The low-field remanent magnetization of the disordered antiferromagnets K2Fe1-xInxCl5.H2O and K2Fe(Cl1-xBrx)5.H2O

Abstract

Magnetization measurements on single crystals of the site-diluted easy-axis antiferromagnet K2Fe1-xInxCl5.H2O were carried out at very low magnetic fields applied along the easy axis. The data reveal that a remanent magnetization Mr develops below the Neel temperature TN. This remanent magnetization is parallel or nearly parallel to the easy axis. For all concentrations x studied, 0.03<or=x<or=0.14, the sign and magnitude of Mr(T) in a given sample and for a given temperature are governed only by the axial field (Haxial) present when cooling through TN. The remanent magnetization is observed in fields as low as 10-3 Oe. For a given Haxial, Mr increases with decreasing T. At a given T, Mr versus Haxial is very nearly saturated even at approximately 1 Oe. The magnitude of the saturated remanent moment increases with x in the range of x studied. The normalized remanent magnetization Mr(t)/Mr(0), where t=T/TN is the reduced temperature, follows a universal curve, i.e., it is independent of Haxial for a given sample (in fields up to several oersted), and it is also the same for all K2Fe1-xInxCl5.H2O samples. Moreover, the r dependence of Mr(t)/Mr(0) in K2Fe1-xInxCl5.H2O is the same as in Mn1-xZnxF2. Values for the effective critical exponent beta r, obtained from the temperature dependence of Mr close to TN, are close to 0.4. (The extrapolated values at TN are between 0.35 and 0.39.) Temperature cycles following cooling through TN in zero field, and isothermal field cycles below TN, suggest that a domain-like structure is obtained when the sample is cooled in zero field through TN. Magnetization measurements were also carried out on the random-bond system K2Fe(Cl1-xBrx)5.H2O with X equivalent to 0.25. A remanent moment also develops in this case, but it is two orders of magnitude smaller than in the site-diluted system. The temperature dependence of Mr is also distinct in this case: with decreasing T, Mr first increases but then decreases. The mechanism that causes the low-field remanent magnetization is yet to be fully identified. The effect cannot be attributed to random fields (i.e., the excess magnetization of random-field-induced domains) because Mr saturates at very low fields. Domains that exist even in the absence of random fields are a likely source for the low-field remanent magnetization, but the details are still unclear. An explanation based on the volume effect, due to the statistical imbalance between the numbers of up and down spins in each of the domains, which exist even in the absence of random fields, seems to fail. Domain-wall magnetization, and domain magnetization due to the piezomagnetic effect, are other possibilities that remain to be explored. Domain formation may be facilitated by non-magnetic cations that act as vacancies in the magnetic lattice; this may be the reason why Mr in K2Fe1-xInxCl5.H2O is much larger than that in K2Fe1-xInxCl5.H2O.