Improved Magnetic Performance of Near-Stoichiometric Pr-Fe-B Alloys with Hf Addition

Abstract

This manuscript reports the influence of Hf substitution for Fe on the magnetic properties and microstructure of near-stoichiometric Pr-Fe-B alloys. Melt-spun ribbons with nominal compositions of Pr_26.7Fe_72.3B₁, Pr_26.7Fe_71.8Hf_0.5B₁, and Pr_26.7Fe_71.3Hf₁B₁ (wt%) are synthesized with optimized wheel speed. Transmission electron microscopy analysis reveals that Hf addition effectively refines the grain structure in terms of grain size. Magnetic characterization at 300 K demonstrates that the partial Hf addition significantly enhances the hard magnetic performance. The pristine alloy (Pr_26.7Fe_72.3B₁) exhibits an intrinsic coercivity (H_cj) of 11.95 kOe, a remanence (B_r) of 8.23 kG, and a maximum energy product ((BH)_max) of 12.6 MGOe. With 0.5% Hf addition, the properties improve to H_cj of 11.47 kOe, B_r of 8.5 kG, and (BH)_max of 15.33 MGOe. A further increase to 1.0% Hf leads to a slight reduction in properties, with H_cj of 11.66 kOe, B_r of 8.37 kG, and (BH)_max of 13.32 MGOe, though they remain superior to the pristine alloy. Furthermore, Hf addition improves the high-temperature magnetic stability. The results indicate that optimal Hf addition is a promising strategy for enhancing the magnetic properties of near-stoichiometric Pr-Fe-B ribbons through microstructural refinement and reducing the volume fraction of the soft magnetic phase.

1. Introduction

Permanent magnets (PMs) constitute a critical class of functional materials that underpin modern technology, enabling the unprecedented miniaturization, and high efficiency of devices ranging from electric motors and generators to data storage systems and medical equipment [1,2,3]. Among PMs, rare-earth (RE) based magnets, particularly the Nd₂Fe₁₄B-type magnets, have remained in dominance since their invention because of their very high maximum energy product ((BH)_max) reaching near 60 MGOe [4,5]. The isostructural Pr₂Fe₁₄B compound is also very significant, offering a higher magnetocrystalline anisotropy field, i.e., high H_A, which provides the basis for achieving superior intrinsic coercivity (H_cj) [6]. In fact, the room temperature H_A of the Pr₂Fe₁₄B compound is higher than that of the Nd₂Fe₁₄B compound, but the H_A of Pr₂Fe₁₄B deteriorates faster at high temperature [1,3,7]. However, a fundamental limitation of all the RE₂Fe₁₄B magnets is their relatively modest Curie temperature (T_C), and high dependence of H_A on temperature, which lead to a pronounced temperature sensitivity of magnetic properties and restrict their application in high-temperature environments [7,8].

Significant research efforts have been dedicated to improving the high-temperature performance of RE-Fe-B magnets. The partial substitution of Fe by Co is a well-established method to elevate the T_C, though this often concurrently reduces the magnetocrystalline anisotropy constant and can complicate the microstructure [9,10]. An alternative and complementary strategy involves micro-alloying with refractory elements such as Nb, Zr, Ti, V, and Cr [11,12,13]. These additions typically do not enter the 2:14:1 lattice in large quantities, but form fine precipitates to segregate at grain boundaries. The primary role of the refractory elements is to refine the grain structure, suppress the formation of the soft magnetic phases such as α-Fe, and potentially pin the domain wall motion, thereby enhancing the intrinsic coercivity and thermal stability [14,15].

Hafnium (Hf) has emerged as an intriguing element among the refractory elements. In the Nd-Fe-B type alloys, Hf addition has been shown to be beneficial. Previous works indicate that Hf effectively inhibits the grain growth in nanocomposite alloys, leading to a finer and more uniform nanostructure [16,17,18]. Furthermore, Hf forms nanoscale precipitates, which may act as pinning centers, contributing to increased intrinsic coercivity [17,19]. In Nd-Ce-Fe-B magnets a (BH)_max of 12.3 MGOe was obtained by adding 1% Hf [19]. Hf has also shown great potential in enhancing the magnetic properties in Sm-Co-type alloys [20,21]. These microstructural modifications translate to improved hard magnetic properties, including enhanced room-temperature coercivity and better thermal stability. The favorable influence of Hf is not limited to Nd-based systems. Research on Pr-Fe-B-based nanocomposites has demonstrated that microalloying with Hf (and Ti) promotes the formation of a nearly ideal, fine-scale mixture of Pr₂Fe₁₄B and α-Fe grains, as confirmed by advanced microstructural characterization [22]. In Pr₂(Fe,Co)₁₄B/α-(Fe,Co) nanocomposites, the partial substitution of Fe with 1 at.% Hf is reported to optimize both the intrinsic coercivity and maximum energy product, outperforming other elemental additions like Al and Ti [23]. Hf has also been reported to help in the continuation of the Cu-rich phase in Alnico 8 alloys, thus enhancing the intrinsic coercivity [24].

Despite these useful findings, a systematic investigation focusing on the impact of Hf on stoichiometric Pr-Fe-B alloys—compositions designed to closely approximate the Pr₂Fe₁₄B phase—remains un-attempted in the literature. Most prior work has concentrated on either RE-lean nanocomposite compositions, where the interplay between hard and soft phases dominates behavior, or on Nd-based analogues [25,26]. The stoichiometric alloy provides a fundamental baseline, primarily comprising the single hard magnetic phase, and is thus an ideal platform to isolate and study how Hf influences the magnetic properties and intergranular interactions without the complicating factor of a significant soft magnetic phase fraction. Understanding these effects is crucial for the targeted design of high-performance Pr-Fe-B magnets.

Previously we explored the concept of integrating Alnico-type semi-hard phases into Pr-Fe-B and Didymium-Fe-B matrices, successfully achieving concurrent improvements in the Curie temperature and maximum energy product via the formation of complex nanocomposite structures [27,28]. In a separate, detailed study on Nd-Fe-B alloys, the role of Hf addition was investigated across a range of Nd contents, concluding that Hf’s efficacy in refining microstructure and boosting intrinsic coercivity is most pronounced in alloys near the stoichiometric 2:14:1 composition [16]. These findings motivate the need for the current paper to investigate the influence of Hf on the magnetic properties, high-temperature performance, and microstructure of a stoichiometric Pr-Fe-B alloy.

In this study, we systematically investigate the effects of Hf addition (0, 0.5, and 1.0 wt%) on Pr_26.7Fe_72.3B₁ melt-spun ribbons. The room-temperature and high-temperature magnetic properties (H_cj, B_r and (BH)_max) are studied. The Curie temperature, interaction mechanisms, and minor loops are investigated in detail. By providing a comprehensive analysis of how Hf modifies the properties of stoichiometric Pr-Fe-B, this work aims to furnish essential insights for the development of advanced Pr-based permanent magnets with tailored performance for demanding applications.

2. Materials and Methods

Ingots with the nominal compositions of Pr_26.7Fe_72.3B₁, Pr_26.7Fe_71.8Hf_0.5B₁, and Pr_26.7Fe_71.3Hf₁B₁ (wt%) were prepared by arc-melting the constituent elements (Pr: 99.95%, Fe: 99.99%, Hf: 99.95%) under a high-purity argon atmosphere. Each ingot was re-melted at least five times in order to obtain homogeneity. The as-cast ingots were then crushed into small slivers of approximately 8–9 g. These precursor pieces were inductively melted in a quartz tube under highly pure argon gas and rapidly solidified via melt-spinning onto a copper wheel rotating at surface velocity ranging from 18 to 35 m/s. Preliminary magnetic screening indicated that the optimal comprehensive magnetic properties for all three compositions were obtained in the ribbons produced at wheel speeds between 24 and 28 m/s. These optimally quenched ribbons were selected for all the subsequent characterizations.

The phase constitution and structural analysis of the melt-spun ribbons were investigated at room temperature using X-ray diffraction (XRD) on a PANalytical X’Pert Pro diffractometer (Malvern Panalytical, Almelo, Netherlands) with Cu-K_α radiation. The magnetic properties were characterized using a vibrating sample magnetometer (VSM, 9 T, PPMS, Quantum Design, San Diego, USA). Hysteresis loops (M-H curves) were measured at 300 K and 400 K with an applied field of up to ±3.0 T to determine the saturation magnetization (M_s), remanence (B_r), and intrinsic coercivity (H_cj). The Curie temperature (T_C) was determined from thermomagnetic (M-T) curves. The T_C was measured by heating the samples from 300 K to 700 K under a constant applied field of 0.10 T. The thermal stability of coercivity was assessed by measuring temperature-dependent hysteresis loops. The nano-structural morphology and phase distribution of the selected ribbons were examined using transmission electron microscopy (TEM, Tecnai G² F20, FEI, Hillsboro, OR, USA) operated at 200 kV.

3. Results and Discussion

The XRD patterns of the investigated alloys are shown in Figure 1. Figure 1 shows that the pristine alloy, Pr_26.7Fe_72.3B₁ exhibits relatively sharp and intense diffraction peaks which correspond to the tetragonal Pr₂Fe₁₄B phase, indicating a relatively large crystallite size, and high structural coherence. With the addition of Hf, a progressive broadening and intensity reduction of the Pr₂Fe₁₄B reflections is observed, suggesting grain refinement and increased micro-strain [17,29]. No crystalline Hf-containing secondary phases are detected. Furthermore, the relative intensity of α-Fe peaks, observed in the pristine alloy, decreases with increasing Hf content, indicating suppression of α-Fe-rich segregation. These structural modifications are expected to enhance the intrinsic coercivity through grain refinement, reduced soft magnetic phase content, and increased domain wall pinning. It is evident that there is no shift in the XRD peaks, which indicates that Hf atoms, with much larger radius compared to Fe, have not entered into the main phase to replace Fe sites [30].

Transmission electron microscopy (TEM) is employed to investigate the microstructural modifications induced by Hf addition in the near-stoichiometric Pr-Fe-B alloys. The TEM images of the pristine, 0.5 Hf and 1.0 Hf samples are respectively presented in Figure 2a–c, which reveal the evolution in the grain morphology and the size distribution. The pristine alloy shown in Figure 2a exhibits a heterogeneous microstructure with a broad grain size distribution, featuring locally coarsened regions and irregular, elongated grains. The average grain size of the Hf free melt-spun ribbon is 50 ± 5 nm. This morphology suggests less controlled grain growth during processing, which can be detrimental to uniform magnetic properties [31]. With the incorporation of 0.5% Hf, as shown in Figure 2b, a significant refinement effect is observed. The microstructure becomes more homogeneous with a reduced average grain size (35 ± 5 nm), and a transition to a more equiaxed morphology. This indicates that Hf effectively acts as a grain refiner, suppressing grain growth and promoting uniform nucleation, a phenomenon consistent with the role of transition metal additives in rare-earth permanent magnets [32]. The refinement is maximized at 1.0 Hf (Figure 2c), resulting in a very fine microstructure with the narrowest grain size distribution. The average grain size is found to be 25 ± 5 nm. From these observations, it is evident that the Hf-free sample (pristine) displays larger grains and fine grains, thus creating microstructural inhomogeneity. In contrast, the Hf-containing alloys, particularly the 0.5 Hf alloy, demonstrate a much uniform grain distribution.

The microstructural trends observed have direct and significant implications for the magnetic performance. The grain sizes of all the three alloys are in the single domain range for RE-Fe-B type magnets, which result in good magnetic properties, with more homogeneous structures having more advantages [33,34]. The Hf addition gradually refines and homogenizes the microstructure of Pr-Fe-B alloys. This tailored microstructural evolution, encompassing grain size reduction and improved uniformity, combined with less soft magnetic phase observed in XRD patterns, is expected to directly translate into enhanced magnetic stability, making Hf a potent microstructural modifier for high-performance permanent magnets.

The M-H curves of the Pr_26.7Fe_72.3B₁, Pr_26.7Fe_71.8Hf_0.5B₁ and Pr_26.7Fe_71.3Hf₁B₁ alloys are given in Figure 3. The pristine alloy, characterized by sharper diffraction peaks and α-Fe reflections as shown in Figure 1 and relatively bigger and irregular grains as shown in Figure 2a, exhibits a relatively gradual demagnetization process, consistent with the heterogeneous grains. In contrast, the Hf-containing alloys display broadened XRD reflections and reduced α-Fe intensity, showing steeper demagnetization curves and more abrupt reversal, resulting in a squarer demagnetization curve, which is also supported by the finer and more homogeneous microstructure shown in Figure 2b,c. These observations validate that Hf addition effectively modifies the structural framework of the Pr–Fe–B alloy. Figure 3a illustrates the microstructural-dependent magnetic properties such as H_cj and B_r of the melt-spun ribbons at 300 K. The pristine Pr_26.7Fe_72.3B₁ ribbon has an intrinsic coercivity H_cj = 11.95 kOe, remanence B_r = 8.23 kG, and maximum energy product (BH)_max = 12.6 MGOe. The magnetic properties increase to H_cj = 11.47 kOe, B_r= 8.58 kG and (BH)_max = 15.33 MGOe for the 0.5 Hf ribbon. Then the magnetic properties slightly decreased to H_cj = 11.66 kOe, B_r = 8.37 and (BH)_max = 13.32 MGOe, which is still better than those of the pristine alloy. The comprehensive magnetic properties reported here are comparable to or even better than those reported in the literature, despite the fact that much less rare earth has been consumed in the current study [17,35,36,37].

To test the high-temperature performance of the melt-spun ribbons, the M-H curves were obtained at 400 K as shown in Figure 3b. The pristine alloy exhibits an intrinsic coercivity H_cj = 6.90 kOe; the 0.5 Hf alloy shows H_cj = 6.99 kOe and 1.0 Hf shows H_cj = 7.0 kOe. For the pristine alloy, the B_r =7.46 kG increases to B_r = 7.75 for the 0.5 Hf alloy and B_r = 7.39 kG for the 1.0 Hf alloy. Based on the magnetic properties presented in Figure 3a,b, the temperature coefficient of remanence α and temperature coefficient of coercivity β are calculated using the following formulae [38]:

$$\mathsf{\alpha }={\displaystyle \frac{{\mathrm{B}}_{\mathrm{r}}\left({\mathrm{T}}_1\right)-{\mathrm{B}}_{\mathrm{r}}\left({\mathrm{T}}_0\right)}{{\mathrm{B}}_{\mathrm{r}}{(\mathrm{T}}_0\left)\right({\mathrm{T}}_1-{\mathrm{T}}_0)}}\times 100\mathrm{\%}$$

(1)

$$\beta ={\displaystyle \frac{H_{cj}\left(T_1\right)-H_{cj}\left(T_0\right)}{H_{cj}{(T}_0\left)\right(T_1-T_0)}}\times 100\%$$

(2)

where T₁ is 400 K and T₀ is 300 K. The α value of the pristine melt-spun ribbon is found to be −0.094%/K, which changes to 0.096%/K for 0.5 Hf and 0.117 for 1.0 Hf alloy. The β values were calculated to be 0.423, 0.390%/K and 0.400%/K for the pristine, 0.5 Hf and 1.0 Hf alloy. The temperature coefficients, particularly the temperature coefficient of coercivity, clearly improve for the Hf-containing alloys. This can be linked to microstructural refinement including the homogenous grains and reduced soft magnetic phases. As shown in Figure 2, Hf reduces the grain size, and in Figure 1, the Fe phase reduces. Smaller grains and reduced soft magnetic phases decrease the demagnetizing stray fields at grain boundaries, leading to better stability at elevated temperatures [20,39].

Shown in Figure 4 are the M-T curves of the investigated alloys taken in the temperature range of 300 to 675 K. No obvious change in the Curie temperature is observed, which indicates that the Hf element might not have entered into the main phase grains. When a paramagnetic element such as Hf enters in to the main phase to replace the Fe sites, the Curie temperature must decrease [39].

To understand the short-range magnetic interactions, which is expected to exist among the nanograins and various phases, the Henkel plots are obtained using the following equation [40]:

$$\delta M\left(H\right)=\left[M_d\left(H\right)-M_r\left(\infty \right)+2M_r\left(H\right)\right]/M_r(\infty )$$

(3)

where δM is the change in magnetization; M_d(H), M_r(H), and M_r(∞) represent the reduced magnetization, remanence at certain applied magnetic field, and remanence at maximum applied field, respectively. δM > 0 shows the short-range exchange interactions among the grains, δM = 0 indicates the absence of any interaction among the grains, and δM < 0 indicates the long-range interactions [41]. The Henkel plots of the Pr_26.7Fe_72.3B₁, Pr_26.7Fe_71.8Hf_0.5B₁, and Pr_26.7Fe_71.3Hf₁B₁ melt-spun ribbons are shown in Figure 5a. The pristine alloy depicts a δM value of about 0.63, which decreases with the addition of Hf, as the values reach 0.52 for the Pr_26.7Fe_71.8Hf_0.5B₁ and Pr_26.7Fe_71.3Hf₁B₁ melt-spun ribbons. This decrease might be due to the reduction of the ferromagnetic phase (α-Fe), which helps in increasing the short-range interactions. The recoil loops obtained by demagnetizing and re-magnetizing the samples multiple times for the best-performing ribbon Pr_26.7Fe_71.8Hf_0.5B₁, are shown in Figure 5b. The magnetization and demagnetization parts in the recoil loops do not overlap, resulting in closed loops with no inscribed area. This indicates that there will be negligible energy loss while a magnetic field is applied during operation of the magnet [42]. The closed loops and the smooth magnetization and demagnetization curves with no bumps indicate the good practical usage of this alloy in changing magnetic field environments.

4. Conclusions

Investigated in this manuscript are the magnetic properties of near-stoichiometric Pr-Fe-B melt-spun ribbons with small additions of Hf. Melt-spun ribbons with nominal compositions of Pr_26.7Fe_72.3B₁, Pr_26.7Fe_71.8Hf_0.5B₁, and Pr_26.7Fe_71.3Hf₁B₁ were prepared with optimized wheel speed. The pristine alloy (Pr_26.7Fe_72.3B₁) exhibits an intrinsic coercivity (H_cj) of 11.95 kOe, a remanence (B_r) of 8.23 kG, and a maximum energy product ((BH)_max) of 12.6 MGOe. With 0.5% Hf addition, the properties improved to H_cj = 11.47 kOe, B_r = 8.5 kG, and (BH)_max = 15.33 MGOe. A further increase to 1.0% Hf leads to a slight reduction in properties (H_cj = 11.66 kOe, B_r = 8.37 kG, (BH)_max = 13.32 MGOe), though they remain superior to the pristine alloy. The XRD analysis indicated that Hf does not enter into the main phase. Transmission electron microscopy analysis reveals that Hf addition effectively refines the grain structure. Furthermore, Hf doping improves the high-temperature magnetic stability. The results indicate that optimal Hf addition is a promising strategy for enhancing the magnetic properties of near-stoichiometric Pr-Fe-B ribbons through microstructural refinement.