2014 Publications

Interfaces anisotropy in single crystal V/Fe/V trilayer
D. Louis, I. Lytvynenko, T. Hauet, D. Lacour, M. Hehn, S. Andrieu, and F. Montaigne (2014)
J. Magn. Magn. Mater. 372, 233

Value and sign of V/Fe interface anisotropy are investigated. Epitaxial V/Fe/V/Au ultrathin films with different iron thicknesses were grown on single-crystalline (001)MgO substrate byultra-high vacuum molecular beam epitaxy. Magnetometry was used to measure magnetization and out-of-plane anisotropy field. From these values, we quantify the number of dead layers due to V/Fe or Fe/V interfaces, and compare it with the literature. We deduce that dead layers occur mostly at the bottom V/Fe interface. An average value for V/Fe and Fe/V interface anisotropy of 0 +/- 0.1 mJ/m2 (erg/cm2) was thus deduced.

Measurement of magnetization using domain compressibility in CoFeB films with perpendicular anisotropy
N. Vernier, J.-P. Adam, S. Eimer, G. Agnus, T. Devolder, T. Hauet, B. Ockert, and D. Ravelosona (2014)
Appl. Phys. Lett 104 , 122404

Here an original method is presented to map the saturation magnetization of soft ultrathin films without typical magnetomeetry method. We illustrate it to assess the compositional dependence of the magnetization of CoFeB(1 nm)/MgO films. The method relies on the measurement of the dipolar repulsion of parallel domain walls that define a linear domain. The film magnetization is linked to the field compressibility of the domain. The method also yields the minimal distance between two walls before their merging, which sets a practical limit to the storage density in spintronic devices using domain walls as storage entities.

A Single-Chain Magnet Based on {CoII 4} Complexes and Azido/Picolinate Ligands
J. Liu, M. Qu, M. Rouzières, X-M. Zhang and R. Clérac (2014)
Inorg. Chem., 53 (15), pp 7870–7875

A new homonuclear single-chain magnet self-assembles as a one-dimensional coordination network of defective dicubane {CoII4} complexes linked by single CoII ions with the assistance of azido and picolinate ligands. Dominating intrachain ferromagnetic interactions, intrinsic Ising-like CoII anisotropy, and negligible interchain magnetic interactions lead to a thermally activated relaxation time of the magnetization below 8 K. Two thermally activated regimes above and below 3.5 K are observed with the following energy barriers: Δτ1/kB = 66 K (τ0 = 3.7 × 10–11 s) and Δτ2/kB = 51 K (τ0 = 2.3 × 10–9 s), respectively. The difference between the two energy barriers of the relaxation time, 15 K, agrees well with the experimental energy, Δξ, to create a domain wall along the chain.

Modification of the magnetic properties of Co films grown on MgO (100) by treatment with NaOH solution
H. L. Liu, T. Hauffman, K. Herdewyn, H. Terryn, M. Van Bael, and C. Van Haesendonck (2014)
Phys. Rev. B 90, 134414

We investigated the surface morphology and magnetic properties of Co films grown on MgO (100) substrates before and after treatment with a NaOH solution. The surface morphology evolves from a continuous film into isolated hexagonal nanodisks resulting from the formation of cobalt hydroxide. In the as-grown Co films the magnetic anisotropy behaves as a superposition of fourfold magnetocrystalline anisotropy and uniaxial magnetic anisotropy (UMA), and the magnetization reversal proceeds by two-step domain-wall motion. After the NaOH treatment the UMA is reduced and the magnetization reversal appears to be dominated by magnetization rotation. In addition, the exchange bias effect, which in the as-deposited films results from the presence of a cobalt oxide layer, disappears after the NaOH treatment, and can be reinduced by annealing the sample. The observed significant changes in the magnetic properties can be related to the disappearance and reappearance of a cobalt oxide top layer.

Relaxor Ferroelectricity and Magnetoelectric Coupling in ZnO–Co Nanocomposite Thin Films: Beyond Multiferroic Composites
D. Y. Li, Y. J. Zeng, D. Batuk, L. M. C. Pereira, Z. Z. Ye, C. Fleischmann, M. Menghini, S. Nikitenko, J. Hadermann, K. Temst, A. Vantomme, M. J. Van Bael, J.-P. Locquet, and C. Van Haesendonck (2014)
ACS Appl. Mater. Interfaces, 2014, 6 (7), pp 4737–4742

ZnO–Co nanocomposite thin films are synthesized by combination of pulsed laser deposition of ZnO and Co ion implantation. Both superparamagnetism and relaxor ferroelectricity as well as magnetoelectric coupling in the nanocomposites have been demonstrated. The unexpected relaxor ferroelectricity is believed to be the result of the local lattice distortion induced by the incorporation of the Co nanoparticles. Magnetoelectric coupling can be attributed to the interaction between the electric dipole moments and the magnetic moments, which are both induced by the incorporation of Co. The introduced ZnO–Co nanocomposite thin films are different from conventional strain-mediated multiferroic composites.

Influence of the bulkiness of the substituent on the aggregation and magnetic properties of poly(3‐alkylthiophene)s
H. Peeters, M. Jivanescu, A. Stesmans, L. M. C. Pereira, L. Dillemans, J‐P. Locquet, M. J. Van Bael, A. Persoons, G. Koeckelberghs (2014)
Journal of Polymer Science Part A: Polymer Chemistry, 52(1), 76-86

A series of poly(3‐alkylthiophene)s (P3ATs) (P1–P5) has been synthesized via a Ni(dppp)‐mediated polymerization, varying the bulkiness of the alkyl side chains in order to investigate the influence of the bulkiness of the alkyl substituent on the aggregation and magnetic properties of P3ATs. UV–Vis spectroscopy, performed in solution as well as in film, shows that the stacking of the polymers becomes more complicated as the bulkiness of the side chains increases. Both the π‐interactions and the planarization of the polymer chains are diminished. While aggregation is absent in poor solvent for the polymer with the most bulky side chains, aggregation was present in film, albeit slowed down. This behavior was also confirmed by X‐ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments. Electron spin resonance (ESR) measurements, performed at 300 K on powders, confirmed the trend of decreasing supramolecular order with increasing bulkiness of the side‐chain. Magnetization measurements, performed at 5 and 300 K, are in line with our hypothesis on the influence of π‐interactions and the fraction of planar polymer chains on the coercivity and saturation magnetization, respectively.

Electric Field‐Induced Oxidation of Ferromagnetic/Ferroelectric Interfaces
S. Couet, M. Bisht, M. Trekels, M. Menghini, C. Petermann, M. J. Van Bael, J‐P. Locquet, R. Rüffer, A. Vantomme, K. Temst (2014)
Advanced Functional Materials, 24(1), 71-76

Composite multiferroics are a new class of material where magneto‐electric coupling is achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The challenge of understanding the chemical and magnetic properties of such interface is a key to achieve good magneto‐electric coupling. The unique possibilities offered by isotope sensitive techniques are used to selectively investigate the interface’s chemistry and magnetism in Fe/BaTiO3 and Fe/LiNbO3 systems during the application of an electric field. With a large enough electric field, a strong oxidation of Fe is triggered, which creates a magnetically dead interface. This leads to an irreversible decrease of the magneto‐electric coupling properties. Material parameters are identified that determine under which electric field the interface may be modified. The results are confirmed on the two systems and are expected to be widespread in this new class of hybrid material.

Oxygen Vacancy Induced Room Temperature Ferromagnetism in Pr-Doped CeO2 Thin Films on Silicon
G. Niu, E. Hildebrandt, M. A. Schubert, F. Boscherini, M. H. Zoellner, L. Alff, D. Walczyk, P. Zaumseil, I. Costina, H. Wilkens, and T. Schroeder (2014)
ACS applied materials & interfaces, 6(20), 17496-17505

Integration of functional oxides on Si substrates could open a pathway to integrate diverse devices on Si-based technology. Oxygen vacancies (Vo··) can strongly affect solid state properties of oxides, including the room temperature ferromagnetism (RTFM) in diluted magnetic oxides. Here, we report a systematical study on the RTFM of oxygen vacancy engineered (by Pr3+ doping) CeO2 epitaxial thin films on Si substrates. High quality, mixed single crystalline Ce1–xPrxO2−δ (x = 0–1) solid solution films were obtained. The Ce ions in CeO2 with a fluorite structure show a Ce4+-dominant valence state in all films. The local crystal structures of the films were analyzed in detail. Pr doping creates both Vo·· and PrO8-complex defects in CeO2 and their relative concentrations vary with the Pr-doping level. The RTFM properties of the films reveal a strong dependence on the relative Vo·· concentration. The RTFM in the films initially increases with higher Pr-doping levels due to the increase of the F+ center (Vo·· with one occupied electron) concentration and completely disappears when x > 0.2, where the magnetic polaron concentration is considered to decline below the percolation threshold, thus long-range FM order can no longer be established. We thus demonstrate the possibility to directly grow RTFM Pr-doped CeO2 films on Si substrates, which can be an interesting candidate for potential magneto-optic or spintronic device applications.

High-Temperature Stability and Saturation Magnetization of Superparamagnetic Nickel Nanoparticles in Microporous Polysilazane-Derived Ceramics and their Gas Permeation Properties
M. S. Bazarjani*, M. M. Müller, H-J. Kleebe, Y. Jüttke, I. Voigt, M. B. Yazdi, L. Alff, R. Riedel, and A. Gurlo (2014)
ACS applied materials & interfaces, 6(15), 12270-12278

Superparamagnetic Ni nanoparticles with diameters of about 3 nm are formed in situ at room temperature in a polysilazane matrix, forming Ni/polysilazane nanocomposite, in the reaction between a polysilazane and trans-bis(aceto-kO)bis(2-aminoethanol-k2N,O)nickel(II). The thermolysis of the Ni/polysilazane nanocomposite at 700 °C in an argon atmosphere results in a microporous superparamagnetic Ni/silicon oxycarbonitride (Ni/SiCNO) ceramic nanocomposite. The growth of Ni nanoparticles in Ni/SiCNO ceramic nanocomposite is totally suppressed even after thermolysis at 700 °C, as confirmed by HRTEM and SQUID characterizations. The analysis of saturation magnetization of Ni nanoparticles in Ni/polysilazane and Ni/SiCNO nanocomposites indicates that the saturation magnetization of Ni nanoparticles is higher than expected values and infers that the surfaces of Ni nanoparticles are not oxidized. The microporous superparamagnetic Ni/SiCNO nanocomposite is shaped as a free-standing monolith and foam. In addition, Ni/SiCNO membranes are fabricated by the dip-coating of a tubular alumina substrate in a dispersion of Ni/polysilazane in THF followed by a thermolysis at 700 °C under an argon atmosphere. The gas separation performance of Ni/SiCNO membranes at 25 and 300 °C is assessed by the single gas permeance (pressure rise technique) using He, H2, CO2, N2, CH4, n-propene, n-propane, n-butene, n-butane, and SF6 as probe molecules. After hydrothermal treatment, the higher increase in the hydrogen permeance compared to the permeance of other gases as a function of temperature indicates that the hydrogen affinity of Ni nanoparticles influences the transport of hydrogen in the Ni/SiCNO membrane and Ni nanoparticles stabilize the structure against hydrothermal corrosion.

B1‐Mobilstor: Materials for Sustainable Energy Storage Techniques – Lithium Containing Compounds for Hydrogen and Electrochemical Energy Storage
B. Böhme, C. B. Minella, F. Thoss, I. Lindemann, M. Rosenburg, C. Pistidda, K. T. Møller, T. R. Jensen, L. Giebeler, M. Baitinger, O. Gutfleisch, H. Ehrenberg, J. Eckert, Y. Grin, and L. Schultz (2014)
Advanced engineering materials, 16(10), 1189-1195

New material concepts for hydrogen storage and lithium ion batteries are investigated in the joint ECEMP project B1 – Mobilstor. Chemical composition are essential for the performance of a storage material. A certain state of material can be effectively analyzed by in situ methods to obtain a maximum of information. For hydrogen storage the LiNH2MgH2 system and as lithium ion battery anode the Ge(cF136) allotrop are highlighted under this issue.

Modeling of Nd-Oxide Grain Boundary Phases in Nd-Fe-B Sintered Magnets
G. Hrkac, K. Butler, T. G. Woodcock, L. Saharan, T. Schrefl, and O. Gutfleisch (2014)
JOM, 66(7), 1138-1143

We use atomistic and micromagnetic simulations combined with atomic-scale, aberration-corrected transmission electron microscopy to study the anisotropy profile of the interface between a Nd2O3-hP5 and a Nd2Fe14B phase. It is shown that a hybrid Morse-Buckingham potential approach can be used to study mixed-metallic and oxide-rich systems and to calculate surface-energy-induced large strains and potentially large relaxation effects on the adjacent grains. These are used to derive a magnetoelastic anisotropy energy from a first-order perturbation of the magnetoelastic Hamiltonian and are used to evaluate its effect on coercivity. It is shown that the change in coercivity originates from these distortions in the Nd2Fe14B crystal lattice close to the grain boundary, and the coercivity can be evaluated using such a multiscale modeling approach.

Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets
S. Bance, H. Oezelt, T. Schrefl, G. Ciuta, N. M. Dempsey, D. Givord, M. Winklhofer, G. Hrkac, G. Zimanyi, O. Gutfleisch, T. G. Woodcock, T. Shoji, M. Yano, A. Kato, and A. Manabe (2014)
Applied Physics Letters, 104(18), 182408

The coercive field and angular dependence of the coercive field of single-grain Nd2Fe14B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where an Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour.

High energy product in Battenberg structured magnets
S. Bance, H. Oezelt, T. Schrefl, M. Winklhofer, G. Hrkac, G. Zimanyi, O. Gutfleisch, R. F. L. Evans, R. W. Chantrell, T. Shoji, M. Yano, N. Sakuma, A. Kato, and A. Manabe (2014)
Applied Physics Letters, 105(19), 192401

Multiphase nano-structured permanent magnets show a high thermal stability of remanence and a high energy product while the amount of rare-earth elements is reduced. Non-zero temperature micromagnetic simulations show that a temperature coefficient of remanence of −0.073%/K and that an energy product greater than 400 kJ/m3 can be achieved at a temperature of 450 K in a magnet containing around 40 volume percent Fe65Co35 embedded in a hard magnetic matrix.