JOURNAL OF APPLIED PHYSICS
VOLUME 89, NUMBER 11
1 JUNE 2001
Oscillation of the coercive force for ultrathin AgÕCoÕCu„111… films J. S. Tsaya) Department of Physics, Tunghai University, Taichung, Taiwan 407, Republic of China
Y. D. Yao Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
J. Y. Lin Department of Physics, National Chung Cheng University, Chiayi, Taiwan, Republic of China
C. S. Yang Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
The magnetic properties of ultrathin Ag/Co/Cu共111兲 films have been investigated using the surface magneto-optical Kerr effect technique. Along polar direction with a harder axis of magnetization, Ag overlayer-induced oscillation of the coercive force shows a strong in-plane anisotropy for complete Ag atomic layers. It is consistent with the enhanced longitudinal Kerr intensity. A more significant change of the magnetic properties for a thinner film was observed and this should be attributed to a larger ratio of the magnetic–nonmagnetic interface to the volume. The relatively easy axis of the magnetization is along the longitudinal direction for the Ag/Co/Cu共111兲 system. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1354587兴
I. INTRODUCTION
III. RESULTS AND DISCUSSIONS
The influences of interface and nonmagnetic layers on the magnetic properties of ultrathin films are very important in material science and have received intensive attention.1–8 This topic is of great scientific and technological interest. Switching the easy axis of Co films 关up to 20 monolayers 共ML兲 thick兴 with a submonolayer amount of Cu has been reported.1 The analogous situation occurs for other magnetic films covered with submonolayers of Ag, Au, and oxygen.2 In the present work we have studied the variation of the magnetic properties of Co/Cu共111兲 films upon Ag deposition up to a few ML. We observed an Ag overlayer-induced oscillation of the coercive force as the thickness of the Ag overlayer increases.
When Ag overlayers were deposited on the top of a 2.3 ML Co/Cu共111兲 film at room temperature, Kerr signals versus magnetic field were measured and are shown in Fig. 1. As can be seen in the figure, the polar hysteresis loop is broader than the longitudinal one before a deposition of the Ag overlayer. The polar coercive force is about five times of magnitude larger than the longitudinal one. This means that the relatively easy axis of the magnetization is along the longitudinal direction for a 2.3 ML Co/Cu共111兲 film. However, it is possible to magnetize the film on both the polar and longitudinal configurations using magnetic fields of several tenths of kOe. After the deposition of Ag overlayers, significant change occurs for the width and highness of the loops on both the longitudinal and polar configurations. The coercive forces as functions of Ag thickness are summarized in Fig. 2. After the deposition of the Ag overlayer, the longitudinal coercive force remained constant while an oscillation behavior for the polar coercive force was observed. The oscillation period is around one atomic layer. Comparing the magnitudes of the coercive forces on both the polar and longitudinal configurations, the polar coercive force is larger. This means the magnetic easy axis is relatively along the longitudinal direction. By way of measuring the polar coercive force, one can get the information of the strength of an in-plane magnetic anisotropy. The oscillation behavior with a period of one ML should be due to the morphology change of the Ag layer. This change causes a magnetic anisotropy oscillation. For a complete Ag atomic layer, the observation of a larger polar coercive force indicates a stronger in-plane anisotropy. This should be attributed to the overlayer-induced change in the electronic structure.3,4 Figure 3 shows the thickness dependence of Kerr intensities for Ag/2.3 ML Co/Cu共111兲 films. At the initial stage of
II. EXPERIMENT
The experiments were conducted in an ultrahigh vacuum 共UHV兲 chamber with a background pressure below 1 ⫻10⫺10 Torr. The UHV chamber was equipped with instruments for Auger electron spectroscopy 共AES兲, surface magneto-optic Kerr effect 共SMOKE兲, low-energy electron diffraction 共LEED兲, and x-ray photoemission spectroscopy 共XPS兲 measurements. The various components are described in detail elsewhere.8,9 The Cu共111兲 surface was cleaned by cycles of Ar⫹ ion bombardment and subsequent annealing at 700 K until a well-ordered 1⫻1 LEED pattern with sharp spots and a low background was observed. The deposition rates of Co and Ag were determined by AES and confirmed by a SYCON thickness monitor of quartz balance. A rotatable electromagnet in the UHV chamber was used to apply a magnetic field; the maximum magnetic field reached 1 kOe. a兲
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© 2001 American Institute of Physics
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J. Appl. Phys., Vol. 89, No. 11, 1 June 2001
FIG. 1. Kerr signals versus magnetic field for Ag/2.3 ML Co/Cu共111兲 films grown at room temperature. After the deposition of Ag overlayers, a significant change of the hysteresis loop occurs on both the longitudinal and polar configurations.
Tsay et al.
FIG. 3. Kerr intensities of Ag/2.3 ML Co/Cu共111兲 films as a function of the Ag overlayer thickness on both the polar and longitudinal configurations.
the Ag deposition, we observed a strong enhancement in the longitudinal Kerr intensity. This is attributed to surface morphology and the locally ordered growth that affects the magnetic anisotropy.1,3 After this region, a monotonic variation of the Kerr intensity is expected. However, a significant oscillation behavior of the Kerr intensities was observed. The period of the oscillation is also around one atomic layer. The oscillating amplitude of the longitudinal Kerr intensity is larger than the polar one. This indicates that it is easier to magnetize the film on the longitudinal configuration for a complete Ag layer. This result coincides with the oscillating behavior in Fig. 2 where the polar coercive force peaks around complete Ag layers.
FIG. 2. Coercive forces of Ag/2.3 ML Co/Cu共111兲 films as a function of the Ag overlayer thickness on both the polar and longitudinal configurations. The longitudinal coercive force remains constant while the polar coercive force shows an oscillation behavior. For a complete Ag atomic layer, the observation of a larger polar coercive force indicates a stronger in-plane anisotropy.
FIG. 4. LEED patterns for 共a兲 a clean Cu共111兲 surface, 共b兲 a 1 ML Ag/ Cu共111兲 film, and 共c兲 a 3 ML Ag/Cu共111兲 film. The Ag overlayer shows a 1⫻1 structure.
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Tsay et al.
J. Appl. Phys., Vol. 89, No. 11, 1 June 2001
FIG. 5. The changes of the polar coercive forces (⌬H C ) as functions of the Ag overlayer thickness for Ag/Co/Cu共111兲 films. The oscillation of ⌬H C is much more significant for a thinner Co layer.
The growth of Co and Ag films on a Cu共111兲 surface, according to our SMOKE measurements, has been systematically investigated using the LEED technique. For Co/ Cu共111兲 films, a 1⫻1 LEED pattern was observed. As the thickness of the Co layers increase, the LEED pattern becomes diffused. This is consistent with the morphology change from two-dimensional growth by bilayer island formation to three-dimensional morphology with many atomic levels simultaneously exposed as reported before.10,11 In general, the Ag overlayer shows a 1⫻1 structure as grown on the top of Co/Cu共111兲 films up to several-ML-Ag thickness studied by the LEED technique. As an example, Fig. 4 depicts LEED patterns of Ag overlayers grown on a Cu共111兲 surface. Although the intensities of the 1⫻1 LEED spots become dim, the spots are still visible as the thickness of the Ag overlayer increases up to 3 ML. The growth of the first-
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few-ML Ag is in an ordered state. The evolution of the LEED patterns for the Ag overlayer grown on Co/Cu共111兲 is similar. We did not show them here because of a technical problem for low intensities of the spots. The changes of the polar coercive forces (⌬H C ) as functions of the Ag overlayer thickness for Ag/Co/Cu共111兲 films are shown in Fig. 5. As the Ag thickness increases, ⌬H C oscillates with a period of 1 ML. The oscillation of ⌬H C is much more significant for a thinner Co layer. We propose that these oscillations are attributed to the periodic variation of the film morphology between the filled and unfilled Ag layer. The magnetic–nonmagnetic interface is more important for a thinner film because of a larger ratio of the interface to the volume. Therefore the change of an electronic structure is larger and causes a more significant change for the corresponding magnetic properties. In conclusion, the relatively easy axis of the magnetization is along the longitudinal direction for a 2.3 ML Co/ Cu共111兲 film. When Ag overlayers are deposited on the top of a Co/Cu共111兲 film, the longitudinal coercive force remains constant. Along the polar direction with a harder axis of magnetization, Ag-induced oscillation of the coercive force shows a strong in-plane anisotropy for complete Ag atomic layers. This result is consistent with the thickness dependence of the longitudinal Kerr intensity. The magneticnonmagnetic interface is more important for a thinner film because of a larger ratio of the interface to the volume. A more significant change for the corresponding magnetic properties was observed.
1
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