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Submitted on 1 Jan 1988
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KINETICS OF SURFACE OXIDATION AND
RELATED CHANGES IN THE MAGNETISM OF
AMORPHOUS TbFeCo FILMS
S. Klahn, H. Heitmann, M. Rosenkranz, H. Tolle
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, dbcembre 1988
KINETICS OF SURFACE OXIDATION AND RELATED CHANGES IN THE
MAGNETISM OF AMORPHOUS TbFeCo FILMS
S. Klahn, H. Heitmann, M. Rosenkranz and H. J. Tolle Philips Research Laboratory, DBOOO Hamburg, F.R.G.
Abstract.
-
Aging of TbFeCo films at temperatures below 250 OC causes preferential oxidation of the Tb atoms resulting in a decrease of compensation temperature, perpendicular anisotropy, and coercive energy density while the in-plane magnetization increases. By increasing the aging temperature above 250 OC, the films are completely oxidized within 15 hours resulting in a decrease of magnetization. At 350 OC the oxidized films begin to crystallize.by e-beam evaporation and rf diode-sputtering at
PA= = 1 Pa and zero bias. The oxidation of un-
coated fdms was traced in-situ in a climate chamber by measuring electrical conductivity and recording Hall hysteresis loops. -4fter aging, the coercivity, Hc, and the magnetization, M., of the films were obtained at room temperature from vibrating sample magnetome- ter measurements, the anisotropy from torque curve analysis. Secondary ion mass spectroscopy provided oxygen depth profiles. The film composition was de- termined by electron probe microanalysis [4], and the microstructure was characterized by transmission elec- tron microscopy (TEM)
.
Introduction
Amorphous rare-earth (RE) transition-metal (TM) films are successfully applied for reversible magneto- optic recording. Their long-term stability, however, is limited by structural relaxation [I] and oxidation prG cesses [2]. In this paper we report on the oxidation kinetics and related changes of magnetic properties of evaporated and rf diode- sputtered TbFeCo films. Of the latter only films sputtered with zero bias were ex-
Results
amined since the stability of rf diode-sputtered films decreases when applying bias voltage [3].
Experimental
The oxidation of both the evaporated and sput- tered Tb22Fe34Co44 (at % ) films of 80 nm thickness is measured by the change of electrical conductivity
a (t,)
/ a
(0) versus aging time (Fig. 1). The decrease of a shows an Arrhenian temperature dependence with activation energies of 0.31 eV and 0.23 eV for evapo- rated and sputtered films, respectively. Oxygen depth\
TbFeCo ---evaporated 18&mi-..-
--
-sputteredprofiles reveal that an oxide layer grows into the film. Three layers of different oxygen concentrations are vis- ible: a surface layer of about 10 nm thickness [2],
a 7 ~ 50 100
TbFeCo films were deposited onto glass substrates t, hours)
Fig. 1. - Imsitu measurements of the ratio of the conduc- tivity of 80 nm thick TbFeCo films, u ( t , ) , and their initial value, u (0)
,
versw aging time, t,, in a dry oxygen-nitrogen atmosphere.where both the RE and TM atoms are completely oxi- dized; a growing oxide layer in which the TbFeCo film is only partially oxidized; and a remaining bulk layer left nearly unaffected. At temperatures below 250 OC the thickness of the surface layer remains constant, whereas that of the partially oxidized layer grows with the third root of time below 90 OC, and with the loga- rithm of time at higher temperatures. The properties of the partially oxidized layer (oxide layer) is discussed in the following. The totally oxidized surface layer is paramagnetic and is not reflected on further.
The conductivity of the oxide layer, a,, as deduced from oxygen depth profiles and conductivity measure- ments (neglecting the surface layer), is only a fac- tor of two less than in the unoxidized TbFeCo films ( a = 5.6
x
10' Q-' rn-l, a,,= 2.9x
lo5
!X1
m-I).During oxidation the compensation ternperat;re, TcOmp, shifts to lower temperatures which results in a change of Hc (T) and M, (T)
.
The coercivity of films with Tcomp>
Tambient decreases approximately lin-C8 - 1712 JOURNAL D E PHYSIQUE early with the logarithm of aging time, whereas films
with an initial T,,,,
> Tambient show an increase of
Hc to values higher than the maximum applied exter- nal field of 950 kA/m followed by a decrease of Hc. In the latter case, the Hall hysteresis loop is reversed during aging [5], thus revealing a decrease of T,,,, to temperatures below ambient temperature.At torque curve measurements two superposed sig- nals are observed, one belonging t o a perpendicular uniaxial phase and the other one t o an in-plane mag- netic phase. Aging leads t o a decrease of the perpen- dicular and an increase of the in-plane signals. In figure 2 the in-plane magnetic moment of the oxide layer is plotted versus the aging temperature (aging time 15 hours). The magnetic moment increases and reaches a maximum at 250 OC (corresponding t o a magnetization of M, = 850 kA/m) and then-decreases with increasing aging temperature. After aging for 15 hours at 450 OC, the magnetic moment of the films nearly vanishes. A sputtered agrng tome l5h
-
aL a2 '0 100 200 300 LOO 500Fig. 2. - The ratio of the magnetic moment of TbFeCo films aged for 15 hours in air, m, (t,)
,
and the magnetic moment of -the T M atoms of the films in the crystalline state derived from literature 171, m ~ C O , versus aging temper- ature, T,.Discussion
The experimental data can be explained by segrega- tion and preferential oxidation of the highly reactive RE atoms [2,6]. The oxygen concentration in the par- tially oxidized layer is about 20 a t %
,
just enough to oxidize the T b atoms completely (TbzOs+FeCo).
Ox- idized TM atoms are reduced by RE atoms exhibiting a higher binding enthalpy [6]. The TM sublattice is hardly oxidized whereas the RE sublattice magnetiza- tion is weakened by preferential oxidation. Thus Tc0,, decreases resulting in changes of Hc andMs.
The re-maining TM matrix between the RE oxide leads t o the high conductivity and in-plane magnetization, the magnetic moment of the latter (2.1 pB per TM atom) being close t o that of crystalline FesCo4 (2.4 pB) 171. At about 250 OC diffusion of oxygen is so fast that the oxidized phase has penetrated the whole film and the perpendicular anisotropy and coercive energy of the film vanish totally. The torque curves show only a magnetic phase with an in-plane anisotropy constant equal t o the shape anisotropy
OM:
/
2.At aging temperatures above 250 OC the RE atoms are completely oxidized. The TM atoms are no longer reduced by unoxidized RE atoms and the surface layer consisting of RE and TM oxides begins t o grow. The magnetic moment of the films decreases due t o the oxidation of the TM atoms. Arrhenius plots of the increase and decrease of the magnetic moment reveal activation energies of 0.31 eV and 0.36 eV, respectively, which is similar t o the activation energies for the ox- ide growth. The oxidation process is diffusion limited with an activation energy of about 0.3 eV, the same as reported for TbFe 181.
After aging RE-TM films in air a t temperatures above 350 OC TEM diffraction patterns show, that the oxidized films start crystallizing and segregating into different phases, one of which is hematite.
[I] Klahn, S., Hartmann, M., Witter, K. and Heit- mann, H., Magnetic Properties of Amorphous Metals, Eds. Hernando et al. (Elsevier) 1987, p. 176.
[2] van Dover, R. B., Gyrogy, E. M., Frankental, R. P., Hong, M. and Siconolfi, D. J., J. Appl. Phys. 5 9 (1986) 1291.
[3] Heitmann, H., Hartmann, M., Klahn, S., Rosenkranz, M., Tolle, H. J. and Willich, P., J. Appl. Phys. 61 (1987) 3331.
[4] Willich, P., Obertop, D. and Tolle, H. J., X-ray Spectrom. 14 (1985) 84.
[5] Malmhiill, M., J. Appl. Phys. 54 (1983) 5128. [6] Shen, D. H., Mizikawa, Y., Jwasaki, H., Shen,
D. F., Numata, T. and Nakamura, S., Jpn J. Appl. Phys. 20 (1981) L757.
[7] Hohl, M. and Vogt, E., Magnetic Properties I, Eds. K. H. Hellwege and M. Hellwege (Lan- dolt Bornstein) Vol. 1119 (Springer, Berlin) 1962, pp. 1-52.
[8] Allen, R. and Connell, G. A. N., J. Appl. Phys.
