Abstract:
A system and method for realizing vector Kerr magnetometry are disclosed. The system enables simultaneous longitudinal and transverse Kerr effect measurements at each point on a sample surface. An optional component includes a sample platform for achieving precise linear and rotational relocation. The repositionable platform enables complete, 360 degree characterization about a single point. Additionally, the platform control mechanism may be utilized in obtaining longitudinal and transverse Kerr effect measurements at succeeding points on the surface of a sample. Rapid sample characterization is thus achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     This invention pertains to Kerr effect magnetometry in general, and in particular to a system and method for realizing multi-dimensional or vector Kerr effect magnetometry. 
     The Magneto-optic Kerr effect (MOKE), or more simply referred to as the Kerr effect, corresponds to a change in the intensity or polarization state of light reflected from a magnetic material. Since the amount of change in the polarization state or intensity is proportional to the magnetization in the material, it is possible to use this effect to examine magnetic properties of a material sample. 
     The prior art has provided three discrete geometries for characterizing a material sample: longitudinal MOKE; transverse MOKE; and polar MOKE. With a longitudinal MOKE geometry, a magnetic field is applied parallel to the plane of the film and the plane of incidence of the light. With a transverse MOKE geometry, the magnetic field is applied perpendicular to the plane of incidence of the light. With a polar MOKE geometry, the magnetic field is applied orthogonal to the surface of the sample. In all cases, a polarization rotation in the detected light provides an indication of the relative magnetization of the sample under test. 
     Another prior art approach employed for characterizing light transmissive materials is a polar Faraday effect system. A magnetic field is applied parallel to the surface of the material under test. Light is emitted through a point in the material under test and is received on the opposite side of the material. Intensity or polarization changes in the received light are used to characterize the response of the material to the applied field. Practically speaking, the detector can be located in any location as long as the appropriate optics are employed for relaying the received light. 
     Systems employing the longitudinal, transverse or polar geometries may be referred to as scalar systems. Scalar analysis has to-date provided the only non-contact, non-destructive approach for detecting magnetic response. 
     Certain magnetic materials are described as being anisotropic, in that the magnetic response to an applied magnetic field at a point on the surface of the material varies in theta about the point, theta being defined in the plane of the material sample. In general, anisotropic materials exhibit a variation on magnetic behavior in one or multiple directions some of which may or may not be in the plane of the sample. Thus, in the presence of a magnetic field applied in the x direction, a material sample may exhibit one characteristic response in light reflected off the sample in a plane parallel to the x direction, while exhibiting a different characteristic response in light reflected off the sample in a plane parallel to the y direction. 
     In order to measure the degree of anisotropy of a sample using scalar Kerr effect systems of the prior art, it has been necessary to obtain measurements from two separate devices, one longitudinal and one transverse. Proper alignment between instruments becomes difficult, as is the ability to gauge the relative strength of the discrete applied fields. 
     The easy axis of a material is the axis along which the material is most readily magnetized in the presence of an externally applied magnetic field. Certain materials exhibit easy axis dispersion, where the easy axis varies from point to point across the surface of the sample. Scalar detection systems have consequently provided incomplete characterization of anisotropic materials. 
     BRIEF SUMMARY OF THE INVENTION 
     A system and method for realizing vector Kerr magnetometry is disclosed. The system enables simultaneous longitudinal and transverse Kerr effect measurements at a single point on a sample surface. Consequently, anisotropic materials may be more completely, rapidly and accurately characterized. The system may be used with a variety of materials to be tested, including but not limited to magnetic media (e.g. hard drive platters), thin film heads, magnetic random access memory (MRAM) and permanent magnetic wafers. 
     An optional component of the presently disclosed system includes a sample platform with motive elements and an associated controller for achieving precise relocation of the sample under test in x, y, and theta. Thus, even though the present system enables two-dimensional hysteresis loop characterization at a sample location without sample movement, the repositionable platform as disclosed enables complete, 360 degree characterization about a single point, if desired. Alternatively, the highly precise control mechanisms may be utilized in obtaining longitudinal and transverse Kerr effect measurements at succeeding points on the surface of a sample. Rapid sample characterization is thus achieved, including easy axis dispersion measurement with a much higher accuracy as compared to prior art measurement systems. 
     A further embodiment of the presently disclosed system includes the integration of a polar Faraday effect emitter/detector pair in conjunction with the vector Kerr magnetometry system disclosed above, either with or without the x, y, and theta translatable sample platform. More accurate and rapid sample characterizations are thereby achieved. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     These and other features of the present invention are more fully set forth in the detailed description and accompanying drawings of which: 
     FIG. 1 is a perspective view of a scalar magneto-optic Kerr effect sensor according to the prior art; 
     FIG. 2 is a perspective view of a vector magneto-optic Kerr effect sensor according to the present invention; 
     FIG. 3 is a perspective view of the vector magneto-optic Kerr effect sensor of FIG. 2 for use in conjunction with a repositionable sample platform; 
     FIG. 4 is a perspective view of the vector magneto-optic Kerr effect sensor of FIG. 2 for use in conjunction with an integrated Faraday effect sensor; 
     FIG. 5 is a graph of applied magnetic field versus Kerr effect rotation detected along the x axis, wherein the easy axis of the material at the sample point is substantially parallel to the x axis; 
     FIG. 6 is a graph of applied magnetic field versus Kerr effect rotation detected along the y axis, wherein the easy axis of the material at the sample point is substantially parallel to the y axis; 
     FIG. 7 is a graph of applied magnetic field versus Kerr effect rotation detected along the x axis, wherein the hard axis of the material at the sample point is substantially parallel to the x axis; 
     FIG. 8 is a graph of applied magnetic field versus Kerr effect rotation detected along the y axis, wherein the hard axis of the material at the sample point is substantially parallel to the y axis; 
     FIG. 9 is a graph of applied magnetic field versus Kerr effect rotation detected along the x axis, wherein the easy and hard axes are offset from the x axis by approximately 45 degrees; and 
     FIG. 10 is a graph of applied magnetic field versus Kerr effect rotation detected along the y axis, wherein the easy and hard axes are offset from the y axis by approximately 45 degrees. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A prior art longitudinal magneto-optic Kerr effect sensor system  10  is illustrated in FIG.  1 . Magnetic pole pieces  12  are used to apply a magnetic field H across the surface of a sample under test  14 . The applied field is under the control of a control unit  13  such as a programmed computer; the relationship between the control unit  13  and the pole pieces is illustrated in FIG. 1 as a dashed line. It should be appreciated that the actual relative dimensions of the pole pieces and the sample under test should not be inferred from this illustration. Rather, the depiction is intended merely to illustrate the relative arrangements of the components of the system. 
     Also shown in FIG. 1 is a light source or emitter  16  and a detector  18 . The source  16  is typically a laser light source, and the detector  18  is capable of detecting the relative polarization of the received light beam. One or more mirrors  20  are also typically employed for causing the light beam to impinge, at a desired angle, upon a point of interest on the surface of the sample  14 . The actual physical arrangement of the source  16 , detector  18  and mirrors  20  is not of significance as long as the illuminated spot on the sample surface is within a region where the applied magnetic field is substantially parallel with the surface of the sample. When so arranged, the polarization of the impinging light is effected by the magnetization of the sample at the illuminated point, and the relative shift in polarization as detected at the detector is used to identify magnetic characteristics of the sample. A data processor (not shown) in communication with the detector  18  is used to extract characterizing data from detector output data. 
     As previously mentioned, the prior art has also included a transverse detection system, whereby the light source, the detector, and the optical elements  20  are aligned such that the light beam impinges upon the sample in a plane, defined by the y and z axes, orthogonal to the direction of the applied magnetic field. Such an arrangement is not illustrated. 
     Both the longitudinal and transverse systems may be described as scalar systems since they only provide a representation of the magnetization in one direction. As anisotropic materials have both hard and easy axes of magnetization, scalar systems provide an incomplete characterization of such materials. The deficiency of the prior art is exacerbated by the tendency for many anisotropic materials to exhibit easy axis dispersion, wherein the easy axis direction changes from point to point on the surface of the material under test. 
     FIG. 2 illustrates a vector Kerr effect sensor system  100  which is capable of measuring the magnetization characteristics of a sample along multiple axes at the same time. As in FIG. 1, pole pieces  112  are employed to generate a magnetic field H across the surface of the material under test  114 . As with the prior art scalar system, a controller  113  is employed to control the operation of the magnetic field system according to predefined parameters. A first emitter  116  and detector  118  pair, with associated optics  120 , is employed for characterizing the sample  114  along a first axis (the x axis in the illustration), while a second emitter  126  and detector  128  pair, with associated optics  130 , is employed for characterizing the sample along a second axis (the y axis in the illustration). As with respect to FIG. 1, the relative or absolute sizes of the elements depicted are not to be construed from this diagram, and preferred locations for the emitters, detectors and optics are not necessarily as shown. In fact, a variety of considerations including system footprint may dictate the physical arrangement of these elements. 
     A variety of control arrangements may be provided for coordinating the operation of the emitter/detector pairs. For the sake of simplicity, such arrangements are not illustrated in FIGS. 2,  3 , or  4 . For instance, a single controller such as a programmed computer may be in communication with both emitters and both detectors for causing the emission of test radiation and for receiving the detected output. Data processing and/or communication to other computing devices may be carried out by the single controller. Alternatively, discrete controllers may be provided for each emitter/detector pair. Once again, the discrete controllers may be networked together, and may be further networked to other processing elements. 
     Discrete emitter/detector pairs are shown in FIG. 2 for measuring along respective axes. However, in a further embodiment of the presently disclosed invention (not illustrated), a single emitter is utilized for both axes, and in yet another embodiment (not illustrated), a single detector is employed. In the latter embodiment, the detector is capable of differentiating between the reflected signals of the two axes based upon the relative shift between received signals. Alternatively, the reflected signals may be time-division multiplexed, with detection occurring in synchronism with the multiplexing. 
     The detector(s) are capable of detecting the change in polarization in the received signals. As noted above, a processing element such as a personal computer, directly connected or in communication with the detector(s) via a communications pathway including a local or wide area network, is employed to record the output of the detector, and optionally to provide further processing including graphical analysis and extraction of parameters including shift of the free layer loop, coercivity of the free layer, shift of the pinning layer loop, and coercivity of the pinning layer. Importantly, however, this data collection and analysis pertains to data from two scalars—longitudinal and transverse. 
     The detector data produced by the presently disclosed system may be used to generate graphs of applied magnetic field, measured in Oersteds (Oe), versus the detected Kerr rotation, measured in thousands of a degree (mDeg). As discussed previously, anisotropic materials exhibit an easy axis along which the material is easily magnetized in the presence of an applied magnetic field and a hard axis along which the material resists magnetization in the presence of an applied magnetic field. 
     FIG. 5 illustrates a hysteresis response loop for a thin film magnetic read head material (e.g. for giant magneto-resistance or GMR heads) measured along the x axis. Here, the easy axis is aligned with the x axis as evidenced by the sharp change in Kerr rotation in a relatively small applied magnetic field. FIG. 6 illustrates measurements made along the transverse or y axis when the easy axis of the sample, at the point of observation, is substantially aligned with the y axis. 
     FIG. 7 illustrates the response curve for longitudinal or x axis measurements, where the hard axis of the material is substantially aligned with the x axis. Note that as the applied field is increased in either direction from zero, the relative polarization rotation gradually increases until a peak rotation is achieved. FIG. 8 is a plot of transverse or y axis measurements, where the hard axis is substantially aligned with the y axis. The resulting output is characteristic of a transverse, hard axis measurement. 
     Finally, FIG. 9 illustrates an x axis measurement with the easy and hard axes offset by approximately 45 degrees from the x axis. FIG. 10 shows a transverse measurement, also with the easy and hard axes offset from the measurement axis by approximately 45 degrees. 
     These graphs illustrate that the magnetization response for a typical anisotropic material varies significantly depending upon the alignment of the easy and hard axes with respect to the measurement system. Thus, in order to fully characterize a sample, it is necessary to have both longitudinal and transverse measurements. Both such measurements enable the creation of a characteristic vector response, as compared to the prior art scalar response. 
     A further embodiment  200  of the present invention builds upon the system disclosed with respect to FIG. 2, and is shown in FIG.  3 . Here, the platform or stage upon which the material under test  114  is located is capable of precise linear and rotational movement, or in other words precise movement in the x, y, and theta directions. Such a platform may be a multi-axis translation stage as manufactured by Newport Corporation, Irvine, Calif. One or more stepper motors (not shown) under control of a programmed computer or other processing element  210  may be utilized to achieve this movement. Depending upon the embodiment, the control  210  for the platform motors is in communication with the emitters  116 ,  126 , detectors  118 ,  128  and applied field control  113  in order to minimize the dwell time at each point on the sample under test. With highly accurate translation of the sample under test, it is conceivable that characterizing measurements may be made while the sample under test is being translated, thus further speeding the sample characterization process. 
     In a first embodiment of the system having movement in x, y, and theta, the motive means are realized as high precision, computer controllable motors that move the sample in the x, y, and theta directions. By appropriately aligning the center of rotation with the test point on the sample, the computer or motion control firmware associated with the processing element  210  computes the x and y motions that are necessary to bring any measurement point back to its original location after it has been rotated by theta degrees. Thus, a full 360 degree rotation of any point can be achieved and the corresponding longitudinal and transverse Kerr signals can be analyzed. 
     Alternatively, a vector magnet system can be employed in which the sample is translated in the x and y directions and the applied magnetic field is rotated in theta; systems are known which provide a steerable applied field without requiring mechanical movement of the pole pieces or the sample itself. A processing element similar to the processing element  210  is adapted for precisely controlling the linear translations of the sample under test and the rotation of the applied field. 
     A further embodiment  300  of the presently disclosed invention is illustrated in FIG. 4 which, in addition to the elements of the vector Kerr effect sensor system  100  of FIG. 2, includes a polar Faraday effect sensor system comprised of a radiation emitter  310  and detector  320  pair. As know to one skilled in the art, a Faraday detector is employed for observing changes in radiation polarization as it passes through a target material. In the present case, the observed changes will be characteristic of the response of the subject material to an applied magnetic field H. The provision of an integrated Faraday effect sensing system further enables the rapid and accurate characterization of a light-transmissive material. Control over the Faraday system may be provided independently, or may be integrated with that provided for the Kerr effect sensor systems. Such control may be provided through a direct connection or via data network. The resulting data also may be processed independently, or may be integrated into graphical or text results data for subsequent review by an operator or other data processing system. 
     In either the embodiment in which the sample is translated and rotated or the embodiment in which the sample is linearly translated and the applied field is rotated, a user interface (not shown) such as a keyboard, mouse and display is provided for enabling a user to define the parameters of operation. The interface may also be used for reviewing the operation of the system and the results of various tests based upon output from the detectors  118 ,  128 . Such a user interface may be provided in conjunction with a system controller such as a programmed computer or networked or intercommunicating programmed computers for carrying out predefined testing regimens on a series of samples. The use of such a system controller is preferred in order to enable the rapid and automated characterization of successive samples. A sample handling robot (not shown) as known in the art may be provided in communication with the system controller to facilitate this automated operation. 
     These and other examples of the concept of the invention illustrated and described above are intended by way of example and the actual scope of the invention is to be determined by the following claims.