M. Katter

High coercivity in Sm2Fe17Nx magnets

Using mechanical alloying and a subsequent two‐step heat treatment we produced magnetically isotropic microcrystalline Sm2Fe17Nx samples with room‐temperature coercivities up to 24 kA/cm (30 kOe). The remanence and the energy product are equivalent to similarly prepared Nd‐Fe‐B samples, but the properties at elevated temperatures are superior because of the high Curie temperature of 470 °C and the large anisotropy field of 14 T at room temperature. From differential scanning calorimetry it is concluded that the 2:17 nitride is metastable. It decomposes into Sm nitride and α‐Fe above 600 °C.

Structural and hard magnetic properties of rapidly solidified Sm–Fe–N

Sm–Fe alloys have been produced by rapid quenching and the resulting phases have been investigated in the as‐quenched state and after nitriding. Besides the well‐known equilibrium phases of the binary Sm–Fe system (Fe, Sm2Fe17, SmFe3, SmFe2 and Sm), a hexagonal TbCu7‐type phase shows up in melt spun ribbons (a=4.88 A, c=4.23 A). Its stoichiometry is about Sm1Fe9 and it is formed only at wheel velocities above 15 m/s. The Curie temperature and the saturation polarization of this new phase is 210 °C and 1.25 T, respectively. At higher Sm concentrations or lower quenching rates the structure changes to the Th2Zn17‐type. The Th2Zn17‐type ribbons are magnetically soft whereas the TbCu7‐type samples show moderate coercivities of up to 1.7 kA/cm. Nitrogenation leads to an expansion of the lattice and to an overall improvement of the hard magnetic properties for both phases. Their Curie temperatures are increased to 470 °C and the saturation polarizations are raised to 1.40 and 1.51 T for the TbCu7‐ and the Th2Zn...

MAGNETOCRYSTALLINE ANISOTROPY OF SM2FE17N2

Abstract The magnetocrystalline anisotropy of Sm2Fe17N2 was investigated from room temperature up to the Curie temperature, Tc. By introducing interstitial N in Sm2Fe17, which has the Th2Zn17 crystal structure, the easy magnetization direction (EMD) changes from the basal plane to the c-axis. The anisotropy field, μ0HA, at temperature is found to be 14 T. The temperature dependence of the anisotropy field was measured up to 700 K by the singular point detection (SPD) method and by magnetization measurements on aligned powder samples. The anisotropy field, HA, decreases monotonically with increasing temperature up to Tc and thus the EMD lies always parallel to the c-axis. The high anisotropy field in combination with the high saturation magnetization and Curie temperature of this material is very promising with respect to its application as a new permanent magnet.

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...

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.

Mechanically alloyed Sm‐Co materials

The preparation of SmCo5 magnets is based on a high‐temperature heat treatment above 800 °C followed by a rapid cooling to room temperature. By mechanical alloying and a subsequent heat treatment, it is possible for the first time to prepare hard magnetic SmCo5 powder at low temperatures. The process involves first the formation of an amorphous precursor during milling, which then is transformed to the 1:5 structure at temperatures above 500 °C. The magnetically isotropic samples show a coercivity of 24 kA/cm and a remanence of 0.52 T. The process is also applicable to Sm2Co17. However, here the coercivity is less than 5 kA/cm and higher values require the formation of a precipitation‐hardened microstructure. The stability of SmCo5 below 800 °C questions the eutectoid decomposition into Sm2Co17 and Sm2Co7.

Sm‐Fe‐Ti magnets with room‐temperature coercivities above 50 kOe

Using mechanical alloying and an additional annealing, we prepared bulk material of a new Sm20 Fe70 Ti10 phase observed before only in sputtered films deposited in the amorphous state and then crystallized. Crystallization of rapidly quenched and partially amorphous ribbons leads to a two‐phase material with a considerable amount of this phase but with the Fe2 Sm phase as a majority phase. This new phase (named 20:70:10 phase) has a Curie temperature of 380 °C and an estimated saturation magnetization of 6–7 kG. The magnetically isotropic, mechanically alloyed samples show room‐temperature coercivities of up to 50.3 kOe.

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.

Intrinsic magnetic properties of R2Fe17CyNx compounds: (R=Y, Sm, Er, and Tm)

Samples of R2Fe17C(y)N(x) (R = Y, Sm, Er, Tm) were prepared by arc melting appropriate amount of R, Fe, and C, vacuum annealing at 1373 K and finally annealing at 740 K in nitrogen for 10 h. The magnetic properties of these compounds were investigated by means of ac initial susceptibility, magnetization measurements, and x-ray diffraction. The thermal stability of the nitride phase was studied by differential scanning calorimetry. It was found that, when heated above 600 K, R2Fe17C(y)N(x) irreversibly decomposes N which is irrespective of the carbon concentration and rare-earth element. The Curie temperatures of R2Fe17C(y)N(x) are independent of the carbon concentration and are approximately 400 K higher than those of the corresponding pure R2Fe17 compounds. However, the Curie temperatures cannot be correlated to the composition x of the initial R2Fe17C(y)N(x) compounds at room temperature because some N was lost during the heating to T(c). In the Er and Tm compounds spin reorientation transitions were found, marking the change of the easy magnetization direction from the c axis to the basal plane with increasing temperature. The Tm compounds show an additional magnetic transition at low temperatures (below 40 K). A coexistence of the hexagonal and the rhombohedral structural modifications was found in Er2Fe17C(y)N(x) when y < 1.5, characterized by two different spin reorientation temperatures. The anisotropy fields of Sm2Fe17C(y)N(x) are higher than that of Sm2Fe17N(x). Indications of a magnetic phase transition were found also in Sm2Fe17C0.7N(x) and Sm2Fe17C0.9N(x).

Preparation of highly coercive Sm‐Fe‐Ti by rapid quenching

The magnetic properties of the hard magnetic 20:70:10 (ω) phase in melt‐spun Sm20Fe70+xTi10−x (x=−2,0,2) and Sm25Fe65.6Ti9.4 were investigated. The intrinsic coercivity iHc and the magnetization J were measured as a function of both the melt spinning parameters and various heat treatments. The maximum coercivities iHc at room temperature for the compositions Sm20Fe70Ti10 and Sm25Fe65.6Ti9.4 are 44.5 and 58 kOe, respectively. These values are among the highest reported so far for melt‐spun materials. The formation of Sm(Fe,Ti)12 nuclei during rapid solidification is revealed by x‐ray diffraction, and its influence on the formation of the ω phase is discussed. By investigating the section Smx(Fe0.875Ti0.125)100−x of the ternary phase diagram, using differential thermal analysis and x‐ray diffraction, we proved the ω phase to be metastable.