CN108008326B - Method for regulating and controlling damping factor of MRAM material - Google Patents
Method for regulating and controlling damping factor of MRAM material Download PDFInfo
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- CN108008326B CN108008326B CN201610957180.1A CN201610957180A CN108008326B CN 108008326 B CN108008326 B CN 108008326B CN 201610957180 A CN201610957180 A CN 201610957180A CN 108008326 B CN108008326 B CN 108008326B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/12—Measuring magnetic properties of articles or specimens of solids or fluids
- G01R33/1215—Measuring magnetisation; Particular magnetometers therefor
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/62—Record carriers characterised by the selection of the material
Abstract
The invention discloses a method for adjusting a CoFeB film damping factor by changing the growth sequence of MgO/CoFeB/Ta. The structure not only shows better vertical anisotropy, but also has good adjustability of the damping factor, so the structure is a key material for producing a Magnetic Random Access Memory (MRAM) device. The invention utilizes magnetron sputtering to grow two ultrathin films of Ta/CoFeB/MgO and MgO/CoFeB/Ta and utilizes time-resolved magneto-optical Kerr effect (TRMOKE) to test the damping factor of the material. The value of the damping factor (alpha) of Ta/CoFeB/MgO is 0.017, while that of MgO/CoFeB/Ta is 0.027, and the material is very suitable for improving the read-write speed of the magnetic storage medium because the damping factor is greatly changed. The sample structure referred to in the present invention is a substrate, a buffer layer, a magnetic layer, and a cover layer in this order. The femtosecond pulse laser has repetition frequency of 1000Hz, pulse width of 50fs, and pumping power density of 3.54mJ/cm2。
Description
The technical field is as follows:
the invention belongs to the crossing field of the combination of magnetic materials and optical technology measurement. The method is mainly characterized in that under the condition that the magnetism of the material is basically unchanged, the damping factor is obviously changed, and the method can be applied to a magnetic random access memory and a low-power consumption sensor.
Background art:
the magnetic random access memory MRAM is a nonvolatile memory. There are a number of distinct advantages: high integration of memory density, high speed read and write capability, near infinite number of repeatable read and write times, low power consumption, high radiation resistance, and most outstanding non-volatility. Typical MRAM device structures and fabrication processes are complex and not conducive to the fabrication of integrated devices.
Many studies on MRAM are currently being made, with the main focus being on magnetic switching speed, thermal stability and low power consumption. The flipping speed of the magnetic moment mainly depends on the damping factor (alpha) of the material, and when the intrinsic damping factor of the material is increased, the relaxation time of the material is correspondingly reduced, so that the flipping speed is increased. Meanwhile, thermal stability and low power consumption are positively correlated with the perpendicular anisotropy factor (kueff) and negatively correlated with α. While keeping the kueff high, a may be selected as appropriate depending on the environment in which the device operates.
The key material of the magnetic random access memory adopts a CoFeB/MgO structural system mainly because the magnetic random access memory has a high tunneling magneto-resistance TMR ratio. How to adjust and control the damping factor in the system mainly focuses on the buffer layer/cover layer type, the CoFeB thickness of the magnetic layer, high-temperature annealing and other aspects.
The invention content is as follows:
the purpose of the invention is as follows: aiming at the defects in the prior art and design, the invention provides a method for adjusting the damping factor by only changing the growth sequence of the Ta/CoFeB/MgO structure, thereby changing the switching speed, the thermal stability and the low power consumption of the magnetic random access memory.
The technical scheme of the invention is as follows: growing material structures Ta/CoFeB/MgO and MgO/CoFeB/Ta by a magnetron sputtering method: the damping factor of a magnetic material is measured using the time-resolved magneto-optical kerr effect. The method for regulating the damping factor of the MRAM material is realized by the material growth sequence (see figure 1). The damping factor is changed by changing the growth sequence of a Ta/CoFeB/MgO material system, a CoFeB/MgO interface forms vertical anisotropy, and the upper and lower positions formed by the Ta/CoFeB interface have different influences on precession during femtosecond laser irradiation, so that the damping factor is changed.
In the Ta/CoFeB/MgO material system, the thickness of the CoFeB film of the magnetic layer is 1 nm.
In the Ta/CoFeB/MgO material system, the thickness of Ta is 5 nm.
In the Ta/CoFeB/MgO material system, the MgO thickness is 3nm and the crystal orientation is (100).
The detection light power is 20 μ w, and the repetition frequency is 1000 Hz; this causes the area detected by the detection light to be uniformly heated.
The magneto-optical Kerr effect detection device consists of a femtosecond laser, a beam splitting plate, a delay line, a Glan Taylor lens, a chopper, an optical lens, an electromagnet, a half-wave plate, a Wollaston prism, a balance detector and a computer. The variation range of the magnetic field and the delay length of the delay line are set by a computer, and the optical signal which changes along with time can be directly collected.
The magneto-optical Kerr effect is used for detecting and obtaining a magnetization precession signal of the material, damping factors of the material under different growth orders are obtained through fitting (see figure 5), and the regulation and control of the damping factors of the material are realized, so that the regulation and control of the magnetic overturning speed, the thermal stability and the low power consumption of the material are realized.
The theoretical basis of the invention is as follows: the damping factor of CoFeB is measured by utilizing the magneto-optical Kerr effect of time resolution. The magneto-optical kerr effect is: when a beam of linearly polarized light passes through a magnetic sample or a substance in the presence of an external magnetic field, the polarization plane of the linearly polarized light is deflected, and simultaneously the linearly polarized light is changed into elliptically polarized light. The time-resolved magneto-optical kerr effect technology refers to: the electron temperature is raised by the transient heating of the pump light, and then the heat of the electron exchanges relaxation among the three systems of the electron, spin and lattice, and finally reaches an equilibrium, which is a well-known three-temperature model. In the dynamic process, another beam of femtosecond pulse light is used for detecting the change caused by the pumping light. The irradiation of the pulsed laser corresponds to the application of a transient effective field H to the sampleeffIn HeffThe magnetization direction of the magnetic thin film is deviated from the initial equilibrium position. When an external magnetic field is applied, the magnetization after the deviation from the equilibrium position precesses along the effective field. The precession of the magnetization will eventually tend towards the direction of the effective field, where the magnitude of the damping factor plays an important role in the stability of the precession. We use the phenomenological LLG equation to well link precession and damping. By experiment, the image which we can directly obtain is the precession of the magnetization, namely the sine oscillation waveThe process of shape exponential decay. Can be expressed by the following formula:
wherein theta iskRepresenting the measured kerr signal, the first term on the right of the equation represents the exponential decay background under ultrafast demagnetization, v represents the rate of magnetization recovery, c, τ, f, and
representing the magnitude, relaxation time, precession frequency and precession phase, respectively, of the magnetization precession. By solving the LLG equation, we can obtain a simple calculation formula of the damping factor: α ═ 1/ω τ.
It should be noted that the damping factor a is not an intrinsic damping factor, and we refer to it as an effective damping factor, because it varies with the magnitude of the external magnetic field. In this patent, since the applied magnetic field is about 7000Oe, the magnetic anisotropy of CoFeB has negligible effect, so the effective damping factor is about equal to the intrinsic damping factor α0。
The varying damping factor α' can be formulated as:
α′＝(hγ/2πMs)(g↑↓S-1/tCoFeB)
g↑↓and S-1Representing the mixed conductance and the cross-sectional area of the sample. When Ta is sputtered on the CoFeB magnetic layer, Ta is a heavy element and the sputtering kinetic energy is relatively large, and therefore Ta is easily mixed with the magnetic layer. Meanwhile, the Ta layer can easily absorb boron, so that the mixed conductance is increased, the damping factor of Ta/MgO/CoFeB/Ta is increased according to the formula, and the control of the damping factor by the material growth sequence is realized.
Has the advantages that: the growth order modulation method provided by the invention can accurately control the growth of the sample, and realize the regulation and control of the damping factor with accurate positioning and controllable range by regulating and controlling the growth order, thereby influencing the relaxation time of the magnetic material, changing the magnetic turnover time of the material, and realizing higher-speed MRAM reading and writing, high thermal stability and low power consumption.
Description of the drawings:
FIG. 1 is a schematic diagram of a sample structure employed in the present invention.
Fig. 2 shows Hysteresis loop (hystersis loop) information of a sample obtained by a Vibrating Sample Magnetometer (VSM).
FIG. 3 is a simplified diagram of the experimental protocol employed in the present invention: (1) the optical fiber laser comprises pump light, (2) detection light, (3) a beam splitting plate, (4) a delay line, (5) BBO crystal used for doubling the frequency of 800nm light into 400nm detection light, (6) a Glantcast prism, (7) a chopper, (8) an optical lens, (9) a reflector, (10) a half-wave plate, (11) a Wollaston prism, and (12) a balance detector.
Fig. 4 is a schematic view of the precession of magnetization according to the present invention.
Fig. 5 shows the process of the effective damping factor varying with the magnetic field and tending towards the eigen α 0. The experimental result clearly shows that by changing the growth sequence of the material, the intrinsic damping factor of the CoFeB film can be effectively changed within a certain range (the damping value does not change along with the external magnetic field).
The specific implementation mode is as follows:
to achieve the above object, the present invention first prepares material structures of substrate/Ta (5)/CoFeB (1)/MgO (3)/Ta (5) and substrate/Ta (5)/MgO (3)/CoFeB (1)/Ta (5) (the numbers indicate the film thickness in nanometers), as shown in FIG. 1. The magnetic thin film structure has perpendicular magnetic anisotropy and coercive force Hc5.31Oe and 5.66Oe, respectively, saturated magnetization field MsIs 890emu/cm3、836emu/cm3As shown in fig. 2.
The sample involved in the invention is grown by a magnetron sputtering method. First, a surface cleaning treatment was performed on the Si substrate by blowing a nitrogen gas flow over the substrate surface. The vacuum degree was maintained at 10 at the beginning of sputtering the sample-6Pa or so.
To accomplish the above invention, we adopted an experimental scheme using time-resolved magneto-optical kerr effect, using an experimental system as shown in fig. 3.
The femtosecond laser used in the present invention has a pulse width of 50fs and a repetition frequency of 1000 Hz. Since the resolution of the pulsed laser itself in the time domain is 50fs, theoretically we can detect physical processes larger than 50 fs. The detection means are therefore sufficiently accurate for the phenomenon of magnetization precession in the picosecond or even nanosecond range (see fig. 4).
In the invention, in order to detect the magnetic precession phenomenon caused by pumping light, a delay line with the precision of 1 μm and the total length of 100cm is adopted. The delay line converts the optical path difference into time difference by using two beams of coherent light, thereby realizing the physical process tracking on a picosecond scale.
In the present invention, the wavelength of the pump light is 800 nm; the wavelength of the detection light is 400nm (obtained by 800nm under BBO frequency multiplication), and the detection power is 20 μ w. In order to ensure that 400nm of detection light is obtained by frequency doubling in the requirements, a 400nm band-pass filter is needed, and light with the wavelength of 800nm mixed in the 400nm light after frequency doubling is filtered.
To detect magnetic moment precession in the magnetic film, a magnetic field in the near plane (much larger than the saturation field of the magnetic film) was applied to the sample. The magnetic field has the effect of pulling out the magnetic moment of the CoFeB sample from the vertical direction, so that precession can be generated around the external magnetic field direction. Angle theta between external magnetic field and sample planeH30 deg., as shown in fig. 4.
In order to detect a more obvious precession phenomenon, the 800nm pump light is vertically incident to the surface of a sample, so that the sample is subjected to local transient heating. In addition, a probe light of 400nm is made to be nearly perpendicularly incident on the surface of the sample, so that precession of the magnetic moment in a direction perpendicular to the in-plane direction of the sample can be detected.
In order to ensure near-vertical incidence of the probe light in the above invention, a long-focus lens with a large aperture is used to focus the pump light and the probe light, so that the probe light naturally irradiates the surface of the sample at a small angle deviated from the vertical direction (i.e. the incidence direction of the pump light).
In the present invention, in order to meet the above requirements, the electromagnet is placed on a self-designed round rotary table marked with scales, so that a magnetic field with any angle can be accurately applied to the sample (see fig. 3).
The invention adopts a balanced optical bridge formed by combining a half-wave plate, a Wollaston prism and a balanced detector to measure the Kerr rotation angle of the detection light. The initial polarization direction of the detection light is changed by using a half-wave plate, so that the output of the balanced detector is close to zero voltage.
The signal detection in the invention adopts a mode of combining balanced optical bridge detection and phase-locked measurement. The pump light is modulated with a chopper at 333Hz and this frequency is used as a reference frequency for the phase lock measurement.
The time constant of the lock-in amplifier of the present invention takes 300 ms.
The femtosecond laser adopted in the invention is provided by coherent companies, the repetition frequency is 1000Hz, the pulse width is 50fs, and the maximum average power can reach 4 mW.
The above description is only a preferred embodiment of the present invention, and should not be taken as limiting the scope of the invention, which is intended to cover all the modifications and equivalents of the claims and the specification.
Claims (1)
1. A method for regulating and controlling damping factor of MRAM material, the key material of magnetic random access memory adopts CoFeB/MgO structure system, the key material has high tunnel magnetoresistance TMR ratio, characterized in that, CoFeB/MgO, through magnetron sputtering method growth material structure Ta/CoFeB/MgO and MgO/CoFeB/Ta: measuring the damping factor of the magnetic material by using the time-resolved magneto-optical Kerr effect; the damping factor is changed by changing the growth sequence of a Ta/CoFeB/MgO material system, a CoFeB/MgO interface forms vertical anisotropy, the upper and lower positions formed by the Ta/CoFeB interface have different influences on precession during femtosecond laser irradiation, the damping factor is further changed, a magnetization precession signal of the material is obtained by utilizing magneto-optical Kerr effect detection, the damping factors of the material under different growth sequences are obtained by fitting, the regulation and control on the damping factor of the material are realized, and further the regulation and control on the magnetic overturning speed, the thermal stability and the low power consumption of the material are realized;
in a Ta/CoFeB/MgO material system, the thickness of a CoFeB film of a magnetic layer is 1 nm;
in a Ta/CoFeB/MgO material system, the thickness of Ta is 5 nm;
in the Ta/CoFeB/MgO material system, the MgO thickness is 3nm and the crystal orientation is (100).
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| CN111560649A (en) * | 2020-04-16 | 2020-08-21 | 南京大学 | Damping regulation and control method for magnetic semi-metal material of MRAM (magnetic random Access memory) |
| CN113054096B (en) * | 2021-03-03 | 2024-03-19 | 南京大学 | Method for regulating and controlling intrinsic damping factor of magnetic film |
| CN113113444A (en) * | 2021-06-11 | 2021-07-13 | 南京南机智农农机科技研究院有限公司 | Metal-based programmable logic circuit for in-memory computation and preparation method thereof |
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| KR100213332B1 (en) * | 1995-12-29 | 1999-08-02 | 이구택 | Producing method for magneto-optical multi-layered thin film of cobalt/platinum |
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| KR100213332B1 (en) * | 1995-12-29 | 1999-08-02 | 이구택 | Producing method for magneto-optical multi-layered thin film of cobalt/platinum |
| CN102403024A (en) * | 2010-09-08 | 2012-04-04 | 索尼公司 | Memory element and memory device |
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Non-Patent Citations (5)
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