Abstract:
A method for measuring hysteresis curves and anisotropic energy of magnetic memory units is disclosed. It comprises gradually applying different magnetic fields to a single-layer or a multilayer magnetic structure (such as a MRAM memory unit) by extra ordinary Hall effect, and recording the variation of the Hall voltage to obtain the hysteresis curve and anisotropic energy with specific instruments, and calculating the individual anisotropic energy value of the magnetic material of the single-layer or the multilayer magnetic structure.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is based on, and claims priority from, Taiwan Application Serial Number 95109489, filed Mar. 20, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to methods for measuring hysteresis curves and anisotropic energy, and more particularly to methods for measuring hysteresis curves and anisotropic energy of multilayer magnetic memory units. 
         [0004]    2. Description of Related Art 
         [0005]    As applications of magnetic components for everyday life grow day-by-day, developing various kinds of magnetic materials with better physical characteristics become the trend of fashion for researchers. Measuring the hysteresis curves and anisotropic energy of magnetic materials are important for differentiating their characteristics. 
         [0006]    Conventional methods for measuring hysteresis curves include vibrating sample magnetometer (VSM) or the Kerr rotation angle by laser beam. 
         [0007]    The measurement principle of VSM is to vibrate the magnetic sample near the coils, and determine the magnetic properties with the corresponding magnetic flux variations. 
         [0008]    The measuring principle of the Kerr rotation angle by laser beam is either to magnetize the magnetic material with an applied external magnetic field, or spontaneously magnetize a ferromagnetic material, which makes the refraction index of the material magnetically birefringent. That means the refraction index of right-handed and left-handed polarized light are different. The magneto-optic Kerr effect will elliptically polarize a linearly polarized laser normally incident on a sample surface. The Kerr angle is the angle between the linearly polarized light and the elliptically polarized light. The direction of the magnetic moment could be determined using the angle of the elliptically polarized light, and together with the external magnetic field, the whole hysteresis curve can be obtained. 
         [0009]    The conventional method for measuring the anisotropic energy of a single-layer uses an applied external magnetic field along the axis parallel to the direction of the sample magnetization to obtain the coercive field of the hysteresis curve and the variation curve of magnetization per unit of the sample. A second external magnetic field is applied along the axis perpendicular to the sample magnetization, wherein the intensity of the field must be much greater than the magnetization of the sample to obtain the variation curve of the perpendicular anisotropic energy of the sample. Software is then used to calculate the value of the anisotropic energy. 
         [0010]    The aforementioned methods can all be affected by environmental factors. The VSM method is very sensitive. Any adjustment for the position between sample and coil will cause a significant deviation for the measurement. While measuring the Kerr rotation angle, a slight deviation regarding the pathway, intensity, or angle for the laser beam could also create some difficulty. The present invention provides methods that apply to the extra-ordinary Hall effect which can significantly reduce the difficulty to measure the hysteresis curves and anisotropic energy. 
         [0011]    The conventional method can only measure the anisotropic energy of a single-layer material, however, if a magnetic structure consists of a plurality of magnetic materials, the conventional method fails to obtain the anisotropic energy of every magnetic material therein. 
         [0012]    This other shortcoming for the conventional method is especially problematic for magnetic devices consisting of multi-film layers of various materials. 
       SUMMARY 
       [0013]    In order to solve the aforementioned and other problems and to achieve the technical advantages of the present invention, the present invention provides a method for measuring hysteresis curve and anisotropic energy of a magnetic memory unit. The method can measure the anisotropic energy for a magnetic structure, even if the structure is consisting of a plurality of materials. 
         [0014]    Therefore, an objective of the present invention is to provide a method for measuring the anisotropic energy of magnetic materials, which applies different step-by-step external magnetic fields and, records the difference of coercive fields in each step, and then an appropriate software is applied to calculate the anisotropic energy of each material. 
         [0015]    According to the aforementioned objectives of the present invention, a method of measuring the anisotropic energy of a magnetic structure with a plurality of layers is provided. The method includes applying the extra ordinary Hall effect. A Hall voltage is obtained by applying an external magnetic field to magnetic material with a perpendicular magnetization rotating from the normal direction. This phenomenon is called the “extra-ordinary Hall effect” as the Hall voltage obtained by the method of the present invention is different from the ordinary Hall voltage. An ordinary Hall voltage is generated when the electrons and the electronic holes are distributed to the opposite sides of the material with a current passing through the material. 
         [0016]    The different coercive field intensities of different magnetic materials are obtained through the following steps: 
         [0017]    1. Apply an external magnetic field to induce the net magnetization of the magnetic materials to align completely in the same direction. 
         [0018]    2. Apply an external magnetic field that is either parallel or antiparallel to the coercive field of each step by step magnetic material, wherein the intensity of the external magnetic field is slightly greater than the coercive field of each magnetic material. 
         [0019]    After that, external magnetic field gradient perpendicular to the coercive field of each magnetic material is applied step by step, wherein the intensity of the external magnetic field gradient is much greater than the coercive field of each magnetic material, such that the variation between the coercive field intensity and the anisotropic energy is recorded step by step. 
         [0020]    The external magnetic field generates the net magnetization for each specific magnetic material of a magnetic structure, with an opposite direction to other magnetic materials. The method applies the external magnetic field gradient that is perpendicular to the coercive field of each magnetic material step by step, wherein the intensity of the external magnetic field gradient is slightly greater than the coercive field of each magnetic material, the variations of coercive field intensity and anisotropic energy are recorded, and then the calculation for the anisotropic energy of the specific magnetic material is performed. 
         [0021]    The present invention applies vector relations to obtain the cumulative equations of anisotropic energy from each magnetic material in the external magnetic field gradient. The value of anisotropic energy of every magnetic material is obtained by vector analysis. The present invention is suitable for all kinds of horizontally or perpendicularly anisotropic magnetic structures or materials. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
           [0023]      FIG. 1  illustrates a method for measuring hysteresis curve of a magnetic structure comprising a single-layer magnetic material with the extra ordinary Hall effect; 
           [0024]      FIG. 2  illustrates a method for measuring anisotropic energy of a magnetic structure comprising a single-layer magnetic material with the extra ordinary Hall effect; 
           [0025]      FIGS. 3 through 6  illustrate of the first through fourth phases of an anisotropic energy measuring method of the preferred embodiment of the present invention; and 
           [0026]      FIG. 7  illustrates the flowchart of a software calculation process for the preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Reference is now made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0028]    While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention is better understood after considering the following description in conjunction with the figures, in which like reference numerals are carried forward. 
         [0029]    Reference is made to  FIG. 1 , which illustrates a method for measuring the hysteresis curve of a magnetic structure comprising a single-layer magnetic material with the extra-ordinary Hall effect. A magnetic device  100  includes an upper protective layer  110 , a magnetic layer  120  and a lower protective layer  130 . 
         [0030]    The material of the magnetic layer  120  is a perpendicularly anisotropic magnetic material, such as GdFeCo, TbFeCo or DyFeCo. Applying a magnetic field  140  in the Z-axis direction of the magnetic device  100  enables the net magnetization of the magnetic layer  120  to rotate in the same direction. Applying a current  150  and changing the intensity of the magnetic field  140 , the variations in the Hall voltage value are generated. (A coordinate axis illustrated in  FIG. 1 , which shows the direction of X-axis, Y-axis and Z-axis). 
         [0031]    Reference is made to  FIG. 2 , which illustrates a method for measuring anisotropic energy of a magnetic structure comprising a single-layer magnetic material with the extra ordinary Hall effect. A magnetic component  200  consists of an upper protective layer  210 , a magnetic layer  220  and a lower protective layer  230 . 
         [0032]    Applying a magnetic field  240  along with the Z-axis of the magnetic component  200  enables the net magnetization of the magnetic layer  220  to rotate in the same direction. A magnetic field  250  is applied in the Y-axis direction of the magnetic component  200 ; the intensity of the magnetic field  250  is slightly greater than coercive field of the magnetic layer  220 . Hence, an anisotropic energy curve of the magnetic layer  220  can be obtained, and an anisotropic energy value could be calculated through appropriate software. (A coordinate axis illustrated in  FIG. 2  shows the direction of the X-axis, Y-axis and Z-axis). 
         [0033]    Reference is made to  FIG. 3 , which illustrates the first phase of an anisotropic energy measuring method for the preferred embodiment of the present invention. A magnetic device  300  is a multilayer magnetic structure, which includes a lower electrode layer  310 , a first main magnetic layer  320 , a first sub magnetic layer  330 , an oxide layer  340 , a second sub magnetic layer  350 , a second main magnetic layer  360  and an upper electrode layer  370 . 
         [0034]    In this embodiment, the magnetic device  300  is a magnetic tunneling junction (MTJ) of a magnetic memory unit. The net magnetization of the first main magnetic layer  320  and the second main magnetic layer  360  are stronger than the other layers. The first main magnetic layer  320  is a free layer of the MTJ, and the second main magnetic layer  360  is a pinned layer of the MTJ. 
         [0035]    The material of the first main magnetic layer  320  could be GdFeCo, TbFeCo or DyFeCo. In this embodiment, the material of the first main magnetic layer  320  is GdFeCo. The first main magnetic layer  320  includes a net magnetization  321 , and the direction of the net magnetization  321  can be changed easily by an external magnetic field. The material of the second main magnetic layer  360  could be GdFeCo, TbFeCo or DyFeCo. In this embodiment, the material of the second main magnetic layer  360  is TbFeCo. The second main magnetic layer  360  includes a net magnetization  361 , and the net magnetization  361  is perpendicularly anisotropic. The coercive field of the second main magnetic layer  360  is greater than the one on the first main magnetic layer  320 . 
         [0036]    Applying a perpendicularly downward magnetic field  380  to the magnetic device  300  enables the net magnetization  321  and the net magnetization  361  to be aligned in the same direction. 
         [0037]    Reference is made to  FIG. 4 , which illustrates the second phase of an anisotropic energy measuring method of the preferred embodiment of the present invention. A horizontal magnetic field  381  is applied to the magnetic device  300 , and the intensity of the horizontal magnetic field  381  has to be slightly greater than the coercive field of the first main magnetic layer  320  but smaller than the coercive field of the second main magnetic layer  360 . A cumulative function of the anisotropic energy of the first main magnetic layer  320  and the second main magnetic layer  360  can be calculated through appropriate software as described by the following equation: 
         [0000]    
       
      
       Ku 
       A 
       =Ku 
       320 
       +Ku 
       360  
      
     
         [0038]    Under the horizontal magnetic field  381 , Ku A  is the cumulative anisotropic energy of the first main magnetic layer  320  and the second main magnetic layer  360 . Ku 320  is the anisotropic energy of the first main magnetic layer  320 , and Ku 360  is the anisotropic energy of the second main magnetic layer  360 . 
         [0039]    Reference is made to  FIG. 5 , which illustrates the third phase of an anisotropic energy measuring method of the preferred embodiment of the present invention. A perpendicularly upward magnetic field  382  is applied to the magnetic device  300 , then the net magnetization  321  and the net magnetization  361  become antiparallel. 
         [0040]    Reference is made to  FIG. 6 , which illustrates the fourth phase of an anisotropic energy measuring method of the preferred embodiment of the present invention. A horizontal magnetic field  383  is applied to the magnetic device  300 , and the intensity of the horizontal magnetic field  383  must be slightly greater than the coercive field of the first main magnetic layer  320  but smaller than the coercive field of the second main magnetic layer  360 . A cumulative function of the anisotropic energy of the first main magnetic layer  320  and the second main magnetic layer  360  can be calculate through appropriate software as described by the following equation: 
         [0000]    
       
      
       Ku 
       B 
       =−Ku 
       320 
       +Ku 
       360  
      
     
         [0000]    A third equation below can be further obtained from the aforementioned first equation and the second equation: 
         [0000]        Ku   320 =( Ku   A   −Ku   B )/2 
         [0041]    Under the horizontal magnetic field  383 , Ku B  is the cumulative anisotropic energy of the first main magnetic layer  320  and the second main magnetic layer  360 . Ku 320  is the anisotropic energy of the first main magnetic layer  320 , and Ku 360  is the anisotropic energy of the second main magnetic layer  360 . 
         [0042]    Because all of the aforementioned magnetic fields are smaller than the coercive field of the second main magnetic layer  360 , the complete function of anisotropic energy of the second main magnetic layer  360  cannot be obtained. Hence, the second and third phases above are repeated, except that the intensity of the magnetic field is made to be much greater than the coercive field of the first main magnetic layer  320  and the second main magnetic layer  360 . Then, the complete function of anisotropic energy of the second main magnetic layer  360  can be obtained. 
         [0043]    From the first, second and third equations, the anisotropic energy of the second main layer  360  can be shown as a fourth equation below: 
         [0000]        Ku   360 =( Ku   A   +Ku   B )/2 
         [0044]    Under the intensity of the magnetic field which is much greater than the coercive field of the first main magnetic layer  320  and the second main magnetic layer  360 , Ku A  is the cumulative anisotropic energy of the first main magnetic layer  320  and the second main magnetic layer  360 , whose net magnetization  320  and  360  are parallel. Ku B  is the cumulative anisotropic energy of the first main magnetic layer  320  and the second main magnetic layer  360 , while the net magnetization  320  and  360  are antiparallel. Ku 360  is the anisotropic energy of the second main magnetic layer  360 . Thus, the specific anisotropic energy of both the first main magnetic layer  320  and the second main magnetic layer  360  can be obtained. 
         [0045]    The arrangement of the aforementioned anisotropic energy measuring method of the magnetic memory unit is as follows: 
         [0046]    The magnetic memory unit includes the first main magnetic layer as a free layer and the second main magnetic layer as a pinned layer. Applying the perpendicularly downward magnetic field to the magnetic memory unit makes that the multiple net magnetizations of the first main magnetic layer and the second main magnetic layer turn totally downward. After that, the horizontal magnetic field is applied to the magnetic memory unit. The intensity of the horizontal external magnetic field is slightly greater than the coercive field of the first main magnetic layer but it is smaller than the coercive field of the second main magnetic layer. From this, the cumulative function of the anisotropic energy of the first main magnetic layer and the second main magnetic layer can be obtained to define a first equation by software calculation. 
         [0047]    After removing the external magnetic field of the previous phase, a perpendicularly upward magnetic field is applied to the magnetic memory unit. The perpendicularly upward magnetic field makes the net magnetizations of the first main magnetic layer and the second main magnetic layer become antiparallel. A horizontal magnetic field is then applied to the magnetic memory unit. The intensity of this horizontal magnetic field is slightly greater than the coercive field of first main magnetic layer and smaller than the coercive field of the second main magnetic layer. From this, the cumulative function of the anisotropic energy of the first main magnetic layer and the second main magnetic layer can be obtained to define a second equation by software calculation. The anisotropic energy of the first main magnetic layer can be calculated from the first equation and the second equation. 
         [0048]    After removing the external magnetic field of the previous phase, a perpendicularly downward magnetic field is applied to the magnetic memory unit. The perpendicularly downward magnetic field enables the net magnetizations of the first main magnetic layer and the second main magnetic layer both turning downward. Then, a horizontal magnetic field is applied to the magnetic memory unit. The intensity of this horizontal magnetic field is much greater than the coercive fields of the first main magnetic layer and the second main magnetic layer. From this, the cumulative function of the anisotropic energy of the first main magnetic layer and the second main magnetic layer can be obtained to define a third equation by software calculation. 
         [0049]    After removing the external magnetic field of the previous phase, a perpendicularly upward external magnetic field is applied to the magnetic memory unit. The perpendicularly upward magnetic field makes the net magnetizations of the first main magnetic layer and the second main magnetic layer become antiparallel. Then, a horizontal magnetic field is applied to the magnetic memory unit whose intensity is much greater than the coercive fields of first main magnetic layer and the second main magnetic layer. Then, the cumulative function of the anisotropic energy of the first main magnetic layer and the second main magnetic layer is obtained to interpret a fourth equation through an appropriate software. The anisotropic energy of the second main magnetic layer can be calculated through the third equation and the fourth equation. 
         [0050]    Reference is made to  FIG. 7 , which illustrates the flow chart of the software calculation process of the preferred embodiment of the present invention. The flow chart of the software calculation process  400  includes many steps for calculating the value of the anisotropic energy. A step  410  describes the measurement for the variation of anisotropic energy in the first and second part of the magnetic device. A step  420  describes the measurement for saturation magnetization (Ms) of a magnetic device by applying AGM or VSM. A step  430  describes the inputs of the initial value for the anisotropic energy, saturation magnetization, and original function of anisotropic energy for performing software. A step  440  obtains a first and a second anisotropic energy values using the aforementioned first, second, third and fourth equations. A step  450  describes the inputs of the first and the second anisotropic energy value for equations (namely the aforementioned first, the second, the third and the fourth equations) and compares the differences between the original plot and the plot based on equations. A step  460  provides the decisions whether the difference is negligible in the step  450  is substantially equivalent. If the results from the step  450 , which are not equivalent, a step  461  is processed to change the anisotropic energy initial value and return to the step  430  to feed in data again. If the result from the step  450  is substantially equivalent, then a step  470  enables the calculation for the anisotropic energy value, and the anisotropic energy value will be for the magnetic device. 
         [0051]    The arrangement of the steps of the software calculation are listed as follows: 
         [0052]    Instruments like AGM or VSM are applied to measure the first main magnetic layer and the second main magnetic layer, which can obtain the saturation magnetization (M S ) and the original function of anisotropic energy. The saturation magnetization, the original function of anisotropic energy, and the initial value of anisotropic energy are inputs into the software program and the previously derived equations (namely the aforementioned first, second, third and fourth equations) are used to calculate the values of the first anisotropic energy and the second anisotropic energy. 
         [0053]    The values of the first anisotropic energy and the second anisotropic energy are input into the previously derived equations (namely the aforementioned first, second, third and fourth equations), then the original plot and the plot according to the previously derived equations are compared to determine equivalency. If the two plots are not equivalent, the anisotropic energy initial value is changed and the anisotropic energy is calculated again. If the results are substantially equivalent, the calculated first and second anisotropic energy value is namely the anisotropic energy value of the first main magnetic layer and the second main magnetic layer, respectively. 
         [0054]    According to the composition and the embodiments above, there are many advantages of the present invention over the prior art, such as: 
         [0055]    1. When the magnetic device consists of a plurality of magnetic materials, no matter whether the easy axes of the magnetic materials are horizontally or perpendicularly anisotropic, this measuring method of the present invention can obtain the anisotropic energy of each magnetic material. 
         [0056]    2. The interaction among the magnetic materials can be comprehensively to facilitate the magnetization obtaining a magnetic device with better physical characteristics by stepwise when modifying the features of these magnetic materials such as the thickness and composition. 
         [0057]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers the modifications and variations of this invention, provided they fall within the scope of the following claims and their equivalents.