Banerji, S.

Kumar, P.

Mahalanabis, A.

Neogy, D.

IACS Institutional Repository

A CONVENIENT AND SENSITIVE BALANCE FOR MEASURING MAGNETIC ANISOTROPIES OF SINGLE CRYSTALSD. NEOGY*. S. BANERJT, V. KUMAR Am> A. MAHALANABIS P h y s i c a l  L a b o r a t o r i k s , U n i v e r s i t y  o f  H u r d w a n , B u r d w a n , W e s t  B e n g a l * I n d i a .{Received April 10, 1967)ABSTRACT. For measuring magntuic arusotropins of single crystals a new sensitivo balance has been di‘)3cribed. The instrument incorporates olectrodynamic method of halanc ing the couple experienced by a crystal and is operated electrically from a distance. Elimina tion of manual handling of the balance onsuros high reproducibility and ox>orational con venience. I t is well adopted for low temperature work. The performance of the balanco is thoroughly tested and discussed.I N T R O D IT < ^  T 1 O NThe past few decades have witnessed a sharp rise in interest in the magnetic properties of single crystals. Studies on the diamagnetic organic crystals by Krishnan and Bancrji (1935, 1938)* BaiuTji (1938) and recently by Mookherji et ol (1959, 1961) show the close correlation between the magnetic anisotropy and the molecular and crystal structure. In understanding the nature of the ligand fields in the single crystals of the transition m(*tal ions, the magnetic aniso­tropy and its variation with temperature play a very fundamimtal role. Exten­sive investigation on the crystals of the iron group elements undertaken by Bose and co-workers (1961, 1963) points to the sensitive field dependonoo of the mag­netic anisotropy and its thermal variation. Also, in the case of the rare-earth ions where the electrons responsible for giving rise to paramagnetism, arc partially shielded by closed shell of outer electrons, the studies of Neogy (1963), Neogy and Mookherji (1965) show the significance of anisotropy for correct determination of the crystal field parameters. Because of this pivotal importance of the magnetic anisotropy we are faced with the problem of collecting a largo amount of data of fairly high standard on this physical property. To ensure a fast process for doing the same, it is very necessary, that a simple and very sensitive method be evolved to take the drudgery out of the existing methods for measuring anisotropy*A quick survey of the present methods of measuring magnetic anisotropy shows that there are only three of them to bo taken notice of. Out of these, the oscillation method has long been discarded as being rather crude. The only• CSIR Pool Offloer.744two widely used techniques are the well-known Krishnan and Bancrji a spin method and later developed Stout and Griefel's (1950) null method The details of these two methods are available in the original literature and’it wiU suffice to point out here tha t both the procedures require manual handling of the delicate suspension system which is rather inconvenient and puts undue physical strain on the worker. Furthermore, the spin method involves determination of torsional constants of the quartz fibres which is not very easy to handle and a t low temperature when the anisotropies of paramagnetic crystals increase manyfold their room temperature value, the sudden violent spinning often becomes a problem. In  the Stout and Griefel method if torsional constant of the fibre is smaU, there may be appreciable error in fixing the zero position after the crystal is rotated through 45° or any other angle due to the small residual magnetic field. This error may, however, be avoided if the magnet is rotated through the fixed angle instead of the crystal and this incidentally does away with the necessity of using a hexagonal mirror. The accuracy of thes(» methods can obviously bo in­creased by the use of large well-calibrated torsion head as is often done, (Guha Thakurta et al, 1966). A highly suitable yet very simple way to get around the difficulties, is the electrodynamic method of balancing the couple experienced by the crystal in the Stout and Griefel method. This eliminates all kinds of direct manual operation on the suspension system and also, since the restoring couple is not furnished by the application of torsion to the suspension fibre, it is no longer necessary to u.se delicate fibres for this purpo.se. The des­cription of the balance is given in the next section. The linearity of values ob­tained for crystals of different masses and also for different sin 20 values, where O'a are the angles of rotation of the magnet, is examined. Some cr3'.stals which are usually held as standard for magnetic field calibration work are also measured using CUSO4.5H2O as standard and the values are compared with those of previous workers.D E S C E I P T I O N  OF T H E  B A L A N C EA schematic diagram of the balance is shown in fig. 1 . Essentially, it may be looked upon as a modified form of a suspended coil galvanometer. The balance coil (6) is wound upon a light rectangular aluminium frame attached to a thin glass rod (20) carrying a small plane mirror (19). The coil is suspended by means of a pyrex fibre (2 ) from a small torsion head (1 ) and is free to move in the radial field of the local magnet (6). The two leads of the coil are brought out by means of two phosphorbronze spirals (4) attached to the coil. The spirals arc wound in opposite directions to nullify any rotational effect on the suspension system due to the thermal expansion of tho spirals, owing to itemperature fluctuations during the course of measurement.' A sufficiently long glass rod (9) is rigidly attached to the lower end of the glass rod carrying the coil. A small glass hook (11) is fixed to the free end of the long glass rod. The crystal (13) to be studied is attached toBalance for Measuring Magnetic Anisotropies U5746 B c tn e r j i ,  K i im o r  ( in d  J ^ d h c itd fu ib isanother very thin glass rod (12 ) with a hook at one end; the length of the rod is so adjusted that when it is suspended from the upper hook, the crystal comes just at the centre of the pole gap of the rotatable electromagnet (14). A heavy glass bead (10) is fixed to the rod (9) to increase the stability of the system againstFigure ( l ) - ( l )  Torsion head. (2) Pyrex fibre. (3) Perspex case. (4) Phosphorbronze spirals. (5) Aluminium frame. (6) Permanent Mngnot. (7) Oil-sealing glass cup. (8) Watch oil. (9) Glass rod. (10) Glass bead. (11) Glass hooks. (12) Very thin glass rod for crystal suspension. (13) Crystal. (14) Pole-pieces of rotatable olectromagnot. (1.9) Potcntioniclor. (16) Standard resis­tance. (17) Coil leads. (18) Outer casing. (19) Plano Mirror. (20) Glass rod., disturbances during the process of changing specimens to be measured. The Ught spot deflected by the mirror is detected by a differential phototube arrangement.The balance may easily be adapted for low temperature work with the intro­duction of an oil-sealing mechanism ‘7’ and ‘8’ which hermetically separates the crystal chamber from the upper part of the balance and thus prevents the humidity from entering the balance box.M EA SU K EM EN T OF O U R B EN T As th e  couple experienced b y  th e  crystal is proportional to  th e  square of the m agnetic field i t  is essential th a t  th e  la tte r  should be m ain tained stric tly  constant.The change in the magnetic field due to small variation in the coil current is avoided by using an electronically regulated power supply capable of maintaining the cur­rent within 0.06% of the desired value, up to 5 amperes. For fine adjustment of the current a screw motion rheostat is connected in scries with the magnet The constancy of the current is checked by moans of the arrangement showing in fig. 2 wherein the voltage drop across a standard resistance (1 ) is balanced byllie potentiometric arrangement shown. I t  is not necessary to measure the actual voltage drop across (1 ) : the constancy of this voltage drop is essential. The absence of any deflection in the galvanometer (3) ensures this constancy. The current in the potentiometer circuit is checked front time to time by means of the standard cell (2). This arrangement is sensitive enough to give a deflection of 10 cm, in the ‘Multiflex’ galvanometer for 1 mA change in the coil current.O P E R A T I O NThe procedure adopted to align the direction of the maximum susceptibility of the crystal in the horizontal plane with the magnetic field direction, is similar to that of Stout and Gricfel, the only difference lying in the fact that the magnet is rotated instead of the torsion head. The exciting coils of the electromagnet are energised and if the crystal is not at the zero position a deflection of the suspension system is observed. The magnet is rotated until there is no deflection on make and break of the current. This gives the ‘zero position’ of the balance and the corresponding position of the magnet is noted. From this position the magnet is rotated through any angle. This angle of rotation should be less than 45“ and preferably greater than 30“. When th(  ^magnetic field is switched on the crystal will experience a couple and will try to align in the direction of the magnetic field. The crystal is restored to its original position by passing a current through the balancing coil. Tn order that the deflection from the ‘zero-position’ be less both the currents viz. the current through the electromagnet and that through the balancing coil should bo increased or decreased simultaneously. Because the upper permanent magnet is placed high above the lower electromagnet and has no interaction with it, the torque acting on the coil is directly proportional to the current in it. This current is determined by measuring the potential drop acro.ssBalance for Measuring Magnetic Anisotropies 747Figure 2—(1) Standard resistance. (2) Standard cell. (3) Multifiex Galvanometer. (4) Ihiving cell. (6) To current regulated "power supply. (6) Magnet,748 Neogy, Banerji, Kumar and MahcUanabisa standard resistance with the help of a precision potentiometer capable of measur­ing upto microvolt.For calibrating the balance the current J , required for balancing the couple due to a standard specimen is found out in the abovemontioncd way. For measuring anisotropy of any substance the standard sample is replaced by it and the coil brought to the original position by rotating the torsion head, if required. We have seen during measurement that if the hook-joint is made rigid by a drop of wax and opened by melting the wax in the joint with a hot brass rod carefully, the position of the coil is not at all altered on changing the crystal. Now, the current required for balancing the couple for the unknown specimen is deter­mined.The couple experienced by the standard sampler ,  oc J , =  sin 28.M ,and that by the test sampler„  oc /„  sin 2831(1)(2)where A  is the constant of proportionality and m, M  correspond to magnetic anisotropy, mass and molecular weight of the crystal. 0 denotes the angle through which the magnet has been rotated in both cases. The subscripts s and u stand respectively for the standard and test sample.Tf the angle of rotation 6 is the same for both the cases then... (3)R E S U L T SThe performance of the balance was examined by measuring the current required to balance the couple acting on the crystal for different angular positions of the crystal with respect to the field direction. According to the relation (1) or (2) the ratio of sine 20 and that of the couple should be constant. We measured the couples acting on the CUSO4, SHgO crystal by rotating the magnet through 30°, 35®, 40® and 44® from its zero position and the linearity is observed to be within0.05% which demonstrates the excellent response of the balance. The graph in fig. 3 shows the linearity between sin 20’s and the corresponding balancing e.xn.f’s as the measure of the couples. I t  may be pointed out here that this check should be taken as the true test of the reproducibility of the balance itself since in these measurements only the couple acting on the crystal is changed by rotating the magnet keeping all other factors constantf However, the overall accuracy tomeasurement of anisotropy depends on various factors, e.g., perfection of the crystal, the correct mode of suspension of the crystal and the homogeneity of the magnetic field over the region. The reproducibility and accuracy of our balance is also chocked by measuring the balancing currents for crystals of different massesB clIclucb f o r  JU cuduriug M u gu etic  A u iso tfo p ic s  749Fio. 3—The linearity in the variation of o.m.f. with values of sin 2$,Absoissae—^Values of sin 2. Ordinate—e.m.f. in /iV.of the Specimen in the same magnetic field. I t  is evident from the expression of the anisotropy tha t the variation of e.m.f. with mass should be linear. Using different crystals of C11SO4, SHgO we find that the linearity is within 0.2%. The higher error in this case is obviously more due to lack of perfection in the crystals and mode of suspension and inhomogcncity of the fields rather than the repro­ducibility of balance. A graph between different masses of CUSO4, 5H2O crystal with the corresponding e.m.f.'s is shown in fig. 4.Fia. (4)—The linearity in the variation of e.m.f. with mass of the crystal. Abscissae—Mass in mg. Ordinate—e.m.f. in ^V.The magnetic anisotropy of three different crystals belonging to the mono clinic system were measured with the balance. For this purpose the field was calibrated first with OuSO ,^ 6HgO crystal (with c-axis vertical). Measurement760 Banerji, Kumar and Mahalanahiawas dono by suspending the sample with &-axis vertical. The results are given in table 1 . The overall accuracy of these values was estimated as less than0.5%.As is obvious from the description and operation of the balance it is extremely simple in construction and easy to operate. The changing of samples is also effected very quickly. If  the pliototube null detecting system is made enough sensitive the same balance is capable of giving highly accurate values both forTABLE 1Crystal (Xi -X qIx IO®Present value Previous workers* valueTemperatureCuKa(SOJaNiKa(S04)aCoKa(S04)2327.7133.8 2530322.3* 131.2* 2540* *SOO'^ K♦  Dutta (1964).** Guha Thcbkurta et al (1966).D I S C U S S I O Ncrystals of low and high anisotropy since it only involves measurement of e.m.f. which can be done pretty accurately by a good potentiometer with a sensitive galvanometer.A few points o f  caution may, however, be mentioned at this point. During the course of measurement it was discovered that the error due to the anisotropy of the pyrex suspension rod which lies between the pole gap may cause appreciable error. Hence it is absolutely necessary to use as thin rod as possible and check each of them before use. Another factor which needs special attention for ensuring satisfactory working of the balance is the elimination of zero shift. Various factors are responsible for such shift in these sensitive instruments. If, however, particular care is bestowed on the neatness in constructing the balance this can be reduced  to negligible proportions. The suspension fibre and phosphorbronze spirals should be scrupulously cleaned; the soldered joints should be as neat as possible and one should avoid any sharp bend in the bronze spirals : finally one should avoid any drastic temperature change during the course of the experiment as far as possible.The distance between the coil and the electromagnet should be made large enough to avoid any appreciable stray field a t the position of the coil compared to the field due to the local magnet. This is necessary since the angular position of the magnet has to be changed in course of measurement and this would entail a corresponding change in the effective field on the coil, The comparative strengthof the stray field could be detected by keeping a small current running through the coil and noting whether tln>re is any change in the deflected position of the mirror by rotating the electromagnet.The figures 3 and 4 display tht' fine performance of tli(‘ balance. High linearity of o.m.f with the mass and sin 20 sliows the high reproducibility of the instrument. The UNICO electromagnet, probably the only Indian iiroduct in this category, was used in the measurement. It was found tht tlie homogeneity of the field with a polo gap of 4 cm was restricted within 1.2 cm around the centre of the pole gap.A C K N O WL E D G ME N T SVVe take great pleasure in thanking Prof. A.- Mukherji for eonstant eneourage- nu-nt during the progress of the work. Thanks arc al.so due to Dr. T. Mookherji and S. P. Chachra for helpful discussions.Balance Measuring Magnetic Anisotropies 761R E F E R E N C E SBaiiorjoe, S., 1938, Z, Kmtallogr., AlOO, 316.Bose, A., Chakraborty. A. 8. and Chattorjeo, K., 1961. Proc, Boy. Soc., A261, 43, 207. Hose, A., Dutta Roy, 8. K., Ghosh, i ’. K. and Mitra, 8., 1963, Inlian J, Phys., 37, 605. Butta, 8. K., 1954, India?} J. Phys., 28, 239.Guha Thakurta, D. and Mukliopadliyay, D., 1966, Indian J. Phys., 40, 69.Krishnan, K. S. and Banerjeo, S., 1935, Phil. Trans,, A245, 265.--------------------------------------- , 1938, Z, Kristallogr., A99, m .Neogy, D. and Mookerji, A., 1965, J, Phys. Soc. (Japan), 20, 3332.1965. Phjska, 31, 1325.Neogy, D., 1963, Physica, 29, 974.Mookerji, A,, Mookherjee, S. N. and Neogy, D., 1961, Bull. Chm. Soc, (Japan), 34, 233. Mookerji, A., Mittal, R. L. and Neogy, D., 1959, Bull. Nat. Jnst. Sc. (India), 14, 54. Stout, J. W. and Griefel, M., 1950, J. Ghem. Phys. 18, 1449.

A Convenient and Sensitive Balance for Measuring Magnetic Anisotropies of Single Crystals

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A Convenient and Sensitive Balance for Measuring Magnetic Anisotropies of Single Crystals

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