Materials Letters North-Holland
13 ( 1992) 32 l-324
of the intermetallic
A. Handstein Institut,fir Festkdrperforschung im Institut fir Festkiirprrphyslk Helmholtzstrasse 20. O-8027 Dresden, Germany
and G. Martinek
Instttut,ftir Phvsik irn Max-Planck-lnstitut Received
I. I+‘-7000 Stuttgart
The nitrogenation of nearly single-phase SmzFe ,, results in an interstitial intermetallic compound of SmzFe,,N,. The time dependence of mass gain is logarithmic. The investigation of the domain pattern reveals a sharp transition from the Sm*Fe,, to the SmZFe,,N, phase within bulk material and larger particles, respectively. The initial magnetization curves of virgin and dcdemagnetized samples are not very different. This points to a different demagnetization behaviour as compared to Nd-Fe-B magnets.
manent magnet powders.
1. Introduction The search for novel permanent magnetic materials got a large impulse by the discovery of the beneficial influence of nitrogenation and/or carbonization of rare earth-transition metal compounds of typeslikeR,TM,,,R(Fe,TM),,orR,(Fe,TM),,B. The RzFe17 series of compounds can irreversibly absorb large amounts of nitrogen. This results in a new series of R2FeI 7Nx interstitial intermetallic compounds (x< 3). This process, by which the intrinsic magnetic properties of iron-rich alloys with Th,Ni,, or ThzZr,, structures can be vastly improved has been djscovered by Coey et al. [ 1,2]. For the SmzFe,, compound the nitrogenation process results in a large increase of lattice parameters, which causes an enhancement of the Curie temperature of about 365 K and of the spontaneous magnetization [ 3,4]. In addition, an easy axis anisotropy is formed with an anisotropy field approximately twice as large as that of NdzFe14B [ 51. Because of these properties the SmzFe,,N, compound has a good chance for permanent magnet application. In this Letter we report on the nitrogenation process of SmzFe,, and on changes in metallographic structure and domain patterns as well as on the per0167-577x/92/$
05.00 0 1992 Elsevier Science Publishers
2. Experimental The Sm,Fe,, compound was melted from elemental Fe and an Fe-50 wt% Sm master alloy in an induction furnace under argon atmosphere using an Al,O, crucible. The investigated alloy was melted in the induction furnace twice. The cast ingots were homogenized at 1000” C for one week in a sealed quartz glass ampoule filled with argon gas. The density of material was 7.54 g/cm3. The composition of material was checked at different processing steps by EDX using a dual stage scanning electron microscope ISI-DS130. For calibration we used the SmzFe,, phase inside the sample. It was found that the samples were nearly single phase with only a very small content of a-Fe. Its composition is about Sm, , zFess.zAlo.h. The ingots were coarsely ground by a mortar. This powder was milled by a vibration ball mill to obtain finer grain fractions. For nitrogenation we used only particle sizes smaller than 20 pm. The powder was annealed in a nitrogen atmosphere at 480°C. During this process the nitrogen
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gas pressure was measured for determination of the quantity of the absorbed nitrogen. In addition, we checked the nitrogen content of the powder by weight gain and by the heat extraction method (LECO 436 device). To investigate the changes in metallographic and magnetic domain structure, respectively, we also annealed bulk Sm2Fe,, material in a nitrogen atmosphere at 480’ C. For magnetic domain observation by the Kerr effect, a polarization microscope POLYVAR (Reichert-Jung) was used. The surfaces of polished bulk samples and the nitrided powders fixed with epoxy resin, respectively, were evaporated with a ZnS film to improve the magnetic contrast. For magnetic measurements the powder was cold compacted by different devices for isotropic samples (px 1590 MPa) and for slightly anisotropic samples by pressing perpendicular to a magnetic field of = 0.5 T (p= 530 MPa). Magnetization measurements were carried out in a vibrating sample magnetometer up to poH= 5 T at room temperature. The density of the samples was determined from the geometric volume and the mass for calculation of polarization J (see table 1 below).
Fig. 1.Time dependence of mass gain of Sm2Fe,, powder the nitrogenation process.
3. Results and discussion During the nitrogenation process the change in the nitrogen pressure was measured for an estimation of the mass gain Am, of the powder. A logarithmic time dependence for Am, was found (fig. 1). We assume that this behaviour describes approximately the diffusion of nitrogen into nearly spherical particles (the arrows at the two curves in fig. 1 mark the achievement of the reaction temperature 480°C). According to the investigations of Coey et al. [ 21, the diffusion kinetics of nitrogen is sluggish. Using their results we estimated the diffusion constant 0,=7.34x lo-i6 m2/s for this thermally activated process at 480°C. A rough estimation of the penetration depth according to x2 = 2tD, yields about 5.4 urn after a reaction time of 5.5 h. This means that the nitrogen atoms penetrate only about 1 urn/h at 480°C into the particles. The domain structure of partially nitrided Sm,Fe,, particles reveals this behaviour (fig. 2, particle A). The penetration depth of nitrogen for these particles is comparable with the 322
Fig. 2. Domain pattern of Sm,Fe,,N, a Sm*Fe,, core and B - polycrystalline
powder: A - particle particle.
value estimated above. The Sm*Fe,, core of particles has no domains or a coarser domain structure. The calculated mass increase of 3.3 wt% from fig. 1 could be confirmed by the weight gain of the powder and by the heat extraction method not only in the case considered but also for other runs and at lower temperatures. By means of X-ray investigations we could not observe additional reflections besides the expanded phase. Therefore we assume a nominal formula of Sm,Fe,,N-,. The observed nitrogen content corresponding to x=3 is higher than those found by other authors: x=2 [ 51, 2.3  or 2.7 [ 61. This fact is probably connected with the very small portion of a-Fe in our SmzFe,, alloy. Other phases could not be found by SEM and X-ray diffraction. For Sm,Fe,,N, an increase in the unit cell volume of about 6.2% was found due to the interstitial nitrogen atoms [ 1 1. The increasing volume causes mechanical stresses inside the material. Because of the brittleness of the homogenized Sm,Fe,, compound the metallographic structure of bulk material is destroyed due to nitrogenation. Fig. 3a shows a sharp transition from SmzFe,, (A) to Sm,Fe,,N, (B). The Sm*Fe,, phase has only a coarse domain structure. The domain pattern of the Sm*Fe,,N, phase is only visible at higher magnifications. Figs. 3b and 3c show domains of grains, the easy axes of which are oriented nearly parallel and perpendicular to the plane, respectively. These pictures show that the transition between the two phases happens across a small region of the material. This observation confirms that there is no large amount of the Sm,Fe,,N, phase with xz 1 to 2. Magnetization curves of cold-compacted samples made from the same powder are shown in figs. 4a and 4b for an isotropic and anisotropic magnet respectively. In table 1 the magnetic properties are compared. Although the coercivities are small compared to the huge anisotropy field of SmzFe,,N3 their values are larger than for other cold-compacted samples (cf ref. [ 2 ] ), probably due to a smaller fraction of a-Fe. Besides the outstanding results of mechanically alloyed Sm,Fe,,N, powder magnets [ 71, recently metal-bonded Sm,Fe,,Nx magnets with higher coercivities up to ,u~JY,= 1.7 T could be prepared by the conventional route using Zn powder and an additional heat treatment [ 8,9].
Fig. 3. Structural and domain pattern of bulk material in the transition region from SmZFe,, (A) to SmZFe,,N1 (B): (a) overview; (b) grains oriented parallel and (c) perpendicular to the plane.
susceptibility (fig. 4b). As shown in fig. 2 (particle B), there are also polycrystalline particles in the powder, which reduce the texture and the values of magnetization and remanence.
The authors are grateful to Mrs. T. Dragon and M. Kelsch for domain pattern and SEM observation, respectively, as well as to Dr. K.-H. Mtiller for stimulating discussions. One of the authors (AH) would like to thank the Max-Planck-Gesellschaft for financial support. Fig. 4. Magnetization curves of cold-compacted ples: (a) isotropic and (b) anisotropic sample.
References Table 1 Characteristic
values of cold-compacted
The intial curve of the virgin sample (th) in fig. 4a shows that any existing domain walls can move easily within a particle. The initial curve of the virgin sample lies very close to that of the dc-demagnetized one. Therefore the demagnetization process is not controlled by nucleation of easily movable domain walls [ lo]. At this point there are differences compared to the initial magnetization behaviour of sintered or melt-spun Nd-Fe-B magnets [ 111. The difference in remanence B, of these two samples is not as high as usual between isotropic and anisotropic magnets. This shows the poorly developed texture of the anisotropic sample. This fact is also proved by the relatively large value of the high-field
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