Neel temperature is proportional to the molecular field constant (and therefore proportional to the strength of the molecular field of the two sublattices in the antiferromagnetic material).  Accordingly, a weak molecular field is associated with a low value of the molecular field constant, and consequently low PM-AFM transition temperature. The origin of the molecular field is the quantum-mechanical exchange forces according to Heisenberg. Thus, weaker exchange interactions indicate weaker molecular field and smaller transition temperature.

For reference, books or chapters on magnetic  interactions and magnetic phenomena in magnetic materials should be helpful. For example, see: B.D. Cullity and C.D. Graham, Introduction to Magnetic Materials, 2nd ed. (Wiley, Hoboken, NJ, 2009).

Dear Prof Mahmood,

Thanks

i have 2 samples, Nd3+ and Pr3+ which Pr3+ like La3+ shows antiferromagnetic with neel temperature and large negative weiss constant but for Nd 3+ MH no neel transition appear but weiss constant is very small -2K.

1-is any example like this?

2- Nd3+ without neel teperature but with small negative weiss constant is weak anti ferromagnetic?

What is your compound? Nd was reported to induce a decrease in the transition temperature in perovskite manganites, which means weakening the superexchange interactions. Did you check the particle size and morphology in each sample (using electron microscopy)?

poly crystalline compounds (micro meter)

ًWhat is the chemical formula of the compound and how was it prepared?

Generally, a negative intercept of 1/chi with the temperature axis implies antiferromagnetic coupling. The small value of the intercept indicate weak coupling

I appreciate

.... and does not necessarily imply ordering to occur at finite temperature. However, if antiferromagnetic order is observed at some TN>0, then from having both, the paramagnetic CW temperature and the Néel temperature, you can estimate the (mean field) coupling constants.