Neutral beam microscopy apparatus and method

Abstract

Apparatus 100 for neutral beam microscopy comprises a neutral particle beam generator, a polariser 104 to polarise the beam 102 to align their magnetic moment along a predetermined direction, a gradient magnetic field generator 106 to manipulate the magnetic moment of the particles to encode the spatial position thereof within the beam, a sample holder for holding a sample 108 in the beam path, a detector apparatus 112 for detecting intensity of the particle beam with a signal intensity proportional to their magnetic moment. The amplitude of the gradient magnetic field is varied and signal intensities are recorded for each amplitude, wherein results are analysed by Fourier transform to obtain a profile of a surface property of the sample. By measuring a signal which depends on the magnetic moment orientation of the beam particles, a one dimensional profile or a two dimensional image of the specimen can be reconstructed.

Description

Technical Field [0001 ] The present application is concerned with an apparatus and method for undertaking neutral beam microscopy of a surface to determine a surface property of said surface. Background Art

[0002] By "microscopy" we mean the technical field of using a microscope to analyse the properties of a material on a very small scale, not visible to the naked eye. Microscopy includes both determination of the surface structure (providing a "view" of surface features too small to be visible to the human eye) as well as other properties such as the surface magnetism (magnetic microscopy; magnetic properties not being visible to the human eye at any scale).

[0003] Many types of microscopes exist. The term 'microscope' is used herein to mean a device for undertaking microscopy. Electron microscopes were developed as an alternative to light microscopes for visualisation of features on a smaller scale. Beneficially, electron microscopes have a much higher resolution than light microscopes due to the fact that electrons can have a much lower wavelength than light, allowing resolutions down to the sub-nanometre scale, however, electron microscopes are not suitable for all applications.

[0004] Delicate surfaces are one example of systems where electron microscopy is problematic, as the charge of the electrons and their relatively high-energy lead to interaction and potential damage to the sample. One solution to study such surfaces is to replace the electrons with neutral higher mass particles- such particles can be at a lower energy at a similar wavelength (wavelength of a matter wave being inversely proportional to the square root of both mass and energy). A particle that is particularly well-suited for neutral beam microscopy is the Helium (He) atom.

[0005] Over the last four decades significant achievements have been made towards developing scanning helium microscopes (SHeM). The surface is probed in a SHeM using a beam of neutral low-energy helium atoms, making it suitable for imaging surfaces which are either damaged, altered or simply cannot be studied using more conventional microscopy techniques. Such materials may be insulating, transparent and I or photosensitive.

[0006] A common aspect of all the SHeM approaches used to date is their reliance on small apertures to achieve a high spatial resolution, whether it is micron sized beam sources in instruments based on focusing elements or a combination of micron sized sources and detector apertures in pin-hole technology. The loss of signal associated with micron sized apertures often results in compromising on the resolution, for the sake of obtaining high enough signal to noise ratios within acceptable acquisition times.

[0007] It is an aim of the present invention to provide an improved neutral beam microscopy apparatus and method. Summary of Invention

[0008] According to a first aspect of the present invention there is provided a method of neutral beam microscopy comprising the steps of: generating a beam of neutral particles, the beam of neutral particles being polarised to align their magnetic moment along a predetermined direction; passing the beam of neutral particles through a gradient magnetic field to manipulate the magnetic moment of the particles to encode the spatial position thereof within the beam; directing the beam of encoded particles towards a sample to be imaged; allowing the beam of encoded particles to interact with the sample to be imaged; detecting a signal which depends on the direction of the magnetic moment of the beam particles; repeating the above steps, for varying values of the amplitude of the gradient magnetic field and recording the signal intensity for each gradient magnetic field amplitude; analysing the results by performing a Fourier transform of the signal intensities as a function of magnetic field gradient amplitudes, to obtain a profile of a surface property of the sample.

[0009] The invention is based on magnetically manipulating the magnetic moment of the neutral particles to encode the spatial position of the beam particles before or after interacting with the sample. The image is obtained by performing a series of measurements in reciprocal space, with all the beam particles contributing to the signal in every measurement, which dramatically reduces the scaling of the measurement time with the spatial resolution in comparison to pin-hole imaging.

[0010] As would be understood by the skilled person, the present invention is preferably undertaken in a vacuum.

[0011] The step of polarization may be carried out subsequent to beam generation, or simultaneously therewith.

[0012] In one embodiment, the sample transmits the particles in a transmission microscopy configuration. In an alternative embodiment the sample scatters the particles in a reflection microscopy configuration.

[0013] The method may comprise the steps of: using two sequential, perpendicular gradient fields to encode the position of the particles along two axes; repeating the steps for varying values of the amplitude of both gradient fields; and, analysing the results by performing Fourier transforms to produce a two dimensional map of the surface property of the sample.

[0014] Optionally the step of passing the beam of neutral particles through a gradient magnetic field comprises the step of passing the beam of neutral particles through a gradient field to encode the phase in polar coordinates to achieve two dimensional mapping of the sample; the method comprising the steps of: repeating the steps for varying values of the amplitude of the gradient field and the angle between the gradient field and the sample; analysing the results by performing a Fourier transform and a back projection transformation to produce a two dimensional map of the surface property of the sample.

[0015] In one embodiment the step of passing the beam of neutral particles through a gradient magnetic field occurs pre-interaction. Alternatively the step of passing the beam of neutral particles through a gradient magnetic field occurs post-interaction.

[0016] In one embodiment the method comprises the step of passing the post-interaction beam of encoded particles through a spin analyser to a particle detector with a transmission probability related to the projection of their magnetic moment on a specific axis. Preferably the neutral particles are 3He, although other particles could be used, for example D2 or metastable He*. [001 7] In an alternative embodiment: the sample exhibits surface magnetism; and, the beam particles are excited to a metastable state; and the interaction between the beam particles and the surface of the sample leads to a reaction where the metastable atom de-excites and the sample is ionised with the reaction rate dependent on the relative angle between the metastable spin and the surface magnetisation; wherein the step of detecting comprises the step of measuring the current related to the ionisation of the surface; wherein the measured property of the sample is surface magnetism.

[0018] Preferably the neutral particles are 4He in a metastable state (He*).

[0019] According to a second aspect of the invention there is provided a neutral beam microscopy apparatus comprising: a neutral particle beam generator; a polariser configured to polarise the beam of neutral particles to align their magnetic moment along a predetermined direction; a gradient magnetic field generator configured to manipulate the magnetic moment of the particles to encode the spatial position thereof within the beam; a sample holder for holding a sample in the path of the beam; a detector apparatus for detecting the intensity of the particle beam with a signal intensity proportional to their magnetic moment.

[0020] The generator and polarizer may be the same apparatus, or may be different, sequential apparatus.

[0021] The geometry of the apparatus (angle between source and detector) may be configured such that the particles are transmitted to a sample held therein in a transmission microscopy configuration.

[0022] The geometry of the apparatus (angle between source and detector) may be configured such that the particles are scattered by the sample held therein in a reflecting microscopy configuration.

[0023] The apparatus may comprise: two sequential, perpendicular gradient field generators configured to encode the position of the particles along two axes.

[0024] In one embodiment there is only one gradient field generator, but the angle between the gradient field and the sample can be rotated to provide a rotating gradient field to achieve two dimensional mapping of the sample.

[0025] In one embodiment the gradient magnetic field generator is upstream of the sample holder.

[0026] Alternatively the gradient magnetic field generator is downstream of the sample holder.

[0027] In one embodiment a spin analyser and a particle detector are used, wherein the spin analyser is configured to pass the beam to the particle detector with a transmission probability related to the projection of their magnetic moment on a specific axis.

[0028] In another embodiment, where surface magnetism ofthe sample is measured, there is provided a current meter between the sample and earth potential to measure current related to the ionisation ofthe surface ofthe sample resulting from interaction.

[0029] As an alternative to measuring the current between the sample and earth potential, the current of electrons ejected from the sample may be measured by detectors. Brief Description of Drawings

[0030] An embodiment of the present invention will now be described with reference to the following figure in which: FIGURE 1 is a schematic view of the basic principle of the present invention in one dimension; FIGURE 2 is a block diagram of a first apparatus and method according to the present invention; FIGURE 3 is a block diagram of the post-processing steps for one dimensional imaging according to the present invention; and, FIGURE 4 is a block diagram of a second apparatus and method according to the present invention. Detailed description The basic principles of magnetic encoding beam microscopy according to the invention [0031 ] The imaging technique of the present invention uses the response of the magnetic moments of the beam particles to external fields, as a way of encoding their positions within the plane (e.g. XY) perpendicular to the beam axis (e.g. Z).

[0032] Figure 1 shows a highly simplified example of the present invention forthe purposes of understanding the operating principle of the invention. The method illustrated in Figure 1 and in the equations below, describe the invention using a classical picture. This approach is accurate for particles such as 3He atoms where the relevant magnetic moment is the nuclear spin. The methodology described in this application works also for other magnetic moments (electronic spin, orbital spin and rotational magnetic moment), however, the equations to explain it becomes more complex (require a quantum mechanical description and taking account of coupling between different magnetic moments). The adjustments required in the theoretical description of the spin dynamics will be readily evident to the reader skilled in the art.

[0033] Referring to Figure 1, a schematic of a 1 d imaging scheme 100 is provided. The scheme is illustrated in an XYZ coordinate system. A particle beam 1 02, propagating along the Z axis is passed through a beam polariser 104 which selects a particular spin projection along the Z axis.

[0034] An encoding magnetic field 106, oriented along the Y axis, has an overall length L, and an amplitude which varies linearly as function of the X coordinate of the beam particles, dB i.e. Bencoding = (0, ~^~x> 0). Within the encoding field, which acts like a phase gradient in magnetic resonance imaging, the spins precess within the XZ plane at a Larmor frequency which depends on their position along the x axis.

[0035] A3He atom moving with velocity v, displaced by x from the centre of the beamline, will at the end of the encoding field accumulate a total classical spin phase with respect to dB h the x axis given by 0 = a)Lt = y-^-x- where « l is the Larmor frequency, t=L / v is the time spent within the field, and y is the gyromagnetic ratio of 3He.

[0036] The beam then interacts with a target / sample 1 08 to image. In Figure 1, the sample 108 is shown as an object that blocks a strip of particles along the Y axis (i.e. a strip of particles within a certain range of classical spin phase). The density distribution function, p (x,y) of the continuing beam, which could be moving straight forward in a transmission configuration or reflected back at some angle in a reflecting configuration, is a product of the initial density distribution of the beam D(x,y) and a second function P(x,y) which describes either the probability of passing through the target in a transmission experiment or scattering into the detector angle in a reflection experiment. It is P(x,y) which contains the information about the sample and how it interacts with the probe, and is what is to be determined in a microscopy experiment.

[0037] The beam (transmitted or scattered) then continues through a spin analyser 110 which transmits particles towards a particle detector 112 with a probability related to the projection of their spin on a specific axis.

[0038] The experiment is repeated for a range of discrete gradient field values between kY . and kr

[0039] After subtracting any constant contributions to the beam intensity measured, we can define a complex signal, S = So + i S90 which has real and imaginary components which correspond to beam intensity measurements performed using beam analysers designed to pass particles according to the projection of the magnetic moment along the X and Y axes correspondingly. If we use an integral to sum the contributions from the beam particles we obtain a Fourier transform relation, S(kx) oc ff p(x,y)e2mkxXdxdy = f plde2mkxXdx , where kx = -±-y is an experimentally controlled variable calculated for the average velocity, v, and Pid(x) = Jp(x,y)dy is a one dimensional projection of the density distribution function p(x,y). If the complex signal is measured for a range of gradient field values (between kx . and kXm ) , an inverse Fourier transformation can be used to reconstruct the projection Pid(x) oc f^Xmax S{kx)e^2mkxX dkx. Kxmin

[0040] When the sample is anisotropic, a 2-dimensional image is more useful. Such an image can be obtained using an additional encoding field along the beam line, characterised by a gradient along the y coordinate, which can be characterised by ky = 1 L dBy . . i ■ r- l l l - l l „ , . r r z X — y=-^-. Using both gradient fields, the signal becomes a 2d Fourier transform of p(x,y) and the latter can be reconstructed by measuring for a range of different kx, ky values and inverse transforming the 2 dimensional signal S(kx, ky), in direct analogy to a single slice MRI phase gradient scan. Description of the first embodiment Configuration

[0041] To undertake the phase encoding imaging method according to the present invention, several other magnetic fields are needed in addition to the gradient field which encodes the spatial position of the beam atoms, as described in more detail below.

[0042] Referring to Figure 2, the main elements of an apparatus 200 according to the present invention are shown schematically, in which there is provided: a supersonic source 202 creating a beam 203 of low energy 3He atoms; a hexapole magnet 204 which focuses particles with one spin projection to a parallel beam 205 and defocuses the other towards a plurality of vacuum pumps 206; a hexapole to dipole transition element 208 which aligns the chosen component along an axis denoted as x. The hexapole to dipole transition element is a magnetic field configuration which includes a strong dipole field at the end of the hexapole field and ensures the magnetic moments undergo an adiabatic transition from being aligned with respect to various different field orientations inside the hexapole to being aligned along the same direction dictated by the dipole field; an electromagnet 210 producing a controllable homogenous magnetic field, Bi, oriented along the x axis. The field is used to both measure the real and imaginary components of the signal and cancel the velocity spread effect created by B2; an encoding gradient device 214 (consisting of both Bencoding and B2) which is used to encode the spatial position of the beam particles as explained earlier. Bencoding is the gradient field which is changed during the acquisition procedure to change the kx value, whereas B2 is a relatively large homogenous component which is used to cancel any orthogonal unwanted components produced by the gradient field device; a sample holder where a sample 21 5 is held for interaction; a second hexapole analyser 216, which focuses particles 217 with a spin oriented along X towards the detector chamber and defocuses particles 219 with the opposing spin state; a particle detector 218 (such as a mass spectrometer) positioned in a detector chamber measuring final beam intensity S(k).

[0043] With reference to the encoding gradient device 214, it is within the skilled person's knowledge to create a gradient field required to the present invention. The desired outcome is a magnetic field which best resembles a perfect gradient throughout the volume occupied by the particle beam. One example of such an apparatus is to pass the beam particles through, a multiple wire configuration. The wires each carry a current, and are aligned coaxially with and surrounding the beam path. The magnitudes and polarities of the current vary to create the desired field, which may, for example, follow a current pattern of I = Io cos(20). It will be noted that this is only one way of producing a gradient field, other ways are known in the art and will be evident to the skilled reader.

[0044] A common problem of gradient devices is that in addition to the required gradient, dB dB (for example) they also produce a second component which can spoil the imaging scheme. A solution to this, is to add a second much larger homogenous field, B2, along the Y direction (i.e. parallel to the direction ofthe gradient). Because ofthe orthogonality ofthe strong field to the unwanted Bx component, the effect of the latter can be reduced dramatically.

[0045] Whilst the strong homogenous field, B2, solves the problem of the unwanted orthogonal gradient, it also introduces a complication. Particle beams will always have some spread of velocities (typically a FWHM of a few percent in supersonic beams), leading to slightly different flight times and phase accumulation within a magnetic field. One subtle but still important effect of the velocity spread is on the encoding itself, i.e. the definition of kx above, which uses the average velocity, is not accurate and the phase depends not only on the position but also on the velocity, this leads to a limitation on the imaging resolution.

[0046] A second more dramatic effect of the velocity spread is due to B2, which is a much larger magnetic field, leading to a complete dephasing of the magnetisation of particles with different velocities and a loss of ability to coherently manipulate the spins and obtain a signal. Fortunately, this effect can be completely reversed by passing the beam (either before or after B2) through another homogenous magnetic field with a magnetic field integral magnitude which is identical to that produced by B2, leading to a refocusing condition known as a spin echo signal; a concept which underlies neutron, helium and molecular spin echo beam experiments. The refocusing field can be implemented with a solenoid electromagnet Bx . Viewing Figure 2, Bi acts to "prepare" the particle spins in advance (before they enter the gradient field / B2) by making them precess through a similar strength field (usually with opposing direction) . Fast spins precess less, slow ones more, and the differences in their final phase are then compensated by the reverse precession later in B2.

[0047] Finally, there is another difference between the simplistic encoding scheme described above with reference to Figure 1 and the realistic setup required to implement the present invention. The direction in which the beam is initially polarised and the direction along which the spins are analysed can be difficult to change on the time scale of the experiment, making it difficult to measure both the real and imaginary components of S(k). Measuring just the real (or imaginary) part of the signal converts the Fourier transform relations derived above to Cosine (or Sine) transforms, creating an artificial symmetry in the reconstructed image. To address this problem without major modifications to the setup, instead of changing the spin selection axis of the detector, a 90° rotation of the spins is induced by the first homogenous field, Bi. In this scheme, the real and imaginary parts of the complex signal are obtained by performing gradient measurements with 0 and 90° rotations respectively. While this effective rotation technique is velocity dependent, the very small rotation needed (^) between the two components, means that the deviations due to the velocity spread are small and do not significantly distort the reconstructed profile or image. Reconstructing the surface structure from the measurement

[0048] Referring to Figure 3, a method of use 250 of the apparatus of Figure 2 is shown as a flow chart.

[0049] At step 252 the particle beam is initiated and interacts with the sample throughout the next stages.

[0050] At step 254, the value of B1 is set to the 0° condition. This is the condition at which the field integral B1 cancels exactly the velocity depend dephasing in B2, i.e. a spin echo condition .

[0051] At step 256 the amplitude of the gradient field along the x axes, is set to a first predetermined level (i).

[0052] At step 258, the beam intensity is measured by the particle detector and stored as S_real(i).

[0053] At step 260, the B1 value is changed to the 90° condition. This is the condition at which a small additional field is added to B1 to simulate a 90° angle between the polarising and analysing devices.

[0054] At step 262, the beam intensity is measured by the particle detector and stored as S_imag(i).

[0055] At step 264 the process is repeated for a different value of i. and the process is repeated for i=1 to N. In each step the gradient field value, and correspondingly kx, is changed.

[0056] At step 266, once N such scans have been completed, there exists a complex vector with N elements: Sfkx) = S_real + V—T x S_imag

[0057] At step 268, a Fourier transform is then performed on S(kx) and the absolute of the result is calculated : PidW « abs fK^max I S(kx)e~2mkxXdkx kxmin

[0058] At step 270 the image is scaled using relations between x and hkx and Ax and maximum^k^ to obtain a profile of the sample. Description of the second embodiment Configuration

[0059] Referring to Figure 4, the main elements of an apparatus 300 according to the present invention are shown schematically.

[0060] Instead of neutral 3He, helium 4 is used in a metastable state (He*) in the triplet 23S state. There are various ways to produce a He* beam, including electron impact ionization and plasma discharge at the nozzle which are known in the art and will not be described here. [0061 ] In the following embodiment, The He* beam is spin polarized by focusing the low-field seeking spin state. A center stop element is used to clear out any other states which exist in the beam. It is very similar to what is done in the spin 72 3He case, where the main difference is thatthe magnetic moment is three orders of magnitude larger, making it easier to bend the trajectories and enables the use of either higher energy beams or weaker magnetic field gradients.

[0062] The next step, following the scheme in figure 2 is spatial encoding using a gradient field. Again the basic principle follows what was described above for 3He, where the main difference is thatthe magnetic moment which is encoded is that of the electron spin. As the Larmor frequency, which governs the precession of the spin projection onto the different lab frame axes, is three orders of magnitude larger than that for 3He the specifications of the encoding device are relaxed (length and strength).

[0063] Whereas the polarization and encoding stages of He* are quite similar to the 3He case, there are significant differences in the interaction with the sample and the measured signal.

[0064] When the He* Atoms reach the surface there is a significant probability of a deexcitation reaction ofthe helium atom accompanied by an ionization of the surface, i.e. an electron is ejected from the surface towards the vacuum.

[0065] The electron ejection is the basis of a surface magnetism measurement technique called spin-polarized metastable-atom deexcitation spectroscopy, which makes use ofthe fact that the yield of electrons ejected from the surface depends on the relative orientation ofthe metastable atom spin and the magnetization ofthe sample it hits. The sample region is enclosed by an external magnetic field which is used to both define the direction ofthe magnetization in the sample and its amplitude through the magnetic properties, B(H), of the sample.

[0066] While the flux ofthe ejected electrons are measured in spin-polarized metastableatom deexcitation spectroscopy experiments by analysing the emerging electron beam, a variant of this experiment has been developed where the electron flux is measured by monitoring the current on the sample itself, which allows the analysis to be made within high external magnetic fields. This is described in Japanese patent application JP2005300402A (granted as JP4374445B2). Both of these techniques allow the characterisation of the overall (average) surface magnetism but do not contain local information ofthe surface, i.e. a surface magnetism image.

[0067] Regardless of how the ejected electron flux is measured, integrating this type of measurement with the spin phase encoding scheme ofthe present invention can be used to produce a surface magnetism image.

[0068] One important difference when using 3He and He* beams, is that in the latter case, the reconstructed map p(x,y} is a product ofthe initial density distribution ofthe beam D(x,y) and a second function P'(x,y) which is proportional to the local surface magnetism rather than the probability of reflection (or transmission), i.e. the method maps magnetism rather than the structure or topography ofthe surface.

[0069] It is important to note that the surface magnetism map can be measured for different external magnetic fields surrounding the sample, introducing a 3rd dimension to the image which is the magnetism, B(H), allowing the determination of local hysteresis curves.

[0070] Referring to Figure 4 there is provided: a source 302, consisting of a supersonic expansion of helium atoms followed by electronic excitation of the helium atoms creating a beam 303 of metastable He atoms; a hexapole magnet 304 which focuses the metastable particles with one spin projection to a parallel beam 305 and defocuses the other towards a plurality of vacuum pumps 306. The hexapole also includes a central beam stop to remove metastable atoms with zero spin projection from the continuing field and stop energetic photons from reaching the sample; a hexapole field to dipole field transition element 308 which aligns the chosen magnetic moment component along an axis denoted as x; a solenoid 310 producing a controllable homogenous magnetic field, Bi. The field is used to both measure the real and imaginary components of the signal and cancel the velocity spread effect created by B2; a magnetic gradient device 314 (Bencoding and B2) which is used to encode the spatial position of the beam particles as explained in the first embodiment Bencoding is the gradient field which is changed during the acquisition procedure to change the kx value, whereas B2 is a relatively large homogenous component which is used to cancel any orthogonal unwanted components produced by the gradient field device. a sample holder where a sample 315 is held for interaction; a variable external magnetic field generator 316. This generates a magnetic field to magnetise the sample 315 and is used to define the magnetisation direction in the sample as well as perform local magnetism measurements , i.e. B(H) curves. The magnetic field profile (field strength and spatial profile) is chosen to ensure the transition of the magnetic moment into the external field conserves the phase encoded within BenCoding; a sensitive current meter 318 which is connected between the sample and ground potential and is used to measure the current of electrons leaving the surface due to a deexcitation - ionisation reaction when the beam interacts with the sample 315. Reconstructing a surface magnetism image from the measurement

[0071] For the surface magnetism imaging embodiment, the current is measured while varying the amplitude of Bencoding to produce the signal S(k). The reaction rate depends on the relative alignment of the spin of the metastable atom and the magnetisation of the surface region of the sample the atom hits. This leads to the same cosine / sine dependence in the equation relating p(x,y) to S(kx,ky^ derived for the first embodiment, and the same post-processing technique of Figure 3 can be used to map the surface magnetism of the sample 315, with the difference being simply the property which is measured, i.e. the interpretation of the image which is reconstructed from the measurement

[0072] It will therefore be understood that the present invention provides another imaging mode producing a magnetism map of the surface (magnetism microscopy). Variations

[0073] In a variation, the Ber,coding I B2 generator 214, 314 may be positioned downstream of the sample (post-interaction).

[0074] The magnetic encoding technique is not restricted to Helium-3, or metastable helium. In theory, any neutral particle, e.g. an atom or a molecule, with an electronic, nuclear or rotational magnetic moment can be manipulated to resolve the spatial position using the schemes presented above. However, coupling between the different magnetic moments, and multiple overlapping oscillation frequencies can make the analysis far more complicated.

[0075] A relatively simple case is using D2 molecules, where it has been shown thatthe nonrotating quintet I = 2,J = 0 state, which dominates a cold D2 beam, can be magnetically manipulated in a way which is analogous to experiments with 3He . In addition to the fact that D2 is a more readily available gas, it also scatters from surfaces differently from helium, leading to a different contrast of the image and could supply complimentary information to an image produced from a helium beam.

[0076] It is important to note, that while the magnetic encoding method could be applied on its own to achieve spatial resolution, it can also be used as an add on to pin-hole or focusing microscopes, providing an additional boost to the resolution with a much smaller price in terms of measurement time.

[0077] The present invention also provides for an apparatus and method with two sequential, perpendicular gradient field devices to encode the position of the particles along two axes. The method is then repeated for varying values of the amplitude for each field devices. This allows a two-dimensional map of the sample to be generated.

[0078] Alternatively, one gradient field can be used if the angle between the gradient field and the sample may be rotated. The method is repeated for both varying field amplitude and rotational angle.

[0079] To achieve high spatial resolutions, the blurring of the image which will be caused by the non-zero width of the velocity distribution needs to be addressed. This could be done by either using pulsed / chopped beams and time of flight detection or using spinecho techniques, both of which will be readily evident to the reader skilled in the art.

[0080] The project leading to this application has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 772228).

Claims

1. A method of neutral beam microscopy comprising the steps of:generating a beam of neutral particles, the beam of neutral particles being polarised to align their magnetic moment along a predetermined direction;passing the beam of neutral particles through a gradient magnetic field to manipulate the magnetic moment of the particles to encode the spatial position thereof within the beam;directing the beam of encoded particles towards a sample to be imaged;allowing the beam of encoded particles to interact with the sample to be imaged;detecting a signal which depends on the direction of the magnetic moment of the beam particles;repeating the above steps, for varying values of the amplitude of the gradient magnetic field and recording the signal intensity for each gradient magnetic field amplitude;analysing the results by performing a Fourier transform of the signal intensities as a function of magnetic field gradient amplitudes, to obtain a profile of a surface property of the sample.

2. A method according to claim 1, wherein the sample transmits the particles in a transmission microscopy configuration.

3. A method according to claim 1, wherein the sample scatters the particles in a reflection microscopy configuration.

4. A method according to any preceding claim, comprising the steps of:using two sequential, perpendicular gradient fields to encode the position of the particles along two axes;repeating the steps for varying values of the amplitude of both gradient fields; and,analysing the results by performing two dimensional Fourier transforms to produce a two dimensional map of the surface property of the sample.

5. A method for neutral beam microscopy according to claim 1, wherein:the step of passing the beam of neutral particles through a gradient magnetic field comprises the step of passing the beam of neutral particles through a gradient field to encode the phase in polar coordinates and achieve two dimensional mapping of the sample;the method comprising the steps of:repeating the steps for varying values of both the amplitude of the gradient field and the angle between the gradient field and the sample;analysing the results by performing Fourier a transform and a back projection transformation to produce a two dimensional map of the surface property of the sample.

6. A method for neutral beam microscopy according to any preceding claim, wherein the step of passing the beam of neutral particles through a gradient magnetic field occurs pre-interaction.

7. A method for neutral beam microscopy according to any preceding claim , wherein the step of passing the beam of neutral particles through a gradient magnetic field occurs post-interaction.

8. A method for neutral beam microscopy according to any preceding claim, comprising the step of passing the post-interaction beam of encoded particles through a spin analyser to a particle detector with a transmission probability related to the projection of their magnetic moment on a specific axis.

9. A method for neutral beam microscopy according to claim 8, wherein the neutral particles are 3He.

10. A method for neutral beam microscopy according to any of claims 1 to 7, wherein:the sample exhibits surface magnetism;the beam particles are excited to a metastable state; and,the interaction between the particles and sample leads to a reaction where the metastable atom de-excites and the sample is ionised with the reaction rate dependent on the relative angle between the metastable spin and the surface magnetisation;wherein the step of detecting comprises the step of measuring the current related to the ionisation of the surface;wherein the measured property of the sample is surface magnetism.

12. A method for neutral beam microscopy according to claim 11, wherein the neutral particles are 4He in a metastable state (He*).

13. A neutral beam microscopy apparatus comprising:a neutral particle beam generator;a polariser configured to polarise the beam of neutral particles to align their magnetic moment along a predetermined direction;a gradient magnetic field generator configured to manipulate the magnetic moment of the particles to encode the spatial position thereof within the beam;a sample holder for holding a sample in the path of the beam;a detector apparatus for detecting the intensity of the particle beam with a signal intensity proportional to their magnetic moment.

14. An apparatus according to claim 13, wherein the sample holder is configured such that the particles are transmitted to a sample held therein in a transmission microscopy configuration.

15. An apparatus according to claim 13, wherein the sample holder is configured such that the particles are scattered by the sample held therein in a reflecting microscopy configuration.

16. An apparatus according to any of claims 13 to 15, comprising:two sequential, perpendicular gradient field generators configured to encode the position of the particles along two axes.1 7. An apparatus according to any of claims 13 to 15, wherein:the angle between the gradient field and the sample rotated to perform the encoding in polar coordinates and achieve two dimensional mapping of the sample.

18. An apparatus according to any of claims 13 to 1 7, wherein the gradient magnetic field generator is upstream of the sample holder.

19. An apparatus according to any of claims 13 to 1 7, wherein the gradient magnetic field generator is downstream of the sample holder.

20. An apparatus according to any of claims 13 to 1 8, comprising a spin analyser and a particle detector, wherein the spin analyser is configured to pass the beam to the particle detector with a transmission probability related to the projection of their magnetic moment on a specific axis.

21. An apparatus according to any of claims 13 to 18, comprising a current measurement device to measure the current related to the ionisation of the surface of the sample resulting from de-excitation reaction of the metastable beam interacting with the sample.

22. An apparatus according to claim 21, wherein the current measurement device measures the current between the sample and earth potential.

23. An apparatus according to claim 21, wherein the current measurement device measures the current of electrons ejected from the sample.

Images