Eyup Duman

Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys.

The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials1,2,3. Here we report on an inverse situation in Ni–Mn–Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect4,5. However, the entropy change found for some ferromagnetic Ni–Mn–Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.

Effect of Co and Fe on the inverse magnetocaloric properties of Ni-Mn-Sn

At certain compositions Ni-Mn-X Heusler alloys (X: group IIIA–VA elements) undergo martensitic transformations, and many of them exhibit inverse magnetocaloric effects. In alloys where X is Sn, the isothermal entropy change is largest among the Heusler alloys, particularly in Ni50Mn37Sn13, where it reaches a value of 20 J kg−1 K−1 for a field of 5 T. We substitute Ni with Fe and Co in this alloy, each in amounts of 1 and 3 at % to perturb the electronic concentration and examine the resulting changes in the magnetocaloric properties. Increasing both Fe and Co concentrations causes the martensitic transition temperature to decrease, whereby the substitution by Co at both compositions or substituting 1 at % Fe leads to a decrease in the magnetocaloric effect. On the other hand, the magnetocaloric effect in the alloy with 3 at % Fe leads to an increase in the value of the entropy change to about 30 J kg−1 K−1 at 5 T.

Coexisting ferro- and antiferromagnetism in Ni2MnAl Heusler alloys

The structural and magnetic properties of stoichiometric Ni2MnAl are studied to clarify the conditions for ferromagnetic and antiferromagnetic ordering claimed to occur in this compound. X-ray and magnetization measurements show that although a single phase B2 structure can be stabilized at room temperature, a single L21 phase is not readily stabilized, but rather a mixed L21+B2 state occurs. The mixed state incorporates ferromagnetic and antiferromagnetic parts for which close-lying Curie and a Neel temperatures can be identified from magnetization measurements.

Magnetic properties and martensitic transition in annealed Ni50Mn30Al20

We have studied the effect of heat treatment on the magnetic properties and on the martensitic transition of the Ni50Mn30Al20 alloy. A mixed L21+B2 state is obtained in the as-prepared sample, while no L21 order is retained in the sample quenched from high temperature. For the two heat treatments, the samples order antiferromagnetically, but there is evidence of coexisting ferromagnetic interactions. A martensitic transition occurs below the magnetic one for quenched samples. However, the martensitic transition is inhibited in the as-prepared sample.

Hysteresis effects in the magnetic-field-induced reverse martensitic transition in magnetic shape-memory alloys

We study the mechanism of magnetic-field-induced reverse martensitic transformations by considering the field-induced temperature-shifts in the thermal hysteresis loop of the magnetization and critical fields determined from magnetization isotherms. We consider first the temperature-shifts in a symmetric thermal hysteresis loop and extend the discussion to the case of the Heusler-based Ni50Mn34In16 magnetically superelastic alloy having a nonsymmetric thermal hysteresis loop. We obtain two different temperature-behaviors for the critical field depending on whether the initial measurement temperature lies on the forward or on the reverse transformation path of the thermal hysteresis.

Large spontaneous magnetostrictive softening below the Curie temperature of Fe3C Invar particles

Fe3C incorporates only Fe as a metallic element and is an Invar material with valence electron concentration 8.67. We have studied the equation of state up to 20GPa of Fe3C particles (d∼40nm) below and above the Curie temperature (room temperature and 550K, respectively) to search for a softening associated with a magnetovolume instabilities characteristic of Invar-type material. The isothermal bulk modulus of Fe3C particles at room temperature was found to be about 13% smaller than that of the value at 550K. This difference is comparable with the relative difference in the bulk modulus of Fe65Ni35 Invar in a similar temperature range around the Curie temperature.

Magnetic properties of Pr1−xGdxMn2Ge2

The structural and magnetic properties of Pr1−xGdxMn2Ge2 were studied by x-ray diffraction and magnetization measurements. The substitution of Gd for Pr leads to a linear decrease in the lattice constants and the magnetic interactions in the Mn sublattice cross over from a ferromagnetic character to an antiferromagnetic one. At low temperatures, the rare-earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The spins in the Mn sublattice are arranged parallel to the Pr sublattice and antiparallel to the Gd sublattice. The results are collected in a phase diagram.

Magnetic phase transitions in PrMn2-xCrxGe2

Abstract The structural and magnetic properties of PrMn 2− x Cr x Ge 2 (0⩽ x ⩽1.0) were studied by X-ray diffraction and magnetization measurements. The powder samples crystallize in the ThCr 2 Si 2 -type structure, and the lattice constants at room temperature show almost no variation as Cr substitutes Mn. The observed phase transitions are summarized in a proposed magnetic x − T phase diagram and compared with previous Moessbauer spectroscopy and neutron diffraction results for x =0.

Magnetic transitions in Nd1-xDyxMn2Ge2

Abstract The various magnetic structures of the ThCr 2 Si 2 structured RMn 2 Ge 2 (R: rare-earth) compounds can lead to numerous magnetic configurations by gradually substituting the rare-earth element by another lanthanide. In this study, the magnetic properties of Nd 1− x Dy x Mn 2 Ge 2 (0⩽ x ⩽1) compounds were investigated by means of temperature-dependent DC magnetization measurements in low external magnetic fields. The substitution of Dy for Nd leads to a linear decrease in the lattice constants, and the magnetic interactions in the Mn sublattice cross over from a ferromagnetic character to an antiferromagnetic one. At low temperatures, the rare-earth sublattice orders in various configurations that depend on the composition. The results are collected in a phase diagram.

Disorder-induced critical phenomena in magnetically glassy Cu-Al-Mn alloys

Measurements of magnetic hysteresis loops in Cu-Al-Mn alloys of different Mn content at low temperatures are presented. The loops are smooth and continuous above a certain temperature, but exhibit a magnetization discontinuity below that temperature. Scaling analysis suggest that this system displays a disorder-induced phase transition line. Measurements allow one to determine the critical exponents