Abstract

We studied magnetic responses at low temperatures of Au/Co (t_Co=0.7nm) /Au deposited on SiO₂. By using Polar Magneto-Optical Kerr Effect (PMOKE) magnetometry, we determined the magnetization easy axis at 300 K. We used Superconducting QUantum Interference Device (SQUID) magnetometer, in polar configuration, to study the quasi-static parameters between 4-200K. Our results showed that the easy magnetization axis changes direction according to the temperature.

1. Introduction

Research on magnetic materials evolved in a spectacular way during the two last decades this tendency was reinforced with the miniaturization of the devices using magnetic materials. This research is directed either towards the comprehension of very fundamental mechanisms or towards an important prospects for applications such as ultra-high density storage ^{1, 2} or magnetic refrigeration ^{3, 4, 5, 6, 7, 8, 9}.

To understand the fundamental mechanisms of ferromagnetic materials, for example, the cobalt ultra-thin films are good candidates. In these materials magnetization reversal plays a very important part in the technological applications. The intrinsic parameters such as coercivity, the anisotropy and the spontaneous magnetization have an undeniable effect on the magnetization reversal. Several studies are devoted to the comprehension and the control of the behaviour of these parameters to the room temperature ^{10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29} but very little are devoted to the comprehension of the behaviour of these parameters to low temperatures ³⁰.

The aim of this paper is to study the evolution of coercivity, spontaneous magnetization and the anisotropy, of cobalt ultra-thin film, at low temperatures.

2. Material and Methods

Si (100) substrate is beforehand cleaned by ultrasounds in an acetone bath. After the cleaning, this substrates is thermally oxidized in a furnace at 1200°C during 2 hours. This time is sufficient for the formation of an oxide layer on the silicon surface substrate.

Au/Co/Au film were prepared by electron beam evaporation in an ultrahigh vacuum chamber, with a base pressure of about 10^-9 Torr and approximately 10^-8 Torr during deposition on SiO₂, at room temperature.

A first 25 nm thick Au film is deposited on the substrate at a deposition rate of 2.5 nm/min, as calibrated with a quartz microbalance, followed by annealing at during to reduce the surface roughness.

The Au film is (111) textured, as shown by X-ray diffraction (Figure 1(a)). Figure 1(b) shows the 2D AFM image of the Au buffer layer after annealing. The surface roughness (root mean square: rms) was measured to be about 0.2 nm. Using the surface corrugation obtained from 2D AFM, we estimate a lateral grain size of 40–60 nm.

Cobalt layer with thicknesses (t_Co) equal to is then deposited on the Au/SiO₂ at a deposition rate of 0.2nm/min. Finally, a second Au layer with a thickness about of 5nm is deposited on top of the cobalt layer.

The (111) texture of the Au buffer layer suggests a possible epitaxial growth of the cobalt layer with the Hexagonal Close-Packed (0001) structure ^{31, 32, 33}.

Magnetic hysteresis loop, at a field sweep rate of , were recorded at room temperature (RT) by polar magneto-optical Kerr effect magnetometry (PMOKE) using a He–Ne laser . Figure 2 shows PMOKE hysteresis loop for this sample. The full remanence (M_r/M_s) = 1 indicates that its magnetic anisotropy of this sample is perpendicular to the magnetic layer.

The anisotropy of this sample was determined by PMOKE with a method described in our previous works ^{19, 21}.

We used Superconducting QUantum Interference Device (SQUID) magnetometer, in polar configuration, to study the quasi-static parameters of this magnetic layer at low temperatures. On the Figure 3 are presented magnetic responses of this sample at low temperatures.

In Table 1 below are summarized data deduced from quasi-static characterizations.

3. Results and Discussion

The hysteresis loops presented on Figure 2 and Figure 3 show that the coercive field depends on the temperature. vs is plotted on Figure 4. On this Figure 4, one can see that between , which we call zone (I), the coercive field vary very slightly according to the temperature but beyond 100 K, in zone (II), decrease quickly according to the temperature. This kind of behaviour at low temperatures is observed on other Au/Co/Au systems where the thicknesses of cobalt are higher than 1nm ³⁰.

Indeed the temperature dependence of the coercivity can be expressed as ⁴:

(1)

where is local fluctuations of the wall energy, is the wall energy, with .

By using the concept of dimensional analysis we can put the equation (1) in this form:

(2)

is the coercive field at . In zone (I) equation (1) is applicable because this zone is between . In the zone (II) the equation is reduced to:

(3)

With .

We used the equations (2) and (3), respectively for the zones (I) and (II) to deduce , and . These adjustments are presented on Figure 5.

From the fittings we found in the zone (I) , and in the zone (2) . The value of found at is practically the same as that found for at . This result confirms the fact that below the effects of the temperature are almost null. The decrease of the coercivity with the temperature shows that the process the magnetization reversal is thermally activated in this sample. For parameter b one can see that it increases with the temperature.

The magnetization of crystallized matter is directed preferentially according to certain crystallographic directions. This is call magnetic anisotropy which is explained by the symmetry of local environment of the magnetic atoms. In the case of Hexagonal Close-Packed (HCP) system the anisotropy is along only one axis and one speaks about uniaxial anisotropy. In the case of Face Centered Cubic structure (FCC) there may be several easy magnetization axis. If one record a hysteresis loop along the easy magnetization axis one has a square loop ³⁴.

Our hysteresis loops presented on Figure 2 and Figure 3 were all carried out in polar configuration. The loop, recorded at 300 K, presented on Figure 2 is square. The squareness of the loop shows that the easy magnetization axis is perpendicular to the magnetic layer at 300 K. The loops presented in Figure 3 are realized by SQUID at 200, 100 and 4 K. In the three cases of temperature the loop is not square but present rounded corner. The rounding of the loop is accentuated with the temperature lowering. Indeed the loop carried out at 200 K presents an abrupt transition from the up to down state whereas at 4 K this abrupt transition disappeared. The change of the shape of the loop could be related to the change of the easy magnetization axis. Since the easy magnetization axis orientation is related to the structure of the magnetic layer ³⁴ we deduce thus that the change of the loop shape would be due to a probable phase change during the temperature lowering.

With regard to magnetization we notice that the greatest value is found at 4 K, what confirms the fact that the effects of the temperature are almost null around 4 K.

4. Conclusion

The study of magnetic responses of ultra-thin film of cobalt (0.7 nm) at low temperatures. The evolution of coercivity according to the temperature revealed below 100 K the thermal effects are weak and from 4 K and below these effects are negligible. The hysteresis loop shape according to the temperature let’s think that the temperature lowering induce a change phase of the crystalline structure. Thus, we show that the anisotropy change according to the temperature.

Acknowledgments

This work is the result of a collaboration between the LMOP laboratory (Tunisia) and the University of Kara, Togo. We acknowledge the help of LMOP in Tunisia and that of “équipe Micro et Nano-Magnétisme de l’Institut Néel à Grenoble, France”.

References