Magnetic properties and microstructural analysis of rapidly quenched FeNdBGaNb permanent magnets

ELSEVIER Journal of Magnetism and Magnetic Materials 139 (1995) 323-334

~ 4 Journal of magnetism and magnetic

~ l ~ materials

Magnetic properties and microstructural analysis of rapidly quenched FeNdBGaNb permanent magnets

J. Bauer, M. Seeger, H. Kronmiiller * Max-Planck-lnstitut fiir Metallforschung, Institut fiir Physik, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Received 2 May 1994; in revised form 24 July 1994

Abstract

The magnetic properties and the microstructure of rapidly quenched FeNdBGaNb magnets have been investigated. The grain size distribution was varied by changing the quenching rates and applying different annealing treatments. From the initial magnetization curves the fractions of single domain and multidomain particles were determined in differently quenched magnets. In order to optimize the magnetic properties several annealing treatments were carried out. From the temperature dependence of the critical field the microstructural parameters a K und Nee r were determined. These parameters describe the effect of a non-ideal microstructure, i.e. magnetically imperfect grain surfaces and internal stray fields, on the magnetic properties of the magnet. The microstructures of several as-quenched and annealed magnets were investigated using TEM technique.

I. Introduct ion

Technologically important high-energy permanent magnets based on the rare earth intermetallic com- pound FelaNd2 B are usually prepared either by sin- tering [1] or by melt-spinning [2]. Another viable technique is mechanical alloying (e.g. Ref. [3]). The excellent hard magnetic properties of these magnets are mainly due to the high magnetocrystalline anisotropy energy of the tetragonal lattice of the phase Fel4Nd2 B. Nevertheless, the efficiency to which this high anisotropy leads to good permanent magnetic properties depends on the microstructure of the particular material. Besides the size and the shape of the grains the volume fraction and the phase

* Corresponding author. Fax +49-711-689 1932.

compositions of the nonmagnetic boundary phases have to be chosen very carefully to optimize the permanent magnetic properties. Using the melt-spinning technique the microstructural properties, e.g., the grain size, can be varied from the nm to the txm range. Furthermore, a large number of additives have been added to the FeNdB alloys to enhance their permanent magnet characteristics. Small amounts of Nb and Ga were found to increase the coercivity rather effectively. A summary of these results is given in the review of Herbst [4]. The most important role of these additive elements is the formation of new intergranular phases improving the microstructure of the magnets. Bemardi et al. [5] found within the Nd-rich intergranular phase in sintered magnets of the composition Fe74NdasB6GalNbl, which is comparable to the compositions analysed in the present paper, the following phases: two phase regions composed of Nd3(Ga, Fe) and Nds(Ga , Fe)3,

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324 J. Bauer et al. //Journal of Magnetism and Magnetic Materials 139 (1995) 323-334

and the phases Nd6Fel3_xGal+ x [ x = 1 - 2 ] and FeNbB. Furthermore spherical Nb precipitates were found within the hard magnetic Fel4Nd2B grains.

The dominant process for the magnetization reversal is the nucleation of reversed domains [6-9]. Nevertheless, a drastic discrepancy between the theoretical nucleation field H N, described by Brown's equation [10]

2K 1 /*oH N Ms ( N i l - N ± ) J s (1)

(K 1 denotes the first anisotropy constant, Js =/-toMs the spontaneous polarization and Nil , N . the demagnetization factors parallel and perpendicular to the rotational symmetry axis of an ellipsoidal particle) and the experimentally realised coercive fields /z 0 H c are a well known facts [6,7,11]. The experimental results are almost a factor 3-5 smaller than the theoretical predictions. This drastic reduction of H c

known as Brown's paradoxon, demonstrates the dominant role of the microstructure. The experimental results can adequately be described by a slight modification of Brown's Eq. (1) taking into consid- eration the effect of the microstructure on the coercive field [6,11]:

2K1 /~0Hc = a K a ~ - - - N e f f J S . (2)

Ms

The so-called microstructural parameters aK, %

and Nef f a re related to different microstructural effects. Nef f is an average effective demagnetization factor describing the internal stray fields acting on the grains, as: is related to the reduced surface anisotropy of nonperfect grains, a , describes the reduction of the nucleation field due to the misorien- tation of the grains. For grains coupled by exchange or dipolar interactions the expression a , can be

min describing the most unfavourably replaced by a ,

Table 1

Cr i t ica l f ie ld and remanent polar iza t ion va lues o f d i f ferent mel t spun magne t s at T = 300 K

C o m p o s i t i o n v s A n n e a l i n g t rea tment

( m / s ) T a (°C) t a (min)

JR (T)

~LO ncrit (T)

Fe72 NdI7B7.5Gal .5 Nb2 14 - - 0 .62 2.30

14 700 12 0.63 2.48

22 - - 0 .64 2.30

22 700 12 0.67 2.48

30 - - 0.65 2.18

30 700 12 0.61 2.42

Fe72 N d l 7 B 8 . s G a l . s N b l 14 - - 0.61 2.12

22 - - 0 .64 2.12

Fe72 N d l 7 B 6 . s G a l . s N b 3 14 - - 0.57 2.18

22 - - 0.58 2.18

Fe70 Nd19 BT. 5 G a 1.5 Nb2 14 - - 0 .60 2.24

14 700 12 0.58 2.36

22 - - 0 .60 2.24

22 700 12 0.60 2.48

Fe71Nd19B7GaaNb2 14 - - 0 .56 2.24

14 700 12 0.50 2.24

22 - - 0.58 2.18

22 700 12 0.56 2.24

Fe74 N d l s B 6 G a l N b l 14 - - 0 .64 2.00

14 700 12 0.67 2.30

22 - - 0 .72 2.00

22 700 12 0.70 2.30

30 - - 0 .74 2.00

30 700 12 0.63 2.30

J. Bauer et al. /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334 325

aligned grains in the magnet. In sintered magnets domain pattern investigations have shown that ide- ally decoupled grains cannot be realised [12]. There- fore the misaligned grains when reversing its magnetization induce reversion of Js also in the neighbour- ing grains thus leading to cascades of grains with reversed magnetization. So the coercive field is determined by the minimum nucleation field. The expression rain a~ 2 K 1 / M s can be defined as a minimum nucleation f ie ld H ~ in determined only by the intrinsic material parameters K1, K 2 and Js. The expression of H~ in including the second anisotropy constant K 2 has been given in [13]:

H ~ nin -- 2V~-Js K1 + W - --K2 + 3

(3) with

W= + 1 + 8 .

Using the above mentioned definition Eq. (2) yields

/xoHc( r ) = a K/x0H~in(T) -Nef f J s (T ) . (4)

Plotting the experimental data t xoHc/J s vs. the m i n theoretical ones t%H N / J s leads to a straight line

with the slope a K and the intersection ( - )Nef f. The required intrinsic material parameters K], K 2, and Js for the pure Fe14Nd2B phase have been given by Hock [14,15].

2. Experimental procedure

A series of alloys in the system FeNdBGaNb were prepared by arc melting under argon atmosphere from the constituent elements (purity > 99.9%) and a FeB prealloy. The six nominal compositions which we have chosen are summarised in Table 1. The ingots were remelted for at least seven times to achieve good homogeneity. Rapidly quenched ribbons were prepared by conventional melt-spinning onto a rotating CuCrZr wheel (O =

200 mm) in helium atmosphere using surface velocities of 14, 22 and 30 m/s . The annealing treatments were performed in an evacuated quartz tube at annealing temperatures from T a = 600°C up to 740°C. The annealing time was varied between t a = 8 min and 1 h.

The magnetic measurements at room temperature were performed on single ribbon flakes (m = 2 mg) in a SQUID magnetometer (Quantum Design Model MPMS) capable of applied fields up to /x0H = 5.5 T. The magnetic properties between 140 and 580 K were measured on cold compacted cylinders (m = 200 mg) using a vibrating sample magnetometer (PAR 155) with a maximum field of 9 T.

The microstructural characterisation of the melt- spun ribbon flakes was carried out by transmission electron microscopy (TEM).

3. Experimental results and discussion

3.1. Magnetic properties at room temperature

Fig. 1 shows a typical hysteresis loop of a melt spun FeNdBGaNb magnet. The critical field ]d,0ncrit which is marked in Fig. 1 is defined as the maximum susceptibility of the demagnetization curve. The virgin magnetization curve in the first quadrant shows a stepwise magnetization. This S-shape is caused by

Fe72Nd17B7.5 Ga 1.5 Nb2

d

if3

7

' .2...-

- /aoHerft I ~ ~ 'I' ~ I

t I

-5 0 5

/.4,0 H [T]

Fig. 1. Virgin magnetization curve and hysteresis loop of an as-quenched Fe72Nd17B7.sGaI.sNb 2 magnet which is quenched with u s = 22 m/ s .

3 2 6 J. Bauer et al. /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334

the contributions of single domain and multidomain particles to the magnetization [7]. In Section 3.2 these two types of contributions are used to determine the volume fraction of single domain and multidomain particles in the magnet.

As shown in Fig. 1, the critical field of the as-quenched Fe72 Nd17BT.sGa1.sNb2 magnet is rather high and has a value of ~L/,0ncrit -~" 2.30 T. In order to optimize the magnetic properties of the as-quenched sample (v s = 22 m / s ) different annealing treatments were tested. In a first run the annealing temperature (T a) was varied between 600 and 740°C. The annealing time (t a) in this investigation was kept constant (10 min). The resulting / . /~0ncri t is shown in Fig. 2. For annealing temperatures between 680 and 700°C we found a flat maximum of the critical field.

Using the optimized annealing temperature of T a = 700°C the annealing t i m e (t a) was varied between 8 min and 1 h. As shown in Fig. 3 the maximum critical field ~ / , 0 n c r i t = 2.48 T is achieved with an annealing time of 12 min.

In order to investigate the dependence of these optimized annealing conditions (T a = 700°C and t a =

12 min) on the microstructure of the magnet, the same annealing experiments were made for differently quenched magnets (v s = 14, 22 and 30 m/s ) . For all samples the same optimized annealing conditions were determined.

Furthermore, the nominal magnet composition as well as the quenching rate were varied. The results of the magnetic measurements at room temperature

,d-

E ~

1-

¢q

eq

t a = lOmin

I i i i i I i i i i I i i i i

600 650 700 750

annealing temperature [°C]

Fig. 2. Influence of the annealing temperature T a on ~u, oncri t (annealing time ta = 10 min)

t ' N

I - . cq

112

T a = 700°C

k

go , I i i i I

0 20 40 60

annealing t ime [rain]

Fig. 3. Influence of the annealing time t a o n /~0 ncrit (annealing temperature T a = 700°C).

of all samples prepared are summarized in Table 1. The critical field of the as-quenched samples varies between ~ 0 n c r i t = 2.00 and 2.30 T. The optimizing annealing treatment leads to an increase of the critical field of about 0.1-0.3 T. An annealing treatment is found to be not necessary to prepare a highly coercive melt-spun permanent magnet in the case where the additives Ga and Nb are used. It is as- sumed that Nb acts as nucleation centers during the melt-spinning process and leads to a fine-grained, homogeneous microstructure even in the as-quenched state. The remanent polarization JR shown in Table 1 varies between 0.50 and 0.74 T. These values are below the theoretical limit for isotropic FeNdB magnets of JR s° = 0.8 T. This reduction of the remanence is caused by a fraction of about 10-30% of nonmagnetic boundary phases in the magnet. Because of the very fine microstructure the chemical compositions of these boundary phases could not be detected but we suppose a similar boundary phase distribution as in comparable sintered magnets [5] (see Introduction) and some-additional metastable phases. It should be noted that generally those magnets with high Fe concentrations have large values for the remanence and therefore small amounts of boundary phases, i.e. these compositions are closer to that of the pure F%aNd2B phase. Regarding the critical field / , z 0 n c r i t , there is only a weak dependence on the amounts of boundary phases. The composition Fe74NdlsB6GalNb1 with about 10% boundary phases reaches / .Z0nc r i t = 2.30 T whereas the mag-


Fe72Nd17B7.5 Go 1.5Nb2

oo ci

i! o

0 1 2 3 4

U.o H [t]

Fig. 4. Virgin magnetization curve of a Fe72Nd17BT.5Gal.5Nb2 magnet which is quenched with v s = 22 m/s (T a = 700°C, t a = 10 min) in reduced quantities.

nets of the composition Fe70NdlqB7.sGal.sNb2 (30% boundary phases) yield a critical field of /x0Hcrit = 2.48 T. From this result we conclude that in Ga- and Nb-containing melt-spun magnets a relatively small volume fraction of boundary phases of about 10% is sufficient to obtain a nearly perfect magnetic decou- piing between the hard magnetic grains and therefore large critical fields. This fact is beneficial with respect to technical applications, where high values for the remanence and therefore for the energy product (BH)ma x are demanded without losing coercivity.

3.2. Analysis o f the virgin magnetization curve and microstructural investigations

In Fig. 4 the virgin magnetization curve is plotted in reduced quantities. The contribution of the reversible polarization due to reversible rotations out of the easy axis is subtracted from the initial magne-

tization curve. This reversible polarization is determined from the recoil curve in the first quadrant. Additionally the reduced polarization is normalized to the saturation value of the polarization. The S- shape of the initial magnetization curve is caused by the contributions of single domain and multidomain particles in the melt-spun ribbon flakes. The first step in the low field range is due to Bloch wall displacements in multidomain particles whereas the second step is caused by the reversal of the magnetization of single domain particles by a nucleation process [7]. From the height of the first step the volume fraction of multidomain particles can be determined. The volume fraction of multidomain particles determined from the initial magnetization curve in Fig. 4 is 40%. The results of the volume fraction of multidomain particles for different quenched magnets are summarized in Table 2.

A decrease of the volume fraction of multidomain particles is found with increasing quenching rates due to a reduction of the average grain size in the magnet. A comparison between the data of the as- quenched samples and the heat treated ones (T a = 700°C, q = 12 min) shows an increase of the average grain size with the annealing process due to a grain growth process by an annealing temperature above the crystallization temperature (Tcryst ~ 590°C for F%aNd2B [16]).

In order to check these results the microstructure of the ribbon flakes was investigated using the transmission electron microscopy (TEM). Figs. 5 - 7 show the microstructure of differently quenched magnets. In the ribbons which are quenched with u s = 14 m / s we found different regions where the grain size varied between ~ 50 and 400 nm (Figs. 5a, b). The ribbons quenched with surface velocities of u s = 22 m / s also show different regions with a grain size distribution between ~ 20 and 200 nm (Figs. 6a, b). In the ribbons which are quenched with the highest

Table 2 Volume fraction of multidomain particles and average grain sizes of differently quenched magnets at T = 300 K

Composition Wheel speed v s multidomain particles (%) average grain size (nm)

(m/s) as-quenched 700°C, 12 min as-quenched 700°C, 12 min

Fe72 Nd17B7.sGal.sNb2 14 44 47 50-400 50-650 22 29 40 20-200 - 30 0 27 20 20-50

328 3". Bauer et aL /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334

Fig. 5. TEM micrographs of as-quenched Fe72 Ndl7B7.sGal.sNb2 magnets quenched with u s = 14 m / s .

wheel speed (u s = 30 m / s ) we found a very fine grain structure where the mean grain size is approximately 20 nm (Fig. 7). The results for the average grain size are summarized in Table 2. As an important result we may conclude from the values of

/,.£0ncrit shown in Table 1 that the room temperature critical field is nearly independent of the grain size in the range 10 n m - 1 /zm. This behaviour is pre- sented in Fig. 8.

The TEM investigation of the microstructure

J. Bauer et aL /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334 329

shows qualitatively the same dependence of the mean grain size and the quenching rate as the result of the initial magnetization curve. Furthermore, the microstructure of the ribbons quenched with u s - - 3 0 m / s is so fine that all grains are smaller than the

critical size of single domain particles [7]. This result is also found in the analysis of the virgin magnetization curve (see Table 2). Therefore a good agreement between the TEM investigation and the evaluation of the virgin magnetization curve has been found.

Fig. 6. TEM micrographs of as-quenched Fe72NdI7B7.sGal.sNb2 magnets quenched with u s = 22 m/s.

330 J. Bauer et al. /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334

Fig. 7. TEM micrograph of an as-quenched Fe72Nd17B7.5Gal.5Nb2 magnet quenched with v s = 30 m/s .

3.3. Temperature dependence of Hcrit and microstructural parameters a r and Nef f

Fig. 9 shows the temperature dependence of ncrit

for a series of magnets with different quenching rates. In all cases the critical field decreases monotonously with increasing temperature and van- ishes at the Curie temperature. The critical fields for the differently quenched magnets possess nearly the same values over the whole temperature range, so

2

~l,,ll,,,, ITI

I

10' 102 lb+ " d r°ml

Fig. 8. Influence of the grain size on the room temperature critical field ~/,0 ncrit .

that no dependence of the critical fields on the quenching rate and therefore on the average grain size in the magnets is found.

The effect of the annealing treatment on the temperature dependence of ]£0ncrit is demonstrated in Fig. 10. An improvement of [/,0ncrit is observed at temperatures below 400 K. This improvement is approximately 1 T at a temperature of 140 K and decreases monotonously with increasing temperature.

Fe72Nd t 7B7.5 G° 1.5Nb2

to . . . . i . . . . i . . . . i . . . . i . . . .

v , = 14 m / s

- * - * " v , = 22 r n / s

100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0

m [K]

Fig. 9. Temperature dependence of the critical field for differently quenched magnets.

J. Bauer et al. /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334 3 3 1

Fe72Nd17B7 .S GO t.sNb2

~o . . . . i . . . . i . . . . i . . . . i . . . .

\~. -~ -~ ~ a s - q u e n c h e d \ ~ - a - . - annealed 700°C, 12min

o ' • ' 1 O0 2 0 0 3 0 0 4 0 0 5 0 0 600

t l K]

Fig. 10. Influence of the annealing t r e a t m e n t ( T a = 7 0 0 ° C , t a = 10 min) on the temperature dependence of iu,0ncrit.

-r

Fe72Nd17B7.sGol.sNb2

• i • - / / z / I

.. : .. 14rn//s: C(K=0.91 Neff.=0,80

a-* -~- 22rn//s: C(K=0.88 Neff.=0.78

-O-C-e- 30 rn /s : C(K=0.89 Neff.=0.77

, I , I , 2 4 0

H ~"/J /~ON S

Fig. 11. IJ, oHct i t /Js versus goH~'n/Js to determine a K and Nef f for differently quenched magnets.

For temperatures above 400 K the critical fields of the as-quenched and of the annealed samples are nearly the same. Furthermore, the value of /x 0 ncrit at the higher temperature of about 450 K is nearly 1 T. This result is important for technical high-temperature applications. Therefore a high coercive permanent magnet can be produced without an expensive heat treatment using the additives Ga and Nb.

From the experimental result of the temperature dependence of Befit the microstructural parameters ct K and Nef f can be determined plotting the experimental results,/x 0 ncr i t / / J s , vs . the theoretical quantities, min /z0HN /Js" This plot should yield a straight line with the slope a K and the intersection Nef f. Fig. 11 shows these analyses for differently quenched magnets. A nearly linear behaviour is found in the temperature range 200 K < T < 560 K. In the low temperature range T < 200 K deviations from the straight line are observed. The same deviations are found in all sintered and melt-spun FeNdB based magnets [17-19]. The reason for these deviations is the decrease of the anisotropy constant K 1 and the increase of K 2 exceeding K 1 for T < 190 K [14,15]. In this temperature range K 2 is the dominant contribution for the anisotropy in Fe14Nd2B and therefore the expressions for tx0H c explained in Section 1 should be modified. This suggestion is supported by the fact that these deviations do not appear in FePrB magnets where K1 > K 2 holds in the whole temperature range [17].

The microstructural parameters a K and Noff determined from the slope and the intersection of the balance straight lines (see Fig. 10) have similar values for the different magnets within reasonable error bars. With respect to the microstructure this results means that the quenching rate has neither an influence on the grain quality nor on their shape and therefore on the demagnetization effects. It should be noted that the mean grain size shown in Table 2 decreases with increasing quenching rate from d 50-400 nm to d = 20 nm, and accordingly the grain size has no remarkable influence on the magnetic properties (see Fig. 9) but neither on the microstructural parameters c~ K and Nef f.

Fe72Nd17B7.5Go 1.5Nb2

/ "1- +

J

: ~" ; as -quenched aK=0.91 Neff.=0.80

- e . 4 - t - annealed 700=C, 12min =K=1.02 Neff = 0 . 9 5

I I T O 2 4

# o H . ~ ' / d s

F i g . 12 . I n f l u e n c e o f t h e a n n e a l i n g t r e a t m e n t ( T a = 7 0 0 ° C , t a = 1 2

min) on a K and Nef f.

3 3 2 J. Bauer et al. /Journal of Magnetism and Magnetic Materials 139 (1995) 323-334

The influence of the annealing treatment on the

microstructural parameters a K and Nef f is shown in mi n Fig. 12 where the ~0ncr i t / Js vs. ~0HN / J s plot

shows an increase of a K and Nef f during the anneal-

ing treatment. The increase of a K means an improvement of the grain surfaces. Accordingly the grains become more perfect. Furthermore, the raise of Nef f indicates stronger demagnetization effects

Fig. 13. Comparison of the microstructure of (a) as-quenched and (b) annealed (T a = 700°C, t a = 12 min) F e 7 2 N d 1 7 B 7 . 5 G a l . 5 N b 2 ribbon flakes quenched with ~s = 30 m/s.


acting on the grains. These increased effects are due to the formation of edges and corners during the grain growing process which takes place preferably along the low indices crystallographic directions. Therefore the better surface quality produced with the annealing treatment is correlated to a worse shape of the grains. It should be noted that a K exceeds the ideal value of a K = 1, which indicates

min has lead to an that the assumption of a,~ = a , underestimation of a , .

In order to check these results, the microstructure of the as-quenched and annealed samples are investigated with the TEM technique. Figs. 13a,b shows the comparison of an as-quenched and an annealed (T a = 700°C and t a = 12 min) ribbon flake. The microstructure of these two samples are completely different. The shape of the grains which are found in the as-quenched sample (see Fig. 13a) are spherical in contrast to the elongated grains in the annealed sample (see Fig. 13b). This investigation shows that the grains grow along fixed cristallographic directions so that the grains develop more edges and comers correlated with stronger demagnetization effects due to a higher Nef f value. To summarize we have found a good agreement between the results of the magnetic measurements and the microstructural investigations.

4. Summary

Magnetic and microstructural investigations of rapidly solidified FeNdBGaNb permanent magnets have been carried out. The nominal magnet composition as well as the quenching rate was varied in order to optimize the magnetic properties. Furthermore, different annealing treatments were applied to im- prove the magnetic properties after the quenching process. From these investigations we obtain the following results:

• An optimum annealing treatment with respect to the critical field ]Zoncrit is found with T a = 700°C and t a = 12 min. Using these annealing conditions the critical field at T = 300 K increases from /%ncrit = 2.30 T up to /L/,0ncrit = 2.48 T. Nevertheless, an expensive annealing treatment is not necessary to produce a high coercive permanent magnet using the additives Ga and Nb.

• There is only a slight dependence of ]z0ncr i t on the volume fraction of the boundary phases. A variation of the fraction of boundary phases between approximately 10-30% increases the critical field from ]Z0ncrit = 2.30 T up to ~ 0 n c r i t = 2.48 T, an improvement of about 7%.

• The average grain size in the magnets decreases with increasing quenching rate.

• The quenching rate has no remarkable influence on the temperature dependence of the critical field.

• The coercive field is not influenced signifi- cantly within the range of grain sizes between 10 nm-1 /zm.

• The optimum annealing treatment leads to an improvement of the critical field in the temperature range T < 400 K. This improvement is approximately 1 T at a temperature of 140 K and decreases monotonously with increasing temperature.

• At the high temperature of 450 K the critical field has a value of nearly 1 T.

• The microstructural parameters a K and Nef f are nearly independent of the quenching rate. This result means that the quenching rate has neither an influence on the grain quality nor on their shape.

• Both microstructural parameters a K and Nef f increase during the annealing treatment.

• The shape of the grains in the as-quenched sample are spherical in contrast to the elongated grains appearing in the annealed sample. This result explains the increase of Nef f during the annealing treatment.

Acknowledgements

The authors wish to thank Dr. D. K6hler, Dr. R. Reisser and Dipl. Phys. G. Rieger for many fruitful discussions and to M. Rapp for performing the TEM investigations. This work was supported by the Com- mission of the European Community within the BRITE/EURAM programme.

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Magnetic properties and microstructural analysis of rapidly quenched FeNdBGaNb permanent magnets