Optimization of the magnetic properties of nanostructured Y-CoFe alloys for permanent magnets

The structural and magnetic properties of ball-milled Fe-doped Y Co5−xFex(0 ≤ x ≤ 0.5) were investigated. The magnetization increases with Fe-doping up to the solid solubility limit, x = 0.3 without destroying the crystal structure or degrading the coercivity. A special magnet array is designed using ring magnets for pressing the powders under magnetic field in order to achieve magnetic alignment. A dramatic increase in magnetization is observed for magnetically aligned Y Co4.8Fe0.2 pressed ingots.


I. INTRODUCTION
2][3] The main concern in the material choice is the energy product, which quantifies the ability of the magnet to store magnetostatic energy and create flux in the surrounding space. 4Improving extrinsic and intrinsic properties by nanostructuring and atomic structuring (crystal structure, composition) is the way to maximize the energy product. 5The maximum possible value (BH) MAX = 1 /4 µ 0 M 2 s is reached with a demagnetization factor N = 1 /2.It can be approximated with a compact cylindrical magnet with height equal to radius (r ≈ h) that exhibits a rectangular M(H) loop.For this H c must be greater than M s /2 where M s is related to the intrinsic atomic and crystal structure and H c originates from the magnetic anisotropy K. Typically its value will not exceed 25% of the anisotropy field H a = 2K/µ 0 M s . 4,6Furthermore, H c depends on grain size, microscopic defects and the thermal history of a sample.Usually it is inversely related to the grain size, and large coerciviy is obtained when the crystallites are small and single-domain. 7This can be understood by considering that smaller grains have fewer potential nucleation centres.In the case of sintered Nd-Fe-B magnets, coercivity is proportional to the inverse square root of the grain size. 8Another way to maximize the energy product is to induce a texture to enhance the remanence M r by aligning the particles' easy axes or by pressing the aligned powder to high density in order to approach M r to M s . 7,9These will increase the ratio M r /M s , which is ≈ 0.5 for isotropically oriented, magnetically uniaxial and non-interacting single domain particles. 7he challenge here is produce an optimized permanent magnets taking care of all the requirements discussed above.Vacuum annealed and magnetically aligned YCo 4.8 Fe 0.2 dry ball-milled powder is a good candidate starting material with a 140 kJ/m 3 maximum energy product for the powder. 10However the reproducibility of these ball-milled powders with controlled structure and grain size distribution is important for large-scale production.Various effects such as the milling method, container, speed, time, powder to balls weight ratio for high-energy ball miling, extent of filling the milling vial, milling atmosphere and temperature all influence the milling process.The optimum grain size for maximum energy product can be obtained by thermal treatment to eliminate the defects associated with plastic deformation induced during milling. 11,12n this work, we will discuss the effect of rapid thermal annealing and vacuum annealing on grain size, crystal structure and magnetic properties of Fe-doped YCo 5 ball-milled powders and the reproducibility of ball-milling of YCo 4.8 Fe 0.2 .Also, pressing the powders with and without 056016-2 Tozman, Venkatesan, and Coey AIP Advances 6, 056016 (2016) magnetic field will be investing in order to obtain a hard magnet which will be in the gap between oriented ferrite (34 kJ/m 3 ) and Nd-Fe-B (350 kJ/m 3 ).

II. EXPERIMENT
YCo 5−x Fe x (0 ≤ x ≤ 0.5) alloys were prepared by arc melting in high-purity argon.The ingots were melted four times to ensure homogeneity.Milling was carried out under an Ar atmosphere for 4 h in a Spex 8000 mixer/mill with stainless steel balls and a ball to powder charge ratio of 15:1, which gives the highest maximum energy product of YCo 5 in the litterature. 13The as-milled YCo 5 powder was wrapped in Ta foil and then subjected to rapid thermal annealing under flowing Ar with an infrared lamp (Ulvac-Riko MILA-5000) at temperatures (T a ) between 800 • C and 1050 • C for 1-5 min and then cooled naturally.The iron-doped YCo 5−x Fe x 0 ≤ x ≤ 0.5 powder samples were rapidly annealed at 800 • C for only 1-3 min in order to find best magnetic properties.Alternatively, these samples were annealed in a tube furnace preheated at 800 and 850 • C for 2-3 minutes under a vacuum of 10 −6 -10 −7 Torr, followed by quenching.Several ball-milled YCo 4.8 Fe 0.2 powders were prepared in order to investigate the reproducibility.The vacuum annealed YCo 4.8 Fe 0.2 powders were pressed in a 3 mm die with tungsten carbide pistons with and without applied field.Structural characterization was carried out by X-ray diffraction (XRD) with a PANalytical X'Pert Pro diffractrometer using Cu-K α radiation.Rietveld analysis of the diffraction patterns was performed using FullProf.The microstructure and composition were analysed with a scanning electron microscope (SEM Carl Zeiss Evo) for magnetically aligned ball-milled powder after vacuum annealing at 850 • C for 2 min.Powder samples for room temperature magnetic measurements were prepared by mixing the annealed powder with Lecoset 7007 cold-curing resin inside a 4 mm × 4 mm cylindrical Perspex bucket and for pressed ingots, sample pieces were firmly placed between two gel caps.Some powders were oriented in a 5 T field and all the magnetic measurements were carried out using a 5 T Quantum Design superconducting quantum interference device magnetometer.

A. Ball-milled YCo 5
Achieving the optimum grain size is important in order to obtain highest coercivity.Grain size can be controlled by ball-milling time, powder to ball ratio, annealing temperature and annealing time.5][16] Fig. 1 shows the influence of rapid thermal annealing temperature and time on (BH) max , σ max and µ 0 H c at room temperature.The highest value of µ 0 H c , 0.88 T is obtained for a sample annealed at 900 • C for 1 min.However (BH) max = 52 kJ/m 3 is obtained after annealing at 800 • C for 1 min.Further increase of the annealing temperature leads to a drop in (BH) max .
Increasing the annealing time decreases the maximum energy product overall and the maximum value of 66 kJ/m 3 is obtained for a 2 min annealed samples.Increasing the annealing time creates a step in demagnetization curve due to a small amount of a secondary soft ferromagnetic Y 2 Co 17 phase which indicates ineffective exchange coupling and is unfavorable for energy product. 17,18

B. Ball-milled Fe-doped YCo 5
The XRD patterns of rapidly thermal annealed (800 • C for 3 min) YCo 5−x Fe x for 0 ≤ x ≤ 0.5 show that the main phase has the hexagonal CaCu 5 structure (P6/mmm) with a secondary rhombohedral Th 2 Zn 17 phase (R3m).The secondary phase (300), ( 303), ( 006) and ( 223) peaks are separated from the main phase for x = 0 but they overlap with main phase with increasing Fe-doping.After reaching the solid solubility limit (x = 0.3), the amount of the secondary phase increases as shown in Fig. 2(a).
Grain growth controls the coercivity by annealing and grain size effect.Coercivity of rapid thermally annealed (800 • C -1050 • C for 1 min) YCo 5 powders is proportional to the inverse of the crystallite size <L>, computed from Scherrer broadening (Fig. 2(b)), as it is for Nd-Fe-B magnets. 8n the other hand, the crystallite size and coercivity have the same dependence on Fe-doping x.After the solid solubility limit, the crystallite size and coercivity drop dramatically due to the increase of the amount of secondary phase.
The hysteresis curves show same trend up to x = 0.3 after rapid thermal annealing at 800 • C for 1-3 min.However beyond x = 0.3 the saturation magnetization increases, and coercivity decreases due to exchange coupling between soft (<L> ≈ 25 nm) and hard (<L> ≈ 15 nm) phases, which makes the demagnetizing curve more convex (Fig. 3(a)).
Rapid thermal annealing of Fe-doped YCo 5 at 800 • C for 1 to 3 min decreases the coercivity due to a decrease of anisotropy constant K 1 and it increases saturation magnetization due to presence of Fe. 19 Maximum energy product was deduced assuming the crystallographic density ρ = 7560 kg m −3 and N = 1/3 for for rapid thermally annealed spherical ball-milled powders of Fe-doped YCo 5 (800 • C for 1-3 min).In Fe-doped YCo 5 samples up to the solid solubility limit x = 0.3, the maximum energy product is enhanced dramatically by magnetically aligning the powders' [001] easy axes, as is illustrated in Fig. 3(b).The annealing time didn't influence the energy product for x = 0.1 and x = 0.3, and the maximum energy product is obtained for 2 min rapid thermal annealing.SEM images of spherical powders magnetically aligned under 1.2 T show that they are well aligned in the direction of applied field (Fig. 4(a)) like SmCo 5 flakes. 20The highest maximum energy product is obtained for YCo 4.8 Fe 0.2 after vacuum annealed at 850 • C for 2min. 10The coercivity of YCo 5 decreases from 0.99 T to 0.77 T with decreasing the temperature from 850 • C to 800 • C for 2 min.We successfully reproduced vacuum annealed YCo 4.8 Fe 0.2 (850 • C for 2 min) in 15 different ball-milling process.The average coercivity and maximum magnetization are obtained 0.81 T and 100 Am 2 /kg respectively.

C. Pressed magnets
The powder of vacuum annealed YCo 4.8 Fe 0.2 powder was pressed into pellets using tungsten carbide pistons with 84% and 66% of crystallographic density, without and with the 0.4 T applied field.The field is obtained with using seven NdFeB N52 magnet rings and one NdFeB Magnet N40 magnet cap with axial magnetization direction through the 3 mm and 10 mm bores, respectively, as is shown in Fig. 4(b).This array was simulated by the MANIFEST program and the maximum field, 0.4 T is found for 15 mm ring height.The maximum energy product of powder ingot, pressed under magnetic field, is 61 kJ/m 3 (for N = 1/2; ρ = 5000 kg/m 3 ; 66% of density and µ 0 H = 0.4 T) and it increases from 52 kJ/m 3 (for N = 1/2; ρ = 6337 kg/m 3 ; 84% of density and µ 0 H = 0).The increase in maximum energy product can be attributed to the enhancement in magnetization (≈ 30%) under magnetic field alignment.Density of the field-aligned magnet was limited by the fragility of the tungsten pistons, but it should be quite possible to achieve higher values for denser magnets under magnetic field.

FIG. 1 .
FIG. 1.The effect of (a) temperature and (b) time dependence of coercivity, maximum magnetization and maximum energy product of rapid thermal annealed, ball-milled YCo 5 .Dash line indicates the trend.

FIG. 4 .
FIG. 4. (a) SEM image of vacuum annealed (850 • C for 2 min) and magnetically aligned YCo 4.8 Fe 0.2 where arrow indicates the field direction.(b) Magnet array around and top of die in order to obtain 0.4 T field in downward direction.(c) Hysteresis of vacuum annealed (850 • C, 2 min) pressed magnet YCo 4.8 Fe 0.2 which is pressed with and without field at room temperature.