Peculiarities of magnetoelastic coupling in Ni–Fe–Ga–Co ferromagnetic martensite

Abstract

The reversible stress-induced magnetization (inverse magnetostriction or Villari effect) has been measured in a polycrystalline Ni–Fe–Ga–Co ferromagnetic martensite. The samples were mechanically excited using longitudinal resonant oscillations at frequencies close to 100 kHz, and experiments were performed over the temperature range 170–350 K under variable polarizing fields. It has been found that the reversible inverse magnetostriction changes its sign under low polarizing fields over a certain temperature range with its upper limit close to the Curie temperature. We argue that the variations of sign of the reversible inverse magnetostriction effect are related in the present experiments with the change in the sign of magnetostriction, as has additionally been verified in test measurements performed for pure Ni and Fe. The observed peculiarity of magnetoelastic coupling is also reflected in the temperature dependence of electrical resistance and even produces a minor effect in calorimetry scans. Possible origins of these features of magnetoelastic coupling are discussed.

Low Field Magnetic and Thermal Hysteresis in Antiferromagnetic Dysprosium

“…An unusual feature is the formation of central hysteresis for heating scans for temperatures slightly below T VP , Figure 3b. Similar behavior has been found close to the Villari point in ferromagnetic Ni-Fe-Ga-Co alloys [28]. Figure 2c.…”

“…Indeed, the condition B 0 = 0 in general marks only an extremum in the B(σ) dependence. Field dependences B 0 (H), which are used to identify the Villari point [28], cannot be registered over the temperature range where B 0 < 0, since the magnetic state of Dy during heating from the FM phase is strongly non-equilibrium and B 0 relaxes towards positive values [25]. heating from the FM phase are qualitatively different, Figure 2c.…”

“…Mechanomagnetic spectroscopy is a method to detect the RVE at ultrasonic frequencies [26][27][28]. The technique employs a classical three component resonant Piezoelectric Ultrasonic Composite Oscillator [29,30] with an additional channel to register periodic flux density induced in a sample by oscillatory stress/strain.…”

“…The experimental setup operated at a frequency around 90 kHz, covering the ranges of ε 0 between 10 −7 and 10 −4 , temperature T from 77 to 400 K and the axial magnetic field H up to 18 kA/m. A description of the method and experimental setup can be found elsewhere [27][28][29][30]. In the present work B 0 (T) continuous spectra were registered between 250 and 80 K (cooling the sample below T C ) and between 250 and 95 K (thermal cycle in the AFM phase) for constant values of H, and ε 0 fixed at 10 −5 .…”

Giant magnetocaloric effect from reverse martensitic transformation in Ni–Mn–Ga–Cu ferromagnetic shape memory alloys

“…Ferromagnetic Shape Memory Alloys (FSMA), another class of functional materials, also show large MCE. Sharp change in magnetization associated with the structural transformation from high-temperature austenite (cubic) phase to low-temperature martensite phase of lower crystallographic symmetry, results not only in large MCE [9][10][11] in FSMAs, but also leads to many interesting functional properties like shape memory effect, magnetic field induced strain (MFIS) [11][12][13][14][15], magnetic field induced reverse transformation (MFIRT) [11] and kinetic arrest effect [16][17][18]. Coincidence of first order structural martensitic transformation and second order magnetic transition can turn MCE into GMCE.…”