INTERNAL FRICTION AND MAGNETIC LOSSES IN CoPt ALLOY DURING PHASE TRANSITION

Transcription

1 INTERNAL FRICTION AND MAGNETIC LOSSES IN CoPt ALLOY DURING PHASE TRANSITION E. KLUGMANN, Depatment of Solid - State Electonics, Technical Univesity of Gdansk, Poland B. SOVILJ, Faculty of Technical Sciences, Univesity of Novi Sad, SCG S u m m a y The vaiations of intenal fiction, magnetic susceptibility and loss angle with time and tempeatue of disoedeed CoPt equi - atomic alloy, duing annealing induced phase tansition fom cubic to tetagonal ae shown. The pape pesents esults of investigation of magnetic and anelastic elaxation of as-quenched CoPt alloy measued fom K to 8 K vesus annealing induced odeing stating fom 6 K. Intenal fiction and magnetic susceptibility/ time and tempeatue signatues can enable a bette undestanding of odeing pocesses and mechanisms.. INTRODUCTION Atomic odeing can affect the anelastic and feomagnetic popeties of metals and alloys. The intenal fiction (IF) level, the initial magnetic susceptibility, its magnetic afte-effect (MAE) and magnetic loss angle can be used as an indication of the degee of ode. CoPt has a cubic (A) stuctue in the asquenched (disodeed) phase. It tansfoms duing annealing into the tetagonal (Ll ) phase. The tansfomation is of diffusion like type and can be attibuted to the odeing pocesses and establishing of shot - ange ode. In the disodeed phase CoPt exhibits a high level of IF (magneto- elastic damping) and soft feomagnetic popeties, while in the odeed phase it show a low magnetic susceptibility and IF level. The aim of the pesent pape is to investigate the change of IF, magnetic susceptibility and loss angle duing continuous o successive annealing of disodeed samples fom the oom tempeatue up to 8K. Reason fo this type of eseach is to study the magnetic and anelastic popeties of pue, equiatomic CoPt alloy by contolling the heat teatment and tempeatue duing the odeing and phase tansition pocesses.. EXPERIMENTAL PROCEDURE The measue of IF in the case of a wie is the logaithmic decement δ of fee tosional vibations of a CoPt sample: δ Q = () π The inveted tosion pendulum is woking at low fequency, f =6 Hz, and at stain amplitude of. -. Electonic time with the accuacy of the ode of µs has been used. Magnetic measuements - pefomed by diffeential ac technique based - on a Wilde bidge [] at a fequency of khz in a field diected paallel to the wie has been also done. Afte demagnetization of the sample at t = s in ac field of about Am -, MAE measuements stated at t = s and ae continued up to 8 s. The measuing field amplitude of. Am - has been chosen. 8 th Intenational Tibology Confeence, Oktobe., Belgade, Sebia 9

2 Expessions fo the loss angle χ'' tan ϕ = () χ' following fom the complex initial magnetic susceptibility and fo the MAE : χ = χ' iχ'' () = ( t,t) ( t,t) ( t,t) () ae used, whee the eluctivity = /χ and χ is the eal pat of the complex initial susceptibility; t = s, and t vaied fom s to 8 s; T is the tempeatue of the sample.. Sample pepaation A high puity, equi- atomic CoPt (5 at % Pt) sample in the fom of a wie, the diamete of. mm has been sealed off unde a vacuum of - 5 Pa in a quatz tube, annealed above the citical tempeatue of K (see Fig. ) fo five hous, and than ice - cold wate quenched in ode to obtain the disodeed fcc phase. IF and magnetic measuements have been taken on the as- quenched (disodeed) samples duing heating up to the Cuie point. The ate of tempeatue change duing these measuements has been. K/min. In samples quenched fom tempeatue T> T C, a high IF level is obseved in Fig.. The high intenal fiction level in as-quenched samples is due to the magneto-mechanical damping of the Bloch domain walls as obseved in the mico-scale, o due to the losses caused by the magneto- elastic hysteesis as measued in the maco- scale[]. Q -. 8 T c T[K] Fig.. The change of IF level of CoPt alloy as function of the quenching tempeatue. T C is the citical tempeatue [].. RESULTS AND DISCUSSION The IF measuing esults fo two diffeent heating ates of CoPt duing odeing ae shown in Fig.. The high level of intenal fiction as seen in Fig. at T < 5K is obseved afte quenching the samples fom K to 7 K. Between K and 5 K, IF changes only a little, but fom 5K to 65 K, it deceases significantly - up to the level of patly odeed sample. In the second tempeatue egion, whee the odeing pocess stated, we see in Fig., a maximum of / and simultaneously a decease of the eal pat of the initial susceptibility χ. 9 8 th Intenational Tibology Confeence, Oktobe., Belgade, Sebia

3 Q Fig.. The tempeatue dependence of IF of disodeed CoPt alloy fo two diffeent heating ates: K/min. - cuve, and 6 K/min. - cuve. Cuves and epesent damping fo satuated and odeed sample espectively [6]. (%) χ ' 8 s 5 s s 9 s χ ' s s 8 s 6 s s Fig.. The magnetic afte-effect and the initial magnetic susceptibility of disodeed CoPt sample vesus annealing tempeatue duing a un fom K to 556 K, as detemined at t = s afte demagnetization; t vaied fom s to 8 s. 8 th Intenational Tibology Confeence, Oktobe., Belgade, Sebia 95

4 ' χ 5 χ' χ ' A 75 B Fig.. Vaiation of the eal pat of initial susceptibility of disodeed CoPt sample, measued duing two successive annealing uns A and B; χ - at t =s and χ - at t =min. afte demagnetization [7]. We suppose that duing annealing of disodeed sample in this step of odeing the fozen in vacancies, poduced duing quenching, ae moving to the patially odeed egions, inceasing the level of ode, as obseved by the initial susceptibility measuement duing two successive annealing uns: A fom K to 556 K and un B fom K to 65 K as seen in Fig., and as calculated in pape [] and [5]. The maximum of the initial susceptibility obseved at about 6 K anneal and has completely annealed out by measuements in the un B. In [5] we have shown, that the MAE maximum at 5 K in Fig. is due to the otation of the easy magnetization vecto fom [] to the [] axis, its position depends upon the heating ate and it disappeas in the couse of the two successive annealing uns A and B as seen in Fig.. The Richte type, evesible magnetic elaxation maximum at 5 K, seen in Fig. has an activation enegy of E R =. ev, and is ascibed to the eoientation of Co - Pt atomic pais [6]. The high tempeatue IF peak at 7 K, whose amplitude incease with deceasing heating ate, as seen in Fig., and huge incease of magnetic losses, seen in Fig.5, ae due to a stuctual elaxation pocess that occu duing annealing induced phase tansition fom cubic to tetagonal. In Fig. we obseve at T> 8 K a high tempeatue backgound (HTBG) of IF. The HTBG is highe fo highe heating ates. The chaacteistics of the backgound ae basically dependent fom inteaction between dislocations, point defects and othe defects like esidual fccphase o stacking faults. By applying a steady magnetic satuation field it is possible to educe the IF to the level of satuated magneto-mechanical damping (cuve in Fig.), which is vey close to the IF level of odeed sample (cuve in Fig.). The backgound of IF follows the function: E Q BG = a exp (5) kt with E=.5 ev fo Q - BG =. - at T = 5 K, and a = /ωτ [] at τ = - s and ω=πf = 77 Hz. As seen in Fig.5, at T >5 K, the stuctual changes stated influencing the incease of the magnetic losses th Intenational Tibology Confeence, Oktobe., Belgade, Sebia

5 5 tan ϕ Fig.5. Loss angle vesus annealing tempeatue of disodeed CoPt alloy afte quenching the sample fom 5 K to 7 K.. CONCLUSION Annealing fom K up to 8 K of disodeed by quenching equi- atomic CoPt alloy esults in: - phase tansition fom cubic to tetagonal, - decease (about ten times) of the initial magnetic susceptibility, - apid, themally - activated eoientational elaxation of Co-Pt atomic pais, leading to shot- ange ode, - a slowe, migational, stuctual odeing pocess, which esults in eduction of Co - Co atomic pais with feomagnetic bonding, leading to long - ange ode. Ou appoach could be extended up to the Cuie point (T A = 8 K) [7], theefoe we could obseve a huge incease of magnetic losses (in Fig.5). 5. REFERENCES [] E. Klugmann, H.J. Blythe, F. Walz, Investigation of Themomagnetic Effects in Monocystalline Cobalt nea the Matensitic Phase Tansition, phys. stat. sol (a) 6 (99), 8-8 [] W. Chomka, Intenal Fiction in Odeed Mg- Cu, Ag-Mg and Co-Pt Systems, Sci. Papes of the Technical Univesity of Gdansk (Poland), Fizyka, No 76 (97), -66 [] W. Chomka, The Mechanism of Intenal Fiction in Metals, Sci. Papes of the Univesity of Katowice (Poland), No 65 (977), 8- [] E.V. Gomonaj, E. Klugmann, Atomic and Magnetic Odeing in CoPt Alloy, J. Appl. Phys. 77 (5) (995), 6-65 [5] E. Klugmann, Relationship Between Stuctue and Intenal Fiction in CoPt and FePd Alloy, in: Fundamentals of Tibology and Bidging the Gap Between the Maco- and Mico/ Nanoscales, Ed. B. Bhushan, Kluwe Academic Publishes, NATO Sci. Seies, Pinted in Nethelands,, pp. 99- [6] E. Klugmann, H.J. Blythe, B. Augustyniak, Kinetic of Odeing in Equi-Atomic CoPt Alloys, J. de Phys. Colloq. C8, 8 (987), 5-57 [7] E. Klugmann, H.J. Blythe, Atomic and Magnetic Odeing in CoPt Alloys Studies by Magnetic and Intenal Fiction Measuements, J. Magn. Magn. Mate. (99), 99-8 th Intenational Tibology Confeence, Oktobe., Belgade, Sebia 97