Priyanka Manchanda

Ultrahard magnetic nanostructures

The performance of hard-magnetic nanostructures is investigated by analyzing the size and geometry dependence of thin-film hysteresis loops. Compared to bulk magnets, weight and volume are much less important, but we find that the energy product remains the main figure of merit down to very small features sizes. However, hysteresis loops are much easier to control on small length scales, as epitomized by Fe-Co-Pt thin films with magnetizations of up to 1.78 T and coercivities of up to 2.52 T. Our numerical and analytical calculations show that the feature size and geometry have a big effect on the hysteresis loop. Layered soft regions, especially if they have a free surface, are more harmful to coercivity and energy product than spherical inclusions. In hard-soft nanocomposites, an additional complication is provided by the physical properties of the hard phases. For a given soft phase, the performance of a hard-soft composite is determined by the parameter (Ms - Mh)/Kh.

2D transition-metal diselenides: phase segregation, electronic structure, and magnetism.

Density-functional theory is used to investigate the phase-segregation behavior of two-dimensional transition-metal dichalcogenides, which are of current interest as beyond-graphene materials for optoelectronic and spintronic applications. Our focus is on the behavior of W1-x V x Se2 monolayers, whose end members are semiconducting WSe2 and ferromagnetic VSe2. The energetics favors phase segregation, but the spinodal decomposition temperature is rather low, about 420 K. The addition of V leads to a transition from a nonmagnetic semiconductor to a metallic ferromagnet, with a ferromagnetic moment of about 1.0 μ B per V atom. The transition is caused by a p-type doping mechanism, which shifts the Fermi level into the valence band. The finite-temperature structure and magnetism of the diselenide systems are discussed in terms of Onsager-type critical fluctuations and Bruggeman effective-medium behavior.

Magnetism of dilute Co(Hf) and Co(Pt) nanoclusters

An investigation of the magnetic properties of Co-rich nanoparticles alloyed with a small fraction of Pt and Hf is presented. Co(Hf) and Co(Pt) nanoparticles with less than 15 at% of dopants were produced using a cluster-deposition method. The nanoparticles have sizes of less than 10 nm and show improved magnetic properties upon doping. Maximum coercivities of 900 Oe (at 300 K) and 2000 Oe (at 10 K) were observed for Co nanoparticles alloyed with 14.1 at% of Hf. Doped nanoparticles also exhibit high anisotropies, such as K1 = 9.98 Mergs/cm3 (14.1 at% of Hf) and K1 = 8.24 Mergs/cm3 (9.5 at% of Pt), as compared to Co nanoparticles (K1 = 6.21 Mergs/cm3).

Anisotropy of heavy transition metal dopants in Co

Evaluating prospects for new transition-metal-rich and lanthanide-free permanent magnets, we calculate the magnetocrystalline anisotropy of dilute Co1−xPtx and Co1−xPdx alloys. The ab initio calculations are done by using the full-potential linear augmented plane wave method, treating exchange and correlation within the generalized gradient approximation. The anisotropy contributions, 11.9 kJ/m3 per at. % Pd and 10.8 kJ/m3 per at. % Pt, are in a range suitable for permanent magnets application.

Anisotropy of W in Fe and Co

The magnetization and magnetic anisotropy of iron-rich Fe-W and W-Co intermetallics and nanostructures are investigated by first-principle calculations. The W atom is antiferromagnetically coupled to nearest neighbor Co atom and interface Fe atom in the case of Fe-W multilayers. The first anisotropy constants are positive, nearly zero, or negative, depending on the atomic structures. Typical anisotropy energies per atom are -0.513 meV for a dilute W-Co alloy, -0.06 meV for W2Fe2, and 0.44 meV for W2Fe4 (0.1 meV/atom ~1MJ/m3). These values are of interest for permanent-magnet applications but also indicate the need for careful structural control and for a more detailed investigation of structure-property relationships in Fe-W nanostructures.

Transition-metal and metalloid substitutions in L10-ordered FeNi

The effect of atomic substitutions on the magnetization, exchange, and magnetocrystalline anisotropy energy of L10-ordered FeNi (tetrataenite) is computationally investigated. The compound naturally occurs in meteorites but has attracted renewed attention as a potential material for permanent magnets, and elemental additives will likely be necessary to facilitate the phase formation. Our density functional theory calculations use the Vienna ab-initio simulation package, applied to 4-atom unit cells of Fe2XNi and 32-atom supercells (X = Al, P, S, Ti, V, Cr, Mn, Fe, Co). While it is found that most additives deteriorate the magnetic properties, there are exceptions: excess substitutional Fe and Co additions improve the magnetization, whereas Cr, S, and interstitial B additions improve the magnetocrystalline anisotropy.

Multiscale micromagnetism of Co-Pd multilayers

The interplay between atomic and micromagnetic effects in Co-Pd multilayers is investigated by model calculations and numerical simulations. By minimizing the total exchange energy, an effective exchange stiffness is obtained. The stiffness depends on the superlattice periodicity, on the wave vector of the magnetization variation, and on the exchange coupling through the Pd, which is calculated from first principles (J = 7.66 mJ/m2). The net magnetic anisotropy, Keff = 0.71 MJ/m3, which is also obtained from first principles, contains two parts, namely the Pd-Co interface anisotropy Kif = 0.45 mJ/m2 and the bulk anisotropy KCo = −0.28 MJ/m3 of the strained fcc Co. For vertical and lateral magnetization variations, we find domain-wall thicknesses of 5.1 nm and 6.9 nm and domain-wall energies of 5.94 mJ/m2 and 6.66 mJ/m2, respectively.

Magnetoelectric Effect in

The effect of an applied electric field on the magnetic properties of L 10-ordered CoPd thin films is investigated by first-principle calculations. Both the magnetic moment and the magnetocrystalline anisotropy of the surface atoms are changed by the electric field, but the net effect depends on the surface termination. The magnetocrystalline anisotropy switches from in-plane to perpendicular in the presence of external electric field. Typical magnetic-moment changes are 0.1 μB per eV/Å. The main mechanism is the shift of the Fermi level, but the anisotropy change also reflects a crystal-field change due to incomplete screening.

L 1_{0}

Magnetic nanoparticles are widely used in biomedical and oil-well applications in aqueous, often harsh environments. The pursuit for high-saturation magnetization together with high stability of the molecular coating that prevents agglomeration and oxidation remains an active research area. Here, we report a detailed analysis of the criteria for the stability of molecular coatings in aqueous environments along with extensive first-principles calculations for magnetite, which has been widely used, and cementite, a promising emerging candidate. A key result is that the simple binding energies of molecules cannot be used as a definitive indicator of relative stability in a liquid environment. Instead, we find that H+ ions and water molecules facilitate the desorption of molecules from the surface. We further find that, because of differences in the geometry of crystal structures, molecules generally form stronger bonds on cementite surfaces than they do on magnetite surfaces. The net result is that molecular coatings of cementite nanoparticles are more stable. This feature, together with the better magnetic properties, makes cementite nanoparticles a promising candidate for biomedical and oil-well applications.

-CoPd Thin Films

Ab-initio calculations are used to determine the parameters that determine magnonic band structure of PdnFem multilayers (n = 2, m ≤ 8). We obtain the layer-resolved magnetization, the exchange coupling, and the magnetic anisotropy of the Pd-Fe structures. The Fe moment is 3.0 μB close to the Pd layers and 2.2 μB in the middle of the Fe layers. An intriguing but not usually considered aspect is that the elemental Pd is nonmagnetic, similar to Cu spacer layers in other multilayer systems. This leads to a pre-asymptotic ferromagnetic coupling through the Pd (about 40 mJ/m2). Furthermore, the Pd acquires a small moment due to spin polarization by neighboring Fe atoms, which translates into magnetic anisotropy. The anisotropies are large, in the range typical for L10 structures, which is beneficial for high-frequency applications.