S. Fähler

Exchange spring media for perpendicular recording

A novel type of exchange spring media is proposed for magnetic recording systems consisting of a hard/soft bilayer. Finite element micromagnetic simulations show that the reversal modes induced by the external write field are significantly different from the thermally activated switching processes. Thus, the bilayers can be optimized in order to achieve a high thermal stability without increase of coercive field. In grains with identical size and coercivity an optimized bilayer reaches an energy barrier exceeding those of optimized single phase media by more than a factor of two. Additionally the lower angular dependence of coercivity of exchange spring media will improve the signal to noise ratio.

Adaptive modulations of martensites.

Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape-memory alloys. To understand the origin of these phases, we employ and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase, and tetragonal martensite in Ni-Mn-Ga magnetic shape-memory alloy epitaxial films. We show that the modulated martensite can be constructed from nanotwinned variants of the tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes metastability, the sequence of 6M-10M-14M-NM intermartensitic transitions, and the magnetocrystalline anisotropy.

Calculations and experiments of material removal and kinetic energy during pulsed laser ablation of metals

Numerical calculations are presented, which describe the processes of target heating and ablation of pure metals (for instance Fe) during irradiation by 30 ns laser pulses at 248 nm. The following effects are taken into account: the absorption of the laser radiation and the heat conduction within the target, the evaporation of material from the target surface, the cooling of the target surface by the heat of evaporation and the partial absorption of the incident laser beam in the evaporated material. As results of the calculations, the temperature profile in the target and the ablated material can be obtained. Furthermore, by energy balance, an energy of the ablated material of up to some 100 eV is obtained. Experiments performed concerning the threshold energy of ablation, the removed mass from the target and the kinetic energy of the deposited ions (by time-of-flight measurements) are in good agreement with the performed calculations.

Voltage-controlled epitaxial strain in La0.7Sr0.3MnO3∕Pb(Mg1∕3Nb2∕3)O3-PbTiO3(001) films

Epitaxially grown La0.7Sr0.3MnO3 thin films show resistance modulations induced by the inverse piezoeffect of the employed Pb(Mg1∕3Nb2∕3)O3-PbTiO3(001) (PMN-PT) substrates. The in-plane strain state of the films can continuously be tuned by application of a piezovoltage to PMN-PT. The lattice deformation of a PMN-PT(001) substrate was quantified by x-ray measurements under an electric field. Variation of in-plane lattice parameters by ∼0.06% reversibly changes the resistance of the manganite films by up to 9% at 300 K and shifts the magnetic Curie temperature. Films of different thicknesses from 50 to 290 nm, offering different as-grown strain states, have been studied.

Magnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO(001)

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45 relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing microstructural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.

Highly coercive electrodeposited FePt films by postannealing in hydrogen

The properties of electrodeposited films subsequently annealed in H2 are reported and compared with those of vacuum-annealed samples. Annealing in hydrogen reduces the oxygen content incorporated during electrodeposition, resulting in significantly higher magnetization values. Phase formation is enhanced by hydrogen and L10 ordering starts at temperatures as low as 350°C. In addition, grain growth is hindered. These effects contribute to the high coercivity of 1.1T achieved after annealing at 600°C in H2.

Full tunability of strain along the fcc-bcc bain path in epitaxial films and consequences for magnetic properties.

Huge deformations of the crystal lattice can be achieved in materials with inherent structural instability by epitaxial straining. By coherent growth on seven different substrates the in-plane lattice constants of 50 nm thick Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches 8,3 % relative to the fcc lattice. The in-plane lattice strain results in a remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. This has dramatic consequences for the magnetic key properties. Magnetometry and X-ray circular dichroism (XMCD) measurements show that Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad ranges.

Epitaxial Ni–Mn–Ga films deposited on SrTiO3 and evidence of magnetically induced reorientation of martensitic variants at room temperature

Epitaxial Ni–Mn–Ga films were grown on SrTiO3 by sputter deposition. The films deposited at 673K are ferromagnetic and martensitic at room temperature. Pole figure measurements indicate that the twinned orthorhombic martensite microstructure of the film has a lower symmetry compared to bulk. Magnetically induced reorientation or magnetic shape memory effect is indicated by magnetization curve measurements. Though the overall extension of the film is constrained by a rigid substrate, the reorientation is possible due to the additional degree of freedom in the orthorhombic phase.

Textured growth of highly coercive L10 ordered FePt thin films on single crystalline and amorphous substrates

FePt films have been prepared by pulsed laser deposition at a temperature of 800 °C onto (001) oriented single crystalline (MgO, SrTiO3) as well as amorphous substrates (Si3N4, glass) with and without a (001) textured MgO buffer layer. Although all films have been prepared under identical conditions, significant differences in coercivity are observed. Very high values of up to 5.6 T have been measured for the epitaxial films, while on the amorphous substrates the coercivities range between 0.8 and 2.0 T. We analyze whether changes in intrinsic or extrinsic properties due to a modified microstructure are responsible for these differences.

Resputtering during the growth of pulsed-laser-deposited metallic films in vacuum and in an ambient gas

To determine the effective sputter yield during pulsed-laser deposition a method by measuring the deposition rate on tilted substrates is proposed. Under vacuum conditions, sputter yields of up to 0.17 and 0.55 were found at a laser fluence of 4.5 J/cm2 for Fe and Ag, respectively. These strong resputtering effects are induced by the large fraction of energetic ions occurring during deposition. With decreasing laser fluence or increasing Ar gas pressure, the sputter yields are reduced due to a decrease of the kinetic energy of the ions. For the deposition of stoichiometric films, an optimum Ar partial pressure of about 0.04 mbar exists, where the deposition rate is highest and the sputter yield is reduced.