It's mainly a problem of detection. In the M(H) curve, you either detect the irreversibility

associated with the pinning of vortices and look where this irreversibility disappears. It gives the lowest value Hirrev for a so called Hc2. You also can look by ac susceptibility meausurements in very low fields where the -1/4 Pi slope disappears, it approaches the Hc2. In magnetoresistance depending on the definition (90% as you say, 95% ..) you also approach Hc2 > Hirrev. Finally the best method  are specific heat measurements which detect the superconducting volume, disappearing above the real (and highest) Hc2. But they are more demanding!

There is no sharp border between SC and normal state. So there is no reason to anticipate that Hc2 values determined by different criteria may be the same.  The correct way is to study unlinear dependence of resistivity on temperature, magnetic field and current. This dependence allows solving any electrodynamic problem and predict any conditional critical value in dependence on criteria choice, if you prefer to use this way to describe properties of superconductors.

Thank you both of you. In one of my experiment I found from M-H curve Hirr = 0.45T at 2K however Hc2(0)=4.6T by 90% criteria and  H(0)= 3.1T by 10% criteria (some people in literature say it Hirr). These studies on a 4.5K Tc(R=0) superconductor are done for polycrystalline  sample. I am unable to understand this two order difference in these values. Please suggest some reference where these two studies M-H and R-T for different H are compared and explain possible mechanism.

Thanks

I guess that your sample was rather inhomogenious. The properties of  such superconductors can be described with Gauss distribution. The result  depends on the type of inhomogeneity, if it volume one or linear(longitudinal).

I believe that Hirr has no real physical sence in spite of a lot of researches. The history of high pinning superconductors is reach with such false mental ideas. The most known  example of the idea is thermal activaited movement of vortices.

http://link.springer.com/article/10.1134/S1063783411050052

This article concerns the irreversibility fields on M(H) and R(H) curves.

Dear Dr. Brajesh,

As Dr. Jean suggest you to do the heat capacity measurement (C vs. T at different fields), which can provide you the reliable  HC2 value at different constant magnetic fields. i also want to add that the determination of HC2 by R vs. T measurement is rather conditional as pointed out by Dr. klimenko. 

Generally R vs. T measurement is surface sensitive, i mean, if there is a presence of surface superconductivity in your system then R vs. T will provide you HC3 value rather than HC2 at constant field of measurement. Generally HC3 = 1.7 HC2.

you can fixed your criteria for the determination of HC2 from R vs. T, as taking the temperature value corresponding to the peak in the temperature derivative of resistivity. if you can find the HC2 values from heat capacity measurement than plotting them with the HC2 estimated from R vs T (as i written above), you can get conjecture about the presence of surface superconductivity in your system because heat capacity gives the information of bulk superconductivity and is rather insensitive for surface superconductivity.

one more thing i want to add that the Hirr in superconductors has  some physical meaning related to the pinning of vortices in the mixed state of superconductors. which can be understood physically as the Hirr is the field below which the flux vortices get pinned in the superconductor and above which  it shows the flux flow in the system, which give rise to flux flow resistivity in the system (although my comment is not generalise for all kind of superconductors but you can check this possibility in your case whether this follow in your case or not).

Dear Dr. Brajesh Tiwary,

      You have received some quite different  opinions. The reason is that the counsellors have experience in very different superconductors. The opinions could be more specified if you report more detailed information about yours specimens.  As I have understood from your papers you are concerned with HTSC specimens and rather smooth SC-normal transitions.  So you cannot use any criteria suitable for sharp transition superconductors.

Thank you so much to Mr. Shyam Sundar  and Dr. E.Yu. Klimenko for such a valuable information.

To Dr. E.Yu. Klimenko , The transition is sharp enough (

Dear Dr. Brajesh,

May i know what criterion did you follow to estimate the HC2(T) value from isothermal M vs. H measurement.

Dear Dr. Brajesh Tiwari,

       It is a lesson to me and others never  to try explain something knowing nothing on details of the experiment.  So your question should carry this information. You have corresponded nothing about your specimen. So my latest hypothesis is that it could be critical field anisotropy if you had not watched carefully magnetic field direction in every measurement.

In the M(H) curves, you must look where it reaches the normal state M(H) value which can be on the paragnetic side, to get Hc2. The problem is that the change of slope is faint.

M(H) = 0 is valid only if the normal state Magnetization  is 0. Then it depends how you accuratly you detect a zero magnetization!

A.c. susceptiblitymeasured in H is more sensitive to detect the vanishing of superconductivity

Sir thank you so much @ Dr. E.Yu. Klimenko. Sorry for for not giving the information details about specimen. Magnetization and Magneto-resistivity (linear four probe)are recorded using PPMS (14T). Sample was cut from sintered polycrystalline pellet into size 4mm X1mm X1.5mm size which was used first for Magnetization measurements. On the same linear four probe was used for magneto-resistivity. Magnetic field is along the longest size of the sample. The sample is of Nb2PdS5 J. Appl. Phys. 115, 213903 (2014). The demagnetizing fields are important but I do not understand how its related so sensitively to the M-H curves.  

They must be different because the upper critical field (Hc2) determined by M-H curve  corresponds to the situation when the whole volume is filled by vortices M(H)=0 while when you do measurement of magnetoresistivity , you will see resistivity as result of vortices movement and this value will be less

@Olga Ivanenko, Thank you so much. But sir In my sample I found opposite trend i.e. by magnetoresistivity measurements Hc2 is almost 10 times higher than by M-H measurements. This trouble me a lot though these measurements are made on pollycrystalline  bulk samples. I convinced myself to understand that it can be due to some kind  filaments of superconducting materials may exist even in presence of strong magnetic field (> Hc2 as determined by M-H). But why they should exist in samples of Nb2PdS5 still I am not comfortable with. Please suggest some explanation and whether I am thinking correct.      

Do not forget that in dc resistivity and ac susceptibility measurements in magnetic field parallel to the sample surface you'll measure Hc3 not Hc2. D. Saint-James, P.G. de Gennes Phys. Lett. vol.7, 306 (1963). Hc3/Hc2 = 1.695 for smooth sample surface.

Hi Brajesh,

You may find the answer in a paper attached (V. Kozhevnikov et al., PRB 95, 174509, 2017).

V.K.