N. Scheerbaum

Reversibility of magnetostructural transition and associated magnetocaloric effect in Ni–Mn–In–Co

By analyzing isothermal magnetization curves under magnetic field cycling, the reversibility of the magnetostructural transition was investigated in Ni–Mn–In–Co in form of bulk sample and melt-spun ribbons. The hysteresis of thermally/magnetically induced martensitic transformation plays an important role in the reversibility of the magnetostructural transition. In ribbons with a large hysteresis of 18K, residual field-induced austenite is present after removing the magnetic field, while, in the bulk sample, the magnetostructural transition is reversible at moderate temperatures due to a relatively smaller hysteresis of 8K. Additionally, the magnetocaloric effect strongly depends on the sample history due to the occurrence of the irreversible magnetostructural transition, especially for the ribbons.

Absence of magnetic domain wall motion during magnetic field induced twin boundary motion in bulk magnetic shape memory alloys

A detailed study of twin boundary motion in NiMnGa single crystals together with in situ magnetic domain observation is presented. Optical polarization microscopy in connection with a magneto-optical indicator film technique was used to investigate the reorganization of the magnetic domains during twin boundary motion over a wide magnetic field range. Images at different field strengths demonstrate that no magnetic domain wall motion within the twins takes place, even during the structural reorientation by twin boundary movement. This absence of interaction of magnetic and structural domains is different from currently proposed models, which assume domain wall movement under an external field.

Large magnetostrain in polycrystalline Ni–Mn–In–Co

This letter reports the magnetic shape memory effect in a polycrystalline Ni45.2Mn36.7In13Co5.1 alloy having the martensitic transformation around 310 K. An application of magnetic field shifts the transformation to lower temperatures by 6.7 K/T. Originating from the magnetically induced martensite-austenite transformation, a large magnetostrain of 0.25% at 310 K was observed in a textured polycrystalline Ni45.2Mn36.7In13Co5.1 sample. Temperature, anisotropy, and cycling effects on the magnetostrain are discussed.

Magnetostructural transformation in Ni–Mn–In–Co ribbons

A series of Ni50−xMn37In13Cox (x=0–9) ribbons was synthesized using the melt-spinning technique. Different ordered structures (5M, 7M, B2, and L21) were formed depending on composition. Both martensitic transformation temperature and entropy change increase with increasing Co for 0⩽x⩽3, while they decrease with further increasing to 3<x⩽9. Increasing Co increases the Curie temperature of the austenite but decreases that of the martensite. An optimized composition range of 4.5⩽x⩽5.5 is obtained where a magnetic-field-induced reverse transformation from nonmagnetic martensite to ferromagnetic austenite is realized.

Magnetic Shape Memory Phenomena

Giant magnetically induced strain up to 50 times larger compared to the strain of giant magnetostriction was observed in some Heusler alloys, particularly in Ni–Mn–Ga. In analogy with the shape memory phenomenon this effect was called magnetic shape memory effect. The effect includes two different phenomena: a magnetically induced structural phase transformation (usually a martensitic transformation) and a magnetically induced structural reorientation occurring in the martensitic phase. Transformation behavior, structure of the martensite, and phenomenology of the magnetically induced reorientation are described. The description is based mainly on the well-studied compound Ni–Mn–Ga.

Perpendicular magnetization of long iron carbide nanowires inside carbon nanotubes due to magnetocrystalline anisotropy

Single crystal iron carbide nanowires contained in multiwalled carbon nanotubes have been prepared by aerosol-based thermal chemical vapor deposition. Investigations by transmission electron microscopy reveal the crystallographic [010] axis of the orthorhombic Fe3C nanowires to be predominantly aligned along the nanotube axis. Despite the high aspect ratio of the Fe3C nanowires, magnetic force microscopy measurements imply single domain behavior with the easy magnetic axis of the nanowires perpendicular to the wire axis. In agreement with the structural results, these findings show that the magnetic behavior is dominated by the magnetocrystalline anisotropy contribution, causing the easy axis to be along the [001] direction.

Effect of pressure on the magnetocaloric properties of nickel-rich Ni–Mn–Ga Heusler alloys

Nickel-rich Ni–Mn–Ga Heusler alloys were prepared by arc melting and subsequent homogenization by annealing. A large magnetic entropy change was observed around 291 K in the alloy where martensite-austenite structural and ferro-para magnetic transitions almost coincide with each other. The effect of hydrostatic pressure of up to 8 kbar on magneto-structural transitions, magnetocaloric effect, and magnetic hysteresis was studied. The martensitic transition temperature as well as the Curie temperature TC was found to increase, whereas the magnetic entropy change |ΔSM| decreases slightly due to the application of hydrostatic pressure. The large hysteresis observed in M versus H curve at the ambient pressure almost vanishes due to the application of 8 kbar pressure.

Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation

Composites consisting of magnetic shape memory (MSM) particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni50.9Mn27.1Ga22.0 (at. %) fibers. The crystal structure of the martensite is tetragonal (5M) with c<a=b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression (height reduction), the MSM particles have a (004)-fiber texture in the direction of compression. This is unambiguous evidence for s...

Ni–Mn–In–Co single-crystalline particles for magnetic shape memory composites

Single- and oligo-crystalline magnetic shape memory (MSM) Ni45.2Mn36.7In13Co5.1 particles were prepared by mechanical grinding the melt-spun ribbons. The influence of annealing and grinding on the crystal structure and magnetic properties of the ribbons and particles was systematically investigated. The obtained Ni45.2Mn36.7In13Co5.1 single-crystalline particles exhibit a field-induced martensitic transformation above room temperature, thus are used for MSM-polyester composites.

Texture Formation during ECAP of Aluminum Alloy AA 5109

The technical aluminum alloy AA 5109 with a strong cube rolling texture has been deformed at room temperature by equal channel angular pressing (ECAP) using three passes of route A. Samples for ECAP have been cut parallel and at 45° with respect to the rolling direction yielding different starting textures. The local texture after ECAP has been investigated by highenergy synchrotron radiation. It is characterized by typical shear components of face-centred cubic (fcc) metals which deviate from their ideal positions. The texture with respect to intensity and deviation from ideal positions of the components depends on the distance from the top of the extruded billet and changes from pass to pass. It is also strongly influenced by the starting texture. The texture gradient has been discussed in the light of Tóth’s flow line model. The texture results have also been compared with those of other fcc metals with different stacking fault energy.