Giant magnetic anisotropy of Fe3Pt thin films at low temperature

The Fe 3 Pt alloy thin films were grown on MgO substrates at the various growth temperatures by e-beam evaporation in a high vacuum. The magnetic properties such as saturation magnetization, coercivity, magnetic anisotropy etc. of the Fe 3 Pt alloy thin films were studied as a function of temperature varying in the range of 80K to 600K. At low temperature (80K), the Fe 3 Pt films possessed giant cubic magnetic anisotropy (K 1 7×10 erg/cc and K 2 2.2×10 erg/cc). The magnetic anisotropy constants of Fe 3 Pt films were found to decrease slowly with temperature and vanished at Curie temperature. The Curie temperature of Fe 3 Pt thin films were also studied as a function of ordering. It was observed that higher ordered sample exhibited lower Curie temperature.  2012 Trade Science Inc. INDIA


INTRODUCTION
Among the various binary alloys, Fe 3 Pt alloys are interesting alloy to study because of its interesting behavior and properties.The Fe 3 Pt alloy was reported to exhibit the so-called invar properties of anomalous thermal expansion coefficients and the large magneto-volume effect [1] .The compounds close to Fe 3 Pt displayed unusually sharp ferromagnetic anomalies in the thermal expansion coefficient and strong magnetostrictive properties [2] .Giant magnetostriction was observed in an ordered Fe 3 Pt single crystal [3] .The Curie temperature of Fe 3 Pt was reported to be sensitive to pressure.The previous experimental observation suggested that the pressure caused to change the degree of ordering and thus the Curie temperature [4] .It was reported that exchange coupled nano-composite magnets made from Fe 3 Pt and FePt showed large energy product, which exceeded the theoretical limit for non-exchange-coupled FePt [5] .Theoretical band structure calculations on ordered Fe 3 Pt revealed strong hybridization between Fe and Pt d-states and a resultant unusually high density of states (DOS) at the Fermi level compared to other strong ferromagnets such as Fe [6][7][8] .Large cubic magnetic anisotropy was observed in Fe 3 Pt thin films at room temperature [9] .Double switching hysteresis curve were obtained in Fe 3 Pt alloy thin films [10,11] .Due to these many interesting phenomena, Fe 3 Pt has been the focus of study by many researchers.In spite of these extensive works carried out, there are very few works focusing on the temperature dependent magnetic properties especially magnetic anisotropy of Fe 3 Pt thin films.This breeds the scientific curiosity to investigate its nature and origin.Therefore, the temperature dependent

EXPERIMENTAL
The Fe 3 Pt thin films were fabricated by electron beam evaporation onto MgO(100) and MgO(111) substrates using Fe and Pt separate targets in a vacuum better than 10 -7 Torr.The substrate deposition temperature (T S ) was varied from ambient to 550C.The combined deposition rate was 0.7Å/sec and thickness of the samples was about 50nm.The samples were rotated in order to get the better uniformity of thickness.Film stoichiometry was achieved by controlling the deposition rate of individual elements, Fe ( Fe =0.47Å/ sec) and Pt ( Pt =0.23Å/sec).All the films were analyzed by the Energy Dispersive X-ray diffraction and the film compositions were found within the range of 1% from the stoichiometric composition.The in-plane magnetic anisotropy of Fe 3 Pt alloy thin films were measured by in-plane torque measurements.The entire samples were made circular in shape in order to avoid the shape magnetic anisotropy.The magnetic anisotropy constants, K 1 of Fe 3 Pt films were obtained from the <001> oriented samples grown onto MgO(100) substrate while the magnetic anisotropy constants K 2 were obtained from the <111> oriented samples grown onto MgO(111) substrate.The torque measurements were carried out in the temperature range (T) of 80K to 600K.The Curie temperature (T C ) of the samples was determined from the temperature dependent magnetization curve.

RESULTS AND DISCUSSIONS
In our earlier paper, we demonstrated the close relationship of magnetic anisotropy constants of Fe 3 Pt thin films with the structural parameter [9,12] .The Fe 3 Pt films grown at 400C were partially ordered and exhibited maximum magnetic anisotropy at room temperature as reported earlier [9] .In this paper, the temperature dependent magnetic properties such as saturation magnetization, coercivity, magnetic anisotropy constant etc. of Fe 3 Pt thin films were studied.Measurements were carried out in the range from 80K to 600K. Figure 1 shows the temperature dependence of saturation mag-netization (M S ).At 80K, the saturation magnetization, M S was about 1300 emu/cc and diminished at about 625K.The coercivity (H C ) of Fe 3 Pt alloy thin films as a function of temperature was depicted in Figure 2. The maximum coercivity was obtained at 80K of about 2 KOe.The magnetization and the coercivity fell in similar fashion with the temperature.The Curie temperature was determined from the inflection point of M vs. 1/T plot.The Curie temperature, T C for the partially ordered (S=0.3)case was found about 625K (352C).The result of T C as a function of the ordering parameter, S is shown in Figure 3.The ordering parameter was estimated according to ref [9] .It was observed that the Curie temperature decreased with the degree of ordering, although, there was a scat-

Materials Science Materials Science
ter in the data at S=0.The reason of this decrement of T C with the ordering was not clearly understood.However, the nearest neighbor occupancy probabilities and the change of atomic interactions from disordered to ordered state [13] or the pair-ordering mechanism might be the origin of the change in Curie temperature.It is known that pair-ordering originates from the unbalanced distribution of the atoms [14,15] .In Fe 3 Pt alloy, three kinds of atomic pairs are possible such as Fe-Fe, Fe-Pt or Pt-Pt.As only Fe is magnetic, the interaction energy (dipolar and quadrapolar) is believed to depend mainly on the number of Fe-Fe pairs.The numbers of pairs depend on the order parameter that is controlled by growth temperature.The number of Fe-Fe planer pairs in Fe 3 Pt alloy is found to decrease with the ordering by simple calculations [12] , which might be related with this decrement of the Curie temperature in the present case.The Curie temperature for the highest ordered sample (S=0.67) was obtained of about 513K (240C), a little bit larger than that (435K, 162C) for the bulk ordered Fe 3 Pt.anisotropy constants, K 1 and K 2 were found to decrease with the increasing T as shown in Figure 4.When the measurement temperature approached towards the Curie temperature (measured value of Curie temperature of the Fe 3 Pt film was about 600K), both the magnetic anisotropy constants K 1 and K 2 gradually vanished similar to other ferromagnetic material.According to the previous theory, the power law dependence of magnetization can be expressed as [16][17][18] , K 1 (T)/ The temperature (T) dependence of K 1 and K 2 of Fe 3 Pt thin films grown at 400ºC were investigated over a range from 80K to 600K.The 4-fold and 6-fold symmetry torque curves were observed over the entire temperature range for the two-oriented samples grown on MgO substrates.However, a change in the torque amplitude was noticed with temperature.The magnetic  An Indian Journal Materials Science Materials Science K 1 (0)=M(T)/M(0) n , where M(0) and M(T) are the magnetization at temperature 0K and T. We employed K 1 (T)/ K 1 (80K)=M(T)/M(80K) n and numerically fitted our data.From the best fitted value as shown in Figure 5, the exponent, n=1.3 was obtained.This value of n was smaller as predicted by Zener and Carr.According to those theories, the value of n would be 3 and 10 for uniaxial and cubic cases respectively.This kind of unexpected dependence of magnetic anisotropy was also observed in other materials [19][20][21] .Further theoretical framework is necessary to explain this behavior.However, from the nature of the slow decrement, one can say that the magnetic anisotropy energy of Fe 3 Pt films is more stable than other transition metal alloys in a wide range of temperature.

CONCLUSIONS
In this manuscript, the temperature dependent magnetic properties of the Fe 3 Pt thin films grown on MgO substrates were studied in the temperature range varying from 80K to 600K.At 80K, the saturation magnetization (M S ) and coercivity (H C ) of Fe 3 Pt films (grown at 400ºC) were obtained about 1300 emu/cc and 2000 Oe.Both M S and H C dropped in similar fashion with temperature.The Curie temperature of Fe 3 Pt films as a function of ordering was investigated and it was found to decrease with the ordering.The reason was yet to be cleared up.However, we believe that the pair ordering mechanism might be the origin of this decrement.The important result was in the case of magnetic anisotropy.The Fe 3 Pt films (grown at 400ºC) at 80K possessed a very large magnetic anisotropy constant.The in-plane magnetic anisotropy constants, K 1 and K 2 obtained were about -7×10 6 erg/cc and 2.2×10 7 erg/cc respectively.Therefore, the magnetic anisotropy constants of Fe 3 Pt films at lower temperature (80K) were little bit larger in compare to the room temperature values.Both the magnetic anisotropy constants of Fe 3 Pt thin films dropped with temperature with the 1.3 power of magnetization.It was relatively slower rate in the case of cubic magnetic anisotropy as predicted by the theory.Thus, the magnetic anisotropy constants were found stable in a wide range of temperature.

Figure 1 :
Figure 1 : The variation of magnetization as a function of temperature of the sample Fe 3 Pt/MgO(100) grown at 400C.

Figure 2 :
Figure 2 : The variation of coercivity, H C as a function of temperature (T) of the sample Fe 3 Pt/MgO(100) grown at 400C.

Figure 3 :
Figure 3 : The Curie temperature, T C of Fe 3 Pt alloy thin films grown onto MgO(100) substrate as a function of ordering parameter, S.

Figure 4 :
Figure 4 : The variation of magnetic anisotropy constant, K 1 and K 2 as a function of measurement temperature (T) of the Fe 3 Pt alloy thin films grown onto MgO substrates.

Figure 5 :
Figure 5 : Thetemperature dependence of normalized magnetic anisotropy constant, K 1 of Fe 3 Pt films.(The solid line is a measured value while the dashed line is a fitted curve from the magnetization).