Temperature dependence and temperature limits of magnetic shape memory effect

Abstract

We study the temperature dependence and low and high temperature limits of the magnetic shape memory effect (MSME) in five-layered tetragonal Ni–Mn–Ga martensite. Using a simple model we show that, additionally to the limits posed by transformation to austenite or an intermartensitic transformation, the temperature dependence of the magnetic anisotropy, tetragonality of the lattice, and twinning stress play important role when considering the temperature limits of the MSME. With decreasing temperature, the lattice distortion and magnetic anisotropy increase, but saturate in a low temperature region. The twinning stress does not saturate and its temperature dependence has exponential-like character that increases rapidly in the low temperature region. The model predicts that the low-temperature limit of the MSME is 165 K for Ni49.7Mn29.1Ga21.2 composition. This agrees very well with the value of 173 K determined from direct measurements. The high temperature limit is transformation to austenite at 315 K. The interval of the MSME existence is therefore 173–315 K. Quasistatic measurements of the MSME in the range 203–313 K up to the 1.15 T magnetic field show that the onset of the MSME shifts to a higher field and that a maximum field-induced strain increases with decreasing temperature.

Magnetocrystalline anisotropy and twinning stress in Ni-Mn-Ga ferromagnetic shape memory alloys

“…Considering Schmidt factor of about 1/2, the value of τ req has been evaluated. Temperature dependences of τ req for the 10M and 14M martensites thus obtained are shown by closed squares in Figure 3 from the previous report for 10M martensite in off-stoichiometric Ni-Mn-Ga [9]. Comparing τ mag at H s with τ req , we know that the value of τ mag at H s is almost equivalent to τ req at any temperature at which the RMV by magnetic field occurs.…”

Magnetic Shape Memory Materials with Improved Functional Properties: Scientific Aspects

“…For this reason, the observation of a large (about few percents) MFIS occurred possible only in some ferromagnetic SMAs, after the elaboration of the complicate modes of production and pretreatment of the samples. It was shown, moreover, that MFIS is observable only if the temperature of ferromagnetic SMA (the Ni-Mn-Ga alloys were used for experiments) lies in a certain temperature interval below MT temperature [11]. It happened that the threshold stress, σ th , which starts the martensite reorientation, increases with a decreasing of temperature quicker than the magnetostress, σ eq .…”

“…For this reason, the lower limit of MFIS observation can be found from a condition σ eq < σ th . It was demonstrated, however, that the width of the temperature interval of MFIS existence (~100 K) is large enough for the practical applications [11].…”

“…The temperature dependence of magnetostress and limits of MFIS were studied in Ref. [11]; the contributions of different factors to this dependence were analyzed in Ref. [12].…”

Ni-Mn-Ga single crystal Shape Memory Alloy magneto-thermomechanical modeling

“…The concept of memorized point was introduced and a difference appears between partial loops and major loops. Moreover, π cr (T ) is a function of the temperature as it was shown in [11]. A linear dependence is used in this paper as π cr (T ) = π A numerical simulation was done with the help of the Matlab/Simulink software.…”