THE EFFECT OF COBALT CONTENT ON MAGNETIC PROPERTIES OF CoFe ALLOYS

Hard Disk Drive (HDD) as a data storage device when operated with high temperatures (around 66C), its function will be constrained. The CoFe alloys have a large coercivity field and can be patterned in very small sizes that are suitable for HDD devices. In this study, Co1-x Fex cube alloy was used (x = 0.25; 0.30; 0.50; 0.75). Samples were treated with temperature changes to get the Curie temperature. The coercivity field value is obtained by giving the external field and temperature below Curie temperature and also above Curie temperature to the samples. The VAMPIRE software is a micromagnetic simulation program based on atomistic models. The results showed that Curie’s temperature decreased when Co content increased. The composition of Co0.25 Fe0.75 has the highest Curie temperature that is equal to 1075 K. The temperature Curie is not affected by the size of the cube. When the sample is given a temperature rise below the Curie temperature, the value of the coercivity field decreases. The value of the coercivity field is very difficult to determine when the temperature used is above the Curie temperature. The percentage of composition does not affect the coercivity field value. Therefore, cube-shaped CoFe material is very suitable for use as a material data storage device operated at temperatures below the Curie.


INTRODUCTION
Hard disk drive (HDD) magnetic recording media has problems in their use related to the physical characteristics of materials, such as high-temperature resistance and the occurrence of unstable data stored conditions due to super-paramagnetic effects. Super-paramagnetic effects are properties that appear on nano-sized materials and one-order magnetic domains, so the particles will be very reactive to external magnetic fields [1]. However, if the external magnetic field is slowly removed, the properties will be similar to paramagnetic material. If the temperature rises in a state of decreased magnetization energy, there will be a process of demagnetization and a change in the orientation of the magnetization [2]. This results in unstable data stored information, and the worst possible damage will occur.
HDD temperature conditions when the computer starts up between 30 -50 o C, whereas when it starts processing data, the HDD temperature reaches 50 -58 o C. When the HDD works continuously, the temperature will reach a maximum called the overheat temperature, which is around 66 o C [3,4]. If the HDD always works at overheat temperatures or even exceeds it, it can make the HDD work slower, and the worst possibility will shorten the HDD's life due to permanent damage.
To make magnetic recording media that is resistant to heat, ferromagnetic material that has a high Curie temperature and high anisotropic magnetic material needs to be chosen. Materials that are suitable and have the potential to be applied as magnetic recording media are ferromagnetic magnetic materials. Curie temperature values are obtained from the relationship between magnetization and susceptibility to temperature. The hysteresis curve is obtained from the relationship between magnetization and external fields. CoFe alloys material that has a high Curie temperature value, a coercivity field, and a good saturation field is a material that can be used as a basic material for making a good HDD. Due to variations in temperature input and the influence of the critical size. So based on the Curie temperature value and hysteresis curve characteristics, it can be used to choose component HDD that can be decreased from the problem of high-temperature resistance [5,6].
The CoFe alloys are one example of ferromagnetic alloys suitable for HDD materials that can improve the magnetic properties of materials with varying degrees of quality. Each of these quality levels differs in their composition and heating treatment to obtain the properties of ferromagnetic materials that are appropriate to their needs or use. The magnetic material of the CoFe alloy is an important hard magnet because it has unique magnetic properties, including high magnetization saturation, high permeability, high coercivity, and good thermal stability [7,8]. The Curie temperature value is influenced by the percentage composition of the CoFe alloy. To gain an understanding of the macroscopic properties of CoFe magnetic materials such as anisotropic surfaces, spin dynamics, and microstructural effects, it can be done with a Vampire micromagnetic simulation program, an open-source software that uses atomic modeling based on the classic Hamiltonian spin and the Landau-Lifshitz-Gilbert (LLG) equation. [9,10].

METHOD
This research was conducted with a simulation method, using VAMPIRE public domain software that can determine magnetic properties such as curie temperature and hysteresis curves. Atomic modeling is based on the spin of neighboring atoms following the Heisenberg exchange, so that the energy from the system is obtained Hamiltonian as follows: Where : Jij : energy exchange between spin i and j Where : i : Gilbert damping parameters given starts from -6 to 6 T with a stage of field change of 0.1 T. The hysterical curve display, as shown in FIGURE 3. Hc is coercivity [12].

RESULT AND DISCUSSION
Based on the simulation results obtained by the magnetization relationship curve to the temperature, which shows the Curie temperature. FIGURE 4 shows the Curie temperature values of the CoFe material with variations in composition at 5 nm in size.    Alloy coercivity field values table, as shown  in TABLE 2. The increase in temperature of the magnetic material shows that it can reduce the coercivity field of the material. This is due to the transition from the ferromagnetic phase to the paramagnetic phase in the CoFe material. Because the value of the coercivity field is above 10 kA/m (0.0125 T), the CoFe material can be classified as hard magnets. The greater the coercivity field value, the magnetic properties of material will be stronger. Co composition 25% and Fe 75% has the highest average field coercivity value compared to other compositions.

CONCLUSION
The Co1-x Fex material has stable magnetizing properties, although the Curie's temperature decreases with increasing Co content, the size of the side of the cube does not affect the Curie's temperature. Changes in temperature result in a decrease in the coercivity field value of the material, but when there is a change in temperature above the overheat temperature (66 o C or 339 K), Co1-x Fex material still has a coercivity field value above 10 kA /m (0.0125 T). It can be concluded that the cube-shaped Co1-x Fex material can be used as a data storage device for operations below the Curie temperature.