce the Korringa-Kohn-Rostocker non-local coherent potential approximation (KKR-NLCPA) for describing the electronic structure of disordered systems. The KKR-NLCPA systematically provides a hierarchy of improvements upon the widely used KKR-CPA approach and includes non-local correlations in the disorder configurations by means of a self-consistently embedded cluster. The KKR-NLCPA method satisfies all of the requirements for a successful cluster generalization of the KKR-CPA; it remains fully causal, becomes exact in the limit of large cluster sizes, reduces to the KKR-CPA for a single-site cluster, is straightforward to implement numerically, and enables the effects of short-range order upon the electronic structure to be investigated. In particular, it is suitable for combination with electronic density functional theory to give an ab-initio description of disordered systems. Future applications to charge correlation and lattice displacement effects in alloys and spin fluctuations in magnets amongst others are very promising. We illustrate the method by application to a simple one-dimensional model.

cond-mat.mtrl-sci↗

arXiv2002

Anisotropic and incommensurate spin fluctuations in hcp iron and some other nearly magnetic metals

We present an ab initio theoretical formalism for the static paramagnetic spin susceptibility of metals at finite temperatures. Since relativistic effects, e.g. spin-orbit coupling, are included, we can identify the anisotropy or easy axes of the spin fluctuations. Our calculations find hcp-iron to be unstable to in ab-plane, incommensurate anti-ferromagnetic (AFM) modes (linked to nested Fermi surface) below T_N =69K for the lowest pressures under which it is stable. T_N swiftly drops to zero as the pressure is increased. The calculated susceptibility of yttrium is consistent with the helical, incommensurate AFM order found in many rare-earth-dilute yttrium alloys. Lastly, in line with experimental data, we find the easy axes of the incommensurate AFM and ferromagnetic spin fluctuations of the normal state of the triplet superconductor Sr2RuO4 to be perpendicular and parallel with the crystal c-axis resepctively.

cond-mat.mtrl-sci↗

arXiv2001

The onset of magnetic order in fcc-Fe films on Cu(100)

On the basis of a first-principles electronic structure theory of finite temperature metallic magnetism in layered materials, we investigate the onset of magnetic order in thin (2-8 layers) fcc-Fe films on Cu(100) substrates. The nature of this ordering is altered when the systems are capped with copper. Indeed we find an oscillatory dependence of the Curie temperatures as a function of Cu-cap thickness, in excellent agreement with experimental data. The thermally induced spin-fluctuations are treated within a mean-field disordered local moment (DLM) picture and give rise to layer-dependent `local exchange splittings' in the electronic structure even in the paramagnetic phase. These features determine the magnetic intra- and interlayer interactions which are strongly influenced by the presence and extent of the Cu cap.

cond-mat.mtrl-sci↗

arXiv2000

Spin fluctuations in nearly magnetic metals from ab-initio dynamical spin susceptibility calculations:application to Pd and Cr95V5

We describe our theoretical formalism and computational scheme for making ab-initio calculations of the dynamic paramagnetic spin susceptibilities of metals and alloys at finite temperatures. Its basis is Time-Dependent Density Functional Theory within an electronic multiple scattering, imaginary time Green function formalism. Results receive a natural interpretation in terms of overdamped oscillator systems making them suitable for incorporation into spin fluctuation theories. For illustration we apply our method to the nearly ferromagnetic metal Pd and the nearly antiferromagnetic chromium alloy Cr95V5. We compare and contrast the spin dynamics of these two metals and in each case identify those fluctuations with relaxation times much longer than typical electronic `hopping times'

cond-mat.str-el↗

arXiv2000

Fermi surface origin of the interrelationship between magnetocrystalline anisotropy and compositional order in transitional metal alloys

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. {\bf 83}, 5369 (1999)\} and showed that compositional order enhances the magnitude of magnetocrystalline anisotropy energy (MAE) of Co$_{0.5}$Pt$_{0.5}$ 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 Co$_{0.25}$Pt$_{0.75}$ alloy we find that the perfect $ L1_2 $ 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 Co$_{0.5}$Pt$_{0.5}$ 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.

cond-mat.mtrl-sci↗

arXiv2000

Ab-initio Theoretical Description of the Interrelation between Magnetocrystalline Anisotropy and Atomic Short-Range Order

The cubic lattice symmetry of ferromagnetic homogeneously disordered alloys is when a compositional modulation is imposed. This can have a profound influence on the magnetocrystalline anisotropy energy (MAE). We describe our ab-initio theory of this effect and use the framework of concentration waves with the electronic structure described within the spin-polarised relativistic Korringa-Kohn-Rostoker coherent-potential approximation. We find that ordering produces a 2 order of magnitude increase in the MAE as well as altering the equilibrium direction of magnetisation. Using the same theoretical framework we also examine directional compositional order produced by magnetic annealing with an explicit study of permalloy.

cond-mat.mtrl-sci↗

arXiv2000

Incommensurate and Commensurate Antiferromagnetic Spin Fluctuations in Cr and Cr Alloys from Ab Initio Dynamical Spin Susceptibility Calculations

A scheme for making ab-initio calculations of the dynamic paramagnetic spin susceptibilities of solids at finite temperatures is described. It is based on Time-Dependent Density Functional Theory and employs an electronic multiple scattering formalism. Incommensurate and commensurate anti-ferromagnetic spin fluctuations in paramagnetic Cr and compositionally disordered Cr95V5 and Cr95Re5 alloys are studied together with the connection with the nesting of their Fermi surfaces. We find that the spin fluctuations can be described rather simply in terms of an overdamped oscillator model. Good agreement with inelastic neutron scattering data is obtained.

cond-mat.str-el↗