A modified version of Chevron type Magnetoresistive Recording Heads (MRH) sensor is described. Two shielded ferromagnetic thin-film vertical-type MRH

are biased at the linear region simultaneously using a small piece of permanent magnet chip. The biasing rotates the magnetisation vector Ms of each MRH .

through 4S· with respect to their respective current vectors J resulting in an

effective Chevron orientation of 90·. This configuration is shown to be most

effective to detect field vectors in applications such as non-destructive detection

of grain structures and loss variation in electrical steels, and in packing more

bits of information in magnetic disc storage

Mohd Ali, Borhanuddin

Universiti Putra Malaysia Institutional Repository

Pertanika J. Sci. & Techno!. 1(2): 239-248 (1993)ISSN: 0128-7680© Universiti Pertanian Malaysia PressAn Improved Chevron Configuration for theDetection of Magnetic Field VectorsBorhanuddin bin Mohd AliFakuUi Kqurut~aa~Univmiti Pertanian Malaysia,43400 UPM S~dang, Selangor Darul Ehsan, MalaysiaReceived 19 January 1993ABSTRAKSuatu bentuk konfigurasi Magnetoresistive Recording Heads (MRH) yangterubahsuai adalah diperihalkan di sini. Dua MRH selaput-nipis, berperisai,jenis menegak telah dipincang di kawasan lelurus dengan serentak,menggunakan suatu serpihan kecil magnet kekal. Pemincangan tersebutmemutarkan vektor pemagnetan Mssetiap MRH tersebut sebanyak 4S· daripadavektor arus J yang menatijahkan orientasi konfigurasi Chevron sebanyak 90·.Konfigurasi begini adalah paling berkesan untuk mengesan vektor. medandalam aplikasi seperti pengesanan tanpa-musnah struktur ira dan perubahankehilangan dalam keluli elektrik, dan untuk penstoran lebih banyak maklumatdalam cakera magnet.ABSTRACTA modified version of Chevron type Magnetoresistive Recording Heads (MRH)sensor is described. Two shielded ferromagnetic thin-film vertical-type MRHare biased at the linear region simultaneously using a small piece of permanentmagnet chip. The biasing rotates the magnetisation vector Msof each MRH .through 4S· with respect to their respective current vectors J resulting in aneffective Chevron orientation of 90·. This configuration is shown to be mosteffective to detect field vectors in applications such as non-destructive detectionof grain structures and loss variation in electrical steels, and in packing morebits of information in magnetic disc storage.Keywords: magnetoresistive recording heads (MRH), chevron, magnetic field,non-destructive detection, magnetic disc storage, grain structures,electrical steel.INTRODUCTIONMagnetoresistive Recording Heads (MRH) have been widely used over thepast two decades. This is largely due to their proliferation in magneticstorage discs and tapes (Davies and Middleton 1975). There are, however,many other useful commercial applications of MRH, such as magneticcard readers, bubble domain detectors (Almasi et at. 1971), coin valida-tion systems (Kwiatkowski and Tumanski 1986), and security systems.It can be shown that the resistivity of a ferromagnetic thin film MRHis given as (Thompson et at. 1975)Borhanuddin bin Mohd. Alip = Po + ~P max (1 - sin 28) (1)where Po is the isotropic resistivity acting transverse to the direction of theMRH strip, !':,. Pmaxis the maximum change of resistivity in the MRH, ande is the angle the magnetisation vector makes with the direction of theconstant current vector J flowing through the device. It can be seen thatthis behaviour is quadratic and hence non-linear.It can also be shown that this resistivity can be represented as (Hunt1971)(2)When a bias field HB is applied to the device to linearise its characteristicsthen, neglecting quadratic terms since they are smallor(3)(4)where 1, t and ware the length, thickness and width of the devicerespectively; Hois the anisotropy acting to restrain the Msalong the lengthof the device; and H y is the external field applied across the width of thedevice.With the constant current I passing through the device the change inresistance can easily be detected as a change in voltage, !':,.V = I !':,. R.This has been the basis of the application of the MRH, i.e. detectionof the change in resistivity of the device inside a varying magnetic field.In most applications, the device merely detects the magnitude of thefield without taking into account the relative orientation between theexternal field and the device. However, when the device is linearly biasedit becomes sensitive to changes in the orientation of the external field.Several applications have stemmed from this behaviour, one of which isthe detection of the grain structures in grain-oriented electrical steels(Mohd Ali and Moses 1989).In the above application, a pair of thin-film monolithic ferromagnetichorizontal type MRH (known as the Chevron) configured at 1200 to eachother, was adapted as the sensor detecting the field on the surface of amagnetized Grain-Oriented Electrical Steel. Because the relative orienta-240 Pertanika J. Sci. & TeChno!. Vo!. 1 No.2, 1993An Improved Chevron Configuration for the Detection of Magnetic Field Vectorstion of the magnetization vector Msand the current vector J runningthrough a linearly biased MRH is 45·, the effective angle of the Chevronbecomes 30·, instead of 120·.Even though satisfactory computation of the resultant field vectors hasbeen achieved in the above investigation this configuration is by no meansideal, as the Chevron angle of 30· makes it less sensitive to changes indirections of field orientation, particularly at small angles. Moreover, thecomputation of the resultant magnetic field vector from the readings ofeach member of the Chevron is also more intensive because it involves alarge number of terms. This factor is critical when a large array of datadetected from the surface of the electrical steel is to be processed in real-time.The resultant vector detected by a Chevron pair is given as (Mohd Aliand Moses 1988)(5)(6)where HAand H B are the fields detected by the two sides of the MRH pairrespectively constituting the Chevron.For the Chevron employed in Mohd Ali and Moses (1989), theresultant magnitude and direction of the field detected by the Chevronare given as(7)(8)DERIVATION OF THE MAGNITUDE AND ANGLEOF APPLIED FIELDIn the present investigation, a pair of vertical-type ferromagnetic MRH wasbiased simultaneously in such a way that the relative angle between theindividual magnetization vector M was 90·. With this configuration, itcould be shown that th.e response of the two MRHs is as follows:Pertanika J. Sci. & Techno!. Va!. 1 No.2, 1993 241Borhanuddin bin Mohd. AliHA = H sin (45· - a )H B = H sin (45· + a)Expanding and adding(9)(10)2H (sin 45° cos a)2H (sin 45° sin a)(11)(12)Therefore(13)The computation of the field in equation (Mohd Ali and Moses 1988)reduces to(14)This equation is simpler than that obtained in equation 7.This paper describes an investigation to confirm that the sensitivity ofthe MRH is with respect to the Ms' and that the best configuration of theMRH pair for the computation magnetic field is 90· obtained by arrangingthe two MRHs parallel. A possible application in increasing the packingdensity of magnetic disc recording in computer systems is also described.EXPERIMENTAL PROCEDURESTwo vertical-type thin-film ferromagnetic MRHs (Ni-Fe 81:91 % by wt)were employed in this investigation. The MRHs have dimensions of:length = 1mm, width = 20J..lm, and thickness = 0.4 J..lm. A dc current of2mA was passed through each device independently, to convert thechange in resistivity when the MRH is subjected to an applied field, to achange in voltage which could be observed, after some signal condition-ing.242 Pertanika J. Sci. & TeChno!. Vo!. 1 No.2, 1993An Improved Chevron Configuration for the Detection of Magnetic Field VectorsThe MRHs (labelled as MRHA and MRHB) were biased simultaneouslyby means of a ferrite permanent magnet chip, sandwiched between thetwo MRHs with double-sided tapes, as shown in Fig. 1. The MRHs werearranged parallel because linear biasing is anticipated to rotate the M,through 45° such that the final configuration would thus be at right anglesto each other. The biasing was performed inside a solenoid excited fromthe 50 Hz ac power supply from the mains, and the hysteresis loop wasobserved on the oscilloscope. Linear biasing was deemed to have beenobtained when the zero field region of the loop was linear over a sufficientfield range.Anticlockwise Clockwise+HM,PermanentMagnetM,ReferenceFig. 1. Schematic representation of the MRHss being biasedsimultaneously fly a permanent magnet chip inside anapplied field. The resulting orientation of the M, is shownto be at right anglesThe pair of MRHs were then mounted on the axis of a graduatedrotatable stage placed at the centre of the solenoid. The MRHs were thenrotated at 15° intervals, through clockwise (+) and anticlockwise (-) sense;this simulates the rotation of the applied field around the Chevron. AB theChevron is rotated clockwise, say, the magnetic field vector turns into theupper half of the circle, which in the convention adopted here, is referredto as positive angle. Likewise, anti-elockwise rotation of the Chevronresults in a negative angle .of orientation of the applied field.The positive peak responses of the two MRHs taken at zero angleposition were plotted against the relative orientation of the applied field,with respect to the horizontal axis reference.Pertanika J. Sci. & Techno!' Vo!. 1 No.2, 1993 243Borhanuddin bin Mohd. AliRESULTS AND DISCUSSIONWhen the Chevron pair is subjected to an external ac field, the resistivityof the MRH changes in response to the applied field. As the Chevron isrotated on its axis inside the field, the amplitude of the voltage detectedfluctuates according to the relation, H= Hasin (45° + a ), where H is thedetected field, Hais the applied field, and a is the angle of orientation ofthe applied field. A sample of the waveforms obtained from the MRHsis shown in Fig. 2(A-H) for a at 0°, +/- 45°, +/_90°, +/- 135° and -180°.These are the positions where significant changes in the detected field occur.At the initial position, the peak responses of the two MRHs wereconsidered equal even though they were actually opposite in phase. Thepeak values were taken as positive in both cases and this was taken as thedatum for subsequent readings.As the pair is rotated clockwise (positive angle), MRHB increasessteadily until 45° when the response is at its maximum. At the same time,the response of MRHA diminishes to zero. This is due to the configura-tion of the biased pair as shown in Fig. 1. As the Chevron rotates in theclockwise direction, the magnetisation vector Msof MRHB gradually ap-proaches the angle 90° with respect to the applied field, at the angle a =45°. This is the position of maximum sensitivity following the relationship,HB= Hasin (45° + a), as stated in equation 5, where H B is the field detectedby MRHB• In this case, a is considered positive when the Chevron isrotating clockwise, as a lies in the positive half of the circle.This behaviour is summarized in Fig. 3 showing the peak values of theMRH waveforms, against the orientation of the applied field, a.It can be seen that after +/- 45°, the response of one of the MRHs turnsnegative (i.e. MRHA at +45° and MRHB at -45°). This comes about becausethe output of that MRH which diminished to zero, changed phase by 180°,so that the peak value of the waveform when measured at a fixed point inB: dV = -4.08 V cit =3D.Oms Vpp = 4.72 V,...., J. ,......\ v r\ 1\ 1\ 1\ 1\A !' 1)\ j\ )\'-- "I-' ,'-- - 1'-..--A-dV-575V dt=300ms Vpp=-6.80V.II' Ip r-\ / \ / \'\ / \ / \I\;;::; I'- ~ 1\..../CONTROL CALC ZOOM SELECT RETURN Vpp RMS MEAN ASS RETURNA (a = DO) B (a = 45°)244 Pertanika J. Sci. & Techno!. Vo!. 1 No.2, 1993An Improved Chevron Configuration for the Detection of Magnetic Field VectorsA dV = 80 Om. dt = 30.0ms Vpp = -240mV A: dV = -3.92 V dt = 30.0ms Vpp = 4.40 VIr 1/ Ir/ \ I \ // \ / \ I~ \ ....r=K - r\ V l'J JJ \ / \ 1/"I:;::: 'e1) (a = 13T)Vpp RMS MEAN ABSC (a = 90°)RETURN Vpp RMS MEAN ABS RETURNA: dV = 3.60 V dt = 3O.0ms Vpp = 4.24 VI\. v I\. f"= K, 1/ \ 1/ \...." I"t-=' ~A dV ~ 240m. dt - 30 Oms Vpp - 320mV- --1/ I/'\ J \ I \~ / \ / -\I\..-V I'-.,Y [\.....11Vpp RMS MEAN ABSE (a = -45)RETURN Vpp RMS MEAN ABSF (a = _90·)RETURNA:dV=-418V dt=300ms Vpp-496V- ....... .... .."..-[\, f\ 1\ f\) 1\) 1\\)\ 1;\ 1;\ 1;\ 1;\"-'"t- '-" ~ '-"-I-.A' dV --5 92 V dt-30 Oms Vpp - 6 96 V-.Ip\-- II" IrI 1\ J 1\ i/ \ / \ /1/ I~V \"-:::~Vpp RMS MEAN ABS RETURN Vpp RMS MEAN ABS RETURNG (a = -135) H (a = -180·)Fig. 2. The waveform traces of the MRHs when rotated inside anapplied field (ex = 0·, +/_45·, +/_90·, +/-135", -180)Pertanika J. Sci. & Techno!. Vo!. I No.2, 1993 245······"""··"",,··,,·,,·,,··,, .... ··90Borhanuddin bin Mohd. Alitime, would be negative. The change of phase happened because, at thatcritical angle of +/- 45° , the Msswitched to the opposite direction tomaintain the position of minimum energy.Detected Field (mV) Computed Angle4 r------------------"'-----=----, 1803~~~~~~~='!-,.,.-~----.....--~==:-::*=~~------+2~ ~:::::;z:~==:::::;4~:::.::~~;::==~~=~ 0-1 " "... -90-2-180_3 I--L-j--L-j---'-----t--'-----+-...L..........-t--'---t----'----t--'--t--'--t--'--t-----'----t------'--j180 150 120 90 60 30 0 -30 -60 ·90 -120 -150 ·180Orientation AngleMRH AComputed HMRH BComputed AngleFig. 3. Detected field and computed angle vs orientation angleA computation of the resultant magnitude and a of the applied fieldfrom equations (13) and (14) shows that the values obtained are fairlyconstant throughout the whole excursion, since the fields are kept con-stant throughout the whole experiment. The computed angles also agreewith the orientation angle a as seen by the linear relationship (Fig. 3).However, at angles greater than +/_90· , a discontinuity in equation 9occurs. This is taken care of by adding 180· to the term in equation 3.The stability of the measurement depends very much on the stabilityof the biasing point. In this investigation, the biasing magnet has beenattached temporarily to the Chevron, by means of a double-sided tape orplasticine, to facilitate subsequent reuse. Because of this, the biasingpoint could displace as the Chevron is moved around, such that thebiasing may no longer be linear. This will introduce some errors in thefinal computation of the resultant field. However this is overcome bytaking the mean of 3 sets of readings for each point in the graph.However, in applications for detecting the surface field of grain-oriented electrical steels as described in [1], the vertical type MRH is notsuitable, because it detects the vertical component of the surface fieldonly, which only occurs at grain boundaries, and not the body of the grain246 Pertanika J. Sci. & Techno!. Vo!. 1 No.2, 1993An Improved Chevron Configuration for the Detection of Magnetic Field Vectorsareas. In order to detect the horizontal component of the surface field apair of horizontal MRH should be used instead.For this type of application where the medium is sensitive to externalfield interference, the most appropriate type of biasing is obviously theself-biasing technique (barber-pole or anisotropy) where the biasing isperformed by rotating the current vector with respect to the anisotropy, orvice versa.CONCLUSIONSAn improved geometry for the detection of field vectors has been pre-sented. Arranging two MRHs parallel to each other, and simultaneouslybiasing them at the linear region, results in an orthogonal configuration,whereby the magnetisation vectors Msare effectively lying at 90" to eachother. This is the ideal configuration for vector sensors.With this configuration, the computation of the magnitude and direc-tion of H is more efficient, compared with configurations at other angles.If a self-biasing pair of MRH is arranged in this manner, an efficientfield detector could be obtained in applications such as the non-destruc-tive detection of grain structures in grain-oriented electrical steel (MohdAli and Moses 1989).This technique could also be used to pack more bits of informationin the magnetic disc or tape storage. The information could be encodedsuitably using the technique popular in digital modulation such asQuadrature Amplitude Modulation (QAM) whereby each bit is modu-lated using Quadrature Phase Shift Keying (QPSK) and Amplitude Modu-lation (AM). A separate inductive recording head would be needed tostore the information in the magnetic medium.The information to be stored in the magnetic storage medium isgrouped into a 4-bit group (nibble) first. The actual combination of the4-bit group of information would magnetise the medium into appropriatedirection and magnitude (Fig.4).For example, if the information is 0110, then the storage is in thedirection of 135", with the magnitude below the threshold set in thesystem. If the information is 1101, then the storage is in the direction of- 90", with the magnitude set higher than the threshold.The two recording heads would record their appropriate components;i.e. for 0110, recording head A would have 0 field, and recording head Bmaximum field strength. Likewise, for 1101 recording head A wouldrecord with half of its field strength and recording head B would also havethe same field strength. However, the phase of the recording head Bwould be 180" to that of recording head A.Hence, each range of angles and amplitude of the fields detected bythe Chevron carry 4-bits of information instead of just one bit. ThisPertanika J. Sci. & Techno!. Vo!. I No.2, 1993 247Borhanuddin bin Mohd. Ali01010111OliO01000010ooll10011000101111011I110001ChevronFig. 4. Encoding technique to increase the packing density of magnetic storage.advantage would of course be at the expense of increased processing timedue to source and channel encoding required. More experiments areneeded to investigate its feasibility and to evaluate its performance againstfactors such as access time, processing time, and costs. Performance of thesystem in the presence of noise should also be evaluated.ACKNOWLEDGEMENTSThe authors would like to express their gratitude to Samsul, Goh andGhamrawi for the laboratory assistance in carrying out this investigation.REFERENCESALMASI, G.S., G.£. KEEFE, V.S. LIN and D.A. THOMPSON. 1971. Magnetoresistive detector forbubble domains. J Appl. Phys. 42: 1268-1269.DAVlES,A.V. and B.K. MIDDLETON. 1975. The resolution ofverticle magneto-resistive readoutreads. IEEE Trans Mag. MAG-ll: 1689-1691.HUNT, R.P. 1971. Amagnetoresistive readout transducer. IEEE Trans. Mag. MAG-7: 150-154.KWIATKOWSKI, W. and S. TUMANSKI. 1986. The permalloymagnetoresistive sensors - propertiesand applications. J Phys. E: Sci. Instrum. 19: 502.MOHD ALI, B.B. and AJ. MOSES. 1988. The magnetoresistive recording heads (MRH) as amagnetic field vector detector. J Fiz. Mal. 9: 75-80.MOHDALI, B.B. andAJ. MOSES. 1989. A grain detection system for grain-oriented electricalsteels. IEEE Trans. MAG-22(6): 4421-4426.THOMPSON, D. A., L.T. ROMANKIW and A.F. MAYADAS. 1975. Thin film magnetoresistors inmemory storage and related applications. IEEE Trans. Mag. MAG-ll: 1039-1050.248 Pertanika J. Sci. & Techno!. Vo!. 1 No.2, 1993

An Improved Chevron Configuration for the

Detection of Magnetic Field Vectors

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An Improved Chevron Configuration for the Detection of Magnetic Field Vectors

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