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A 2D Heisenberg ferromagnet with exchange J and random magnetic anisotropy of strength ${D}_{R}\ll J$ has been studied. Analytical theory for the dependence of the average size of a pinned skyrmion on the magnetic field H , and for stability of such skyrmions on a lattice, has been developed. It has been complemented by numerical studies of 2D lattices containing up to 40 million spins. At low fields the average size of the skyrmion, λ , is determined by the average size of Imry–Ma domains. On increasing the field the skyrmions first shrink, with $\lambda \propto {D}_{R}/H$ , and then collapse at fields distributed around ${H}_{c}\propto {D}_{R}^{4/3}$ . The concentration of the skyrmions goes down with the field as $\exp [-\tfrac{1}{2}{(H/{H}_{c})}^{3/2}]$ .

The MPS family of layered magnetic materials ( M = Fe, Ni, Mn, etc) shows many unusual properties. We have recently observed time-dependent magnetisation and two magnetic phase transitions in FeNiPS, and here we use neutron diffraction and Mössbauer spectroscopy to explore the magnetic and structural ordering. Neutron diffraction shows that the staggered magnetisation lies closest to the Brillouin curve for J = 1, which is the spin for quenched Ni. In agreement with neutron diffraction, Mössbauer spectroscopy shows a magnetic ordering temperature of ∼120K. It does not show any evidence of a second, low temperature (re)ordering, suggesting that the low temperature transition seen previously is a result of the time dependence of the magnetisation and is not apparent when the sample is given time to relax between measurements. The presence of three magnetic Fe-site environments when four chemical environments (Fe, FeNi, FeNi and Ni) are possible may indicate that the mixture is not random, but shows some local ordering; the neutron results show evidence for a similar conclusion. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of Mn-site doping by Ru has been studied in La1.24Sr1.76Mn2-y Ru y O7. [•] Electrical resistance, magnetoresistance and magnetic properties were measured. [•] Ru substitution enhances the ferromagnetism and metallicity. [•] Results were interpreted by the ferromagnetically coupled Ru with Mn ions in Mn–O–Ru network. [ABSTRACT FROM AUTHOR]

Abstract: is a dipolar-coupled Ising ferromagnet with . If sufficient dilution is introduced into the system by substituting magnetic Ho atoms with nonmagnetic Y, it becomes glassy. Even though theoretical predictions state that the glassy state should persist down to the lowest Ho concentrations, it has been observed that the system runs into what has been called the “antiglass” phase. The nature of the “antiglass” state is still not understood, and not much experimental data is available. We studied this system at the microscopic scale using . Zero field and longitudinal field measurements were performed from room temperature down to 20mK. We found that the Ho spins remain dynamic down to 20mK with a fluctuation rate equal to 2.6MHz in a longitudinal field of 16G. [Copyright &y& Elsevier]

Abstract: The modifications in electrical and magnetic properties of polycrystalline bulk La0.7Ca0.3Mn1−x T x O3 (T=Fe, Ga) samples at relatively higher doping concentration (x=0.08–0.12) are investigated. All the synthesized, single phase samples were subjected to resistivity measurements in the temperature range 50–300K. No insulator–metal transition (T P) was observed for Fe doped samples with x=0.12. For all the other samples the transition temperature decreased with increase in doping concentration. The small polaron hoping energy was found to increase, rather slowly, with increase in doping concentration. The effect on magnetic properties is also prominently observed with respect to doping element and doping concentration. Interestingly, with the increase in doping concentration, the Curie temperature (T C) and T P separate out significantly indicating decoupling of electric and magnetic properties. Changes in these properties have been analyzed on the basis of magnetic disorder introduced in the system due to the magnetic and nonmagnetic nature of these ions rather than strong lattice effects which is insignificant due to similar ionic radii of Fe+3 and Ga+3 when compared to that of Mn+3. [Copyright &y& Elsevier]

We have fabricated new perovskite multiferroic PbFe $_{1/2}$ Sb $_{1/2}$ O _3 with a high degree (up to 0.9) of chemical ordering and unexpectedly high-temperature magnetic relaxor properties, which can barely be described within concepts of conventional spin glass physics. Notably, we found that the field-temperature phase diagram of this material, in the extremely wide temperature interval, contains the de Almeida–Thouless-type critical line, which has been the subject of long debates regarding its possible experimental realization. We explain our findings by the creation, at high temperatures of not less than 250 K, of giant superspins (SSs), owing, curiously enough, to the antiferromagnetic superexchange interaction. We show that these SSs are capable of strong high-temperature magnetic relaxation in the relaxor phase, down to about 150 K, where they transform into a SS glass phase. On further cooling, the material experiences another striking transition, this time, into an ordinary (single-spin) antiferromagnetic phase. We comprehensively analyze the above complex physical picture in terms of three complimentary theoretical approaches. Namely, the ab initio calculations elucidate the microscopic mechanism of giant SS formation, the high-temperature expansion accounts for the morphology of these clusters, and the random field approach provides the description of disorder-related characteristics.

Abstract: Magnetization measurements were performed on metal polycarbosilazane complexes polymers doped with Fe(II) and Fe(III)-ions. Zero field cooled (ZFC) and field cooled (FC) magnetization for Fe(II) doped polymer were found to bifurcates at a freezing temperature T g, showing typical spin glass behavior but at much higher fields (∼kOe) than what is commonly observed in metallic spin glass(∼Oe). However, the ZFC magnetization of Fe(III) doped polymer did not reveal any spin-glass behavior. The magnetization curves showed very little Hysteresis effects and saturate at about 40kOe at 2K then drops sharply at 20K. The saturation magnetization continues to drop gradually up to room temperature ∼300K. [Copyright &y& Elsevier]