1 / 13

PRESENTATION FILE: CMDAYS2011 Gauhati University: August 24-26 th 2011

PRESENTATION FILE: CMDAYS2011 Gauhati University: August 24-26 th 2011. CONDENSED MATTER DAYS 2011 PROGRAMME. A Slide show with automated animation timings. The show duration for this presentation file 33 mins. http://www.ugc-inno-nehu.com/cmdays_abstract_accepted.html

lizina
Télécharger la présentation

PRESENTATION FILE: CMDAYS2011 Gauhati University: August 24-26 th 2011

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PRESENTATION FILE: CMDAYS2011 Gauhati University: August 24-26th 2011 CONDENSED MATTER DAYS 2011 PROGRAMME A Slide show with automated animation timings. The show duration for this presentation file 33 mins. http://www.ugc-inno-nehu.com/cmdays_abstract_accepted.html http://www.ugc-inno-nehu.com/cmdays2011_gu.html S.Aravamudhan CMDAYS2011

  2. When A Magnetic Moment Is Subdivided, Do The Fragmented Moments Interact Among Themselves? ABSTRACT It has been possible to improve the validity of point dipole approximation by appropriately subdividing the magnetic moment of a magnetized material to calculate the induced fields within the material and calculate the demagnetization factors by a convenient summation procedure (1). Subdividing the magnetic moment in such a way that the vector addition of the fragmented moments yields the total magnetic moment value is followed by distributing the subdivided moments over the entire sample. In such a case will there be an interaction among the fragments? This would entail accounting for induced fields at one fragment due to the other fragments and the corresponding interaction energy. One way to explain this process is to uphold that the subdivision is only hypothetical and the boundaries between fragments are not real but only a mathematical convenience. On the other hand, the demagnetization factor by a simple procedure is possible only by this subdivision necessarily and the demagnetization obtained have physical significance. Hence it seems it has more basis to find out whether a subdivision of this kind which results in a physical quantity closer to true value, must be such that the interaction among the fragmented and distributed moments must be such as to cancel the effects of the interaction from all the others at the location of every fragment. A procedure to calculate such induced field values at a fragment would be reported and discussed for the above perspective. References: Proceedings of the 96th Indian Science Congress, 2009; http://www.ugc-inno-nehu.com/isc2009nehu.htmlhttp://saravamudhan.tripod.com/id6.html "When a Magnetic Moment is Subdivided, do the Fragmented Moments Interact with Each Other?" The contents of this presentation file would be subjected to alterations as and when it is necessary. Download the updated file from this LINK S.ARAVAMUDHAN NORTH ESATERN HILL UNIVERSITY, SHILLONG Chemistry Department CMDAYS 2011 Gauhati University Aug 24-26, 2011 Additional materials for this presentation: WebPage for the participation in CMDAYS2011: http://www.ugc-inno-nehu.com/cmdays2011_gu.html S.Aravamudhan CMDAYS2011

  3. E3 is the discrete sum at the center of the spherical cavity; does not depend upon macroscopic specimen shape. (Lorentz field)E2is usually for only a spherical Inner Cavity; with Demagnetization factor=0.33 ; E2 = [NINNER or DINNER] P E1isthe contribution assuming the uniform bulk susceptibility and depend upon outer shape E1 =[NOUTER or DOUTER]P E0 is the externally applied field Discrete summation Additional materials for this presentation: E3= intermolecular E2= Ninnerx P E1=Nouterx P C.Kittel, book on Solid State Physics Pages 405-409 Lorentz Relation: Eloc = E0 +E1 + E2 +E3 S.Aravamudhan CMDAYS2011

  4. Evaluation requires solving integrals set up for appropriate shapes INDUCED FIELDS,DEMAGNETIZATION,SHIELDING The difference between induced moments and permanent moments: Induced moments arise when the specimen is exposed to magnetic field and the induction phenomena ensures the orientation of the moment is such a way the moment should be aligned (diamagnetism) for minimum potential energy. No further alignment and changes in orientation of the moment is necessary. The induced moments arise due to the polarization (P) of the electronic (static) charge cloud. Since it is the Currents (flowing charges) give rise to magnetic fields, time de[pendent changes in polarization are invoked and the Polarization currents (∂P /∂t) appear in the expressions for current densities and charge densities in the context of Magnetism.. The permanent moments are present even in the absence of the magnetic field applied externally, and could be oriented in a direction that may require a realignment when the field is applied. In the context of the allowed discrete orientations of a spin, the ensemble of spins undergo a redistribution of the orientation of the spins and this phenomenon is referred to as the polarization of the spin-orientations. The vectorial direction of a Field happens to be opposite conventionally to the vector direction of the Moment. Links below are or getting clarifications on the conventions http://www.youtube.com/user/aram1121944/#p/u/8/LctfaPFRjJ8 http://www.av8n.com/physics/electric-dipole.htm http://en.wikipedia.org/wiki/Dipole http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dipole.html http://www.youtube.com/watch?v=513aIefo6QA Induced Field inside a hypothetical Lorentz’ cavity within a specimen = H``  Shielding Factor = Demagnetization Factor = Da H`` = -  . H0 = - 4 .  . (Din- Dout)a .  . H0 out  These are Ellipsoids of Revolution and the three dimensional perspectives are imperative in 0.333  = 4 .  . (Din- Dout)a .  When inner & outer shapes are spherical Moment direction polar axis ‘a’ N + Field: Lines of Force Din= Dout m = a/b Induced Field H`` = 0 equatorial axis ‘b’ Induced Field /4 .  .  . H0 = 0.333 - Dellipsoid Only induced diamagnetic moment is considered Thus it can be seen that the the ‘D’-factor value depends only on that particular enclosing-surface shape ‘innner’ or ‘outer’ S - = b/a polar axis ‘b’ Discrete summation References to ellipsoids are as per the Known conventions-------->>>> equatorial axis ‘a’ S.Aravamudhan CMDAYS2011

  5.  is the susceptibility which is inherent characteristic of the electronic structure of molecules/materials. Magnetization Mparising due to the interaction of the material at the Spot with the externally applied magnetic field of strength H0 When the induced field, due to such native moments, at a distant point is to be calculated, would it be of any consequence to know whether the native moments interact with each other? Would such an interaction if it is present alter the generic field (local filed) value at a site within the specimen? 2-dimensional lattice Each point is occupied by a molecule. with susceptibility p Magnetic moment μp = p H0 A possible way to answer the query stated as the title of this presentation is included in Slide #12 Molecular to molar or volume susceptibility  requires summation over the appropriate number of molecules;  = n. p. Till now no interaction due to the μpat other sites has been considered (the native property). The native field at the molecular site, due to the molecular susceptibility p should be equal to |μp| = p· H0 = |H”p| The Magnetization M=Induced moment per unit volume; M= n.μp ; where n is the number of molecules per unit volume Inquire for the appropriate units of Moment and Field Can the qualitative difference between the physical quantity “Moment” and the “Field” traced to account of r the necessity of the “H” field and the “B” field macroscopically ? (Only) shape dependent value (a gemetricalpremultiplying factor)of the Demagnetization factor D arises due to the induced field from all the other molecules. Thus the demagnetization factor D ( a fraction, <1, ) can be pre-multiplying μp to result in a reduced moment; D xμp The Electro-dynamic aspects of the micro- & macroscopic fields, and the generating currents and current densities for H and B fields find a reinterpretation in the review: http://www.ugc-inno-nehu.com/cmdays2011/micros_magntzn_llhirst_revmodphys69_607_1997.pdf S.Aravamudhan CMDAYS2011

  6. Do these moments interact with each other? That these moments should be interacting and accounting for this interaction in the discrete regime is all about the Lorentz sphere and the Field. Slide#3 Indfld vs distance 6.0E-08 χ||=-90 x 10-6 [cgs units (molar)] Per molecule would be The above value divided by Avagadro number: 6.023 x 1023 =14.9427 x 10-29 = 1.49427 x 10-30 At a distance of 2 Angstrom from this moment (per unit field= χ|| x 1G) The secondary field value would be -1.49427 x 10-30 / (2 x 10-8 )3 = -1.8678 x 10-5 For χ||= -100 x 10-6 ; Secondary field would be -2.07533 x 10-5 Further detailed consideration of the inter and intra molecular shielding contributions in terms of molecular, bond and atomic charge circulations (currents) are in the linkand in the side #8 of this file Indfld vs distance 2 A˚ equal spacing Indfld vs distance Benzene Molecule & Its magnetic moment {χM . (1-3.COS2θ)}/(RM)3 Indfld vs distance S.Aravamudhan CMDAYS2011

  7. μp It is precisely at this point of knowing the microscopic matter as it prevails, the Proton Magnetic Resonance seems to be answering to the details on the paradoxical situation: THE MICRO-MACRO PARADOX ! Hp = μp/Rp3 http://nehuacin.tripod.com/id5.html The shielding parameter measured by the proton magnetic resonance spectroscopy is a measure of the generic local field at the nuclear site within a molecule. This shielding has contributions from the neighboring moments present, and in favorable cases these can become significant contributions to alter the pattern of the spectra obtained. How these changes (to the spectral patterns) also become significant in the net macroscopic field distributions within the magnetized material: it is a matter of numerical values of induced fields at the neighboring moments compared to the native moment induced by the interaction with the externally applied (possibly) strong magnetic fields. It becomes necessary to correct for the bulk susceptibility effects depending on the shape. ∑p Hp Rp This is merely shape dependent μx∑p1/Rp3 S.Aravamudhan CMDAYS2011

  8. H0 These neighbor moment contributions are of the order of ppm of the strength the external fields as much as the native moments are. The (χvsusceptibility values are ppm cgs units) molecular susceptibility values (χmolecular = χv / no. molecular units within volume ‘V’ ) are of order of 10-30 CGSunits to be multiplied by the external filed strength to arrive at the natively induced moments. Thus the native moments μ are themselves of the order of 10-30 CGS units. If these moments are to induce secondary fields at a neighboring point, (~ μ /R3) which is of the order of ppm, this induced secondary field would add to the native moment only insignificantly in comparison. Thus the interaction of the native moments would not be much consequence for the induced filed values at any location from all the other locations. ppm 10-36 10-36 Same 10-30 10-42 Further detailed consideration of the inter and intra molecular shielding contributions in terms of molecular, bond and atomic charge circulations (currents) are in the linkand in the side #8 of this file R These estimates have been possible since the validity of point dipole approximation has been assumed 10-37 10-36 μ THE MICRO-MACRO PARADOX IN INDUCED FIELD CALCULATIONS AND THE ROLE OF HR SOLID STATE NMR http://nehuacin.tripod.com/id5.html S.Aravamudhan CMDAYS2011

  9. When the magnetic moment is divided then, the divided elements are not molecular moments as considered in the previous slide. The magnetic moments of the divided elemental volumes are semi-micro magnitudes, each element comprising of several molecules. The magnitude of the moments is also larger and hence the induced fields at the neighboring element may also be more, depending on the element to element distance being larger in comparison to intermolecular distances. When the induced field is calculated at a farther point than the neighboring element, how much would be the inter-element contributions to alter the originally divided moment-magnitudes? Line defined by Polar angle θ / direction of radial vector This demarcated typical region is considered in the next slide It is to be pointed out at this juncture, the total induced field at a point values within the spheroidal specimen results only in a shape (and not the size) dependent pre-multiplying factor to the value of the induced moment (generated by the interaction with external field). It would be true that the individual elemental moments interact among each other but what matters is the effective total interaction. It is only in a spectroscopic analysis as for the Proton Magnetic Resonance Spectroscopy, it is possible to demarcate the intra molecular versus the intermolecular around a particular site and distinguish the strengths and significance of the long-range & short-range interaction scales. And, disentangle the microscopic and macroscopic consequences and observe the effects distinctly as if these are two different physical quantities even though it is all induced fields. Vector map: Slide#4 of 0_2_12Aug2011.ppt S.Aravamudhan CMDAYS2011

  10. Refer to graph in next slide Appropriate radius and distances are noted and the calibration graph is used to estimate local field A calibration graph in next slide obtainable from the data below Click here for an xls file Refer to previous slide The numerical net value at the center amounts to nearly zero. Click for elaboration This favorable numerical result indicates that the close packing criterion and the associated distances can be applied to know the actualities of contributions besides the afortiori reasons. Induced field at a point outside the magnetized specimen S.Aravamudhan CMDAYS2011

  11. 0.793832, 2.90E-06 A calibration graph is slide obtainable from the data in this URL 0.0419, 2.00E-07 link Calculating the graphical plot: Click here for an xls file 7.2 / 2.5 = 2.88 ; 1 / (2.88)3 = 0.0419 2.7/2.5=1.08 ; 1/ (1.08)3 = 0.793832 Distance = 7.2 mm Radius 2.5 mm S.Aravamudhan CMDAYS2011

  12. The following conclusive part is based on the contents of the materials in slide #5 of this ppt file. Click…….. As explained, the Magnetization is the induced moment per unit volume. And, in terms of the molecular moments, the native moments of the molecules are simply summed up over all the molecules in the unit volume to get this magnetization (or induced magnetic moment) and no interaction between the molecular moments is taken into account. And it is such a simple sum of moments which stands for the Total Magnetic Moment “M” of the specimen. It is this interaction (spelt out as unconsidered above) which is accounted for while calculating the induced secondary field (demagnetization factor) at distant point within the specimen; and, for this calculation, the total moment is divided and distributed. If the M is multiplied by the Demagnetization factor (shape-determined), then, dividing the resulting magnetic moment value cannot be subjected to the division and distribution in such a simple and straight forward manner. The way the magnetic moment is divided and distributed in the summation method is precisely to account for the interaction among the moments, hence the reply to the query (as in the title) could be that non interacting moments-sum is divided to consider the interaction among themselves. At this juncture it can be inferred that the subdivided fragments are being so considered as the consideration of molecular native moments within the Lorentz Sphere, the discrete regime. Further considerations and details of summation within the discrete volume element (typically the Lorentz sphere) have been made available as internet resource and the same can be tractable from the webpage at: http://www.ugc-inno-nehu.com/cmdays2011_gu.html S.Aravamudhan CMDAYS2011

  13. Download the PowerPoint Files from the Web Directory http://www.ugc-inno-nehu.com/cmdays2011/cmdays_gu/ and save these files in your resident disk in one and the same new directory created (may be named cmdays_gu) in your disk. More materials related to this symposium can be available from the Web subDirectory http://www.ugc-inno-nehu.com/cmdays2011/ Also, you may display the URL: http://www.ugc-inno-nehu.com/cmdays_abstract_accepted.html http://www.ugc-inno-nehu.com/cmdays2011_gu.html S.Aravamudhan CMDAYS2011

More Related