Theory of defect-induced crystal field perturbations in rare earth magnets

We present a theory describing the single-ion anisotropy of rare earth (RE) magnets in the presence of point defects. Taking the RE-lean 1:12 magnet class as a prototype, we use first-principles calculations to show how the introduction of Ti substitutions into SmFe$_{12}$ perturbs the crystal field, generating new coefficients due to the lower symmetry of the RE environment. We then demonstrate that these perturbations can be described extremely efficiently using a screened point charge model. We provide analytical expressions for the anisotropy energy which can be straightforwardly implemented in atomistic spin dynamics simulations, meaning that such simulations can be carried out for an arbitrary arrangement of point defects. The significant crystal field perturbations calculated here demonstrate that a sample which is single-phase from a structural point of view can nonetheless have a dramatically varying anisotropy profile at the atomistic level if there is compositional disorder, which may influence localized magnetic objects like domain walls or skyrmions.Comment: 14 pages, 3 figure

Revisiting N\'eel 60 years on: the magnetic anisotropy of L10\mathrm{L}1_0 FeNi (tetrataenite)

The magnetocrystalline anisotropy energy of atomically ordered $\mathrm{L}1_0$ FeNi (the meteoritic mineral tetrataenite) is studied within a first-principles electronic structure framework. Two compositions are examined: equiatomic Fe$_{0.5}$Ni$_{0.5}$ and an Fe-rich composition, Fe$_{0.56}$Ni$_{0.44}$. It is confirmed that, for the single crystals modelled in this work, the leading-order anisotropy coefficient $K_1$ dominates the higher-order coefficients $K_2$ and $K_3$. To enable comparison with experiment, the effects of both imperfect atomic long-range order and finite temperature are included. While our computational results initially appear to undershoot the measured experimental values for this system, careful scrutiny of the original analysis due to N\'{e}el et al. [J. Appl. Phys. 35, 873 (1964)] suggests that our computed value of $K_1$ is, in fact, consistent with experimental values, and that the noted discrepancy has its origins in the nanoscale polycrystalline, multivariant nature of experimental samples, that yields much larger values of $K_2$ and $K_3$ than expected a priori. These results provide fresh insight into the existing discrepancies in the literature regarding the value of tetrataenite's uniaxial magnetocrystalline anisotropy in both natural and synthetic samples.Comment: 9 pages, 3 figures, 2 table

Large-scale synthesis of high moment FeCo nanoparticles using modified polyol synthesis

Binary alloys of Fe and Co have among the highest magnetizations of any transition metal alloy systems, but their affinity to form oxides act to reduce the magnetization of nanoparticles as their size is reduced below ∼30 nm. Here, we demonstrate the synthesis of single phase, size-controlled FeCo nanoparticles having magnetization greater than 200 emu/g via a non-aqueous method in which ethylene glycol served as solvent and reducing agent as well as surfactant. Experiments indicated pure-phase FeCo nanoparticles, having saturationmagnetization up to 221 emu/g for sizes of 20–30 nm, in single batch processes resulting in \u3e 2 g/batch. Post-synthesis oxidation of nanoparticles was investigated until very stable nanoparticles were realized with constant magnetization over time

Stoichiometry-anisotropy connections in epitaxial L1(0) FePt(001) films

The order parameters and anisotropy constants of a series of epitaxial L1(0) FePt films with compositions in the range of 45-55 at. % Fe and nominal thicknesses of 50 nm have been characterized. The films were made by cosputtering the elements onto single crystal MgO(001) substrates. The substrates were coated with 1 nm Pt/1 nm Fe bilayer seeds prior to alloy deposition. Both the bilayer seed and the alloy film were deposited at 620 degreesC. Lattice and order parameters were obtained by x-ray diffraction. Film compositions and thicknesses were determined by Rutherford backscattering spectrometry, and room-temperature magnetocrystalline anisotropies were determined with a torque magnetometer. It was found that the order parameter had a maximum for the film composition closest to the equiatomic composition, whereas the magnetocrystalline anisotropy increased as the Fe content increased from below to slightly above the equiatomic composition. These results imply that nonstoichiometric FePt compositions, with a slight excess of Fe, may in fact be preferred for applications that require high anisotropy

Giant multicaloric response of bulk Fe49Rh51

We report on the multicaloric response of the Fe49Rh51 alloy under the combined application of hydrostatic pressure and magnetic field. Experimental data are complemented by a mean field model that takes into account the interplay between structural and magnetic degrees of freedom. A large multicaloric strength has been found for this alloy, and it is shown that a suitable combination of pressure and magnetic field enables the sign of the entropy change to be reversed and thus the multicaloric effect can be tuned from conventional to inverse. It is also shown that an extended temperature window for the multicaloric effect can be achieved by taking advantage of the coupling between structure and magnetism which enables a cross response of the alloy under the application of different external fields. Mean field calculations remarkably reproduce experimental results

Survival and Presenting Complaint of Canine and Feline Paediatric Emergencies Presenting to UK Emergency Clinics

Objective: To describe the presenting problems for paediatric dogs and cats presented to emergency clinics, and to determine what factors influence mortality.Background: There is a lack of published literature available on canine and feline paediatric emergencies, their survival rates and factors that contribute to mortality. Experience and opinion largely replace evidence-based medicine when treating this subset of the emergency population.Evidentiary value: Evidentiary value for primary and emergency care practitioners is strong as the data is based on a large primary emergency care database. The paediatric population analysed consisted of 3142 paediatric canine and feline patients.Methods: A retrospective review of canine and feline emergency admissions to emergency out-of-hours clinics (May 2012-January 2014) was conducted. Data recorded included age, nature of complaint, euthanasia/death during treatment, and survival to discharge. Survival to discharge included all patients who presented to the clinic for examination, received treatment and were subsequently discharged; this includes those patients who were discharged  home or to their primary vets care. Patients were divided into four groups: canine patients ≤ 3 months of age and 3-6 months and feline patients ≤ 3 months of age and 3-6 months.Results: Paediatric patients represented around 3% of the emergency cases identified in the database. Survival to discharge in patients ≤3 months of age was significantly reduced in comparison to patients between 3-6 months of age, in both species. Feline paediatric patients had reduced survival to discharge compared to their canine counterparts.Conclusion: Paediatric patients have a low survival rate, indiscriminate of species and reason for presentation, with lower survival in younger animals; with a significant difference in survival between canine and feline patients.Application: The results of this study are important to small animal veterinarians in both emergency and non-emergency practice. <img src="https://www.veterinaryevidence.org/rcvskmod/icons/pr-icon.jpg" alt="Peer Reviewed" /