Origin of orbital magnetization and magnetocrystalline anisotropy in TX ordered alloys (where T=Fe,Co and X=Pd,Pt)

Thin film epitaxial [111] Co[Formula: see text]Pt[Formula: see text]: structure, magnetisation, and spin polarisation

Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ferromagnetic multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain Co[Formula: see text]Pt[Formula: see text] alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic Co[Formula: see text]Pt[Formula: see text] and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the [Formula: see text] and [Formula: see text] ordered compounds. Primarily, interest in Co[Formula: see text]Pt[Formula: see text] has been in the [Formula: see text] crystal structure, where layers of Pt and Co are stacked alternately in the [100] direction. There has been less work on [Formula: see text] crystal structure, where the stacking is in the [111] direction. For the latter [Formula: see text] crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former [Formula: see text] crystal structure, the magnetic anisotropy is perpendicular to the [100] plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the [Formula: see text] and [Formula: see text] Co[Formula: see text]Pt[Formula: see text] to be [Formula: see text].

Chemical order-disorder nanodomains in Fe3Pt bulk alloy

Chemical ordering is a common phenomenon and highly correlated with the properties of solid materials. By means of the redistribution of atoms and chemical bonds, it invokes an effective lattice adjustment and tailors corresponding physical properties. To date, however, directly probing the 3D interfacial interactions of chemical ordering remains a big challenge. In this work, we deciphered the interlaced distribution of nanosized domains with chemical order/disorder in Fe₃Pt bulk alloy. HAADF-STEM images evidence the existence of such nanodomains. The reverse Monte Carlo method with the X-ray pair distribution function data reveal the 3D distribution of local structures and the tensile effect in the disordered domains at the single-atomic level. The chemical bonding around the domain boundary changes the bonding feature in the disordered side and reduces the local magnetic moment of Fe atoms. This results in a suppressed negative thermal expansion and extended temperature range in Fe₃Pt bulk alloy with nanodomains. Our study demonstrates a local revelation for the chemical order/disorder nanodomains in bulk alloy. The understanding gained from atomic short-range interactions within the domain boundaries provides useful insights with regard to designing new functional compounds.

Topological-chiral magnetic interactions driven by emergent orbital magnetism

Two hundred years ago, Ampère discovered that electric loops in which currents of electrons are generated by a penetrating magnetic field can mutually interact. Here we show that Ampère's observation can be transferred to the quantum realm of interactions between triangular plaquettes of spins on a lattice, where the electrical currents at the atomic scale are associated with the orbital motion of electrons in response to the non-coplanarity of neighbouring spins playing the role of a magnetic field. The resulting topological orbital moment underlies the relation of the orbital dynamics with the topology of the spin structure. We demonstrate that the interactions of the topological orbital moments with each other and with the spins form a new class of magnetic interactions [Formula: see text] topological-chiral interactions [Formula: see text] which can dominate over the Dzyaloshinskii-Moriya interaction, thus opening a path for realizing new classes of chiral magnetic materials with three-dimensional magnetization textures such as hopfions.

Elaboration of Nanomagnet Arrays: Organization and Magnetic Properties of Mass-Selected FePt Nanoparticles Deposited on Epitaxially Grown Graphene on Ir(111)

The moiré pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2 nm diameter have been mass selected in a gas phase and deposited with low energy on the hexagonal moiré pattern. Their morphology and organization have been investigated using grazing incidence small angle x-ray scattering, while their magnetic properties have been studied by x-ray magnetic circular dichroism, both pointing to a FePt cluster-graphene surface specific interaction. The spatial coherence of the nanoparticles is preserved upon annealing up to 700 °C where the hard magnetic phase of FePt is obtained.

Giant contribution of the ligand states to the magnetic properties of the Cr2Ge2Te6 monolayer

The magnetic properties of Cr₂Ge₂Te₆, an important two-dimensional ferromagnetic material, are investigated at the molecular level by constructing and solving realistic models extracted from first-principles electronic structure calculations.

Formation of the layered conductive magnet CrCl2(pyrazine)2 through redox-active coordination chemistry

The unique properties of graphene, transition-metal dichalcogenides and other two-dimensional (2D) materials have boosted interest in layered coordination solids. In particular, 2D materials that behave as both conductors and magnets could find applications in quantum magnetoelectronics and spintronics. Here, we report the synthesis of CrCl₂(pyrazine)₂, an air-stable layered solid, by reaction of CrCl₂ with pyrazine (pyz). This compound displays a ferrimagnetic order below ∼55 K, reflecting the presence of strong magnetic interactions. Electrical conductivity measurements demonstrate that CrCl₂(pyz)₂ reaches a conductivity of 32 mS cm^-1 at room temperature, which operates through a 2D hopping-based transport mechanism. These properties are induced by the redox-activity of the pyrazine ligand, which leads to a smearing of the Cr 3d and pyrazine π states. We suggest that the combination of redox-active ligands and reducing paramagnetic metal ions represents a general approach towards tuneable 2D materials that consist of charge-neutral layers and exhibit both long-range magnetic order and high electronic conductivity.

Induced perpendicular magnetization in a Cu layer inserted between Co and Pt layers revealed by x-ray magnetic circular dichroism

We used x-ray absorption spectroscopy and x-ray magnetic circular dichroism to investigate the effects of inserting Cu into Co/Pt interfaces, and found that a 0.4-nm-thick inserted Cu layer showed perpendicularly magnetized properties induced by the proximity effect through the Co and Pt layers. The dependence of the magnetic properties on the thickness of the Cu layers showed that the proximity effects between Co and Pt with perpendicular magnetic anisotropy can be prevented by the insertion of a Cu layer with a nominal threshold thickness of 0.7 nm. Element-specific magnetization curves were also obtained, demonstrating that the out-of-plane magnetization is induced in the Cu layers of the Co/Cu/Pt structures.

Giant perpendicular magnetic anisotropy energies in CoPt thin films: impact of reduced dimensionality and imperfections

The impact of reduced dimensionality on the magnetic properties of the tetragonal L1 ₀ CoPt alloy is investigated from ab initio considering several kinds of surface defects. By exploring the dependence of the magnetocrystalline anisotropy energy (MAE) on the thickness of CoPt thin films, we demonstrate the crucial role of the chemical nature of the surface. For instance, Pt-terminated thin films exhibit huge MAEs which can be 1000% larger than those of Co-terminated films. Besides the perfect thin films, we scrutinize the effect of defective surfaces such as stacking faults or anti-sites on the surface layers. Both types of defects reduce considerably the MAE with respect to the one obtained for Pt-terminated thin films. A detailed analysis of the electronic structure of the thin films is provided with a careful comparison to the CoPt bulk case. The behavior of the MAEs is then related to the location of the different virtual bound states utilizing second order perturbation theory.

Spin-Fluctuation Mechanism of Anomalous Temperature Dependence of Magnetocrystalline Anisotropy in Itinerant Magnets

The origins of the anomalous temperature dependence of magnetocrystalline anisotropy in (Fe_{1-x}Co_{x})_{2}B alloys are elucidated using first-principles calculations within the disordered local moment model. Excellent agreement with experimental data is obtained. The anomalies are associated with the changes in band occupations due to Stoner-like band shifts and with the selective suppression of spin-orbit "hot spots" by thermal spin fluctuations. Under certain conditions, the anisotropy can increase, rather than decrease, with decreasing magnetization due to these peculiar electronic mechanisms, which contrast starkly with those assumed in existing models.

Magnetic order and phase transitions in Fe50Pt50–xRhx

Polarized and unpolarized neutron diffraction techniques have been applied to study the temperature-dependent magnetic and structural properties of four 200 nm-thick Fe50Pt50−xRhxfilms withx= 5,x= 10,x= 17.5 andx= 25. Similar to the bulk system, an antiferromagnetic to ferromagnetic transition can be found in the films with decreasing Rh concentration. The application of structure factor calculations enables one to determine the microscopic magnetic configuration of the different films as a function of temperature and Rh concentration. The developed models indicate a magnetic transition from a dominant antiferromagnetic order in the out-of-plane direction to a dominant ferromagnetic order in the in-plane direction with decreasing Rh concentration. The different magnetic configurations can theoretically be described by a phenomenological model which includes a two-ion and a one-ion interaction Hamiltonian term with different temperature dependencies of the anisotropy constants.

Magnetocrystalline anisotropy energy for adatoms and monolayers on non-magnetic substrates: where does it come from?

The substrate contribution to the magnetic anisotropy energy (MAE) of supported nanostructures can be assessed by a site-selective manipulation of the spin-orbit coupling (SOC) and of the effective exchange field Bex. A systematic study of Co adatoms and Co monolayers on the (1 1 1) surfaces of Cu, Ag, Au, Pd and Pt is performed to study common trends in this class of materials. It is found that for adatoms, the influence of the substrate SOC and Bex is relatively small (10-30% of the MAE) while for monolayers, this influence can be substantial. The influence of the substrate SOC is much more important than the influence of the substrate Bex, except for highly polarizable substrates with a strong SOC (such as Pt). The substrate always promotes the tendency to an out-of-plane orientation of the easy magnetic axis for all the investigated systems.

The origin of perpendicular magneto-crystalline anisotropy in L1(0)-FeNi under tetragonal distortion

We investigated the origin of perpendicular magneto-crystalline anisotropy (MCA) in L1(0)-ordered FeNi alloy using first-principles density-functional calculations. We found that the perpendicular MCA of L1(0)-FeNi arises predominantly from the constituent Fe atoms, which is consistent with recent measurements of the anisotropy of the Fe orbital magnetic moment of L1(0)-FeNi by means of x-ray magnetic circular dichroism. Analysis of the second-order perturbation of the spin-orbit interaction indicates that spin-flip excitations between the occupied majority-spin and unoccupied minority-spin bands make a considerable contribution to the perpendicular MCA, as does the spin-conservation term for the minority-spin bands. Furthermore, the MCA energy increases as the in-plane lattice parameter decreases (increasing the axial ratio c/a). The increase in the MCA energy can be attributed to further enhancement of the spin-flip term due to modulation of the Fe d(xy) and d(x(2) - y(2)) orbital components around the Fermi level under compressive in-plane distortion.

Quadratic scaling of intrinsic Gilbert damping with spin-orbital coupling in L10 FePdPt films: experiments and Ab initio calculations

The dependence of the intrinsic Gilbert damping parameter α(0) on the spin-orbital coupling strength ξ is investigated in L1(0) ordered FePd(1-x) Pt(x) films by time-resolved magneto-optical Kerr effect measurements and spin-dependent ab initio calculations. Continuous tuning of α(0) over more than one order of magnitude is realized by changing the Pt/Pd concentration ratio showing that α(0) is proportional to ξ(2) as changes of other leading parameters are found to be negligible. The perpendicular magnetic anisotropy is shown to have a similar variation trend with x. The present results may facilitate the design and fabrication of new magnetic alloys with large perpendicular magnetic anisotropy and tailored damping properties.

Charge-orbital density wave and superconductivity in the strong spin-orbit coupled IrTe2:Pd

Using transmission electron microscopy, the anomalies in resistivity and magnetic susceptibility at ~262 K in IrTe2 are found to accompany the superlattice peaks with q[over q=(1/5,0,-1/5). The wave vector is consistent with our theoretical calculation for the Fermi surface nesting vector, indicating that the ~262 K transition is of the charge-orbital density wave (DW) type. We also discovered that both Pd intercalation and substitution induce bulk superconductivity with T(c) up to ~3 K, which competes with DW in a quantum critical pointlike manner.

Fully relativistic pseudopotential formalism under an atomic orbital basis: spin-orbit splittings and magnetic anisotropies

We present an efficient implementation of the spin-orbit coupling within the density functional theory based SIESTA code (2002 J. Phys.: Condens. Matter 14 2745) using the fully relativistic and totally separable pseudopotential formalism of Hemstreet et al (1993 Phys. Rev. B 47 4238). First, we obtain the spin-orbit splittings for several systems ranging from isolated atoms to bulk metals and semiconductors as well as the Au(111) surface state. Next, and after extensive tests on the accuracy of the formalism, we also demonstrate its capability to yield reliable values for the magnetic anisotropy energy in magnetic systems. In particular, we focus on the L1(0) binary alloys and on two large molecules: Mn(6)O(2)(H -sao)(6)(O(2)CH)(2)(CH(3)OH)(4) and Co(4)(hmp)(4)(CH(3)OH)(4)Cl(4). In all cases our calculated anisotropies are in good agreement with those obtained with full-potential methods, despite the latter being, in general, computationally more demanding.

Structural and magnetic properties of ternary Fe(1-)MnPt nanoalloys from first principles

BACKGROUND

Structural and magnetic properties of binary Mn-Pt and ternary Fe(1-) (x)Mn(x)Pt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared.

RESULTS

The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices.

CONCLUSION

Addition of Mn effectively increases the stability of single crystalline L1(0) particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications.

Orbital polarization in itinerant magnets

We propose a parameter-free scheme of calculation of the orbital polarization (OP) in metals, which starts with the strong-coupling limit for the screened Coulomb interactions in the random-phase approximation (RPA). For itinerant magnets, RPA can be further improved by restoring the spin polarization of the local-spin-density approximation through the local-field corrections. The OP is then computed as the self-energy correction in the static GW method, which systematically improves the orbital magnetization and the magnetic anisotropy energies in transition-metal and actinide compounds.

Temperature dependent magnetic anisotropy in metallic magnets from an ab initio electronic structure theory: L1(0)-ordered FePt

Using a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy K with magnetization M in metallic ferromagnets. We apply the theory to the high uniaxial K material, L1(0)-ordered FePt, and find its magnetic easy axis perpendicular to the Fe/Pt layers for all M and K to be proportional to M2 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 ab initio results for this important magnetic material agree well with recent experimental measurements, whereas the single-ion anisotropy model fails to give the correct qualitative behavior.