POSITIVE MAGNETORESISTANCE IN THE AMORPHOUS GdCo3 FILMS : COULOMB INTERACTION EFFECT ?

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POSITIVE MAGNETORESISTANCE IN THE

AMORPHOUS GdCo3 FILMS : COULOMB

INTERACTION EFFECT ?

W. Maj, M. Serbanescu

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplkment au no 12, Tome 49, dkcembre 1988

POSITIVE MAGNETORESISTANCE IN THE AMORPHOUS GdCog FILMS:

COULOMB INTERACTION EFFECT?

W. Maj (I) and M. D. Serbanescu (2)

(I) Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warsaw, Poland (') Institute of Physics and Technology of Materials, P.O. Box MG-6, Bucharest, Romania

Abstract. - We report preliminary measurements of the magnetoresistance in sputtered GdCo3 thin films from 4.2 to 100 K and in magnetic fields up to 6 T. Beyond technical saturation, the magnetoresistance is positive, varies linearly with the applied field and is nearly temperature independent. A possible contribution from electron interadions is discussed.

GdCos target in an Ar atmosphere onto water-cooled

We have studied the resistivity, magnetoresistance

-

- and Hall effect in the amorphous ferrimagnet GdCo3

[I]. We were initially motivated in this investiga- 4 tion by the work of Przysrupski and Raduszkiewicz,

who observed a large zero-bias anomaly in the tunnel- ing resistance of A1/A1203/a-GdCo3 junctions [2]. The anomaly was explained on the basis of electron- electron interaction (EEI) theory [3]. This theory pre-

glzq substrates without dc bias voltage. The thick- ness was measured by an interference microscope. The films were 2 700

fi

thick. The lack of crystalline struc-

1

..**! -

.

...** _{4 2 K}

_-

.* .**

.

C . . .**'

....

...*

....*'

I

~ O O K - ,...** *.-...**** a - ~d CO,

ture was confirmed by X-ray analysis. A stripe do- main structure was observed a t room temperature by the polar Kerr effect.

dicts also peculiar behaviour of the low temperature o 2

4 6

transport properties. In particular, a positive, nonsat- H [TI

u r a t i n ~ magnetoresistivity is predicted. In this note Fig, 1. - Norhalized transverse magnetoresistance

we discuss the EEI contribution to the magnetoresis- A R / R (0) = [ R (H)

-

R (O)] /R (0) of a-GdCos at 4.2 and

tivity of aGdCoa thin films. A detailed report will be 100 K. The field was applied perpendicularly t o the film

published elsewhere [4]. plane. For clarity, the curves have been zero shifted. The films were rf sputtered from an arc melted

Transport measurements were taken in a flow cryo- stat with a superconducting coil of 6 T and in a conven- tional * ~ e cryostat with a superconducting coil of 4 T by a standard dc four-probe technic. The estimated value of the resistivity was about 220 ,uRcm. The low- field galvanomagnetic properties were typical for a-

GdCo3 films. In particular, the negative magnetoresis- tance described by Berger [5] was ARIR = - 1 . 5 ~ 1 0 ~ ~ at 4.2 K to compare with about -2

x

reported by Okuno et al. [6]. However, beyond technical sat- uration the magnetoresistance is isotropic, varies lin- early with the applied field and is nearly temperature independent. The value of the slope d In R/dH 0.5 x T-' obtained by the least square fitting of data points beyond saturation was constant (to within experimental error) over a temperature range from 1.5 up t o at least 48 K. Figure 1 shows the transverse mag- netoresistance measured in fields applied perpendiu- larly to the film plane at 4.2 and 100 K. The anomaly is unlikely to be due to the orbital motion of electrons because of the extremely short elastic mean free path

(N interatomic distances). The ordinary magnetore- sistance should be negligibly small in a high-resistivity amorphous metal. This assumption is favored by ex- periments [7, 81.

Since the EEI theory provides a good description of the resistivity of *GdCo3 films at low temperatures [I], it seems reasonable to discuss its contribution to the magnetoresistivity. According to Lee and Ramakr- ishnan [3], in an amorphous ferromagnetic metal a pos- itive magnetoresistivity arises from the splitting of the spin-up and spin-down bands and is given by

provided that k ~ T / g p ~ < < H ~ f f . A constant

F is the bare screening parameter (0

5

F

<

I ) , D is the diffusion constant, he^ is the effective mag- netic field feIt by the conduction electrons. Following Olivier et al. [9] we put Hetr=H

+

Hint, where Hint is the internal exchange field. Thus, Hint may be quite large here (-- 100 T or more). Consequently, when ~ B T / ~ P B , H

<<

Hint, which can be easily met in our case, the magnetoresistivity shows a linear-field depen- dence and is nearly temperature independent. Using D

--

0.1 cm2/s estimated from the resistivity mea- surements [I] and Hint>40 T , to account for the ab-

C8

-

1356 JOURNAL DE PHYSIQUE

sence of temperature dependence of d In R/dH below found here may be fortuitous and a detailed analysis 48 K, we obtain the observed magnitude of d In R/dH of all possible contributions to the magnetoresistance for E 0.5. The bare screening parameter F is then of a-GdCos is necessary.

--

0.55.

Before we conclude, at least three points need a com- Acknowledgements ment. Firstly, it should be noted that the applied field

must add to Hint with the same sign, if this effect is t o be responsible for the observed anomaly. However, it has been found that the resistivity in a-GdCoa is dominated by d-electrons of Co [I]. Moreover, it is a well known fact that the magnetization of the GdCo3 alloy is dominated by the Gd sublattice at low temper- atures (here a t least up to the room temperature). In what follows, the applied field would have to enhance the exchange splitting of d electrons of Co, though the magnetic moment of Co sublattice points opposite to the applied field. Secondly, taking H i n t ~ 4 0 T we ob- tain an upper limit of d In R/dH. In a real system Hint can be much larger. The exchange splitting of d- electrons of Co is of the order of l eV. Then, formally, Hint which enters equation (1) would be

--

lo4

T (!). Thus the contribution to the magnetoresistivity from the EEI might be strongly reduced or even suppressed. Finally, one really does not know what is a reason-

able value of F in a 3d ferromagnetic metal. The bare

screening parameter may be much larger than 1

[lo].

To conclude we have observed anomalous, positive, isotropic high-field magnetoresistance in the amor- phous ferrimagnet GdCo3. The anomaly may be ex- plained by the Coulomb interaction theories, provided that the magnetic moment of the Co sublattice is ex- chapge enhanced by the Gd sublattice in the applied magnetic field. However, a quantitative agreement

The authors are grateful to Dr. J. Rauluszkiewicz and Dr. V. Florescu for constant rrupport in this in- vestigation. Thanks are given to M:rs. J. Gbrecka for X-ray analysis, Dr. J. Nowak for t,hickness measure- ments, and Dr. W. Dobrowolski for help in measure- ments in a flow cryostat.

[I] Maj, W., Serbanescu, M. D. and Zajbt, T., Int. Symp. on Magn. Properties of Amorphous Met- als (Benalmadena, Spain, 1987') Abstracts p. 59 &d Maj, W., to be published.

[2] Przyslupski, P. and Raduszkiewicz, J., Solid State Commun. 50 (1984) 789.

[3] For a review see Lee, P. W. and Ramakrishnan, T. V., Rev. Mod. Phys. 57 (1985) 287.

[4] Maj, W., to be published.

[5] Berger, L., A I P Conf. Proc. 29 (1975) 165. [6] Okuno, H., Matsushita, L. and Sakurai, Y

.,

IEEE

Trans. Magn. 17 (1981) 2831.. (Erratum: the minus sign is missing before the normalized mag- netoresistance A p / p in Fig. 6).

[7] Bergrnann, G., Phys. Rev. B 15 (1977) 1514. [8] Kuz'mienko, V. M. and Mel'nikov, V. I., Fiz. Met.

Metalloved. 50 (1980) 984.

[9] Olivier, M., Strom-Olsen, J. C). and Altounian,

Z., Phys. Rev. B 35 (1987) 333.