Im provem ents in spin v a lv e perform ance (G M R , e x c h a n g e field , sen sitiv ity ) are co n tin u a lly pursued both through the d e v elo p m en t o f n e w m aterials and the d esig n o f n ew spin v a lv e g eo m etries. T he b asic F M /N M /F M /A F spin v a lv e has e v o lv e d into m any variants [43, 4 4 ].
T he standard spin v a lv e is co m p o se d o f at least six layers; buffer layer, free layer, spacer layer, p in n ed layer, antiferrom agnetic layer and cap p in g layer. T he bu ffer layer is u sually 2 -5 nm o f Ta and is u sed to increase the quality o f the sub seq u en t layers and reduce rou gh n ess. The rela tiv ely high resistiv ity (p = 160 x 10'* Q m ) o f tantalum m ak es it a su itab le ch o ic e as shunting e ffe c ts through it are n e g lig ib le . The cap p in g layer is also u su ally Ta and protects the structure from ox id a tio n and corrosion . T he free layer in ex ch a n g e b iased spin v a lv e s gen erally c o n sists o f tw o ferrom agn etically co u p led layers w ith different co m p o sitio n s. The m ost frequently u sed com b in ation is NigoFe2o/Co9oFeio. The p erm alloy layer is used to reduce the co e r c iv ity w h ereas the C oF e g iv e s a higher M R ratio d u e to the larger scattering potential for m in ority electron s at the C oF e/C u interface. U su a lly the antiferrom agnetic layer is at the top o f the spin v a lv e structure, h en ce the nam e top spin v a lv e . The antiferrom agnet can a lso be gro w n at the bottom o f the stack - a bottom spin v a lv e. T he bottom spin v a lv e is m ore d iffic u lt to rea lize exp erim en tally as the structure gen erally n eed s to b e annealed in a m a g n etic field to estab lish reasonable e x c h a n g e bias. T yp ical M R valu es for top and bottom spin v a lv e s w ith IrMn range from 5 - 10% .
The dual (or sym m etric) spin v a lv e takes ad van tage o f the fact that spin dependent scattering o f the co n d u ction electron s takes p la ce at the m a g n etic/n o n -m a g n etic interfaces. T herefore it is p o ssib le to increase the M R ratio by in creasin g the num ber o f interfaces. 31
Dual spin valves consist o f essentially three FM layers separated by two non-m agnetic spacers. The m agnetisation o f the outer two FM layers are pinned by an AFM layer, w hereas the inner FM layer is free. An exam ple o f a dual spin valve structure is N iO /C o/Cu/Co/Cu/CoN iO . GM R ratios as large as 24.8% [45,46] have been reported for these spin valves but the increased thickness o f the structure can m ake it unsuitable for read head applications.
A further advance in spin valve design was the developm ent o f the synthetic antiferrom agnet (SAF). The SAF is a three-layer stack consisting o f two ferromagnetic layers (usually Co or CoFe) separated by a thin layer o f non-m agnetic spacer (usually Ru). The coupling betw een the ferrom agnetic layers oscillates betw een ferrom agnetic and antiferrom agnetic as a function o f the Ru thickness and is strongly antiferrom agnetic in the range 0.5-1.0 nm [47]. One o f the ferrom agnetic layers in the SAF is coupled to an antiferrom agnet to give a spin valve o f the form shown in Figure 1.11. The antiparallel coupling across the Ru layer is m uch stronger than the interfacial coupling between the FM /AFM layer and the two FM layers in the SAF rem ain antiparallel up to quite large fields. The m agnetoresistance curve in Figure 1.11 shows that the pinning fields in the SAF spin valve can be twice as large as that o f the standard spin valve and the G M R value is the same. A nother advantage o f using the SAF is that, in a patterned device, stray field created by the pinned layer on the sensing layer is reduced because o f the antiparallel alignm ent o f the tw o FM layers in the pinned layer. An analytical calculation o f the m agnetic response o f the SAF spin valve has been carried out [48].
5 n mT a 1 0 nm IrMn 2 nm C Q Fe~ 0 . 7 nm R ~ 2 . 5 nm C o F e 2 . 9 nm Cu 1. 5 nm C o F e 3 . 5 nm MiPe 5 n mTa 8 6 4 2 0 -1.0 -0.5 0.0 0.5 1.0 M-oH( T )
Figure 1.11 Magnetotransport curve o f SAF spin valve with stack composition shown.
Spin valves with nano-oxide layers (NOL) w ere introduced in 1999 [49]. Very thin specularly reflecting oxide layers were introduced inside the pinned layer and near the free layer o f the spin valve resulting in enhanced M R ratios o f up to 18 %. This was attributed to specular reflection at the NOL interface. A specularly reflected electron at an interface in a CIP spin valve conserves its m omentum parallel to the interface and its perpendicular com ponent changes sign. The result is that the electrons m ove repeatedly though the active region in a m anner that is equivalent to increasing the num ber o f repeats in the structure. This affords an increase in the G M R o f the structure w ithout dram atically increasing the thickness. N ano-oxide layers are form ed by the oxidation o f an already deposited layer, or by the deposition o f a m agnetic oxide layer. N O Ls inserted in the pinned layer should be thin enough such that a large effective exchange bias field is retained. Deposition is norm ally followed by an anneal step [50]. Specular reflection has
a lso been reported in spin v a lv e s w ith o x id e antiferrom agnets such as N iO [46] and a - F e2 0 3 [51], N O L insertion into a spin v a lv e w ith a m eta llic antiferrom agnet such as IrMn can b e m ore u sefu l due to the high er therm al stab ility and lo w er critical th ick n ess o f the antiferrom agnet.
Spin filter spin v a lv e s have the structure A F M /F M /N M /F M /B w h ere B is a con d u ctiv e n o n -m a g n etic back layer. The result is an increased m ean free path and reduced resistan ce for m ajority spin electron s in parallel align m en t and h en ce an increased G M R ratio [52], O ptim isation is a trade o f f b etw een the increased m ean free path and increased current sh u n tin g through the back layer. The advantage o f the spin filter spin v a lv e is that the th ick n ess o f the free layer can be reduced to le v e ls that w o u ld be sub-optim al w ith ou t a back layer. A thinner free layer is ad van tageou s for read head applications as the con tin u ed reduction in the bit siz e o f m agn etic record in g heads m ean s sm aller m agn etic flu x co m in g from each bit. T his m ean s that the m o m en t o f the free layer m ust be d ecreased in order to m aintain the sam e am plitude o f rotation.