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Submitted on 1 Jan 1988
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ANOMALOUS MAGNETIZATION PROCESSES AT
LOW TEMPERATURES IN COMPOSITIONALLY
MODULATED Co/Mn THIN FILMS
H. Sakakima, M. Tessier, R. Krishnan, E. Hirota
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, dhcembre 1988
ANOMALOUS MAGNETIZATION PROCESSES AT LOW TEMPERATURES IN COMPOSITIONALLY MODULATED Co/Mn THIN FILMS
H. Sakakirna (I), M. Tessier (2), R. Krishnan ( 2 ) and E. Hirota (I)
(I) Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., 570 Moriguchi, Osaka, Japan
(2) Laboratoire de Magndtisme, C.N.R.S., 92195 Meudon Principal Cedex, France
Abstract. - Compositionally modulated Co/Mn thin films were prepared by UHV evaporation. The artificial periodicity
was confirmed by small angle X-ray diffraction. The CMF composed of 10
A
thick Co layers and 13A
thick Mn layers showed anomalous magnetization processes below 60 K which resemble those observed in superconducting ferromagnetic alloys.CMF (Compositionally Modulated Films) with layer thickness ranging from 7-70
a
were grown on Corning glass or Si wafer substrates by alternately de- positing Co and Mn layers using two electron-beam guns in an UHV evaporator [I]. Prior t o the growth of the CMF, about 200A
of Ma was deposited as a buffer layer. The samples are designated by (tc,, t ~ ~ ) " with t as the thickness (A) of each Co and Mn layer and nas the number of repetitions of the bilayer. The ar- tificial periodicity, A (= t ~ , + tMn)
,
was confirmed by low angle X-ray diffraction peaks as shown in figure 1.Figure 2 shows high angle X-ray diffraction patterns of the CMF. The arrows denote calculated peak positions assuming that the CMF have a-Cola-Mn stacking tex- ture [I]. The crystal structure becomes obscure with decrease of the layer thickness and the CMF(c) showed a broad diffraction peak at around a-Mn(330).
Temperature dependence of the magnetization was measured between 10 K and RT. The magnetization of the CMF with Co layer thickness less than 30
A
(a) (b) (c)
28 (deg.)
Fig. 1. - Low angle X-ray diffraction patterns with F e K
for (a): (24, 36), (b): (12, 17), and (c): (7, 9).
2 8 (deg.)
Fig. 2. - High angle X-ray diffraction patterns with Cu-K for (a): (24, 36), (b): (12, 17), and (c): (7, 9).
was found to decrease below a certain transition tem- perature
Tt,
which varied depending upon the layer thickness between 70 and 200 K [2]. The phenomenon seems to be a ferromagnetic-antiferromagnetic transi- tion with onset temperature Tt, as Mn or Col-,Mn, compounds with x>
0.35 are reportedly antiferromag- netic at low temperatures [3]. Figure 3 shows magneti--5 0 5 -5 0 5
H (KOe)
Fig. 3.
-
Magnetization curves of the sample A at low tem- peratures.JOURNAL DE PHYSIQUE
-
6 0 6 - 6 0 6 -6 0 6 -6 0 6H I K O e )
Fig. 4. - Magnetization curves of the sample B at low temperatures. zation curves of the CMF A with (10, 13)~' measured
with VSM one month after the film preparation. 4nM, of the CMF normalized to the amount of Co present was estimated t o be 9 0 0 0 ~ 1 1 000 G at room tem- perature for the CMF with Co layer thickness of 10
A
[I]. 4nM, of the sample A decreased below 200 K and was almost zero near 0 K. The film showed anoma- lous magnetization process a t low field below 50 K as shown in the figure.We have examined the reproducibility with sample
B composed of almost same layer thickness of Co and Mn layers. Figure 4 shows the magnetization curves measured with VSM about two months after the film preparation. The CMF again showed anomalous mag- netization processes below 60 K, as if the CMF were composed of ferromagnetic phase that decreased be- low
T
t
and unknown phase that appeared at lower temperatures and caused the anomalous magnetiza- tion process at low fields. The magnetization curves shown in the figure resemble those reported for super- conducting and ferromagnetic alloys such as Co7Y9 [4]or GdRuz-CeRu2 [5].
Resistivity measurement and magnetization mea- surement with SQUID have been carried out 2 years and a half after the sample preparation and the VSM measurement above mentioned. But the sample does
not show the anomalous magnetization processes any more and no anomaly is observed in the temperature dependence of the resistivity. We believe that the CMF have been changed by aging effect as the layer thick- ness and the total thickness of the samples are so small. The authors intend to re-investigate the magnetic and electric properties of the CMF.
Acknowledgments
The authors wish to thank Dr. K. Wasa, Dr. S. Hatta, Mr. H. Hisgashino and Dr. Y. Ichikawa for the help of resistivity and SQUID measurements.
[I] Sakakima, H. et al., J. Appl. Phys. 57 (1985)
3651.
[2] Sakakima, H. et al., J. Magn. Magn. Mater. 54-
57 (1986) 785.
[3] Matsui, M. et aL, J. Phys. Soc. Jpn 35 (J973) 419.
[4] Yamaguchi, Y. et al., J. Magn. Magn. Mater.
31-34 (1983) 513.
