J Supercond Nov MagnDOI 10.1007/s10948-014-2525-0

ORIGINAL PAPER

Structural and Magnetic Properties of NanostructuredPr0.75Y0.25Co5 Powders Obtained by Mechanical Millingand Subsequent Annealing

Abid Hussain · Tahir Abbas · Amer Bashir Ziya ·Zubair Ahmad · Hafiz Ahmad Raza

Received: 18 December 2013 / Accepted: 18 February 2014© Springer Science+Business Media New York 2014

Abstract Pr0.75Y0.25Co5-based as-cast alloys were pro-cessed by high-energy ball milling to obtain nanostructuredpowders with high coercivity. The powders obtained after4 h of milling exhibited nearly amorphous behavior in X-ray diffraction patterns. DSC scans of the as-milled powdersindicated a process of crystallization by broad, exothermictransition peak at 503 ◦C. Annealing of the milled powdersat 850 ◦C for 2.5 min in high vacuum produced fine grainsof size ranging 15–30 nm with optimal microstructure andhard magnetic properties. Magnetic measurements of theannealed powders evaluated a high intrinsic coercivity, iHc

of 9.3 kOe, and a remanence ratio, Mr/Mmax of 0.72. Themagnetic hardening was attributed to higher anisotropy fieldof the powders and microstructural uniformity achieved bythe processing methodologies.

Keywords Ball milling · Coercivity · X-ray diffraction ·Magnetic hardening

1 Introduction

Nanostructured R-Co5 (R = rare earth) compoundshave attracted much attention due to their unique sur-face/interface effects causing unusual scientific and tech-

A. Hussain (�) · T. Abbas · A. B. ZiyaDepartment of Physics, Bahauddin Zakariya University,Multan 60800, Pakistane-mail: ahussainbzu@gmail.com

Z. AhmadInstitute of Industrial Control System,P. Box 1398, Rawalpindi, Pakistan

H. A. RazaDepartment of Physics, Quaid-i-Azam University,Islamabad 45320, Pakistan

nological advantages [1–3]. These compounds are usuallyproduced by mechanical alloying or mechanical milling fol-lowed by thermal annealing. Mechanical milling is a moresuccessful process to produce high-coercivity nanostruc-tured powders with better magnetic properties [4]. PrCo5

and YCo5 with high maximum energy product (BH)max

are excellent candidates for the development of high-energy permanent magnets. Like SmCo5, they have highersaturation magnetization, Curie temperature, and uniax-ial anisotropy [5, 6]. Furthermore, metallic Y and Pr aremore abundant and less expensive than Sm. They havenot been commercialized so far, as the production ofsintered or powdered magnets through conventional pow-der metallurgy route causes some technical difficulties indeveloping well-reproducible and high enough coerciv-ity [7–9]. These problems might be overcome throughmicrostructural refinements by mechanical milling or meltspinning and rapid quenching techniques [10, 11]. Nanos-tructured Y0.5Pr0.5Co5, Pr0.5Sm0.5Co5, and Sm0.5Y0.5Co5

compounds with better magnetic properties produced bymechanical milling have earlier been reported [12–14].There is no published data about the nanostructuredPr0.75Y0.25Co5 compounds obtained by mechanical milling.In this work, we discuss the structural and magnetic proper-ties of the nanostructured Pr0.75Y0.25Co5 powders producedby high-energy ball milling and subsequent annealing.High coercivity achieved for nanostructured Pr0.75Y0.25Co5

powders stimulates interest in these materials for possiblepermanent magnet applications.

2 Experimental Procedures

Alloys with stoichiometric composition Pr0.75Y0.25Co5

were prepared by arc-melting the raw material ingots of Pr

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(99.9 %), Y (99.9 %), and Co (99.8 %) under Ar atmo-sphere. The ingots were homogenized by turning over andremelting for four times. The as-cast ingots were thencoarsely pulverized and subjected to mechanical millingunder Ar atmosphere using a high-energy Spex-8000 ballmill for a period of time ranging 0.5–4 h, keeping the ballto powder ratio at 8:1. Evaluation of crystallization processin the as-milled powders was carried out using differentialscanning calorimeter (DSC), and the curve was recordedunder high-purity Ar at the heating rate of 5 ◦C/min. Theas-milled powders were sealed in quartz ampoules underhigh vacuum and annealed at temperatures ranging T =750–950 ◦C for times ranging t = 1–5 min, followed byquenching in cold water. Structural and microstructuralstudies were carried out by a Rigaku X-ray diffractometer(XRD) using Cu-Kα radiation (λ = 1.54056 A) and JEOL,JEM-2010 transmission electron microscope (TEM). Mag-netization measurements were made at room temperatureusing a Quantum Design Physical Measurement System(PPMS) with a maximum applied magnetic field of 16 and40 kOe.

3 Results and Discussion

Figure 1a shows an X-ray diffraction pattern of as-castPr0.75Y0.25Co5 alloy. The pattern was indexed to a hexago-nal phase with a CaCu5-type structure. The Rietveld refine-ment was carried out using MDI Jade, which confirmedthat 1:5 phase was the overwhelming majority phase in the

Fig. 1 X-ray diffraction pattern of the (a) as-cast Pr0.75Y0.25Co5alloy, (b) Pr0.75Y0.25Co5 powders milled for 4 h, and (c)Pr0.75Y0.25Co5 powders milled for 4 h and annealed at 850 ◦C for2.5 min

Pr0.75Y0.25Co5 system. No impurity peak was observed inthe pattern and the as-cast alloy was used as a precursorfor further mechanical processing. Figure 1b shows an X-ray diffraction pattern of powders milled for a 4-h duration,exhibiting low intensity and broad peaks corresponding tothe strongest reflections of the 1:5 phase, indicating analmost amorphous state of Pr0.75Y0.25Co5 powders. Subse-quent annealing of the powders was carried out at highertemperatures for better crystallinity and enhanced magneticproperties [15]. In order to investigate the dependence ofmagnetic properties on annealing parameters (time t, tem-perature T), the as-milled powders were sealed in quartzampoules under high vacuum. The sealed ampoules wereannealed at temperatures T = 750, 850, and 950 ◦C forthe annealing times t = 1, 2.5, and 5 min, respectively.Each time, for microstructural optimizations and develop-ment of high coercivity in the annealed powders, annealingwas followed by quenching in cold water. It was foundthat the nanostructured Pr0.75Y0.25Co5 powders obtained byannealing at 850 ◦C for 2.5 min exhibited the best magneticproperties. Figure 1c represents the X-ray diffraction pat-tern of the powders annealed at the temperature of 850 ◦Cfor 2.5 min. Broadening of the peaks indicated the desirednanostructured hexagonal Pr0.75Y0.25Co5 phase with smallcrystallite size. For the estimation of average grain size(< D >= 16 nm) by the Scherrer formula [16], the fullwidth at half maximum (FWHM) of intensity of the Braggpeak (111) was calculated by the corresponding peak pro-file fitting to a combination of Lorentzian and Gaussianfunctions. Figure 2 shows the DSC curve recorded for theas-milled Pr0.75Y0.25Co5 powders milled for 4 h. A broad,exothermic transition peak in the curve with a maximum at503 ◦C was attributed to the process of crystallization takingplace in the amorphous powders [17]. Figure 3 shows thedependence of maximum magnetization, Mmax, and intrin-sic coercivity, iHc, on milling time. An initial sharp risein iHc resulted due to defects accumulation and grain sizerefinement during the milling process. Elongated millings

Fig. 2 DSC curve for as-milled Pr0.75Y0.25Co5 powders milled for 4 h

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Fig. 3 Dependence ofmaximum magnetization, Mmax,and intrinsic coercivity, iHc, onmilling time for thePr0.75Y0.25Co5 alloy

caused thermal fluctuations and random anisotropy, destroy-ing the magnetic order, so the iHc was reduced in increasedmilling hours [18]. An abrupt increase in Mmax for shortmilling times was due to intensive destruction of the long-range order in crystallites along with gradual formation ofthe amorphous phase together with rapid magnetic softening[5]. Figure 4 shows TEM micrograph for the powders milledfor 4 h and annealed at 850 ◦C for 2.5 min. It was observedthat almost spherical grains with size D = 15–30 nmwere uniformly distributed [12]. In TEM observations, thegrain size was slightly dissimilar from the average grainsize obtained by Scherrer’s formula which was ascribed tothe fact that only a minute amount of particles could beobserved with TEM, whereas with Scherrer’s formula, moreaccurate estimation was possible [5]. Figure 5 demonstratesthe effect of annealing time on coercivity iHc and rema-nence ratio Mr/Mmax, at different annealing temperatures. Itwas observed that the coercivity exhibited a stronger depen-dence on the annealing time than the remanence ratio. Itwas worth noting that all the samples exhibited enhancedremanence (Mr/Mmax > 0.5) because of their nanomet-ric average grain size [19]. It has earlier been establishedthat the 1:5 phase with metastable character [20] mightundergo decomposition at elevated temperatures, loweringthe hard magnetic phase proportion by leading to the for-mation of secondary phase and also the annealing-inducedgrowth of grains strongly influenced the coercivity iHc [21,22]. Figure 6 demonstrates the hysteresis loop obtainedfor Pr0.75Y0.25Co5 powders milled for 4 h and annealed at850 ◦C for 2.5 min. The convex shape of the virgin curveindicated a pinning-type magnetic hardening mechanism,similar to the materials reported earlier [23]. It was deducedthat the correlation lengths in the nanostructured powders

were above the average grain size, leading to the forma-tion of interacting, multigrain domains and the location ofpinning sites were at the hard-hard interfaces among theadjacent nanograins [24]. It was observed that even with thehigher applied magnetic field, Hmax, the maximum magne-tization, Mmax did not reach its saturation value, followingthe characteristic of the nanostructured magnets [25]. Thesmoothness of the hysteresis loop indicated a uniform, finecrystallite size [22], yielding significant values for coer-civity, iHc = 9.3 kOe and remanence ratio, Mr/Mmax =0.72 with energy product, (BH)max = 4.3 MGOe whichwas comparable to the nanostructured magnetic compoundsof the same family [12]. The observed high remanence

Fig. 4 TEM micrograph for the Pr0.75Y0.25Co5 powders milled for4 h and annealed at 850 ◦C for 2.5 min

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Fig. 5 Remanence ratio Mr/Mmax and coercivity iHc dependence onthe annealing time t at different annealing temperatures

ratio was ascribed to the exchange interaction among theneighboring nanograins earlier demonstrated with micro-magnetic calculations [26]. In an assembly of uniaxialmagnetic nanocrystalline grains, the exchange interactionamong the nanograins causes a deviation in spontaneousmagnetization from the easy axes of magnetization, result-ing in the enhanced remanence ratio greater than 0.5, a valuefor noninteracting, uniaxial, and single-domain assembly ofparticles predicted by the Stoner-Wohlfarth model [13, 26–29]. The magnetic hardening in the powders was attributedto higher anisotropy field and uniformity of microstructureachieved by the adopted methodologies.

Fig. 6 Hysteresis loop for the Pr0.75Y0.25Co5 powders milled for 4 hand annealed at 850 ◦C for 2.5 min

4 Conclusion

In summary, the results demonstrated above conclude thatnanostructured Pr0.75Y0.25Co5 powders with significant val-ues of coercivity iHc and remanence ratio Mr/Mmax weresuccessfully obtained by arc-melting, mechanical millingand subsequent annealing in high vacuum. It was found thatiHc exhibited a stronger dependence on the annealing timethan Mr/Mmax. The best magnetic powders were obtainedafter 4 h of milling and annealing at 850 ◦C for 2.5 minshowing the grain size of 15–30 nm and medium strengthpermanent magnetic properties with iHc = 9.3 kOe andMr/Mmax = 0.72 emu/g. The observed high remanence ratiowas ascribed to the exchange interaction among the neigh-boring nanograins. The magnetic hardening in the powderswas attributed to higher anisotropy field of the 1:5 phaseand uniformity of microstructure achieved by the adoptedmethodologies.

Acknowledgments This research was financed by the HigherEducation Commission (HEC) of Pakistan. The first author is thankfulto the World Federation of Scientists (WFS) for financial support.

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