Magnetic and thermal properties of (NdxFe1−x)82.5B17.5 glasses

Materials Science and Engineering, 99 (1988) 157-159

157

Magnetic and Thermal Properties of (NdxFe _x)a2.sB17.5 Glasses* Z. ALTOUNIAN and D. H. RYAN

Physics Department, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8 (Canada)

Abstract

2. Experimental methods

The effects of iron substitution by neodymium on the structural and magnetic properties of glassy Fe-B at the eutectic composition have been studied. Metallic glasses of composition (NdxFel _x.)82.sB17.5 w e r e obtained by melt spinning for 0 <~x <~0.25. The crystallization temperature (measured at a heating rate of 40 K min I) rises to a maximum of 956 K at x = O.I0 and then falls as x ~ 0.25 because of the appearance of tetragonal Nd2Fe14B as the primary crystallization product. The ordering temperature and iron moment are both extremely sensitive to alloying with neodymium, decreasing with d Tc/dx = 950 K (at. % N d ) - 1 and d P F J d X = 4. 7#B (at. % Nd) l respectively, and even by x = 0.2 the neodymium dominates the magnetic properties.

The (Fe~ _xNdx)s2.sBlT.5 alloys were prepared by arc melting the pure elements (all starting metals were of 99.9% purity and were obtained from Alfa Products, Danvers, MA) in the required stoichiometry under titanium-gettered argon and subsequently melt spinning the resultant I g buttons on a copper wheel rotating at a tangential speed of 60 m s- J in a 30 kPa helium atmosphere. The samples ranged in quality from continuous ribbons 1 m long for x = 0 to flakes 2-3 mm long for x = 0.25. All the neodymium-containing ribbons were extremely brittle. X-ray diffraction measurements employing CuKct radiation showed only a broad feature centred around 20 -- 44 °, with no evidence of contamination by crystalline phases. The crystallization temperatures Tx measured at the peak of the exotherms and magnetic ordering temperature Tc were determined in a calibrated Perkin-Elmer DSC-2C differential scanning calorimeter under a flow of oxygen-free argon gas at a heating rate of 40 K min-l. Magnetization measurements were performed at 4.2 K on a Foner-type vibratingsample magnetometer with a field provided by a 5.5 T superconducting solenoid. Room temperature Mrssbauer spectra were obtained on a conventional constant-acceleration spectrometer with a 0.2 GBq 57CoPd source. The spectra (Fig. l(a)) are broadened sextets, typical of magnetically ordered amorphous systems. The average hyperfine fields (Bhr) were calculated from distributions of Bhf obtained using a window Fourier deconvolution program [3]. The isomer shifts are quoted relative to ct-Fe at room temperature.

1. Introduction The recent discovery of a new class of powerful permanent magnets based on Nd-Fe-B has attracted a great deal of attention. A large amount of data has been published on the crystallographic and magnetic properties of these alloys [ 1, 2]. While the tetragonal hard magnetic phase, Nd2Fe14B, may be obtained by rapid solidification of the melt, at sufficiently high quench rates glassy phases are formed. Very little is known about the magnetic and thermal properties of glassy Fe-Nd-B alloys. The crystallization of these alloys may also shed some light on the various stable and metastable crystalline compounds in the F e - N d B ternary phase diagram. For this reason, we have started a detailed investigation of ternary Fe-Nd-B glasses. In this paper, we report the results on the pseudobinary glassy alloys (NdxFel_x)s2.sBl7.5 (0 ~
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50

3. Results and discussion The substitution of neodymium for iron in glassy (Ndx Fe~ _ x)s2.sBl7.5 initially leads to an increase in the crystallization temperature (Fig. 2), which reaches 957 K at a heating rate of 40 K min -1 in the x = 0.10 sample, an enhancement of over 200 K when compared with Fes7.sB17.~, which crystallizes at 746 K. Further additions of neodymium reduce Tx and lead © Elsevier Sequoia/Printed in The Netherlands

158 I

I+

U

U

U

I

(~

i

+

I

I

u

u 150

x=O.O

100

50

I

I

I

I

I

-6

-3

0

3

6

I

I

I

zOO

Velocify (ram~s] I

0

n .05

o

.10

.15

.20

I

25 X

--.. L ~

~~~~'~jo,

I

o

I

,

l

I

!

b

\ai

2.0

200 q \\ 2O

1.5

~-x-\x\xO~ \\\

150

/-0 -

\

0

0

I

I

I

I

.05

.1O

.15

.20

I\

x

100

.25

Fig. 1. (a) Typical room temperatureMfssbauer spectra;(b)

variation in the average hyperfine field (Bhf> in glassy (NdxFel ~)82.5B~7.5. I

I

I

I

9OO

800

/

,

70~ I

I

I

I

I

• 05

.10

.15

.20

"25

J

X

Fig. 2. Variation in the crystallization temperature with neodymium content in glassy (NdxFe ~_x)S2.sB]7.5.

I

I

I

I

I

•05

,1O

.15

.20

"25 X

Fig. 3. Magnetic properties of glassy (NdxFen_x)a2.~B17.5 showing (a) ordering temperature Tc (O) and coercivity Be at 4.2 K ([El), (b) saturation magnetization Ms at 4.2 K (O) and average iron moment/~Fc(II) derived as described in the text.

to the appearance of tetragonal Nd2Fet,B as the primary crystallization product. In the composition range studied, Tc is linear (Fig. 3(a)), is given by T¢ = (614 _ 6) - (950 +__40)x K and extrapolates to zero at x = 0.64 + 0.02. (Bhf) measured at room temperature (about 290 K) also falls rapidly with increasing x (Fig. l(b)). Some of, but by no means all, the decrease is due to measuring at progressively larger fractions of Tc as x is increased; a significant reduction in the iron moment is indicated. Magnetization curves at 4.2 K show that all the alloys are collinear ferromagnets which saturate easily in fields of less than 0.3 T. The initial decline in saturation magnetization Ms observed for small values of

159 TABLE 1

x, 40Kmin

0 5 I0 15 20 25

Magnetic properties of (NdxFe I _ x)s2.sBiT.s

i

Be, 4.2K

M s, 4.2K

#/metal

#/Fe

(K)

(Bhr), 300K (T)

6, 300K

(K)

(mms -I)

(mT)

(JT l k g - I )

(,us)

(~B)

746 856 957 929 881 849

613 558 531 469 427 370

25.2 23.7 24.0 18.7 17.4 13.3

-0.16 -0.18 -0.22 -0.23 -0.24 -0.23

1.0 2.8 15.4 56 114

202 182 170 139 116 114

2.10 2.04 2.04 1.78 1.57 1.64

2.10 1.87 1.70 1.27 0.89 0.79

Tx, 40Kmin

i

Tc

x (Fig. 3(b)) seems to level off for x / > 0.2, and the coercivity rises dramatically in these high neodymium samples (Fig. 3(a)). These effects arise because the magnetic order in this system switches over from being iron dominated for x < 0.2 to being neodymium dominated for x >t 0.2. On the assumption of a constant neodymium moment of 3.4#B as found in tetragonal Nd2Fe~aB [1] the concentration-dependent average iron moment in these alloys may be calculated. This is plotted in Fig. 3(b). The average iron moment is roughly linear with increasing x and extrapolates to zero at x = 0.47__+ 0.02 (or about 44 total at,% Fe, as is found in a wide range of amorphous iron-based alloys [4]). The alloy with x = 1 should have a saturation magnetization of 130 J T -~ kg ~. The broken line in Fig. 3(b) was calculated using a constant neodymium moment and a linearly decreasing iron moment and reproduces the observed trend quite well. These results are summarized in Table 1.

4. C o n c l u s i o n s

Glassy (NdxFel_x)s2.sBlT.5 alloys are ferromagnetic in the composition range studied (0 ~< x ~< 0.25). Alloying with neodymium greatly reduces both the magnetic ordering temperature and the iron moment. The saturation matnetization of the alloys, while falling rapidly at small x, starts to level off for x > 0.2 as the contribution from the neodymium starts to dominate. The crystallization temperature increases rapidly with the addition o f neodymium up to a maximum Tx of 9 5 6 K at a composition where the neodymium-to-iron ratio is close to that in tetragonal Nd2FeL4B. References

1 K. H. J. Buschow, Mater. Sci. Rep., 1 (1987) 3. 2 J. M. D. Coey, J. Less-Common Met., 126(1986) 21. 3 B. Window, J. Phys. E, 4 (1971) 401. 4 J. M. D. Coey and D. H. Ryan, IEEE Trans. Magn., 20 (1982) 1280.