J. Kamarád

Co and In Doped Ni-Mn-Ga Magnetic Shape Memory Alloys: A Thorough Structural, Magnetic and Magnetocaloric Study

In Ni-Mn-Ga ferromagnetic shape memory alloys, Co-doping plays a major role in determining a peculiar phase diagram where, besides a change in the critical temperatures, a change of number, order and nature of phase transitions (e.g., from ferromagnetic to paramagnetic or from paramagnetic to ferromagnetic, on heating) can be obtained, together with a change in the giant magnetocaloric effect from direct to inverse. Here we present a thorough study of the intrinsic magnetic and structural properties, including their dependence on hydrostatic pressure, that are at the basis of the multifunctional behavior of Co and In-doped alloys. We study in depth their magnetocaloric properties, taking advantage of complementary calorimetric and magnetic techniques, and show that if a proper measurement protocol is adopted they all merge to the same values, even in case of first order transitions. A simplified model for the estimation of the adiabatic temperature change that relies only on indirect measurements is proposed, allowing for the quick and reliable evaluation of the magnetocaloric potentiality of new materials starting from readily available magnetic measurements.

VOLUME DEPENDENCE OF MAGNETIC PHASE TRANSITIONS OF THE NOVEL ND3'FETI)29, PR3(FETI)29 AND TB3(FETI)29 COMPOUNDS

Abstract The effect of pressure up to 8 kbar on the Curie temperature Tc and on the spin reorientation transition temperature Tsr of the Nd3(FeTi)29, Pr3(FeTi)29 and Tb3(FeTi)29 compounds has been measured. The temperatures Tc and Tsr decrease under pressure with initial slopes dTc/dp = -2.9 K/kbar and dTsr/dp = -3.6 K/kbar for Nd3(FeTi)29, dTc/dp = -3.1 K/kbar for Pr3(FeTi)29 and dTc/dp = -2.6 K/kbar for Tb3(FeTi)29. The compressibility of Nd3(FeTi)29, ϰ = 9.5 × 10-4kbar-1, was determined by the strain gauge method and hence an increase in volume of 1% will produce an increase of 32 K in Tc.

Reverse Magnetostructural Transitions by Co and In Doping NiMnGa Alloys: Structural, Magnetic, and Magnetoelastic Properties

We review the composition dependence of the structural and magnetic properties of the Co-doped Ni–Mn–Ga Ferromagnetic Shape Memory Alloy around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a “paramagnetic gap” between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. The magnetic and structural properties display noticeable discontinuities across the martensitic transformation: remarkable values of the saturation magnetization jump at the transformation (DM), of the field dependence of the martensitic transformation temperature (dTM/dH), and of the crystalline volume change (DV/V) are reported, and are considerably enhanced by additional Indoping of the quaternary alloy. These properties give rise to a remarkable phenomenology which is of interest for multifunctional applications; magnetic superelasticity and high values of reversible strain are found.

Magnetoelastic and pressure effects at the antiferro–ferromagnetic transition in Hf1−xTaxFe2−y alloys

Magnetostriction measurements using high‐pulsed magnetic fields up to 14 T and thermal expansion measurements under hydrostatic pressure up to 12 kbar have been performed in Hf1−xTaxFe2−y alloys with x=0.17 and 0.15 and y=0.02. These compounds have a first‐order magnetic phase transition from an antiferromagnetic (AF) high‐temperature state to a ferromagnetic (F) low‐temperature state at TAF–F=246 and 290 K, respectively. This transition is accompanied by a volume expansion from the low‐volume AF state to the high‐volume F state of ΔV/V≊0.56% and 0.38% for x=0.17 and 0.15, respectively. Hydrostatic pressure decreases significantly the AF–F transition temperature, whereas the thermal expansion anomaly becomes more pronounced. The dependence of TAF–F on pressure follows a quadratic behavior and TAF–F reaches a value of 106 K at 10.5 kbar for x=0.17. In search of high magnetostrictive materials, we have found a giant volume magnetostriction value (e.g., ω≊0.7% for the x=0.17 compound) in these alloys at a mo...

Magnetization and magnetocrystalline anisotropy of R2Fe17 intermetallics under pressure

The measurements of magnetization and magnetic anisotropy of R2Fe17 (R=Y, Er, Ho, and Gd) single crystals were performed under hydrostatic pressure up to 1 GPa, in temperature range 5–300 K and at magnetic fields up to 5 T, using the SQUID magnetometer with a miniature pressure CuBe cell. A pronounced decrease of spontaneous magnetization M of the R2Fe17 intermetallics has been observed under pressure. The values of dM/dp vary in an interval from −0.32μB/f.u. per kbar (Y2Fe17) up to −0.1μB/f.u. per kbar (Ho2Fe17). The R elements in the R2Fe17 intermetallics have a strong influence on the d(ln mFe)/dp values at 5 K. A remarkable decrease of the anisotropy constant with pressure, d ln K1/dp=−1.7×10−2 kbar−1 at 5 K, was observed on the Y2Fe17 compound. In the Gd2Fe17 compound, |K1(T)| increases with increasing pressure below 150 K and decreases with increasing pressure above this temperature value. The critical field HC(T) of the FOMP in the Er2Fe17 compound exhibits a similar behavior, it increases under pr...

Invar behaviour of Y2Fe17 and YFe11Ti single crystals: magnetic moment of Fe under pressure

Abstract The magnetization of Y 2 Fe 17 and YFe 11 Ti single crystals has been measured under pressure up to 10 kbar. The decrease of Fe magnetic moment under pressure is more pronounced in Y 2 Fe 17 ( − 0.017 μ B /Fe per kbar) than in YFe 11 Ti (− 0.006μ B /Fe per kbar). The absolute value of the anisotropy constant K 1 decreases under pressure more rapidly in Y 2 Fe 17 than in YFe 11 Ti.

Magnetovolume effects and anomalous compressibility of Nd2Fe17 and Nd2(FeTi)17 compounds

We present the results of direct measurements of the isothermal compressibility up to 13 kbar on Nd2Fe17 and Nd2(FeTi)17 compounds and the results of measurements of the linear thermal expansion on Nd2Fe17 compound under hydrostatic pressure up to 6 kbar in temperature range 200–400 K covering both ferromagnetic and paramagnetic region using a modified strain gauge method. A large magnetic contribution to the compressibility κm∼0.15 Mbar−1 was observed in the ferromagnetic state of the Nd2Fe17 compound. The pronounced lambda-shaped anomalies in the thermal expansion coefficient α (negative values of α) and the maximum of compressibility were simultaneously observed nearby the Curie temperature. The corresponding thermodynamic relations are used to analyze the pressure behavior.

Magnetic phase transitions and magnetovolume anomalies in DyCo2 and GdMn2 compounds under pressure

Abstract A pronounced effect of pressure on the volume changes at the first-order magnetic phase transitions has been observed in DyCo2 and GdMn2 compounds. Using a strain gauge technique, compressibility, spontaneous magnetostriction and the transition temperatures were determined under high pressure. The results are compared with the effect of ‘chemical’ pressure in the relevant series of these compounds with substituted elements.

Magnetovolume effects and magnetic anisotropy in Ce3(FeM)29 compounds (M=Ti, Cr)

The magnetovolume effects and magnetic anisotropy of the new Ce3(FeTi)29 and Ce3(FeCr)29 compounds have been studied by measuring their linear thermal expansion (LTE), pressure dependence of the Curie temperature Tc, compressibility, and anisotropy field. The value of Tc, its pressure dependence dTc/dp, and anisotropy field HA for the Ce(FeCr)12 phase are also reported. The linear thermal expansion of these compounds exhibits Invar‐like behavior with large positive values of the spontaneous magnetostriction at Tc [e.g., ωs(0)≊1.2% in Ce3(FeTi)29]. Different values of the compressibility have been observed in the ferromagnetic and paramagnetic states of Ce3(FeTi)29 (κf=1.25×10−3 kbar−1 and κp=1.12×10−3 kbar−1) in qualitative agreement with the large magnetovolume effect observed in the LTE. Tc for all the phases studied decreases under pressure with initial slopes dTc/dp=−3.75, −3.6, and −2.7 K/kbar for Ce3(FeTi)29, Ce3(FeCr)29, and Ce(FeCr)12, respectively. Two anisotropy fields have been detected in Ce3(...

The effect of pressure on the magnetic phase transitions in Nd2(FeTi)17 and Nd(FeTi)12 compounds

Abstract We describe the effect of high pressure on the Curie temperature T C and spin reorientation transition temperature T SR of Nd 2 (FeTi) 17 and Nd(FeTi) 12 compounds. Both T C and T SR decrease significantly with pressure. We measured the isothermal compressibility and using these results we found the same rate of change of T C with respect to the relative volume change, d T C /dln V ≈ 3000 K, for four different Fe-rich Nd-based intermetallics having T C values ranging from 330 to 555 K.