E. F. Wassermann

Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In

Applying a magnetic field to a ferromagnetic Ni{sub 50}Mn{sub 34}In{sub 16} alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field, giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first-order structural transition are studied by magnetization, strain, and neutron-diffraction studies under magnetic field.

Modelling the phase diagram of magnetic shape memory Heusler alloys

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

Quantitative determination of effective dipole and monopole moments of magnetic force microscopy tips

We present experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field. Well defined magnetic stray fields are produced using current carrying rings with radii ranging between 603 and 2369 nm fabricated by electron-beam lithography directly imaged by MFM. Treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic stray field and the lift height allows for a quantitative determination of effective magnetic dipole and monopole moments of the tip as well as their imaginary location within the real physical tip. Our systematic study gives a quantitative relationship on how absolute values of the magnetic dipole and monopole moments and their location within the tip depend on a characteristic decay length of the z component of the magnetic field being detected. From this we can estimate the effective tip volume of the real physical thin film tip relevant in MFM...

Magnetic properties of FePt nanoparticles

Abstract We have prepared FePt nanoparticles with concentrations in the vicinity of the stoichiometric Fe50Pt50 composition by means of a gas-phase-based process. The preparation technique employed allows to anneal (‘sinter’) the particles in the gas-phase prior to their deposition. Depending on the gas-pressure during preparation, the structure of the unsintered particles is icosahedral or polycrystalline face-centered cubic (FCC). Temperature-dependent magnetization measurements reveal that these particles are superparamagnetic at room temperature. Gas-phase sintering of polycrystalline FCC FePt nanoparticles results in the formation of predominantly single crystal particles which are face-centered tetragonal due to an increased degree of L10 order at elevated sintering temperatures TS. Concurrently, we observe an increase of both the blocking temperature TB and the coercivity HC with increasing TS. This enhancement of TB and HC is more pronounced in iron-rich off-stoichiometric samples, for which the concentrations are still within the L10 stability range of the FePt phase diagram. Fe62Pt38 particles, which are sintered at T S =1273 K , reach T B =530 K and H C 0 =H C (T=0)=4.7 kOe . Whereas the elevated blocking temperature is mainly due a somewhat increased particle size at elevated sintering temperatures, the higher coercivity is attributed to the enhanced degree of L10 order in the gas-phase sintered particles.

Tailoring magnetic and magnetocaloric properties of martensitic transitions in ferromagnetic Heusler alloys

Ni50Mn34In16 undergoes a martensitic transformation around 250K and exhibits a field induced reverse martensitic transformation and substantial magnetocaloric effects. We substitute small amounts Ga for In, which are isoelectronic, to carry these technically important properties to close to room temperature by shifting the martensitic transformation temperature.

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.

FABRICATION OF LARGE SCALE PERIODIC MAGNETIC NANOSTRUCTURES

We report on the fabrication of large scale periodic magnetic nanostructures designed as possible candidates for a future application in magnetic data storage technology. The nanostructures are prepared on glass substrates by electron beam evaporation of Fe or Co onto photoresist masks, which are periodically patterned using optical interference lithography with an Ar+ laser (λ=457.8 nm) in combination with subsequent selective etching. We present our first results on the fabrication of periodic arrays of isolated magnetic dots with an average diameter of 600 nm and periodicity of 900 nm over areas of 5 cm2.

Preparation of size-classified PbS nanoparticles in the gas phase

We report on the synthesis of size-classified PbS nanocrystals by differential mobility analysis of a polydisperse aerosol formed by nucleation and aggregation processes in a furnace reactor. The sublimation temperature employed is below the stoichiometric evaporation temperature, thus enabling direct and simple synthesis by sublimation. The irregularly shaped and amorphous agglomerate particles are then sintered in a second furnace, resulting in the formation of monocrystalline and quasispherical particles with sizes adjustable between 3 and 20 nm and a standard deviation of 1.13.

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.

Magnetization reversal in nanostructured Co/Pt multilayer dots and films

We report on the magnetic properties of large scale periodic magnetic nanostructures designed as dot-arrays with periodicity ranging between 300 nm and 1100 nm, consisting of Co/Pt multilayer dots with respective diameters ranging between 230 nm and 740 nm. The nanostructures are fabricated by using optical interference lithography allowing for large area (cm/sup 2/) fabrication. The Co/Pt multilayer dots show a pronounced out-of-plane magnetic anisotropy. The magnetic properties are determined from magneto-optical Kerr effect measurements in polar geometry and results are compared to respective continuous Co/Pt multilayer thin films. The micromagnetic properties of both, films and nanostructures are investigated with magnetic force microscopy allowing for the application of an external perpendicular magnetic field of up to B=0.15 T.