(Pr, Ho)-Fe-B magnets for low-temperature applications

We have investigated the effect of HoH2 hydride addition on the hysteresis loop parameters of sintered Pr-Fe-Ti-Al-Cu-B magnets. The magnets were prepared by traditional powder metallurgy technology, and 3 wt.% HoH2 was added to the powder at the fine-milling stage. The magnets exhibited a monotonic increase in all hysteretic parameters with decreasing temperature down to 4.2 K. The coercive force and maximum energy product at 295 K (4.2 K) were 1344 (5402) kA/m and 221 (336) kJ/m3, respectively. The structure of the magnets was studied in detail by scanning electron microscopy and energy dispersive X-ray spectroscopy, which demonstrated the formation of the so-called “core-shell” structure, which is assumed to favor the marked improvement in the hysteretic properties of the samples analyzed. The surface domain structure was measured in the directions perpendicular and parallel to the magnet texture using magnetic force microscopy. The data obtained indicated fine labyrinth-like and strip domain patterns in the directions perpendicular and parallel to the magnet texture, with an average domain width of 1.2–1.8 µm.We have investigated the effect of HoH2 hydride addition on the hysteresis loop parameters of sintered Pr-Fe-Ti-Al-Cu-B magnets. The magnets were prepared by traditional powder metallurgy technology, and 3 wt.% HoH2 was added to the powder at the fine-milling stage. The magnets exhibited a monotonic increase in all hysteretic parameters with decreasing temperature down to 4.2 K. The coercive force and maximum energy product at 295 K (4.2 K) were 1344 (5402) kA/m and 221 (336) kJ/m3, respectively. The structure of the magnets was studied in detail by scanning electron microscopy and energy dispersive X-ray spectroscopy, which demonstrated the formation of the so-called “core-shell” structure, which is assumed to favor the marked improvement in the hysteretic properties of the samples analyzed. The surface domain structure was measured in the directions perpendicular and parallel to the magnet texture using magnetic force microscopy. The data obtained indicated fine labyrinth-like and strip domain patterns ...


I. INTRODUCTION
In recent years, permanent magnets based on (Pr, Nd) 2 Fe 14 B compounds have become potential candidates for applications at low temperatures (undulators, magnetic bearings, wigglers, etc.). 1-3 Nd 2 Fe 14 B-based magnets with a high remanence cannot be employed for the construction of undulators because of their weak coercivity at room temperature (RT). During cooling to 135 K, magnets exhibit a high resistance to demagnetization and a marked increase in their remanence and coercivity due to spin reorientation. At 135 K, the easy-axis magnetic anisotropy changes to easy-axis cone anisotropy and the spin-reorientation transition (SRT) takes place. The tilting angle increases with decreasing temperature and, as a result, the remanence drops below values corresponding to RT. Thus, the natural low-temperature-induced improvement in hysteretic properties cannot be used adequately, and, therefore, most Nd 2 Fe 14 B-based magnets must operate in the temperature range 200-450 K.
Previously, Pr 2 Fe 14 B-based magnets were not applied in practice despite the fact that most of their magnetic parameters at RT are better than or comparable with those of Nd 2 Fe 14 B. For example, the anisotropy field of Pr 2 Fe 14 B (8.7 T) is higher than that of Nd 2 Fe 14 B (6.7 T), 4 indicating the possible higher intrinsic coercivity ( j Hc) of the first-mentioned alloy, while the Curie temperature and the ARTICLE scitation.org/journal/adv saturation magnetization (569 K and 1.56 T, respectively) 5,6 are only slightly lower than those of Nd 2 Fe 14 B (586 K and 1.60 T). Moreover, the Pr 2 Fe 14 B-based magnets exhibit no SRT down to 4.2 K. 7,8 All these factors make Pr-Fe-B alloys attractive for wide temperature range applications. On the other hand, these alloys exhibit the worst temperature and time stability. 9 The Pr-Fe-B alloys are more viscous than the Nd-Fe-B alloys. This fact determines the higher quenching rates and assumes the application of hydrogen decrepitation. However, it was shown in Reference 10 that the presence of hydrogen changes the anisotropic nature of Pr l5 Fe 79 B 6 hydrogenated powders from uniaxial to planar in the range 4250-4500 ppm H 2 . Thus, if the hydrogenated Pr 2 Fe 14 B powder is compacted in a magnetic field, no texturing along the easy-magnetization axis takes place, and the powder behaves similarly to an isotropic system at RT. In this case, the hysteretic parameters of the magnets decrease abruptly. The authors of Reference 10 suggest either dehydrogenating the powder before compacting or alloying the composition with dysprosium (Dy).
The considerable interest shown by researchers in reducing the usage of Dy in Nd-Fe-B-based magnets has stimulated interest in the application of Ho-containing compounds. In Reference 11, the coercivity of Dy-free magnets was investigated through intergranular addition of eutectic Ho 63.4 Fe 36.6 powders. The coercivity enhancement was explained by microstructural observations and elemental distribution analysis. However, Ho 2 Fe 14 B exhibits spin reorientation at 58 K, 12 which is shifted to 90 K as a result of hydrogen absorption with increasing H 2 content.
The aim of the present study was to investigate the effect of HoH 2 additions, which were subjected to fine milling together with the hydrogenated PrFeBHx alloy, on the texture formation and properties of permanent magnets over the wide temperature range 4.2-295 K.

II. EXPERIMENTAL
The alloy containing (wt.%) Pr-33, Ti-0.9, Al-0.3, Cu-0.15, B-1.3, Fe-balance was prepared by induction melting in an argon atmosphere and cast into a water-cooled copper mold. The alloy was subjected to hydrogen decrepitation using a bell-type furnace, whose chamber was preliminarily washed with nitrogen gas and flashed with hydrogen. The hydrogenation process was then performed during heating to 473 K for 1.5-2 h where it was held for 1 h. The furnace chamber was then washed with nitrogen gas, and the alloy was subjected to furnace cooling to RT. Powders of the Pr 2 Fe 14 B-based alloy and its mixture with 3 wt.% HoH 2 were prepared by milling in an isopropyl alcohol medium using a vibratory ball mill. Magnet blanks were prepared by compacting at a force of 300 kg/cm 2 using a hydraulic press with a loading rate no faster than 10 mm/s. A texturing magnetic field of 1.6 T was applied perpendicular to the pressing force direction. The blanks were dried and sintered at 1375 K for 1 h (single-cycle technological operation) and subjected to low-temperature treatment at 775 K for 1 h using vacuum resistance furnaces. The resulting sample is denoted as (Pr,Ho)-Fe-B or (Pr,Ho) 2 Fe 14 B magnet in the following text.
The microstructure and chemical composition of the phases were studied by scanning electron microscopy (SEM) using a QUANTA 450 FEG equipped with an energy dispersive X-ray (EDX) APOLLO X analyzer. The phase composition of the magnets was investigated by X-ray diffraction (XRD) analysis using an Ultima IV (Rugaku, Japan) diffractometer equipped with a D/teX detector and Cu Kα radiation. XRD patterns were taken in a 2θ angular range of 3 ○ -100 ○ at a scanning step of 0.001 ○ . The qualitative and quantitative analyses of diffractograms were performed using the simplified Rietveld method, PHAN, and PHAN% software.
Magnets were first magnetized in a pulse magnetic field up to 12 MA/m. Additionally, a MH-50 hysteresisgraph with a close magnetic circuit and a vibrating-sample magnetometer (VSM) were used for measurements of the magnetic characteristics in magnetic fields of up to 2 MA/m (2.5 T) and 7.2 MA/m (9 T), respectively. Hysteresisgraph measures bulk magnetic properties of prepared magnets (30 mm in diameter and 10 mm high) at RT, while only small piece of magnet (mass about 100 mg) is analyzed using the VSM in the temperature range 4.2-295 K. The domain structure of the samples was observed in directions perpendicular and parallel to the magnet texture by magnetic force microscopy (MFM) using Solver Pro EC (NT MDT) equipment.

III. RESULTS AND DISCUSSION
A. Structural studies  (Table I).
The main magnetic phase (Pr,Ho) 2 Fe 14 B-based (2-14-1) is depleted of rare-earth metals (REMs) because the (Pr+Ho+Nd) content corresponds to only 32 wt.% and Nd is present as an impurity in the starting Pr. It is a tetragonal Nd 2 Fe 14 B-type phase with space group P4 2 /mnm. Phase 2 appears as (Pr,Ho) rich , with a relatively high amount of Cu that was found mainly at the ternary junctions of 2-14-1 grains. Finally, phases 3 and 4 are Ti-based and (Pr, Ho) 2 O 3 oxide phases, respectively.
Since the Ho-containing powder is introduced from the grain boundaries, the distributions of Ho in the 2-14-1 phase grains and the grain boundaries of the final magnet are critical for coercivity enhancement. The Ho distribution within the main magnetic phase grains was studied in detail and is shown in Figures 1(c-g). We found that Ho was distributed nonuniformly, that is, the cores of grains were Ho-depleted, whereas their edges were Ho-enriched. Therefore, we could observe typical "core-shell" structure formation (Fig. 1a) when HoH 2 hydride additions were used in the powder mixture.
Powders at different stages of the preparation process were studied by XRD to determine precisely the lattice parameters of the main magnetic phase (Table II). The base alloy (wt.%) Pr-33, Ti-0.9, Al-0.3, Cu-0.15, B-1.3, Fe-balance, which was first subjected to hydrogen decrepitation, and its mixture with 3 wt.% HoH 2 were subjected to dehydrogenation during heating at 775 K for 1 h in a vacuum of 1.33 × 10 −2 Pa. The results show that (i) the lattice parameters of the alloy prepared with holmium hydride are slightly higher  (Table I) Table III), despite the fact that Ho atoms are antiferromagnetically ordered with respect to Fe atoms. 13 Measurements performed at low temperatures indicate the progressive increase in the values of the magnetic induction and the maximum energy product (Table IV). Due to slow increase of Br with decreasing temperature (inset of Figure 2(a)), we can state that no SRT takes place at temperatures down to 4.2 K.

B. Magnetic measurements
It might seem that an improvement in the magnetic properties of the Ho-rich magnet could be related to the presence of Ho in the Pr 2 Fe 14 B crystal lattice and the formation of the "coreshell" structure of the phase grains. It is known that the alloying of Nd 2 Fe 14 B-based magnets with Dy and Tb leads to an increase in the coercive force and is related to the higher anisotropic fields of the Dy 2 Fe 14 B and Tb 2 Fe 14 B compounds. 7 An analogous explanation can be given for alloying with Ho since the anisotropic field of Ho 2 Fe 14 B is slightly higher than that of Nd 2 Fe 14 B, 11 but almost identical to or slightly lower than that of Pr 2 Fe 14 B, 7 and cannot be responsible for such a marked increase in the coercive force of (Pr,Ho)-Fe-B magnets. Our explanation is based on the conclusions reached in the literature, 14,15 which are that alloying of the Nd 2 Fe 14 B structure with heavy REMs (Dy and Tb) results in its stabilization, in ARTICLE scitation.org/journal/adv contrast to the effect of light REMs (La, Pr). Similarly, we can assume that Ho, as a heavy REM, also stabilizes the Nd 2 Fe 14 B-type structure and favors the higher resistance to the formation of crystal lattice stacking defects. The importance of the role of the structural state of the main magnetic field was discussed by our group in Reference 16.