Normas transitorias atributivas

The choice and study o f antiferromagnets for spin valves has been largely driven by the requirements o f read head applications. The main requirements are as follows:

■ Large exchange bias field

■ High resistivity in order to reduce parasitic shunting o f resistance in CIP geometry ■ Low processing temperature so that the structural integrity o f the rest o f the stack is not

affected during deposition or post deposition anneal ■ Corrosion resistance

■ Thermal stability - high blocking temperature Tb, the temperature below the antiferromagnetic ordering Neel temperature at which the exchange bias goes to zero.

■ Low critical thickness - the m inimum thickness o f antiferrom agnetic m aterial for which exchange bias can be established.

The m ost com m only studied antiferrom agnetic layers are m anganese-based alloys such as FeM n, IrM n, NiM n, PdMn and PtMn. FeM n was for a long time the m ost intensively studied alloy. Film s o f com position Fei.xMnx with x ~ 0.5 were im plem ented in some o f the first spin valves [13]. Further studies indicated a low blocking tem perature in the range 140 to 190 °C rendering the m aterial unsuitable for disk-drive applications [33]. Other disadvantages include low stability with respect to Mn diffusion and poor corrosion resistance [34], Iri.xMnx with x ~ 0.8 has been intensively studied since its introduction as an exchange bias m aterial in 1996 [35] and is the m aterial used in spin valves discussed in this work. One o f its advantages is ease o f processing. For spin valves in which the IrMn is grown on top o f the FM layer (top spin valves), room tem perature deposition in a small m agnetic field is enough to establish reasonable exchange bias. For spin valves in which the IrMn is grown beneath the FM layer (bottom spin valves), a m agnetic anneal is norm ally required although a small exchange bias at room tem perature can be obtained. A detailed discussion o f the effects o f m agnetic annealing on top and bottom IrMn spin valves is presented in Chapter 4. The advantages o f IrMn over FeM n include higher blocking tem perature, larger exchange field, lower critical thickness, higher resistivity and better stability against Mn diffusion [34, 38].

NiM n, PtM n and PdM n are ordered AFM com pounds with very high Neel tem peratures ranging from 540°C for PdM n to 800°C for NiM n. This m akes them good candidates for application as exchange bias materials. U nfortunately in the as-deposited state these m aterials are random alloys with low exchange bias and a long annealing process is required to induce a phase transform ation to the ordered state. In the case o f NiM n, for

exam ple, an n ealin g pro cesses from 10-40 h o u rs at tem p eratu res aro u n d 280 °C h av e been re p o rted [36], N ev erth eless, these m aterials h av e very h ig h b lo ck in g tem p eratu res and ex c ellen t therm al stability [37,38], w hich m ak es th em v ery suitable fo r use in spin valves for read h ead applications.

O xide antiferro m ag n ets, such as N iO and a - F e2 0 3 h av e also been im p lem en ted in spin valv es [39,40]. T he o bvious advantage o f an o xide an tife rro m ag n e t is the elim in a tio n o f cu rren t shunting though the layer due to its intrinsically high resistivity. A n added adv an tag e o f increased G M R ratio due to sp ec u la r re flectio n at the oxide interface w as d isco v ered w hen studying N iO spin valves. T his has re su lted in high G M R ratios o f u p to 28 % [41]. U nfortunately both N iO and a - F c203 su ffe r from low ex c hange b ias fields and large critical thicknesses. An overview o f the m ain p ro p e rtie s o f the an tife rro m ag n e ts used in spin valves from [42, 44] is given in T able 1.1.

A FM F ilm s FeM n IrM n N iM n P tM n N iO a - F c2 0 3 C ritical thickness, tc (nm ) 7 - 1 0 5 - 8 25 1 0 - 1 5 50 50 N eel T em p, Tn (°C ) 230 420 800 702 250 680 B lock in g tem p, Tb (°C ) 140-190 240-290 3 6 0-400 350-400 180-230 250 E x ch an g e energy, J (m J/m ^) 0.10- 0.15 0.12- 0.40 0.30- 0.40 0.20- 0.30 0.02- 0.12 0.03- 0.12

Table 1.1 Characteristics of different types of antiferromagnets. Values of J(mJ/m^) are given for systems with CoFe as the ferromagnetic layer.