KINETICS OF ORDERING IN EQUI-ATOMIC CoPt ALLOYS

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KINETICS OF ORDERING IN EQUI-ATOMIC CoPt ALLOYS

E. Klugmann, H. Blythe, B. Augustyniak

To cite this version:

E. Klugmann, H. Blythe, B. Augustyniak. KINETICS OF ORDERING IN EQUI-ATOMIC CoPt AL- LOYS. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-513-C8-517. �10.1051/jphyscol:1987880�.

�jpa-00227184�

KINETICS OF ORDERING IN EQUI-ATOMIC Copt ALLOYS E. KLUGMANN, H.J. BLYTHE* a n d B. AUGUSTYNIAK

Institute of Electronic Technology, Technical University of

~ d d s k , PL-80-952 ~ d a s s k , Poland

" ~ e p a r t m e n t of Physics, University of Sheffield, GB-Sheffield 537

R M ,

Great-Britain

On a recherche le frottement intdrieur et le trainage magndtique qui se trouvent dans un alliage en d6sordre de Copt. La gamme de temperature etait 300K B 840K. Que corrglation dtroite a 6td dtablie entre le procgdd de mettre en ordte et le changement des procbs comportments englastique et ferromagn6tique. Audessus de 400K il y a deux procSdSs qui se passent: la mise en ordre atomique activee

thermiquement et la mis en ordre magnetique. Ces procgdgs conduisent ?i la fois 2 la diminution temps dependante de l'amortisement magndtoelastique et pour la

d'esaccommodation magngtique.

Abstract

Internal friction and magnetic after-effect have been investigated in a disordered, equi-atomic Copt alloy in the temperature range 300K to 840K. A close correlation between the ordering process and the change of anelastic and

ferromagnetic properties has heen established. Above 400K two processes occur:

thermally activated atomic and magnetic ordering. These processes are responsible for the time-dependent decrease of the magnetoelastic damping and for the magnetic disaccommodation.

INTRODUCTION

Neutron diffraction studies of ordering kinetics in the CoFe alloy system show that the highest rate of ordering occurs in the case of the equi-atomic structure [I]. The equi-atomic alloy Copt has the highest critical temperature (1100K) and exhibits, in the "as-quenched" fcc phase, soft ferromagnetic properties [ 2 ] . The ordered Copt alloy belongs to the hardest ferromagnetic materials. In order to obtain comprehensive information about atomic and magnetic ordering in "as-quenched"

Copt samples we can investigate the dependence of various physical phenomena such as internal friction and magnetic after-effect on the ordering process.

Magnetic properties of the disordered Copt alloy have been previously reported by several workers [3-71; however, there are only few reports of magnetic

after-effect measurements in this alloy [8]. The aim of the present work is to investigate the ordering process which occurs during heating up to the Curie point by means of magnetomechanical damping, shear modulus and magnetic relaxation measurements.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987880

JOURNAL

DE

PHYSIQUE 2 MPERIMENTAL PROCEDURE

Sample preparation

A high purity equi-atomic polycrystalline Copt alloy was prepared by melting together 5N Co and 4N Pt obtained from Johnson Matthey Chemicals Ltd. The Co had a toial metallic impurity content of less than 13 ppm and the Pt had impurity levels of Zn, Si, Cu, Rh

<

⁵ ppm and Ir

<

50 ppm. Samples, in the form of wires 1 mm diameter, were sealed- off under vacuum in quartz tubes, annealed for 5 hours at 1430K and water-quenched.

Measuring techniques

Internal friction measurements were made using an inverted torsional pendulum at a frequency of 60Hz and the strain amplitude was ZXIO-~. During measurements, the temperature of the "as-quenched" sample was increased at a constant rate of heating and a steady magnetic field of 1x10-= Am-I was applied to saturate it. The shear modulus, G, was determined by means of frequency measurements (G

-

^{f2). The}

magnetic after-effect was observed as a time decrease of the real component of the initial susceptibility x(t,b) [ 9 ] . The results are presented as isochronal relaxation curves of the susceptibility

(x)

or reluctivity (r). Ar = r(tp,T)

-

r(tl ,T), where r = 1/x. (tl = 1s and t2 varied from 4s to 1800s).

3 RESULTS AND ANALYSIS

At room temperature, the disordered sample exhibits high magnetomechanical damping strongly dependent on the amplitude of vibration and the magnetic field intensity [ 7 ] . Fig 1 shows the internal friction curves and the relative change of

Temperature

IK)

+

Fig 1 The temperature dependence of internal friction and shear modulus defect for two different heating rates: 1 : 2K/min, 2 : 6K/min; curves 3 and 4 are damping for saturated and for ordered samples respectively.

shear modulus during heating. The internal friction of "as-quenched" samples was measured at two heating rates: 2Kfmin (curve 1) and 6K/min (curve 2). By applying a steady magnetic saturation field it was possible to reduce the magnetomechanical damping (curve 3) to the level observed in the ordered alloy (curve 4). The damping decreases for T

>

400K and increases f0r.T

>

800K. The temperature change of damping is greater for the slower heating rates. This means that the

magnetomechanical damping is time-dependent. The small temperature increase of damping observed for the saturated and ordered sample is due to the background internal friction. Above the Curie point the magnetomechanical damping disappears and curves la and 2a exhibit the same temperature dependence as curves 3 and 4.

Temperature

( K ) -

Fig 2 Variation of ~-l(Ti) and f2(Ti) during anneal: 1 : t = 3s, 2 : t = 15 min after demagnetization, 3 : after a second demagnetization.

Fig 2 shows the isochronal curves of internal friction and shear modulus of an

"as-quenched" sample as measured during a step-wise annealing programme: 1

-

immediately after attaining the required temperature and following a demagnetization, 2

-

after 15 min and 3

-

immediately after a second

demagnetization. A str6ng AC magnetic field was applied during the increase from one temperature to the next. One can see from these curves that the decrease of magnetomechanical damping is only partially reversible.

150 125 100

t

⁷⁵

X SO 2 5

0 300 LOO 500 600 700 800

Temperature (K1

-

Fig 3 Spectra of susceptibility during annealing: 1 : t = Is, 2 : t = 30 min after demagnetization, 3 : after a second demagnetization.

Fig 3 gives the results of similar measurements of the initial susceptibility of a disordered sample heated from room temperature up to 840K and then cooled to 706K. The susceptibility was measured: Is after demagnetization

-

curve 1, at 1800s

-

curve 2 and 1s after a second demagnetization

-

^{curve 3.} In the temperature range from 450K to 560K a significant, irreversible magnetic after-effect is observed.

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PHYSIQUE

Temperature ( K I

-

Fig 4 Isochronal relaxation spectrum of a disordered Copt sample.

(tl = ls, t2 varied from 4s to 1800s)

Fig 4 shows the isochronal relaxation spectrum of an identical sample. Above 430K the magnetic relaxation intensity increases and reaches a maximum at 530K. A quantitative analysis of this maximum is difficult as an anneal occurs during the course of measurement. After subtracting the linear background relaxation and using the frequency factor fo = 1013 Hz (equivalent to the Debye frequency of Copt) it was possible to determine the activation energy of this relaxation process, but only for the low-temperature flank and for short relaxation times (tg

<

128s); a value of Q = 1.4 eV was obtained [8]. A second relaxation process occurs at 700K; this is superimposed on the shoulder of the rapidly increasing slope of a third maximum at 816K just below the Curie point. During cooling from 840K to 700K a small

reversible relaxation peak is observed at 780K.

Results of susceptibility measurements obtained for a disordered sample during step-wise heating from 300K up to maximum temperatures incremented by about lOOK are given in fig 5. It can be seen that the magnetic after-effect maximum observed at 530K (fig 4) anneals out during heating up to 556K (second sweep, B).

Temperoture I K I ⁺

Fig 5 Variation of initial susceptibility of a disordered Copt sample during successive annealing runs: o : t = Is, : t = 30 min.

properties of the disordered Copt alloy has been established by investigation of the annealing behaviour of the magnetomechanical damping and the magnetic after-effect.

From an inspection of the results, we conclude that there is a close correlation between anelastic and magnetic properties of Copt alloy at the first stage of ordering. The development of the long-range order at this stage is small because of the low annealing temperature; there are also only few thermal vacancies. Electron microscopy [lo] shows that in quenched Copt samples, small partially-ordered regions

exist of diameter 208 to 308; these increase in size up to 408 until the final ordered state at 870K is obtained. The decrease in the time-dependent

magnetomechanical damping, which occurs at temperatures from 400K to 650K, can be analysed in terms of the decrease of the domain wall mobility. This is caused by magnetic, directional ordering in the adjacent domains and by an increase of the

long-range order in the partially-ordered regions. The magnetic relaxation is due to a rapid, short-range, thermally activated process (reorientation of Copt pairs) and to a long-time and long-range point defect migration leading to a long-range order. The second magnetic relaxation peak at 700K (fig 4) is related to an internal friction maximum (fig 1). This maximum is acompanied by an inflection of the shear modulus defect. The general conclusion is that the magnetomechanical internal friction and the initial susceptibility provide very useful information, especially about the first stage of the ordering process in this alloy.

REFERENCES

[I] Deringin, E E, Lotkow, A J, Gaidikowa, L J, Izwiestia WUZOW, Fizika (USSR), 23 (1980), 76.

- [2] Newkirk, J B, Geisler, A M, Martin, D L, Smoluchowski, R, J Appl Phys,

2

(1951), 290.

[3] Inden, G, Alloy Phase Diagrams, Symposium held 1982 in Boston (USA), ed Moscow,

"Mir", 1986, p114-127.

[4] McCurie, R A, Gaunt, P, Phil Mag,

12

(1966) 567.

151 Eurin, P, Pauleve, J, IEEE Trans on Magnet, (1969), MAG-5, 216.

[6] Rouchy, A, Waintal, A, Solid State Com,

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(1975), 1227.

[7] Augustyniak, B, Chomka, W, Acta Physica Pol,

2

(1983), 515.

[8] Klugmann, E, Blythe, H J, Augustynik, B, "Internal friction and Magnetic Relaxation in Solids", ed Silesian University, Katowice (1985), p209-219.

[9] Blythe, H J, phys stat sol (a),

92

(1985), 193.

[lo] Eurin, P, Thesis, University of Grenoble, France (1973).