Voltage-controlled magnetic device operating over a wide temperature range

1. A voltage-controlled magnetic device comprising:

a magnetic layer extending along a reference plane and having a variable direction magnetisation and an effective anisotropy K eff ;

a non-magnetic insulating layer extending onto the magnetic layer;

a contact layer extending onto the non-magnetic insulating layer;

a polarisation voltage device configured to apply a polarisation voltage between the contact layer and the magnetic layer, through the non-magnetic insulating layer;

said magnetic layer having an anisotropy switching threshold such that the application of a polarisation voltage V max through the non-magnetic insulating layer enables switching of the effective anisotropy K err from a direction perpendicular to the reference plane to a direction in the reference plane or vice versa,

wherein the magnetic layer comprises:

a first layer with thickness t B , having a first volume anisotropy K VB ;

a second layer with thickness t A , having a surface anisotropy K SA and a second volume anisotropy K VA , the second layer being situated between the first layer and the non-magnetic insulating layer;

a composition and a thickness of the second and first layers being chosen in order that the surface anisotropy K SA and the first and second volume anisotropies K VB and K VA respect, over a given operating temperature range, the following inequality:

Min=( K SA ( V= 0), K SA ( V=V max ))<− K VB t B +K VA t A )<Max( K SA ( V= 0), K SA ( V=V max ))

 where K SA (V=0) is the surface anisotropy when no polarisation voltage is applied; K SA (V=V max ) is the surface anisotropy when the polarisation voltage V max is applied.

2. The magnetic device according to claim 1 , wherein the non-magnetic insulating layer is made of MgO, AlOx, AlN, SrTiO 3 , HfO x or any other insulating oxide or nitride having a dielectric polarisability greater than or equal to 6.

3. The magnetic device according to claim 1 , wherein

the effective anisotropy K eff is in a direction perpendicular to the reference plane when no polarisation voltage is applied;

the effective anisotropy K eff is in a direction in the reference plane when the polarisation voltage V max is applied.

4. The magnetic device according to claim 3 , wherein

the surface anisotropy K SA of the second layer is in a direction perpendicular to the reference plane and the surface anisotropy K SA decreases when the polarisation voltage V max is applied;

the total volume anisotropy K VB t B +K VA t A is in a direction in the reference plane.

5. The magnetic device according to claim 3 , wherein

the surface anisotropy K SA of the second layer is in a direction in the reference plane and the surface anisotropy K SA increases when the polarisation voltage V max is applied;

the total volume anisotropy K VB t B +K VA t A is in a direction perpendicular to the reference plane.

6. The magnetic device according to claim 1 , wherein

the effective anisotropy K eff is in a direction in the reference plane when no polarisation voltage is applied;

the effective anisotropy K eff is in a direction perpendicular to the reference plane when the polarisation voltage V max is applied.

7. The magnetic device according to claim 6 , wherein

the surface anisotropy K SA of the second layer is in a direction perpendicular to the reference plane and the surface anisotropy K SA increases when the polarisation voltage V max is applied;

the total volume anisotropy K VB t B +K VA t A is in a direction in the reference plane.

8. The magnetic device according to claim 6 , wherein

the surface anisotropy K SA of the second layer is in a direction in the reference plane and the surface anisotropy K SA decreases when the polarisation voltage V max is applied;

the total volume anisotropy K VB t B +K VA t A is in a direction perpendicular to the reference plane.

9. The magnetic device according to claim 1 , wherein the second layer is made of an alloy based on Co, Fe, Ni or any other material leading, in combination with the insulating layer, to a surface anisotropy K SA perpendicular to the reference plane and having a variation greater than 5% as a function of the application or not of the polarisation voltage V max .

10. The magnetic device according to claim 1 , wherein the first layer having the first volume anisotropy K VB is a multilayer stack of n elementary patterns of type F1/N1 or F1/N1/F2/N2 or F1/F2, with F1 and F2 two different ferromagnetic materials and N1 and N2 two different non-magnetic materials.

11. The magnetic device according to claim 1 , wherein the first layer having the first volume anisotropy K VB is an alloy having a tetragonal structure L1 0 .

12. The magnetic device according to claim 1 , wherein the first layer having the first volume anisotropy K VB is a monolayer of an alloy of type F1F2F3N1N2, with F1, F2 and F3 three different ferromagnetic materials and N1 and N2 two different non-magnetic materials.

13. The magnetic device according to claim 1 , wherein

the contact layer comprises, in contact with the non-magnetic insulating layer, a magnetic layer having a fixed magnetisation direction serving as reference direction for the magnetisation;

the non-magnetic insulating layer is a tunnel barrier enabling a current to circulate by tunnel effect between the contact layer and the magnetic layer;

the device then behaving like a magnetic tunnel junction.

14. The magnetic device according to claim 13 , wherein

the magnetic layer of the contact layer is an alloy of CoFeB;

the non-magnetic insulating layer is made of MgO;

the second layer of the magnetic layer is an alloy of CoFeB.