Rudolf Schäfer

Novel Design of La(Fe,Si)13 Alloys Towards High Magnetic Refrigeration Performance

www.MaterialsViews.com C O M M Novel Design of La(Fe,Si) 13 Alloys Towards High Magnetic Refrigeration Performance U N IC By Julia Lyubina , * Rudolf Schäfer , Norbert Martin , Ludwig Schultz , and Oliver Gutfl eisch A IO N Magnetic refrigeration near room temperature has great potential to become an energy-effi cient alternative to the conventional vapor-compression technology. [ 1–3 ] Attractive candidates for use as magnetic refrigerants are materials that undergo a fi rstorder magnetic phase transition, [ 4–7 ] which is accompanied by a change of crystal symmetry and/or volume. The latter leads inevitably to fracture during thermal and magnetic fi eld cycling and is a signifi cant challenge for the integration of these materials in the emerging magnetic cooling technology. [ 8 ] Here, we present a novel approach to controlling the magnetocaloric effect (MCE) in these materials by introducing porosity. In particular, we show that LaFe 13 − x Si x magnetic refrigerants with porous architecture do not degrade mechanically during cycling, and thus maintain their excellent cooling performance. This very signifi cant improvement is attributed to partial removal of grain boundaries that restrain volume expansion in the bulk material. We demonstrate that the fi rst-order magnetic phase transition is retained in porous LaFe 13– x Si x and the volume effects lead to particle size–dependent transition parameters. These results provide insight into how size-tuning can enhance the MCE and have exciting technological implications for the development of high-performance magnetic refrigerants. Magnetic cooling relies on the reversible magnetization and demagnetization of a magnetic material (magnetic refrigerant) and the resultant change in temperature in the refrigerant material under adiabatic conditions, Δ T ad . [ 1 , 9 ] The latter phenomenon is known as the magnetocaloric effect (MCE) and is maximized at temperatures corresponding to a magnetic phase transition, for example, the Curie point in ferromagnets. Usually the Curie transition is a phase transition of second order, but there exist magnetic materials in which the transition from one magnetic (disordered or ordered) state to another is a fi rst-order phase transition. [ 10–13 ] The fi rst-order magnetic phase transition gives rise to an abrupt change of the magnetization near the transition point and this, in turn, results in a large (giant) MCE. [ 2–7 ]

Domain observation on nanocrystalline material

Bulk polycrystalline material with a grain size in the nanometer range can be prepared by controlled crystallization of certain metallic glass alloys. The material shows excellent soft magnetic properties which are attributed to an effective averaging of the local crystal anisotropy over the scale of a Bloch wall width. The general appearance of observed domain patterns resembles those of metallic glasses. Wide in‐plane domains as well as the characteristic stress patterns are observed. On annealing beyond the optimum temperature the coercivity rises which manifests itself by an irregular domain appearance. An interesting behavior is found if the samples are observed at temperatures beyond ∼320 °C. At this temperature the amorphous ferromagnetic grain boundary phase which couples the iron‐rich crystallites becomes paramagnetic. The domains reversibly show a similar character as the overannealed samples. This observation indicates that overannealing leads in this material not only to grain growth beyond th...

Quantitative observation of magnetic domains with the magneto-optical Kerr effect

Abstract A new method for the quantitative determination of the magnetization direction at every point of a sample surface is presented. The method uses our digitally enhanced Kerr microscope and is based on a combination of the longitudinal and the transverse Kerr effect. The information from both effects is digitally combined with pictures taken in different saturated states. The resulting map of magnetization directions is displayed by arrows and additionally with the help of a colour code. Examples taken from complicated stress-governed domain structures on a metallic glass show pictures which are clearly easier to interpret than conventional qualitative Kerr images.

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.

Kerr observations of asymmetric magnetization reversal processes in CoFe/IrMn bilayer systems

The magnetization reversal process in the ferromagnetic layer of an exchange-biased Co90Fe10(20 nm)/Ir23Mn77(10 nm) film structure, deposited by dc-magnetron sputtering, is imaged by high-resolution Kerr microscopy. Additionally, high-resolution magnetization loops are measured by deriving the magnetization signal from the average image intensity. The magnetization reversal occurs first by magnetization rotation under the development of ripple-like structures. The modulated structures then partially switch, generating complicated multidomain configurations, which finally annihilate by large angle domain wall movement. The amount of magnetization rotation at different field directions is quantified by measuring the transversal magnetization components during reversal. A strong asymmetry, both in domain behavior and magnetization loop, between the forward and recoil branch of the magnetization reversal is found. The magnitude of asymmetry strongly depends on small angle misalignments between the direction o...

Two–dimensional modelling of soft ferromagnetic films

We examine the response of a soft ferromagnetic film to an in–plane applied magnetic field by means of both theory and experiment. In the thin–film limit, we uncover a separation of scales in the rough energy landscape of micromagnetics: leading–order terms generate constraints which eliminate degrees of freedom, terms of second order in the film thickness lead to a (new) reduced variational model, higher–order terms are related to wall, vortex and anisotropy energies. We propose a new strategy to compute low–energy domain patterns, which proceeds in two steps: we determine first the magnetic charge density by solving a convex variational problem, then we construct an associated magnetization field using a robust numerical method. Experimental results show good agreement with the theory. Our analysis is consistent with prior work by van den Berg and by Bryant and Suhl, but it goes much further; in particular it applies even for large fields which penetrate the sample.

Anisotropy pinning of domain walls in a soft amorphous magnetic material

Ribbons were annealed in the demagnetized state with one wall along the ribbon middle. This wall becomes pinned during the heat treatment. Reentrant reversal occurs when reverse domains are nucleated at the ribbon edge with a threshold field larger than the demagnetizing field; this wall does not annihilate when it meets the pinned wall but leaves a line of reverse domains stabilized by ripple in the anisotropy. These domains permit a regular smooth reversal for the demagnetization process until the ribbon returns to the pinned configuration. The regular loop appears when the ribbon has been completely saturated by a large field. Mobile walls are nucleated on both sides of the pinned wall so that the ribbon does not return to the pinned configuration. Reversal now follows the usual demagnetization curve over the entire cycle. Kerr magnetooptical domain and domain wall observations are used in this investigation. All of the possible wall structures predicted by the model of asymmetric flux closed Bloch walls were identified. >

Investigation of the domain structure of amorphous Fe40Ni40P14B6-alloys

Abstract In amorphous Fe 40 Ni 40 P 14 B 6 -alloys (Allied Chem. Corp., no. 2826), the domain structure is composed of wide, wavy laminae and islands of rather narrow laminar domains which are due to compressive stresses. From the field dependence of the domain width, we derived an effective anisotropy constant of K =6.2 × 10 2 J/m 3 ≡ 6.2 × 10 3 erg/cm 3 , and a specific domain wall energy γ B = 0.17 × 10 -3 J/cm 2 ≡ γ B = 0.17 erg/cm 2 .

Dynamic anisotropy in amorphous CoZrTa films

The high-frequency response of amorphous CoZrTa thin films was measured by using a pulsed inductive microwave magnetometer. The anisotropy of the magnetic films was varied by magnetic field annealing. Static anisotropy field values ranging from Hk=100 to 1920 A/m were obtained. The dynamically determined anisotropy field is shifted to higher values compared to the static anisotropy by an additional isotropic internal field Hadd. This internal field is independent of the strength of the static anisotropy field. We determined a value of about Hadd=510 A/m.

Domains in 'extremely' soft magnetic materials

Abstract The magnetic microstructure of high-permeability materials (crystalline NiFe-sheets, amorphous and nanocrystalline ribbons) is reviewed. For the majority of such ‘extreme’ soft magnets identifiable domains and walls are found due to residual anisotropies. Only in cases of vanishing anisotropy conventional domains are replaced by a smoothly varying magnetization as demonstrated for the first time in high-quality Permalloy sheets and properly treated amorphous ribbons.