IRON SPIN REORIENTATION IN MULTILAYERED Fe/RARE EARTH METAL FILMS

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IRON SPIN REORIENTATION IN MULTILAYERED

Fe/RARE EARTH METAL FILMS

N. Hosoito, K. Yoden, K. Mibu, T. Shinjo

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988

IRON SPIN REORIENTATION IN MULTILAYERED Fe/RARE ,EARTH METAL

FILMS

N. Hosoito, K. Yoden, K. Mibu and T. Shinjo

Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu, 611 Japan

Abstract. - In multilayered Fe/Dy and Fe/Nd films, it is observed from 5 7 ~ e Massbauer spectra that direction of the Fe magnetic moments changes from in-plane t o perpendicular to the film plane with decreasing temperature. The relation of the spin reorientation and other magnetic properties is discussed in the Fe/Nd case.

Introduction

Multilayered transition metal (TM)/rare earth metal (RE) magnetic films with artificial superstruc- tures offer us a new magnetic system in which two magnetic layers with different characteristics interact with each other through the interfaces. In Fe/Gd [I] and Co/Gd [2] superstructures, ferrimagnetic align- ments of TM and RE layer magnetic moments were observed. In Fe/Tb superstructures, a strong perpen- dicular magnetic anisotropy was observed [3]. These facts indicate that magnetic interactions and/or mag- netic anisotropies induced at the interfaces play im- portant roles in magnetic properties of artificial super- structured films. In other words, magnetic properties of TM/RE artificial superstructures are not a simple contribution of TM and RE layers.

We carried out systematic investigations on Fe/Dy and Fe/Nd superstructures and found a new magnetic property, spin reorientation of Fe magnetic moments, with 5 7 ~ e Mossbauer spectroscopy. We discuss a rela- - -

tionship of the spin reorientation and other magnetic properties of the superstructures.

Experimental

The multilayered films were prepared by alternate deposition of Fe and RE in the vacuum of

lo-'

Torr range. Since rare earth metals are easily oxidized, the ultrahigh vacuum is a necessary condition to obtain metal films. The metals are alternately evaporated by electron beam heating and deposited on a kapton sheet cooled down to

-

50 O C to inhibit interdiffusion of Fe

and RE.

We confirmed the formation of the layered structure in the films and examined the structures of constituent layers by X-ray diffraction. Magnetic properties of the superstructures were studied with 5 7 ~ e Mossbauer spectroscopy and magnetization measurements with a SQUID magnetometer.

Results and discussion

The spin reorientation of Fe magnetic moments was first observed in Fe/Dy artificial superstructures with

bcc Fe layers and amorphous Dy layers. Since the de- tails were shown elsewhere [4, 51, we only introduce the main points here. The spin reorientation starts at around 200 K where the Dy layer magnetization begins to increase rapidly. We interpret the phenomenon as a competition of the shape anisotropy of the Fe layer and a perpendicular anisotropy induced by the Dy atoms through the interaction of Fe and Dy moments at the interface.

In the case of the Fe/Nd superstructures, we also observed the Fe spin reorientation. Hereafter we discuss the results on the sample of [ ~ e (39

A)

/

Nd (28

A)]

40 in which the Fe and the Nd layers have

their bulk crystal Structures. The Mossbauer spectrum for [ ~ e (39

A)

/

Nd (28

A)]

,,

shows a magnetic split six-line pattern between 4.2 K and 300 K. Relative in- tensities of six lines in Mossbauer spectra are generally expressed as 3 : X : 1 : 1 : X : 3. The value of X is related to the angle 0 between the film normal and the Fe magnetic moment with the following equation,

in our experimental condition. The temperature de- pendence of X is plotted in figure 1. The value of X is 4 a t room temperature. This fact indicates that magnetic moments of the Fe layer are in-plane direc- tion, which is usual in thin films because of the shape anisotropy. With decreasing temperature, X becomes smaller and at 4.2 K, X is 0.74, indicating that the angle 0 is 34'. From these facts, we conclude that the magnetic moments of the Fe layer in the sam- ple of [ ~ e (39

A)

/

Nd (28

A)]

40 rotate from in-plane

t o perpendicular direction when the temperature be- comes low. The spin reorientation phenomena are ob- served in the samples in which both the Fe and the Nd layers are in crystal states.

We carried out magnetization measurements using the same sample as the Mossbauer measurements to compare the spin reorientation phenomena in Fe/Nd superstructures with that in the Fe/Dy case. The sat- uration magnetization of [ ~ e (39

A)

/

Nd (28

A)]

,,

is plotted against the temperature in figure 2a. The mag- netization is normalized with the total volume of the

C8 - 1778 JOURNAL DE PHYSIQUE

Fig. 1. - Temperature dependence of relative intensity

X for Am = 0 Mijssbauer absorption lines. The relative intensity ratio of six lines is expressed as 3 : X : 1 : 1 : X : 3. The corresponding angles are indicated in the right side of the ordinate. The sample has a nominal structure of

[ ~ e (39

A)

/

Nd (28

A)]

40

.

sample. At room temperature the observed magneti- zation is explained well if we assume that the Fe layer has full magnetic moments and there are no contribu- tion from the Nd layer moments. The magnetization increases monotonously with decreasing the tempera-

Fig. 2.

-

a) Saturation magnetization vs. temperature. MFe means the saturation magnetization of the Fe layer. The sample is the same as in figure 1. b) Anisotropy energy

W vs. temperature. The definition of W is given in the text. The direction of the magnetic field is parallel (0) or perpendicular (A) to the film plane.

ture. We attribute the increase to the magnetic mo- ments of the Nd layer. No drastic change is observed at around 220 K where the angle 13 shows rapid change. This is in contrast to the case of Fe/Dy superstruc- tures.

To clarify the origin of the spin reorientation in the Fe/Nd case, magnetization curves were measured with applying magnetic field parallel or perpendicular to the film. Since the curves do not show the typical uniaxial behavior, we discuss the following quantity W,

where H is magnetic field, M is the magnetization of the sample and M, is the saturation magnetization. In figure 2b, we plot the temperature change of WII where the magnetic field is applied parallel and W l where the magnetic field is applied perpendicular to the film. We point out that the behavior of W l can be explained with the shape anisotropy energy 2aM:. This implies that the Nd moment induces no intrinsic anisotropy in the direction perpendicular to the film. It is clear that the energy difference of WL and WII becomes smaller at low temperatures because of the more rapid increase in Wli. We suppose that this behavior is responsible for tl;k spin reorientation phenomena in Fe/Nd super- structures. The rapid increase in WII suggests that the Nd moment has such a magnetic anisotropy that makes the in-plane direction a magnetic hard axis.

Finally we stress that magnetic interactions of TM and RE layers are of great importance for the spin re- orientation of TM/RE superstructures and the effect of magnetostriction is less important because our pre- liminary Mossbauer measurements on a reference sam- ple of Fe/Y superstructure show no spin reorientation phenomena.

[l] Morishita, T., Togami, Y. and Tsushima, K., J. Phys. Soc. Jpn 54 (1985) 37.

[2] Webb, D. J., Walmsley, R. G . , Parvin, K., Dick-

inson, P. H., Geballe, T. H. and White, R. M.,

Phys. Rev. B 3 2 (1985) 4667.

[3] Sato, N., J. Appl. Phys. 59 (1986) 2514. [4] Hosoito, N., Yoden, K. and Shinjo, T., Hyperfine

Interactions 41 (1988) 583.