Structure and magnetic properties of mechanically alloyed SmFe2Clive D. Milham Citation: Journal of Applied Physics 75, 5659 (1994); doi: 10.1063/1.355628 View online: http://dx.doi.org/10.1063/1.355628 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/75/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of Mn substitution for Fe on magnetic and magnetostrictive properties of SmFe2 compound J. Appl. Phys. 111, 07A901 (2012); 10.1063/1.3669914 Magnetic properties of amorphous Sm–Fe and Sm–Fe–B thin films fabricated by radio-frequencymagnetron sputtering J. Appl. Phys. 83, 7270 (1998); 10.1063/1.367616 Magnetic properties and microstructure studies of Sm–Fe magnetic thin films J. Appl. Phys. 81, 328 (1997); 10.1063/1.364114 Investigation of crystallization of a mechanically alloyed SmFe alloy J. Appl. Phys. 71, 6146 (1992); 10.1063/1.350423 Magnetic properties of a metastable SmFe phase synthesized by selectively thermalized sputtering J. Appl. Phys. 55, 2611 (1984); 10.1063/1.333253

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Structure and magnetic properties of mechanically alloyed SmFe2 Clive D. Miiham Research Cfentre j%r Advanced Mineral and hfateriais Processing, Tlze University of Western Australia, Nedlands \%I &Xl?, Awifralia

Mechanical alloying has been used to prepare SmFe,, from p0wdere.d Sm and Fe. A maximum remanence iif,.Z of 40.9 emu/g and a maximum coercivity &fc of 3.09 kOe were measured for samples anneaIed at 500 and 600 “C, respectively. Above these temperatures both &fri and ,,H, decrease monotonically with increasing grain size. Annealing at temperatures between 500 and St10 “C lead to the formation of second-phase SmFe, affecting saturation magnetization M, , which ranges from 55.7 to 61.9 emu/g, and resulted in constriction of the hysteresis curves. Activation volumes tl were determined from the results of magnetic viscosity measurements and range from 3.3 X 1 (I- Is to 10.5 X .I 0 - ‘s cm3 for samples annealed at 500 and 800 “C, respectively. An estimate of the anisotropy energy coefficient k’ of 5.6X10” ergs/cm for mechanically alloyed SmFe, was calculated using an estimate of the exchange energy coefficient A and the experimentally determined value of v .

1. INTRODUCTION

Mechanical alloying has been extensively investigated as a method for the synthesis of permanent magnetic materials.“” Research into the production of rare-earth tran- sition metal alloys indicates that not only is mechanical al- loying a useful method for materials preparation due to the small number of total production steps compared with other techniques, it also has a pronounced effect on the magnetic properties of such materials. Very high coercivities, rema- nence enhancement effects, and large energy products have been reported.‘,”

SmFe? is one of the highly magnetostrictive RE-Fe2 compounds which have attracted considerable research interesL5 Like commercially available anisotropic magneto- strictive alloys, SmFe, is usually prepared by conventional melting and solidification techniques. In comparison. the me- chanical alloying process may prove to be a useful and cost effective method for the production of isotropic polycrystal- line magnetostrictive materials. To this end, the effect of post-milling heat treatment on the structure and magnetic properties of mechanically alloyed SmFeZ is reported here.

Il. EXPERIMENTAL

Starting materials Sm E’99.9%, -40 mesh) and Fe (99.9$& -3% j mesh) were sealed in a cylindrical hardened steel vial under an argon atmosphere with ten 12.7-mm-diam hardened steel balls. A 10% excess of Sm was added to knit the formation of phases more iron rich than SmFe, and a ball to powder nmss ratio of J.0 was used. The powders were milled for 24 h in a Spex 8000 mixer/mill and after comple- tion toluene was added and the mixture was milled for a further 5 min to increase recovery. As-milled powders were annealed in vacuum-seal& silica tubes for 2 h at tempera- tures TG1 in the range sOi&-801) “C.

Samples were studied using a Siemens D5000 x-ray dif- fractometer with monochromatic Cu Kcu radiation. Magnetic measurements were made at 298 K using a vibrating sample magnetometer in conjunction with a 5 T superconducting solenoid.

Measurement of magnetic viscosity was carried out and the magnetic viscosity coefficient 12. was determined accord- ing to a phenomenological theory of magnetization kinetics.b Magnetic viscosity is due to thermal activation of domain processes which are responsible for irre.versiblc changes in magnetization. The. activation Ltnergy barriers over which thermal activation occurs are. functions of field and the acti- vation volume v is the fundamental volume swept out as the activation barrier is overcome. u can be related to A by the following equation:

A=-$. SP

where k=Boltzrndnn’s constant and ~W,~=spontaneous mag- netization. Values of u were determined for samples an- nealed at 500, 600, 700, and 800 “C.

Ill. RESULTS AND DISCUSSION

The x-ray diffraction (XRD) patterns for the as-milled and heat-treated powders are shown in Fig. 1. The as-milled

A SmFe, D smo, &4 + SmFe, . u-Fe

3.5 3 2.5 2 1.5 1 d (A)

FIG. 1. S-ray diffraction patterns of mechanically alloyed and heat treated SmFe, . (* Sample not milted in toluene.)

J. Appl. Phys. 75 (IO), 15 May 1994 0021-8979/94/75(10)/5659/3/$6.00 8 1994 American Institute of Physics 5659 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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-as-milled -----T,=500T

I 5 10 15 20

Hi (kOe)

FIG. 2. Hvsteresis loops and initS magnetization curves of as-milled and annealed samples.

powder shows a broad peak corresponding to the major dif- fraction line of n-Fe. No other phases are present in the diffraction pattern. Annealing for 2 h at 500 “C crystallized the SmFez phase and produced large changes in the materi- al’s magnetic properties which can be seen in Figs. 2 and 3. Coercivity IlfHc was found to more than double from 0.58 to 2.1 kOe with an associated increase in remanence ,V,.i from 23.5 emu/g to a maximum value of 40.9 emu/g.

Saturation magnetization M, remains relatively unaf- fected by the annealing treatment and ranges from a maxi- mum value of 61.9 emu/g for the as-milled powder to a minimum of 55.7 emu/g for the sample annealed at 700 “C. This compares favorably with the value of M,=59.7 emu/g reported for SmFez by Dublon et ~1.~ While the variation in M,Y is slight, the effect of annealing on phase formation is significant. Annealing enables the free cu-Fe present in the as-milled powder to combine to form the SmFe2 phase thereby lowering M, slightly. Loss of Sm by vaporization for T,~=600 “C leads to the formation of SmFe, as a second phase. SmFe, possesses an M,, much larger than that of

0.7

g oJ3 - .^ 4 0.5 -

0.4 - L i I I I ' '20

0 200 400 600 800 T, K3

FIG. 3. Magnetic properties of as-milled (7’, -0 “C) and annealed samples.

-8;

4,

B Y 3-

-6;

-4

11 ’ , 1 I t 12 0 2m 450 600 800

T, (*Cl

FIG. 4. Average activation volume u and anisotropy energy coefficient K of as-milled (T, = (1 “C] and annealed samples.

SmFe, (Ref. 8) and consequently M, increases with increas- ing SmFe3 concentration as T, is raised to X00 “C.

Annealing at 600 “C produced the largest coercivity, MH,=3.09 kOe. *[Hc was found to then decrease monotoni- cally with increasing T, and this can be attributed to the increase in grain size. As a result ,H, decreases with the decreasing density of centers impeding magnetization rever- sal. This is evident in the changing shape of the initial mag- netization curves in Fig. 2.

The XRD pattern indicate.s that a small proportion of SmO-C phase’ formed as a result of the brief milling in toluene. The diffraction pattern of a sample mechanicaLLy alloyed without toluene and annealed at 800 “C showed the same crystalline structure without the presence of the SmO-C phase. Magnetic properties were not significantly af- fected by its presence nor did its inclusion affect magnetiza- tion mechanisms.

When the as-milled material containing a 10% excess of Sm is annealed above its eutectic temperature of 720 “C (Ref. 10) liquid Sm is expected to form. The high concentra- tion of SmFe, indicated by XRD did not result in a large change in the saturation magnetization of the bulk material indicating that the I:3 phase tends to form at the surface of the sample where preferential loss of Sm results from vapor- ization. While the amount of SmFe, in the bulk material remains small, complex two phase magnetic behavior results and the hysteresis loops become increasingly constricted as T, increases. Two phase magnetic behavior was also indi- cated by a second maxima in the intrinsic irreversible sus- ceptibility xi,, at an applied field of -- 10 kOe which is close to the reported coercivity of mechanically alloyed SmFe, .ll Demagnetization processes become less reversible with increasing SmFe3 concentration and remanence de- creases accordingly.

Activation volumes determined from magnetic viscosity measurements are plotted against T, in Fig. 4. The activation volume for the as-milled material is 4.4X lo-‘” cm.:. Anneal- ing at 500 “C decreases u to 3.3X 10 -rs cm3. This may be due to microstructural changes in the bulk material which cannot be confirmed without detailed microstructural infor-

5660 J. Appl. Phys., Vol. 75, No. 10, 15 May 1994 Clive D. Milham [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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mation. v changes little for TUG600 “C but as the amount of second-phase SmFe, is increased by annealing at higher tem- peratures, u increases considerably to a value of 10.5X lo-” cm at 800 “C. The magnitude of u is very similar to that reported for DyFe2 prepared by mechanochemical reduction’” and is estimated to be an order of magnitude less than that of bulk Tb,,,,Dya.;l.;Fez at room temperature.13 This suggests that the microstructure arising from preparation by mechanical alloying may have a considerable effect on the mechanisms responsible for magnetization in these materials, Further study is required to confirm this.

‘The activation volume of a ferromagnetic material can be related to the domain wall thickness 6 by 8 3=~.14 This relationship and the expression for 8 of a ferromagnetic ma-

- teriai with a simple cubic crystal structure S = r+t/K were used to provide an estimate of the first anisotropy energy coefficient K for SmFe2. A value for the exchange energy coefficient A= 1.26X10-” erg/cm was estimated from the Curie temperature Tc:.=6’75 K and lattice parameter R =7.417 A (Ref. Cl) using the expression A = (kT,)/n. The estimated variation in anisotropy energy for the samples studied is plot- ted in Fig. 4 which gives an estimate of K=5.6X lo6 ergs/cm for mechanically alloyed and heat treated SmFe,. The esti- mated domain wall thickness for a sample annealed at 500 “C is -15 nm which, when compared to an average crystallite size determined by transmission electron micros- copy iTEM of 80 nm, indicates that these materials are mul- tidomain.

IV. CONCLUSIONS

The preparation of single-phase SmFe, by mechanical alloying is dependent upon post-milling annealing condi- tions, Some SmFes crystallizes in the material and is thought to form by preferential loss of Sm at the sample surface despite the addition of Sm in excess of stoichiometric re- quirements. .M, remains unaffected by the formation of SmFe, indicating that it is only present as a minor phase. The concentration of SmFes increases with annealing temperature and gives rise to two phase magnetic behavior which mani- fests itself in constriction of the hysteresis loop and a second maxima in xf, at a field corresponding to the coercivity of the harder SmFea phase. Annealing at 600 “C produces a maximum #,=3.09 kOe which then decreases monotoni- cally with increasing annealing temperature. This decrease in

MHc correlates with the increase in grain size brought about by annealing at higher temperatures. Annealing at 500 “C produced a maximum value of Mrvi=40.9 emu/g with a cor- responding M,iIM,=O.69. Demagnetization was found to become less reversible with increasing SmFea concentration and as a result M,,i decreases with increasing T, .

Activation volumes ranging from 3.3X10-‘s cm3 for a sample annealed at 500 “C to lOSXlO~‘* cm for a sample annealed at 800 “C we.re determined from the results of mag- netic viscosity tests. The magnitude of u is similar to that of DyFe, prepared by mechanochemical reaction and an order of magnitude less than that of bulk Tb0.27Dy0.73Fe2. This suggests that similar magnetization mechanisms dev>oF% Laves phase rare-earth iron alloys prepared by mechanical alloying. The anisotropy energy coefficient K of SmFe, pre- pared by mechanical alloying is estimated to be of the order of 5.6X 10” ergs/cm.

Measurement of the magnetostriction of mechanically alloyed materials is currently in progress.

ACKNOWLEDGMENT

T would like to thank L. Folks for use of the software developed to analyze the results of magnetic viscosity tests.

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Magn. Mater. 116, L320 (1992). ‘J. Ding, P. G. McCormick, and R. Street, J. Magn. Magn. Mater. 124, LI

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