Journal of Magnetism and Magnetic Materials 83 (1990) 251-253 North-Holland

251

LARGE ANISOTROPY FIELDS IN HEXAGONAL R,_ xT17+2x

Hong-Shuo LI, Bo-Ping HU, J.M.D. COEY, J.P. GAVIGAN I, G. MARUSI 2, L. PARETI 2, G. CZJZEK 3 and R. KMIEC 3

Physrcs Department, Trinity College, Dublin 2, Ireland

A series of alloys with the hexagonal Th,Ni,,-structure (Pr,,, Sm,,,M,,,)(Fe,,_ Co,,) where M = (Zr + Fe) or (Ti + Fe),

have Curie temperatures 2 700 K when y 2 5. The excess iron is located on the 4e dumbell site. The average iron hyperfine

field is about 31.5 T. All alloys have an easy c-axis with a room temperature anisotropy field in the range 3-4 T. The sign of

A,, is opposite for the 2b and 2d rare-earth sites: A,,(2b) = (-170*20) KaG2 and A,,(2d) = (+100+20) Kac2, and the

anisotropy is attributed to some appropriate preferential occuption of the two sites by rare-earths with opposite sign of the

second-order Stevens coefficient (Ye.

1. Introduction

The discovery of Nd,Fe,,B by Sagawa et al. [l] in 1983 focussed permanent magnet research on this series of compounds. Nevertheless, the better thermal stability of Sm-Co based permanent magnets is still advanta- geous for some practical applications. Energy products as high as 500 K.lmm3 may, in principle, be achieved in

(PrSm),(FeCo)r, compounds [2,3], but the necessary coercivity has not yet been observed.

In our earlier analysis, we showed that appropriate rare-earth site-preferences are, in fact, obtained in

(R’R”),(FeCo),, compounds with the hexagonal Th,Ni,, structure, where there are two inequivalent rare-earth crystallographic sites [4]. The larger rare-earth ions occupy 2d sites and the smaller ones occupy 2b sites, with positive and negative A,,, respectively [4-61. When the rare-earth ions at 2d sites have a negative second-order Stevens coefficient LYE (Pr3+, Nd3+, Tb3+, Dy3+ and Ho3’) and those at 2b sites have a positive (Ye (Sm3’, Er3+, Tm3’ and Yb3+), the contribution to the uniaxial anisotropy field due to the rare-earths reinforce eachother. It is interesting to seek hexagonal- Th,Ni,, compounds containing two light rare-earths having opposite sign of (Ye, e.g. (Pr, Sm) or (Nd, Sm), in order to maximize simultaneously the uniaxial ani- sotropy field and the bulk magnetization. A difficulty arises because light rare-earths tend to crystallize in the rhombohedral Th,Zn,,-structure, where there is only one rare-earth site. However, the Zr and Hf substitution for the transition metal help to stabilize the hexagonal structure, while Ti and V substitution leads to a dis- ordered Th,Zn,,-structure (TbCu,-type [7]) [2,8,9].

’ Laboratoire Louis N&l, CNRS, 38042 Grenoble, France.

* Istituto MASPEC de1 CNR, 43100 Parma, Italy.

s Kernforschungszentrum Karlsruhe, D-7500 Karlsruhe 1, Fed. Rep. Germany.

0304-8853/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)

Furthermore, Zr-substitution leads to a surprising in- crease in anisotropy fields [2,10,11]. An excess of transi- tion metal, where the rare-earths at 2b sites are partially replaced by transition-metal dumbells on 4e site, also makes the hexagonal structure more favourable [12,13].

Here, we present the magnetic properties of alloys with the general formula of (Pr,,,Sm,,,M,.,)(Fe,,_,- Coy), where M = (Zr + Fe) or (Ti + Fe) and y 2 5. Large uniaxial anisotropy fields and high magnetizations have been observed in these alloys.

2. Results and discussion

Three alloys: I) (Pr,,Sm,_,Zr,,Fe,,)(Fe,Co,), II)

(PrO.sSmo.~Zro.~Feo.,)(Fe&o,) and III) (Pr,.sSma.,- Ti,,,Fe,,,)(Fe,Co,), were prepared by arc-melting 99.9% pure metals. Powder X-ray diffraction patterns showed that the Zr-containing compounds crystallize in the pure hexagonal Th,Ni,,-structure while the Ti-contain- ing compound has a TbCu,-type structure, in accor- dance with previous results observed by Satyanarayana

r T

6-

I-

m”

4

I

T(K)

Fig. 1. Anisotropy fields measured by SPD as function of

temperature.

252 H. -S. Li et al. / Large anlsotropy fields in R J , T, , + ?,

Table 1 Magnetic properties of three alloys. (BH),,, is theoretical value

Com- (I c r, !-%M, B* (Bhr) K, (BH),,, pound (A) (A) (K) (T) (T) (T) (MJm ‘) (kJm 3 )

I 8.450 8.289 950 1.60 2.96 32.11 1.88 508 II 8.454 8.304 698 1.58 4.10 30.80 2.58 497 III 8.487 8.286 930 1.57 3.92 31.53 2.45 492

et al. [9]. The lattice parameters obtained are given in table 1.

The Curie temperatures were measured thermomag- netically and the anisotropy fields were measured, in the temperature range 77-300 K, by the singular point detection technique (SPD) in pulsed magnetic fields in Parma. Thermomagnetic analysis showed that all three samples are magnetically single phase, with the values of Curie temperature listed in the table. Anisotropy fields measured by SPD as function of temperature are plotted in fig. 1; the alloys all have an easy c-axis anisotropy and values at room temperature are also included in the table. The room temperature magnetiza- tion curves were measured on random polycrystalline

I I I I I I 1 1 I

-8 -4 0 4 a

V Imm/sl

Fig. 2. s’Fe MGssbauer spectra at room temperature for the three alloys with the n-Fe spectrum, for comparison. Vertical

bars represent 1% absorption. The arrow marks line 6 of the 4e site spectrum.

samples in fields up to 7 T in the Laboratory Louis NCel, Grenoble. The values obtained for the saturation magnetization, together with the corresponding K, (=

p,,M,H,/2) values are given in the table. The room temperature 57Fe Mossbauer spectra with

theoretical fits for three samples are sketched in fig. 2. Average iron hyperfine fields are in the range of 30.8-32.1 T, corresponding to an iron moment of 2.05-2.14~~. The subspectrum with the largest hyper- fine field of = 35.8 T, increases in intensity when the iron content is increased, and is attributed to the transi-

tion-metal dumbells. A quantitative estimation of the second-order crystal

field coefficient A 2,, , was made from a ls5Gd Mossbauer spectrum taken on a polycrystalline sample of (Gd,,,,- Lu,,,,Fe,,)(Fe,Co,) at 4.2 K at the Kernforschungs- zentrum, Karlsruhe. The spectrum and the theoretical fit are shown in fig. 3, results are: A,,(2b) = (- 170 i 20) Ku,’ and A,,(2d) = (+lOO + 20) Ka;‘. The large anisotropy fields can now be understood in terms of appropriate preferential occupation of two sites by rare-earths having opposite sign of (Ye: Pr ( OL,, < 0) and Sm (aJ > 0). Recently. Chen et al. [14] have also shown independently that Sm substitution for Pr in Pr,- (Fe’,_,Co,) increases the uniaxial anisotropy field.

Fig. 3. “‘Gd Miissbauer spectrum for (Gd,,Lu, (,Fe,,)(Fe,- Co,) at 4.2 K, fitted by assuming two rare-earth sites with different quadrupolar interactions; the resulting parameters

are: AQ(2b) = (+0.22&0.02) mm/s and Ao(2d) = (-0.13+

0.03) mm/s, (Bhf) = (23.4k0.2) T, u( B,,) = (4.4kO.2) T,

6,s = (0.226 + 0.004) mm/s and x2 = 1.03. The vertical bar represents 1% absorption.

H. 4. Li et al. / Lurge anisotropy fielak in R J _ x T, , + 2x 253

The difference in the room temperature anisotropy field of different iron compositions of Zr-containing compounds (see table 1) indicates that as the transition metal composition in R,(Fe,,_,Co,) passes from cobalt-rich (y = 17) to iron-rich (y = 0), the difference of A,, between 2b and 2d sites of hexagonal structure increases, on the other hand, the higher anisotropy field observed in the Ti-containing compound compared with the corresponding Zr-containing compound cannot be attributed to the marginally smaller magnetization; it

may be because the TbCu,-type structure favours stronger site-preferential occupation for different size of

rare-earth ions.

3. Conclusion

We have stablized hexagonal Th,Ni,,-type of (Pr,,Sm,,,M,,s)(Fe,,_,Co,) compounds by using Zr or Ti substitutions for transition-metal and an excess of transition-metal. Large uniaxial anisotropy fields, = 4 T and high magnetizations, = 1.6 T were observed. The theoretical energy products (HI),, are about 500 kJme3, similar to that of Nd,Fe,,B [15], while Curie points are higher due to the presence of cobalt. To make useful hard magnets, the coercivity needs to be greater than 1.0 T, or about one quarter of the largest ani- sotropy field we have yet observed in this series.

This work forms part of the Concerted European Action on Magnets’, a project supported by the Com- mission of the European Communities.

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