Abstract:
An apparatus and method of testing a magnetic medium at temperatures of interest is disclosed. Properties of the magnetic medium are determined by focusing light from a source of polarized light on a magnetic surface of the magnetic medium; measuring polarization of resulting reflected light due to the magneto-optical Kerr effect, using, for example a measuring subsystem; and varying the light source to heat the magnetic material where incident to pre-defined temperatures, thereby allowing determination of the magnetic properties using the magneto-optical Kerr effect at said pre-defined temperatures.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 61/353,902 entitled, “METHOD AND APPARATUS FOR THERMAL MAGNETIC PROPERTIES OF MAGNETIC MEDIA” and filed Jun. 11, 2010, the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to magnetic materials, and more particularly to methods and devices for measurement of thermal magnetic properties of magnetic media at different temperature using Magneto-Optical Kerr Effect (MOKE). 
       BACKGROUND OF THE INVENTION 
       [0003]    The performance of magnetic storage media depends largely upon magnetic properties of the recording layer. Important properties include coercivity and remanance of the material. 
         [0004]    The coercivity (typically expressed in Oersteds) is the minimum magnetic intensity of an applied magnetic field sufficient to cause the magnetic media to undergo a transition from a state of magnetic saturation to a non-magnetized state. The remanance (typically expressed in Ampere/M) indicates magnetization left behind in a sample after an external magnetic field is removed and thus relates to strength of electrical signal recoverable from a magnetic-electrical transfer. 
         [0005]    There are several approaches to testing magnetic media for their magnetic properties: one includes the use of a Vibrating Sample Magnetometer (VSM), another uses a Superconducting Quantum Interference Device, yet another makes use of a Magneto-Optical Kerr Effect (MOKE) system. 
         [0006]    The MOKE is the phenomenon that light reflected from a magnetized material has a slightly rotated plane of polarization. The degree of polarization depends on the magnetic properties of the material and the applied magnetic field. 
         [0007]    A typical MOKE system includes a single laser source configured to provide a probing beam to detect Kerr signal dependence on an applied magnetic field at room temperature only. A hysteresis loop of a magnetic media is then plotted to obtain its magnetic properties. To measure the magnetic properties at elevated temperature, such as for the research of the magnetic media for heat assisted magnetic recording, a sample is cut and heated to a required temperature before measurement. This method is time-consuming and is also destructive in that it requires the cutting a magnetic medium and a well designed heating unit. 
         [0008]    Therefore, there is a need for a MOKE system that can perform measurement of thermal magnetic properties of magnetic media at different temperature without an additional heating unit and without destroying the magnetic media. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with an aspect of the present invention, an apparatus for testing a magnetic medium at multiple temperatures of interest, comprises a light source to provide polarized light incident on a magnetic surface of the magnetic medium; a measuring subsystem to measure polarization of reflected light due to the magneto-optical Kerr effect, the reflected light reflected from the magnetic medium in response to the polarized light incident on the magnetic surface. As such, the polarized light heats the magnetic surface where the polarized light is incident, to the multiple temperatures of interest, to allow determination of magnetic properties of the magnetic medium at the multiple temperatures of interest using the magneto-optical Kerr effect. 
         [0010]    In accordance with another aspect of the present invention, a method of testing a magnetic medium at temperatures of interest, the method comprises focusing light from a source of polarized light to be incident on a magnetic surface of the magnetic medium; measuring polarization of reflected light due to the magneto-optical Kerr effect, the reflected light reflected from the magnetic medium as a result of the light where incident; and varying the light source to heat the magnetic material where incident to pre-defined temperatures, to allow determination of the magnetic properties of the magnetic medium using the magneto-optical Kerr effect at the pre-defined temperatures. 
         [0011]    Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the figures which illustrate by way of example only, embodiments of the present invention, 
           [0013]      FIG. 1A  is a schematic diagram of a MOKE system with a single laser source use to both probe and heat, exemplary of an embodiment of the present invention; 
           [0014]      FIGS. 1B and 1C  are schematic diagrams of the system of  FIG. 1A , in operation; 
           [0015]      FIG. 1D  is a schematic diagram of the MOKE system of  FIG. 1A , further including a temperature calibration subsystem; 
           [0016]      FIG. 2  is a schematic diagram of a further MOKE system exemplary of an embodiment of the present invention; 
           [0017]      FIG. 3  is a schematic diagram showing the adjustment of incident laser power exemplary of an embodiment of the present invention; 
           [0018]      FIG. 4  is a flow chart showing a method for measurement of thermal magnetic properties of a magnetic medium at pre-determined temperatures in accordance with an embodiment of the present invention; 
           [0019]      FIG. 5  is a schematic diagram showing a method for calibrating temperature with laser power according to an embodiment of the present invention; 
           [0020]      FIG. 6  is a schematic diagram showing the infra-red sensor used to monitor surface temperature of a magnetic media, in accordance with an embodiment of the present invention; and 
           [0021]      FIGS. 7A-7F  are graphs showing hysteresis loops of a magnetic medium under different temperatures, measured in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Exemplary of embodiments of the present invention, an apparatus for measuring thermal magnetic properties of magnetic media at different temperatures uses the Magneto-Optical Kerr Effect (MOKE). Embodiments of the present invention advantageously may use only one laser beam to both heat a measured medium and probe the Kerr signal. 
         [0023]      FIGS. 1A and 2  are schematic diagrams of MOKE systems exemplary of embodiments of the present invention. A single laser source ( 112 ,  222 ) provides a laser beam ( 102 ,  230 ), which functions as both a heating source and as a probing signal. Therefore, an additional heating source is not required. 
         [0024]    Conveniently, laser source ( 112 ,  222 ) running with an output of linear polarization beam of 200 milli-wattage (mW) may be used. The power incident to a measured medium may be adjusted in the range of 0.1-200 mW with an external adjustment unit for heating the medium to a temperature from room temperature to 700 K. If a higher temperature is desired, it can be realized by focusing the laser beam more tightly. This power level is higher than the laser power (several mW to tens of mW) used in a typical MOKE system. 
         [0025]    Laser source  122 ,  222  may conveniently be a continuous wave laser. Conveniently, with a continuous wave laser, synchronization with data acquisition is not required and implementation is simplified and more cost effective. Alternatively, a pulsed laser may be used as source  122 ,  222 , and data acquisition may be synchronized with each laser pulse. 
         [0026]    Different temperatures can be achieved on the surface of the magnetic media by fine-tuning the incident laser power. Since the Kerr rotation angle is calculated using ratio of detected intensity, a change in incident laser power will not affect the Kerr effect. A plot of the Kerr signal against applied magnetic field forms a hysteresis loop of the magnetic media at the heated temperature. This hysteresis loop determines the magnetic properties of the magnetic media at such temperature. 
         [0027]    As will become apparent, the surface of the magnetic media may be heated to multiple temperatures of interest—for example between 293 K to 700 K, ranging from about room temperature to the Curie temperature of the media. 
         [0028]    In an embodiment, there is provided an apparatus  100  for measurement of thermal magnetic properties of magnetic media at different temperatures using Magneto-Optical Kerr Effect (MOKE), as schematically depicted in  FIG. 1A . Apparatus  100  may include a laser focusing and collimating arrangement configured to focus energy of an incident optical beam  102  from a laser source  112  towards a small focused spot  104  at a surface  106  of a magnetic medium  108 . 
         [0029]    Laser light from source  112  is passed through a polarizer  128 , and a polarizing beam splitter  118 . Polarized light is directed by polarizing beam splitter  118  to lens  110 . A focused laser beam may be realized with a focusing lens  110  and a collimating laser beam may also be realized with the same focusing lens  110 . Lens  110  thus may play roles both in focusing the laser beam and collimating the beam. With the arrangement, the incident optical beam  102  is focused to high intensity by focusing lens  110  so that the small spot  104  on the surface  106  of magnetic medium  108  is heated to a predetermined temperature, even though the power of the laser source  112  may be of similar range as that of a conventional MOKE system. Only a small fraction of the laser beam passes through splitter  118  and is received by detector  116 , which records the records laser power, and may be used for temperature calibration. Conveniently, the heated point is exactly the same point as a measurement point. A magnetic field may be applied to magnetic medium  108  by poles  122  and  124 . The magnetic field may be time varying. 
         [0030]    As temperature will be dependent on the power of the applied beam  102 , and duration of application, a function correlating the times of application for a particular laser source  102 /magnetice medium  108  with temperature of the medium may be determined experimentally. 
         [0031]    The reflected optical beam  114  from surface  106  of magnetic medium  108  is reflected toward polarizing beam splitter  118  after having been collimated by collimating lens  110  to a parallel beam and then towards an analyzer  126 . Now, as will be appreciated, as a result of the MOKE, the polarization of reflected light will be changed—a so-called Kerr rotation will occur in the reflected beam. A light component belonging to Kerr signal in the reflected optical beam  114  from the surface  106  of the magnetic medium  108  is allowed to almost fully pass through a polarizing beam splitter  118  and enter analyzer  126  while a light component with original polarization determined by a main polarizer  128  in reflected beam  114  is mostly reflected in the direction of laser source  112  by polarizing beam splitter  118 . 
         [0032]    The incident optical beam  102  may be incident substantially vertically on surface  106  of magnetic medium  108 . The reflected optical beam  114  from surface  106  of magnetic medium  108  is collimated to a parallel beam again by focusing lens  110  and then split by a polarizing cube beam splitter  118  ( FIG. 1A ). As noted, the Kerr signal component of the reflected optical beam  114  passes through polarizing beam splitter  118  to analyzer  126 . At analyzer  126  the so-called Kerr signal reaches a detector  120 , which may record signal intensity change when the magnetic field intensity varies. 
         [0033]    The laser power may be monitored with a detector  116 . By reading the detector  116 , the temperature of the medium heated may be calibrated. 
         [0034]    Detectors  116  and  120  may be in communication with, or part of a computing device (not shown) programmed to record the Kerr signal intensities/magnetic field intensity at various temperatures. Likewise, laser  112  and magnetic poles  122  and  124  may be in communication with, and controlled by the computing device (not specifically illustrated). The computing device may, for example, cause the magnetic field between magnetic poles  122  and  124  to sweep from a positive maximum value to a negative value, and back. 
         [0035]    The recorded Kerr signal- magnetic field intensity dependence will form a hysteresis loop if the magnetic field is swept at enough intensive amplitude from positive to negative and then back to positive. The part with original polarization in the reflected optical beam is mostly reflected back by the polarizing beam splitter  118 . Conveniently, vertical incidence of optical beam  102  allows only one lens  110  to be used for both focusing and collimating a laser beam. This makes system easier to implement because of very limited space around magnetic poles  122 , 124 . 
         [0036]    To avoid some resonance resulting from co-axis reflection, a small angle between the incident beam and the normal of medium surface may be used, as depicted in  FIG. 1C . The angle may be adjusted to ensure that the incident optical beam  102  is located on one side of the optical axis of lens  110  and the reflected optical beam  114  is located on the other side of the axis. As will be appreciated, the reflected optical beam  114  need also not travel the same path as the incident optical beam  102 . The incident optical beam  102  may, for example, be incident at an angle on the surface  106  of the magnetic medium  108 , as exemplified in  FIG. 1B , to ensure that the incident beam  102  and reflected optical beam  118  take different paths. In this case, a pair of lenses may be used: focusing lens  110  is on the path of incident beam, and a further collimating lens  110 ′ may be inserted in the path of reflective beam. 
         [0037]    Optionally, as illustrated in  FIG. 1D , apparatus  100  may further include a detector  113  that may measure the intensity of the reflected beam (sampled by a sampler or partial mirror  111 ). The ratio of signal at detector  113  to that at detector  116  may evidence the reflectivity of surface  106 . As will be appreciated, as the temperature increases, the reflectivity of surface  106  will decrease. Reflectivity may therefore be used to calibrate the temperature of sample  106 . As required, temperature as a function of reflectivity T=f(R) may be experimentally determined. 
         [0038]    In an alternate embodiment depicted in  FIG. 2 , an apparatus  200  for measurement of thermal magnetic properties of magnetic media  220  at different temperatures using MOKE may further include a signal detection arrangement  202  configured to monitor the Kerr signal resulting from an incident optical beam when a magnetic field is applied at a temperature heated on the spot on the surface of the magnetic medium. 
         [0039]    As in the embodiment of  FIG. 1 , a laser source  222  provides a laser beam  230  to be focused on magnetic material  220 . Laser beam  230  is passed through main polarizer  210  and beam splitting polarizer  248  to arrive at magnetic material  220 , where a small fraction of the beam may be passed to detector  244 , to monitor laser power and for temperature calibration. Again, the beam may be focused by lens  215 . Reflected light from material  220  will be optically polarized as a consequence of the magneto-optical Kerr effect. Reflected light will pass to beam splitter  248 , where a component is directed to detector  244  of detection arrangement  242  and detector  208  of detection arrangement  202 . Beam  230  thus again heats and probes magnetic material  220 . 
         [0040]    Signal detection arrangement  202  may include an analyzer  204 , a laser light filter  206  and a photo-detector  208 . Analyzer  204  may take the form of an optical polarizer, configured almost vertically to main polarizer  210  in optical axis, to allow the Kerr signal component in the optical beam to almost fully pass through and the component with original polarization in the optical beam to be mostly blocked. Laser light filter  206  blocks light from other sources. The signal received by photo-detector  208  is thus the Kerr signal resulting from the magnetic field applied to the magnetic material  220 . The Kerr signal against the applied magnetic field plots a hysteresis loop of the magnetic medium at the temperature heated, then at least one magnetic property of the magnetic medium can be determined from the hysteresis loop. A general purpose computing device (not shown), in the form of a personal computer, controller, or other data processing apparatus, under software control may control the overall operation of apparatus  200 , and may be in communication with signal detection arrangement.  202  for recording of the magnitude of the Kerr signal component at various temperatures, and in the presence of applied magnetic fields. Likewise the general purpose computing device may again monitor the temperature of magnetic material  220 . 
         [0041]    Optionally apparatus  200  may also include a magnetic field generation arrangement  212  configured to apply a magnetic field of a time-varying strength to a portion of the magnetic medium. Magnetic field generation arrangement  212  includes a magnetic field driver (not shown), a magnetic coil  214 , magnetic poles  216 ,  218  and an optional magnetic field meter (not shown). Magnetic field generation arrangement  212  is used to generate a magnetic field that is applied to a region of a magnetic medium  220 , where measurement is taken. The strength, orientation and sweep duration of the magnetic field are determined by the magnetic field driver, and may for example be controlled by the above described computing device. 
         [0042]    Optionally, apparatus  200  may further include a light source  222  having a laser source  224  and an external laser power adjustment unit comprising a half wave plate  226  and a polarizing beam splitter  228 . A main polarizer  210  for generating pure linear polarizing beam to probe Kerr effect is configured to direct a polarized optical beam  230  towards the portion of magnetic medium  220  that is in the magnetic field, wherein the optical beam is reflected by the surface of magnetic medium  220  at a point of incidence in the magnetic field. 
         [0043]    Laser power adjustment may be realized by a pair of a half wave plate  302  and polarizing beam splitter  306  if the laser beam is of a linear polarization, as shown in  FIG. 3 . 
         [0044]    Half wave plate  302  may be rotated manually or by motor (not shown). As a consequence, the laser power  304  delivered to main polarizer  210  direction through polarizing beam splitter  306  will be changed accordingly. In this way, laser power and/or intensity of the incident polarized light is adjusted very conveniently. A black hole  308  is used to collect unused laser power. Once again, half wave plate  302  and light source  222  may be in communication with, and controlled by, the computing controlling overall operation of apparatus  200 . 
         [0045]    Optionally, apparatus  200  may further include a vision unit  232  configured to check the optical beam focusing status. The vision arrangement includes an imaging lens  234 , a CCD camera  236 , a lighting source  238 , and a beam splitter  240 . The vision unit is used to monitor the focusing status of the laser beam at the surface of the magnetic medium  220 , and to find a measurement spot on the magnetic medium if it is necessary. Again, the vision unit  234  may be in communication with the computing controlling overall operation of apparatus  200 . 
         [0046]    In alternate embodiments, the duration, intensity or frequency of the laser source  224  may be varied to heat the surface of the magnetic medium to multiple temperatures of interest. 
         [0047]    Also, apparatus  200  may optionally further include a laser power and temperature monitoring arrangement  242 . Laser power used to heat magnetic medium  220  is monitored with a photodiode  244 , combining with a laser line filter  246 , which blocks light from other source. Polarized laser beam  230  is directed to the polarizing beam splitter  248 , where most of the laser power is guided to the surface of the magnetic medium  220  for heating and probing, and only a very small part of the laser power goes through the polarizing beam splitter  248  and into photodiode  244 . Using the laser power recorded, a temperature of the magnetic medium  220  heated at the laser spot can be calibrated. 
         [0048]    Examples of magnetic properties that may be determined using apparatus  200  or  100 , include but are not limited to, are coecivity (H e ), nuclei field (H a ), saturation field (H s ), remanence (M r ), and saturation remanence (M s ). 
         [0049]    The above presented embodiments are configured for use with magnetic media for perpendicular recording. Embodiments of the present invention may be advantageously adopted for use with perpendicular recording media. However, embodiments of the present invention may be applicable to use with longitudinal recording media. 
         [0050]    As illustrated in  FIG. 4 , the method includes applying a magnetic field of a time-varying strength (using, for example poles  122 ,  124 — FIG. 1A ) to a portion of the magnetic medium in block  402 . The method may further include directing a polarized incident optical beam (e.g. beam  102 ) towards a surface of the magnetic medium (e.g. medium  106 ) that is in the magnetic field, wherein the optical beam is reflected by the surface of the magnetic medium at a point of incidence in the magnetic field in block  404 . The method may further include adjusting and focusing the energy of an incident optical beam (e.g. using lens  110 ) towards a small spot at the surface of the magnetic medium, wherein the surface of the magnetic medium is heated to one or more pre-determined temperature of interest in block  406 . The method may further include monitoring the applied energy of the incident optical beam (e.g. using detector  113 ), by which the temperature of the magnetic medium at the optical spot may be calibrated in block  408 . The method may further include generating, analyzing and recording a series of Kerr signal from the reflected optical beam of the magnetic medium in block  410  (e.g. using detector  120 ). The method may further include plotting hysteresis loop of the magnetic medium at the pre-determined temperature in block  412  where at least one magnetic property of the magnetic medium from the hysteresis loop may be determined in block  414 , using an interconnected computing device. Of course, the above mentioned method may not necessarily be carried out in the order as presented. 
         [0051]    An example of the temperature calibration mentioned above can be illustrated using the schematic diagram shown in  FIG. 5 . A magnetic medium is cut into two parts. The first part is measured with a suitable measuring apparatus, such as Vibrating Sample Magnetometer (VSM), to get its coercivity dependence on temperature. Reversing the relationship, a temperature dependence on coercivity of the magnetic medium is obtained, namely an expression of temperature-coercivity T=f(H c ) is obtained. Then the second part of the magnetic medium is measured in manners exemplary of the present invention. A coercivity dependence on laser power, namely an expression of coercivity—laser power H c =f(P L ) is obtained. Using the temperature-coercivity expression and the coercivity—laser power expression, a temperature—laser power expression T=f(P L ) is obtained. Therefore, the calibration of temperature with laser power is complete. 
         [0052]    As an alternative, the temperature of the medium can also be monitored with measurement of near infra-red (NIR) radiation from the spot heated  602  of the surface of the magnetic medium  604 , as shown in  FIG. 6 . The NIR radiation is directed with an infra-red (IR) mirror  606  to an IR detector  608 , that acts as a temperature sensor, before which a laser line notch filter  610  with IR pass through is used to block laser line. In this way the temperature can be calibrated with the intensity of the IR irradiation. 
         [0053]    Using exemplary methods, the coercivity of a magnetic medium is measured at different temperature, as shown in  FIGS. 7A-7F . 
         [0054]    Of course, the above described embodiments, are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention, are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.