In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries

A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions-spin, charge, lattice, and/or orbital-are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior.

http://science.sciencemag.org/content/sci/309/5732/257.full.pdf

Complexity in Strongly Correlated Electronic Systems

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

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Electronic Transport In Mesoscopic Systems

Results of a detailed investigation of the structure and electron-transport properties of $La_{1-x}A_xMnO_3$ (A =Ca, Sr) over a wide range of compositions are presented along with those of parent $LaMnO_3$ containing different percentages of $Mn^{4+}$. The electrical resistivity $(\rho)$ and magnetoresistance (MR) of polycrystalline pellets have been measured in the 4.2–400 K range in magnetic fields up to 6 T and the Seebeck coefficient (S) from 100 to 400 K. The electrical measurements were supplemented by ac susceptibility and magnetization measurements. MR is large and negative over a substantial range of compositions and peaks around temperatures close to the ferromagnetic transition temperatures $(T_c)$. An insulator to metal-like transition occurs near the $T_c$ and the temperature dependence of $\rho$ below $T_c$ is related to the magnetization although $\rho$ in the metallic state is generally much larger than the Mott’s maximum metallic resistivity. The occurrence of giant magnetoresistance is linked to the presence of an optimal proportion of $Mn^{4+}$ ions and is found in the rhombohedral and the cubic structures where the Mn-O distance is less than 1.97 \AA and the Mn-O-Mn angle is $170^o\pm10^o$. The field dependence of MR shows the presence of two distinct regimes. The thermopower S shows a positive peak in the composition range at a temperature where MR also peaks; S becomes more negative with increase in $Mn^{4+}$.

Structure, electron-transport properties, and giant magnetoresistance of hole-doped LaMnO 3 systems

Effect of particle size on the electron transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated. While the ferromagnetic Tc, low field magnetic susceptibility, and insulator‐metal transition are markedly affected by the particle size, the maximum magnetoresistance exhibited by the samples near Tc is not sensitive to the particle size. However, the magnetoresistance at 4.2 K increases with decrease in particle size, suggesting a substantial contribution by the grain boundaries. Preliminary measurements on La0.7Sr0.3MnO3 samples of different particle sizes also corroborate the above conclusions.

Effect of particle size on the giant magnetoresistance of La0.7Ca0.3MnO3

Giant Magnetoresistance in Bulk Samples of La1-xAxMnO3 (A = Sr or Ca)

We have investigated the magnetoresistance MR! of the perovskite oxide $La_ {1-x}Sr_xCoO_3$ for $0{\leq}x{\leq}0.4$ which shows a spin-state transition from low-spin $Co^{III}$ to high-spin $Co^{3+}$ as a function of temperature. In the metallic compositions $(x\geq0.25)$ appreciable MR occurs only near the ferromagnetic Curie temperature. For the most metallic composition x=0.4, there is a small positive contribution to the MR near the $T_c$ . In the insulating samples $(x\leq0.2)$ the MR shows a large hysteresis which depends on the temperature. The value of the MR is negative and large in the insulating compositions and shows a maximum near the temperature where magnetic studies showed a spin-glass-like transition. We also find a strong role of the spin-state transition of the $Co^{3+}$ ions in the electronic transport leading to a characteristic behavior of the MR in the $Co^{3+}$-rich insulating samples.

Magnetoresistance of the spin-state-transition compound La1-xSrxCoO3.

We report a detailed study of steplike metamagnetic transitions in polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset below a critical temperature, are extremely sharp (width <2x10(-4) T), and occur at critical fields which are linearly dependent on the absolute value of the cooling field in which the sample is prepared. Similar transitions are also observed at low temperature in non-Co doped manganites, including single crystal samples. These data show that the steps are an intrinsic property, qualitatively different from either previously observed higher temperature metamagnetic transitions in the manganites or metamagnetic transitions observed in other materials.

https://arxiv.org/pdf/cond-mat/0211337.pdf

Ramanathan Mahendiran

Papers

Structure, electron-transport properties, and giant magnetoresistance of hole-doped LaMnO 3 systems

Effect of particle size on the giant magnetoresistance of La0.7Ca0.3MnO3

Giant Magnetoresistance in Bulk Samples of La1-xAxMnO3 (A = Sr or Ca)

Magnetoresistance of the spin-state-transition compound La1-xSrxCoO3.

Ultrasharp magnetization steps in perovskite manganites.