GB2388915A

GB2388915A - Anisotropic magnetoresistive sensor

Info

Publication number: GB2388915A
Application number: GB0211845A
Authority: GB
Inventors: Stuart Nicholas Holmes
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2002-05-22
Filing date: 2002-05-22
Publication date: 2003-11-26
Anticipated expiration: 2022-05-22
Also published as: GB2388915B; GB0211845D0

Abstract

An anisotropic magnetoresistive (AMR) sensor comprises a Heusler alloy i.e. an alloy of a non-ferromagnetic material and manganese that exhibits ferromagnetism. A preferred alloy has the composition XMnZ or X2MnZ where X is a transition metal such as Co, Cu, Ni, Fe or Pt, and Z is Ge, Si, Ga or Sb. A particularly preferred alloy is Co2MnGa. The Heusler alloy 23 may be deposited on a substrate 21 e.g. a layer of GaAs, and the Heusler alloy may be a single crystal or polycrystalline. A patterned layer with a plurality of stripes may be provided adjacent to the Heusler alloy layer to influence the direction of current flow through the sensor (figure 13). The AMR sensor exhibits an AMR ratio of 6% at 300K.

Description

( 23889 1 5

An Anisotropic Magnetoresistance sensor and a Method of detecting a Magnetic field

The present invention relates to the field of magnetic sensors and methods of detecting a

magnetic field. More particularly, the present invention relates to so called Anisotropic

magneto resistance (AMR) sensors which operate utilising the Anisotropic magneto resistance (AMR) effect.

Magneto resistance sensors are a class of sensors whose resistance changes in the presence of a magnetic field. AMR sensors are a subclass of these sensors where the

resistance of the sensor changes dependent on the angle between the magnetization direction of the sensor and the direction of current flow through the sensor.

Such AMR sensors are widely used in magnetic read heads for reading magnetic data storage media' magnetic position sensors etc. The following properties are desirable in an AMR sensor: À The AMR coefficient, which is the change in magneto-resistivity divided by the resistivity in zero applied field (Ap/p), should be as large as possible, to maximise

the sensitivity.

À The resistivity, p, should be as large as possible for any given Ap/p, so that the sensor can be of smaller dimensions and have a smaller power consumption.

À The anisotropy field Hk should be small, because the sensitivity of a sensor is

inversely proportional to the anisotropy field.

À The coercive field Hc should be small, so that low fields can reliably be measured.

À A reliable material should be used, which is corrosion resistant, has parameters stable in time, etc.

À Ideally, the material should show an AMR effect at voltages of greater than 1 volt, and preferably of around 5 volts, to make integration with standard semiconductor technology easier.

Easy fabrication is a big advantage.

Previously known AMR materials include Ni-Co alloys and Ni-Fe alloys. Details of the AMR at various compositions are shown in table i below.

Material AMR300K (%j AMR 1.6K (%) Ni90CoO 4.9 NisoCo20 6.5 20 Ni70Co30 6. 6 Ni92Fe 5.0 NisFer9 (permalloy) 2-4 (1) 6 The particular composition Ni'Feg is known as permalloy. Perrnalloy has traditionally been the AMR material of choice for magnetosensors, even though the AMR is only 2-

4%. This is because it has several other important properties which are desirable in an AMR sensor. At around x=81, NixFeoo-x has zero crystalline anisotropy, so is isotropic with no preferred easy or hard axes. This means that the coercive field can be very low

(HC1 G). The anisotropy field Hk is very low, typically 1-10 G. An easy axis can be

defined in Permalloy by annealing the film in a magnetic field or by growth in an

orientating field.

However, as perrnalloy has a fairly low AMR of 2-4%, there is a need to develop new AMR sensors which have higher AMR values and which also have desirable properties such a low anisotropic fields and low coercive fields.

( Recently, there has been interest in Heusler alloys which are a family of ferromagnetic ternary materials. The Heusler alloys are ternary alloys of Mn and other non-

ferromagnetic materials which exhibit ferromagnetism. They have general formula X2YZ or XYZ. The X2YZ alloys are known as the full Heusler alloys, and the XYZ alloys are known as half Heusler alloys.

The use of Heusler alloy layers is already known in giant magnetoresistive (GMR) devices. See for example: Caballero et al in J. Magn & Mag Mat 198-199, 55 (1999) which describes a GMR device based on NiMnSb; WOOD/13 194 which describes GMR sensors which contain a Co based Heusler alloy; and US 2002/0012812 which describes the use of a half Heusler alloy as a specular reflection layer in a GMR device.

GMR devices differ considerably from AMR devices. GMR devices comprise at least two ferromagnetic layers separated by a non-ferromagnetic conductive layer. Applying a magnetic field to a GMR device aligns the magnetization of the ferromagnetic layers,

thus changing the resistivity of the device.

Unlike AMR, in GMR devices the resistivity decreases with the applied magnetic field

and the resistivity is independent of the direction of the applied field with respect to the

direction of current flow. GMR devices generally require higher magnetic fields than

AMR devices and their multilayer structure is more complicated to fabricate than a single layer AMR device. The GMR effect disappears with applied voltages in excess of a few hundred millivolts. Additional circuitry is then needed to integrate the devices with standard semiconductor devices operating at several volts, considerably increasing the cost and complexity. Finally, in general, GMR devices use very thin layers, e.g. in the range of 15 to 40 A. It is also known to use a half Heusler alloy as magneto-optical device, as described in US4876144.

However, none of the above documents allude to the surprising anisotropic magneto-

resistance characteristics of Heusler alloys which have been discovered by the applicant.

( At room temperature, a 6% change in the anisotropic magnetoresistance has been observed which increases to 8% at 1.6 K. Further, in addition to the large magnetoresistance, the Heusler alloys have been found to have a small anisotropy field and a low switching field of around 20 to 30 G at 300K.

Thus, in a first aspect, the present invention provides an anisotropic magnetoresistive sensor comprising a Heusler alloy.

Preferably, a full Heusler alloy is used which has the composition X2YZ, where X is a transition metal, Y is Mn, and Z is selected from the group of Ge, Si, Ga and Sb.

Preferably, X is selected from the group of Co, Cu. Ni, Fe and Pt and more preferably Co. Preferably Z is Ga.

The Heusler alloy preferably possesses a structure from one of the group: Lid, DO3, B2 using the Structurbericht designation.

Said Heusler alloy is preferably a thin film layer with a thickness of at least 40 nm.

More preferably at least 60 nm.

Many of the Heusler alloys have lattice parameters which are close to those of standard substrate materials such as GaAs, Si and InGaAs, thus, the thin film may be provided on a single crystal substrate which is more preferably a GaAs, Si or InGaAs substrate.

Also, the Heusler alloy may be a single crystal layer. However, it should be noted that surprisingly good properties have also been observed from a polycrystalline Heusler alloy. polycrystalline structure may be fabricated using a standard evaporator at a pressure of approximately I Ohm Bar. and does not require epitaxial growth techniques.

The magnetoresistive sensor may comprise a plurality of thin film Heusler alloy layers or just one such layer.

! To operate an AMR sensor, a current is passed through the sensor and the resistance is measured. To improve operation, the sensor is configured such that the direction of current flow is non parallel to the easy magnetization axis. Preferably, the direction of current flow is provided at an angle of between 20 and 70 to the easy axis, more preferably between 40 and 50 , and ideally at 45 .

In order to achieve the above condition, a patterned layer having a lower resistance to that of the Heusler alloy may be provided over the Heusler alloy to influence the direction of current flow through the Heusler alloy.

Preferably, the pattern of said patterned layer has a plurality of stripes formed non parallel or perpendicular to said easy magnetization axis, a so called 'Barber pole' pattern. In a preferred configuration of the magnetoresistive sensor said Heusler alloy is formed into four anisotropic magnetoresistive elements, the four elements being arranged in a bridge configuration.

The magnetoresistive sensor may be used in a magnetic memory device.

In a second aspect, the present invention provides a method of detecting a magnetic field, comprising:

passing a current through a Heusler alloy in a first direction, said first direction being non parallel or perpendicular to the easy axis of the Heusler alloy; and measuring the resistance using said current to determine the presence of a magnetic field.

Preferably, the Heusler alloy has the above described composition.

Preferred embodiments of the present invention will now be described with references to the accompanying drawings, in which:

( Figures I A and I B show a schematic diagram of a material exhibiting an AMR effect in accordance with an embodiment of the present invention; Figure 2 shows the layer structure of an AMR device according to a further embodiment of the present invention; Figure 3 shows the AMR in a series of CO2MnGa:5aAs films of different thickness at 1.6K; Figure 4 shows the AMR in a series of Co2MnGa:GaAs films of different thickness at 300K; Figure 5 shows a graph of AMR versus thickness for the CO2MnGa:GaAs films at temperatures of 1.6K and 300K; Figure 6a, 6b, 6c, 6d, be, 6f, 6g and 6h shows MOKE loops for CO2MnGa:GaAs films of different thickness along the easy axis and along the hard axis; Figure 7 shows the resistance versus magnetic field of a CO2MnGa:GaAs films of

thickness 73.1 rim at 300K, for two different geometries of field and current;

Figure 8 shows the dependence of coercive field Hc on thickness for CO2MnGa:GaAs

films at 1.6K and at 300K; Figure 9 shows the AMR in a series of Ni2MnGa:GaAs films of different thickness at 1.6K; Figure 10 shows the AMR in a series of Ni2MnGa:GaAs films of different thickness at 300K; Figures 1 1 a, I 1 b and 1 1 c shows MOKE loops for Ni2MnGa:GaAs films of 50 rim thickness, with the applied field along selected directions;

( Figure 12 shows the coercive field versus film thickness for Ni:MnGa:GaAs films at

300 K and at 1.6K; Figure 13 schematically illustrates a sensor in accordance with a preferred embodiment of the present invention; and Figure 14 schematically illustrates the sensor of figure 13 in a bridge configuration.

Figure IA schematically illustrates an anisotropic magnetoresistance sensor 1, comprising a Heusler Alloy 3 having a generally elongate shape. The Heusler alloy 3 has an easy axis 5 along its elongate direction.

The sensor I is in zero magnetic field, thus the Magnetisation direction M (shown as

axis 7) is parallel to the easy axis 5.

A current is passed through the Heusler alloy 3 along direction 9, such that there is an angle O' between the current direction 9 and the magnetization direction 7.

Figure 1 B schematically illustrates the sensor I of figure 1 A in the presence of a magnetic field H (shown along axis I I perpendicular to the easy axis 5). Due the

presence of the external magnetic field I 1, the magnetization axis 7 is rotated such that

it now forms an angle 02 with the current direction 9.

In a magnetic material showing an AMR effect, such as Heusler alloy 3, the electrical resistivity p of the material is dependent on the angle between the magnetization 7 and the direction of the electric current 9. This can be described by the equation: P() = Pi + Ap cost (I) (1) where PI is the resistivity when the magnetic field and applied current are

perpendicular. is the angle between the applied field and the applied current.

( 8 UP = Pit - P1, is the magnetoresistance value. The AMR is usually quoted as-, and PI expressed as a percentage value. Thus, since the angle 0 between the magnetization 7 and direction of the electric current varies depending on the field, the resistance varies

depending on the strength of the field.

The AMR effect can arise due to electron spin-orbit coupling. Under an applied magnetic field, the spin-orbit coupling causes a spin dependent shift in the energy levels

of electrons, leading to a shift in the Fermi levels. The result is that the electron scattering is dependent on the angle between the electron wavevector and the magnetization direction.

Figure 2 schematically illustrates a layer structure of an AMR sensor in accordance with an embodiment of the present invention.

A Polycrystalline wafer 23 of CO2MnGa was grown at a pressure of I -2x 107 mbar with a nominal temperature of 20 C on an GaAs(001) epi-ready substrate 21.

Four wafers were prepared in the above manner. The thickness of the polycrystalline layer for each of the wafers was 12.9 urn, 17.0 nm, 53.9 nm and 73.0 nm.

X-ray diffraction measurements using a Bede 200 system indicate that the films are polycrystalline. The GaAs 0 0 4 diffraction peak is observed to broaden in a 300 urn film of Co2MnGa:GaAs although in the thinner films there are no clear reproducible diffraction peaks close to the GaAs 0 0 4 or 0 0 2 peaks. The wafer thicknesses of were determined using an in-situ crystal monitor and verified with a Dimension 3000 AFM Nanoscope in tapping mode. All the wafers have a Curie temperature > 300K.

The lattice parameter of the CO2MnGa film is 5.743}L which is a 1.5% mismatch with GaAs. Thus, there is little strain at the interface between substrate 21 and the Heusler alloy layer 23.

( The films were measured and were found to have a small uniaxial magnetic anisotropy.

The easy axis is parallel to [0,-1,1]. The temperature dependence of the resistance shows a metallic behaviour Figure 3 is a plot of AMR P) against rotation angle for the four wafers having PI thicknesses 12.9 nm, 17.0 nm, 53.9 nm and 73.0 rim at 1.6K.

The AMR for all four wafers follows a cos20 dependence on rotation angle, as expected.

The largest AMR is 8% at 1.6 K measured for the 73.1 rim wafer. This is remarkably large considering the polycrystalline nature of the wafers. In the thin wafers (t<20 nm) the AMR is weakly temperature dependent, but is still large, 1-2% due to scattering from the surface and the GaAs interface.

The AMR decreases as the thickness of the wafer decreases.

Figure 4 is a plot of AMR against angle of magnetic field for the same four wafers as

Figure 3. However, here the measurements were made at room temperature (300K).

Again, the thickest wafer shared the largest magnetization and all plots followed a cos20 dependence. A 6% AMR is measured for the thickest wafer 73.1 nrn.

Figure 5 is a plot of AMR against layer thickness. AMR and layer thicknesses are plotted on logarithmic scales. The maximum AMR is plotted for the four wafers. The upper trace corresponds to measurements at 1.6K, the lower trace at 300K.

The trend of increasing AMR with increasing layer thicknesses suggests that a higher AMR may be achieved by using thicker wafers.

Figures 6a, 6b, 6c, 6d, be, 6f, 6g and 6h illustrate hysteresis loops achieved by plotting the determined magnetization M scaled to the saturation magnetization Ms against field.

The measurements were made using the Magneto-optical Kerr effect (MOKE). Figures

( 6a, 6b, 6c and 6d illustrates MOKE measurements along the [0,-1, I] axis for the 12.9 rim wafer, 170 nm wafer, the 53.9 run wafer and the 73. 1 nm wafer respectively.

Figures 6e, 6f, 6g and 6h show MOKE measurements along the [0,-1,-1] axis for the 12.9 nm wafer, the 17.0 nrn wafer, the 53.9 nrn wafer and the 73. 1 wafer respectively.

The figures demonstrate that the easy axis is along the [0,-1,1] direction with a weak uniaxiai anisotropy. ne easy axis hysteresis loop is nearly square, with squareness MR/Ms values up to 0.97i0.2 where MR iS the remanent magnetization. A uniaxial magnetic anisotropy is observed with the [0,-1,-1] direction a hard axis characteristic.

He values are in the region of 20 to 30 G with no clear trend with thickness at 300 K. A uniaxial anisotropy has also been observed for CO2MnGa grown on a Si(001) surface and an InAs(001) surface but in thick wafers (t>300 nrn) the anisotropy disappears with an increase in coercive field to 170 G at 300 K.

Figure 7 is a plot of the resistance in ohms against magnetic field in G for the 73.1 urn

thick wafer.

The upper trace corresponds to sweeping the magnetic field positive then negative with

the magnetic field parallel to the current. The lower trace corresponds to sweeping the

field positive then negative with the field perpendicular to the current direction.

The coercive fields determined from the easy axis AMR switching agree with the

coercive fields determined by the magneto-optical Kerr effect (MOKE) at 300K.

The low coercive field means that the alloy can be used to detect small magnetic fields

and can hence be used as a very sensitive detection.

Figure 8 is a plot of He against layer thicknesses for the four layers described with reference to Figure 2. The upper trace is for measurements at a temperature of 1.6K, the lower trace for measurements at 300K. It can be seen that at low temperature He is dependent on thickness, varying from 80 G at 73 nrn up to 400 G at 12.9 rim thickness and has a very similar behaviour to Nix0Fe20. This indicates that thermal activation of

( domain rotation is important in this material, particularly in the thin film form where surface and interface structure appear to strongly pin domains.

A uniaxial anisotropy characteristic has also been observed in Co2MnGe:GaAs with an easy axis along <l 10>, although Co2MnIn:GaAs(OOI) does not have an in-plane anisotropy. Figures 9 to 12 illustrate results for Ni2MnGa. The films were grown in a similar way to the CO2MnC;a films and have thicknesses 12 nm, 50 nm and 100 nary..

Figure 9 is a plot of AMR against rotation angle for the three Ni2MnGa wafers at 1.6K.

The upper trace is for the 50 rim wafer, the middle trace for the 100 nm wafer and the lowest trace for the 12 rim wafer. Again, the expected cos20 dependence can be seen. In this material, the maximum AMR is only around 2% at 1.6 K. Figure l O is a plot of AMR against rotation angle for the three Ni2MnGa wafers at 300K. The upper trace is for the 12 rim wafer, the middle trace for the 50 rim wafer and the lowest trace for the 100 rim wafer. Again, the expected cos20 dependence can be seen. In this material, the maximum AMR is only around 2% at 300K.

Figures I 1 a, 1 1 b and I I c shows the hysteresis loops for the 50 nm wafer. The determined magnetization over the saturated magnetization is plotted against magnetic field. The data was obtained using MOKE measurement. Figures l la, l lb and 1 lc

show results for the applied field B along the [0,-1,-1], [0,-1,-1] and [0,- l. l] directions

respectively. There is little difference in the hysteresis curve between the three directions, indicating that there is no in-plane anisotropy.

Figure i2 is a plot of the coercive field against film thickness for the three films, both

the x axis and y axis are logaritlunic. The upper trace is for a temperature of INK, the lower trace for a temperature of 300K. In comparison with the CO2MnAs films, there is not a pronounced thickness dependence.

Thus, from the data illustrated in Figures 3 to 12, it is clear that a Heusler alloy can be used as a highly sensitive AMR sensor.

Figure 13 illustrates an AMR sensor in accordance with a further embodiment of the present invention. A patterned aluminium layer 31 is provided overlying the Heusler alloy 33. The aluminium layer 33 is patterned to forte a plurality of parallel stripes arranged at an angle of 45 to the easy axis 35 of the alloy 33.

In an AMR sensor, as previously described, there is a costs relationship relating the resistance and the magnetization direction, where is the angle between the magnetization and the current. The resistance therefore exhibits a strongly non-linear dependence on the external field. This can be linearised by proper biasing.

If the easy axis lies along the same direction as the current will flow, there is also the problem that the sensitivity dR/dH is very small in the proximity of the origin, and disappears entirely for H = 0 (where H is an applied field along the y direction). A

farther disadvantage is that the sign of H cannot be determined since R is a function of H2. One solution would be to set up the thin film so that the easy axis is at 45 to an axis of the thin film. Then, if current is passed along an axis of the film, there will be an angle of 45 between the current direction and the easy axis. The sensitivity will then be at a maximum at fields close to zero, and there will be no problem determining the sign of

H. as a field in one direction will increase the resistance, and a field in the opposite

direction will decrease the resistance.

However, the easy axis may be fixed with respect to the substrate. In this situation, the direction of current flow may be fixed by using patterned layer 31. The patterned layer 31 of Figure 13 is in a so-called "barber pole" pattern. The aluminium layer 31 has a higher conductivity than the Heusler alloy layer 33. The position of the aluminiurn stripes at 45 to the easy axis causes the current to flow through alloy 33 in the direction of arrows 37.

For measurement of external fields H of less than Ho/2 (where Ho is the total anisotropy

field), a barber pole arrangement can provide a fairly linear resistance versus applied

field, with a non-linearity of less than 5%. The change of resistance changes its sign if

the spontaneous magnetization is flipped by 180 . The value of the resistance can be found more accurately by flipping the spontaneous magnetization and determining the value as the arithmetic mean value of the two resistance values before and after flipping.

It is commonly known to use magnetic sensors in a Wheatstone bridge configuration with four individual resistors, to convert the resistance changes into a voltage without a dc component. By using Barber poles with angles of 45 and 135 , resistors with a positive and a negative DR in the linear range can be realised. In order to obtain a maximum voltage output, two diagonally opposite resistors have barber poles under 45 , and the other two, under 135 . This set-up also compensates for a temperature dependence of the resistors.

Such a configuration is schematically illustrated in Figure 14. Here, four magnetoresistance elements 41, 43, 45 and 47 are arranged in a bridge configuration.

The first pair of diagonally opposing elements 41 and 47 have the same current orientation as each other. The second pair of diagonally opposing elements 43 and 45 have the same current direction as each other. The current direction of the first pair of elements is perpendicular to the current direction of the second pair of elements.

Claims

( 14 CLAIMS:

An anisotropic magnetoresistive sensor comprising a Heusler alloy.
2. A magnetoresistive sensor according to claim 1, wherein said Heusler alloy has a composition X:YZ? where X is a transition metal, Y is Mn, and Z is selected from the group of Ge, Si, Ga and Sb.
3. A magnetoresistive sensor as claimed in claim 2, wherein X is selected from the group of Co, Cu. Ni, Fe and Pt.
4. A magnetoresistive sensor as claimed in either of claims 2 or 3, wherein Z is Ga.
5. A magnetoresistive sensor as claimed in claim 4, wherein X is Co.
6. A magnetoresistive sensor as claimed in any one of the previous claims, wherein said Heusler alloy possesses a structure from one of the group: Lid, DO3, B2.
7. A magnetoresistive sensor as claimed in any one of the previous claims, wherein said Heusler alloy is a thin film layer with a thickness of at least 40 nm.
8. A magnetoresistive sensor according to claim 7, wherein said thin film has a thickness of at least 60 nm.
9. A magnetoresistive sensor according to any preceding claim, wherein said Heusler alloy is provided on a single crystal structure.
10 A magnetoresistive sensor according to claim 9, wherein said Heusler alloy is provided on a GaAs (001) substrate.
11. A magnetoresistive sensor according to any preceding claim wherein said Heusler alloy has a polycrystalline structure.

(
12. A magnetoresistive sensor according to any of claims 1 to 10, wherein said Heusler alloy is a single crystal layer.
13. A magnetoresistive sensor as claimed in any one of the previous claims, comprising no more thar' one ferromagnetic layer.
14. A magnetoresistive sensor as claimed in any one of the previous claims, wherein said Heusler alloy is configured such that its easy axis is non-parallel to the direction configured for current flow.
15. A magnetoresistive sensor according to claim 14, further comprising a patterned layer having a lower resistance than that of said Heusler alloy provided on said Heusler alloy to influence the direction of current flow through said Heusler alloy.
16. A magnetoresistive sensor according to claim 15, wherein said pattern of said patterned layer has a plurality of stripes formed non-parallel or perpendicular to said easy magnetization axis.
17. A magnetoresistive sensor according to any of claims 14 to 16, wherein easy magnetization axis forms an angle of between 20 and 70 with the direction configured for current flow.
18. A magnetoresistive sensor according to claim 17, wherein the easy magnetization axis forms an angle of between 40 and 50 with the direction configured for current flow.
19. A magnetoresistive sensor as claimed in any one of the previous claims, wherein said Heusler alloy is formed into four anisotropic magnetoresistive elements, the four elements being arranged in a bridge configuration.
20. A magnetic memory device comprising the anisotropic magnetoresistive sensor of any one of the previous claims.

/
21. A method of detecting a magnetic field, comprising:

passing a current through a Heusler alloy in a first direction, said first direction being non parallel to the easy axis of the Heusler alloy; and measuring the resistance using said current to determine the presence of a magnetic field.
22. A method as claimed in claim 21, further comprising calibrating said element by measuring said voltage difference under a plurality of different magnetic fields.
23 A method as claimed in claim 21, wherein said method is performed at room temperature.
24. A method as claimed in any of claims 21 to 23, wherein said magnetic field

applied to said magnetic sensor is less than I mT.
25. A magnetoresistive sensor as substantially hereinbefore described with reference to any of the accompanying drawings.
26. A method of detecting a magnetic field as substantially hereinbefore described

with reference to any of the accompanying drawing