CULL2001]
Chapter 4 Thickness dependent properties of single-layer CoFeB
4.3. Results and discussion 1. Structural properties
4.3.2. Magnetic properties
4.3.2.2. Temperature dependent magnetic properties
ZHUT2015]. The first one could primarily be related to the stress induced in higher thickness films [PLAT2001] and the latter one is mainly originating from the interfaces between CoFeB and other stacking films, which was further supported through micromagnetic modelling [WANG2014].
Figure 4.11: M-H loops measured at different temperatures (30−300 K) for CoFeB442 (t nm) films with four different thicknesses.
Figure 4.12: M-H loops measured at different temperatures (30−300 K) for CoFeB262 (t nm) films with four different thicknesses.
In order to quantify the variations in magnetic parameters with temperature, the values of HC, HSat and MS are extracted from the M-H loops measured at different temperatures and shown in Figure 4.13. HC versus T exhibits two distinct regions of variation with temperature for lower thickness films (~20 nm), i.e., HC increases at a slower rate of 137 (129) mOe/K down to 150 K followed by a larger rate of 1149 (1394) mOe/K at temperature below 100 K for CoFeB442 (CoFeB262) films, respectively. This, in total, provides a maximum relative change of HC between 300 K and 30 K as 4.61 (3.98) for CoFeB442 (CoFeB262) films, respectively. Such behaviors can be attributed to the development of additional interfacial stress by the substrate on the lower thickness films with decreasing temperature.
With increasing CoFeB thickness, the maximum relative change in HC decreases from 4.61 to 1.49 and from 3.98 to 2.72 for CoFeB442 and CoFeB262 films respectively.
This indicates that the CoFeB442 films exhibit a large reduction in the maximum relative change in HC with temperature as compared to CoFeB262 films. While the values of HSat
increase with decreasing temperature for all the films as expected for typical ferromagnetic materials, HSat for CoFeB262 (200 nm) film decreases slightly with decreasing
temperature. This unusual behavior may be attributed to the change in the magnetic domain structure with decreasing temperature caused by the change in Keff with temperature.
Figure 4.13: Temperature dependent variations of HC, HSat and MS for CoFeB442 (t nm) films [(a), (b) and (c)] and CoFeB262 (t nm) films [(d), (e) and (f)].
On the other hand, it is clear from Figures 4.13(c) and (f) that the values of MS
change weakly with temperature for all the films in both the compositions. This is possibly due to the high thermal stability of CoFeB films with temperature and high Curie
temperature (TC) of the CoFeB films [YAMA2011]. Nagasaka et al. [NAGA2000] have reported similar behavior of MS with temperature for CoFeB film and extracted Curie temperature for CoFeB film was reported as 1040 °C. To study the temperature dependence of Keff in higher thickness CoFeB films, the calculation of Keff was extended as a function of temperature using the eqn.(4.02) and by substituting the parameters obtained from temperature dependent M-H loops.
Figure 4.14: Temperature dependent effective magnetic anisotropy (Keff) for 100 nm and 200 nm thick (a) CoFeB442 and (b) CoFeB262 films.
Figure 4.14 depicts the variations of Keff as a function of temperature for 100 and 200 nm thick CoFeB442 and CoFeB262 films. It is revealed that even though the room
temperature values of Keff are similar for 100 and 200 nm thick CoFeB442 films, their temperature dependence depends significantly on the thickness of the films. However, for the CoFeB262 films, the temperature dependent Keff exhibits increasing and decreasing nature with decreasing temperature for 100 and 200 nm thick films, respectively. This is mainly due to the strong temperature dependent variation in the values of HSat with thickness (see Figure 4.13), resulting from the large variation in the angle between the bulk and surface domains with decreasing temperature. The temperature dependent analysis of magnetic domain structure would provide more insights for such unusual variation of Keff
in composition dependent CoFeB films.
Figure 4.15: High temperature M-T data recorded under the applied field of 500 Oe for 20 nm and 50 nm thick (a) CoFeB442 and (b) CoFeB262 films.
To study the magnetic properties of the films up to Curie temperature, high temperature M-T data were measured at a constant applied field of 500 Oe for all the films using VSM magnetometer. Figure 4.15 illustrates M-T data above room temperature for 20 nm and 50 nm thick (a) CoFeB442 and (b) CoFeB262 films. In order to compare all the magnetization data in a single graph, M-T data are normalized with respect to the room temperature magnetization data of that particular sample. It could be clearly seen from the Figure 4.15 that (i) CoFeB442 (20 nm) film shows an incessant decrease in magnetization up to 590 K. The rate of decrease of magnetization decreases with further increase in temperature. (ii) A similar behavior is observed for CoFeB262 (20 nm) film but in this case the film shows faster decrease in magnetization up to 575 K. (iii) With increasing t to 50 nm, the temperature at which the magnetization starts decreasing shifts slightly towards high temperature. The above results clearly indicate that with increasing temperature above room temperature, CoFeB films exhibit a clear magnetic phase transition from ferromagnetic state to paramagnetic state. The values of TC were obtained from the thermal derivative of the magnetization data and found to be 539 K and 562 K for CoFeB442 (t nm) films and 517 K and 553 K for CoFeB262 (t nm) films with t = 20 and 50 nm, respectively. The obtained values of TC are found to be significantly lower than those reported in literature for similar alloys [YAMA2011, PELL2015]. For instance, Yamanouchi et al. [YAMA2011] have reported TC value of 728 K and 1026 K for as- deposited and annealed Co20Fe60B20 films, respectively. However, they determine TC by extrapolating the temperature dependent domain wall surface energy. Pellegren et al.
[PELL2015] have shown that TC of CoFeB films can be decreased progressively by alloying CoFeB with Ta and Hf. The reported results were in good agreement with bulk materials as well [LUCA2011]. These results suggest that the low value of TC in the presently investigated films could be correlated to the complete amorphous form of the films [LIUY2008, SAKU2013].