D. Goll

Melt-spun precipitation-hardened Sm2(Co, Cu, Fe, Zr)17 magnets with abnormal temperature dependence of coercivity

Rapidly quenched Sm(CobalCu0.08Fe0.22Zr0.02)8.5 (Cu-/Fe-rich) and Sm(CobalCu0.05Fe0.10Zr0.03)8.5 (Cu-/Fe-poor) ribbons have been prepared by means of the melt-spinning technique. By applying an appropriate annealing procedure a microstructure similar to that of sintered magnets can be obtained. The energy dispersive x-ray microanalysis of the compositional dependence near the cell boundaries suggests a model for the profile of the crystal anisotropy constants responsible for the magnetic hardening. The Cu-/Fe-rich alloy shows a normal temperature dependence of coercivity with a negative temperature coefficient, but the Cu-/Fe-poor ribbons show a positive temperature coefficient in the temperature range from 400–700 K. The different temperature coefficients are discussed in terms of a pinning model.

Experimental realization of graded L10-FePt/Fe composite media with perpendicular magnetization

A concept is suggested and experimentally realized to fabricate graded media for ultrahigh density magnetic recording where the material parameters vary gradually in the interfacial region between the hard magnetic part and the soft magnetic part of epitaxial L10-FePt/Fe exchange spring nanocomposites with perpendicular magnetization. A graded interface between the L10-FePt phase and the Fe phase is formed by depositing part of the Fe layer at elevated temperatures. The existence of the graded interface is verified by electron energy-loss spectroscopy. The influence of the character of the graded interface on the magnetic properties is studied. With increasing thickness of the graded interface the coercivity continuously decreases, which can be used for a fine tuning of the coercivity of exchange spring composite media.

Coercivity of ledge-type L10-FePt/Fe nanocomposites with perpendicular magnetization

Exchange-coupled ledge-type L10-FePt/Fe composite systems with out-of-plane anisotropy composed of nanostructured L10-FePt films covered by Fe are prepared to analyze the influence of the soft magnetic layer thickness on the magnetic properties. By the soft magnetic layer thickness dFe the coercivity can be tailored according to a 1/dFe1.38 relation. This result can be used to realize recording media with coercivities in the range which are afforded by conventional write heads.

Micromagnetism and the microstructure of high-temperature permanent magnets

Sm2(Co,Cu,Fe,Zr)17 permanent magnets with their three-phase precipitation structure (cells, cell walls, and lamellae) show two characteristic features which so far are difficult to interpret but which are the prerequisites for high-temperature applications: (1) The hard magnetic properties only develop during the final step of the three-step annealing procedure consisting of homogenization, isothermal aging, and cooling. (2) Depending on the composition and on the annealing parameters, the temperature dependence of the coercivity can be easily changed from the conventional monotonic to the recent nonmonotonic behavior showing coercivities up to 1T even at 500K. The magnetic hardening during cooling is due to the fact that the cell walls order chemically and structurally during the cooling process. From an analysis of electron diffraction patterns of the superimposed structures existing before and after cooling it could be proven that a phase transition from a phase mixture of defective phases 2:17, 2:7, a...

Micromagnetic analysis of pinning-hardened nanostructured, nanocrystalline Sm2Co17 based alloys

Abstract In Sm(Co bal Cu 0.07 Fe 0.22 Zr 0.04 ) 7.4 the chemical composition of the 2:17 cells and the 1:5 cell walls has been analysed by high-resolution TEM-EDX for different annealing stages. From the measured Cu-profile the profile of the first anisotropy constant has been determined and applied to determine the pinning of domain walls at the phase boundary.

Micromagnetic analysis of nucleation-hardened nanocrystalline PrFeB magnets

Abstract The coercive field of exchange-coupled nanocrystalline melt-spun and bonded composite magnets of Pr 2 Fe 14 B+ α -Fe is shown to obey the fundamental relation μ 0 H c =(2 K 1 / M s ) α − N eff J s . The microstructural parameter α is analysed in terms of misaligned grains, imperfect grain boundaries and exchange coupling between neighbouring grains.

Thermal stability of ledge-type L10-FePt/Fe exchange-spring nanocomposites for ultrahigh recording densities

Long-term thermal stability is crucial for magnetic nanoparticles in ultrahigh density magnetic recording. For ledge-type exchange-spring nanocomposites consisting of a hard magnetic L10-FePt part and a soft magnetic Fe part of more extended horizontal size, the minimum energy barriers for thermal reversal between equilibrium states of the hysteresis loops are determined using a nudged-elastic-band path approach. The field dependence of the energy barrier can be described approximately by power laws. It turns out that nanopatterns of isolated ledge-type L10-FePt/Fe composite elements or isolated L10-FePt nanodots covered by a thin Fe film are well-suited in realizing ultrahigh recording densities.

High-throughput search for new permanent magnet materials

The currently highest-performance Fe-Nd-B magnets show limited cost-effectiveness and lifetime due to their rare-earth (RE) content. The demand for novel hard magnetic phases with more widely available RE metals, reduced RE content or, even better, completely free of RE metals is therefore tremendous. The chances are that such materials still exist given the large number of as yet unexplored alloy systems. To discover such phases, an elaborate concept is necessary which can restrict and prioritize the search field while making use of efficient synthesis and analysis methods. It is shown that an efficient synthesis of new phases using heterogeneous non-equilibrium diffusion couples and reaction sintering is possible. Quantitative microstructure analysis of the domain pattern of the hard magnetic phases can be used to estimate the intrinsic magnetic parameters (saturation polarization from the domain contrast, anisotropy constant from the domain width, Curie temperature from the temperature dependence of the domain contrast). The probability of detecting TM-rich phases for a given system is high, therefore the approach enables one to scan through even higher component systems with one single sample. The visualization of newly occurring hard magnetic phases via their typical domain structure and the correlation existing between domain structure and intrinsic magnetic properties allows an evaluation of the industrial relevance of these novel phases.

Analysis of the temperature dependence of the coercive field of Sm2Co17 based magnets

In Sm2Co17 based magnets the coercive field is determined by the repulsive or attractive interaction of domain walls with the 1:5 cell walls. With increasing temperature the coercivity mechanism changes from repulsive to attractive pinning and above the Curie temperature of the cell walls a nucleation mechanism may be dominant.

Temperature dependence of the magnetic properties of L10-FePt nanostructures and films

Hard magnetic L10-Fe51Pt49 thin films with out-of-plane texture and film thicknesses between 3 and 200 nm have been prepared on MgO(001) single crystalline substrates by co-sputtering and have been magnetically investigated by SQUID magnetometry at different temperatures ranging from 40 K up to the Curie temperature. The Curie temperature is found to be 660 K for film thicknesses down to 8 nm and decreases almost linearly for thinner films. For the whole ferromagnetic temperature range the intrinsic magnetic material parameters (saturation polarization Js, magnetocrystalline anisotropy constant K1, exchange constant A) and the coercivity μ0Hc are determined as a function of the film thickness. Furthermore the microstructural parameters are ascertained by analyzing the temperature dependence of the coercivity within the framework of micromagnetism leading to a deeper understanding of the magnetic reversal process.