Reducing extrinsic hysteresis in first-order la (Fe,Co,Si)13 magnetocaloric systems

Reducing extrinsic hysteresis in first-order la (Fe,Co,Si)13 magnetocaloric system

Low-temperature specific heat in hydrogenated and Mn-doped La(Fe, Si)(13)

It is now well established that the paramagnetic-to-ferromagnetic transition in the magnetocaloric La(FeSi)13 is a cooperative effect involving spin, charge, and lattice degrees of freedom. However, the influence of this correlated behavior on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of LaFexMnySiz samples, with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature, and the spin-wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced d-band electronic interactions at play. The Sommerfeld coefficient is found to be 90–210mJmol−1K−2, unusually high compared to that expected from band-structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin-wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly and provide a basis for future theoretical development

Grain boundary oxide layers in NdFeB-based permanent magnets

The microstructure of grain boundaries (GBs) in the commercial NdFeB-based alloy for permanent magnets has been studied. It is generally accepted that the unique hard magnetic properties of such alloys are controlled by the thin layers of a Nd-rich phase in Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs. These GB layers ensure the magnetic isolation of Nd$_{2}$Fe$_{14}$B grains from each other. It is usually supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial NdFeB-based permanent magnets frequently contain a tangible amount of neodymium oxide Nd$_{2}$O$_{3}$ at the triple junctions between Nd$_{2}$Fe$_{14}$B grains. The goal of this work was to check whether the Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs could also contain the thin layers of Nd$_{2}$O$_{3}$ oxide phase. Indeed, the screening with EELS-based elemental analysis permitted to observe that some of these Nd-rich layers in Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs contain not only neodymium, but also oxygen. More detailed analysis of such GBs with high-resolution transmission electron microscopy (HR TEM) showed these GB layers are crystalline and have the lattice of neodymium oxide Nd$_{2}$O$_{3}$. In turn, the Lorentz micro-magnetic contrast in TEM permitted to observe that the Nd-oxide GB layers prevent the migration of domain walls from one Nd$_{2}$Fe$_{14}$B grain to another during remagnetization. This finding proves that the GB oxide layers, similar to those of metallic Nd or Nd-rich intermetallic compounds, can ensure the magnetic isolation between Nd$_{2}$Fe$_{14}$B grains needed for high coercivity. Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets

Determining the first-order character of La(Fe,Mn,Si)(13)

Definitive determination of first-order character of the magnetocaloric magnetic transition remains elusive. Here we use a microcalorimetry technique in two modes of operation to determine the contributions to entropy change from latent heat and heat capacity separately in an engineered set of La ( Fe , Mn , Si ) 13 samples. We compare the properties extracted by this method with those determined using magnetometry and propose a model-independent parameter that would allow the degree of first-order character to be defined across different families of materials. The microcalorimetry method is sufficiently sensitive to allow observation at temperatures just above the main magnetic transition of an additional peak feature in the low field heat capacity associated with the presence of Mn in these samples. The feature is of magnetic origin but is insensitive to magnetic field, explicable in terms of inhomogeneous occupancy of Mn within the lattice resulting in antiferromagnetic ordered Mn clusters

Evaluation of the reliability of the measurement of key magnetocaloric properties: a round robin study of La(Fe,Si,Mn)Hδ conducted by the SSEEC consortium of European laboratories

Managing refrigeration of our homes, our food and our work environments in energy efficient ways is of increasing importance. Refrigeration using solid state magnetic cooling is one of a number of technologies that may make a significant contribution to addressing this problem. In order to develop materials that may enable commercial development of this increasingly relevant field it is important to review the reliability of methods used to extract key physical properties, so that as the field matures the community can develop recognised standards of measurement. Here we measure key physical properties in one composition taken from a series of La(Fe,Si,Mn)Hδ 1:13‐type samples grown by a source laboratory and measured independently in a consortium of European laboratories using both commercial and bespoke facilities

Atomic structure and domain wall pinning in samarium-cobalt-based permanent magnets

A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm₂Co₁₇-type pinning-controlled permanent magnets. These are of importance for high-temperature industrial applications due to their intrinsic corrosion resistance and temperature stability. Here we present model magnets with an increased iron content based on a unique nanostructure and -chemical modification route using Fe, Cu, and Zr as dopants. The iron content controls the formation of a diamond-shaped cellular structure that dominates the density and strength of the domain wall pinning sites and thus the coercivity. Using ultra-high-resolution experimental and theoretical methods, we revealed the atomic structure of the single phases present and established a direct correlation to the macroscopic magnetic properties. With further development, this knowledge can be applied to produce samarium cobalt permanent magnets with improved magnetic performance

Towards engineering the perfect defect in high-performing permanent magnets

Permanent magnets draw their properties from a complex interplay, across multiple length scales, of the composition and distribution of their constituting phases, that act as building blocks, each with their associated intrinsic properties. Gaining a fundamental understanding of these interactions is hence key to decipher the origins of their magnetic performance and facilitate the engineering of better-performing magnets, through unlocking the design of the "perfect defects" for ultimate pinning of magnetic domains. Here, we deployed advanced multiscale microscopy and microanalysis on a bulk Sm2(CoFeCuZr)17 pinning-type high-performance magnet with outstanding thermal and chemical stability. Making use of regions with different chemical compositions, we showcase how both a change in the composition and distribution of copper, along with the atomic arrangements enforce the pinning of magnetic domains, as imaged by nanoscale magnetic induction mapping. Micromagnetic simulations bridge the scales to provide an understanding of how these peculiarities of micro- and nanostructure change the hard magnetic behaviour of Sm2(CoFeCuZr)17 magnets. Unveiling the origins of the reduced coercivity allows us to propose an atomic-scale defect and chemistry manipulation strategy to define ways toward future hard magnets