Role of the (H,C,N) interstitial elements on the magnetic properties of iron-rare earth permanent magnet alloys

Journal of

ALLOYS AND COM?OU;HDS ELSEVIER

J o u r n a l of Alloys a n d C o m p o u n d s 219 (1995) 16-24

Role of the (H,C,N) interstitial elements on the magnetic properties of iron-rare earth permanent magnet alloys J.L. Soubeyroux, D. Fruchart, O. Isnard, S. Miraglia, E. Tomey Laboratoire de Cristallographie du CNRS, 166X, 38042 Grenoble, France

Abstract

A comparison is made between the intermetallic compounds R2Fe~4B, R2Fe~7 and RFele-xMx (R=rare earth metal and M=Ti, Mo) and their interstitial derivatives (hydrides, carbides and nitrides). The role of the different interstitial elements on the magnetic properties will be reviewed through their crystal structure (in particular the localization of the interstitial) and through experimental work done by magnetic susceptibility, Curie point measurements and neutron diffraction. Keywords: Magnetic properties; Crystal structure; Neutron diffraction; Magnetic susceptibility

1. Introduction Alloys and intermetallic compounds obtained by combining rare earth metals (R) with 3d metals have attracted a lot of interest for their hard magnetic properties [1]. In general it can be said that the 3d sublattice is responsible for high values of magnetization and magnetic ordering temperature, while the 4f sublattice provides the magnetic anisotropy. If the RCo5 or RzFe14B (2-14-B) compounds can be directly used as permanent magnets, the properties of the R2Fe~7 (2-17) and RFelz_xMx (1-12) compounds must be improved by the insertion of light elements such as H, C or N. The new ternary compounds have properties mainly controlled by electric charge, valence electron density, change in intermetallic distances, band structure effects, volume increase,... The description of the structure of the new interstitial compounds will be reviewed through experimental work mainly done by powder neutron diffraction. The role of the different interstitial elements on the magnetic properties will be clarified by comparison with the Curie temperature, the magnetization and the magnetocrystalline anisotropy of the host alloys.

2. Experimental The alloys are prepared by melting the metals using a high frequency furnace in a cold copper crucible under argon atmosphere. The interstitial compounds

0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)05017- 1

are prepared from powders of the starting alloys by action of different interstitial vectors. The hydrides of 2-14-B, 2-17 and 1-12 alloys are prepared in the temperature range 20-100 °C by applying a low H2 gas pressure (0.1-0.5 MPa), thus forming rather stable compounds at room temperature [2,3]. The nitrides of 2-17 and 1-12 alloys are synthesised by N2 or NH3 solid-gas reaction, by heating for a few hours in the temperature range 400-500 °C under a medium gas pressure (0.1-2.0 MPa) [4,5]. The nitrogenation of 2-14-B compounds leads to the decomposition of the alloy and to the formation of rare earth nitrides, a-iron and borides [6]. The carbides of 2-17 and 1-12 alloys can be synthesized by solid state diffusion at high temperature (800-1000 °C) [7,8] or by arc melting [9]. The reaction of alcans or other benzenic compounds in the temperature range 400-500 °C leads to the formation of monophased compounds [10]. Mixed reactions ( H + N , C + N or H + C ) are also possible but the interpretation of the role of the interstitial is more difficult and will not be discussed in this paper. Powder neutron diffraction has been widely used before and after interstitial charging but also with in situ solid-gas reactions [11,12]. This technique is a particularly powerful tool to locate the light elements and also to measure accurately the changes in intermetallic distances and to determine the magnetic moments and their orientations.

ZL. Soubeyroux et al. / Journal of Alloys and Compounds 219 (1995) 16-24

Magnetic susceptibility measurements have been made by the extraction .technique in the 4-300 K temperature range ( H = 1 0 e , v = 120 Hz) or in a Bittercoil-type magnetometer (0-20 T) at the Laboratoire des Champs Magnrtiques Intenses in Grenoble. The Curie point determinations have been made with a thermomagnetic torquemeter in the 20-1000 °C temperature range on samples sealed in evacuated silica tubes. Aligned samples were obtained by applying a magnetic field of 0.5 T on the powder compound mixed with a resin at room temperature. The magnetic field was applied parallel or perpendicular to the fiat surface of the sample. X-ray powder data were analysed to determine the preferred orientation of the sample so induced and to determine the magnetocrystalline orientation.

(RFeB-Fe-Fe-Fe). The structure shown in Fig. l(a) comprises six crystallographic non-equivalent Fe sites and two Nd sites. The boron occupy a six-coordinated prismatic iron site. Hydrogen forms a solid solution with R2Fe14B compounds and the hydrides retain the host alloy structure with a volume increase of 2.1 ~3 (H atom)-1. The structure can accommodate up to 5.5 H (formula unit) -1 depending on the nature of the rare earth element [2]. Powder neutron diffraction experiments have shown that the hydrogen occupies four different crystallographic sites, i.e. D1 (8j), D2 (16kl), D3 (16k2) and D4 (4e), with metal coordinations formed by 3R-1Fe for D1 and 2R-2Fe for the others (Fig. l(b)). The filling of the different interstitial sites as a function of composition shows that at low hydrogen concentration the sites D1 and D2 are almost equally occupied and are the most favourable, but, at the highest hydrogen concentrations, all the sites contain hydrogen. At low temperature the symmetry of the alloys and the hydrides of Nd and Ho compounds is lowered to Cm owing to high magnetostrictive forces [13,14]. The volume expansion of the alloys and their hydrides for the R series follows the lanthanum series reduction of the metal radii for all the compounds. The volume expansion is of the order of 4.2% for the most charged hydride.

3. Results and discussion

3.1. Structure of the compounds

R2Fe14B

and

17

R2Fe14BHx

The structure of the alloys at room temperature for all the R series is tetragonal (space group, P4z/mmm; Z = 4) forming a sequence along the c axis of planes

RE2 Re5

F%

g ~ F e c(~ F e e (DFe j l ( ~ Fe J 2 ~ F e k 1 ~ F e k 2 ~ B g

H_4.40

R/E

_~ Fe6

:e3

Fig. 1. Structure of the 2-14-B compounds: (a) alloy; (b) environment of the hydrogen interstitial sites.

18

J.L. Soubeyroux et al. / Journal o f Alloys a n d C o m p o u n d s 219 (1995) 16-24

3.2. Structure of the R2Fe17 and R2Fe17Z x compounds

(Z=H,C,N) Binary R2Fel7 compounds crystallize in the hexagonal ThENi17 structure (space group, P63/mmm; Z =2) or in the rhombohedral ThEZn~7 structure (space group, R3m; Z = 3 ) . They are derivatives of the CaCus-type structure and can be represented by replacing onethird of the R atoms by a dumb-bell pair of Fe atoms. There are two ways in which these substitutions can be done leading to the two ordered structures shown in Fig. 2. The light rare earth elements crystallize in the rhombohedral structure and the heaviest in the hexagonal structure. Hydrogen forms a solid solution with all the REFer7 intermetallics leading to stable hydrides at room temperature [15-17]. The determination of the hydrogen location has been made by powder neutron diffraction for the rhombohedral compounds [17]. The hydride retains the host metal symmetry with 5 H (formula unit)-1 for the most charged hydride. Two different interstitial sites are occupied: D1 (9e) which is six coordinated 2R--4Fe can accommodate 3 H (formula unit)-~, and D2 (18g) which is four-coordinated 2R-2Fe can accommodate 2 H (formula unit)-L The D1 site is filled before the D2 site [11]. The carbides synthesized at high temperature (T= 1000 °C) can accommodate up to 1.5 C (formula unit) - 1. They form solid solutions with the parent alloy. There is a carbon-induced structural transformation from the hexagonal type to the rhombohedral type [18]. The carbides synthesized at lower temperature (T= 500 °C) by the hydrocarbon route accommodate up to 3 0

O

O

O

'~ O ) Ih2\il7 ~ilrt' t, a r | h O O

( b ) Th2Znl7

I Icxagtmal

Iron t h n n |l-lwll

Rholnl)ohedr;d

(h)

..........

(Ol

@

' L

.~ $

r'

. ........

Fig. 2. Structure of the 2-17 compounds (upper diagrams show schematic representations of the Fe dumb-bell substitutions): (a) the hexagonal-type structure; (b) the rhombohedral-type structure.

C (formula unit) -1, they retain the structure of the host alloy [10], and they form a defined compound with a stoichiometry close to 3.0 [12]. The difference in final phase structure between the two routes can certainly be explained by the low diffusibility of the metal atoms at 500 °C in comparison with the syntheses performed at 1000 °C. X-ray and neutron diffraction experiments have shown that the carbon atoms occupy the 9e sixcoordinated site in the rhombohedral structure [19,20]. The nitrides synthesized under N2 or NH 3gas pressure preserve the host alloy structure and can accommodate up to 3 N (formula unit)-1. By in situ powder neutron diffraction it has been shown that nitrogen forms defined compounds R2FeaTN=3.o in equilibrium with the parent RzFe17 alloy [12]. Nitrogen atoms are located in the six-coordinated 9e site [21]. Higher nitrogen concentrations have been reported, but the compounds formed are not single phases and no site determination of the extra nitrogen positions has been proposed. The volume expansion of the alloys and their hydrides for the R series follows the lanthanum series reduction of the metal radii for all the compounds. The volume expansion for the hydrides is of the order of 4.3% for the most charged hydride, 4.4% for the carbides with 1 C (formula unit) -~, 6.5% for the carbides with 3 C (formula unit)-x and 7.0% for the nitrides with 3 N (formula unit)-L

3.3. Structure of the RF12_,M. and RFe~2_xMxZx compounds (Z=H, C, N) Rare earth elements and manganese form intermetallics with the ThMn12 tetragonal structure. The corresponding Fe compounds do not form if they are not allied with a third element to give RFe12_xM, where M represents Ti, V, Cr, Mo, W, A1 or Si [22]. The stability range depends on the substituted M element; see Ref. [23] for phase diagrams. The structure is tetragonal (space group, I4/mmm; Z = 2); there is only one rare earth site and there are three iron sites. Xray diffraction as well as neutron diffraction have shown that the elements M=Ti, V and Mo have a strong preference for occupying the 8i site [24,25]. The structure reported in Fig. 3 is strongly correlated to the CaCu,type structure by the substitution of half the rare earth metal atoms by a dumb-bell pair of iron atoms. We will discuss in this article the results obtained on two substituted series, RFellTi and RFem.sMol.5. Hydrogen forms a solid solution with the host alloys. A maximum of 1.8 H (formula unit)-~ can be accommodated in the structure, depending on the M or R elements [26,27]. Powder neutron diffraction experiments have shown that hydrogen can accommodate either two four-coordinated sites D1 (161) and D2 (320) 1R-3FeM or a six-coordinated site D3 (2b) 2R-4FeM [28-30]. The full occupation of the D3 site corresponds

19

J.L. Soubeyroux et al. / Journal of Alloys and Compounds 219 (1995) 16-24 o

750

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

(a) 700 3c

ct

~" ~-~

O

a~'3 •

Rare earlh

'rh2Zrll7

~ (

(

)

(

(

O eT:alI\L l

Rhombohedral

O ThMnl2

650 600 550

I t.lral~llnill O O

Iron dumb-bell

500 450 400 40

i

,

,

i

~

i

LaCe PrNd I

I

I

I

,

,

Sm I

I

F

,

,

p

,

~

i

i

GdTbDyHo ErTmYbLu Y I

I

I

I

I

I

I

I

I

I

,

,

,

,

,

,

,

~ 35. R 02. F~ lDar @8, ~ 8 j N

•

3O

2b

Fig. 3. Structure of the 1-12 compounds (upper diagrams show schematic representation of the Fe dumb-bell substitutions).

25

y d r i d e s

20-15-

to 1 H (formula unit) -1. So, for compositions above 1 H (formula unit) -1, one must imagine that another site is occupied. The carbides synthesized at high temperature can accommodate 1 C (formula unit) -1 and preserve the host alloy structure. Powder neutron diffraction experiments have located the carbon on the six-coordinated 2b site [31]. The nitrides formed by N2 or NH3 solid-gas synthesis can accommodate 1 N (formula unit)-1. They preserve the alloy structure. As for carbon, nitrogen is located on the 2b site [32,33]. As for RzFe17 nitrides, higher nitrogen concentrations have been reported, but the compounds formed are not single phases and no site determination of the extra nitrogen positions has been proposed. The volume expansion of the alloys and their hydrides for the R series follows the lanthanum series reduction of the metal radii for all the compounds. The volume expansion for the hydrides is of the order of 1.8% for the most charged hydride, 3% for the carbides with 1 C (formula unit) -1 and 3.2% for the nitrides with 1 N (formula unit)-1.

4. Magnetic properties The compounds R2Fea4B, R2Fe17 and RFe12_xMx and their hydrides, carbides and nitrides are all ferromagnetic. For the light rare earth elements (up to Sm), the R and Fe moments are coupled parallel while for the heavy rare earth compounds they are coupled antiparallel.

10

Alloys ~

,

,

,

,

,

LaCe PrNd

,

,

,

,

Sm

GdTbDyHoErTmYbLu Y Rare-Earth Fig. 4. (a) Evolution of the Curie temperature along the lanthanum series for the R2Fet4B alloys and hydrides. (b) Evolution of the saturation magnetization at 4 K along the lanthanum series.

4.1. Curie temperatures

The variation in the Curie temperature along the lanthanide series is reported on Fig. 4(a) for the R2Fe14B and R2Fei4Bnmax compounds. The effect of the hydrogen is to enhance the Curie temperature by about 20%. For the RzFe17, RzFe17H.... RzFelTCmax and R/Fe17Nm~, compounds the variation in the Curie temperature along the lanthanide series is reported on Fig. 5(a). With increasing interstitial size (N > C > H) there is an increase in the Curie temperature of 40% for the hydrides, 100% for the carbides and 120% for the nitrides. For the RFe11Ti, RFellTiHm~,, RFellTillTiCmax and RFe11TiNm~, compounds the variation in the Curie temperature along the lanthanide series is reported in Fig. 6(a). With increasing interstitial size there is an increase in the Curie temperature of 8% for the hydrides, 20% for the carbides and 30% for the nitrides. In order to clarify the role of the Fe-Fe coupling on the Curie temperature we have plotted in Fig. 7, for various rare earth elements, the Curie temperature vs. the iron reduced volume. The latter value is the

20

J.L. Soubeyroux et al, / Journal of Alloys and Compounds 219 (1995) 16-24

~, 800

L

L

I

I

I

I

J

I

I

i

J

I

I

I

(a)

8 0 0

I

I

I

I

J

I

I

I

L

I

I

I

I

L

L

I

I

(a)

750

700-

700 600

Hydrides

~,

. ."

650

Carbides

500 600 400

S

300 200 45

550 500 450

. . . . . . . . . . . . . . . . LaCe PrNd Sm GdTbDyHoErTmYhl,u Y t

I

"-~ 40

I

I

I

I

I

L

i

I

I

~ i d e s

i

24

I

(b )

. . . . . . . . . . . . . . . . LaCe PrNd Sm GdTbDyHo ErTm I

I

I

I

I

I

I

I

22

L

I

I

h

s

I

La Y I

L

I

i

,

" "

20 18 16

(~rbides~

14

25

12 20 15

10 ,

i

:

i

LaCe PrNd

i

i

Sm

i

i

i

i

,

i

i

:

i

,

GdTbDyHoErTmYbl,u Y Rare-Earth

Fig. 5. (a) Evolution of the Curie temperature along the lanthanum series for the R2FeI7 alloys, hydrides, carbides and nitrides. (b) Evolution of the saturation magnetization at 4 K along the lanthanum series.

volume of the crystallographic cell reduced to one iron atom in the formula RxFeZy. All the compounds derived from the same rare earth element lie approximately on a line, meaning that the Curie temperature increases linearly with the volume available for one iron atom in the structure. In fact, by precise neutron diffraction experiments performed on the series of the neodymium compounds, we have shown that Tc is more closely related to the mean iron-iron distance [34]. The reduced iron volume is easier to determine but reflects the same phenomena.

4.2. Magnetization From the parallel coupling of the rare earth and of the 3d moments the total magnetization is enhanced for the light rare earth. At low temperature the saturation magnetization has been plotted along the rare earth series in Figs. 4(b)-6(b). The maximum value is obtained for the Pr, Nd and Sm compounds. For all the rare earth-Fe intermetallic compounds, an important contribution to magnetization is given by the iron atoms. The moment value is very sensitive to

Alloys i

i

r

q

LaCe PrNd

i

i

Sm

i

r

i

i

i

i

r

GdTbDyH0 ErTm Rare earth

i

Lu Y

Fig. 6. (a) Evolution of the Curie temperature along the lanthanum series for the RFelz-,Mx alloys, hydrides, carbides and nitrides. (b) Evolution of the saturation magnetization at 4 K along the lanthanum series.

local coordination and nearest-neighbour distances. If iron is surrounded by other iron there is an increase in magnetization; on the contrary, the presence of rare earth elements or interstitials as nearest neighbours decreases the magnetization. In the 2-14-B series there are six different iron sites. The iron site Fe(4) (8j2) has the longer iron-iron distance and in general has the strongest moment ((2.6-3.0)~B), then Fe(2) (16kl) and Fe(3) (16k2), and finally Fe(1) (4c), Fe(5) (8jl) and Fe(6) (4e). In the hydrides, the same hierarchy is found with a slight increase in the total magnetization [14,29]. In the 2-17 series, the moment of the dumb-bell atoms (Fe(6c)) is the strongest ((2.6-3.0)/zB), then the Fe(9d) and finally the Fe(18h) and Fe(18f), following the hierarchy of iron-iron distances [21,29]. For the hydrides there is a consequent increase for the Pr, Nd and Sm compounds, coming mainly from an increase in the iron moments, the rare earth moments being unchanged. For the nitrides and carbides, there is a net increase in the saturation magnetization at 4 K, coming mainly from the volume increase that enhanced the 3d band

J.L. Soubeyroux et al. / Journal of Alloys and Compounds 219 (1995) 16-24

21

Tc (K) 800

.

.

700

.

.

.

.

.

.

' ....

•

Tc

2/14/B

• • o

Tc 2/17 Tc 1/12/Mo Tc 1/12/Ti

' ....

' ....

' ....

800 . . . . . . . T c (K)

'

/

,

/ o Q////

*

S

500

i ....

600

m

i,

i

....

i

o

500

400

/

~

~

'

•

400 V / F e (~3)

300

, 14.5

14

800

....

700

/o

600

,,,i

,

.... 15

, .... , .... ~ .... , .... , .... 15.5 16 16.5 17 17.5 18

300 14

• , .... ~ • • • . 14.5 15 15.5

7OO

700

•

oo / ~

Pr

600

V / Fe (,~3) - . .... . .... , .... 16.5 17 17.5 18

. 16

800 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T c (K) o t

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T c (K)

o

....

Ho

•

600

•

500 * .

500 400

~

/

o

~

40O

300

V / F e (~3)

200

....

14

, ....

14.5

, ....

15

, ....

15.5

, ....

16

, ....

16.5

, .....

17

300

• . • •

17.5

18

.... 14

V / F e (A s) , .... , .... , .... , .... , .... , .... , .... 14.5 15 15.5 16 16.5 17 17.5 18

Fig. 7. Variation in the Curie temperature vs. the iron reduced volume for some rare earth elements (the lines are guides for the eye) for all the RxFeZy compounds (R=Sm, Nd, Pr, Mo).

splitting. The individual moments of the Fe(18h) and F e ( 1 8 f ) sites are, however, reduced as they are the nearest neighbours of the carbon or nitrogen atoms. T h e other moments are increased owing to an increase in the iron-iron distances. At room temperature, the saturation of the interstitial compounds presents a significant increase in comparison with the alloys, but the effect is mainly due to the formidable increase in the Curie temperature. In the 1-12 series, the iron Fe(8i) presents the higher moment, then the Fe(8j), and the Fe(8f), following the iron-iron distance order, but also the hierarchy in number of iron atoms as nearest neighbours [31]. A s for the 2-17 compounds there is a net increase in magnetization for Pr, Nd and Sm hydride compounds coming from an increase of the iron moments. The same remarks for the carbides and nitrides apply here too with a reduction in the moment of the iron 8j site that is nearest neighbour of the interstitial and is hybridized.

Table 1 Anisotropy easy axis directions for the alloys and hydrides of the R2Fez4B series (I = inclined)

4.3. Magnetic anisotropy

this property. The magnetocrystalline anisotropy results from a competition between the R and 3d sublattice anisotropies, the electrostatic crystal field acting on the rare earth elements being the main source of anisotropy in these compounds. The modification of the local environment produces a crystal field modification, so the nature and the interstitial-rare earth distances strongly act on the anisotropy.

The magnetic ability for the moments to be aligned along the crystallographic c axis is of fundamental importance for hard magnetic properties. We will not discuss here the origin of the anisotropy in the intermetallic compounds 2-14-B, 2-17 and 1-12; we will only point out the role of the interstitial in modifying

R

Alloys o (deg) (4.2 K)

La

Ce Pr Nd Sm

Gd Tb

Dy Ho Er Tm Yb Lu y

Hydrides TSR (K)

0

0 0 32

130

90 90 0

0 22 90 90 90

0 0

58

O (deg) (300K) 0 0 0 0 90 90 0 0 0 90 90 90 0 0

O (deg) (4.2 K)

90 90 32 90 90 0 I 50 90 90 90 90

TsR (K)

125 355 45 90

O (deg) (300 K)

0 90 0 90 I 0 0 0 90 90 90 90

J.L. Soubeyroux et al, / Journal of Alloys and Compounds 219 (1995) 16-24

22

Table 2 Anisotropy easy axis directions for the alloys, hydrides, carbides and nitrides of the R2Fe17 series R

La Ce Pr

Nd Sm Gd Tb Dy Ho Er

Tm Yb Lu Y

Alloys

Hydrides

Carbides

Nitrides

(9 (deg)

TSR

6) (deg)

O (deg)

TSR

O (deg)

6) (deg)

TSR

O (deg)

O (deg)

TSR

O (deg)

(4.2 K)

(K)

(300 K)

(4.2 K)

(K)

(300 K)

(4.2 K)

(K)

(300 K)

(4.2 K)

(K)

(300 K)

90 90 90 90 0 90 90 90 90 0 90 90 90

90 90 90 90 0 90 90 90 90 90 90 90 90

90 90 90 90 0 90 90 90 90 90 90 90 90

90 90 90 0 0 90 90 90 0 0 90 90 90

90 90 90 0 0 90 90 90 90 90 90 90 90

90 90 90 0 90 90 90 90 0 0 90 90 90

90 90 90 90 0 90 90 90 90 90 90 90 90

74

111 180

125 205

90 90 90 0 90 90 90 90 90 90 90 90 90

Table 3 Anisotropy easy axis directions for the alloys, hydrides, carbides and nitrides of the RFelo.sMoL5 series (I = inclined) R

Alloys ~9 (deg) (4.2 K)

La Ce Pr

Tm

0 90 90 0 0 90 I 0 54 0

Yb Lu Y

0 0

Nd Sm Gd Tb Dy Ho Er

Hydrides TSR

O (deg)

O (deg)

(K)

(300 K)

(4.2 K)

0 90 1 0 0 0 0 0 0 0

I 90 90 0 0 90 90 1 26 0

0 0

0 0

160

190 170 58

Carbides TSR (K)

120

160 38

O (deg) (300 K)

! 90 I 0 0 90 90 0 0 0 0 0

Most of the 2-14-B compounds are easy axis (Mllc), except for Sm, Er, Tm and Yb which give in-plane compounds. Nd and Ho (easy axis) are slightly tilted from c at low temperature but present a spin reorientation transition (SRT) towards an easy axis structure (Table 1). It has been shown that the anisotropy is given by the rare earth element and that, for the peculiar elements that present an SRT, there is a competition between the anisotropies of the two rare earth sites. The Study of the hydrogen effect on the magnetic anisotropy has been made by Pareti et al. [35] and Coey et al. [36]. The hydrogen acts mainly by reducing the magnetic anisotropy of the Fe sublattice but also that of the rare earth. However, the effect is not sufficient to change the anisotropy from easy axis to in plane.

(9 (deg) (4.2 K)

Nitrides TSR

O (deg)

O (deg)

TSR

~9 (deg)

(K)

(300 K)

(4.2 K)

(K)

(300 K)

0 0 90 90 0

0 0 90 90 0

0 0 90 90 0

I

80

0 0 90 90 0

0

0

0

0 90 90

0 90 90

0 90 90

0 90 90

90 90

90 90

90 90

90 90

Only for the Nd and Ho hydrides is there an increase in the tilt angle and an increase in the temperature TSR for spin reorientation. In the 2-17 series, most of the compounds are in plane, except of the Gd and Tm compounds (Table 2). The in-plane anisotropy is low and due to the iron atoms. The hydrogen insertion decreases the iron anisotropy, but the anisotropy of the hydride compounds is unchanged compared with that of the alloys. The hydrogen is mainly located close to the rare earth, with R-H distances of the order of 2.4-2.5 /~. Nitrogen and carbon act on the anisotropy of the rare earth. The distances R-N and R-C are of the order of 2.5 /~ and those of Fe-N and Fe--C are in the range 1.85-1.95 /~ (almost identical to the iron

J.L. Soubeyroux et al. / Journal of Alloys and Compounds 219 (1995) 16-24

23

Table 4 Anisotropy easy axis directions for the alloys, hydrides, carbides and nitrides of the RFetiTi series (I = inclined) R

Alloys 0 (deg) (4.2 K)

La Ce Pr Nd Sm Gd Tb Dy Ho Er Tm Yb Lu Y

I 0 0 I 90 I I 0 0 0

Hydrides TSR (K)

190

327 200 50 50

0 (deg) (300 K)

0 0 0 0 0 90 0 0 0 0

0 (deg) (4.2 K)

0 I 90 90 I

Carbides TsR (K)

340 210 90 40

0 (deg) (300 K)

0 (deg) (4.2 K)

Nitrides Tsrt (K)

0 (deg) (300 K)

0 0 0 90 0 0 0 0 0 0

0 90 0 0 0

0 0

0 (deg) (4.2 K)

I

TsR (K)

45

0 (deg) (300 K)

0 0 0 90 0 0 0 0 0 0

0 0

nitride and carbide binaries). The effect is mainly reflected by the samarium compound that becomes easy axis; the erbium and thulium compounds present SRTs. In the 1-12 series., the compounds are mainly easy axis at room temperature (Tables 3 and 4), the anisotropy of the iron sublattice being stronger than the rare earth lattice. However, at low temperature the anisotropy of the rare earth prevails, and most of the compounds present SRTs. The effect of the hydrogen is to reinforce the iron anisotropy, so at room temperature most of the hydrides are easy axis; in general, the SRT temperatures of the hydrides are lowered. The carbon and nitrogen atoms are located in the 2b site, in the close vicinity of the rare earth atoms, thus modifying strongly the anisotropy of the rare earth atoms. The effect is sufficient to change the anisotropy of some of the compounds from easy axis to in-plane compounds. A model to explain the anisotropy changes has been developed by Coehoorn and Daalderop, originating from Miedema's theory of stability of alloys [37]. The model allows us to understand qualitatively the changes in sign of the magnetocrystalline anisotropy as a function of the size and nature of the interstitial.

The volume increase is also at the origin of the magnetization increase for the interstitial compounds, by reducing significantly the state density at the Fermi level for the 3d band. The local magnetic moments of the iron atoms close to nitrogen or carbon are reduced owing to hybridization, the other atoms compensating this effect to produce globally a higher magnetization. The effect of the interstitial on the magnetocrystalline anisotropy is to modify the crystal field parameters of the rare earth and iron atoms. The volume increase does not act on the crystal field parameters. The model of Coehoorn and Daalderop is well adapted to describe the influence of the different interstitial on the magnetocrystalline anisotropy. For application as hard magnetic materials, very few compounds present intrinsic "good" magnetic characteristics such as high Curie temperature, high magnetization at room temperature and easy axis anisotropy. Only the nitrides and carbides of Sm2Fe17 compounds, the SmFellTi hydrides, the NdFe11Ti carbides and nitrides and the Nd2Fe~4B hydrides can present some interest as magnetic powders in the plastic magnet processes [1]. However, these compounds must also present extrinsic property such as coercivity that is not easy to obtain for powdered materials.

5. Conclusion

References

The insertion of light elements such as H, C or N in the holes of the R-Fe intermetallic compounds leads to a strong increase in the Curie temperature. The effect has been mainly correlated with a volume increase and with a Fe-Fe distance increase acting on the exchange interactions. The larger is the size of the interstitial, the larger is the Curie temperature increase.

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