Laser parameters L aser w avelength 532 nm
7.6. The effect of composition on the MR of thin films
Figure 7.10 shows the applied field dependence of the m agnetoresistance at 4.2K for the CoxAgi.x films grown at a fixed fluence of 3J/cm^ and annealed at 150°C for 15 m inutes for com positions w here x=0.10, 0.30 and 0.50. The largest M R ratio was obtained for values o f x = 0.30, suggesting that there exists an optim um com position for approaching a maximum magnetoresistance values.
0
-10 -20 i -30O f t
ft
o
o
•
#
o
° D « °□
□
□
• x = 0 . 1 0 o V .□
□ x = 0 .3 0 □ O x = 0 .5 0 ^ -40 -50 -5 -3 - 1 0 1 3 5 H (T e sla )Fig. 7.10. The applied fie ld dependence o f the m agnetoresistance o f Coy^Agj.x film s deposited at room temperature and annecded at 150°C for 15
m inutes fo r x = 0.1, 0.3, and 0.5 at 4.2K.
The tem perature dependence o f the m agnetoresistance for these three COxAgi_x films under a m agnetic field o f 4.7 T is show n in F i g .7 .1 1. It can be seen that the m agnetoresistance ratios for all films decrease with increasing temperature. W hile the samples exhibit attractive m agnetoresistance values at reduced temperatures, the values at 290K are negligible.
C h apter Seven Growth o f m etallic gran ular Co-Ag film s by pu lsed laser deposition 40 o o
0.10
0.50 100T em p eratu re (K)
200
Fig. 7.11. The temperature dependence o f the m agnetoresistance o f film s f o r X = 0.1, 0.3, and 0.5. E u Cl ’> '7 D 35 30 25
20
15 10 5 0 100 Co volume fraction (x)Figure 7.12. The values o f resistivity as function o f Co conten t in vol.% measured
or 300/^.
The dependence o f resistivity ( p o ) on the volum e fraction is shown in Fig. 7.12. The value of initially rises when Co is first introduced, reaching a plateau for a broad range of Co contents and decreases towards the pure Co limit. The measured resistivity is
C h apter Seven G rowth o f m etallic gran u lar Co-Ag film s by p u lsed laser deposition
co n sisten t w ith w hat is know n about the m agnetic b eh avio ur o f Co. B u t w hat d is tin g u is h e s the G M R e ffe c t e x h ib ite d in C o -A g g ra n u la r m a te ria ls from m agnetoresistance (M R) appeared in m agnetic m etal Co. B oth G M R in C o-A g and A M R in C o are alw ays n egative, i.e., resistan ce d ecreases w ith in creasin g field. H ow ever G M R in Co-A g is irrespective o f the direction o f the applied m agnetic field w ith respect to the current, A M R in Co is sensitive to the direction o f applied m agnetic field. Also nearly 50% change in resistivity under the field for Co-A g system is much larger than that o f 0.5 ~ 1% change in resistivity for pure Co.
7.7. D iscussion
The m echanism s responsible for the G M R observed in the CoxAgi_x system , as with m agnetic multilayers, can be attributed to spin-dependent electron scattering with or at a heterogeneous interface. The M R ratio is largest for a particular com position and optim um annealing step, and is m ost likely associated w ith the size o f the particles form ed in the film s. T herm al annealing w ill act to reduce the degree o f structural disord er, en larg in g the p article size and in creasin g the in terp article separation. Consequently the electron mean free path will increase, causing the absolute resistivity, p, to decrease. A nnealing at higher tem peratures can also suppress the field-dependent change in resistivity, p, m ainly due to the reduction o f the spin-dependent scattering also as a result o f the enlarged particle size.
T he com position dependence o f the m ag netoresistive behaviour ob serv ed in this m agnetically dilute system can be associated w ith the concentration o f m agnetic scattering centres. W hen the Co concentration in the CoxAgi.x granular films increases from X = 0.10 to 0.30 the M R increases and reaches a m axim um value. H owever, with further increases in x, the M R strongly decreases, and approaches a very small value. This effect can be related to increasing coalescence of the magnetic particles, and hence a reduction in the num ber o f scattering centres, resulting in a decrease in the scattering of spin-dependent electrons. O ur results are consistent with the phenom enological model for heterogeneous systems recently presented by Berkow itz et al.[7.1].
T he tem perature dependence o f the m agnetoresistance o f the C o-A g film s can be associated with phonon scattering. The resistivity o f a granular m etal can be expressed as [7.7]:
C h apter Seven G rowth o f m etallic gran ular Co-Ag film s by p u lse d la ser deposition
w here po is the tem perature-independent contribution from crystal defects, p p h o n o n ( T ) is the temperature dependent phonon contribution and is quasilinear in T; M and Ms are the field dependence o f the m agnetisation and the saturation m agnetisation values respectively, F is a function o f (M /M s)2, and p m ( T ) is the contribution due to scattering o f the conduction electrons from the ferrom agnetic Co particles and their interface responsible for GMR. The fractional G M R is:
M R=100% X { Pm(T) F[(M /M s)2]/[ po - k P p h o n o n ( T ) + Pm(T)]} ( 2 )
w h e re p p h o n o n ( T ) is strongly dependent, and p m ( T ) only w eakly dependent, on tem perature [8.7]. Therefore m ost o f the tem perature dependence o f the resistivity p ( H , T ) is carried by the p p h o n o n ( T ) factor, explaining why the M R decreases with increasing temperature.
The highest m agnetoresistance ratio found in our studies, for film s deposited at a laser fluence of 3J/cm2, is m ost likely determ ined by the onset o f phase segregation. PLD, in contrast with conventional sputtering, is associated with m uch higher quenching rates as well as very high instantaneous deposition rates. A lthough the kinetic energies o f laser ablated particles are o f the same order of m agnitude as those o f conventionally sputtered particles (around 10 to 100 eV ), the in stan tan eo u s grow th rate during PLD is substantially higher than the grow th rate during sputter deposition as exhibited in F ig .2.4 [7.8]. These properties enable the form ation o f solid solutions with a very high degree o f saturation and produce phase segregation. C onsequently electrons will experience more scattering from the rougher interface between the magnetic particles and the metallic matrix, and thus enhance the m agnetoresistance values.
The decrease in the m agnetoresistance ratio for laser fluences above 3J/cm2 can be attrib u ted to the effect o f the p a rtic le sizes fo rm ed b ein g asso ciated w ith a superparam agnetic phase. A t high laser fluences (>3J/cm2), the large vapour density formed directly above the ablated target surface is generated in the first few nanoseconds o f the laser pulse, and creates a shielding plasm a layer. The interaction betw een the rem aining portion o f the laser pulse and the plasm a m ay result in the ionisation o f the m olecules, m olecular ions, and even clusters in the plasm a [7.9]. This effect w ould be expected to break up the large ejected particles into sm aller ones, leading to sm aller particles being deposited on to the films. The effect of higher fluences on the formation o f sm aller particles has been confirm ed in our X R D results as show n in F ig. 7.4. Although the small particle size favours an increase in the concentration o f magnetic
C h apter Seven G rowth o f m etallic gran ular Co-Ag film s by p u lsed la ser deposition
scattering centres, the magnetic particles tend towards becom ing superparamagnetic with further decreases in particle size [7.5]. The random ly oriented small m agnetic mom ents are m ore difficult to align with an external m agnetic field, and hence the M R strongly decreases.