The structural and magnetic properties of nickel manganite (NiMn(_2)O(_4)) have been investigated, for bulk, and thin film samples. NiMn(_2)O(_4) has partially inverted spinel structure, Mn(_v)Ni(_1-v)[Ni(_v)Mn(_2-v]O(_4), where v is the inversion parameter. Bulk samples were produced from co-precipitated metal hydroxides at various firing times and temperatures. Particular attention was given to determining the optimum preparation route. Nickel oxide was the major impurity encountered due to sub-optimal preparation conditions, but was difficult to detect using diffraction, due to considerable Bragg reflection overlap with those NiMn(_2)O(_4). Nickel oxide is believed to have been present in most samples studied by other researchers in the field. Pure material formed in air after firing for 48 hours in the region 780 C to 820 C; a much smaller range than previously reported. By controlling the cooling rate after firing, 0.7483(19) ≤ v ≤ 0.8830(22) was obtained; as determined by neutron diffraction measurements. Ferrimagnetic Curie temperatures (T(_c)) between 100 K and 147 K were obtained for the range of v studied; somewhat lower than previously reported. The magnetization below T(_c) exhibits P-type behaviour, which has hitherto not been observed in this compound. Evidence compatible with a local canted state at temperatures below -10 K was observed using muon spectrometry. The magnetic properties of electron-beam evaporated thin films of NiMn(_2)O(_4) were investigated with a custom built Alternating Gradient Field Magnetometer. The AGFM was initially constructed for a study of Dilute Magnetic Semiconductor (DMS) materials, and was capable of temperatures down to 77 K, and resolution of 14 pJT(^1). This instrument used a mechanically resonant quartz fibre sample holder, and piezoelectric detection. The response of the instrument to temperature drift, applied magnetic field, and differing sample properties is reported. The T(_c) of a typical thin film sample was measured, and v= 0.788(8) inferred from the relationship with T(_c), as determined for bulk material.