The characteristic temperatures related to phase transitions were obtained from the M – T data. Figure 5.19 displays the M – T curves of as-spun and heat treated R2 ribbons recorded during cooling (solid symbols) and heating (open symbols) cycles in a magnetic field of 0.1 T. Inset of Figure 5.19 shows M – T curves corresponding to the as-spun ribbons R1, R2 and R3, which portrays the influence of solidification rate on the temperature dependent magnetization of the as-spun ribbons. It can be observed from the M – T curves that magnetization values of the ribbons vary with temperature in the following manner: (i) As the temperature is increased from 100 K, magnetization decreased to almost zero around 240 K and then increased to a maximum value near 285 K. (ii) On further increasing the temperature, magnetization again decreased to zero. (iii) On the other hand, while decreasing the temperature from 300 K, magnetization first increased to a maximum value near 277 K.
(iv) After reaching the maximum value, magnetization decreased to almost zero and then continuously increased on further decrease in temperature. (v) A careful inspection of the data around the range 235 K – 288 K confirms that the field cooled (FC) and field heated (FH) data do not follow the same path. A thermal hysteresis (¨Thys), due to the structural (martensite ↔ austenite) first-order phase transition is observed between the FC and FH curves for all the samples in the temperature range of 235 K – 288 K. The data reveal that all the samples exhibit one structural (martensite ↔ austenite) first-order phase transition and
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two magnetic transitions: one is in austenite phase (at TC,A) and another in martensite phase (at TC,M). Since MCE near TC,M is considerably smaller and is not of interest to this work, only TC,A would be referred in the remaining part of this chapter. It can also be observed that the samples are ferromagnetically ordered in austenite phase below TC,A, until they begin to transform into martensite phase at TMs.
100 200 300
0 10 20 30
Temperature (K) Magnetization (Am2 kg-1 )
TC,M
TMs
TC,A as-spun
as-spun 823 K R2
1073 K
100 200 300
0 6 12 18 24
R1
R3 R2
Figure 5.19: M – T curves of as-spun and annealed R2 ribbons recorded under an applied field of 0.1 T. Inset shows the M – T curves of as-spun ribbons.
Characteristic temperatures of the alloy ribbons, viz., TMs, TAs, TMf and TAf extracted from the M – T data are summarized in Table 5.6. Characteristic temperatures of the ribbons decreased (increased) with increasing solidification rate (annealing temperature). ∆Thys
increased with increasing solidification rate. It can be noticed that ∆Thys does not change upon annealing R1 ribbon, whereas it decreases with increasing annealing temperature for R2 and R3 ribbons. Thermal hysteresis in FSMA is known to be a complicated process. From the microscopic view point, it is generally related to the nucleation of the new phase and the interaction of interfaces with defects. But from the mesoscopic view, it originates from the formation, annihilation, and rearrangement of elastically interacting domains [HUFX09a]. In
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addition, the friction from domain rearrangement and phase boundary motions also contribute to the hysteresis. Thus, ∆Thys characterizes the strength of the friction during the transformation. In the present case, the effects of solidification rate and heat treatment on
∆Thys confirm that the higher solidification rate can induce large internal stresses as well as defects in the alloy ribbons. As the annealing temperature is increased, internal stresses get relieved and defects get reduced resulting in an increase in the grain size. This results in a reduction in ∆Thys in annealed samples as indicated in Table 5.6. On the other hand, the friction (caused by domain rearrangement and phase boundary motion during first-order phase transition) results in an incomplete transformation between martensite and austenite phases.
Table 5.6: Structural and magnetic parameters of (as-spun and heat treated Ni51Mn34In14Si1 alloy ribbons) obtained from M – T data recorded at 0.1 T.
Sample TMs (K) TMf (K) TAs (K) TAf (K) TC,A (K) ∆∆∆∆Thys (K)
R1 (as-spun) 276 254 271 286 290 13.5
R1 (823 K) 278 256 274 287 294 13.5
R1 (1073 K) 279 265 280 291 294 13.5
R2 (as-spun) 274 245 269 285 288 17.5
R2 (823 K) 277 252 273 287 290 16.5
R2 (1073 K) 279 265 284 291 294 15.5
R3 (as-spun) 269 243 268 284 287 20.0
R3 (823 K) 275 246 271 286 289 18.0
R3 (1073 K) 278 254 278 289 293 17.5
The nature of magnetism in the intermediate temperature range TC,M < T < TMf has evoked considerable interest among researchers in recent years. For an FSMA exhibiting complete transformation between the martensite and austenite phases, one would expect ideal paramagnetic state in this regime [RAMA10a]. Mossbauer studies on Ni-Mn-Sn alloy with
57Fe has shown paramagnetic phase in this regime [UMET08a]. However, neutron
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polarization analysis of Ni-Mn-Sn alloy indicates the presence of antiferromagnetism in this regime [AKSO09a]. In order to understand the magnetic behavior in the present data in this temperature regime, we recorded isothermal magnetization curve at T = 235 K, where TC,M <
T < TMf for our samples. Figure 5.20 shows the M – H curves obtained for both as-spun and heat treated R2 ribbons. Inset in Figure 5.20 portrays the data corresponding to the three as- spun ribbons. The nonlinear behaviour of the data at low field region indicates the presence of a small fraction of spontaneous magnetization (or ferromagnetism) in the intermediate temperature regime. Straight lines in the figure denote linear fits to the data in the high field region. Linear fits extrapolated to zero field yield the spontaneous magnetization of the impurity ferromagnetic phase present in the samples. The fraction of ferromagnetic phase present in the intermediate temperature (TC,M < T < TMf) regime decreased as solidification rate and annealing temperature are increased. The impurity ferromagnetic phase is the residual (untransformed) austenite phase present in the samples.
0.0 0.3 0.6 0.9 1.2 0
1 2 3
0.0 0.3 0.6 0.9 1.2
0 1 2
3 T = 235 K
Field (T) M (Am2kg-1)
R1 R2 R3 as-spun
R2 (as-spun) R2 (ann. 823 K) R2 (ann. 1073 K)
T
= 235 KMagnetization (Am2 kg-1 )
Field (T)
Figure 5.20: Initial magnetization curves recorded at T = 235 K for R2 ribbons annealed at different temperatures. The inset presents the data corresponding to as-spun ribbons.
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It is worthy to point out here that Aksoy et al. [AKSO09a] have also pointed out the existence of austenite fraction in this temperature regime in Ni-Mn-Sn alloy. Considering the evidence given by them for antiferromagnetism in this regime, presence of antiferromagnetic interaction cannot be ignored in the present case. However, given the convex nature of the nonlinearity near the origin in the M-H data shown in Figure 5.20, only observe the signature of weak ferromagnetism due to the untransformed austenite phase could be observed in the present samples in this temperature regime.
It can be also noted from Figure 5.19 that the magnitude of ∆M at the austenite to martensite transition and the magnetization values at the ferromagnetic state of the martensite phase decrease with increasing solidification rate. Annealing at lower temperature (823 K) improves magnetization at structural phase transition. On the other hand, higher annealing temperature (1073 K) decreases the magnetization drastically and shifts the phase transition temperatures to higher temperature as reported earlier [ROSA12a]. These dramatic magnetic and structural behaviours upon heat treatment could be attributed to increase in grain size, reduction of internal stresses accumulated during the rapid solidification process and reduction in Mn-Mn distance. In full Heusler (X2YZ) alloys such as Ni50Mn25In25, Mn atoms occupy the (½ ½ ½) position in the L21 unit cell. In off-stoichiometric Heusler compositions such as Ni51Mn34In14Si1, the excess Mn atoms occupy other positions such as (0 0 0) which is usually occupied by In atoms. Mn-Mn exchange interaction is very sensitive to interatomic distance. Annealing tends to modify the Mn atomic position, leading to reduction in interatomic distance [ROSA12a].