Magnetic properties of R2Fe14B hydrides

VALVES EXHAUST GAS

Appendix 2 Appendix 2

3. Magnetic properties of RzFel4B compounds

3.5. Magnetic properties of R2Fe14B hydrides

106 E. BURZO and H.R KIRCHMAYR

The paramagnetic behaviour of other systems has been also studied. In case of ferrimagnetic Gd2Fe14_xAlxB (Burzo et al. 1986a) and Gd2_xYxFe14B (Burzo 1985a) compounds, the reciprocal susceptibilities follow a non-linear temperature dependence described by the relation (2). In Y2Fe14_xTxB ( T = Co, Si, Cu) compounds Curie-Weiss dependence for X values are found (Burzo et al. 1985c, Pedziwiatr et al. 1987a).

Considering a two sublattice ferrimagnet and using the molecular field approxi- mation, the mean values of the exchange interactions inside and between magnetic sublattices were determined (Burzo et al. 1985a). The exchange interac- tions between lanthanides as well as those between iron and the lanthanide are smaller than those inside the iron sublattice. The exchange fields, Hexch , acting on the lathanides and iron were estimated. For example the exchange field acting on erbium in Er2Fe14B is 2/~BHe×ch = 250 K (Pedziwiatr et al. 1987b) and on Gd in Gd2Fe14B of 370 K (Bog6 et al. 1985). In Nd2Fe14B we have 2~Hex~h = 385 K (Givord et al. 1986b). These values are close to that obtained in GdFe2, 2/XBHexch = 300 K (Burzo 1975), suggesting that the mechanism involving mag- netic interactions in ternary R - F e - B compounds is not significantly modified from that in the binary lanthanide-transition metal systems. The exchange fields acting on the lanthanides are of the same order of magnitude as the overall splitting of the multiplets (Givord et al. 1986b). This suggests that the lanthanide moments are not significantly influenced by the crystal field effects. The exchange field acting on the iron sublattice is substantially greater, 2/~Hexch --- 1.5 X 103 K.

Consequently, the iron magnetization as function of temperature dimin- ishes more slowly than that of the lanthanide.

From the effective iron moments, the mean spin values, Sp, were computed according to the relation M2ff(Fe)= 2 gFeSp(Sp +

1)

(Burzo et al. 1985d). The Sv values thus determined are listed in table 6. These are somewhat greater than the spin values, So, obtained from the mean saturation iron moments.

A measure of the localization of iron moments is given by the ratio r =

Sp/So,

between the number of spins deduced from the Curie constants and those obtained from saturation data (Burzo 1978a). In case of Fe, Co and Ni metals, the r values are 1.05, 1.34 and 1.46, respectively. The r values for iron moments in R2FeI4B compounds are around 1.22 (table 6). These data do not suggest a great degree of itineracy.

PHYSICAL PROPERTIES OF R2Fe~4B-BASED ALLOYS 107 substantially reduces the anisotropy contribution of the Nd sublattice. The spin reorientation temperature is shifted at lower values. The occupation numbers of certain Fe sites deviate from those found in the parent compound. This together with the selective oxidation of Nd may cause the formation of free metallic iron.

L'Heritier and Fruchart (1985) and L'Heritier et al. (1985) studied also the magnetic properties of R2Fe14BH x compounds with R = Y , Lu and Dy. The presence of hydrogen increases the lattice parameters, Curie temperatures and the magnetizations. Ferreira et al. (1985) and Fruchart et al. (1984) studied by magnetization measurements and M6ssbauer effect on 57Fe and 16IDy, the Dy2Fe~4BH x system with 0~<x~<4.7. The 57Fe spectra in DyFe~4BH3. 3 and ErFeaaBH2.6 reveal both a significant line broadening and an increase in the average field compared with the original materials (Friedt et al. 1986a). The saturation hyperfine field Hhf is increased by --2% in the hydrides as compared with the original materials. The significantly enhanced Hhf observed in the hydrides at room temperature by reference to the original alloys arises primarily from the higher ordering temperatures. The local Fe monlents depend sensitively on the Fe coordination number, on the nature of R element and on the presence of hydrogen. This is connected with the changes of the magnetic exchanges coupling, which depend on the F e - F e interatomic distances. The enhanced ordering temperature of the hydride probably arises from the lattice expansion, i.e. decreased negative exchange interactions. The structure and magnetic prop- erties of NdzFe14BH2. 7 were investigated by Oesterreicher and Oesterreicher (1984a). The hydrogen decrepitation of an NdlsFe77B 8 magnetic alloy was reported by Harris et al. (1985).

4. Rare earth-iron-boron magnets 4.1. Sintered R - F e - B magnets

The N d - F e - B type magnets have been obtained since 1983, by a procedure similar to that used in manufacturing rare-earth-cobalt ones (Sagawa et al. 1984a).

The production of the magnets involves certain steps (Ormerod 1985). After casting, the samples are crushed under a nitrogen atmosphere prior to the final milling, that is when N d - F e - B alloys involve dimensions smaller than 500/xm.

The alloys prepared by calciothermic method are generally of particle size suitable for direct milling. The fine powder may be obtained by ball milling in on organic liquid (cyclohexane, toluene, freon), under an inert atmosphere, the powder being then dried under vacuum, or by heating under argon. Another frequently used method is jet milling. The milling conditions are selected in such a way to give the required particle size distribution and at the same time to pick-up the smallest possible quantity of oxygen.

The effect of milling time in the vibration ball in the presence of freon, on the oxygen pick-up, is given in fig. 20a and in fig. 20b, the correlation between the milling time and particle dimensions is shown (Ormerod 1984, 1985). The oxygen

108 E. B U R Z O and H.R. KIRCHMAYR

1.0

°-e0. 6 o 0.2 ,,

0 1 2 3 4

MiEting t i m e ( h ) (a

5.0 - 4 . 0

. o

~ 3.0

5a2

^2.0

13. I I I I --

1 2 3 4

Mi{ting time (h) (b)

Fig. 20. Effect of milling time during vibration ball-milling in freon on the oxygen pick-up (a) and on the particle size of N d - F e - B and Sm(Co, Fe, Cu, Zr)~ alloys (b).

pick-up in N d - F e - B alloys is higher than in S m - C o magnets, while the size reduction is more pronounced than in S m - C o alloys.

The powder compaction is made by die-pressing or by isostatic pressing. In the first case the external field necessary for alignment is set up in the cavity of a non-magnetic die with its axis lying either in the direction of pressing or perpendicular to it. The pressure used for compacting is thus selected to ensure to a powder compact with sufficient mechanical strength to be handled, but not. so high to lead to misorientation of the particles. Isostatic pressing is normally carried out on powders prealigned in a pulsed magnetic field, 3 - 4 time greater than used in uniaxial pressing. This improves the degree of particle alignment which is maintained during the isostatic pressing.

The sintering of N d - F e - B magnets is performed in vacuum or in inert atmosphere. Commonly, the density is greater than 95% from theoretical value.

The dependence of magnetic properties on the sintering temperature has been analysed by Sagawa et al. (1984a). The optimum magnetic properties may be obtained for a large range of sintering temperatures (fig. 21). The coercivity of N d - F e - B magnets further increases after a post sintering thermal treatment, which may include several different steps at temperatures between 900°C and 600°C and different cooling rates in between. The magnets are e.g. kept at 630°C for 1 h. Following Sagawa et al. (1984a), the enhancement of the coercivity is due

PHYSICAL P R O P E R T I E S OF R~Fe14B-BASED AI,LOYS 109

200~ NdlsFe77B s

150 , i I I i

1.2 =

a~ 1.0

0.8 L I L~_ I

12001 l

_I< lOOO I - - - = ,

80 0~- - " - " - - -" -- --,,,

6001- L .... , , ,

~ " - 7.2 _{C r2~}

c~ ~ 7. 01300 1340

L _ _ I I

1380 1420

Sintering temperoture (K)

Fig. 21. The influence of the sinter- ing temperatures on the magnetic properties of Nd15Fe77Ba magnets.

to the removal of damaged particle surfaces by the action of a grain boundary liquid phase. Stadelmaier et al. (1983) suggest that the liquid phase, during sintering, inhibits grain growth and maintains or even reduces the milled particle size.

Strnat (1985) reviewed the spectrum of modern permanent magnet materials with emphasis on those based on N d - F e - B . Falconnet (1984) analysed some industrial and economical problems of N d - F e - B magnets in the short and long term. Ervens (1984) compared the properties of N d - F e - B magnets with those of Sin-Co-type, while Grand (1984) made some technico-economic considerations on N d - F e - B magnets. The current patent situation in the field of N d - F e - B permanent magnets was given by Herget (1984, 1985b).

The most recent results in the science and technology of N d - F e - B based magnets have been listed by Capellen et al. (1986) and are contained in the proceedings of the I N T E R M A G 87 conference, April 1987 Tokyo, Japan (to appear in September 1987 in the "Transactions on Magnetics") and in the proceedings of the "Ninth International Workshop on Rare-Earth Magnets and the Fifth International Symposium on Magnetic Anisotropy and Coercivity in Rare-Earth-Transition Metal Alloys". August/September 1987, Bad Soden (near Frankfurt) Federal Republic of Germany.

The structure inside the Nd2Fe14B matrix grain is clean and faultless. The average grain size is 5 - 2 0 / z m , much larger than the single domain particle (-0.26/xm). Thus, the multidomain structure is most stable (Sagawa et al.

1984a, b, Durst and Kronmfiller 1985).

Generally, the microstructure studies on N d - F e - B magnets reveal at least three phases (Livingston 1985b, Ormerod 1985, Fidler and Yang 1985, Oester- reicher and Oesterreicher 1984b, Kostikas et al. 1985):

-Nd2Fe14B which occupies a volume fraction of 80-85%.

110 E. BURZO and H.R. KIRCHMAYR

- A boron-rich phase, Ndl+~Fe4B4 (or Nd2Fe7B6) irregularly distributed, which occupies a volume fraction of 5-8%.

- A neodymium rich phase which, together with pores and neodymium oxide, has a volume fraction of 10-15%.

Fidler and Yang (1985) reported the presence of ~-Fe in an as-cast sample as well as of the Nd(FeCo)2 Laves phase in cobalt-containing magnets. The structure of neodymium rich phase is still a controversial matter. It has been reported that this is of fcc-type (Sagawa et al. t984b, Chang and Qiang 1986), bcc (Stadelmaier et al. 1983, Sagawa et al. 1985a) or hcp (Fidler 1985, Fidler and Yang 1985). This phase appears heavily faulted. As mentioned the melting temperature of this phase is --920K (-650°C) (Sagawa et al. 1984a, Oesterreicher 1985). The eutectic reaction between Nd-rich phase and NdzFe14B is believed to enhance the liquid phase sintering (Sagawa et al. 1984a). The distribution of the neodynium rich phase within the grains may effectively reduce the grain size (Oesterreicher and Oesterreicher 1984b, Handstein et al. 1985).

We now present some data on the magnetic properties of sintered magnets.

Sagawa et al. (1984a) prepared NdlsFe77B8 magnets by sintering the aligned powders at 1100°C, the samples being then rapidly cooled. After a post-sintering thermal treatment at 600°C, magnets with B r = 1.23T, MHc = 0.96 M A / m and (BH)max = 290kJ/m 3 have been obtained. The partial replacement of iron by cobalt (Sagawa et al. 1984b) raise the Curie temperature of NdzFe14B compound, and thus improves the temperature coefficient of remanence. The intrinsic coercive force, however, is decreased by the cobalt addition. For example this is the case for Ndls(Fe0.sCo0.2)77B 8 samples, where the magnetic characteristics are Br = 1.21 T, MHc = 0.82 M A / m and (BH)max = 260 kJ/m3; the reversible co- efficient of magnetization decreasing in absolute magnitude for the above sample from - 0 . 1 2 3 % / K for Nd~sFe77B 8 to - 0 . 0 7 4 % / K (Sagawa et al.

1984b). Tianduo and Xijian (1985) also investigated the magnetic properties of Nd16.7(Fel_xCox)75.sB7.8 alloys. The coercivity and energy product show a rapid decrease at x = 0.4. The reversible coefficient of magnetization around room temperature decreases in absolute magnitude from - 0 . 1 3 % / K for x = 0 to - 0 . 0 5 8 % / K for x = 0.4. N d - F e - B based magnets having similar properties are now already the industrial standard.

Arai and Shibata (1985) obtained magnets having the nominal composition

Nd23 yFe77 2xCO2xBy

with x = 0 to 10 and y = 5 to 10. After crushing, the powders were aligned in a magnetic field of 0.96 k A / m , pressed at p = 500 MPa, sintered and thermally treated. In fig. 22, the composition dependence of the magnetic properties of Nd15Fe7w_2xCo2xB 8 alloys is plotted. The maximum energy product of 336 k J / m 3 has been obtained in case of Nd16Fe66Coa~B 7 alloy. The reversible coefficient of remanent induction, in this case, is - 0 . 0 2 % / K . It has been suggested that highly heat resistant magnets may be produced when cobalt distributes only in the tetragonal matrix phase.

Lidong et al. (1985) studied the system Pr16Fese_yBy in order to produce permanent magnets. Energy products in excess of 300 k J / m 3 were obtained.

The effect to DY20 3 sintering additive on the coercivity of Nd15Fe77B8 magnets was studied (Ghandehari 1986). The MHc values increase from 0.936 to 1.08 and

PHYSICAL PROPERTIES OF R2FelzB-BASED ALLOYS 111

3 2 0 ] ^{- - , , I I}

/

2881~ (gH)max

x

12q

1 9 '

0,64

0,4,..4

E 0,32- 0,16 - -%

:.:c 0 ~ i i

0 2 4 6

/ Composition x

'500

4 0 0

300

l_) - 200 ~-d

I--

!1°° l

I

B 10 Fig. 22. The influence of cobalt on the mag- netic properties of NdlsFe77 2xCo2xB alloys.

1 . 2 8 M A / m a s D Y 2 0 3 concentration is increased from 0 to 2 and 4 w t % , respectively. There is also a decrease in remanence and a corresponding diminu- tion of the energy product, characteristic of the Dy-containing magnets.

Wang et al. (1985) prepared sintered magnets having the composition Ndx55 16.sFe77.5_75.sB7_8. The optimum magnetic properties a r e B r = 1.32T,

~H c = 0.692MA/m. The demagnetization curve has a perfect squareness. To- kunaga and Harada (1985) investigated the properties of Pr(Fe, B)z and Pr(Fe, Si)z. The magnetic properties of sintered Pr(Fe0.gB0.1) 5 magnets are comparable with those of N d - F e - B alloys. Cumulative aging enhanced MHc to 1.16 M A / m . High MHc values were not obtained in Pr(Fel_xSix) 5 alloys.

Narasimhan (1985a, b, c) prepared sintered N d - F e - B magnets having the composition RF%.3B0. 3 by a similar method as used in manufacturing the SmCo 5 ones. The maximum energy product was 360 kJ/m 3. By a slight increase of the intrinsic coercive field the energy product would be of 380 k J / m 3, corresponding

t o (BH)max = B r / 4 l % . 2 Stadelmaier et al. (1985a) prepared N d - F e - B magnets by sintering the Nd2Fe14B compound or starting from neodymium and iron particles having d = 100/xm and d ~- 38/xm, respectively mixed with Fe2B powders (d <

38/xm). By compacting the above powders at p = 700 MPa and sintering in quartz tubes under vacuum, the powders react and form the Nd2Fe14B phase. The high coercive field is attributed to small dimensions of particles resulting from the above reaction. The dimensions depend essentially on temperature and duration of the process.

The demagnetizing curves for NdtsFe77B 8 as well as for Nd13.sDY1.sFe77B 8 magnets are plotted in fig. 23 (Sagawa et al. 1984b). The coercive field increases

112 E. B U R Z O and H.R. K I R C H M A Y R

Nd%Fe77B 8 Nd13.5 DyI-sF°77B 8 f

-1600 -1200 800 - 4 0 0 0

H ( k A / m )

1.2

b-

0.8

0.4

Fig. 23. Some demagnetizing curves for N d - F e - B magnets.

substantially by partial replacing of Nd by Dy. This substitution has negative effect on the remanent induction. The saturation inductions of the sintered Nd15 xDyxFe77B8 magnets were calculated from those of intermetallic Nd2Fe14B and Dy2Fe14B compounds (Rodewald 1985a, b). The experimental data coincide with the calculated values and indicate that other phases in the alloy do not carry appreciable magnetic moments.

Lidong et al. (1985) in case of the magnets with nominal composition Pra6Fe76B 8 obtained B r = 1.29 T, MH~ = 0.936 M A / m and (BH)max = 296 k J / m 3.

The magnetocrystalline anisotropy constant, K 1 = 4.2 x 1 0 6 j / m 3, is somewhat higher than in N d - F e - B magnets. Okada and Homma (1985), Okada et al.

(1985) and Homma et al. (1985) studied the possibility to replace neodymium by didymium ( N d - 1 0 w t % Pr) and cerium in N d - F e - B magnets. In fig. 24 the magnetic properties of some magnets with the composition (32.5-34.5) di- dymium-5 Ce-(1-1.6) B are plotted as function of sintering temperature. Better magnetic characteristics are evidenced in case of Fe-33.5 wt% (Di-5 Ce) and i wt%B namely B I. = 1.32 T, MHc = 0.816 M A / m and

(BH)

... = 320 k J / m 3. The cerium enhances the wettability of Nd-rich liquid phase to RzFe14B compound. In addition, it helps to form the reactive surface of hard magnetic phase by deoxidation effects. At 70°C, the coercive force of (Ce, D i ) - F e - B magnets is by about 15% greater than in alloys without cerium and at 150°C by about 20%.

Thus, the presence of cerium in N d - F e - B permanent magnets is desirable. The microstructure of didymium-iron-boron was also studied (Ramesh et al. 1986).

The main phase is R2Fe14B. In all samples, a triple junction fcc phase which frequently extended into two-grain junctions, was observed with a lattice parame- ter of 5.24 A enriched in Ce, Pr and Nd. The magnets with poorer properties have considerable amounts of rare-earth oxides.

For NdlsFe75B 8 + 4wt% Nd, magnets with (BH)max = 248 kJ/m 3, ~H~ = 0.656 M A / m and B r = 1.18 T were obtained (Deryagin et al. 1984).

The effect of various substitutions on the properties of Nd15(Fel_xTx)77B 8 sintered magnets has been studied (Maocai et al. 1985b). The substitution of iron by cobalt lowers the reversible coefficient of induction but increases the irrevers-

P H Y S I C A L P R O P E R T I E S O F R 2 F e , 4 B - B A S E D A L L O Y S 113

32O 28O 2 4 0 E 2 0 0 1 6 0

×

~ 1 2 0 _E

" r -

m 80

I - - 1.3 '-" 1.2

r n

1.1

I t r I

C

V" ~ . .-, • - - • - V " -~ ~7

o F e - 3 2 . 5 R-1.0B

- • F e - 3 3 . 5 R - 1 . 0 B

z~ F e - 3 4 . 5 R-1.0 B - R : 5 C e + D i d y r n ° A F e - 3 4 , 5 R - 1 . 3 B

v F e - 3 4 . 5 R - 1 . 6 B

v . - - - . . . . V . . . . 7

< 8 0 0 " .',

L )

m 600 A v ~ "-'~" " ~ - - . ~ . . . . A

E o-- 70

... _[ ^{L _}

1040 1060 1 0 8 0 1100

S i n t e r i n g t e m p e r a t u r e ( ° C )

Fig. 24. M a g n e t i c p r o p e r t i e s a n d d e n s i t y as f u n c t i o n o f s i n t e r i n g t e m p e r a t u r e s f o r F e - ( 3 2 . 5 to 3 4 . 5 ) 5Ce d i d y m i u m - ( 1 to 1 . 6 ) B alloys.

- 4 0 - 3 6

- 32

- 2 8 "~

- 2 4 9

O

-20 ~_.

-16 1 - 1 2

- 3

- 1 2 ° _2~

- 1 1 I

- 10

- 8 o _{._M} - 6 t - 7 5

c ~

E o

7 0 c ,

ible loss of the open-circuit remanent induction. Aluminium and molybdenum enhance the intrinsic coercive field. The addition of Y, Er, Cr, Zr, Ti and especially Ni, Mn and Cu deteriorate the magnetic properties of N d - F e - B magnets. When 50% Nd is replaced by mischmetal, the magnets still exhibit rather good magnetic properties with ( B H ) .... ranging from 160 to 2 0 0 k J / m 3.

The magnetic behaviour of Nd16(Fe~_,Alx)76B 8 alloys are plotted in fig. 25 (Maocai et al. 1985b). The coercive fields, MHc, increase nearly linearly by increasing the aluminium content. The energy product decreases as result of the diminution of the remanent induction.

Tokunaga et al. (1985) analysed the effect of heat treatment on the coercive force of R - F e - B magnets. The following heat treatments were used: (1) heating at 900°C for 2 h, followed by the continuous cooling at the rate of 1.3°C/min at room temperature, (2) heating at a temperature smaller than 700°C for l h and quenching. When using this heat treatment, the following magnetic properties were obtained for Nd(Fe0,92B0.o8)6, B, = ] .38 T, BHc = 734 k A / m and (BH)max = 352 k J / m 3.

114 E. BURZO and H.R. KIRCHMAYR

1,6 - 20 1,44 1,8 E 1,28~. 1,1~

1,12 c 1,4

~

^{0,96 1,2}

0,8- 1,C

I I l I , I _~ I 1 I_J~

.02

.04

.06 .08 .10 X

r - n

3 2 0 ~

r-')

240 i

16o

Fig. 25. The composition depend- ence of B, MHc and (BH)max val- ues in Nd16(Fe1 xAlx)y6B 8 sin- tered magnets.

Bolzoni et al. (1985) analysed the carbon influence on the magnetic properties of Nd2Fel4BI_xC x system. The room temperature anisotropy field increases with the carbon content from 6.84 M A / m (x = 0) to 7.72 M A / m (x = 0.75). Liu and Stadelmaier (1986) show also that isostructural R - F e - C systems have high magnetocrystalline anisotropy. In a preliminary study an intrinsic coercivity of 25 T has been obtained in D y - F e - C alloy. The advantages of these materials are:

(1) it is possible to develop high coercivities without recourse to special proces- sing such as powder metallurgy or melt spinning, and (2) unlike sintered boride magnets, the carbides are not sensitive to coercivity loss by comminution.

The microstructures of R - F e - B magnets were also analysed. Rong et al.

(1985) studied the phase composition of the NdxFel00 x_yBy alloys with x = 15, 16 and y = 4 to 10, by means of the M6ssbauer spectroscopy. The alloy without B consists of a-Fe and NdzFe17 phases. By adding 4 at% B, the NdzFe17 phase disappears and the tetragonal Nd2Fe14B phase forms. Increasing the amount of B up to 7 a t % , the a-Fe disappears and in addition to the Nd2Fe24B phase a Nd-rich phase and a B-rich phase emerge. By further increasing the B content, the quantity of B-rich phase tends to increase. Fidler (1985) and Fidler and Yang (1985) showed that the rare-earth-iron magnets produced by powder metallurgb cal process exhibit "single phase" microstructure of R2Fe14B-type, with additional grains and precipitates of soft magnetic phases. These soft magnetic phases lower the remanence and, therefore, the energy product of the final magnet, but they also act as nucleation sites for reversed domains and therefore, as center for the magnetization reversal. The intrinsic coercive force is determined by the nuclea- tion and expansion of reversed domains. The coercive force is dependent on the external field (Hadjipanayis et al. 1985d, f). Both in sintered and melt-spin samples the pinning of domain walls at the grain boundaries has been observed (Hadjipanayis et al. 1985d, f, Mishra et al. 1986). The formation of thin inter- granular Nd-rich layer extending to pockets of polycrystalline fcc Nd crystals has

PHYSICAL PROPERTIES OF R2FeI4B-BASED ALLOYS 115

been evidenced in sintered magnets annealed at 650°C (Mishra et al. 1986). The Ndl+~Fe4B 4 phase is an unavoidable product of the sintering process but for good coercivity it is not necessary to be present.

Hiraga et al. (1985) examined the grain boundaries of sintered NdasFe77B 8 magnets. A bcc phase with lattice constant a = 2 . 9 A was found at most of boundaries of Nd2Fe14B grains. The bcc phase and Nd2Fe14B grains are joined with smooth interfaces in the samples annealed at 870 K, which have a coercivity as high as 1 M A / m . In samples quenched from 1350 K with a coercivity of about 0.5 M A / m , however, thin plates of bcc phase extend from the interfaces to the inside of the Nd2Fe14B grains and deform the lattice spacing of (001) planes of the NdaFe14B matrix. These thin plates are considered to act as nucleation centers of reverse magnetic domains which reduce the coercivity of the samples.

Harris and Bailey (1984) show a two stage precipitation process in the Nd~sFe77B8 alloy on aging at 600°C, this process involves fine precipitates. Ogilvy et al. (1984) analysed the influence of cooling rate during solidification and boron concentration on the constitution and microstructure of N d - F e - B alloys. The approximate minimum cooling rate required to avoid the presence of the ferrite iron was also determined for some N d - F e - B magnets.

Studies were performed to analyse the domain-wall surface energy, Yw. For Nd2FelgB compound values of 3.5 × 1 0 - 2 j / m z (Livingston 1985a) or 4.2 × 10 2 j / m 2 (Szymczak et al. 1985) were determined. In the Pr2Fe14B the domain- wall surface energy is 3.3 x 10 .2 J / m 2 (Szymczak et al. 1985) or 3.16 x 10 2 j / m 2 (Shouzeng et al. 1986). These values are about half of the value determined in SmCo 5 magnet.

Domain patterns of a sintered N d - F e - B permanent magnet with different magnetic states at H = 0 were observed on the pole and side surfaces using the Kerr effect and the Bitter technique, respectively (Tiesong et al. 1986). During the magnetization process, most of the single domain grains persist as single domains apparently only reversing the direction of magnetization (Li and Strnat 1985), while reverse domains in multidomain grains grow. As the value of the demagnetization field approaches and exceeds that of the coercivity, some reverse domains in multi-domain grains extend across grain boundaries. It seems that the growth and shrinkage of reverse domains with the variation of the field character- izes the recoil lines. Suzuki and Hiraga (1986) analysed static and dynamical walls in NdlsFe77B 8 sintered magnets. A 180 ° wall near a triple junction of the grains constituting of a bcc phase boundary region can be a strong pinning site. Other walls by increasing the field easily moved and disappeared. The observation of 180 ° walls moving under the influence of the applied field is also discussed. The magnetic domains and the changes in magnetic properties of sintered P r - F e - B magnets daring the course of annealing at 580°C have been investigated (Shouzeng et al. 1986). The mean width of surface domains in the tetragonal phase is about 0.9/xm. The magnetic properties are related to the concentration gradient in the phase boundaries.

Otani et al. (1986) show that the hysteresis loop of NdlsFe77B 8 can be decomposed in hysteresis loops of two magnetic phases; one with a comparatively