A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

http://science.sciencemag.org/content/sci/264/5157/413.full.pdf

Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films

Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Why Are There so Few Magnetic Ferroelectrics

http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

A technique for reduction of the Dirac equation, which initially omits the spin-orbit interaction (thus keeping spin as a good quantum number), but retains all other relativistic kinematic effects such as mass-velocity, Darwin, and higher order terms is presented. The spin-orbit interaction can be included as a perturbation once the 'relativistic' spin-polarised bands and wavefunctions have been obtained. The technique is used together with the local spin density approximation for exchange and correlation to calculate the self-consistent charge and spin density of a neutral Gd atom. The calculated magnetic form factor agrees extremely well with experiment. Comparison with a paramagnetic RAPW calculation shows the procedure should be accurate and fast for general band structure determinations.

https://iopscience.iop.org/article/10.1088/0022-3719/10/16/019/pdf

A technique for relativistic spin-polarised calculations

Diffraction experiments with polarized neutrons have been carried out on a Tm${\mathrm{Co}}_{2}$ single crystal, at various temperatures, in a field of 57.2 kOe applied parallel to the [$01\overline{1}$] direction. The localized moments on Tm and Co sites have been determined at 4.2 and 100 K from the magnetic densities obtained by Fourier projections. The Tm moment ($5.4{\ensuremath{\mu}}_{B}$ at 4.2 K) is reduced by crystal-field effects. A $3d$-type Co moment is coupled antiparallel to the Tm one. The magnetic scattering amplitudes measured for the reflections to which the $4f$ Tm moment does not contribute give direct evidence for the existence of a diffuse magnetic density. This diffuse density is oscillatory with distance and remains similar at all temperatures. The $4f$ Tm and the $3d$ Co moments have been evaluated at eight temperatures from 4.2 to 250 K. The Co susceptibility above 25 K is temperature independent. The Co behaves as a Pauli paramagnet under the action of two opposite fields: the molecular field due to Tm atoms and the applied field. A collective-electron model is therefore appropriate for the description of paramagnetic susceptibility. At 4.2 K a large increase of the Co susceptibility in the molecular field leads to a value of the Co moment of $0.8{\ensuremath{\mu}}_{B}$. This result can be related to the increase of the susceptibility observed in Y${\mathrm{Co}}_{2}$ in high applied fields and to the first-order ferri-paramagnetic transition of some $R{\mathrm{Co}}_{2}$ compounds, where $R$ stands for rare-earth metals.

Polarized-neutron study of TmCo2

Neutron diffraction measurements were made on polycrystalline samples of a 75% Pd---25% Mn alloy. It was found that long-range positional order could be obtained by appropriate heat treatment and that this order was of the one-dimensional antiphase domain type with a period of 4 cubic unit cells. Below 170\ifmmode^\circ\else\textdegree\fi{}K antiferromagnetic reflections were observed for this ordered alloy. A magnetic structure is proposed which satisfactorily accounts for the observed magnetic reflection intensities. On the basis of this structure the atomic magnetic moments are $(4.0\ifmmode\pm\else\textpm\fi{}0.2){\ensuremath{\mu}}_{B}/\mathrm{Mn}$ and $(0.2\ifmmode\pm\else\textpm\fi{}0.1){\ensuremath{\mu}}_{B}/\mathrm{Pd}$.

Antiferromagnetism of the Antiphase Domain Structure of Pd 3 Mn

The antiferromagnetic structure of MnBr/sub 2/ (T/sub N/ = 2.16 deg K) has been determined by single-crystal neutron diffraction measurements nt temperatures down to 1.35 deg K and with magnetic fields (0-- 13 kilo-oersteds) applied to the sample. The antiferromsgnetic structure appeared to have hexagonal symmetry in the absence of a magnetic field but this was found to be associated with a structure domain-growth property. The true magnetic structure was found to be associated with one or another of three hexsgonal axes, the particular growth direction being determined by the direction of an applied magnetic field. Information relating to indirect magnetic exchange via the bromine ions in this crystal was obtained from a study of the domain growth properties. The antiferromagnetic ordering transition in this compound was found to be nearly first order rather than of the usual second order type. A small lowering (~2%) of T/sub N/ was observed for an applied field of 13.1 kolooersteds. Short-range order data were obtained from powder patterns. (auth)

NEUTRON DIFFRACTION STUDY OF THE MAGNETIC PROPERTIES OF MnBr$sub 2$

The coherent neutron scattering cross section of Ho as determined from neutron diffraction measurements on ${\mathrm{Ho}}_{2}$${\mathrm{O}}_{3}$ is found to be (9.1\ifmmode\pm\else\textpm\fi{}0.5) b with amplitude of positive phase. The total nuclear scattering cross section is estimated to be about 13 b. The magnitude and angular dependence of the differential cross section for paramagnetic scattering by the trivalent holmium ion in ${\mathrm{Ho}}_{2}$${\mathrm{O}}_{3}$ have been measured at room temperature. For small scattering angles the observed cross section is equal, within experimental error, to that calculated for the free-ion moment, i.e., 10.6 Bohr magnetons, and the angular dependence of the differential scattering cross section reflects the full contribution of the orbital as well as of the spin contribution to the total moment of the ion.

W. C. Koehler

Papers

Polarized-neutron study of TmCo2

Antiferromagnetism of the Antiphase Domain Structure of Pd 3 Mn

NEUTRON DIFFRACTION STUDY OF THE MAGNETIC PROPERTIES OF MnBr$sub 2$

Paramagnetic and Nuclear Scattering Cross Sections of Holmium Sesquioxide

Neutron Diffraction Investigation of the Magnetic Order in MnI 2