The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe${}_{1-x}$Co${}_x$Ga${}_3$ was studied by means of ab initio band-structure calculations and nuclear spin-lattice relaxation $1/T_1$ of the ${}^{69,71}$Ga nuclei. The ${}^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of 2–300 K for the concentrations $x=0.0$, 0.5, and 1.0. In the parent semiconducting compound FeGa${}_3$, the temperature dependence of the ${}^{69}(1/T_1)$ exhibits a huge maximum at about $T\sim 6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa${}_3$, demonstrates a metallic Korringa behavior with $1/T_1\propto T$. In Fe${}_{0.5}$Co${}_{0.5}$Ga${}_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1\propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with ab initio band-structure calculations, where a ferromagnetic state with an ordered moment of 0.5 ${\mu }_B$/f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.

The spatial spin-modulated structure in BiFeO₃ is retained and the anharmonicity parameter decreases with crystallite size decreasing, followed by a change in the anisotropy type from an “easy axis” to an “easy plane” at about 50 nm.

BaV${}_{\text{3}}$O${}_{\text{8}}$ contains magnetic V${}^{\text{4+}}$($S=1/2$) ions and also nonmagnetic V${}^{\text{5+}}$($S=0$) ions. The V${}^{4+}$ ions are arranged in a coupled Majumdar-Ghosh chainlike network. A Curie-Weiss fit of our magnetic susceptibility $\chi \left(T\right)$ data in the temperature region of $80–300$ K yields a Curie constant $C=0.39$ cm${}^{\text{3}}$K/mole V${}^{\text{4+}}$ and an antiferromagnetic Weiss temperature $\theta =-26\mathrm{K}$. The $\chi$(T) curve shows a broad maximum at $T\simeq 25$ K indicative of short-range order (SRO) and an anomaly corresponding to long-range order (LRO) at $T_{\mathrm{N}}$ $\sim 6$ K. The value of the “frustration parameter” ($f=|\theta /T_{\text{N}}|\sim 5$) suggests that the system is moderately frustrated. Above the LRO temperature, the experimental magnetic susceptibility data match well with the coupled Majumdar-Ghosh (or $J_{\mathrm{nn}}-J_{\mathrm{nnn}}$ Heisenberg) chain model with the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic coupling $\alpha =2$ and $J_{\mathrm{nnn}}/k_B=40$ K. In a mean-field approach when considering the interchain interactions, we obtain the total interchain coupling to be about 16 K. The LRO anomaly at $T_{\mathrm{N}}$ is also observed in the specific heat $C_{\mathrm{P}}\left(T\right)$ data and is not sensitive to an applied magnetic field up to 90 kOe. A ${}^{\text{51}}$V NMR signal corresponding to the nonmagnetic vanadium was observed. Anomalies at 6 K were observed in the variation with temperature of the ${}^{51}$V NMR linewidth and the spin-lattice relaxation rate $1/T_1$ indicating that they are sensitive to the LRO onset and fluctuations at the magnetic V sites. The existence of two components (one short and another long) is observed in the spin-spin relaxation rate $1/T_2$ data in the vicinity of $T_{\mathrm{N}}$. The shorter component seems to be intimately connected with the magnetically ordered state. We suggest that both magnetically ordered and nonlong-range-ordered (non-LRO) regions coexist in this compound below the long-range-ordering temperature.