What is claimed is:
1. A magnetic head comprising:
a first spin valve element;
a second spin valve element; and
a non-magnetic spacer layer formed between the first spin valve element and the second spin valve element;
the first spin valve element including:
a first antiferromagnetic film
a first ferromagnetic film;
a first non-magnetic film;
a second ferromagnetic film;
a second non-magnetic film of which magnetization direction can be rotated in respect to an external magnetic field; and
a first soft magnetic film;
the second spin valve element including:
a second antiferromagnetic film;
a third ferromagnetic film;
a third non-magnetic film; and
a second soft magnetic film of which the magnetization direction can be rotated in respect to an external magnetic field; wherein:
the magnetization direction of the first ferromagnetic film and the magnetization direction of the second ferromagnetic film are in an antiparallel state;
the magnetization direction of the second ferromagnetic film and the magnetization direction of the third ferromagnetic film are in an antiparallel state; and
the first soft magnetic film and the second soft magnetic film are arranged to be symmetrical with respect to the non-magnetic spacer layer.
2. A magnetic head according to claim 1, wherein said first non-magnetic medium layer consists of any one of metal layers selected from a group of Ru, Rh, Ir, Cr, and Cu or consists of an alloy containing some of these metals.
3. A magnetic head according to claim 1, wherein said second non-magnetic film and said third non-magnetic film comprise a Cu layer.
4. A magnetic head according to claim 1, wherein the product of the film thickness and saturation magnetization of said first ferromagnetic film is larger than the product of the film thickness and saturation magnetization of said second ferromagnetic film.
5. A magnetic head according to claim 1, wherein the magnetization direction of said first ferromagnetic film and said third ferromagnetic film is prescribed to be in the same direction.
6. A magnetic head according to claim 1, wherein the magnetization direction of said first ferromagnetic film and said third ferromagnetic film is prescribed by antiferromagnetic films or hard magnetic films which are respectively in contact with both said first ferromagnetic film and said third ferromagnetic film.
7. A magnetic head according to claim 1 in which a film thickness of the second ferromagnetic film is smaller than a film thickness of the first ferromagnetic film.
8. A magnetic head comprising:
a first spin valve element;
a second spin valve element; and
a non-magnetic spacer layer formed between the first spin valve element and the second spin valve element;
the first spin valve element including:
a first antiferromagnetic film
a first ferromagnetic film;
a first non-magnetic film made of Ru;
a second ferromagnetic film;
a second non-magnetic film of which the magnetization direction can be rotated in respect to an external magnetic field; and
a first soft magnetic film;
the second spin valve element including:
a second antiferromagnetic film;
a third ferromagnetic film;
a third non-magnetic film; and
a second soft magnetic film of which the magnetization direction can be rotated in respect to an external magnetic field; wherein:
the magnetization direction of the second ferromagnetic film and the magnetization direction of the third ferromagnetic film are in an antiparallel state; and
the first soft magnetic film and the second soft magnetic film are arranged to be symmetrical with respect to the non-magnetic spacer layer.
9. A magnetic head according to claim 8, wherein said first non-magnetic medium layer consists of any one of metal layers selected from a group of Ru, Rh, Ir, Cr, and Cu or consists of an alloy containing some of these metals.
10. A magnetic head according to claim 8, wherein said second non-magnetic film and said third non-magnetic film comprise a Cu layer.
11. A magnetic head according to claim 8, wherein the product of the film thickness and saturation magnetization of said first ferromagnetic film is larger than the product of the film thickness and saturation magnetization of said second ferromagnetic film.
12. A magnetic head according to claim 8, wherein the magnetization direction of said first ferromagnetic film and said third ferromagnetic film is prescribed to be in the same direction.
13. A magnetic head according to claim 8, wherein the magnetization direction of said first ferromagnetic film and said third ferromagnetic film is prescribed by antiferromagnetic films or hard magnetic films which are respectively in contact with both said first ferromagnetic film and said third ferromagnetic film.
14. A magnetic head according to claim 8 in which a film thickness of the second ferromagnetic film is smaller than a film thickness of the first ferromagnetic film.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A method for authenticating a user, comprising the steps of:
receiving a claim from an individual claiming to be an enrolled user;
retrieving a stored electroencephalographic response profile associated with the enrolled user, the electroencephalographic response profile being generated as a result of supervised enrollment process based a shared secret, wherein the shared secret comprises a sequence of visual elements which follows an explicit rule known by the enrolled user during the supervised enrollment process, wherein the explicit rule specifies at least one of a relationship between the visual elements and an allowed sequence of the visual elements;
presenting perceptory stimuli to the individual, the perceptory stimuli comprising the shared secret;
obtaining at least one electroencephalographic response from the individual in response to the perceptory stimuli comprising said shared secret presented to the individual; and
authenticating the individual’s claim as being the enrolled user based on the obtained electroencephalographic response, wherein authenticating comprises:
detecting a pattern of P300 event-related potentials in the obtained electroencephalographic response, wherein a P300 event-related potential comprises a prominent peak detectable in an electroencephalographic signal after about 300 milliseconds (ms) following a perceptory stimulus;
time-aligning the detected pattern of P300 event-related potentials with the perceptory stimuli presented to the individual;
comparing the detected pattern of P300 event-related potentials with an expected pattern of P300 event-related potentials as provided by the stored electroencephalographic response profile associated with the enrolled user, wherein an unexpectedly produced P300 event-related potential in the detected pattern is indicative of an imposter;
determining an authentication score based on results of said comparing; and
comparing the determined authentication score to a predetermined authentication threshold value to positively authenticate the individual as being the claimed enrolled user when the authentication score exceeds the predetermined authentication threshold or to reject the individual as an imposter when the authentication score does not exceed the predetermined authentication threshold.
2. The method of claim 1, further comprising performing the supervised enrollment process, wherein the supervised enrollment process comprises:
presenting a set of perceptory stimuli to a user to be enrolled, wherein the set of perceptory stimuli comprises one or more sequences of visual elements that follow the explicit rule and one or more sequences of visual elements that violate the explicit rule, and wherein the user is informed of the explicit rule before being presented with the set of perceptory stimuli;
recording a set of electroencephalographic responses of the user in response to the set of perceptory stimuli presented to the user, wherein the set of electroencephalographic responses comprises at least one P300 event-related potential which corresponds to the occurrence of a violation of the explicit rule; and
analyzing the set of recorded electroencephalographic responses to generate a set of results which are stored as part of the electroencephalographic response profile of the enrolled user.
3. The method of claim 2, further comprising the step of confirming the consistency of the set of results by repeating the supervised enrollment process.
4. The method of claim 1, wherein the authentication score comprises a negative cumulative distance measure obtained using a dynamic time-warping process.
5. An article of manufacture comprising a non-transitory computer readable storage medium comprising program instructions which, when executed by a computer, implement the method of claim 1.
6. A method for authenticating a user, comprising the steps of:
receiving a claim from an individual claiming to be an enrolled user;
retrieving a stored electroencephalographic response profile associated with the enrolled user, the electroencephalographic response profile being generated as a result of an unsupervised enrollment process based on an implicit rule, said implicit rule being learned from electroencephalographic signals obtained in response to the user being presented with one or more training sequences of visual elements, wherein the implicit rule specifies at least one of a relationship between the visual elements and an allowed sequence of the visual elements;
presenting perceptory stimuli to the individual, the perceptory stimuli comprising a sequence of visual elements that follow the implicit rule;
obtaining at least one electroencephalographic response from the individual in response to the perceptory stimuli presented to the individual; and
authenticating the individual’s claim as being the enrolled user based on the obtained electroencephalographic response, wherein authenticating comprises:
detecting a pattern of P300 event-related potentials in the obtained electroencephalographic response, wherein a P300 event-related potential comprises a prominent peak detectable in an electroencephalographic signal after about 300 milliseconds (ms) following a perceptory stimulus;
time-aligning the detected pattern of P300 event-related potentials with the perceptory stimuli presented to the individual;
comparing the detected pattern of P300 event-related potentials with an expected pattern of P300 event-related potentials as provided by the stored electroencephalographic response profile associated with the enrolled user, wherein an unexpectedly produced P300 event-related potential in the detected pattern is indicative of an imposter;
determining an authentication score based on results of said comparing; and
comparing the determined authentication score to a predetermined authentication threshold value to positively authenticate the individual as being the claimed enrolled user when the authentication score exceeds the predetermined authentication threshold or to reject the individual as an imposter when the authentication score does not exceed the predetermined authentication threshold.
7. The method of claim 6, further comprising performing the unsupervised enrollment process, wherein the unsupervised enrollment process comprises:
presenting a set of training perceptory stimuli to a user to be enrolled, wherein the set of training perceptory stimuli comprises one or more training sequences of visual elements;
recording a set of electroencephalographic responses generated in response to the user being presented with the one or more training sequences of visual elements;
analyzing the set of recorded electroencephalographic responses to learn at least one rule;
presenting, to the user, at least one perceptory stimuli that is based on the at least one learned rule;
recording a second set of electroencephalographic responses generated in response to the user being presented with the at least one perceptory stimuli based on the at least one learned rule; and
using the recorded second set of electroencephalographic responses to perform a consistency check for the at least one learned rule.
8. The method of claim 7, wherein during the unsupervised enrollment process, the user is not informed of the one or more rules that are learned from the training set of perceptory stimuli.
9. The method of claim 7, wherein during the unsupervised enrollment process, the user selects one of the one or more implicit rules that are learned from the training set of perceptory stimuli.
10. The method of claim 6, wherein the authentication score comprises a negative cumulative distance measure obtained using a dynamic time-warping process.
11. An article of manufacture comprising a non-transitory computer readable storage medium comprising program instructions which, when executed by a computer, implement the method of claim 6.
12. An apparatus, comprising:
a memory configured to store program instructions; and
a processor coupled to the memory and configured to execute the stored program instructions to execute a process to authenticate a user, the process comprising:
receiving a claim from an individual claiming to be an enrolled user;
retrieving a stored electroencephalographic response profile associated with the enrolled user, the electroencephalographic response profile being generated as a result of supervised enrollment process based a shared secret, wherein the shared secret comprises a sequence of visual elements which follows an explicit rule known by the enrolled user during the supervised enrollment process, wherein the explicit rule specifies at least one of a relationship between the visual elements and an allowed sequence of the visual elements;
presenting perceptory stimuli to the individual, the perceptory stimuli comprising the shared secret;
obtaining at least one electroencephalographic response from the individual in response to the perceptory stimuli comprising said shared secret presented to the individual; and
authenticating the individual’s claim as being the enrolled user based on the obtained electroencephalographic response, wherein authenticating comprises:
detecting a pattern of P300 event-related potentials in the obtained electroencephalographic response, wherein a P300 event-related potential comprises a prominent peak detectable in an electroencephalographic signal after about 300 milliseconds (ms) following a perceptory stimulus;
time-aligning the detected pattern of P300 event-related potentials with the perceptory stimuli presented to the individual;
comparing the detected pattern of P300 event-related potentials with an expected pattern of P300 event-related potentials as provided by the stored electroencephalographic response profile associated with the enrolled user, wherein an unexpectedly produced P300 event-related potential in the detected pattern is indicative of an imposter;
determining an authentication score based on results of said comparing; and
comparing the determined authentication score to a predetermined authentication threshold value to positively authenticate the individual as being the claimed enrolled user when the authentication score exceeds the predetermined authentication threshold or to reject the individual as an imposter when the authentication score does not exceed the predetermined authentication threshold.
13. The apparatus of claim 12, wherein the process further comprises performing the supervised enrollment process, wherein the supervised enrollment process comprises:
presenting a set of perceptory stimuli to a user to be enrolled, wherein the set of perceptory stimuli comprises one or more sequences of visual elements that follow the explicit rule and one or more sequences of visual elements that violate the explicit rule, and wherein the user is informed of the explicit rule before being presented with the set of perceptory stimuli;
recording a set of electroencephalographic responses of the user in response to the set of perceptory stimuli presented to the user, wherein the set of electroencephalographic responses comprises at least one P300 event-related potential which corresponds to the occurrence of a violation of the explicit rule; and
analyzing the set of recorded electroencephalographic responses to generate a set of results which are stored as part of the electroencephalographic response profile of the enrolled user.
14. The apparatus of claim 12, wherein the authentication score comprises a negative cumulative distance measure obtained using a dynamic time-warping process.