MAGNETIC STUDIES OF FeNdB COMPOSITIONALLY MODULATED THIN FILMS

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MAGNETIC STUDIES OF FeNdB

COMPOSITIONALLY MODULATED THIN FILMS

B. Martinez, M. Moreu, J. Tejada

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, dBcembre 1988

MAGNETIC STUDIES OF FeNdB COMPOSITIONALLY MODULATED THIN FILMS

B. Martinez (I>, M. A. Moreu (2) and J. Tejada (2)

(I) I C M A B C.S.I.C., Marti i Franques s/n, 08028 Barcelona, Spain

(') Faeultad Fisica, U. Barcelona, Diagonal 649, 08028 Barcelona, Spain

Abstract. - In this paper we present a magnetic study of several NdFeB compositionally modulated thin films. We have done an analysis of the temperature dependence of magnetization in the scope of collective excitations. It is shown that the spin waves modes depend on both the thickness and modulation of the multilayers. The values of the spin-wave stiffness constant in multilayers are smaller than those obtained in crystalline and glassy alloys.

Introduction

Magnetic superlattices and compositionally modulated films have attracted much attention recently [I, 21.

In this paper we present a systematic series of magnetization measurements for different NdFeB compositionally modulated thin films. Our attention will be focused on the existence of spin waves and its dependence on the geometrical parameters of the samples.

Experimental

The magnetization measurements were recorded using a SHE SQUID magnetometer in applied magnetic fields up t o 5.5 T working in the temperature range between 1.8 and 200 K.

Our NdFeB compositional modulated thin films were deposited onto glass substrates held at room temperature by using a special triode sputtering system. The modulated structure of the films was verified by low angle X-ray scattering. In table I we summarize the thickness and composition of the samples. Table I. - Sample characteristics i n the present paper.

The sample denoted with the number 6 i s a iibulk'7 sam- ple of composition NdlaB6Feso and 2 000

k

thickness.

20000 40000

H (Gauss Fig. 1. - Magnetization dependence at T = 4.2 K, on the applied magnetic field.

Sample I 2 3 4 5 6

t o the substrate. In figure 2 we show a typical hysteresis loop and in table I1 we summarize the values of the coercitivity, H,, remanence, M, and the extrapolated

saturation magnetization to zero field, Mo. Ndz6B6Fess single layer thickness (A) 2.3 4.6 9.1 18.2 36.5

-

In the multilayers the presence of spin waves can be understood as collective excitations of the structure as whole and consequently they may have different modes than those of the any one constituent.

Results and discussion For that reason we have carried out a careful study of

the temperature dependence of the "saturation" mag- In figure 1 we show the magnetization dependence, 'netization,

Ms.

This has been done by applying a field

Fe92B8 single layer thickness (A) 1.1 2.3 4.6 9.1 18.2 -

at constant temperature T = 4.2 K, on the applied of 5 T and looking for the change of magnetization magnetic field, parallel (H

11)

and perpendicular ( H I ) when the samples are heated from 1.8 K to a well con-

Characteristic modulation length X (A) 3.4 6.9 13.7 27.3 54.7 - -

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C8 - 1808 JOURNAL DE PHYSIQUE

-3000 0 3000

Fig. 2. - Hysteresis loop for sample No 2 with H I to the sample plane.

Table 11. - Hzsteresis loop characteristics for H I to the substrate.

trolled new temperature. As a consequence of the rel- ative large diamagnetic signal of the glass substrate, which is very similar t o that of the samples at T

2

50 K for H = 5 T, we have decided to consider only the Ms ( T ) data for T

5

30 K . At this temperature range

the incertitude in the temperature is less than 0.1 K and we are able to detect changes in M to one part in lo4.

The M, ( T ) data have been fitted by using the equation

where the T ~ / ' and T ~terms reflect the existence / ~

of magnons and their mutual interaction and the

T

'

accounts for Stoner excitations. The F functions are the Bose-Einstein integral functions and the Riemman zeta functions are denoted by

C.

The field values used

Table 111. - B and C values of equation ( 1 ) obtained from the our least squares fit procedure.

in evaluating the F functions contain the applied exter-

nal field, the anisotropic and the demagnetizing fields. No improvements in the least-squares fitting resulted from use of a non-zero value of A. In table I11 we sum- marize the values of B an C obtained from our fitting procedure.

The main results are summarized as follows: Sample 1 2 3 4 H

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5 6 1 2 3 4 H I 5 6

i) our fit procedure provide remarkably good sup- port for equation (1);

ii) the values of both B and C parameters are much larger than those corresponding t o ordinary ferromag-. nets;

iii) the B values deduced from the M I ( T ) data are larger than those deduced from M

11

( T ) which may reflect the antiferromagnetic coupling between Nd and Fe within the NdBFe. Therefore when the applied field

B ( K - ~ / ~ ) 4.82

x

8.84

x

7.82 x 6.64 x 6.22 x 4.97 x 1.73 x 1.70 x lo-' 1.73 x lo-' 1.94 x lo-' 1.92 x lo-' 1.44 x lo-'

is in the perpendicular direction a canting angle between the Fe and Nd magnetic moments appears pro- ducing a faster decreasing of magnetization;

C ( K - ~ / ~ ) -7.89 x low5 -1.72 x -1.38 x -1.14 x -1.07 x -8.89

x

-3.15 x -3.11 x -3.27 x

low4

-3.65 x -3.78

x

-2.69 x

iv) for the saniple having X = 3

A

we have obtained similar B and C values that for the "bulk" sample, 6 , and for the samples with X

>

3 both B and

C values are larger and depend on A. Therefore we may conclude that the collective excitations of these multilayers depend on the geometry of the samples and the interface region is less than 2

A

in agreement with Mogsbauer results [3].

[ I ] Puzniak, R. and Dmowski, W., J. Phys. F 17 (1987) 1437.

[2] Chien, C. L. and Hasegawa, R., Phys. Rev. B,16

(1977) 2115.

[3] Martinez, B., Moreu, M., Ruiz, A., Labarta, A., Obradors, X., Briones, F. and Tejada, J., I.E. E.E.