V. Neu

Domain structure of epitaxial Co films with perpendicular anisotropy

Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single crystal substrates or with an intermediate Ruthenium buffer layer. The crystal structure and epitaxial growth relation was studied by XRD, pole figure measurements and reciprocal space mapping. Detailed VSM analysis shows that the perpendicular anisotropy of these highly textured Co films reaches the magnetocrystalline anisotropy of hcp-Co single crystal material. Films were prepared with thickness t of 20 nm < t < 100 nm to study the crossover from in-plane magnetization to out-of-plane magnetization in detail. The analysis of the periodic domain pattern observed by magnetic force microscopy allows to determine the critical minimum thickness below which the domains adopt a pure in-plane orientation. Above the critical thickness the width of the stripe domains is evaluated as a function of the film thickness and compared with domain theory. Especially the discrepancies at smallest film thicknesses show that the system is in an intermediate state between in-plane and out-of-plane domains, which is not described by existing analytical domain models.

A comparison of the magnetic properties and deformation behaviour of Nd-Fe-B magnets made from melt-spun, mechanically alloyed and HDDR powders

Commercial melt-spun and HDDR Nd-Fe-B powders as well as mechanically alloyed Nd-Fe-B powders have been used for hot pressing and subsequent die upsetting. A comparison of all three types of magnets with respect to their magnetic properties and deformation behaviour is discussed in this paper. The highest values of remanence, , and energy density, , found for the die-upset melt-spun material can be explained in terms of its small Nd content of 14 at% and its high degree of texture connected with the strong shape anisotropy of the deformed nanocrystalline grains. Fracture surfaces of this material have coarse grains at the former flake boundaries which results in a coercivity, , of only 1 T. An energy density of has been measured for a die-upset mechanically alloyed material. In spite of a Nd content of 16 at% in this alloy, a remanence of 1.2 T and a coercivity of 1.6 T have been attained. The large coercivity is due to (i) a very homogeneous distribution of the Nd-rich intergranular phase, (ii) a grain size of only observed after die upsetting and (iii) the fact that there is no formation of faceted grains during hot deformation. The microstructure of HDDR magnets (14 at% Nd) has larger average grain sizes and some faceted grains. Consequently these magnets have the smallest values of the degree of texture, remanence, , and energy density, . By addition of further elements the coercivity of this material could be held at . After thermal demagnetization the three types of hot-deformed Nd-Fe-B magnets have a relatively large initial susceptibility, which is due to the presence of classical magnetic domains as well as interaction domains. For the investigated fine-grained Nd-Fe-B materials the deformation mechanism can be described by using a solution-precipitation creep model, governed by interface-reaction-controlled creep.

Epitaxial SmCo5 thin films with perpendicular anisotropy

Epitaxial SmCo5 thin films with strong perpendicular magnetic anisotropy have been developed by pulsed laser deposition on heated Ru buffered Al2O3(0001) substrates. Pole figure measurements on thicker films reveal the existence of two epitaxial variants, 30° in-plane rotated to each other, which reduces to a single epitaxial relation for smaller film thickness. The presence of a single SmCo5 phase with perpendicular direction of the c-axis is established by multiple peak analysis of x-ray data in pole figure geometry and is confirmed by a numerical fit of hysteresis measurements, which results in an anisotropy constant of Ku=7.6 MJ/m3.

Intrinsic and extrinsic properties of epitaxial Nd2Fe14B films

Epitaxial Nd2Fe14B films with the c axis (easy magnetization axis) perpendicular to the film plane were prepared on a Ta(110) buffer on single crystalline Al2O3(0001) substrates using pulsed laser deposition. Due to the epitaxial film growth an almost perfect alignment of all magnetic moments perpendicular to the film plane is achieved. Coercivities up to 2 T are obtained for Nd-rich films where the Nd2Fe14B grains are magnetically decoupled. An analysis of the coercivity mechanism shows that the switching mechanism is nucleation dominated and the high coercivity is achieved by avoiding nucleation within the grains. Intrinsic properties like the spin reorientation temperature of 135 K and spin reorientation angle of 30° at 4.2 K of these films are in very good agreement to single crystal data.

Growth of epitaxial SmCo5 films on Cr∕MgO(100)

Although the hard magnetic SmCo5 phase has very attractive and well-known intrinsic magnetic properties, it has not been grown as an epitaxial thin film, so far. This letter reports the epitaxial growth of SmCo5 films by pulsed-laser deposition on Cr(100) buffered MgO(100) single-crystal substrates. The phase purity, crystal structure, epitaxial relation to the substrate, and magnetic properties have been determined by careful energy-dispersive x-ray analysis, pole figure measurements, transmission electron microscopy, and vibrating sample magnetometry. Compared to the formerly studied Sm2Co7 films, the preparation of the SmCo5 phase improves the remanent magnetization by 38%.Although the hard magnetic SmCo5 phase has very attractive and well-known intrinsic magnetic properties, it has not been grown as an epitaxial thin film, so far. This letter reports the epitaxial growth of SmCo5 films by pulsed-laser deposition on Cr(100) buffered MgO(100) single-crystal substrates. The phase purity, crystal structure, epitaxial relation to the substrate, and magnetic properties have been determined by careful energy-dispersive x-ray analysis, pole figure measurements, transmission electron microscopy, and vibrating sample magnetometry. Compared to the formerly studied Sm2Co7 films, the preparation of the SmCo5 phase improves the remanent magnetization by 38%.

Effect of rare earth content on microstructure and magnetic properties of SmCo and NdFeB thin films

Thin films of highly anisotropic rare earth intermetallic compounds increasingly come into the focus of interest for application in magnetic miniature devices. We prepared hard magnetic SmCo and NdFeB films by pulsed laser deposition and investigated the influence of rare-earth content on phase formation, microstructure, and magnetic properties. The phase evolution with rare earth content, as observed by x-ray diffraction, shows a distinct difference for the two systems. For SmCo a disordered SmCo7 structure is found at low Sm content and a change in the lattice constants is observed for a Sm-rich sample. Simultaneously, the coercive field improves whereas the magnetic in-plane texture found for Sm-poor films vanishes for higher Sm content. For NdFeB the increase in Nd content leads to an additional Nd phase because of the small homogeneity range. Contrary to the bulk, coercivity is reduced, which can be attributed to a significant increase of grain size and roughness.

High performance permanent magnets made by mechanical alloying and hot pressing

The nonequilibrium process of mechanical alloying was used to prepare nanocrystalline powder. The crystallization of the -type structure in mechanically alloyed Sm - Fe(Ga) - C material begins at about C. The single phase structure can be obtained after homogenization for one hour at temperatures between C and C. The coercivity increases to a maximum value of about 1.5 T after annealing at a temperature of C. The nearly square shape of the demagnetization curve suggests a very homogeneous microstructure of the highly coercive powder. By hot pressing, a fully dense magnet was successfully obtained with a density of 7.6 g and a coercivity of 1.5 T. No texture could be observed after a 75% hot deformation at C.

Epitaxial growth,,of highly coercive Sm-Co thin films using pulsed laser deposition

Hard magnetic materials with a uniaxial magnetocrystalline anisotropy can be most efficiently used if the easy axis is well aligned along one crystallographic direction in the entire sample volume. Epitaxial growth is one suitable method to achieve this aim and therefore Sm–Co thin films on Cr-buffered single-crystal MgO (100) substrates were deposited. Pulsed laser deposition from elemental Sm and Co targets was used to prepare films of nominal Sm2Co7 stoichiometry. Pole figure measurements and magnetization measurements reveal an epitaxial growth of Cr on MgO onto which the c axis of the Sm–Co layer is aligned in plane, viz., MgO(001)[100]‖Cr(001)[110]‖Sm–Co(110)[001]. At higher deposition temperatures an additional Sm–Co (1 1 16) texture was observed. This corresponds to an additional epitaxial orientation relation, where the c axis is tilted 60° out of the substrate plane. However, at low deposition temperatures and with smaller pulse repetition rates a reduction in the amount of this unwanted compone...

Fully epitaxial, exchange coupled SmCo5/Fe/SmCo5 trilayers

Fully epitaxial SmCo5(25 nm)/Fe/SmCo5(25 nm) trilayer films were prepared, in which the easy magnetization axis of both hard magnetic SmCo5 layers is uniquely aligned along one orientation within the film plane. The interlayer exchange coupling between the hard and soft magnetic phase is studied by hysteresis measurements in the magnetic hard and magnetic easy orientation and by dc-demagnetizing loops for different thicknesses of the soft Fe layer. The good coupling together with the optimum texture provided by the epitaxial growth lead to excellent magnetic properties. Fully coupled trilayers are achieved for 6 nm thin Fe layers, which maintain a square hysteresis with high coercivity of μ0Hc = 1.5 T and a 30% remanence enhancement over a single hard magnetic layer. For thicker Fe layers (16 nm) the exchange coupling leads to reversible rotation processes within the soft phase before the trilayer switches irreversibly at a field of 0.9 T. The still acceptable coercive field and the further improved remanence lead to a maximum energy density of 224 kJ m−3 (28 MG Oe). The specific epitaxial relation between SmCo5 and Fe with its unique orientation of the SmCo5 easy axis throughout the whole layer stack presents the essential step in the realization of thick, textured, exchange coupled multilayers with high energy density.

Pulsed laser deposited epitaxial Sm–Co thin films with uniaxial magnetic texture

Sm–Co thin films have been grown epitaxially with pulsed laser deposition on Cr buffered MgO(110) substrates. The buffer microstructure plays a significant role in controlling the growth and hence the magnetic properties of the Sm–Co film. High deposition temperatures of the Cr buffer result in a rough and discontinuous microstructure, thereby resulting in an x-ray amorphous or nanocrystalline Sm–Co layer, as has been confirmed by transmission electron microscopy studies. By lowering the buffer deposition temperature from 700to300°C, the roughness decreases from 5to0.6nm. Sm–Co films grow epitaxially on these low temperature buffers with the epitaxial relation MgO(110)[001]∥Cr(211)011∥SmCo(100)[001] which implies a single in-plane orientation of the c axis along the MgO[001] direction. The strong in-plane crystallographic texture seen in the pole figure measurements leads to a very pronounced magnetic texture, quantified by a remanence ratio JrMgO[110]∕JrMgO[001] as low as 0.08 and a high remanence of 0.8...