Non-stoichiometric alloy Ni_1-x-yMn_xGa_y with x = 0.2817, y = 0.2136 is studied using magnetic resonance spectroscopy: ferromagnetic resonance (FMR) and conduction electron spin resonance (CESR). Temperature dependence of the resonance field value H_res(T), the line width ΔH(T), and the integral intensity I(T) is measured across the wide temperature interval T= 4.2 K ÷ 570 K. Three phase transformations are found: paramagnetic↔ferromagnetic with Curie temperature of T_C= 360 K, austenite-to-martensite transformation (direct with M_s = 312 K and reverse with A_s = 313 K, and a transformation to a new state at T = 45 K. It is suggested that the low temperature transition proceeds through the spin glass state since the temperature dependence of the I(T) has the ‘cusp’- like character. The angular dependence of the FMR signals was measured in the martensitic and austenitic states before and after martensite-austenite transition. Experimental data were used for determination of the magnetisation 4π I_s and anisotropy parameters K₁ = -0.77 kG² , K₂= 0.85 kG² for the martensitic state. The scattering of the values and disorientation of the magnetization directions over the sample are estimated using the analysis of the angular dependence of the line width. The temperature dependence of the resonance signals was investigated in the temperature range significantly higher than T_C where FMR is transformed to the conduction electron spin resonance. In the paramagnetic state (above T_C) the alloy reveals an intensive signal of CESR, which is an evidence for a high concentration of free electrons and correlates with the large magnetic-field-induced strain observed in the alloys of such composition. The temperature dependence of the skin layer is found from the sharp decay of the CESR signal with temperature, which is related to disappearing the large magnetic resistance after transformation to the paramagnetic state.

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