Abstract
The applicability of the values of the strength \(H_{\textrm{irr}}\) of the irreversibility field and the strength \(H_{c2}\) of the upper critical field in pinning mechanism determination in a sample of the family of iron pnictides of the class 111, NaFe \({}_{1-x}\) Co \({}_{x}\) As, was analyzed in this work. Methods for calculating \(H_{\textrm{irr}}\) and \(H_{c2}\) were also considered. The first method for determining the strength \(H_{\textrm{irr}}\) of the irreversibility field is based on extrapolation of the zeros of the graph \(J_{c}(H)\) . The second method, considered exclusively from a theoretical perspective, is Kramer’s method. It is based on extrapolation of the zeros of the graph \(J_{c}^{1/2}B^{1/4}(B)\) . Each of the methods requires qualitative identification of the graph zeros in order to obtain the most accurate result. The strength of the upper critical field is determined by extrapolating the points obtained at the intersection of the graph \(M(T)\) with the line at the level \(M=0\) . Due to the limited operating range of the measuring device, the values of \(H_{\textrm{irr}}\) for the NaFe \({}_{1-x}\) Co \({}_{x}\) As sample were obtained by approximating data from previously acquired results. When using \(H_{\textrm{irr}}\) , a volume pinning type was identified. The values of \(H_{c2}\) were also calculated based on already obtained data and allowed the surface pinning mechanism to be determined, which is consistent with earlier studies. This indicates that the pinning mechanism determined using \(H_{\textrm{irr}}\) was established incorrectly. It was found that the use of irreversibility field strength values for the NaFe \({}_{1-x}\) Co \({}_{x}\) As sample may lead to incorrect identification of the pinning type.
