Available online at www.sciencedirect.com

08) 2640–2642www.elsevier.com/locate/matlet

Materials Letters 62 (20

High temperature magnetic properties of mechanically alloyed Fe–Zr powder

Debabrata Mishra, A. Perumal ⁎, A. Srinivasan

Department of Physics, Indian Institute of Technology Guwahati, Guwahati — 781 039, India

Received 10 November 2007; accepted 3 January 2008

Available online 11 January 2008

Abstract

We report the preparation and characterization of amorphous/non-equilibrium solid solution Fe100− xZrx (x=20–35) alloys by mechanicalalloying process. The microstructure and magnetic properties of milled powders have been studied as a function of Zr substitution. The effectivemagnetic moment of as-milled powders decreases as concentration of Zr is increased. Thermomagnetization measurements confirmed that theFe80Zr20 sample exhibits two clear magnetic phase transitions due to the co-existence of an amorphous phase and a Fe rich non-equilibrium solidsolution. All the other samples exhibiting an amorphous structure showed a single magnetic phase transition with Curie temperature of ~570 °C,which did not vary much with different composition. The Curie temperature of the mechanically alloyed powders is noticeably higher than thoseof melt-spun amorphous ribbons.© 2008 Elsevier B.V. All rights reserved.

Keywords: Amorphous phase; Mechanical alloying; Curie temperature; Internal stress; Magnetic moment

1. Introduction

Fe rich metallic alloys with disordered structure have stimulateda keen interest over a few decades due to their complex magneticbehaviour as well as their potential application in various magneticdevices [1,2]. These materials can be obtained through two diverseprocesses viz., melt-spinning [2] and mechanical alloying [3].Mechanical alloying (MA) has been successful in producingvarious disordered alloy [4] powders since the pioneering study byKoch et al. [5]. MA has been largely employed to obtainnanometer sized crystallites from microcrystalline phases and tosynthesize composite materials with interesting properties [6].However, there is still a lack of detailed investigation on the phasetransformation, thermodynamics, and resulting magnetic proper-ties of these powders obtained by MA. In the present study, wereport the preparation and characterization of amorphous and/ornon-equilibrium solid solution Fe100−xZrx (x=20–35 at.%) alloypowders by MA process. The effect of stress induced during theMA process on structural and high temperature magnetic proper-ties of Fe100−xZrx powders are also investigated.

⁎ Corresponding author. Tel.: +91 361 2582714; fax: +91 361 2690762.E-mail address: perumal@iitg.ernet.in (A. Perumal),

debabrata@iitg.ernet.in (D. Mishra), asrini@iitg.ernet.in (A. Srinivasan).

0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2008.01.004

2. Experimental details

Weighed quantities of elemental Fe and Zr powders (99.9%purity) corresponding to nominal compositions of Fe100−xZrx(x=20–35 at.%) were sealed in a hardened steel vial filled withhigh purityAr gas.Mechanical alloying of the powdermixturewasperformed in a Insmart planetary ball mill under a milling speed of500 rpm using 8 mm diameter tempered steel balls and a ball/powder weight ratio of 20:1. In order to avoid excessive heating,the mill was programmed to halt for 10 min after every 15 min ofoperation. All the alloy compositions were milled for 60 h. Thephases evolved in the as-milled powders were characterized by anX-ray diffractometer (XRD, Seifert 3003T/T), Scanning ElectronMicroscope (SEM, Leo 1430VP) with EDS attachment andTransmission Electron Microscope (TEM, JEOL2100). Themagnetic properties of the powders were characterized using aVibrating Sample Magnetometer (VSM, LakeShore7410) inapplied field range of ±20 kOe.

3. Results and discussion

Fig. 1 shows the XRD patterns of as-mixed Fe65Zr35 powder and as-milled Fe100− xZrx alloy powders. The multiple sharp Bragg peakscorresponding to the starting constituents of Fe and Zr are visible in the

Fig. 1. XRD patterns of as-mixed Fe65Zr35 and as-milled Fe100− xZrx alloypowders.

Fig. 3. Room temperature M–H loops and Inset: Variation of effective magneticanisotropy with Zr substitution in Fe100− xZrx (x=20–35 at.%) as-milled alloypowders.

2641D. Mishra et al. / Materials Letters 62 (2008) 2640–2642

XRD pattern of the as-mixed sample. On milling, the characteristicpeaks of Zr disappear. The broadened low intensity peaks correspond-ing to FeZr amorphous phase centered around 2θ=33° and non-equilibrium solid solution of Fe(Zr) centered around 2θ=44°,respectively, appear. This indicates that milling for 60 h results eitherin the formation of an amorphous phase (for x≥25 at.%) or a structureconsisting of an amorphous phase with a finite non-equilibrium solidsolution (for x=20 at.%) with an average crystallite size of about 8 nm.The MA process introduces a lot of strain in the as-milled powders.EDS analyses performed at selected places show that the deviations inthe measured compositions of FeZr are within 3% of the nominalstarting compositions. This signifies compositional homogeneity in theas-milled powders.

Fig. 2 shows a bright-field plane view TEM image and selected areaelectron diffraction (SAED) pattern of the amorphous Fe65Zr35 alloypowder. The diffused halo ring in the SAED pattern is the typicalsignature of amorphous structure present in the milled Fe65Zr35powders. This is in good agreement with the XRD results depicted in

Fig. 2. TEM bright-field plane view image and the selected area electrondiffraction pattern of amorphous Fe65Zr35 sample.

Fig. 1. Fig. 3 shows the room temperature magnetic hysteresis loops ofthe as-milled Fe100− xZrx powders. The curves depict the typical softmagnetic behavior, but they do not saturate at 20 kOe applied field. Theaverage magnetic moment of FeZr varies from 0.8 μB per formula unitfor x=20 at.% to 0.25 μB per formula unit for x=35 at.%. These valuesare found to be much lower when compared to the value of 2.2 μB forpure iron and somewhat lower than the value of 1.08 μB of themechanically alloyed powders [7]. Such a large decrease in magneticmoment could be due to more than one of the following reasons: (i)change in the average Fe atomic distance caused by the increased Zrsubstitution leads to the formation of finite antiferromagnetic (AFM)sites [8], (ii) charge transfer from the alloying element to Fe, whichpartially fills the d-band of Fe and lowers its magnetic moment [9], (iii)consequence of the change in the hybridization between Fe–Fe d bandsand Fe–Zr d bands [10] and (iv) presence of large topological disorderin the amorphous phase. In order to calculate the saturationmagnetization and to obtain more specific details about effectivemagnetic anisotropy in the system, the experimental high field

Fig. 4. Thermomagnetization curves measured at 1 kOe applied field. Inset:Thermal derivative of magnetization as a function of temperature for as-milledFe100− xZrx (x=20–35 at.%) alloy powders (Only one-third of the data points areshown in the plots for the sake of clarity).

2642 D. Mishra et al. / Materials Letters 62 (2008) 2640–2642

magnetization curves were fitted to the law of approach of saturation,defined as [11],

M ¼ MS 1� affiffiffiffiH

p � bH2

� cH3

: : :� �

þ vHH ð1Þ

where, MS is saturation magnetization, H is applied field, χH is highfield susceptibility, and a, b, c are constant coefficients. The coefficientb is related to the effective magnetic anisotropy through the relation[13], keff ¼ A0MS

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi105 b=8

p. The a=

ffiffiffiffiH

pterm arises from point like

defects with stress fields varying as σ ∝ 1/r3 [12], while the term b/H2

is attributed to magnetoelastic interaction. Long range stresses,accumulated in the powders, can cause stress anisotropy viamagnetoelastic coupling. A detailed least square fitting procedurewas carried out to fit the experimental magnetization data, byconsidering a, b, c, MS and χH as free fitting parameters. Inset inFig. 3 shows the variation of the effective magnetic anisotropy with Zrsubstitution in Fe–Zr. It is to be noted that the value of Keff is found tobe more than one order higher than that of bulk iron (0.414×105 J/m3).

Thermomagnetization (M–T) measurement was carried out up to850 °C under 1 kOe applied field for as-milled Fe100− xZrx powders andthe data are presented in Fig. 4. In order to accommodate all the M–Tcurves and to compare them directly, normalized magnetization [M(T)/M30] with respect to the room temperature magnetization of thecorresponding samples is plotted along the y-axis. The inset in Fig. 4shows the derivative of thermomagnetization for all the samples toidentify the magnetic phase transition temperature(s). With increasingtemperature, all the samples show a similar variation of magnetizationwith temperature except for Fe80Zr20 sample. The Fe80Zr20 sampleclearly shows two magnetic phase transitions: one at 625 °C andanother at 760 °C. These two magnetic phase transitions represent thepresence of both amorphous and non-equilibrium solid solution phasesin this sample, as described in Fig. 1. While the first magnetic phasetransition at 625 °C corresponds to the ferromagnetic (FM) toparamagnetic (PM) phase transition in the amorphous phase, thesecond transition might be due to the FM to PM phase transition (Curietemperature, TC) in Fe rich non-equilibrium solid solution. The valueof TC decreases to about 570 °C with increasing Zr from 20 to 25 at.%and then remains almost constant up to 35 at.%. It can also be observedfrom Fig. 4 that the manner in which thermomagnetization decreases isnot smooth. This might be due to the release of the stresses withincreasing temperature. It is important to note that these FeZr milledpowders exhibit high TC despite having low average magnetic momentat room temperature. Also, the TC of these samples is higher than thoseof amorphous alloys of similar compositions prepared by the melt-spinning technique. Such a difference in TC could be due to thepresence of large internal stresses accumulated during MA process andthe existence of finite AFM sites in FMmatrix. As described earlier, theeffective magnetic anisotropy caused by the induced stresses from theMA process is larger and facilitates improvement of the stability of thelocal magnetic structure with long range magnetic ordering viamagnetoelastic coupling [14,15]. In addition, the presence offluctuations of concentration and exchange integral have alsoconsiderable effect on the higher TC, as described in an earlier reportof micromagnetic theory of phase transition [15]. The increase of Curietemperature on the applied stress has been reported in a similar(FeZrCuB) system [16]. Internal stress is a consequence of the millingprocess, while the existence of finite AFM sites in FMmatrix can result

from a change in the average Fe atomic distance. Both these factorscould have efficiently contributed to the high magnetic stability in thesealloy powders and hence the high TC. However, at this moment, it maybe difficult to quantify exact contribution of various parameters leadingto the observed high TC.

4. Conclusions

In summary, we have prepared and characterized theamorphous and/or non-equilibrium solid solution Fe100− xZrxalloys by MA process. MA of the mixture of elemental powdersled to the formation of amorphous or amorphous/non-equilibrium solid solution depending upon the composition.These samples showed high TC of about 570 °C, which isalmost constant with increasing Zr substitution. The internalstresses accumulated in the samples during the milling processhave strong influences on the magnetic properties.

Acknowledgment

This work was financially supported by DAE-BRNS, Indiathrough a Young Scientist Research Award (2005/20/34/1/BRNS/376) for AP and DST, India vide project No: SR/S2/CMP-19/2006. Permission from CIF, IIT Guwahati, India forthe use of SEM, TEM (100/IFD/6278/2005–2006) and VSM isgratefully acknowledged.

References

[1] D.E. Witkin, E.J. Lavernia, Prog. Mater. Sci. 51 (2006) 1–60.[2] M.E. McHenry, M.A. Williard, D.E. Laughlin, Prog. Mater. Sci. 44 (1999)

291–433.[3] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006)

427–556.[4] J.J. Ipus, J.S. Blaquez, V. Franco, A. Conde, Intermetallics 15 (2007)

1132–1138.[5] C.C. Koch, O.B. Cavin, C.G. Mckamey, J.O. Scarbrough, Appl. Phys.

Lett. 43 (1983) 1017–1019.[6] J. Bednarick, E. Burkel, K. Saksl, P. Kollar, S. Roth, J. Appl. Phys. 100

(2006) 1–6.[7] A. Grabias, D. Oleszak, J. Kalinowska, M. Kopcewics, J. Latuch, M.

Pekala, J. Alloy. Compd. 434–435 (2007) 493–496.[8] J.S. Zhang, M. Ding, F. Pan, J. Phys., D, Appl. Phys. 33 (2000) 185–194.[9] T.D. Shen, R.B. Schwarz, J.D. Thompson, Phys. Rev., B 72 (2005)

014431.[10] D. Stoeffler, K. Ounadjela, J. Sticht, F. Gautier, Phys. Rev., B 49 (1994)

299–309.[11] R. Herz, H. Kronmuller, J. Magn. Magn. Mater. 15–18 (1980) 1299–1300.[12] H. Kronmuller, M. Fahnle, M. Domann, H. Grimm, R. Grimm, B. Groger,

J. Magn. Magn. Mater. 13 (1979) 53–70.[13] S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964, p. 277.[14] Y.N. Mitsay, A. Fridman, D.V. Spirin, C.N. Alexeyev, M.S. Kochmanski,

Phys., B 292 (2000) 83–88.[15] H. Kronmuler, M. Fahnle, Phys. Status Solidi (b) 97 (1980) 513–520.[16] P. Goria, I. Orue, M.L.F. Gubieda, F. Plazaola, N. Zabala, J.M.

Barandiaran, J. Magn. Magn. Mater. 157–158 (1996) 203–204.