GB2604921A

GB2604921A - Ultra-low temperature rotator apparatus

Info

Publication number: GB2604921A
Application number: GB2103843.5A
Authority: GB
Inventors: Bryant Ben; Matthews Anthony
Original assignee: Oxford Instruments Nanotechnology Tools Ltd
Current assignee: Oxford Instruments Nanotechnology Tools Ltd
Priority date: 2021-03-19
Filing date: 2021-03-19
Publication date: 2022-09-21
Also published as: GB202103843D0; WO2022195298A1

Abstract

An ultra-low temperature (ULT) rotator apparatus 101 for effecting and monitoring an orientation of a rotatable member within a magnetic field comprises: a rotator mechanism 103 to control rotation of the rotatable member; and a sensor arrangement 105 mounted to the rotatable member to output a signal representative of the orientation of the rotatable member with respect to a direction 102 of the magnetic field. The sensor arrangement comprises one or more ULT magnetic sensors, such as Hall sensors. Each of the one or more magnetic sensors monitors field strength with a field responsivity of at least 1 x 10-4 VT-1 while generating heat at a rate of less than 1 x 10-8 W. The rotator may be used to rotate a specimen in the field. The apparatus may be used at millikelvin temperatures.

Description

ULTRA-LOW TEMPERATURE ROTATOR APPARATUS

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for monitoring and effecting rotation of a member with respect to a magnetic field in an ultra-low temperature environment.

BACKGROUND TO THE INVENTION

In many low-temperature measurements it is desirable to apply a magnetic field at an arbitrary angle to a sample. Conventional approaches to this involve the use of a multiple-axis 'vector' magnet. However, such magnets typically cannot supply as strong a magnetic field as those which single axis magnets, such as solenoids, are able to generate. Rotating a sample to be analysed relative to a fixed-orientation magnetic field can allow for fields up to 22 T to be applied in any direction. Sample rotators that have been used for such applications are conventionally of two general types: mechanical and piezo rotators. Mechanical rotators use a mechanical linkage, typically driven by a room temperature dial or stepper motor, to rotate a low-temperature sample stage. Piezo rotators have no mechanical linkage from the sample area, but instead use a piezo-driven slip-stick or walker type rotator motor It is necessary to be able to determine accurately the angle of the sample to the magnetic field. Existing rotators for low-temperature environments often use mechanical resistive or conductive encoders to measure rotation angle. Both mechanical and piezo sample rotators suffer from a high degree of uncertainty in terms of measurement of the relative angle of the sample to the magnetic field. Mechanical rotators introduce uncertainty due to slip or backlash in the linkage, while piezo rotators are limited in this regard by their step size not being highly reproducible Therefore, conventionally these approaches have relied on encoders for angle measurement, the accuracy of which is limited.

Further issues arise for rotators operating at ultra-low temperatures (ULT) (for example less than 100 mK), where heat load must be minimised. For mechanical rotators, mechanical feedthroughs to ULT stages represent a major challenge for cryogenic engineering. Piezo rotators provide a more convenient and flexible method for ULT sample rotation, but typically employ resistive encoders which are not suitable for use at ultra-low temperatures due to their substantial heat dissipation. Hall-effect magnetic field sensors, either used individually or in two-or three-axis configurations, may accurately measure sample angle relative to a magnetic field. However, conventional Hall sensors operate at high currents of 1- 100 mA, meaning that their correspondingly high heat dissipation renders them unsuitable for ULT operation.

Additionally, in many applications, accurate temperature measurement is required for precise cryogenic experiments. A common method for measuring temperature is to utilize the resistance of a specialised device, where the variation of resistance with temperature is known or can be calibrated. Common temperature sensors of this type include Cernox, Ruthenium oxide, Silicon diodes and platinum sensors.

It is a long-standing challenge to measure accurately the temperature in a cryogenic apparatus which is also subject to an intense magnetic field. This is because most methods of measuring temperature suffer from a shift in apparent reading in high magnetic fields. A common example would be temperature measurement via the resistance of a specialised device such as a Cernox sensor: the sensor exhibits magnetoresistance, and the apparent temperature reading therefore changes with magnetic field. This effect is particularly pronounced in intense magnetic fields with strengths in excess of 20 T. Some other temperature measurement techniques used at ultra-low temperatures, such as SQUID-based noise thermometry, cannot be used directly in high magnetic fields at all, as the necessary superconducting elements can be driven normal by the applied field.

Therefore a need exists for a device that can enable accurate measurement of the relative angle of a sample to an external magnetic field, which can be integrated into an assembly for rotating the sample relative to the direction of the magnetic field, and where the sensor can be operated at ultra-low temperatures (ULT). Furthermore it is desirable to integrate such a rotation angle sensor into a complete ULT sample rotator product.

It is an aim of this disclosure to provide a sample environment combining mK-temperatures and high magnetic fields in any sample orientation, in which the rotation angle can be accurately measured. This disclosure addresses the further need for a sensor device that can simultaneously additionally measure temperature, and magnetic field strength, and which also works even in highly

intense magnetic fields.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided an ultra-low temperature, ULT, rotator apparatus for effecting and monitoring an orientation of a rotatable member within a magnetic field, the apparatus comprising: a rotator mechanism adapted to control rotation of the rotatable member, a sensor arrangement mounted to the rotatable member and configured to output a signal representative of the orientation of the rotatable member with respect to a direction of the magnetic field, the sensor arrangement comprising one or more ULT magnetic sensors, wherein each of the one or more ULT magnetic sensors is adapted, when in use at ultra-low temperatures, to monitor magnetic field strength with a field responsivity of at least 1 x 10-4 VT-1 while generating heat at a rate of less than 1 x 10-8 W. The inventors have devised an approach to achieving precise measurements of magnetic field orientation in an ultra-low temperature (ULT) environment. It has been found that it is possible to measure a relative angle to an external magnetic field of a sample, where the sample, sensors, and rotating mechanism are operated at such ULT conditions, less than 100 mK. This is facilitated by the use of low-heat dissipation magnetic field sensors, the architecture of which allows for a very low operating current and therefore low heat generation, enabling their use at such temperatures. Sensors of this type may be integrated into an assembly for rotating the sample relative to the direction of the field, enabling high-precision rotation measurement and control in ULT environments.

Typically the rotatable member comprises or is connectable to a sample holder to enable controlled rotation of the sample with respect to the field by way of the mechanism. The magnetic field is typically homogeneous, at least in terms of its direction, throughout a region of interest in which the rotatable member is disposed. If any inhomogeneity exists in the field, with respect to the field strength for example, the effect of any such variations upon measurements can be reduced or eliminated by way of particular sensor arrangements as described later in this

disclosure.

In some embodiments the sensor arrangement comprises a single ULT magnetic sensor. Although the ULT magnetic sensors are adapted to monitor magnetic field strength magnitude, rather than direction, single-sensor arrangements may be used to determine a relative angle with respect to the field. This may be achieved by way of identifying an orientation of the sensor and member corresponding to a maximum or a minimum monitored field strength, and associating that orientation with the sensitive axis of the sensor being parallel to and perpendicular to the magnetic field direction, respectively. Some embodiments may include additional ULT magnetic sensors in the arrangement.

The signal output from the sensor arrangement, whether it includes a single or multiple sensors, can be representative of the orientation of the member with respect to the field direction in a variety of ways. For example, in single-sensor arrangements, the signal may represent the magnitude of the component of the monitored magnetic field that is aligned with the sensitive axis of the sensor. Thus in accordance with a signal monitored by that sensor for either or both of the maximum and minimum monitored field strengths as described above, the orientation angle with respect to the field direction may be derived, and so the signal may be representative of the orientation in that manner. Typically in single-sensor embodiments the rotator mechanism is capable of rotating the member through a full revolution in order for the reference minimum and/or maximum values to be identified.

In multiple-sensor arrangements, the signal output by the arrangement may be representative of the orientation by way of some combination of signals output by multiple sensors, each of those individual sensor signals being representative of a respective magnetic field magnitude, indicating the strength of the field component parallel to the sensitive axis of the respective sensor at a given time.

It is generally not necessary to obtain absolute measurement values of field strength from any of the sensors in the arrangement for the purpose of monitoring orientation. Thus it is typically not required for a particular value of field responsivity, within the operating condition ranges discussed later in this disclosure at least, to be known or predetermined in use.

The representation, by the signal, of angle can be generated or provided in accordance with or based on monitored field strength at more than one orientation, and/or more than one sensor mounted so that they are oriented at different angles with respect to the field direction.

The properties of the ULT magnetic sensor or sensors included in the sensor arrangement are important to achieving the aforementioned advantages. A ULT magnetic sensor as referred to in this disclosure may be thought of generally as a magnetic sensor. The aforementioned field responsivity and heat generation properties of the sensor refer to operating characteristics of the sensor when it is used to monitor magnetic field strength and is at ultra-low temperatures. This typically refers to temperatures less than 1 K. More specifically, the ultra-low temperatures may be thought of as being less than 100 mK or less than 10 mK. Thus when the temperature of the sensor or sensing element thereof is within such a range of ultra-low temperatures, the sensor may monitor the magnetic field strength with the said field responsivity and heat generation levels. As noted above, the sensor need not necessarily measure the magnetic field strength in the sense of providing a numerical value, but rather is typically configured at least to provide a signal responsive to the field strength that can be used to infer or calculate orientation of the sensor or sensors with respect to the magnetic field direction.

It will be understood that the field responsivity refers generally to an electrical response in the sensor to magnetic field strength experienced by or passing through the sensor element thereof. In particular it may be understood as a voltage response to magnetic field strength. More particularly, in the case of Hall-effect sensors, the field responsivity may be defined as a Hall voltage per unit of magnetic field strength. Thus the expression "field responsivity" as used in this disclosure may be thought of, and used interchangeably with, sensitivity, or field sensitivity in particular. It will be understood that responsivity is a known term for quantifying the performance of magnetic field sensors, such as Hall-effect sensors, typically in terms of voltage per unit magnetic flux density. In general, the performance of a magnetic sensor may additionally be described in terms of resolution, which can refer to the smallest resolvable unit of monitored magnetic field. Typically, this is dependent on the accuracy with which a voltage signal from a sensor can be measured. Therefore, for a given voltage measurement capability, greater field responsivity values provide greater precision for monitoring

field strength and thereby orientation.

In some preferred embodiments, the minimum field responsivity with which each of the sensors is adapted to monitor magnetic field strength when in use at ultra-low temperatures is 1.75 x 10-4 VT-1. More preferably it is 2.5 x 10-4 VT-1.

Such high levels of field responsivity, together with minimal heat generation, can enable the apparatus to operate in the advantageous manner described above. The heat generation referred to above typically relates to heat being generated within the sensor itself, or more particularly any sensor elements thereof More generally it may refer to any heat generated within components of the sensor that are in thermal communication with the ULT environment such that the said heat can dissipate into the ULT environment. That is to say, the generated heat typically refers to that which can substantially alter the temperature of a ULT environment of the apparatus in which the sensor arrangement is disposed. Therefore, minimising this generated heat avoids the ULT conditions being disrupted or lost as a result of the sensor operating. The aforementioned upper limit upon the rate of heat generation may alternatively be thought of as an upper limit upon the rate of heat dissipation. Typically it is heat generated by Joule heating within the sensor that gives rise to the deleterious dissipation of heat into ULT environments. The apparatus alleviates this dissipation problem by reducing the heat generation.

In some preferred embodiments, the aforementioned upper limit to the rate of heat generation in use is 5 x 10-8 W. More preferably it is 2 x 10-8 W. More preferably still it is 1 x 10-9 W. The sensor may be adapted to operate with each of these maximum heat dissipation values at each of the aforementioned minimum responsivity values. It may be adapted to have these maximum and minimum properties when operated below any of the temperature thresholds corresponding to ULT conditions noted above.

In practice the particular required degree of field responsivity for a given level of heat generation in use (and for ultra-low temperatures) is typically applicable when the strength of the magnetic field to be monitored is in a given range. For example, these sensor property requirements may be met when the field strength is less than or equal to a threshold value, and might not be met when the field strength exceeds that value. Typically the range in which the defined responsivity and heat generation requirements apply is 0-6 T. Preferably the range is 0-14 T More preferably still the range is 0-30 T. These magnetic field strength values typically refer to absolute values, that is values without regard to the sign of the values.

Each of the one or more ULT magnetic sensors may be defined in terms of a maximum operating current. This maximum current is typically based on a given degree of electrical resistance to the driving current in the sensor element. Typical resistance, at least while in use under ULT conditions, is in the range 1200 x 1030. It will be understood that typically, higher-sensitivity magnetic sensors, such as Hall sensors for instance, have higher resistances. Since heat generation is related to current I and resistance R by I2R, and sensitivity is typically linear with current, high-sensitivity, low-heat generation sensors are typically configured to operate at higher resistance and run at low current. Preferably, each of the one or more ULT magnetic sensors is adapted to operate, or in particular to monitor the magnetic field strength with the aforementioned sensitivity, with an applied current less than 1 x 10-5 A. More preferably, the current is in the range 1 x 10-8A to 1 x 10-5A. In other words, the sensor may be adapted such that the current required to achieve the aforementioned sensitivity may be less than the said upper limit. Low current requirements can advantageously reduce the resulting heat generation and dissipation by the sensor in use. These properties may preferably be exhibited in any of the aforementioned ULT ranges.

Typically each of the one or more ULT magnetic sensors is a Hall-effect sensor.

Such sensors are particularly suited to being adapted to the ULT application. Other sensor types, such as magneto resistive sensors are also envisaged, and may be used in a similar manner to Hall sensors. Other embodiments may include a 3D SQUID (Superconducting Quantum Interference Device) in the sensor arrangement.

In accordance with a second aspect of the invention there is provided an ultra-low temperature, ULT, rotator apparatus for effecting and monitoring an orientation of a rotatable member within a magnetic field, the apparatus comprising: a rotator mechanism adapted to control rotation of the rotatable member, a sensor arrangement mounted to the rotatable member and configured to output a signal representative of the orientation of the rotatable member with respect to a direction of the magnetic field, the sensor arrangement comprising one or more ULT magnetic sensors, wherein each of the one or more ULT magnetic sensors is a Hall-effect sensor, and wherein each of the one or more Hall-effect sensors comprises a sensing element having an electron mobility greater than 1 x 104 cm2V-1s-1 at a charge carrier electron density of 1 x 1011 cm-2 at ultra-low temperatures Any of the properties and features described in relation to the preceding and following embodiments in this disclosure may relate to the apparatus of either or both of the first and second aspects.

It will be understood that in the Hall-effect sensors, the carrier density, mobility, conductivity, and sensitivity of the sensing element are coupled. In general, however, preferably the sensor is provided with a low carrier concentration, in order to achieve a large Hall response. Thus the aforementioned preferred electron mobilities typically relate to a given carrier density with the value also noted above. For that electron density, preferably the material from which the sensing element is formed has an electron mobility greater than 6 x 105mc 2v-1 s-These are preferred values for two-dimensional sensor materials that may be used. Graphene is a typical example of a suitable material from which a sensor having these properties may be formed. It will be understood that these material properties are typically exhibited by the sensor when the temperature of the sensor or sensing element specifically is within a range of ultra-low temperatures. Typically these properties hold at temperatures of 4 K and lower.

Preferably each of the one or more ULT magnetic sensors comprises a sensing element provided as a layer That is the sensing element is typically arranged as a sheet or a plane. It is thus generally components of the external magnetic field that are perpendicular to that plane to which the sensor is sensitive. In addition to graphene, as noted above, the sensing element layer may alternatively comprise a gallium arsenide thin film. More particularly, in some embodiments each of the sensors may comprise a layer of a two-dimensional material. The use of such materials may be advantageous for achieving the precise sensing capabilities described above. In this context, the material being referred to as two-dimensional refers to a single-layer material, that is typically comprised of a single layer of atoms. Graphene is a well-known example of such a material. Thus, in some preferred embodiments each of the one or more ULT magnetic sensors comprises a sensing element comprising a graphene sensing layer Graphene-based sensors are advantageous because they may provide the desired high-sensitivity, low-dissipation operation across a wider range of temperatures than other materials, and additionally typically do not suffer from carrier freeze-out at ultra-low temperatures. Materials such as gallium arsenide as mentioned above may alternatively be employed. However, generally the heat generation within such sensors is greater than the heat generation that occurs in an equivalent graphenebased sensor under equivalent conditions.

In accordance with a third aspect of the invention there is provided an ultra-low temperature, ULT, rotator apparatus for effecting and monitoring an orientation of a rotatable member within a magnetic field, the apparatus comprising: a rotator mechanism adapted to control rotation of the rotatable member, a sensor arrangement mounted to the rotatable member and configured to output a signal representative of the orientation of the rotatable member with respect to a direction of the magnetic field, the sensor arrangement comprising one or more ULT magnetic sensors, wherein each of the one or more ULT magnetic sensors comprising a sensing element formed from a two-dimensional material.

Preferably the two-dimensional material is graphene. Any of the preceding and following features and implementation details described in relation to the embodiments throughout this disclosure may be present in any of the apparatuses according to the first, second, and third aspects as appropriate.

Of the various sensors with differing architectures that may be used at ultra-low temperatures, in practice many may exhibit similar limitations when used under such conditions. These limitations may include a degree of non-linearity and temperature-dependence of magnetic field sensitivity. In some embodiments, therefore, two or more sensors may be used, typically in an orthogonal configuration, in order to monitor rotation angle without being affected by these limitations. Therefore preferably each of the sensors in multiple-sensor arrangements have identical or sufficiently similar non-linearity and/or temperature dependence. In preferred embodiments, wherein two sensors are mounted orthogonally to one another, a ratio of their output voltages may be taken in order to calculate the orientation angle. Although this calculation is simplified by such an orthogonal arrangement, it is also envisaged that other relative orientations between the two sensors may be possible. The first sensor may be mounted at a known or predetermined angle orientation with respect to the second sensor, for instance, and an angle may be calculated in accordance with that information and the output of the first and second sensors. Accordingly, in some embodiments, the sensor arrangement comprises two Hall-effect sensors, mounted such that a sensing layer of a first one of the two Hall-effect sensors is aligned with a first plane that is perpendicular to a second plane, with which a sensing layer of a second one of the two Hall-effect sensors is aligned, and wherein the rotator mechanism is adapted to effect rotation of the rotatable member about first rotation axis, the sensor arrangement being mounted such that the rotation axis is parallel to an intersection of the first and second planes.

In embodiments with a multi-axis sensor arrangement such as these, preferably the spacing between the multiple sensors is smaller than the scale of an inhomogeneity that might be present in the magnetic field. Preferably a maximum distance between any two sensors in the arrangement is 10 mm, more preferably 5 mm. In typical applications such close spacing renders negligible any effects of spatial variation in magnetic field within the monitored environment. Although preferably such embodiments comprise two ULT magnetic sensors, each being a Hall-effect sensor, as noted above, it is also envisaged that each of the sensors may alternatively be a different type of magnetic sensor with a planar or two- dimensional sensing element. Such alternatives apply equally to the below-described embodiments involving Hall-effect sensors where appropriate.

In some other embodiments, the apparatus can be configured to use the output of a third sensor, representing a monitored magnitude of a field component perpendicular to those monitored by the first and second sensors, for example. In this way the apparatus may monitor and hence control rotation about an additional spatial axis. Therefore in some embodiments the sensor arrangement comprises a third ULT magnetic sensor, typically a Hall-effect sensor, mounted such that a sensing layer of the third sensor is aligned with a third plane that is perpendicular to the first and second planes, and wherein the rotator mechanism is adapted to effect rotation of the rotatable member about a second rotation axis perpendicular to the first rotation axis.

In accordance with a fourth aspect of the invention there is provided a system for applying a magnetic field to a specimen in an ultra-low temperature, ULT, environment, the system comprising: an apparatus for maintaining a ULT environment within the specimen chamber, an apparatus according to any of the first, second, and third aspects, wherein the rotatable member is disposed within the specimen chamber and comprises a specimen holder, the specimen holder typically being adapted to hold or attach to a specimen fixedly or such that orientation of the specimen can be fixed with respect to the holder and the rotatable member, and a control module configured to control the rotator mechanism so as to effect a predetermined orientation of the specimen, or the holder, the former typically requiring the specimen mounting/fixing orientation with respect to the holder to be known or predetermined, with respect to the magnetic field, in accordance with the signal.

In accordance with a fifth aspect of the invention there is provided a method for effecting and monitoring an orientation of a rotatable member within a magnetic field in an ultra-low temperature, ULT, environment, the method comprising: monitoring an orientation of the rotatable member using a sensor arrangement mounted to the rotatable member and configured to output a signal representative of the orientation of the rotatable member with respect to a direction of the magnetic field, the sensor arrangement comprising one or more ULT magnetic sensors, wherein each of the one or more ULT magnetic sensors is adapted, when in use at ultra-low temperatures, to monitor magnetic field strength with a field responsivity of at least 1 x 10-4 VT-1 while generating heat at a rate of less than 1 x 10-8 W, controlling rotation of the rotatable member in accordance with the signal, which may be understood as being in accordance with the monitored orientation.

The method may further comprise maintaining a ULT environment within a specimen chamber, wherein the rotatable member is disposed within the specimen chamber and comprises a specimen holder, and controlling, typically, by the said controlling rotation of the rotatable member, the rotator mechanism so as to effect a predetermined orientation of the specimen with respect to the

magnetic field, in accordance with the signal.

In some embodiments the controlling the orientation of the rotatable member is performed while the strength of the magnetic field is less than or equal to a predetermined threshold value. Typically the strength to which this refers is the said monitored field strength. This field strength may alternatively or additionally be a field strength monitored by a further magnetic field sensor that is not one of the one or more ULT magnetic sensors comprised by the arrangement, or may be a field strength value provided indirectly. In any case, typically the magnetic field may be understood as that to which the sensor is exposed, that is at least in the vicinity of the sensor. The predetermined threshold value typically corresponds to an upper bound of the field strength range in which the ULT magnetic sensor is known to operate with the defined sensitivity and heat generation requirements. The threshold may typically be 6 T, or 14 T or 30 T as noted above. In some applications it is desirable or necessary to operate using a specimen in a magnetic field with a strength in excess of that at which the ULT magnetic field sensor can monitor the field with the defined high degree of sensitivity and low heat generation. In such cases, an alternative method of operation may be used whereby the field strength is returned to a lower value, within the range in which the sensor properties defined above hold, in order to make angle measurements and/or adjustments, before being ramped or swept for measurements. Thus the method may further comprise controlling, which typically comprises reducing, the magnetic field such that the strength of the magnetic field is less than or equal to the predetermined threshold value.

In preferred embodiments the sensor arrangement comprises two Hall-effect sensors, that is the sensor arrangement comprises two ULT magnetic sensors, each being a Hall-effect sensor. Other sensor types are envisaged, however.

Typically the two sensors are mounted such that a sensing layer of a first one of the two sensors is aligned with a first plane that is perpendicular to a second plane, with which a sensing layer of a second one of the two sensors is aligned, and the rotator mechanism is adapted to effect rotation of the rotatable member about a first rotation axis, the sensor arrangement being mounted such that the rotation axis is parallel to an intersection of the first and second planes.

Some embodiments additionally include a third ULT sensor in the arrangement. This may be a Hall-effect sensor and is preferably of the same type as, and more preferably is identical in its electrical properties to, the first and second sensors.

The third sensor is typically mounted such that the sensing layer thereof is aligned with a third plane that is perpendicular to the first and second planes. In such embodiments typically the rotator mechanism is adapted to effect rotation of the rotatable member about a second rotation axis perpendicular to the first rotation axis.

In some implementations of the method, the sensors may additionally be advantageously used to measure, simultaneously with the orientation, the temperature in a cryogenic environment. The use of the sensor arrangement to measure temperatures in this way may be applied in cryogenic conditions generally, as well as at ultra-low temperatures specifically. Such an approach may utilise the effects of temperature upon the resistivity of the sensor material. By way of establishing the resistive properties of the sensor, particularly by projecting those properties in the absence of the applied magnetic field, the temperature of the sensor, independent of the magnetic field, may be found. Such approaches may enable temperatures to be accurately measured with any magnetic field, even relatively intense fields greater than 20 T, in addition to measuring the field strength and the rotation angle, thereby providing additional functionality as well as the cryogenic rotation measurements and control. In particular, the method may additionally be for monitoring a temperature within the cryogenic environment. The method thus typically further comprises: obtaining a zero-field resistance value that is representative of the longitudinal electrical resistance of the first Hall-effect sensor when the sensing layer is oriented parallel to the direction of the applied magnetic field; and calculating, in accordance with the zero-field resistance value and temperature calibration data that is representative of a predetermined relationship between electrical resistance and temperature for the first Hall-effect sensor, a sensor temperature value. Preferably this approach uses the arrangement of two magnetic sensors, preferably orthogonally mounted.

Typically these are Hall-effect sensors, although it is envisaged that these embodiments may be implemented with other types of magnetic sensor such as those described earlier in this disclosure.

The obtaining of the zero-field resistance value typically comprises monitoring the electrical resistance of the first Hall-effect sensor. The resistance value typically corresponds to the resistance to the driving current applied to the sensor device. Typically it is monitored in accordance with a monitored electrical potential difference across the first Hall-effect sensor, thus corresponding to a longitudinal voltage. That voltage, in combination with the applied driving current, can be used to provide a monitored resistance. Such approaches typically involve a first and second set of electrical connections, each configured to measure a voltage across the sensor. Typically these first and second sets of connections are additional to the connections by which the Hall voltage output signal, representative of the sensed magnetic field, is produced. Thus the obtaining of the resistance value preferably includes a four-wire resistance-monitoring arrangement, typically in addition to a two-wire magnetic field-monitoring arrangement.

Although this functionality is described in relation to the first Hall-effect sensor, the choice of sensor upon which this measurement is based is preferably arbitrary. This is because it is typically assumed that the sensors are identical in their electrical resistance properties.

In order for the projected value of zero-field resistance to be accurately representative of the resistive properties of the sensor in the absence of the external magnetic field, the sensor preferably has negligible or zero-field responsivity to a magnetic field component parallel to its sensing plane, that is, perpendicular to its sensitive axis. The use of sensors having zero or negligible planar Hall effect, and preferably likewise negligible planar magneto resistance, facilitates this. This requirement may be realised in sensors in which the active element is a two-dimensional material such as graphene. However, it is also envisaged that this may be realised in more conventional sensors.

The temperature calibration data may be understood as resulting from a calibration of the zero-field resistivity-temperature relationship for the sensor. In general this calibration data is obtained by way of empirical measurement, rather than calculation. In principle, no information is needed about the sensor material for this purpose. Typically, the calibration is performed against a known temperature-measuring device, at zero applied magnetic field, for the temperature interval desired. It is possible that all similar devices have substantially identical resistivity/temperature relationships, and therefore that a single, general calibration will be applicable to all such sensors in some embodiments. Typically, in multi-sensor arrangements, the sensors are identical to one another, at least in terms of these characteristics. It is possible that individual devices in some embodiments may differ, and require separate calibration. It will be understood that the sensor temperature value typically refers to the temperature in or of the first sensor, particularly the sensor element.

In some embodiments, it is desirable to calculate the temperature at a given time, and when the rotatable member is at any given state orientation, that is without any additional requirements such as a particular rotation being effected, in order to monitor zero-field resistance directly. Preferably, therefore, the obtaining of the zero-field resistance value comprises calculating, in accordance with the monitored orientation of the rotatable member with respect to the magnetic field, the zero-field resistance value.

The monitored temperature can also be used to monitor simultaneously the magnetic field strength, in addition to the temperature and orientation measurements. Accordingly, the method may, further, be for monitoring magnetic field strength within the cryogenic environment, the method further comprising: calculating, in accordance with the calculated sensor temperature value, the signal output by the sensor arrangement, and field responsivity calibration data that is representative of a predetermined relationship between field responsivity and temperature for the first Hall-effect sensor, a field strength value representative of the field strength of the applied magnetic field. As alluded to above, the signal typically represents orientation with respect to the external magnetic field, and for example may comprise or indicate a ratio of signals of first and second orthogonally mounted sensors. In embodiments wherein the arrangement comprises a third, orthogonally mounted sensor, a ratio including the signal from the third sensor may analogously be calculated, and the three sensor signals may be used in combination to obtain two-axis orientation information representing the orientation of the arrangement or rotatable member in three-dimensional space.

In this way, the magnitude of the field represented by the output of a single sensor, for example a Hall voltage, which may correspond to a sensor plane-perpendicular component of the total field, may be used together with a known sensitivity or responsivity of the sensor in order to calculate the strength of that total field. Since the temperature has been calculated, and by way of providing calibration data by which dependence of responsivity varies with sensor temperature, it is possible to

obtain simultaneous field strength readings.

The predetermined relationship between field responsivity and temperature is typically provided by way of calibration against known standards. The relationship may be complex and is typically difficult to calculate. With regard to the longitudinal resistance, it is possible that all similar devices will have essentially identical properties, or that devices will need to be individually calibrated. As noted above, although this functionality is described in relation to the use of Hall-effect sensors specifically, embodiments are envisaged in which magnetic sensors of different types are used. It will be understood that such variations are possible if the requisite information about the relationships between responsivity, temperature, and resistance for the chosen sensors is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention will now be described, with reference to the accompanying drawings, wherein like reference numerals indicate like features, and in which: Figure 1 is a schematic perspective view of a first example apparatus according to the invention; Figure 2 is a flow diagram showing steps of a first example method according to the invention; and Figure 3 is a schematic perspective view of a second example apparatus according to the invention

DESCRIPTION OF EMBODIMENTS

With reference to Figures 1-3 example apparatuses and an example method are now described. Two perspective views of a schematic representation of a first example apparatus 101 are shown in Figure 1. The apparatus 101 is mounted on a probe inside a cryogenic apparatus (not shown) with a magnet (not shown). A sensor arrangement including a low-dissipation Hall sensor 105 is mounted on a rotatable assembly together with a sample plate 107 on which an experimental sample can be fixed. The assembly can be rotated about the rotation axis r by a mechanical rotator 103. Thus the plate can be rotated relative to the field 102 generated by the magnet. Variants in which the rotator mechanism 103 is alternatively a piezo rotator are also envisaged. The present example is adapted for single-axis rotation of the sample plate 107. However other examples include a two-axis rotator mechanism.

The Hall sensor is used to measure the relative angle of the sample to the external magnetic field 102. The rotation axis r is perpendicular to the magnetic field. The present example uses a single Hall sensor. In order to obtain the relative angle measurement, a 'zero' position for the rotator can be established by rotating the assembly using the mechanism 103 until the Hall voltage is a maximum, thus establishing the orientation at which the direction of the field 102 is perpendicular to the sensitive plane of the sensor 105. Upon further rotation to other orientations, the angle can then be read as the inverse cosine of the ratio of the Hall voltage to the 'zero' position voltage.

In the present example the active element of the low-dissipation Hall-effect sensor 105 is composed of graphene. The sensing plane of the sensor is formed as a square with side length 1.3 mm. Graphene-based Hall sensors have been evaluated for their suitability for magnetic field measurements at ultra-low temperatures. The magnetic Hall voltages of such sensors as a function of magnetic field up to 14 T were tested in a dilution fridge at a base temperature of 10 mK. The rate of heat being generated and dissipated by the sensor was found to be less than 1 nW. The Hall response was found to be approximately linear for field strengths with magnitudes up to 6 T, with a field responsivity in excess of 1 x 10-4 V/T at a drive current of 100 nA in this field strength range and at the aforementioned temperature. The example sensor was found to exhibit some nonlinearity (approximately 1%), and significant dependence of sensitivity on temperature was observed.

A second example apparatus corresponding to a preferred implementation is shown in Figure 3. The apparatus 301 and depiction thereof are similar to those of Figure 1. In the present example, however, in place of the single sensor, mounted to the assembly with the sample plate 307 are two Hall sensors 305. A first one of the sensors is mounted so that its sensitive plane is parallel to the sample plate 107. The second sensor and its sensitive plane are perpendicular to the first sensor and the sample plate. Both sensors are monitored simultaneously. The angle of the sample plate to the magnetic field can be determined directly as the inverse tangent of the ratio of the two Hall voltages.

Using two or more Hall sensors thus provides the advantage of obviating the need to establish a 'zero' position voltage. This configuration also has the advantage that, since a ratio of Hall voltages is recorded, the angle measurement is insensitive to any nonlinearity in the sensor Hall response, and to any temperature dependence of the Hall sensor sensitivity, if first and second sensors having sufficiently similar nonlinearity and temperature dependence are provided.

Further examples can additionally include a third Hall sensor, mounted to the assembly with its sensitive plane orthogonal to those of the first and second sensors. Such a three-axis sensor arrangement enables monitoring of multi-axis rotation in applications wherein the rotator mechanism is so configured, as mentioned above. In such multi-axis variants of these examples, the spacing between sensors is minimized in order to prevent inhomogeneity in the strength of the field affecting the respective readings of the separate sensors. The three-axis sensor arrangement can be provided as a cube with a side length of up to approximately 3 mm for example A method of effecting and monitoring an orientation of a rotatable member within a magnetic field, using an apparatus such as either of those of the first and second examples, is illustrated in Figure 2. An experimental specimen is fixed to the sample holder in a cryogenic specimen chamber, in which an ultra-low temperature of less than 100 mK is maintained 201.

At step 202 the orientation of the rotatable member including the sample plate is monitored using the output of the sensor arrangement. Preferably the orientation is calculated using two voltage signals representative of the magnitude of the magnetic field sensed by the two respective sensors mounted orthogonally to one another, as described above with reference to the second example apparatus.

This reading can be used, while controlling the rotator mechanism, to bring the rotation state of the assembly precisely into a desired or predetermined angle with respect to the direction of the magnetic field 203. Achieving this precision, while maintaining the ULT conditions in the cryogenic apparatus, is facilitated by the high-sensitivity, low-heat generation magnetic sensors employed to measure the sample orientation.

With reference again to Figure 2, additional steps for measuring temperature and magnetic field strength may optionally be performed. These are preferably carried out using a sensor arrangement according to that of the second example apparatus, that is one including at least two orthogonal sensors. As noted above, each of the sensors is sensitive to temperature as well as to magnetic field.

It is assumed for this example method that the magnetic field sensitivity of each sensor is negligible when its sensitive axis is perpendicular to the field direction, and therefore the temperature sensitivity of the sensor when in that orientation is not susceptible to shift in its apparent reading due to the applied magnetic field. The method exploits this, and the magnetic field sensitivity of the sensors, in order to measure, simultaneously and immediately, temperature, magnetic field, and the

orientation of the sensor to the magnetic field.

Two or more sensors are arranged in an orthogonal array: the Hall voltage and longitudinal resistance of each sensor can be measured simultaneously. The field strength, angle of the array to the applied magnetic field, and the temperature are determined as follows. Firstly, the angle of the array to the magnetic field is obtained as the inverse tangent of the ratio of the Hall voltages of the sensors as described above. This calculated value is insensitive to the temperature of the array, assuming that the sensors all have identical dependence of Hall sensitivity on temperature. The zero-field longitudinal resistivity is then projected 204 based on the Hall voltages and the angle to the field. This may be understood as corresponding to the special case in which the rotation angle orients one of the sensors with its sensitive axis precisely perpendicular to the magnetic field. The resistance of that sensor in that case is the "zero-field resistivity".

This zero-field resistivity can be calibrated for temperature, giving a temperature reading 205 which is independent of magnetic field. Finally, with the temperature known, the magnetic field strength can be obtained 206 from the modulus of the Hall voltages and the Hall sensitivity, calibrated for the dependence of Hall sensitivity on temperature.

Claims

CLAIMS1. An ultra-low temperature, ULT, rotator apparatus for effecting and monitoring an orientation of a rotatable member within a magnetic field, the apparatus comprising: a rotator mechanism adapted to control rotation of the rotatable member, a sensor arrangement mounted to the rotatable member and configured to output a signal representative of the orientation of the rotatable member with respect to a direction of the magnetic field, the sensor arrangement comprising one or more ULT magnetic sensors, wherein each of the one or more ULT magnetic sensors is adapted, when in use at ultra-low temperatures, to monitor magnetic field strength with a field responsivity of at least 1 x 10-4 VT-1 while generating heat at a rate of less than 1 x 10-5 W.
2. An apparatus according to claim 1, wherein each of the one or more ULT magnetic sensors is adapted to operate with an applied current less than 1 x 10-5A.
3. An apparatus according to claim 1 or claim 2, wherein each of the one or more ULT magnetic sensors is a Hall-effect sensor.
4. An apparatus according to claim 3, wherein each of the one or more Hall-effect sensors comprises a sensing element having an electron mobility greater than 1 x 104mc 2v-1 -1 S at a charge carrier electron density of 1 x 1011 cm-2 at ultra-low temperatures.
5. An apparatus according to any of the preceding claims, wherein each of the one or more ULT magnetic sensors comprises a sensing element provided as a layer.
6. An apparatus according to any of the preceding claims, wherein each of the one or more ULT magnetic sensors comprises a sensing element comprising a graphene sensing layer.
7. An apparatus according to any of the preceding claims, wherein the sensor arrangement comprises two Hall-effect sensors, mounted such that a sensing layer of a first one of the two Hall-effect sensors is aligned with a first plane that is perpendicular to a second plane, with which a sensing layer of a second one of the two Hall-effect sensors is aligned, and wherein the rotator mechanism is adapted to effect rotation of the rotatable member about first rotation axis, the sensor arrangement being mounted such that the rotation axis is parallel to an intersection of the first and second planes.
8. An apparatus according to claim 7, wherein the sensor arrangement comprises a third Hall-effect sensor, mounted such that a sensing layer of the third Hall-effect sensor is aligned with a third plane that is perpendicular to the first and second planes, and wherein the rotator mechanism is adapted to effect rotation of the rotatable member about a second rotation axis perpendicular to the first rotation axis.
9. A system for applying a magnetic field to a specimen in an ultra-low temperature, ULT, environment, the system comprising: an apparatus for maintaining a ULT environment within a specimen 20 chamber, an apparatus according to any of claims 1 to 8, wherein the rotatable member is disposed within the specimen chamber and comprises a specimen holder, and a control module configured to control the rotator mechanism so as to effect a predetermined orientation of the specimen with respect to the magneticfield, in accordance with the signal.
10. A method for effecting and monitoring an orientation of a rotatable member within a magnetic field in an ultra-low temperature, ULT, environment, the method comprising: monitoring an orientation of the rotatable member using a sensor arrangement mounted to the rotatable member and configured to output a signal representative of the orientation of the rotatable member with respect to a direction of the magnetic field, the sensor arrangement comprising one or more ULT magnetic sensors, wherein each of the one or more ULT magnetic sensors is adapted, when in use at ultra-low temperatures, to monitor magnetic field strength with a field responsivity of at least 1 x 10-4 VT-1 while generating heat at a rate of less than 1 x 10-8 W, the method further comprising controlling rotation of the rotatable member in accordance with the signal.
11. A method according to claim 10, further comprising maintaining a ULT environment within a specimen chamber, wherein the rotatable member is disposed within the specimen chamber and comprises a specimen holder, and controlling the rotator mechanism so as to effect a predetermined orientation of the specimen with respect to the magnetic field, in accordance with the signal.
12. A method according to claim 10 or claim 11, wherein the controlling the rotation of the rotatable member is performed while the strength of the magnetic field is less than or equal to a predetermined threshold value.
13. A method according to claim 12, further comprising controlling the magnetic field such that the strength of the magnetic field is less than or equal to the predetermined threshold value.
14. A method according to any of claims 10 to 13, wherein the sensor arrangement comprises two Hall-effect sensors, mounted such that a sensing layer of a first one of the two Hall-effect sensors is aligned with a first plane that is perpendicular to a second plane, with which a sensing layer of a second one of the two Hall-effect sensors is aligned, and wherein the rotator mechanism is adapted to effect rotation of the rotatable member about a first rotation axis, the sensor arrangement being mounted such that the rotation axis is parallel to an intersection of the first and second planes.
15. A method, according to claim 14, wherein the sensor arrangement comprises a third Hall-effect sensor, mounted such that a sensing layer thereof is aligned with a third plane that is perpendicular to the first and second planes, and wherein the rotator mechanism is adapted to effect rotation of the rotatable member about a second rotation axis perpendicular to the first rotation axis.
16. A method, according to claim 14 or claim 15, and for monitoring a temperature within a cryogenic environment, the method further comprising: obtaining a zero-field resistance value that is representative of the longitudinal electrical resistance of the first Hall-effect sensor when the sensing layer is oriented parallel to the direction of the applied magnetic field; and calculating, in accordance with the zero-field resistance value and temperature calibration data that is representative of a predetermined relationship between electrical resistance and temperature for the first Hall-effect sensor, a sensor temperature value.
17. A method according to claim 16, wherein the obtaining of the zero-field resistance value comprises calculating, in accordance with the monitored orientation of the rotatable member with respect to the magnetic field, the zero-field resistance value.
18. A method according to claim 16 or claim 17, and for monitoring magnetic field strength within the cryogenic environment, the method further comprising: calculating, in accordance with the calculated sensor temperature value, the signal output by the sensor arrangement, and field responsivity calibration data that is representative of a predetermined relationship between field responsivity and temperature for the first Hall-effect sensor, a field strength value representative of the field strength of the applied magnetic field.