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Permanent magnets are applied in many large-scale and emerging applications and are crucial components in numerous established and newly evolving technologies. Rare-earth magnets exhibit excellent hard magnetic properties; however, their applications are limited by the price and supply risk of the strategic rare-earth elements. Therefore, there is an increasing demand for inexpensive magnets without strategic elements. Here, the authors report the intrinsic highly-anisotropic magnetic properties of Co and Si co-doped single crystals (Fe1−yCoy)2P1−xSix (y ≈ 0.09). Co increases Curie temperature TC; Si doping decreases magnetocrystalline anisotropy K1 and also increases TC significantly because of the enhanced interlayer interaction. The maximum room temperature magnetocrystalline anisotropy K1 = 1.09 MJ m−3 is achieved for x = 0.22, with saturation magnetization µ0Ms = 0.96 T and TC = 506 K. The theoretical maximum energy product is one of the largest for any magnet without a rare earth or Pt. Besides its promising intrinsic magnetic properties and absence of any strategic elements, other advantages are phase stability at high temperatures and excellent corrosion resistance, which make this material most promising for permanent magnetic development that will have a positive influence in industry and daily life. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. ; This work was financially supported by the Joint Initiative for Research and Innovation within the Fraunhofer and Max Planck cooperation program, an Advanced Grant from the European Research Council (no. 742068) "TOPMAT," the European Union's Horizon 2020 research and innovation programme (no. 824123) "SKYTOP," the European Union's Horizon 2020 research and innovation programme (no. 766566) "ASPIN," the Deutsche Forschungsgemeinschaft (project ID 258499086) "SFB 1143," the Deutsche Forschungsgemeinschaft (project IDs FE 633/30‐1, RE 1164/6‐1, and LU 2261/2‐1) "SPP Skyrmionics", and the DFG through the Würzburg‐Dresden Cluster ...

The topological Hall effect (THE), induced by the Berry curvature that originates from non-zero scalar spin chirality, is an important feature for mesoscopic topological structures, such as skyrmions. However, the THE might also arise from other microscopic non-coplanar spin structures in the lattice. Thus, the origin of the THE inevitably needs to be determined to fully understand skyrmions and find new host materials. Here, we examine the Hall effect in both, bulk- and micron-sized lamellar samples of MnBi. The sample size affects the temperature and field range in which the THE is detectable. Although a bulk sample exhibits the THE only upon exposure to weak fields in the easy-cone state, in micron-sized lamella the THE exists across a wide temperature range and occurs at fields near saturation. Our results show that both the non-coplanar spin structure in the lattice and topologically non-trivial skyrmion bubbles are responsible for the THE, and that the dominant mechanism depends on the sample size. Hence, the magnetic phase diagram for MnBi is size-dependent. Our study provides an example in which the THE is simultaneously induced by two mechanisms, and builds a bridge between mesoscopic and microscopic magnetic structures. © 2022. ; This work was financially supported by an Advanced Grant from the European Research Council (No. 742068 ) "TOPMAT," the European Union's Horizon 2020 research and innovation programme (No. 824123 ) "SKYTOP," the European Union's Horizon 2020 research and innovation programme (No. 766566 ) "ASPIN," the Deutsche Forschungsgemeinschaft (Project-ID 258499086) "SFB 1143," the Deutsche Forschungsgemeinschaft (Project-IDs FE 633/30-1, RE 1164/6-1 and LU 2261/2-1) "SPP Skyrmionics," the DFG through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project-ID 39085490). I.S. is grateful to Deutsche Forschungsgemeinschaft for supporting this work through project SO 1623/2-1. D.W. has received funding from the European Research ...

The increasing demand for computer data storage with a higher recording density can be addressed by using smaller magnetic objects, such as bubble domains. Small bubbles predominantly require a strong saturation magnetization combined with a large magnetocrystalline anisotropy to resist self-demagnetization. These conditions are well satisfied for highly anisotropic materials. Here, we study the domain structure of thin Nd2Fe14B lamellae. Magnetic bubbles with a minimum diameter of 74 nm were observed at room temperature, approaching even the range of magnetic skyrmions. The stripe domain width and the bubble size are both thickness dependent. Furthermore, a kind of bubble was observed below the spin-reorientation transition temperature that combine bubbles with opposite helicity. In this paper, we reveal Nd2Fe14B to be a good candidate for a high-density magnetic bubble-based memory. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society. ; This paper was financially supported by an Advanced Grant from the European Research Council (No. 742068) "TOPMAT," the European Union's Horizon 2020 research and innovation program (No. 824123) "SKYTOP," the European Union's Horizon 2020 research and innovation program (No. 766566) "ASPIN," the Deutsche Forschungsgemeinschaft (Project-ID 258499086) "SFB 1143," the Deutsche Forschungsgemeinschaft (DFG; Project-IDs FE 633/30-1, RE 1164/6-1, and LU 2261/2-1) "SPP Skyrmionics," the DFG through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project-ID 39085490). I.S. would like to express his gratitude to the DFG for supporting this paper through Project SO ...

Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr$_{0.53}$Ti$_{0.47}$O$_3$–PbFe$_{0.5}$Ta$_{0.5}$O$_3$ (PZT–PFT) have been assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr$_{0.53}$Ti$_{0.47}$O$_3$–PbFe$_{0.5}$Nb$_{0.5}$O$_3$ (PZT–PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe$_{0.5}$Ta$_{0.5}$O$_3$. As for PZT–PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic–tetragonal–monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel–Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT–PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales. ; RUS facilities in Cambridge were established with funding from the Natural Environment Research Council (Grants NE/B505738/1, NE/F017081/1). The present work was supported by Grant No. EP/ I036079/1 from the Engineering and Physical Sciences Research Council. We thank Dr. Sam Crossley for his assistance with dielectric analysis and the use of his software to run those measurements. JAS gratefully acknowledges the hospitality of the Max Planck Institute for Chemical Physics of Solids. The Nanopaleomagnetism lab has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007– 2013)/ERC Grant Agreement 320750. SED and HS acknowledge support from the Winton Programme for the physics of sustainability. HS also acknowledges support from the Funai Foundation for Information Technology and the British Council Japan Association. Part of the work was carried out at the University of Puerto Rico, supported by the DOEEBSCoR project DEG02-ER46526.

This work was supported in part by a grant from the Engineering and Physical Sciences Research Council (EP/I036079/1) which is gratefully acknowledged. The RUS facilities in Cambridge were established with support from the Natural Environment Research Council (grant numbers NE/B505738/1 and NE/F17081/1). HS and SED acknowledge support from the Winton Programme for the Physics of Sustainability, and HS acknowledges support from the Funai Foundation for Information Technology. Funding to support this research (to RJH) was also obtained from the European Union's Seventh Framework Programme (FP/2007-2013)/ ERC grant agreement 3207500. ; Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe0.5Nb0.5)O3 (PFN). Elastic softening begins at ~550 K, which is close to the Burns temperature marking the development of dynamical polar nanoregions. A small increase in acoustic loss at ~425 K coincides with the value of T* reported for polar nanoregions starting to acquire a static or quasi-static component. Softening of the shear modulus by ~30–35% through ~395–320 K, together with a peak in acoustic loss, is due to classical strain/order parameter coupling through the cubic → tetragonal → monoclinic transition sequence of ferroelectric/ferroelastic transitions. A plateau of high acoustic loss below ~320 K is due to the mobility under stress of a ferroelastic microstructure but, instead of the typical effects of freezing of twin wall motion at some low temperature, there is a steady decrease in loss and increase in elastic stiffness below ~85 K. This is attributed to freezing of a succession of strain-coupled defects with a range of relaxation times and is consistent with a report in the literature that PFN develops a tweed microstructure over a wide temperature interval. No overt anomaly was observed near the expected Néel point, ~145 K, consistent with weak/absent spin/lattice coupling but heat capacity measurements showed that the antiferromagnetic transition is actually smeared out or suppressed. Instead, the sample is weakly ferromagnetic up to ~560 K, though it has not been possible to exclude definitively the possibility that this could be due to some magnetic impurity. Overall, evidence from the RUS data is of a permeating influence of static and dynamic strain relaxation effects which are attributed to local strain heterogeneity on a mesoscopic length scale. These, in turn, must have a role in determining the magnetic properties and multiferroic character of PFN. ; Publisher PDF ; Peer reviewed