Magnetic properties of ( misch metal , Nd )-FeB melt-spun magnets

The effect of replacing Nd with misch metal (MM) on magnetic properties and thermal stability has been investigated on melt-spun (Nd1-xMMx)13.5Fe79.5B7 ribbons by varying x from 0 to 1. All of the alloys studied crystallize in the tetragonal 2:14:1 structure with single hard magnetic phase. Curie temperature (Tc), coercivity (Hcj), remanence magnetization (Br) and maximum energy product ((BH)max) all decrease with MM content. The melt-spun MM13.5Fe79.5B ribbons with high ratio of La and Ce exhibit high magnetic properties of Hcj = 8.2 kOe and (BH)max= 10.3 MGOe at room temperature. MM substitution also significantly strengthens the temperature stability of coercivity. The coercivities of the samples with x = 0.2 and even 0.4 exhibit large values close to that of Nd13.5Fe79.5B7 ribbons above 400 K.


I. INTRODUCTION
With the increasing demand for NdFeB permanent magnets and the extensive use of Nd in natural rare-earth resources, the rising price of Nd induces cost pressures on the NdFeB material industry.Developing low-cost rare-earth permanent magnet becomes particularly important.2][3][4][5][6][7][8] This work aims to obtain R 2 Fe 14 B based hard magnets using misch metal (MM) alloys with high La and Ce contents (unseparated rare-earth mixtures: La-Ce-Pr-Nd alloy).Although both saturation magnetization and magnetocrystalline anisotropy of La 2 Fe 14 B and Ce 2 Fe 14 B are inferior to that of Nd 2 Fe 14 B, 9 MM-substituted permanent magnets are potential alternative and economical materials with intermediate magnetic properties filling the gap between ferrites and NdFeB magnets. 10,11Unfortunately, sintered or die-upset magnets with high La or Ce ratios usually exhibit very low coercivities because of their seriously deteriorated microstructure. 11,12By contrast, completely or excessively replaced magnets prepared by the meltspun method can achieve promising magnetic properties.For example, Ko et al. 13 reported that melt-spun MM 12 Fe 82 B 6 alloy exhibited a coercivity of 5.6 kOe and a maximum energy product of 8.9 MGOe, Yamasaki et al. 3 recorded H cj = 9.4 kOe and (BH) max = 8.1 MGOe in a MM 16 Fe 75 B 9 alloy.The MM used in these studies contains approximately 30 wt.% La, 50 wt.%Ce and 20 wt.% Pr-Nd, and MM 2 Fe 14 B can theoretically achieve an increased maximum potential energy product of 10 MGOe as isotropic magnet. 14Further effort, such as optimizing alloy composition or enhancing orientation, can be exerted to improve the magnetic properties of these kinds of low-cost magnets.
In addition, melt-spun magnetic powder used to make isotropic bonded or injection molded magnets should constantly provide sufficient high magnetic properties at or above 400 K. Given the a Author to whom correspondence should be addressed.Electronic mail: shenbg@iphy.ac.cn 2158-3226/2017/7(5)/056207/6 Replacing Nd with MM affords the magnets with enhanced temperature stability of coercivity.The ribbons with x = 0.2 and even 0.4 exhibit the same coercivity as that of NdFeB above 400 K.

II. EXPERIMENT
Ingots having nominal compositions of (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) are fabricated by arc melting under an argon atmosphere of 99.9% Fe, 99.9% Nd, 99.7% MM and 97% ferroboron (FeB).The MM alloy consists of 28.3 wt.% La, 50.5 wt.% Ce, 5.2 wt.% Pr, 15.7 wt.% Nd and 0.3 wt.% others (approximately to the same ratio of rare earth elements found in natural ore).Each ingot is remelted at least five times to ensure composition homogeneity.The melt-spun ribbons are produced by induction melting the ingot in quartz tubes with a 0.5 mm diameter bottom orifice under 1/2 atm of high purity Ar gas.The melted ingot is then ejected at a 60 torr overpressure onto a rotating copper wheel.The optimum tangential wheel velocity ranges from 20 m/s to 25 m/s.Typical ribbons possess a thickness and width of approximately 30-50 µm and 2-3 mm, respectively.The ribbons are then crushed to powders, and these powders are examined with X-ray diffraction (XRD) using Cu Kα radiation.Curie temperatures for all samples are determined by the magnetization of polycrystalline samples as a function of temperature with a vibrating sample magnetometer (VSM) in a static field of 500 Oe.The hysteresis loops of all samples are measured in a Physical Property Measurement System (PPMS) made by Quantum Design Inc. in a magnetic field of up to 70 kOe and temperatures up to 650 K.

III. RESULTS AND DISCUSSION
Fig. 1 displays the XRD patterns of melt-spun ribbons with optimal magnetic properties in (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys.The results show line characteristics of the tetragonal Nd 2 Fe 14 B structure in all cases and no additional phases (rare earth oxides, RFe 2 or α-Fe).These findings confirm that all samples contain a single-phase with MM replacement.According to Scherer's formula, the average grain sizes are estimated within the range of 30-50 nm.Diffraction peaks are slightly broadened, and some peaks are obviously weakened with increasing MM concentration.A reasonable explanation is that amorphous phases and metastable phases at intergranular interface in these ribbons are easily formed and increased because of different phase equilibria in the La-Fe-B, Ce-Fe-B, Pr-Fe-B and Nd-Fe-B systems. 7he variation of intrinsic coercivity (H cj ), remanence (B r ) and maximum energy product ((BH) max ) at room temperature and the Curie temperature (T c ) of all samples are summarized in Table I.These data include the corresponding mass proportions of La and Ce and Pr-Nd in MM metal.The room temperature magnetic properties (H cj , B r , and (BH) max ) and T c all show linear decreases with MM content.The decrease in magnetic properties is attributed to the drop in intrinsic magnetic properties, because the intrinsic magnetic properties of La 2 Fe 14 B and Ce 2 Fe 14 B are lower than that of Pr 2 Fe 14 B and Nd 2 Fe 14 B. The decrease in Curie temperature is ascribed to T c of Ce 2 Fe 14 B being lower than that of La 2 Fe 14 B, Pr 2 Fe 14 B or Nd 2 Fe 14 B. This result also suggests that MM atoms enter into the 2:14:1 phase.6][17][18] Consequently, sintered or die-upset magnets containing high La or Ce content with poor microstructure tend to obtain very low coercivities or even lose their hard magnetic properties.However, large coercivities could be achieved in nanocrystalline magnets fabricated by melt-spun method with enhanced microstructure.This effect is achieved because the unfavorable factors mentioned above can be greatly suppressed by the non-equilibrium process of fast cooling.For example, in high La-and Ce-addition magnets, such as MM 2 Fe 14 B in this work, the stoichiometric MM 12 Fe 82 B 6 melt-spun alloy reportedly exhibits a coercivity of 5.6 kOe. 13Similar to the method of increasing the rare earth or boron content to enlarge coercivity, rich-MM and rich-B based MMFeB magnets obtained higher coercivities, and the maximum values reported reach approximately 25% of magnetocrystalline anisotropy field (H a = 4 T 14 ), such as H cj = 9.4 kOe for MM 16 Fe 75 B 9 3 and H cj = 10 kOe for MM 22 Fe 70 B 8 . 19To avoid low maximum energy product in rich-MM and rich-B based magnets because of the increase in non-magnetic phases, the melt-spun ribbons designed in this work with a nominal composition of MM 13.5 Fe 79.5 B 7 achieve an acceptable intrinsic coercivity and a high maximum energy product (H cj =8.2 kOe and (BH) max =10.3 MGOe).
The remanence and intrinsic coercivity of melt-spun (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) as a function of temperature from 300 K to 500 K are displayed in Figs.2(a) and (b).Their temperature coefficients for remanence (α) and coercivity (β) are listed in Table I.Fig. 2(a) shows that the reduction trend of B r gradually enlarges with the increasing MM content and the remanences of the samples with x=0.8 and 1 drop rapidly as temperature approaches their Curie temperature.These findings indicate that the temperature stability of remanence worsens by MM addition and α decreases from -0.13 %/K to -0.22 %/K.Because α is inversely related to the Curie temperature T c , 20 Ce addition in magnets, which results in the low T c , is the reason for the decrease in the temperature stability of remanence.On the contrary, in Fig. 2(b), H cj show opposite reduction trends with the varying content of MM and β increases from -0.42 %/K to -0.33 %/K.This result suggests that replacing Nd with MM is good for enhancing the temperature stability of coercivity.TABLE I.The content of La, Ce, and Pr-Nd in the total rare earth and the room temperature magnetic properties and the Curie temperatures and the temperature coefficients for remanence (α) and coercivity (β) of (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 ribbons with x=0, 0.2, 0.4, 0.6, 0.8, 1.
Samples La (wt.%)Ce (wt.%)Pr-Nd (wt.%)Interestingly, the intrinsic coercivity of the sample with x = 0.2 does not decrease considerably before ∼400 K compared with that of Nd 13.5 Fe 79.5 B 7 , and it is even slightly larger than that of Nd 13.5 Fe 79.5 B 7 after ∼400 K.Moreover, the intrinsic coercivity above 450 K for the sample with x = 0.4 attains a large value close to Nd 13.5 Fe 79.5 B 7 .The rare earth sites in R 2 Fe 14 B have different crystalline electric field parameters and Fe coordination numbers, which results in different temperature dependence of the magnetocrystalline anisotropy. 9,21H a of Pr 2 Fe 14 B and Nd 2 Fe 14 B arises from the large R moment with an orbital angular moment L 0,which leads to large crystal-field splitting.However, their H a show strong temperature dependence and a monotonic decline with increasing temperature.The L=0 compounds La 2 Fe 14 B and Ce 2 Fe 14 B have low H a which are completely contributed by the iron sublattice, but their H a show weak temperature dependence.Moreover, the temperature dependence of H a above room temperature is anomalous for La 2 Fe 14 B and it presents a trend of increasing first and then decreasing.For a solid solution of R 2 Fe 14 B, temperature dependence of magnetocrystalline anisotropy is expected to follow a rule of mixtures.Thus, the inclusion of La and Ce in solid solution with Pr and Nd in the R 2 Fe 14 B phase should enhance the temperature dependence of H a and accordingly improve the temperature stability of coercivity.3][24] In spite of replacing by La or Ce with low anisotropy field, (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 (84.2wt.% Pr-Nd) and (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 (68.5 wt.% Pr-Nd) ribbons in this work obtain large coercivities at high temperature.
According to the magnetic hardening mechanism, besides on magnetocrystalline anisotropy field, the intrinsic coercivity strongly depends on the microstructure of magnet, which can be phenomenologically expressed by the following micromagnetic equation: 25 Where α k , α ex , α ϕ , and N eff are so called microstructural parameters and related to non-ideal microstructure of the real magnets.The parameter α k reflects the locally reduction of crystal anisotropy of the inhomogeneous surface regions, α ex describes the effect of exchange coupling between the neighboring grain, α ϕ is related to the grain orientation distribution (generally equal to 0.5) and N eff is the effective demagnetization factor due to the enhanced stray fields at the edges and corners of the grains.The values H a (T) of La 2 Fe 14 B, Ce 2 Fe 14 B, Pr 2 Fe 14 B and Nd 2 Fe 14 B phases are collected from the previous work 9 and then are used to calculate H a (T) for each sample according to the percentage of La, Ce, Pr and Nd.By linear fit of H cj /M s vs H a /M s plot (the fitting curves of the samples with x=0, 0.6 and 1 are shown in Fig. 3), α k α ex and N eff for all samples increase from 0.55 and 0.09 to 0.64 and 0.28 with MM increasing, respectively.It is known that Curie temperature is controlled by Fe(3d)-Fe(3d) and R(4f)-Fe(3d) exchange and the exchange constant can be evaluated by the Curie temperature.The decrease in Curie temperature with MM content proves a smaller exchange constant between atoms in the unit cell and it indicates a weaker intergranular exchange coupling.Hence, the increase of α k α ex can be attributed to the weak exchange coupling between the grains, and the increase of N eff is owing to the increase of local stray fields at the edges and corners of the grains because of the weakened smoothing effect of the exchange interaction. 25

IV. CONCLUSION
In this work, melt-spun (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 ribbons with single hard magnet phase are successfully prepared.With the increase of MM content, magnetic properties, such as H cj , B r , and (BH) max , all drop because of the decrease in intrinsic magnetic properties by La and Ce replacement.High percentage of La-Ce melt-spun MM 13.5 Fe 79.5 B 7 ribbons exhibit a high room temperature performance with H cj = 8.2 kOe and (BH) max = 10.3MGOe.The temperature stability of coercivity is strengthened through MM substitution and the replaced magnets with x = 0.2 and even 0.4 maintain large coercivities above 400 K. Using MM can significantly reduce the cost of rare-earth permanent magnets with acceptable performance.

FIG. 3 .
FIG. 3. H cj /M s values plotted against H a /M s values for the samples with x=0, 0.6 and 1.
9 424 K) of Ce 2 Fe 14 B,9excessive substitution of Nd with MM degrades the intrinsic magnetic properties of RFeB magnets at elevated temperature.Most investigations only reported the room temperature magnetic properties of MMFeB magnets, but few studies have been performed on the effects of MM on the temperature stability of magnetic properties.In this work, we prepare (Nd 1-x MM x ) 13.5 Fe 79.5 B 7 ribbons via the melt-spun method.The Curie temperature and magnetic properties above room temperature are investigated with varying MM content.High magnetic properties of H cj = 8.2 kOe and (BH) max = 10.3MGOe are obtained in MM 13.5 Fe 79.5 B 7 alloy.