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Off stochiometric Heusler Alloys

IP.com Disclosure Number: IPCOM000034121D
Publication Date: 2005-Jan-17
Document File: 4 page(s) / 48K

Publishing Venue

The IP.com Prior Art Database

Abstract

Heusler alloys are being considered for GMR and TMR devices as well as other spintronic devices. Many of the alloys are predicted to exhibit 100% spin polarization--i.e. one spin direction has states available for electrons while the other does not. Another way to look at this is that for one spin direction, the band structure and density of states (DOS) is metallic while the other spin direction is semiconducting. Because of this property, these materials should give extremely high values of magnetoresistance.

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Off stochiometric Heusler Alloys

    The Density of States (DOS) for one Heusler compound (Co2CrAl) is shown in Fig. 1. Fig. 1A shows the case for the electrons in one spin direction. In this case, there are no states available at the Fermi level. I.e. DOS=0 at E=0. This is a characteristic of a semiconductor. Fig. 1B shows the DOS for the electron spins antiparallel to the case in Fig. 1a. Here, the DOS is nonzero at E=0, indicating a metallic structure. Because one spin direction is semiconducting and one is metallic, these are often referred to as "Half Metal" structures.

Fig 1a--Density of States (DOS) for Co2CrAl spin 1 The presence of the gap at the Fermi energy (EF) is a sign of semiconducting behavior

Stochiometric Co2CrAl

35

Spin 1

30

25

Density of States

20

15

10

Gap

5

0-2 -1 0 1 2

Energy (eV) relative to EF

Fig 1b--Density of States (DOS) for Co2CrAl spin 1 The presence states (DOS<>0) t the Fermi energy (EF) is a sign of metallic behavior

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Stochiometric Co2CrAl

No Gap

The spin polarization is given by P=(DOS2-DOS1)/(DOS2+DOS1), where DOS1 (2) is the value of the DOS at EF for spin 1 (2). Since DOS1=0 for the case above, P=1 (or 100%).

For a Tunnel system, DR/R is given ideally by:

  2 1

40

35

30

25

20

15

10

5

0-2 -1 0 1 2

Spin 2

Density of States

Energy (eV) relative to EF

    = ∆

Where P1 and P2 are he spin polarization of the free and pinned layers, respectively. If the spin polarization is 1 for both the free and pinned layers, DR/R is infinite. I.e. resistance goes from conduciting to insulating as the free layer is reversed.

As shown in Fig. 1 above for the Heusler Co2CrAl, this material has a spin polarization of 1. Other Heusler materials do as well. It is commonly believed that one has to be exactly on the correct stochiometry (A2BC for a full Heusler and ABC for a half Heusler) in order to obtain P=1. This makes the processing somewhat difficult. Getting targets and the actual sputtered films to match the exact composition can be difficult. This is made very clear in the case of PtMn, where the films can be as much as 10% off the target composition. Also, altering the composition may lead to higher ordering temperatures, lower anneal temperatures, etc.

We have used band structure calculations to show that the Heuslers can have high spin polarization without being exactly stochiometric. We used Co2CrAl as the test Heusler. The Heuslers are ordered structures.

We calculated band stuctures for Co2CrAl with extra Co, Cr and Al. In the calculations, we place take away one atom (e.g. Co) and replace it with another (e.g. Al). We work with an extended unit cell with 16 total atoms (8 Co, 4 Cr and 4 Al) and 32 total atoms (16 Co, 8 Cr, and 8 A...