IES78960B2 - Magnetic switch or sensor - Google Patents

Abstract

A device is described which exhibits a change of electrical resistance in response to a magnetic field. It is composed of two or more regions of half-metallic ferromagnet joined by one or more weak conductance links. The switch can be implemented in thin-film form. It can also be implemented by using powders of the half-metallic ferromagnet which may optionally be diluted with nonmetallic nonferromagnetic powders to improve the magnetoresistive reponse.

Description

Background of the Invention.

Certain magnetic materials and structures are known for which the electrical resistance is sensitive to an applied magnetic field. These include alloys such as permalloy which exhibit an anisotropic magnetoresistance effect where the resistance increases or decreases according to whether the magnetic field is applied parallel or perpendicular to the current direction. The resistance change, which is of order 1 %, may be observed in low fields, of order milliteslas, and it is associated with the alignment of ferromagnetic domains. A larger magnetoresistance effect, known as giant magnetoresistance, appears in multilayer and granular structures of ferromagnetic and nonmagnetic metals such as cobalt/copper or cobalt/silver. Giant magnetoresistance is associated with aligning the magnetization of different ferromagnetic particles or layers. It is isotropic, in the sense that it is independent of the direction of current relative to the applied field, and negative in the sense that the resistance decreases with applied field. Certain oxides such as (Lao.7Caoj)Mn03 exhibit an isotropic negative magnetoresistance effect known as collossal magnetoresitance Unlike giant magnetoresistance,this in an intrinsic effect related to alignment of magnetic moments on an atomic scale in a large external field, especially in the vicinity of the Curie point. The effects may be very large, but large magnetic fields, in the tesla range, are needed to produce them.

Here a new type of magnetoresistive structure is disclosed which may be used as a magnetic switch with no moving parts. It depends for its operation on a half-metallic ferromagnet. A half-metallic ferromagnet is a metallic ferromagnetic material where the conduction electrons have essentially all the same spin orientation. There are only majority-spin electrons at the Fermi level. An example is the oxide C1O2. Other examples include certain manganese-based Heusler alloys and mixed-valence manganites such as (Lai_xAx)MnO3 where R = rare earth ion, A = alkaline earth ion and x is approximately 0.3. The class may be extended to include materials such as magnetite (Fe3O4), which is a ferrimagnet, with only minority-spin carriers at the Fermi level. The essential feature is that the carriers are essentially all of the same spin orientation.

Description of the invention.

The magnetic switch is composed of an array of ferromagnetic elements, at least two in number, but typically many more. Each element has two probable directions of magnetization which lie along an axis in the element. These easy directions may be defined, for example, by shape or magnetocrystalline anisotropy. The easy axes of the elements in the array are arranged to lie largely parallel to one another, and each of the elements is in electrical contact with its neighbours. The nature of the contact is such that it does not facilitate coupling of one element to the next by magnetic exchange interactions. The elements are exchange decoupled, but they may be coupled by magnetic dipole interactions and stray fields. Furthermore, the size of the ferromagnetic elements may be chosen so that each element of the array behaves as a singledomain particle, reversing its magnetization when a magnetic field of sufficient magnitude is applied in a direction opposite to the direction of magnetization of the element.

It is important for effective operation of the switch that the contacts between the elements do not greatly modify the spin state of the conduction electrons or introduce a large amount of spin-flip scattering. The contacts may be narrow tunnel barriers, or else they may be composed of a conducting material which posseses a long spin diffusion length.

In the unmagnetized state, the array will include roughly equal populations of elements with the two directions of magnetization, denoted as f and j. The conduction electrons from the f elements can pass into the neighbouring | elements and the conduction electrons from the 1 elements can pass into the neighbouring J elements, but electrons cannot pass into elements with the opposite spin orientation because no opposite spin states are available in the vicinity of the Fermi level since the ferromagnetic elements are made from a half-metallic ferromagnet. If the geomety and connectivity of the array are chosen so that there are no percolation paths through it which involve only the half of the elements in the array which have | or J, spin, then the switch is closed. It presents a high resistance in the unmagnetized state. On applying a magnetic field of sufficient strength to align the magnetization of the elements in the direction of the field, the electrons may now move among all the elements of the array, not just half of them,.as before. If the geometry and connectivity of the array are chosen so that there are percolation paths through it which involve all or nearly all the elements of the array, then the array is now conducting and the switch is open. It may be closed again by applying a sufficient reverse magnetic field to reduce the magnetization of the array to zero. The field required is the coercive field of the magnetic array. By operating between a value close to the coercive field and a value close to the saturation field, the switch may be reversibly cycled between its off and on states. When operating as a sensor, the device detects changes of magnetic field. The coercive field of the array may be selected by choosing an appropriate magnetization, shape and anisotropy field for the single-domain elements.

Implementation One realization of the the invention is an array of aligned particles of the half-metallic ferromagnet The particles may first be aligned in a magnetic field so that their easy directions of magnetization lie in a determined direction. The array of particles may then be compacted under pressure or subjected to a thermal or chemical treatment which ensures electrical contact between the particles. When the switching field is applied along the orientation direction, the switch is open and | electrons can be conducted across the array, as shown in Figure la. When the field is reversed to the coercive field of the array, half of the elements have reversed their magnetic polarization and the system consists of two channels in parallel, one for f electrons and one for j electrons, as shown in Figure lb.. If the structure of the array is such that percolation is possible in neither of these channels, then the switch is closed. A way to achieve this in the aligned array of powder particles as follows. The number of contacts for each powder particle required for percolation is approximately two. For random dense packing of the powder particles, the number of electrical contacts for each particle may be more than four. For example, if the particles were randomly dense-packed with contact to all neighboring particles then the number of contacts Z will be approximately 12. Reducing this to 6 by operating at the coercive field will increase the resistance of the switch, but will not close it completely. A solution is to dilute the ferromagnetic particles with a fraction f of nonconducting, nonferromagnetic particles, as shown in Figure 2. Provided/ is chosen so that (1-/)/2 < xp < (1-/), where xp is the percolation threshold of the array, then the device will switch between a conducting and a nonconducting state. The value of xp is approximately 2/Z.

Another implementation of the invention is a two-dimensional array of shaped ferromagnetic elements of the half-metallic ferromagnet, as shown in Figure 3. The shape is chosen to define the easy direction of magnetization by shape anisotropy. The array may be produced by standard lithographic techniques. The contacts between the elements may also be modified using techniques common in semiconductor processing to provide the necessary electrical contact without exchange coupling the elements of the array. In the square array shown, the number of contacts Z = 4, and xp for the square lattice is 0.5 so the condition (1-/)/2 < xp < (1-/) is just satisfied for/ = 0. The operation of the device may be improved by breaking some more contacts at random.

The simplest implementaion of the switch consists of just two elements as shown in Figure 4. The switch is open when the two elements have parallel magnetization and it is closed when they have antiparallel magnetization.

Such a device made from two or more elements of a half-metallic ferromagnet could serve as an effective detector of magnetic stray fields, as a sensor or as a read head in magnetic recording.

The elements in the array may be chosen to have an elongated shape in order to define an easy axis of magnetization and achiev appropriate coercivity. An external magnetic field produced by a permanent magnet or an electromagnet may optionally be used to bias the device in order to improve the sensitivity, reversibility or linearity of the magnetoresistive response.

The devices of the invention may be fabricated by lithographic techniques known in the semiconductor industry. They may also be prepared by powder processing techniques inclusing silk-screen printing, spray deposition, cold or hot pressing. The methods used in the magnetic recording industry to manufacture particulate or thin-film media may also be applied to make the sensors and switches of the invention.

Claims (5)

Claims I

1) A magnetic switch or sensor which depends for its operation on percolation of spin-up and spin-down electrons in separate conduction channels

2. ) A magnetic switch or sensor with no moving parts made from monodomain elements of a half-metallic ferromagnetic material controlled by an externally-applied magnetic field.

3. ) A switch or sensor made from aligned powder particles of a half-metallic ferromagnetic material which may be mixed with an amount in the range 0-80 volume per cent of particles of a nonconducting ferromagnetic material

4. ) A switch or sensor made from a planar array of magnetic single-domain elements of a halfmetallic ferromagnetic material which are in electrical contact with their neighbours and which have parallel easy directions of magnetization.

5. ) A switch or sensor made from a pair of domains of a half-metallic ferromagnetic material which are in electrical contact with eachother and which have parallel easy axes of magnetization.