# US 5,703,805 A · Method for detecting information stored in a MRAM cell having two magnetic layers in different thicknesses > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,703,805 A - Document ID: US5703805A - Status: Granted - Kind: A - Application number: 08/646,897 - Filed: 1996-05-08 - Granted: 1997-12-30 - Earliest claimed: 1996-05-08 - Assignees: Motorola, Inc. - Inventors: Saied N. Tehrani; Eugene Chen; Mark Durlam; Xiaodong T. Zhu - Web page: https://patentgrep.com/patent/US5703805A ## CPC - G11C 11/5607: {using magnetic storage elements} - G11C 2211/5616: Multilevel magnetic memory cell using non-magnetic conducting interlayer, e.g. GMR, SV, PSV - G11C: STATIC STORES (semiconductor memory devices H10B) ## Abstract A method for detecting and storing four states contained in a MRAM cell having two layers (11,13) which have different thicknesses is provided. A first magnetic field is applied to the MRAM cell, which causes a magnetoresistive change in the MRAM cell. A first and second states are detected based on the magnetoresistive change. A second magnetic field is further applied to the MRAM cell. A third and fourth states are detected based on the magnetoresistive change due to the second magnetic field. ## Claims ### Claim 1 In a magnetic random access memory (MRAM) cell having two switchable magnetic layers having different magnetic switching thresholds separated by a conductor layer, a method for detecting one of a first state, a second state, a third state and a fourth state stored in the MRAM cell comprising the steps of: applying a first magnetic field to the MRAM cell; sensing one of the first state and the second state in the MRAM cell in response to a change of a resistance due to the first magnetic field; applying a second different magnetic field to the MRAM cell; and sensing one of the third state and the fourth state in the MRAM cell in response to a change of a resistance due to the second magnetic field. ### Claim 2 (depends on 1) The method of claim 1 further including restoring an original state in the MRAM cell where the resistance has changed. ### Claim 3 (depends on 2) The method of claim 2 wherein the original state is restored by applying a magnetic field opposite to the first magnetic field in a direction and having the same strength as the first magnetic field when the first or second state is sensed, and the original state is restored the second different magnetic field and then by applying a magnetic field opposite to the second magnetic field in a direction and having the same strength as the first magnetic field when the third or fourth state is sensed. ### Claim 4 (depends on 1) The method of claim 1 wherein the first magnetic field has a strength which causes a magnetization vector in only one of the two magnetic layers to switch, and the second magnetic field causes a magnetization vector in both of the two magnetic layers to switch. ### Claim 5 (depends on 4) The method of claim 4 wherein the first magnetic field causes the magnetization vector in a thinner magnetic layer to switch. ### Claim 6 (depends on 1) The method of claim 1 wherein the first state is sensed when the resistance in the MRAM cell increases in response to the first magnetic field, the second state is sensed when the resistance in the MRAM cell decreases in response to the first magnetic field, the third state is sensed when the resistance in the MRAM cell increases in response to the second magnetic field, and the fourth state is sensed when the resistance in the MRAM cell does not change. ### Claim 7 (depends on 6) The method of claim 6 wherein the first, second, third, and fourth states represent two bits of 00, 10, 01, and 11, respectively. ### Claim 8 (depends on 1) The method of claim 1 further including applying a third magnetic field to the cell according to a difference between a state sensed and a state to be stored. ### Claim 9 (depends on 8) The method of claim 8 further including applying a fourth magnetic field to the MRAM cell. ### Claim 10 (depends on 8) The method of claim 8 wherein the third magnetic field causes a magnetization vector in either of the two magnetic layers to switch. ### Claim 11 (depends on 9) The method of claim 9 wherein the third magnetic field causes a magnetization vector in both of the two magnetic layers to switch, and the fourth magnetic field causes a magnetization vector in either of the two magnetic layers to switch. ### Claim 12 (depends on 8) The method of claim 8 wherein the third magnetic field causes a magnetization vector in both of the two magnetic layers to switch. ### Claim 13 In a magnetic random access memory (MRAM) cell having two magnetic layers having different thicknesses separated by a conductor layer, a method for detecting one of a first state, a second state, a third state and a fourth state stored in the MRAM cell comprising the steps of: applying a first magnetic field to the MRAM cell; sensing one of the first state and the second state in the MRAM cell in response to a change of a resistance due to the first magnetic field; applying a second different magnetic field to the MRAM cell which is opposite to the first magnetic field in a direction and having the same strength as the first magnetic field; and sensing one of the third state and the fourth state in the MRAM cell in response to a change of a resistance due to the second magnetic field. ### Claim 14 (depends on 13) The method of claim 13 further including restoring an original state in the MRAM cell where the resistance has changed. ### Claim 15 (depends on 14) The method of claim 14 wherein the original state is restored by applying a magnetic field opposite to the first magnetic field in a direction and having the same strength as the first magnetic field when the first or second state is sensed, and the original state is restored by applying the first magnetic field when the third or fourth state is sensed. ### Claim 16 (depends on 13) The method of claim 13 wherein the first and second magnetic fields have a strength which causes a magnetization vector in only one of the two magnetic layers to switch. ### Claim 17 (depends on 16) The method of claim 16 wherein the first and second magnetic field cause the magnetization vector in a thinner magnetic layer to switch. ### Claim 18 (depends on 13) The method of claim 13 wherein the first state is sensed when the resistance in the MRAM cell increases in response to the first magnetic field, and the second state is sensed when the resistance in the MRAM cell decreases in response to the first magnetic field, the third state is sensed when the resistance in the MRAM cell decreases in response to the second magnetic field, and the fourth state is sensed when the resistance in the MRAM cell increases in response to the second magnetic field. ### Claim 19 (depends on 18) The method of claim 18 wherein the first, second, third, and fourth state represent two bits of 00, 10, 01, and 11 respectively. ### Claim 20 (depends on 13) The method of claim 13 further including applying a third magnetic field to the cell according to a difference between state sensed and state to be stored. ### Claim 21 (depends on 20) The method of claim 20 further including applying a fourth magnetic field to the MRAM cell. ### Claim 22 (depends on 20) The method of claim 20 wherein the third magnetic field causes a magnetization vector in either of the two magnetic layers to switch. ### Claim 23 (depends on 21) The method of claim 21 wherein the third magnetic field causes a magnetization vector in both of the two magnetic layers to switch, and the fourth magnetic field causes a magnetization vector in either of the two magnetic layers to switch. ### Claim 24 (depends on 20) The method of claim 20 wherein the third magnetic field causes a magnetization vector in both of the two magnetic layers to switch. ### Claim 25 In a magnetic random access memory (MRAM) cell having two magnetic layers having different thicknesses separated by a conductor layer, a method for storing one of a first state, a second state, a third state and a fourth state in the MRAM cell comprising the steps of: switching magnetic vectors in both layers by applying a first magnetic field to the cell when the first state and the fourth state are stored in the cell; and switching magnetic vectors in both layers by applying the first magnetic field to the cell and then applying a second magnetic field to the cell when the second and third states are stored in the cell.