Contribution To Magnetocrystalline Anisotropy Research Articles

Magneto-transport behavior of disordered three dimensional NixCo1−x inverse opal networks

Herein we report on the magneto-transport properties of disordered three dimensional inverse opals (3D-IOs) fabricated by a standard three-probe electrodeposition technique into the interstices of porous membranes made of 150 nm diameter self assembled poly(methyl methacrylate) spheres. This approach has allowed the synthesis of large scale nanocomposites with exact ferromagnetic NixCo1−x alloy compositions and complex interconnected structure. Particularly, the microstructure of Co-rich 3D-IOs is consistent with the hexagonal close packed hcp texture and its corresponding magnetoresistance is explained in terms of the hcp Co magnetocrystalline anisotropy contribution. Conversely, the magnetoresistive behavior of Ni-rich 3D-IO networks is explained in terms of only their magnetostatic field. The control of these features is made possible by the reduced dimensions of necks and walls, in the 40 nm to 60 nm range, of the 3D-IO structure. Despite the disordered morphology of these 3D-IO nanoarchitectures, their microstructural and magneto-transport properties can be fine tuned due to the reduced nanoscale dimensions of the electrical interconnections. These properties have been found to be comparable to those obtained in other 3D networks, making them interesting systems for their potential use for magnetic sensing and spintronic applications.

Effect of High-Anisotropic Co2+ Substitution for Ni2+ on the Structural, Magnetic, and Magnetostrictive Properties of NiFe2O4: Implications for Sensor Applications.

This work reports on the effect of substituting a low-anisotropic and low-magnetic cation (Ni2+, 2μB) by a high-anisotropic and high-magnetic cation (Co2+, 3μB) on the crystal structure, phase, microstructure, magnetic properties, and magnetostrictive properties of NiFe2O4 (NFO). Co-substituted NFO (Ni1-xCoxFe2O4, NCFO, 0 ≤ x ≤ 1) nanomaterials were synthesized using glycine-nitrate autocombustion followed by postsynthesis annealing at 1200 °C. The X-ray diffraction measurements coupled with Rietveld refinement analyses indicate the significant effect of Co-substitution for Ni, where the lattice constant (a) exhibits a functional dependence on composition (x). The a-value increases from 8.3268 to 8.3751 Å (±0.0002 Å) with increasing the "x" value from 0 to 1 in NCFO. The a-x functional dependence is derived from the ionic-size difference between Co2+ and Ni2+, which also induces grain agglomeration, as evidenced in electron microscopy imaging. The chemical bonding of NCFO, as probed by Raman spectroscopy, reveals that Co(x)-substitution induced a red shift of the T2g(2) and A1g(1) modes, and it is attributed to the changes in the metal-oxygen bond length in the octahedral and tetrahedral sites in NCFO. X-ray photoelectron spectroscopy confirms the presence of Co2+, Ni2+, and Fe3+ chemical states in addition to the cation distribution upon Co-substitution in NFO. Chemical homogeneity and uniform distribution of Co, Ni, Fe, and O are confirmed by EDS. The magnetic parameters, saturation magnetization (MS), remnant magnetization (Mr), coercivity (HC), and anisotropy constant (K1) increased with increasing Co-content "x" in NCFO. The magnetostriction (λ) also follows a similar behavior and almost linearly varies from -33 ppm (x = 0) to -227 ppm (x = 1), which is primarily due to the high magnetocrystalline anisotropy contribution from Co2+ ions at the octahedral sites. The magnetic and magnetostriction measurements and analyses indicate the potential of NCFO for torque sensor applications. Efforts to optimize materials for sensor applications indicate that, among all of the NCFO materials, Co-substitution with x = 0.5 demonstrates high strain sensitivity (-2.3 × 10-9 m/A), which is nearly 2.5 times higher than that obtained for their intrinsic counterparts, namely, NiFe2O4 (x = 0) and CoFe2O4 (x = 1).

Influence of HfO2 interlayers on magnetocrystalline anisotropy in Fe|MgO|Fe magnetic tunnel junction: First-principles investigation

Perpendicular magnetic anisotropy (PMA) is important for MgO based magnetic tunnel junction and magnetic random access memory to be integrated on a large scale due to high thermal stability and low critical switching current. Here, we applied the density functional theory to study the effect of HfO2 inserting layers on PMA of Fe|MgO|Fe tunnel junction. It was found that the magnetocrystalline anisotropy (MCA) of the junction for 5 layers Fe electrode was up to 1.95 mJ/m2 with one unit cell HfO2 interlayer, while it was 1.72 mJ/m2 without the interlayer. More importantly, analyzed by the layer and orbital-resolved MCA based on the second-order perturbation theory, MCA characters and the underlying mechanism of PMA become very different after inserting HfO2. The remarkable difference is the MCA contribution of the second interfacial Fe layers, which is about 0.4 mJ/m2 for Fe|MgO|Fe junctions, while it was larger than 0.7 mJ/m2 for Fe|HfO2|MgO|HfO2|Fe junctions. Furthermore, Fe-dz2 and O-pz hybridization plays a crucial role in MCA contribution from the first interfacial Fe layers since the interfacial Fe–O bond length reduces from 2.20 Å to 1.77 Å with inserting HfO2 layers. Besides, the reduction in Fe–O bond length can redistribute the orbital-resolved electrons of the second and third closest Fe layer to the interface to enhance their absolute values of MCA contributions, which results in the strong dependence of MCA on Fe thickness.

Origin of magnetic anisotropy in doped Ce2Co17 alloys

Magnetocrystalline anisotropy (MCA) in doped Ce$_{2}$Co$_{17}$ and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective effect and contributions from other sublattices may change significantly. Moreover, we found that Zr doping promotes the formation of 1-5 structure that exhibits a large uniaxial anisotropy, such that Zr is the most effective element to enhance MCA in this system.

Thermal effects on the magnetic properties of titanium modified cobalt ferrite

The temperature dependence of the magnetic properties of titanium modified cobalt ferrite is presented. The change of maximum magnetization obtained at H ≈ 2.4 MA/m between any two temperatures increases systematically with composition, which is desirable for applications in devices. Variation in magnetocrystalline anisotropy and coercivity were different from previous studies on cation substituted cobalt ferrite. At lower concentrations, the effect of lower thermal energy dominated the effect of non-magnetic cation substitutions in controlling the anisotropy. The reverse was the case at higher concentrations. The temperature dependence of coercivity is dominated by the contribution of magnetocrystalline anisotropy to coercivity, while the compositional dependence of coercivity is dominated by microstructural contribution through the pinning of domain walls.

High temperature structural and magnetic properties of cobalt nanorods

We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (∼10) at high temperatures (up to 623K) using in-situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500K. The coercivity can be modelled by μ0HC=2(KMC+Kshape)/MS with KMC the magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant and MS the saturation magnetization. HC decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300–500K in good agreement with the absence of particle alteration. Above 525K, the magnetic properties are irreversibly altered either by sintering or by oxidation.

Perpendicular magnetization of long iron carbide nanowires inside carbon nanotubes due to magnetocrystalline anisotropy

Single crystal iron carbide nanowires contained in multiwalled carbon nanotubes have been prepared by aerosol-based thermal chemical vapor deposition. Investigations by transmission electron microscopy reveal the crystallographic [010] axis of the orthorhombic Fe3C nanowires to be predominantly aligned along the nanotube axis. Despite the high aspect ratio of the Fe3C nanowires, magnetic force microscopy measurements imply single domain behavior with the easy magnetic axis of the nanowires perpendicular to the wire axis. In agreement with the structural results, these findings show that the magnetic behavior is dominated by the magnetocrystalline anisotropy contribution, causing the easy axis to be along the [001] direction.

Shape induced in-plane magnetic anisotropy reorientation in epitaxial hexagonal close packed cobalt dots

The in-plane magnetic easy-axis reorientation was investigated for the arrays of (10⋅0) hcp Co rectangular dots 0.2 μm in width with a variable length from 3 to 5 μm perpendicular to the c axis. In the system, the magnitude of the shape anisotropy contribution is determined by a lateral aspect ratio of the dot, while the magnetocrystalline anisotropy contribution is temperature dependent. The torque curves for the dot arrays exhibited a characteristic fourfold symmetry due to the easy-axis reorientation. The reorientation angle from the initial easy axis was found to be dependent on the value of in-plane demagnetization factors as well as the temperature variable first and second magnetocrystalline anisotropy constants. The experimental results are in good agreement with the calculated effective magnetic anisotropy.

Correlation between structure and magnetic anisotropies of Co on Cu(110)

Magnetic anisotropies of molecular-beam-epitaxy-grown fcc Co(110) films on Cu(110) single-crystal substrates have been determined by using Brillouin light scattering and have been correlated with the structural properties determined by low-energy electron diffraction and scanning tunneling microscopy (STM). Three regimes of film growth and associated anisotropy behavior are identified: coherent growth in the Co film thickness regime of up to 13 \AA{}, in-plane anisotropic strain relaxation between 13 and about 50 \AA{} and in-plane isotropic strain relaxation above 50 \AA{}. The structural origin of the transition between anisotropic and isotropic strain relaxation was studied using STM. In the regime of anisotropic strain relaxation long Co stripes with a preferential [1$1\ifmmode\bar\else\textasciimacron\fi{}$0]-orientation are observed, which in the isotropic strain relaxation regime are interrupted in the perpendicular in-plane direction to form isotropic islands. In the Co film thickness regime below 50 \AA{} an unexpected suppression of the magnetocrystalline anisotropy contribution is observed. Symmetry reflections based on a crystal-field formalism and discussed within the context of band theory, which explicitly takes tetragonal misfit strains into account, reproduce the experimentally observed anomalies despite the fact that the thick Co films are quite rough.

Suppression of the magnetocrystalline bulk anisotropy in thin epitaxial Co(110) films on Cu(110).

We report on an unexpected suppression of the magnetocrystalline anisotropy contribution in epitaxial fcc Co(110) films on Cu(110) below a thickness of ${d}_{c}=(50\ifmmode\pm\else\textpm\fi{}10)$ \AA{}. For film thicknesses larger than ${d}_{c}$ the measured anisotropy value agrees with published data. Measurements on films with reduced strain indicate a large strain dependence of ${d}_{c}$. A model calculation based on a crystal-field formalism and discussed within the context of band theory, which explicitly takes tetragonal misfit strains into account, reproduces the experimentally observed anomalies. Our results indicate that the usually applied phenomenological description of anisotropies, assuming additive free energy terms for each anisotropy contribution, fails in this case.

Structural and magnetocrystalline anisotropy contributions to the blocking temperatures of NixCo(1−x)O exchange couples (abstract)

The simple spin and crystal structure of the antiferromagnetic (AF) monoxide NixCo1−xO facilitates structural characterization and modeling of its magnetic properties NixCo1−xO is also receiving technological attention for biasing magnetoresistive sensors as a robust alternative to FeMn. In this work, we studied the thickness dependence of the AF layer on the exchange field created through interfacial exchange coupling between NixCo1−xO and Ni81Fe19 films. The NixCo(1−x)O films were reactively sputtered on silicon substrates and capped with a 300 Å Ni81Fe19 layer. An additional silver layer was deposited as an oxidation barrier. An exchange field was observed on all the exchange couples with the NixCo(1−x)O thickness ranging from 25 to 1000 Å. The exchange couples were characterized using transmission electron microscopy (TEM) to yield information on the orientation, structure, and size of the crystallites. The films were polycrystalline with columnar grains. For oxide thicknesses below 300 Å, the film was randomly oriented but developed a texture with a preferred orientation for thicker films. We found that the blocking temperature, at which the exchange field goes to zero, decreases with decreasing oxide thickness. The blocking temperature is relatively thickness independent for higher thicknesses but decreases rapdily below a certain thickness. The decrease in the blocking temperature does not appear to be consistent with a finite size scaling law. Instead we modeled this behavior to take into account the temperature dependence associated with the changing microstructure of the AF layer with thickness, in particular the size of the crystallites. In thick oxide films, the blocking temperature corresponds to the AF ordering temperature, the Néel temperature. The decrease of the blocking temperature below the Néel temperature suggests at the competing magnetic and thermal energies necessary for exhibiting unidirectional anisotropy. The thickness sensitivity of the blocking temperatures of the AF monoxides are found to be strongly dependent on their magnetocrystalline anisotropy.

Spin-reorientation transition in ultrathin Tb/Co films.

A series of terbium (0--6 \AA{})/Co (16 \AA{}) films deposited on Ru(0001) under ultrahigh vacuum conditions and covered by 40 \AA{} Ru have been investigated ex situ by means of the magneto-optic Kerr effect (MOKE). Combined polar and longitudinal loops (dc-MOKE) show a reversible rotation of the remanent magnetization from out-of-plane to in-plane with increasing temperature. The reorientation is continuous and occurs without loss of long-range order. It is discussed in terms of the competition between dipolar demagnetization energy and second- and fourth-order magnetocrystalline anisotropy contributions. The switching of the easy axis of magnetization is confirmed by the susceptibility peak observed in a 10 Oe oscillating field (ac-MOKE) applied perpendicular to the film plane. The transition temperature varies from 323 K (1 \AA{} Tb) to 277 K (6 \AA{} Tb). \textcopyright{} 1996 The American Physical Society.

Identification of magnetoelastic and magnetocrystalline anisotropy contributions in ultrathin epitaxial Co(110) films (abstract)

Magnetic anisotropies of ultrathin transition-metal films are inherently related to their structural properties. In ultrathin films the large fraction of atoms located at the film interface generates strong interface anisotropies, whereas elastic strain fields caused by the forced registry of atoms at the substrate/film interface induce magnetoelastic anisotropy contributions. So far the experimental confirmation of the transition from these thin-film properties to bulk anisotropy properties, characterized by a dominating magnetocrystalline anisotropy, has not yet been presented. Magnetic anisotropies reflect, depending on their origin, both the crystallographic symmetry and the symmetry of the film geometry. For a clear separation between magnetoelastic, magnetocrystalline and Néel-type interface anisotropy contributions, the film symmetry and thickness must be chosen such that the respective different anisotropy contributions appear with different symmetries and film thickness dependencies. This is the case for (110)-oriented fcc Co films. In the present study we use the Brillouin light-scattering technique for the determination of the anisotropy contributions. An analysis of the spin-wave frequency measured as a function of the in-plane direction of the external field and the film thickness yields information about all relevant anisotropies. The samples used were molecular-beam-epitaxy grown in ultrahigh vacuum. Onto a Cu (110) single-crystal substrate a wedge-type sample and two staircase-shaped samples with distinct thicknesses in the range of 8–110 Å were grown. To obtain symmetric Co/Cu interfaces the Co layers were covered with a 12 Å Cu layer. Finally, a 25-Å-thick Au protective layer was deposited. Low-energy electron-diffraction studies were used to obtain the structural data of the films. All relevant anisotropy contributions—the magnetocrystalline anisotropy, and the uniaxial in-plane and out-of-plane anisotropy contributions—were determined. Three different anisotropy regimes are observed as a function of the Co layer thickness dCo. This thickness regime up to 13 Å is dominated by the magnetoelastic anisotropy contributions as a result of the pseudomorphic film growth of the Co layer. For Co layer thicknesses larger than 13 Å we find a reduction of the magnetoelastic anisotropy contributions. This is structurally correlated to an anisotropic relaxation of the in-plane Co lattice constant. In the regime of dCo≳50 Å we observe a thickness-independent value for the magnetocrystalline anisotropy contribution K1=−8.5×105 erg/cm3. This anisotropy contribution is largely suppressed for dCo<50 Å. This finding might either indicate a breakdown of the usually postulated linear superposition principle of magnetic anisotropy contributions to the free anisotropy energy, or it might point to a subtle modification of the electronic band structure. At the onset of the magnetocrystalline anisotropy we find a change in the easy magnetization direction from 〈001〉 for thin Co films to 〈111〉 for thicker ones. For a more detailed discussion see Ref. .