EP0954754A1

EP0954754A1 - Magnetic field sensor and method for making same

Info

Publication number: EP0954754A1
Application number: EP98955734A
Authority: EP
Inventors: Albert Thomson-CSF P.I Dpt Pro. & Conseil FERT; Frédéric Thomson-CSF P.I D.P & Conseil PETROFF; Luiz F. Thomson-CSF P.I D.P. & Conseil SCHELP; Alain Thomson-CSF P.I Dpt. Pro.&Conseil SCHUHL
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1997-11-25
Filing date: 1998-11-24
Publication date: 1999-11-10
Also published as: US6291993B1; FR2771511A1; FR2771511B1; WO1999027379A1

Abstract

The invention concerns a magnetic sensor comprising a non-magnetic insulating layer (2) including at least a layer of ferromagnetic particles (4), the assembly being enclosed between two ferromagnetic electrodes (2, 3). For its operation, said sensor uses the passage of electrons by tunnel effect between each electrode and the ferromagnetic particles. The tunnel resistance depends on the orientation of the electrode magnetisation and therefore varies in presence of a magnetic field. Compared to the usual type of junctions (without aggregates), the multichannel and multistage characteristic of tunnel conduction eliminates the problems of short circuits caused by porosity, thereby providing a less delicate product with improved resistance to burn-outs. The possible thermal fluctuations of magnetic couples of the aggregates can be eliminated by selecting a magnetic material for the insulating layer portion containing the aggregates.

Description

MAGNETIC FIELD SENSOR AND MANUFACTURING METHOD

OF THIS SENSOR

The invention relates to a magnetic field sensor and a method of manufacturing such a sensor.

Such a sensor can be applied to the reading of magnetic recordings such as magnetic tapes or magnetic disks or for the production of magnetic memory with integrated reading (Magnetic Random

Access Memory) and generally for the detection of weak magnetic fields with high spatial and angular resolution.

It is known that the resistance of a tunnel junction composed of a very thin layer of insulator between two ferromagnetic metal electrodes depends on the relative orientation of the magnetization in the two electrodes. This effect was discovered by Jullière in 1975 (see document [1] at the end of the description), but it is only recently that high magnetoresistance values (up to 30%) and fairly reproducible have been obtained (see, for example, document [2] at the end of the description). We will call this type of junction “planar junction”. Typically, in the planar junctions studied by Moodera et al [2], a layer of 1.5 to 2.5 mm of insulator (AI2O3 for example) separates two electrodes of Cobalt or Cobalt alloy. The deposition through masks makes it possible to produce the conventional geometry of cross junction where the crossing of the two electrodes separated by the insulating layer defines the tunnel effect zone. Compared to magnetic magnetoresistors with giant magnetoresistance (see document [5]), tunnel junctions have the advantage of a much higher resistance, which is interesting for a certain number of applications. Their drawback is their difficulty in manufacturing roughness, porosity (“pinhole” in English) and other imperfections of the ultra-thin insulating layer establishing direct contacts between electrodes which short-circuit the junction, and obtaining reproducible results is generally difficult. The very thin thickness of the insulating layers also makes the planar junctions sensitive to breakdown effects.

A magnetoresistance effect resulting from a tunnel effect between ferromagnetic metals can also be obtained in granular materials made up of small particles (aggregates) of metal ferromagnetic coated in an insulating matrix (see documents [3] and [4]). Conduction takes place by tunneling of electrons from an aggregate to the neighboring aggregate, and the resistance of the material varies when a magnetic field changes the relative orientation of the magnetic moments of the aggregates. These materials are easier to manufacture than planar junctions, are robust and can exhibit significant resistance variations [3 and 4]. However, their disadvantage comes from the high field necessary to orient the moments of small particles and obtain the effect of magnetoresistance. This field is particularly high in the superparamagnetic regime, that is to say when the temperature is higher than the blocking temperature (Tβ) of the thermal fluctuations of the moments.

The invention relates to a junction which combines the properties of planar junctions (weak field response) and those of granular materials (ease of construction and robustness). The invention therefore relates to a magnetic sensor comprising two conductive layers, at least one of which is a ferromagnetic material, these two layers being separated by a layer of non-magnetic insulating material of almost uniform thickness and containing particles of ferromagnetic material. located in a plane parallel to the layer of ferromagnetic material, the magnetization characteristics of the ferromagnetic conductive layer and of the ferromagnetic particles being different.

The invention also relates to a method for producing a sensor characterized in that it comprises the following steps: • production on one face of a substrate of the ferromagnetic conductive layer;

• production of a first layer of non-magnetic insulation;

• production by sputtering of a thin layer of a ferromagnetic conductive material so that said material aggregates into aggregate particles;

• production of a second layer of non-magnetic insulation;

• production of a layer of ferromagnetic conductive material. The various objects and characteristics of the invention will appear more clearly in the description which follows and in the appended figures which represent:

- Figures 1a to 1c, a simplified embodiment of the sensor of the invention;

- Figure 2, a more complete embodiment of the sensor of the invention;

- Figure 3, a sensor comprising several layers of ferromagnetic particles; - Figure 4, an alternative embodiment of the sensor of the invention.

Referring to Figures 1a to 1c, we will therefore first describe an embodiment of a sensor according to the invention. This sensor comprises on a substrate S, a stack of an electrode 1, an insulating layer 2 and another electrode 2. The insulating layer 2 contains particles of a ferromagnetic conductive material. All these particles are located substantially in the same plane parallel to the planes of the electrodes. At least one of the electrodes is made of ferromagnetic conductive material. In the description which follows, it will be considered that the two electrodes are made of ferromagnetic conductive material.

By connecting a current generator I to the electrodes 1 and 3, we will obtain the circulation of a current by tunnel effect between the electrode 3 and each of the particles and between each of the particles and the electrode 1.

FIG. 1b shows the case where the magnetization directions in the ferromagnetic particles 4 and in the electrodes 1 and 3 are parallel. In this case the resistance of the structure is minimal and a maximum current is measured.

On the other hand, in FIG. 1 c, the magnetizations of the particles and of the electrodes are antiparallel. The resistance of the structure is maximum and the current measured will be minimum.

By way of example for particles of Cobalt type material, Cobalt electrodes and particles located at t '= 1.5 nm from electrode 3 and t = 2.7 nm from electrode 1, the field magnetic reversal of the magnetization of the electrodes which produces the resistance variation was measured at approximately 90 ersted. Referring to Figure 2, we will now describe a more detailed embodiment of the sensor of the invention. It comprises, on the substrate S, a buffer layer 6, a ferromagnetic conductive electrode layer 1, an insulating layer 2 comprising ferromagnetic particles, a ferromagnetic conductive electrode layer 3 and a protective layer 7.

The conductive electrode layer 1 is made of soft magnetic material such as Permalloy. Its thickness is not critical and can be between 10 nm and 1 μm. The insulating layer 2 is made of AI2O3. Particles 4 are in

Cobalt, FeNi or CoFe. The particle size is between 2 and 4 nm. The thickness of the insulating layer 2 is such that the outer edges of the particles are distant from the electrodes 1 and 3 by a distance of between 1, 5 and 3 nm. The electrode layer 3 is similar to layer 1. The protective layer can be made of metal such as gold.

The embodiment of FIG. 3 comprises, in the insulating layer 2, several planes of ferromagnetic particles 4. In FIG. 3, three planes of particles are shown.

We will now describe a method for producing a sensor according to the invention.

On one face of a substrate S, an electrode layer 1 is produced, a layer of an insulating material, preferably for example by sputtering of the insulator, or by depositing a metal in an oxidizing atmosphere to obtain a insulating oxide, or by depositing a metal and oxidizing then, preferably by sputtering, by the same process, an ultra-thin layer of a ferromagnetic material (metal, alloy or oxide such as Fe3θ4) is produced.

In fact, due to the thinness of this layer, the material will tend to form aggregates so as to provide a uniform distribution of particles. The quantity of material deposited makes it possible to control the size and the concentration of the aggregates. Then, the deposition of a new insulating layer is carried out, followed by the deposition of an electrode layer 3. The structure of FIG. 1 is thus obtained.

To produce a structure comprising several layers of particles as shown in FIG. 3, alternately depositing layers of insulation and ultra-thin layers of ferromagnetic material. The quantity of insulation deposited makes it possible to fix the average thickness t of insulation between layers of particles. This thickness as well as the average thicknesses of insulation between electrodes (1 and 3) and layers of particles are substantially between 1.5 nm and a few nm. Realistically, the number of particle layers can reach ten.

The three-layer or multi-layer described above can be deposited through masks to achieve, for example, the conventional cross-junction structure (as in reference [2]). Other types of geometry for the current leads and the voltage contacts can also be produced, either by structuring the multilayer deposited by lithography and etching methods, or by using a ramp type junction technology.

The electrical conduction between electrodes 1 and 3, in the type of junction described above, is by tunnel effect from electrode 1 to particle then from particle to particle if the number of layers of particles is greater than 1, and finally of particle with electrode 3. The multi-channel and multi-step nature in each channel for tunnel conduction makes it possible to avoid a possible short circuit by porosity (“pinhole” in English) which is frequent in planar junctions. Indeed, in a planar junction composed of 2 electrodes separated by an ultra-thin insulating layer, a very small number of contacts between electrodes by pinhole short-circuits the tunnel resistance. The need to avoid any pinhole in an insulating layer a few nanometers thick then makes the preparation of planar junction extremely delicate. On the contrary, in the structure according to the invention, a short circuit is improbable because it could only occur by the succession of pinholes on all the tunnel stages of a given channel. This results in less delicate manufacture than that of known planar junctions as well as greater robustness to breakdown effects. The dependence in magnetic field of the tunnel resistance of the proposed structure is linked to the relative orientation of the magnetizations at the entry and at the exit of each tunnel step. However, by exploiting the very rapid increase in tunnel resistance with the thickness of insulation, it is possible to fix the thicknesses of insulation separating the electrodes 1 and 3 from the layers of particles slightly greater than t, so that the resistance total of the junction is essentially controlled by the tunnel resistances between electrodes and particles and therefore controlled by the relative orientation of the magnetization in the electrodes and the aggregates. Planar and aggregate electrodes have very different coercivities. In a sweep in a field, the magnetization of the electrodes varies very quickly, however, that the magnetization of the aggregates will keep its so-called residual value (induced by an initial polarization). The variation of the tunnel resistance will therefore follow the variation of the magnetization of the electrodes, as in a planar junction. A choice for electrodes of a very soft magnetic material,

Permalloy or monocrystalline material, for example, must make it possible to obtain the magnetoresistance effects sought in a few gauss. For aggregates (particles) we then choose for example Cobalt, CoFe, FeNi, ... We then find the performances of weak field sensitivity of planar junctions while keeping the ease of development and the robustness of granular materials .

A condition is however necessary: the existence of a non-zero remanent magnetization of the aggregates, that is to say of a temperature Tβ of blocking of fluctuations (introduced above) higher than the operating temperature. The temperature Tβ is linked to the size of the aggregates and to their interactivities. To obtain a sufficiently high Tβ value (for example Tβ> 300 ° K, one can:

- or increase the particle size by the deposition conditions. In this case, particles of between 4 and 10 nm are provided;

- either increase the interactions between moments of particles in a layer by increasing their density (by depositing a larger quantity of metal);

- Either use a ferromagnetic or antiferromagnetic or ferrimagnetic insulator (ferrite for example) whose interaction with the aggregates will increase Tβ. The material deposited in the part of the insulating layer containing the aggregates will therefore be a magnetic insulator; on the other hand, a non-magnetic insulator must remain between aggregates and electrodes to decouple them magnetically. Such a structure is shown in Figure 4. In this structure, the insulating layer 2 contains a magnetic insulating layer 5

(ferromagnetic, anti-ferromagnetic or ferrimagnetic), which contains the particles 4 (its thickness must be very slightly greater than the vertical dimension of the particles).

Finally, another advantage of the proposed structure compared to planar junctions comes from the possibility of using the Coulomb biocage effects [6]. The energy-related effects of small aggregates can increase tunnel resistance by an important factor. For nanometric particles, this increase remains significant at room temperature. This increase in resistance due to the blocking of

Coulomb can be interesting for obtaining high resistances and therefore important magnetoresistive signals.

In general, the sensor according to the invention uses the passage of electrons by tunnel effect between two electrodes of ferromagnetic material through a thin insulating layer containing particles of ferromagnetic material (aggregates). The tunnel resistance depends on the orientation of the magnetization of the electrodes and therefore varies in the presence of a magnetic field. Compared to the usual type of junctions (without aggregates), the multi-channel and multi-stage nature of the tunnel conduction eliminates the problems of short-circuit by porosity, thus bringing about a less delicate manufacturing and an improved robustness with respect to clicks. The possible thermal fluctuations of the magnetic moments of the aggregates can be eliminated by the choice of a magnetic material for the part of the insulating layer which contains the aggregates.

References

[1] M. Jullière, Phys. Lett. A 54, 225 (1975) [2] J.S. Moodera, L.R. Kinder, T.M. Wrong, R. Meservey, Phys. Rev. Lett. 74, 3273 (1995) [3] H. Fujimori, S. Mitani, S. Ohnuma, Mat. Science and Engineering B 31,

219 (1995) [4] A. Miiner et al, Phys. Rev. Lett. 1996 [5] MN Baibich et al, Phys. Rev. Lett. 61, 2472, 1988 [6] MH Devoret, D. Estève, C. Urbina, Nature 360, 547 (1992)

Claims

1. Magnetic sensor comprising two conductive layers (1, 3) at least one of which is a ferromagnetic material, these two layers being separated by a layer of non-magnetic insulating material (2) of almost uniform thickness and containing particles (4) of ferromagnetic material located in a plane parallel to the layer of ferromagnetic material, the magnetization characteristics (coercivity) of the ferromagnetic conductive layer and of the ferromagnetic particles being different.

2. Magnetic sensor according to claim 1, characterized in that the two conductive layers (1, 3) are both made of ferromagnetic material.

3. Sensor according to one of claims 1 or 2, characterized in that the conductive layers (1, 3) are with soft magnetic magnetization while the ferromagnetic particles are with hard magnetic magnetization.

4. Magnetic sensor according to one of claims 1 or 2, characterized in that the layer of non-magnetic insulating material (2) comprises a layer of magnetic insulating material (ferromagnetic, anti-ferromagnetic or ferrimagnetic) (5) encompassing or covering particles of ferromagnetic material.

5. Magnetic sensor according to one of claims 2 or 4, characterized in that the dimensions of the particles are compπses between 2 and 4 nanometers.

6. Magnetic sensor according to claim 2, characterized in that the dimensions of the particles are between 4 and 10 nanometers.

7. Magnetic sensor according to one of claims 1 or 2, characterized in that particles are located in several parallel planes.

8. Magnetic sensor according to claim 2, characterized in that the conductive layers are made of Permalloy.

9. Magnetic sensor according to claim 2, characterized in that the particles (4) are made of Cobalt, FeNi or CoFe and the insulating layer (2) is made of Al2O3.

10. Magnetic sensor according to claim 4, characterized in that the layer of magnetic insulating material is made of ferrite.

11. Magnetic sensor according to one of claims 2 or 8, characterized in that the distance between a layer of particles and a conductive layer (1, 3) as well as the distance between two layers of particles is between 1 nm and a few nanometers.

12. Magnetic sensor according to claim 4, characterized in that the thickness of the layer of magnetic insulator (5) is between 2 and 4 nm.

13. A method of producing a sensor according to claim 1, characterized in that it comprises the following steps: a) production on one face of a substrate of the ferromagnetic conductive layer (1); b) production of a first layer of non-magnetic insulation; c) producing a thin layer of a ferromagnetic conductive material so that said material aggregates into aggregate particles (4); d) production of a second layer of non-magnetic insulation; e) production of the layer (2) of a ferromagnetic conductive material.

14. Method according to claim 13, characterized in that at least step (c) of producing a thin layer of ferromagnetic conductive material, giving rise to particles of aggregates, is carried out by sputtering.

15. The method of claim 14, characterized in that it comprises the alternation of layers of insulation and thin layers of ferromagnetic conductive material thus providing an alternation of layers of insulation and layers of particles.

16. Method according to claim 14, characterized in that the production of the first layer of insulation is followed by the production of a first layer of magnetic insulation and that the production of the thin layer of ferromagnetic conductive material is followed by producing a second layer of magnetic insulator, the first and second layers of magnetic insulator forming a layer of magnetic insulator (5) including the ferromagnetic particles (4).