David Sellmyer Publications

Coercivity Enhancement in Zr2Co11-Based Nanocrystalline Materials Due to Mo Addition

Wenyong Zhang et al. — Thu, 16 May 2013 10:16:11 PDT

The Mo-content dependence of structure and magnetic properties of Zr₁₆Co_78-xMoxSi₃B₃ (x = 0,2,3,4,5) nanocrystalline materials has been studied. The samples consist of hard-magnetic Zr₂Co₁₁ and soft-magnetic Co phases. The substitution of Mo for Co restrains the formation of Co, raises the content of Zr₂Co₁₁, and increases the mean grain size of Zr₂Co₁₁. Therefore, the coercive force of the sample increases with x. A coercive force of 7.9 kOe, which is a highest value reported among Zr-Co alloys, was achieved for x = 5. The anisotrophy field of Zr₂Co₁₁ remains almost unchanged with increasing Mo content.

Susceptibility of Fe atoms in Cu clusters

Rui Zhang et al. — Thu, 16 May 2013 10:06:21 PDT

By putting a Kondo system of limited dimension inside an insulating matrix, the Kondo screening cloud is confined by the size of the system. This allows us to distinguish the bulk and nano-particle behaviors of such system. We have investigated the magnetic properties of 0.3 at. % Fe-doped Cu clusters with dimensions less than 20nm embedded in a SiO₂ matrix. The magnetic measurements are consistent with the Kondo interaction, and net antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between several Fe atoms in one cluster are estimated to have a very small effect. To understand the low-temperature susceptibility reduction, we compare Brillouin functions with exact quantum-mechanical solutions for interacting spin-1 particles and with the Kondo predictions for confined nanoparticles.

Structure and magnetism of dilute Co(Zr) nanoclusters

B. S. Das et al. — Thu, 16 May 2013 10:02:28 PDT

Co(Zr) nanoclusters having a small fraction of Zr (≤ 7.8 at. %) were produced using a cluster deposition method and aligned using a magnetic field of about 5 kOe prior to deposition. This study shows that Zr addition to Co nanoclusters improves the fraction of hexagonal close-packed structure, magnetic anisotropy, and easy-axis alignment process. Co(Zr) nanoclusters having 7.8 at. % of Zr exhibit a considerably enhanced magnetic anisotropy constant K₁ ≈ 6.7 Mergs/cm³ and coercivity H_c≈700 Oe at 300 K as compared to those of Co nanoclusters (K₁≈ 2.9 Mergs/cm³ and H_c ≈180 Oe).

Magnetism and structure of anatase (Ti1-xVx)O2 films

D. Le Roy et al. — Thu, 16 May 2013 09:50:49 PDT

Anatase TiO₂ is known as a promising host material for a wide-gap ferromagnetic semiconductor as it is a good solvent for numerous transition-metal elements. We report on the structural and magnetic properties of epitaxially grown anatase (Ti_1-xV_x)O₂ layers where x covers the whole range of solubility of V atoms in anatase TiO₂ and beyond the solubility limit of 21%. We measured an average magnetic moment per vanadium as high as 1 µ_B with a magnetic percolation threshold of less than 6% which agrees with recent theoretical predictions. Interestingly, our results show a decrease of the average V magnetic moment as x increases throughout the solubility range. Anatase (Ti_1-xV_x)O₂ is no longer present beyond the solubility limit, where the nonmagnetic VO₂ phase forms and destroys the net magnetization.

Assembly of uniaxially aligned rare-earth-free nanomagnets

B. Balamurugan et al. — Mon, 22 Oct 2012 06:20:47 PDT

We report HfCo₇ nanoparticles with appreciable permanent-magnet properties (magnetocrystalline anisotropy K₁≈10 Mergs/cm³, coercivity H_c ≈ 4.4 kOe, and magnetic polarization J_s≈10.9 kG at 300 K) deposited by a single-step cluster-deposition method. The direct crystalline-ordering of nanoparticles during the gas-aggregation process, without the requirement of a high-temperature thermal annealing, provides an unique opportunity to align their easy axes uniaxially by applying a magnetic field of about 5 kOe prior to deposition, and subsequently to fabricate exchange-coupled nanocomposites having J_s as high as 16.6 kG by co-depositing soft magnetic Fe-Co. This study suggests HfCo₇ as a promising rare-earth-free permanent-magnet alloy, which is important for mitigating the critical-materials aspects of rare-earth elements.

Fourier transform infrared spectroscopy and wide-angle X-ray scattering: Investigations on polypropylene–vapor-grown carbon nanofiber composites

Mircea Chipara et al. — Fri, 31 Aug 2012 08:11:10 PDT

Fourier transform infrared (FTIR) spectroscopy and wide-angle X-ray scattering (WAXS) investigations of isotactic polypropylene (iPP)–vapor-grown carbon nanofiber (VGCNF) composites containing various amounts of VGCNFs ranging between 0 and 20 wt %. are reported. The FTIR investigations were focused on the regularity bands of iPP. The FTIR data indicated a drop in the isotacticity index as the concentration of nanofibers was increased; this suggested a decrease in the crystallinity. WAXS measurements revealed a dominating α₁phase, with a small admixture of γ phase or mesophase. The loading of the polymeric matrix with carbon nanofibers (CNFs) did not induce significant changes in the morphology of the polymeric matrix. A weak decrease in the size of α crystallites upon loading of CNFs was noticed. The experimental data obtained by FTIR spectroscopy supported the WAXS data. Spectroscopic data (a drop in the isotacticity index as estimated by FTIR spectroscopy and the ratio between the crystalline and total areas of WAXS lines assigned to iPP) failed to confirm the enhancement of the degree of crystallinity of polypropylene upon loading by nanofibers. However, whereas both techniques can identify with a high accuracy vibrations in ordered domains (FTIR spectroscopy) and the crystalline structure, including the lattice parameters and the size of crystallites (WAXS), difficulties in the correct assessment of the baseline and of amorphous components may result in important errors (typically >5%) in the estimation of the degree of crystallinity of the polymeric component.

Structural, magnetic, and electron transport properties of MnBi:Fe thin films

Parashu Kharel et al. — Fri, 04 May 2012 13:49:54 PDT

The structural, magnetic, and electron transport properties of Mn55=xFexBi45 (x¼0, 2, 4, 5, 8, 11, 13, 16) films prepared by multilayer deposition and annealing using e-beam evaporation have been investigated. Fe doping has produced a significant change in the magnetic properties of the samples including the decrease in saturation magnetization and magnetocrystalline anisotropy and increase in coercivity. Although the magnetization shows a smooth decrease with increasing Fe concentration, the coercivity jumps abruptly from 8.5 kOe to 22 kOe as Fe content changes from 4% to 5%, but the change in coercivity is small as the concentration goes beyond 5%. The temperature dependence of resistivity shows that the samples with low Fe concentration (<4%) are metallic, but the resistivity increases unexpectedly as the concentration reaches 5%, where the resistance increases with decreasing temperature below 300 K. First-principle calculations suggest that the observed magnetic properties can be understood as the consequences of competing ferromagnetic and antiferromagnetic exchange interactions between the interstitial atom and the rest of the MnBi lattice.

Ultrahard magnetic nanostructures

P. K. Sahota et al. — Fri, 04 May 2012 13:49:51 PDT

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 (M_s - M_h)/K_h.

A quantum-mechanical relaxation model

Ralph Skomski et al. — Fri, 04 May 2012 13:36:11 PDT

The atomic origin of micromagnetic damping is investigated by developing and solving a quantum-mechanical relaxation model. A projection-operator technique is used to derive an analytical expression for the relaxation time as a function of the heat-bath and interaction parameters. The present findings are consistent with earlier research beyond the Landau-Lifshitz-Gilbert (LLG) equation and show that the underlying relaxation mechanism is very general. Zermelo’s recurrence paradox means that there is no true irreversibility in non-interacting nanoparticles, but the corresponding recurrence times are very long and can be ignored in many cases.

Nanomagnetic skyrmions

Ralph Skomski et al. — Fri, 04 May 2012 13:36:10 PDT

Magnetic skyrmions and other topologically protected nanostructures are investigated. Since skyrmions are mathematical rather than physical objects, they describe a wide variety of physical systems, from simple magnetic domain walls to complicated quantum phases with long-range many-body entanglement. Important distinctions concern the skyrmions’ relativistic character, their quantum-mechanical or classical nature, and the one- or many-body character of the wave functions. As specific examples we consider magnetic nanospirals, where the topology of a vortex-like spin state is protected by magnetostatic interactions, and edge currents in dilute magnetic semiconductors and metallic nanodots. Our analysis militates against giant orbital moments created by a mesocopically enhanced spin-orbit coupling.

L10 CrPt phase formation and magnetic properties

Rui Zhang et al. — Fri, 04 May 2012 13:28:42 PDT

L1₀-ordered antiferromagnetic CrPt is of interest as a pinning material in exchange-biased system due to its many intriguing properties and such alloy with a (001) texture has also been used to serve as an underlayer to promote the L1₀ phase formation of other materials. Therefore, it is important to control not only the L1₀ phase formation of such material but also the texture of its ordered phase. A systematic study of the L1₀ phase formation of CrPt thin film was performed. The anisotropy of CrPt L1₀ phase has also been investigated both experimentally using CrPt/Fe bilayer system and theoretically using first principle calculation. The experimental result is in consistent with the theoretical estimation within the present thin film limitation.

Aligned and exchange-coupled L10 (Fe,Co)Pt-based magnetic films

Yi Liu et al. — Fri, 04 May 2012 13:10:38 PDT

Films of aligned L1₀-structure (Fe,Co)Pt with fcc Fe(Co,Pt) are synthesized by co-sputtering Fe, Co, and Pt on an (001) MgO substrate with in situ heating at 830 degrees C. The nanostructures and magnetic properties of the films are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID). The compositions of the samples (Fe,Co)_xPt_1-x are designed to maintain an atomic Fe: Co ratio of 65: 35 while increasing the Fe,Co content in each successive sample. In samples with low Fe and Co concentration, the XRD patterns exhibit three strong peaks, namely L1₀ (Fe,Co)Pt (001), L1₀ (Fe,Co)Pt (002), and MgO (002). A fourth peak is observed in samples with high Fe and Co concentration and identified as fcc (002). The XRD patterns confirm the formation of L1₀-ordered (Fe,Co)Pt and its epitaxial growth on MgO. TEM shows that the (Fe,Co)Pt films form isolated magnetic grains of about 100 nm in diameter. Hysteresis-loop measurements show that the increase of the Fe,Co concentration from 57.3 to 68.3 at% enhances the saturation magnetization M_s from 1245 emu/cm³ to 1416 emu/cm³, and the coercivity decreases from 32 kOe to 8.9 kOe. The nominal maximum energy product per grain is 64 MGOe.

One-step fabrication of L10 FePt nanocubes and rods by cluster beam deposition

W Li et al. — Fri, 04 May 2012 13:04:19 PDT

In this work, single crystal L1₀ FePt nanocubes have been successfully produced by a cluster beam deposition technique without the need of post annealing. Particles have been deposited by dc magnetron sputtering using high Ar pressures on both single crystal Si substrates and Au grids for the measurement of magnetic and structural properties, respectively. The nanocubes have a uniform size distribution with an average size of 5nm. At 1 Torr, the particles have the L1₀ structure with an order parameter of 0.5 and a RT coercivity of 2 kOe with high switching fields observed in the hysteresis loop. Further annealing increased the particle size to 20nm and the RT coercivity to 10.2 kOe with perfect chemical ordering. In addition to these nanocubes, micron size rods with the L1₀ structure have been observed near the cluster gun. SEM analysis showed that these rods consist of nanoparticles with 20nm average size. Surfactant assisted high-energy ball milling has been used to separate the nanoparticles from the rods. After one hour of milling, these 20 nm particles showed a room temperature coercivity of 9 kOe with an order parameter of 0.85. These FePt nanocubes have a potential for use in the development of future high-density magnetic recording media because of their high coercivity, good shape and very narrow size distribution.

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

Balamurugan Balasubramanian et al. — Fri, 04 May 2012 12:55:24 PDT

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 K₁¼9.98 Mergs=cm³ (14.1 at% of Hf) and K₁¼8.24 Mergs=cm³ (9.5 at% of Pt), as compared to Co nanoparticles (K₁¼6.21 Mergs=cm3).

Magnetism of directly ordered Sm-Co clusters

Balamurugan Balasubramanian et al. — Fri, 04 May 2012 12:43:47 PDT

Sm-Co bulk alloys have shown superior permanent-magnet properties, but research on Sm-Co nanoparticles is challenging because of the need to control particle size, size-distribution, crystalline ordering, and phase purity. In the present study, a cluster-deposition method was used to produce Sm-Co nanoparticles having desired crystal structures without the requirement of subsequent high-temperature thermal annealing. Poorly crystallized SmCo₅ nanoparticles exhibit a low room-temperature coercivity of only 100 Oe, whereas crystalline SmCo₅ and Sm₂Co₁₇ nanoparticles show room-temperature coercivities of 2000 and 750 Oe, respectively. The direct synthesis of Sm-Co nanoparticles having sizes of less than 10 nm and a high degree of atomic ordering is an important step toward creating nanoparticle building blocks for permanent-magnets and other significant applications.

Structure and magnetic properties of Co-W clusters produced by inert gas condensation

Farhad Reza Golkar-Fard et al. — Fri, 04 May 2012 06:50:59 PDT

In this article, inert-gas condensation was used to synthesize Co-W clusters. The formation, structure, and magnetic properties of the clusters were investigated. Sub-10-nm clusters were obtained, and the structures and average sizes were strongly dependent on sputtering power. At low sputtering powers, the clusters were predominantly amorphous, while, at high sputtering power, the clusters were crystalline. X ray diffraction and transmission electron microscopy revealed clusters with hcp structure at high sputtering power. The magnetic properties were dependent on the sputtering power and temperature, with the highest coercivity of 810 Oe at 10 K for high sputtering power.

Hysteresis and relaxation in granular permanent magnets

Ralph Skomski et al. — Fri, 04 May 2012 06:46:10 PDT

Some nontrivial aspects of the magnetic and structural characterization of hard-magnetic nanoparticles are investigated. Dilute ensembles are well-described by mean-field theory, although there is an asymmetry between exchange and magnetostatic interaction fields. Corrections to the mean-field approximation are caused by cooperative effects and have the character of Onsager reaction fields, which are much stronger in micromagnetism than in atomic-scale magnetism. The slow dynamics of zero-field-cooled (ZFC) magnetization curves is strongly affected by the particles0 magnetic anisotropy, which reduces the corresponding energy-barrier height from 25 to 19.1 k_BT.

Structural and magnetic properties of Mn_{2 +δ}TiSn

Parashu Kharel et al. — Fri, 04 May 2012 06:40:40 PDT

The structural and magnetic properties of Mn_{2 +δ}TiSn prepared by arc melting and annealing have been investigated. Structural studies show that the compound crystallizes in the hexagonal Ni₃Sn-type structure with a=5.70Å and c=4.55Å . The phase stability of Mn₂TiSn in the hexagonal structure is supported by the first-principle electronic structure calculations where the total energy per unit-cell volume in the hexagonal structure is smaller than that in the cubic structure. Field and temperature dependence of magnetization show that the sample is magnetically ordered with a Curie temperature around 400 K. The anisotropy energy calculated from the high-field data is 4.0×10⁵ ergs/cm³ at 300K but increases by a factor of two (8.6×10⁵ ergs/cm³) as temperature decreases to 10 K. The observed magnetic properties are explained as the consequences of competing ferromagnetic and antiferromagnetic interactions between different magnetic sublattices.

Spin and elastic contributions to isothermal entropy change

T. Mukherjee et al. — Fri, 04 May 2012 06:30:59 PDT

Statistical considerations of ensembles of localized magnetic moments reveal an upper bound of the isothermal entropy change when only the magnetic degrees of freedom are considered. In this case, the maximum molar isothermal entropy change is determined by the spin multiplicity and is equal to Rln(2J+1), where J is the angular momentum of an individual atom. However, in materials with giant magnetocaloric effect, the isothermal field-induced entropy change goes beyond the spin-multiplicity limit due to field-activated elastic degrees of freedom. Recently, we investigated a model of pairs of exchange-coupled Ising spins with variable real-space positions. We showed, within a classical approximation for the elastic degree of freedom, that a vibrational entropy contribution can be activated via applied magnetic fields. Here we quantify the impact of quantum corrections in the low-temperature limit. We compare calculations that include elastic interaction with the rigid exchange model in the high-temperature limit. We find that quantum effects provide quantitative corrections in the low-temperature limit. In addition we show that the elastic contributions to the isothermal entropy change can be additive but, remarkably, it can also give rise to reduced isothermal entropy change in certain temperature regions.

Isothermal entropy changes in nanocomposite Co:Ni₆₇Cu₃₃

Steven A. Michalski et al. — Fri, 04 May 2012 06:27:01 PDT

The temperature-dependent magnetic properties of artificial rare-earth, free-magnetic nanostructures are investigated for magnetic cooling. We consider two-phase nanocomposites, where 2 nm nanoclusters of cobalt are embedded in a Ni₆₇Cu₃₃ matrix. Several composite films were produced by cluster deposition. The average Co nanocluster size can be tuned by varying the deposition conditions. Isothermalmagnetization curves weremeasured at various temperatures 150K