C. Kuhrt

Formation and magnetic properties of nanocrystalline mechanically alloyed Fe‐Co and Fe‐Ni

Fe100−xCox and Fe100−xNix powders were prepared by mechanical alloying of the elements in a planetary ball mill. They were investigated with respect to phase formation and magnetic properties using x‐ray diffraction and measurements of the saturation magnetization and the coercivity. In both systems, disordered solid solutions were formed by mechanical alloying as proved especially by the measurement of the saturation magnetization. Moreover, nonequilibrium microstructures were established consisting in a nanocrystalline state (average minimum grain sizes 20–30 nm) accompanied by the introduction of considerable atomic‐level strain (root mean square strain up to 1%). The soft magnetic behavior of this material shows some features of rapidly quenched nanocrystalline ribbons, but very low coercivities are prevented by a predominant influence of strain via magnetoelastic interaction.

Phase formation and martensitic transformation in mechanically alloyed nanocrystalline Fe—Ni

Fe—Ni alloys were prepared by mechanical attrition of elemental powders in a planetary ball mill in argon atmosphere. A nonequilibrium phase diagram of Fe100−xNix, mechanically alloyed at various milling intensities, is presented. Disordered body‐centered‐cubic and/or face‐centered‐cubic supersaturated solid solutions with a two‐phase region are found. The maximum solubilities are 30 at. % Ni in Fe and 40 at. % Fe in Ni and decrease slightly with increasing milling intensity, i.e., processing temperature. Mechanically alloyed Fe—Ni is in a nanocrystalline state with an average grain size of 25–35 nm. The grain refinement is accompanied by an increase in atomic‐level strain up to about 1% root‐mean‐square. This high internal strain affects predominantly the coercivity which is at least 4 A/cm even for single‐phase alloys. The kinetics of the martensitic γ→α’ transformation in mechanically alloyed nanocrystalline Fe—Ni are significantly modified in comparison to coarse‐grained as‐cast alloys. The nonequilib...

Permanent magnets by mechanical alloying (invited)

Mechanical alloying is applied to prepare Nd‐Fe‐B, Sm‐Fe‐TM type (TM: V, Ti, Zr), and interstitial nitride and carbide permanent magnets. Starting from elemental powders, the hard magnetic phases are formed by milling in a planetary ball mill and a following solid‐state reaction at relatively low temperatures. For Nd‐Fe‐B, the magnetically isotropic particles are microcrystalline, show a high coercivity (up to 16 kA/cm for ternary alloys and above for Dy‐substituted samples), and can be either used for making bonded magnets or compacted to dense isotropic magnets by hot uniaxial pressing. Magnetically anisotropic samples with a remanence up to 1.31 T and an energy product up to 326 kJ/m3 are formed by die upsetting. The mechanical alloying process has also been applied to prepare magnetic material of three new Sm‐Fe‐TM phases: Sm‐Fe‐V with the ThMn12 structure, Sm‐Fe‐Zr with the PuNi3 structure, and Sm‐Fe‐Ti with the A2 structure. They all show high or ultrahigh coercivities (up to 51.6 kA/cm for Sm‐Fe‐Ti...

Remanence enhancement due to exchange coupling in multilayers of hard- and softmagnetic phases

Nd-Fe-B/Fe/Nd-Fe-B trilayers with individual layer thicknesses between 10 and 100 nm have been grown by sputter deposition. The aim of our study is to investigate remanence enhancement due to exchange coupling between the soft- and hard-magnetic layers as a function of the Fe-interlayer thickness d. It is shown that for decreasing d the remanence of the trilayer system is significantly increased due to exchange coupling. The coercivity of the trilayer is also increased for d<40 nm which can be explained with a higher nucleation field for magnetization reversal.

Magnetic properties and thermal stability of Sm2Fe17Nx with intermediate nitrogen concentrations

Abstract The existence of interstitial Sm2Fe17Nx with intermediate nitrogen contents (0 ≤ x ≤ 2.94) is revealed by a continuous increase of the unit cell volume, the Curie temperature, the saturation polarization and the anisotropy field with increasing N concentration. The thermal stability of Sm2Fe17Nx increases also with increasing x. The anisotropy constants K1 and K2 at room temperature were deduced by fitting magnetization curves of oriented powders. The easy magnetization direction changes from the basal plane (x = 0) via an easy cone (δ = 24 ° forx = 0.4) to the c-axis (x ≥ 0.55). The intrinsic magnetic properties of Sm2Fe17N2.94 were found to be Tc = 473 ° C, Js = 1.51 T and HA = 21.0 T where K1 = 8.4 and K2 = 2.1 MJ/m3.

Structural and intrinsic magnetic properties of Sm2(Fe1−xCox)17Ny

Formation and magnetic properties of the interstitial Th2Zn17-type nitrides were investigated over the whole concentration range for Sm2(Fe1−xCox)17. The decomposition temperature, Td, and the nitrogenation kinetics decrease with increasing Co content x. The substitution of Co for Fe, for x < 0.2, leads to an overall improvement of the intrinsic magnetic properties of Sm2Fe17Ny. The Curie temperature, the room temperature saturation polarization and the anisotropy field show a maximum in dependence on composition for x = 0.5 (TC = 901 K), x = 0.15 (Js = 1.55 T) and x = 0.3 (HA = 25.0 T), respectively. For Sm2Co17, nitrogenation considerably decreases the Curie temperature and the saturation polarization whereas the anisotropy field is almost doubled. For x = 0.2 the compound has excellent intrinsic magnetic properties for hard magnetic applications: TC = 842 K, Js = 1.55 T and HA = 23.7 T at room temperature.

Magnetic properties and growth texture of high-coercive Nd–Fe–B thin films

We have investigated the growth texture and magnetic properties of sputtered Nd–Fe–B thin films with thicknesses from 5 to 350 nm. Films deposited directly onto a quartz substrate grow with a pronounced c-axis texture perpendicular to the film plane and have a coercivity of about 160 kA/m. If the films thickness is reduced below 150 nm, a significant part of the Nd is oxidized by substrate oxygen and the coercivity decreases to values below 20 kA/m. The deposition of a 80 nm Cr buffer layer between substrate and Nd–Fe–B film prevents oxidation of the film and decreases the growth texture of the films. This enables us to fabricate Nd–Fe–B films as thin as 20 nm with good hard magnetic properties [Hcj=800 kA/m, Jr=1.1 T, and a maximum energy product (BH)max=190 kJ/m3 perpendicular to film plane]. An inverse relationship between growth texture and coercivity is found which can be understood in terms of domain wall motion. The deposition of a Cr buffer leads to an island type of growth and a high surface roug...

Pressure‐assisted zinc bonding of microcrystalline Sm2Fe17Nx powders

Highly coercive isotropic magnets were prepared by pressure‐assisted Zn bonding of microcrystalline Sm2Fe17Nx powders. Mechanically alloyed and two‐step heat treated powder was mixed with elemental Zn powder and compacted by uniaxial pressing at elevated temperatures. The maximum room‐temperature coercivity of such magnets was 34.7 kA/cm (43.6 kOe), which represents an increase by about 50% relative to that of the starting powder. The density was about 80% of the theoretical value resulting in a relatively low remanence of 0.4 T. This was achieved after compaction using parameters optimized with respect to coercivity, i.e., a Zn content of 20 wt. % (referred to the 2:17:N weight), a temperature of 425 °C and a pressure of 270 MPa. The coercivity increase induced by compaction with Zn presumably originates from improved magnetic decoupling of the 2:17:N grains which is caused by the paramagnetic Fe3Zn7 phase formed at the grain boundaries within the powder particles.

Mechanically alloyed and gas‐phase carbonated highly coercive Sm2Fe17Cx

Microcrystalline Sm2Fe17Cx powders with high carbon content (x≊2) were prepared by mechanical alloying and a subsequent solid‐gas reaction in acetylene atmosphere to introduce carbon atoms interstitially into the 2:17 lattice. Such processing results in magnetically isotropic material with a room‐temperature coercivity of up to 18.5 kA/cm (23.2 kOe) and a maximum energy product of 59 kJ/m3 (7.4 MGOe). It is unstable at high temperatures and decomposes into the equilibrium phases SmC and α‐Fe upon heating to 700 °C in vacuum.

Development of coercivity in Sm2Fe17(N,C)x magnets by mechanical alloying, solid–gas reaction, and pressure‐assisted zinc bonding

Highly coercive isotropic Sm2Fe17Nx and Sm2Fe17Cy powders were prepared by a mechanical alloying of elemental Fe and Sm, a heat treatment in vacuum at 700–800 °C to form microcrystalline Sm2Fe17, and, finally, a solid–gas reaction in N2 and C2H2 atmosphere, respectively, at 450–500 °C to introduce nitrogen and carbon, respectively, on interstitial sites. The solid–gas reaction parameters, such as temperature and time as well as, in the case of carbides, the C2H2 partial pressure, are very critical with respect to dynamical decomposition of the Sm2Fe17 phase during reaction progress. Optimum parameters result in a coercivity of up to 25.6 kA/cm (32.2 kOe) and 18.5 kA/cm (23.2 kOe) for the nitride and carbide, respectively. Zinc bonding of such powders assisted by a uniaxial pressure of 200–300 MPa at a temperature of 425–450 °C leads to further substantial coercivity enhancement. The room‐temperature coercivity of Zn‐bonded Sm2Fe17Nx magnets reached 34.7 kA/cm (43.6 kOe), which represents an increase by ab...