METHOD FOR DETERMINING SWITCHING OF NANOMAGNETS

Abstract

The disclosure concerns a method for characterizing a magnetic device including a plurality of binary nanomagnets, having the steps of: (i) providing a magnetic device having one or more carriers on or in which the plurality of nanomagnets is arranged or embedded, (ii) applying a saturation magnetic field (.sub.0H.sub.sat) having a first direction to a plurality of binary nanomagnets, (iii) applying a second magnetic field (.sub.0H.sub.c) having a second direction to the plurality of nanomagnets, repeating steps (ii) to (iii), determining a first fraction or percentage () and a second fraction or percentage (.sub.1) of nanomagnets which switched orientation in step (iii) and repeated step (iii) respectively, determining a statistical double-switching percentage .sub.ideal based on the determined first and second fractions or percentages and .sub.1, and determining the effective double-switching fraction or percentage () of individual nanomagnets which switched orientation in step (iii) and in the repeated step (iii).

Claims

1. A method for characterizing a magnetic device including a plurality of binary nanomagnets, which may be arranged in an array, comprising (i) providing a magnetic device, which is a binary nanomagnetic array, comprising one or more carrier elements on which the plurality of nanomagnets is arranged or in which the plurality of nanomagnets is embedded; (ii) applying a saturation magnetic field (.sub.0H.sub.sat) having a first direction to a plurality of binary nanomagnets to induce a first magnetic orientation in the plurality of nanomagnets, (iii) applying a second magnetic field (.sub.0H.sub.c) having a second direction, which is different to the first direction, to the plurality of nanomagnets, (iv) repeating steps (ii) to (iii) at least once, (v) determining a first fraction or percentage () of nanomagnets which switched orientation in step (iii) and a second fraction or percentage (.sub.1) of nanomagnets which switched orientation in the repeated step (iii), (vi) determining a statistical double-switching percentage .sub.ideal based on the determined first fraction or percentage and the determined second fraction or percentage .sub.1 of nanomagnets which switched orientation, (vii) determining the effective double-switching fraction or percentage () of individual nanomagnets which have switched orientation in step (iii) as well as in the repeat of step (iii), and (viii) making a statement about the quality of the plurality of nanomagnets is made based on the comparison between the determined statistical double-switching percentage (.sub.ideal) and the determined effective double-switching percentage () of the plurality of nanomagnets.

2. The method according to claim 1, wherein steps (ii) and (iii) are repeated once and wherein the statistical double-switching percentage .sub.ideal is determined as ${}_{\mathrm{ideal}}=*{}_1.$

3. The method according to claim 1, further comprising identifying individual nanomagnets which switched orientation in step (iii) and identifying individual nanomagnets which switched orientation in the repeat of step (iii).

4. The method according to claim 3, wherein the identification of an individual nanomagnet is based on said nanomagnet's local information, such as position.

5. The method according to claim 3, comprising the step of determining, based on the magnetic history of a plurality of nanomagnets in combination with the local information of the individual nanomagnets which switched orientation after step (iii), or which switched after a repeat n of step (iii), the likelihood of a switching of said individual nanomagnets with a defined local information in a subsequent repeat of step (iii), or after a subsequent repeat n+1 of said repeat n of step (iii).

6. The method according to claim 3, wherein steps (ii) and (iii) are repeated more than once, wherein a fraction or percentage (.sub.n) of nanomagnets which switched orientation in repeat n of step (iii) is determined, wherein nanomagnets which switched orientation in repeat n of step (iii) are identified, wherein the statistical probability of multiple switching (.sub.ideal n+1) of the plurality of nanomagnets, is determined as: ${}_{\mathrm{ideal}n+1}=*{}_1*.Math.*{}_n,$ wherein n is the number of repeats and wherein (.sub.n) is the fraction or the percentage of nanomagnets which switched orientation in repeat n of step (iii), wherein the effective multiple-switching percentage (.sub.n+1) is determined cumulatively for all repeats, and wherein a statement about the quality of the plurality of nanomagnets is made based on the comparison between the statistical probability of multiple switching (.sub.ideal n+1) and the effective multiple switching percentage (.sub.n+1) of the plurality of nanomagnets.

7. The method according to claim 3, wherein steps (ii) and (iii) are repeated more than once, wherein a fraction or percentage (.sub.n) of nanomagnets which switched orientation in repeat n of step (iii) is determined, wherein nanomagnets which switched orientation in repeat n of step (iii) are identified, wherein the statistical probability of nanomagnets switching m times in a series of n repeats (.sub.ideal (m,n)) is determined as; $\begin{array}{c}{}_{\mathrm{ideal}\left(m,n\right)}=n!/\left(\left(n-m\right)!m!\right)<{>}^m{\left(1-<>\right)}^{\left(n-m\right)},\\ \mathrm{wherein}<>=\left(+{}_1+.Math.+{}_n\right)/n,\end{array}$ wherein n is the number of repeats, wherein (.sub.n) is the fraction or percentage of nanomagnets which switched orientation in repeat n of step (iii), and wherein m is the number of actual switching events, and wherein a statement about the quality of the plurality of nanomagnets is made based on the comparison between statistical probability of nanomagnets switching m times in a series of n repeats (.sub.ideal (m,n)) and effective multiple-switching percentage (.sub.(m, n), which is the fraction or the percentage of nanomagnets of the plurality of nanomagnets which switched orientation m times.

8. The method according to claim 1, wherein the determined quality of the device decreases with the increase of difference between the determined statistical double-switching percentage (.sub.ideal) and the determined effective double-switching percentage ().

9. The method according to claim 1, wherein the second direction is the opposite direction of the first direction.

10. The method according to claim 1, wherein a scanning or mapping of the plurality of nanopillars is performed following each step (iii) to determine the orientation and the local information of the individual binary nanopillars.

11. The method according to claim 10, wherein the mapping is performed by scanning magnetometry, preferably by scanning nitrogen vacancy magnetometry (SNVM).

12. The method according to claim 1, wherein the method is performed under ambient conditions.

13. The method according to claim 1, wherein the method is performed without electrically contacting the nanomagnets.

14. The method according to claim 1, wherein the magnetic device comprises one or more carrier elements on which the plurality of nanomagnets is arranged or in which the plurality of nanomagnets is embedded.

15. The method according to claim 14, wherein storage density of the plurality of nanomagnets ranges from 1 nanomagnets per (200 nm*200 nm) to 1 nanomagnet per (100 nm*100 nm), or from 1 nanomagnets per (100 nm*100 nm) to 1 nanomagnet per (10 nm*10 nm).

16. The method according to claim 1, wherein the magnetic device is a binary information storage device, such as used for magnetic random access memory (MRAM) wafer.

17. The method according to claim 6, wherein the determined quality of the device decreases with the increase of difference between the determined statistical double-switching percentage (.sub.ideal n+1), and the determined effective double-switching percentage (.sub.n+1).

18. The method according to claim 7, wherein the determined quality of the device decreases with the increase of difference between the determined statistical double-switching percentage (.sub.ideal (m,n)), and the determined effective double-switching percentage (.sub.(m, n).

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0068] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:

[0069] FIG. 1A is an optically detected magnetic resonance scan of a portion of a MRAM wafer obtained with the SNVM system used in this study, the orientation of the spin defect in the sensor probe is indicated by an arrow; the orientation of magnetization of the cylindrical binary nanomagnet is also indicated with an arrow. The circular lines schematically represent the magnetic field lines.

[0070] FIGS. 1B and 1C schematically depict the same portion of a microarray after a switching cycle of applying a first saturated and subsequently a second alternate magnetic field; Nanomagnets which switched once are shown in dark grey, nanomagnets which switched twice, once in each cycle, are shown in white, medium grey nanomagnets did not switch;

[0071] FIG. 1B shows nanomagnets after a first switching cycle FIG. 1C shows nanomagnets after a repeat switching cycle;

[0072] FIGS. 2A, 2B and 2C, schematically depict the flow of subsequent the switching cycles and show the images obtained for each cycle after the application of the second magnetic field for inducing switching behaviour of the nanomagnets;

[0073] FIGS. 2A and 2B depict two examples of two consecutive switching cycles, and

[0074] FIG. 2B depicts three consecutive switching cycles;

[0075] FIGS. 3A, 3B and 3C are images obtained with the SNVM system used in this study of a portion of a MRAM wafer in Full-B mode, the sample magnetic field ranges of the sweep ranged from 6G or 8G to 20G or 22G, corresponding to a range of 0.6 mT or 0.8 mT to 20 mT or 22 mT with respect to the bias magnetic field applied along the NV.sup. centre axis in the range of 15G (0.15 mT);

[0076] FIG. 4A is an image obtained with the SNVM system used in this study of a portion of a MRAM wafer in Full-B mode over a measurement time of 3 hours; the magnetic field B.sub.NV of the sweep ranged from 1.5 mT to 3.5 mT;

[0077] FIG. 4B is an image obtained with the SNVM system used in this study of a portion of a MRAM wafer in dual iso-B mode corresponding to a magnetic field component of B.sub.NV=0.2 m; the grey scale indicates the fluorescence output signal in kilo counts per second (kc/s);

[0078] FIGS. 5A to 5E are images of magnetic sweeps detected with the SNVM system used in this study of a portion of a MRAM wafer; the sweeps were performed to the saturation field, and then back to a different field .sub.0H.sub.c, where a certain proportion of the nanomagnets switch. Measurements were taken around B.sub.NV=0;

[0079] FIGS. 6A to 6D show images of the orientation of the magnetized nanomagnet obtained with the SNVFM system used in this study,

[0080] In FIG. 6A orientation of spin of the NV center of the sensing probe was at an angle upward of 45 with respect to the measured surface of the nanomagnet;

[0081] In FIG. 6B the orientation of spin of the NV center of the sensing probe was in plane with the measured surface of the nanomagnet;

[0082] In FIG. 6C orientation of spin of the NV center of the sensing probe was off plane with the measured surface of the nanomagnet;

[0083] In FIG. 6D the magnetic field applied to the nanomagnet was tilted, wherein the orientation of spin of the NV center of the sensing probe was in plane with the measured surface of the nanomagnet;

EXAMPLES OF EMBODIMENTS OF THE PRESENT INVENTION

[0084] The Figures show results of examples of experiments performed according to the method of this invention.

[0085] In all the experimental set-ups of the results shown magnetic imaging was performed with a commercial SNVM (the Qnami ProteusQ, Qnami AG) operating under ambient conditions.

[0086] A commercial diamond tip hosting a single NV.sup. defect at its apex (Qnami, Quantilever MX) has an integrated quartz tuning fork to allow frequency modulation-based AFM (FM-AFM) and is scanned above an array of nanopillars arranged in a MRAM wafer.

[0087] The magnetic field strength indicated in some of the figures is given in the unit Gauss (G). As the skilled person knows, 10,000 G correspond to 1 Tesla (T). The strength of the magnetic fields measured in the depicted scans can therefore easily be converted into the SI unit T.

[0088] FIG. 1A shows an magnetometry scan of a portion of an MRAM wafer. A first seep to a saturation field .sub.0H.sub.sat was applied to the wafer, which put all the nanomagnets in the direction down, as indicated in the image for an individual nanopillar. A subsequent sweep to a switching field .sub.0H.sub.c was then applied to switch a at least some of the nanomagnets into the direction up, which is also indicated in the image for an individual nanopillar.

[0089] FIG. 1A depicts an as-grown situation, i.e. a situation without magnetic history. Here, about 50% of the nanomagnets have a down direction and the remainder have a up direction. Examples of nanomagnetic arrays with magnetic history are provided in FIGS. 5A to 5E.

[0090] The detected magnetic field strength of the nanomagnets, which are nanopillars in this case, is indicated in grey scale. Each pillar can easily be localised using the cartesian X and Y axes beside the image. Some of the binary nanomagnets which have switched their magnetic orientation are indicated with an arrow. A schematic presentation of the nanomagnet 1 and the tip of the SNVM sensor probe scanning the surface of the nanomagnet is shown on the left to the scan. The white arrow in the nanomagnet indicates the orientation of its magnetization. The black arrow in the sensor tip indicates the orientation of the NV.sup. centre, which is characterized by its polar angle .sub.NV and its azimuthal angle .sub.NV.

[0091] Nanomagnets with mostly white surfaces have one orientation of magnetization, wherein nanomagnets with mostly black surfaces have the opposite orientation of magnetization.

[0092] FIGS. 1B and 1C illustrate schematically an array of 100 nanomagnets after a first switching cycle (FIG. 1B) and after a second switching cycle (FIG. 1C).

[0093] A switching cycle as referred to herein consists of applying a first saturation magnetic field .sub.0H.sub.sat having a first direction and subsequently applying a second magnetic field .sub.0H.sub.c having a second direction, which is different to the first direction, to the plurality of nanomagnets.

[0094] As can be seen in FIG. 1B, 10% of the nanomagnets of the array have switched here polarization during the first switching cycle, which is indicated as a switch from white to black in this Figure. The first percentage of nanomagnets which switched orientation in the first switching cycle is therefore 10%.

[0095] The same array, respectively portion of array, as applicable, is then subjected to a second switching cycle. Following this second cycle 10% of the nanomagnets have switched their magnetic orientation again, as is shown in FIG. 1C. The second percentage .sub.1 of nanomagnets which switched orientation in the second switching cycle is therefore also 10%.

[0096] The nanomagnets shown in black or shown striped in FIG. 1C have switched their polarization following the second switching cycle. The nanomagnets indicated in stripes are those who have switched their magnetic orientation twice, once during the first switching cycle and a second time during the second switching cycle. These nanomagnets are the effective double-switchers.

[0097] In this example of an array of 100 nanomagnets, 4 effective double switchers were identified. The double-switching percentage is therefore 4%.

[0098] The number nanomagnets exhibiting double switching expected to occur based on thermal activation corresponds to the statistical double-switching percentage .sub.ideal. The statistical double-switching percentage .sub.deal is a calculated theoretical value based on the detected number of switches in each switching cycle. It may be calculated as the second fraction or percentage ai applied to the first fraction or percentage of switches which have occurred during the first switching cycle.

[0099] In this study, this ideal statistical double-switching percentage .sub.ideal, was be calculated as

[00004]${}_{\mathrm{ideal}}=*{}_1$

[0100] The statistical double-switching percentage .sub.deal in the example shown is therefore 1%. However, the effective percentage of double switches detected is 4%, 3% more than the thermally expected double-switches. This deviation of switching distribution from the ideal thermally expected double-switching percentage .sub.ideal indicates that the array of nanomagnets has a less than ideal behaviour, which may indicate issues with its quality. The deviation may be attributable to geometrical or physical defects of the array.

[0101] FIGS. 2A and 2B show a sequence of switching cycles and an SNVM scan of a portion of an MRAM wafer comprising an array of nanomagnets obtained after sweeping to the second magnetic field .sub.0H.sub.c of each cycle. The saturation magnetic field .sub.0H.sub.sat is referred to a saturate, and the second magnetic field .sub.0H.sub.c as switch in these figures.

[0102] The total numbers of nanomagnet assessed is indicated underneath each scan as Total. the number of switched nanopillars and their percentage , respectively .sub.1, is also indicated.

[0103] FIG. 2A shows a sequence of two switching cycles. A selection of nanomagnets which have switched at least once, and a selection of nanomagnets which have switched in both cycles are indicated with arrows.

[0104] I FIG. 2B a sequence of three switching cycles and their SNVM scans are shown.

[0105] The value for the statistical double-switching percentage .sub.ideal and for the effective double-switching percentage can easily be determined from these Figures as outlined above. FIGS. 3A to 3C show different magnifications of SNVM scans of a portion of a MRAM wafer in Full-B mode. The sample magnetic field strength of the sweep performed in this study ranged from 6G or 8G to 20G or 22G, which is a range of 0.6 mT or 0.8 mT to 20 mT or 22 mT with corresponding to a range of 0.6 mT or 0.8 mT to 20 mT or 22 mT with respect to the bias magnetic field applied along the NV.sup. centre axis in the range of 15G (0.15 mT).

[0106] The results were taken on an as-grown wafer, i.e. a wafer without any previous magnetic field history. Whereas the overall distribution is entirely statistical, i.e. 50% of nanomagnets point in the same direction, the local distribution of orientations is not entirely random but distinct line-like patterns form. These lines are formed by nearest-neighbour nanomagnets. This is indicative of interactions between nearest-neighbouring nanomagnets, which provides relevant information for the MRAM fabrication process.

[0107] To determine the switching behaviour of the nanomagnets, a SNVM scan can be performed in a Full-B mode, as mentioned above. In FIG. 4A a scan in full-B mode performed over a magnetic field sweep ranging from 1.5 mT to 3.5 mT was performed in 3 h measuring time.

[0108] To obtain a quicker characterization of the magnetic device, respectively the plurality of nanomagnets, it is also possible to use SNVM to image two iso-magnetic field contours that are resonant to two specific microwave frequency f.sub.MW,1, and f.sub.MW,2 hereinafter referred to as dual iso-B mode. The fluorescence counts at f.sub.MW,1, and f.sub.MW,2 are subtracted from each other.

[0109] In the scan shown in FIG. 4B a scan in dual iso-B mode was performed on the same portion of MRAM wafer imaged in FIG. 4A. The dual iso-B mode image shown in FIG. 4B corresponds to the magnetic component of B.sub.NV=2.5 mT. The grey scale in this figure indicates the fluorescence output signal in kilo counts per second (kc/s). The dual iso-B scan was performed in 20 mins retrieving 169 bits, or nanomagnets, wherein on bit comprised 236 pixels. The speed of the scan averages to 1 bit/7 seconds. When the number of pixels per bit is reduced to 4, 1 bit/100 ms can be retrieved. By further optimising the SVNM set-up, optical output detection and count, a further increase the scanning speed of about a factor 100 is possible. A scanning speed of about 1 bit/ms is therefore feasible.

[0110] It was observed during studies performed according to this invention, that a certain percentage of the MRAM nanomagnets switched more easily. The assumption can be made that a certain percentage will switch, because switching is probabilistic, i.e. thermal. The percentage of bits, or nanomagnets, that flip depends on the magnetic history. Which individual bits flip and whether they flip more easily than other bits may depend on critical dimension, defects, nearest neighbour interactions and so on.

[0111] This phenomenon is demonstrated by the images shown in FIGS. 5A to 5E.

[0112] In these images, nanomagnets with mostly white surfaces have one orientation of magnetization, wherein nanomagnets with mostly black surfaces have the opposite orientation of magnetization.

[0113] The orientation of the nanomagnets is easily distinguishable in FIG. 5C, for example. Here, one orientation is characterized by circles that have mostly a white contrast, with a dark-grey shade at the upper half-radius. The other orientation is characterized by circles that are mostly black, with a light-grey contrast at the upper half radius.

[0114] FIG. 5A shows the as-grown status, where half of the nanomagnets point in one, and the other half in the other direction.

[0115] FIG. 5B shows an image after a first saturation magnetic field .sub.0H.sub.sat has been applied. Here all nanomagnets point in the same direction.

[0116] FIG. 5C, FIG. 5D and Figure E show results after a saturation magnetic field and a second magnetic field .sub.0H.sub.c was applied and where now 45%, 14%, or 1% of the nanomagnets point in one direction, as indicated in the respective Figures.

[0117] Based on the magnetic history of the plurality of nanomagnets, which are preferably arranged in an array, the likelihood of a switching of nanomagnets in a defined local area may be estimated. To this end, localisation maps may be used to identify which individual nanomagnets switch their orientation in a system or array of nanopillars having a certain magnetic history. The local information on individual nanomagnets can then be used to determine, if for example nearest-neighbour switching, or if switching at the edge of the array is more likely to occur.

[0118] It was furthermore observed that the orientation of the NV.sup. spin defect influenced the pattern of the image received for individual binary nanomagnets. This is effect is shown in FIGS. 6A to 6C, each schematically indicating the polarization of the nanomagnet with a white arrow and the orientation of the NV.sup. centre in the sensor tip with a black arrow. In each of these Figures, the orientation of magnetization of the nanomagnet is the same, i.e. orthogonal to the upper face of the nanopillar, while the polar angle .sub.NV of the NV.sup. centre changes. The corresponding SNVM images obtained for each of these set-ups is shown underneath the respective illustration.

[0119] In FIG. 6A the orientation of the spin is tilted off plane of the measured surface. The upper face of the nanopillar does not appear homogenous but has a bright and a dark portion in this polarization of the nanomagnet. In a switched polarization the imaged dark and bright portions would be inverted.

[0120] In FIG. 6B the orientation of the spin is in plane with respect to the measured surface. As can be seen in the corresponding SNVM image, about half of the measured face of the nanopillar appears bright, and the other half appears dark. Again, a switching of the binary nanomagnet would result in a flipping of brightness of these halves.

[0121] In FIG. 6C the orientation of the spin is entirely off plane with respect to the measured surface. The orientation of the spin is parallel to the magnetic orientation of the nanopillar. It is notable, that the SNVM image obtained for this kind of arrangement shows a uniform brightness of the measured surfaces of the nanopillars. In this case, all surfaces appear dark. It was noticed, that when used in this set-up, less pixels are required when the SNVM scan is performed in dual iso-B mode. The speed of the scan can therefore be optimized when performed using this set-up.

[0122] FIG. 6D depicts a similar setup as shown in FIG. 6B, however in this experiment the orientation the magnetization of the nanopillar was tilted with respect to its measured surface by the application of a field perpendicular to the nanopillar. The resulting image showed a more complex pattern of bright and dark zones on the face of the nanopillar. Due to the heterogenous distribution of bright and dark zones on the nanopillars, it is more difficult to ascertain the switching of an individual nanopillar. However, the canted magnetization angle of the nanopillar can be calculated from such maps. The canting behaviour under external field contains valuable information for MRAM characterization.