Weak ferromagnets

Parasitic ferromagnetism is a weak ferromagnetism that accompanies antiferromagnetism, eg, in a-ferric oxide [1309-37-1], a-Fe202. Possible causes include the presence of a smaU amount of ferromagnetic impurities, defects in the crystal, and slight deviations in the directions of the plus and minus spins from the original common axis. 

As shown in Fig. 7.6, the Mossbauer data show a reduction in Morin transition temperature with increasing shock severity. At temperatures below the transition, increasing shock severity causes greater retention of the higher temperature, weak ferromagnetic contribution. The measure of weak ferromagnetic (WF) fraction (the high temperature form) is a sensitive indication of shock modification. 

Fig. 7.6. The weak ferromagnetic (WF) fraction (high temperature form) of hematite provides a sensitive measure of shock modification. Sample 31G836 is an 8 GPa experiment. Sample 29G836 is a 17 GPa experiment, while 17G846 is a 27 GPa sample (after Williamson et al. [86W03]).

In contradistinction to this, weak ferromagnetism has been observed in a number of chloro and bromo complexes of the type M2[CrX4] (M = a variety of protonated amines and alkali metal cations, X = Cl, Br), which are analogous to previously known copper(II) complexes (p. 1192). They have magnetic moments at room temperature in the region of 6BM (compared... 

LCo(H20)6] ion, and bidentate /V-donor ligands such as cn, bipy and phen form octahedral cationic complexes [Co(L-L)3] , which are much more stable to oxidation than is the hexaammine [Co(NH3)6l . Acac yields the orange [Co(acac)2(H20)2] which has the tram octahedral structure and can be dehydrated to [Co(acac)2l which attains octahedral coordination by forming the tetrameric species shown in Fig. 26.3. This is comparable with the trimeric [Ni(acac>2]3 (p. 1157), like which it shows evidence of weak ferromagnetic interactions at very low temperatures. fCo(edta)(H20)] is ostensibly analogous to the 7-coordinate Mn and complexes with the same stoichiometry, but in fact the cobalt is only 6-coordinate, 1 of the oxygen atoms of the cdta being too far away from the cobalt (272 compared to 223 pm for the other edta donor atoms) to be considered as coordinated. 

While mononuclear octahedral Ni11 complexes often show relatively broad signals, nuclear relaxation enhancement and sharp signals may be observed in related dimer species 350,351 This has been taken advantage of for a detailed NMR investigation of a series of weakly ferromagnetically spin-coupled dinuclear octahedral Ni11 centers.352... 

Moriya, T. 1960. Anisotropic superexchange interaction and weak ferromagnetism. Physical Review 120 91-98. 

Hematite is paramagnetic above 956 K (Tc). At room temperature it is weakly ferromagnetic and at 260 K (the Morin temperature, Tm), it undergoes a phase transition to an antiferromagnetic state. Particles smaller than about 8 nm display superpara-magnetic relaxation at room temperature. A plot of the dependence of the B f (Hi) of hematite on temperature is shown in Figure 6.7 the plot follows an approximate Brillouin curve. 

Fig. 6.7 Temperature dependence of the magnetic properties of hematite. Tc = Curie temperature,Tm = Morin temperature, pm = paramagnetic region, wfm = weakly ferromagnetic region afm = antiferromagnetic region. The insets show simulated Mossbauer spectra of hematite in the paramagnetic, weakly ferromagnetic and antiferromagnetic states (Murad, 1988, with permission).

Another rutile type fluoride, NiF2 (6P), is also unique in showing weak ferromagnetism below its Ne lpoint (4, 123, 222). As for the interpretation of this behaviour and other questions connected with the spin configurations in antiferromagnetics the reader is referred to reviews of Morija (227, 228) and Bertaut (32). Also some data on antiferromagnetics compiled in tables should be mentioned here (116, 258). 

Bozorth, R. M. Origin of weak ferromagnetism in rare-earth orthoferrites. Phys. Rev. Letters 1, 362 (1958). 

Dzyaloshinsky, I. E. A thermodynamic theory of weak" ferromagnetism of antiferromagnets. J. Phys. Chem. Solids 4, 241 (1958). 

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