Influence of Nd content on magnetic properties of amorphous FeB alloys

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ELSEVIER Journal of Magnetism and Magnetic Materials 146 (1995) 37-41

Influence of Nd content on magnetic properties of amorphous Fe-B alloys

N. Hassanain a, H. Lassri b, R. Krishnan c,., A. Berrada

^a

a Laboratoire de Physique des Matdriaux, Facult~ des Sciences Rabat, Morocco

b Laboratoire de Physique des Mat~riaux et de Microdlectronique, Facultd des Sciences Ain chok, Casablanca, Morocco c Laboratoire de Magn~tisme et Mat~riaux Magndtiques, CNRS 92195 Meudon, France

Received 7 September 1994; in revised form 21 October 1994

Abstract

The influence of the addition of Nd on the magnetic properties (Tc, Mo, H c .... ) of Fe-B amorphous alloys is investigated. Using Chudnovsky's model we have analyzed our data and obtained some fundamental parameters. For instance, with the addition of Nd atoms the local anisotropy is 2.0 × 1 0 7 e r g c m - 3 and the exchange constant A decreases from 38 × 10 -8 to 21.7 X 10 -8 erg cm -1 as the Nd concentration increases from 0 to 15%. The ferromagnetic exchange correlation length also decreases drastically from 353 to 80 .~ in the same concentration range.

1. Introduction

In the last few years the study of rare earth (RE) and transition metal (TM) based amorphous alloys ( R E - T M - B ) has become very intense [1-4]. The reasons for this are on the one hand that their physical properties are very useful in technological applications, and on the other hand that they exhibit random magnetic anisotropy (RMA) and magnetic exchange which are fundamentally very important.

Amorphous alloys containing rare earth metals and particularly those with large spin-orbit coupling pre- sent what is known as random magnetic anisotropy due to topological disorder. In order to study the influence of the addition of Nd on the various mag-

* Corresponding author.

netic properties of amorphous F e - B alloys, we prepared amorphous NdxFe80_xB20 alloys with 0 < x

< 15 and investigated their magnetic properties.

2. Experimental

Amorphous NdxFe80_xB20 alloys with 0 < x < 15 were prepared in the form of ribbons by the usual melt spinning technique in an inert atmosphere of argon. The amorphous structure was characterized by X-ray diffraction using Co Kct radiation. The magnetic moment M was measured using a vibrating sample magnetometer (VSM) with a maximum applied field of 17 kOe, and in the temperature range from 4 to 300 K. The Curie temperature T c was determined from the evolution of the magnetic moment in a weak field (100 Oe) in the temperature range 300 to 1000 K.

SSDI 0304-8853(94)01661-5

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38 N. Hassanain et al. /Journal of Magnetism and Magnetic Materials 146 (1995) 37-41 3. Results and discussion

For all the samples studied, magnetic saturation could be obtained with H < 17 kOe at all tempera- tures. The Nd concentration dependence of T c and the magnetic moment M at 4 and 300 K is shown in Fig. 1. Both the quantities decrease linearly with x.

The above results are attributed essentially to the dilution of Fe in the alloy and a decrease in the Fe moment due to the addition of Nd. The coupling of Nd and Fe moments is ferromagnetic and the alloy moment at 4 K (/z a) can be written as

~a = ((80 - x ) ~Fo + x~'~d)/a00, (1) where /-~F¢ and /Z~d are Fe and Nd moments respec- tively.

For small concentrations (x _< 6) of Nd, the iron moment /zF~ is not perturbed. So taking the value of /~v¢--2.06PB obtained from the alloy with x = 0 and substituting it in Eq. (1), it is possible to deter- mine the Nd moment for x = 4 and 6. The ~Nd is found to be 1.4/zs which is smaller than the theoreti- cal value (3.27~B) and indicates that the Nd moments form a conical spin structure with a half cone angle 0 = 100 °. This reduction in Nd moment arises from the random local anisotropy of Nd atoms which

tends to align the Nd moment along random directions and causes a spread in their directions. Now /XFe for other alloys could be calculated based on the reasonable assumption that /.eNd is independent of x.

Fig. 2 shows that at 4 K, on one hand, /['£a decreases with Nd. On the other hand, it is seen that /xFe deceases only very slightly with increasing x unlike in the case of F e - E r - B - S i amorphous alloys [5].

The coercive field H c is structurally very sensi- tive and is significantly dependent on the technological details of specimen preparation. Hc inceases rapidly with Nd concentration at 4 K, as shown in Fig. 3, which is due to the anisotropy of the Nd atoms.

The approach to saturation of the magnetic moment has been interpreted by Chudnovsky and Serota [6,7]. From the above models, for applied fields less than the exchange field Hex, the magnetic moment is expected to show a linear dependence on H - ~ / e From the slope one can deduce the field H s, using the equation

( M o - M ) / M o = ~ ~ s / H , (2)

H s = H r 4 1 H 3 x , (3)

lOO

5O

0 4 8 12 16

Nd (x)

Fig. 1. The Nd concentration dependences of M (at 4 and 300 K) and T c.

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N. Hassanain et aL / Journal of Magnetism and Magnetic Materials 146 (1995) 37-41 3

39

E 2

o

eg

a s

-q

~'[Ii

l~v+

k I I

0 4 8 12

N d (x)

Fig. 2. The Nd concentration dependences of ~Fe and /~a at 4 K.

16

400

300

0

2OO

100

0

I 1 I

4 8 12

Nd (x)

Fig. 3. The Nd concentration dependence of H c.

16

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40 N. Hassanain et al. / J o u r n a l of Magnetism and Magnetic Materials 146 (1995) 37-41

where H r is the random anisotropy field. The local random anisotropy constant K L is given by

H r =

2 K L / M o.

(4)

From the same model [6,7] one can also relate Hcx and the exchange constant A by the expression

Hex

=2A/(MoRZ~),

(5)

where R is the length over which the local axes show a correlation (10 ,A) [6]. The exchange constant can be obtained from the mean field model proposed by Hasgawa [8] and from the Curie temperature using the relation proposed by Heiman et al. [9]. By neglecting the F e - N d and N d - N d interactions one can derive the following relation [10]:

a = (CSFekBTc)/(4(SFe

⁺

1)rFe_Fe),

(6) where C is the concentration of Nd in atomic per- cent. We found that the exchange constant A decreases from 35 × 10 -8 to 2 1 × 10 -8 erg cm -a when x increases from 2 to 15.

By plotting M as a function of H -°'5 (Fig. 4), one can obtain H s from the slope and the random anisotropy field H t from the relations (3) and (5).

From relation (4) we calculate the anisotropy constant K L which is reported in Table 1.

Table 1

Some magnetic parameters from the approach to saturation at 4 K x A(10-3ergcm -1) KL(107ergcm -3) Rf(,~) A

0 38 1.4 553 0.14

4 33 1.7 283 0.19

10 26.2 2.0 129 0.28

15 21.7 2.1 80 0.35

It is known [6,7] that the magnetic behavior of the random anisotropy system changes drastically with the value of the dimensionless parameter

2 2

A = ~1~ KL Ra/A = ~aa/Rf, (7)

where R e is the ferromagnetic correlation length.

Finally, from

KL,

A and R a and with the help of Eq. (7) h was calculated. It is known that for A < l (weak anisotropy) the ferromagnetic correlation length Rf becomes greater than R a. The various parameters are shown in Table 1. It is interesting to note that R e decreases with Nd addition and the value for x > 10 agrees with the results from neutron measurements by Rhyne [11]. It is found that in our alloys A < 1, which corresponds to a ferromagnet system with high exchange and a weak anisotropy.

220 200 180 160

~E 140 120 100

80

~ x~O

-'-" ~ ~ - o - x--4

~ x = l O

" ' - ~ x = 15

| I I I I

0 0,1 0.2 0.3 0.4 0,5

H ~'' ( kO~ "°')

Fig. 4. The H-1//2 dependence of magnetic moment at 4 K.

0.6

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N. Hassanain et al. / Journal of Magnetism and Magnetic Materials 146 (1995) 37-41 41 4. Conclusions

We have prepared amorphous NdxFeso_xB20 alloys and carried out magnetization studies up to 17 kOe. The Nd moment at 4 K is found to be 1.4/~ B indicating a conical spin structure. The anisotropy studies show that these alloys are weak anisotropy ferromagnets.

References

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