A magnetic phase diagram of the (Nil -,Mg,)O solid solutions is drawn by the results of elastic magnetic neutron scattering and magnetic measurements. The ranges of homogeneous antiferromagnetism (0 5 x 2 0.37), tricritical behaviour or frustrated antiferrornagnetism (0.37 < x 5 0.6), cluster spin glass (0.6 c x 5 0.75) and paramagnetic behaviour of the NiZ+ spin moments in the ground state (x 2 0.8) are determined. It is shown that diamagnetic Mg2+ ions serve as a source of a random field inducing an extra diamagnetic moment of value 0 . 3 5~~. Mit den Ergebnissen magnetischer Messungen und elastischer magnetischer Streuung von Neutronen wird ein magnetisches Phasendiagramm der (Nil -,Mg,)O-Festkorperlosung angegeben. Die Bereiche fur homogenen Antiferromagnetismus (0 5 x 5 0,37), trikritisches Verhalten oder frustrierter Antiferromagnetismus (0,37 < x 5 0,6), Clusterspinglas (0,6 < x 5 0,75) und paramagnetisches Verhalten der Ni'+-Spinmomente im Grundzustand (x 2 0,8) werden bestimmt. Es wird gezeigt, daIj diamagnetische MgZ+-Ionen eine Quelle fur statistische Felder darstellen, die ein zusatzliches diamagnetisches Moment mit dem Wert 0,35pB induzieren.

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A symmetry analysis of possible magnetic structures in crystals with the space group Dih is made for the wave vector of star { k l s } . This analysis allows deciphering the neutron diffraction data on the character of antiferromagnetic ordering in the AF, (below T, = 66 K) and AF, (below T2 = = 99 K) phases. It is shown that the magnetic phase transitions arise mainly from the structural transformations that occur a t T, and T,.Eine Symmetrieanalyse der moglichen magnetischen Strukturen in Kristallen mit der Raumgruppe D& wird fur den Wellenvektor des Sterns {kls} durchgefuhrt. Die Analyse erlaubt die Entzifferung der Neutronenbeugungsdaten in Abhiingigkeit vom Charakter der antiferromagnetischen Ordnung in den Phasen AF, (unterhalb Tl = 66 K) und AF, (unterhalb T, = 99 K). Es wird gezeigt, daB die magnetischen Phasenubergiinge hauptsiichlich aus den strukturellen Ubergiingen herruhren, die bei Tl und T, auftreten.

FeGe 2 , and lightly doped compounds based on it, have a Fermi surface driven instability which drive them into an incommensurate spin density wave state. Studies of the temperature and magnetic field dependence of the resistivity have been used to determine the magnetic phase diagram of the pure material which displays an incommensurate phase at high temperatures and a commensurate structure below 263 K in zero field. Application of a magnetic field in the tetragonal basal plane decreases the range of temperatures over which the incommensurate phase is stable. We have used inelastic neutron scattering to measure the spin dynamics of FeGe 2 . Despite the relatively isotropic transport the magnetic dynamics is quasi-one dimensional in nature. Measurements carried out on HET at ISIS have been used to map out the spin wave dispersion along the c-axis up the 400 meV, more than an order of magnitude higher than the zone boundary magnon for wavevectors in the basal plane.

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