Low-temperature measurements of the magnetocrystalline anisotropy energy K in (Fe 1−x Co x ) 2 B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence K(x) with a maximum near x = 0.3 and a minimum near x = 0.8 is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, K depends strongly on the tetragonality and doubles under a modest 3% increase of the c/a ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.Magnetocrystalline anisotropy (MCA) of a magnetic material is one of its key properties for practical applications, large easy-axis anisotropy being favorable for permanent magnets.1 Intelligent search for new materials requires understanding of the underlying mechanisms of MCA. This can be particularly fruitful for substitutional alloys whose properties can be tuned by varying the concentrations of their components. The analysis is often relatively simple in insulators, where MCA is dominated by single-ion terms which can be deduced from crystalfield splittings and spin-orbital (SO) selection rules. In contrast, in typical metallic alloys the band width sets the largest energy scale, and MCA depends on the details of the electronic structure.The (Fe 1−x Co x ) 2 B solid solution 2-6 (space group I4/mcm 7 ) is a remarkable case in point. Fe 2 B has a fairly strong easy-plane MCA, and Co 2 B, at low temperatures, a small easy-axis MCA. However, the alloy has a substantial easy-axis MCA around x = 0.3, 2 making it a potentially useful rare-earth-free 8 permanent magnet. At x ≈ 0.5 the MCA again turns easy-plane, peaks at x = 0.8, and then turns easy-axis close to x = 1. These three spin reorientation transitions must be related to the continuous evolution of the electronic structure with concentration. The goal of this Letter is to elucidate the origin of this rare phenomenon.First, we report the results of experimental measurements at low temperatures.Single crystals of (Fe 1−x Co x ) 2 B were grown from solution growth out of an excess of (Fe,Co) which was decanted in a centrifuge. 9The single crystals were grown as tetragonal rods which were cut using a wire saw to give them the shape of a rectangular prism. The demagnetization factor was calcua) This article has been accepted by Applied Physics Letters.After it is published, it will be found at http://scitation.aip.org/content/aip/journal/apl . lated using Ref. 10. Field-dependent magnetization measurements were performed in a Quantum Design MPMS at 2 K in fields up to 5.5 T. The MCA energy K was determined as the area between the two magnetization curves, with the field parallel and perpendicular to the c axis, taken at the same temperature.6 The results shown in Fig. 1 Density-functional calculations using several different m...

Single crystals of Fe 5 B 2 P were grown by self-flux growth technique. Structural and magnetic properties are studied. The Curie temperature of Fe 5 B 2 P is determined to be 655 ± 2 K. The saturation magnetization is determined to be 1.72 µ B /Fe at 2 K. The temperature variation of the anisotropy constant K 1 is determined for the first time, reaching ∼ 0.50 MJ/m 3 at 2 K, and it is comparable to that of hard ferrites. The saturation magnetization is found to be larger than the hard ferrites. The first principle calculations of saturation magnetization and anisotropy constant are found to be consistent with the experimental results.

We have grown a series of nickel substituted single crystals of the layered ferromagnet (FM) Fe3GeTe2. The large single crystalline samples of (Fe1−xNix)3GeTe2 with x = 0 − 0.84 were characterized with single crystal X-ray diffraction, magnetic susceptibility, electrical resistance and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only minor structural variation. In addition, FM order is suppressed from TC = 212 K for x = 0 down to TC = 50 K for x = 0.3, which is accompanied with a strong suppression of saturated and effective moment, and Curie-Weiss temperature. Beyond x = 0.3, the FM order is continuously smeared into a FM cluster glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe3GeTe2 and subsequent dilution effects from the Ni substitution. arXiv:1809.03429v2 [cond-mat.str-el]