This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Spin glasses: Experimental facts, theoretical concepts, and open questions

Haldane predicted that the isotropic quantum Heisenberg spin chain is in a “massive” phase if the spin is integral. The first rigorous example of an isotropic model in such a phase is presented. The Hamiltonian has an exactSO(3) symmetry and is translationally invariant, but we prove the model has a unique ground state, a gap in the spectrum of the Hamiltonian immediately above the ground state and exponential decay of the correlation functions in the ground state. Models in two and higher dimension which are expected to have the same properties are also presented. For these models we construct an exact ground state, and for some of them we prove that the two-point function decays exponentially in this ground state. In all these models exact ground states are constructed by using valence bonds.

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Valence Bond Ground States in Isotropic Quantum Antiferromagnets

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

CMR manganites: physics, thin films and devices

Magnetoelectric phase diagrams have been investigated for rare-earth manganites with orthorhombically distorted perovskite structure, $R{\mathrm{MnO}}_{3}$ ($R=\mathrm{Gd}$, Tb, and Dy). A variety of magnetic and electric phases emerge with varying $R$-site ion, temperature, and magnetic field in these systems. The magnetoelectric phase diagram varies sensitively with the direction of a magnetic field relative to the crystallographic axes. Although the ground state of ${\mathrm{GdMnO}}_{3}$ with the largest ionic radius of $R({r}_{R})$ is not ferroelectric in zero magnetic fields $(H=0)$, a ferroelectric phase with electric polarization $(P)$ along the $a$ axis appears by applying $H(g\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ along the $b$ axis. Both ${\mathrm{TbMnO}}_{3}$ and ${\mathrm{DyMnO}}_{3}$ show a ferroelectric order with $P$ along the $c$ axis even at $H=0$ below a lock-in transition temperature where nonzero wave vectors for magnetic and lattice modulations become nearly constant. These systems also exhibit a flop of the ferroelectric polarization ($P\ensuremath{\Vert}c$ to $P\ensuremath{\Vert}a$) when $H$ is applied along the $a$ or $b$ axis. By contrast, the application of $H$ above $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ along the $c$ axis completely suppresses the ferroelectricity in ${\mathrm{TbMnO}}_{3}$. Possible origins of the observed evolution of magnetoelectric phases are discussed in consideration of magnetism and lattice distortion in the perovskite rare-earth manganites.

Magnetoelectric phase diagrams of orthorhombic R MnO 3 ( R = Gd , Tb, and Dy)

Magnetic susceptibility and inelastic neutron scattering experiments have been performed in the nearly ideal one-dimensional Heisenberg antiferromagnet with spin one, Ni(C2H8N2)2NO2ClO4. The experimental results are consistent with the recent theoretical predictions for a quantum energy gap between the ground state and the first excited states.

http://iopscience.iop.org/article/10.1209/0295-5075/3/8/013/pdf

Presumption for a Quantum Energy Gap in the Quasi-One-Dimensional S?=?1 Heisenberg Antiferromagnet Ni(C2H8N2)2NO2(ClO4)

We present magnetoresistance (MR) measurements performed on two Au/Co/Au sandwiches with ultrathin cobalt layers (0.32 and 0.76 nm) and on a bilayer Au/Co/Au/Co/Au (0.75 nm Co). The easy magnetization axis is shown to be perpendicular to the films, in agreement with previous magnetization and ferromagnetic resonance measurements. The hysteretic behavior is easily observed by measuring the MR, leading to a determination of the coercive fields of a few ${10}^{2}$G. The Co bilayer exhibits a drastic enhancement of the MR effect with respect to the corresponding monolayer ($\ensuremath{\delta}\frac{R}{R=1.3%}$ at 300 K and 3% at 4.2 K) and a square hysteresis loop. Different mechanisms for the enhancement of the MR are proposed.

Enhanced magnetoresistance of ultrathin (Au/Co)n multilayers with perpendicular anisotropy.

Single crystals of ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{MnO}}_{3\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ were prepared with various oxygen vacancies from \ensuremath{\delta}=0 to \ensuremath{\delta}=0.055. The $b$ and $c$ lattice parameters were found to increase with \ensuremath{\delta} whereas the $a$ parameter remains practically constant. The oxygen vacancies are expected to reduce the hole concentration, and therefore the Curie temperature and the electrical conductivity. However, these effects are somewhat different from those observed in the hole-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ systems, owing to different lattice distortion. The lattice change of the sample is illustrated by a strong shift in the hyperfine field of ${\mathrm{Mn}}^{4+}$ ions, detected with the NMR method. The oxygen deficiency and all the above properties resulting from it were shown to be reversible.

Influence of controlled oxygen vacancies on the magnetotransport and magnetostructural phenomena in La 0.85 Sr 0.15 MnO 3-δ single crystals

We report the first direct and comprehensive determination of the energy levels involved in the Haldane gap occurrence: the singlet ground state and the first triplet excited state. Precise values of the parameters are obtained from the analysis of the magneto-optical resonance transitions observed in the far infrared for frequencies ranging from 70 to 1000 GHz and magnetic fields up to 18 T.

Direct observation of the Haldane gap in NENP by far-infrared spectroscopy in high magnetic fields.

We have performed magnetic-susceptibility measurements and Cu nuclear-magnetic-resonance studies on single crystals of the quasi-one-dimensional antiferromagnet CuGe1-xSixO3, with 0 ≤ x ≤ 0.1. The Si doping has a drastic effect on the spin-Peierls temperature TSP which decreases at TSP (x)/TSP (0) 1-50x at low Si concentrations. For x ≥ 0.005, three-dimensional antiferromagnetic order with an easy axis along the Cu chains takes place below a well-defined Neel temperature TN. A qualitative interpretation of the phase diagram (T, x) is given.

https://iopscience.iop.org/article/10.1209/0295-5075/30/8/006/pdf

J.P. Renard

Papers

Presumption for a Quantum Energy Gap in the Quasi-One-Dimensional S?=?1 Heisenberg Antiferromagnet Ni(C2H8N2)2NO2(ClO4)

Enhanced magnetoresistance of ultrathin (Au/Co)n multilayers with perpendicular anisotropy.

Influence of controlled oxygen vacancies on the magnetotransport and magnetostructural phenomena in La 0.85 Sr 0.15 MnO 3-δ single crystals

Direct observation of the Haldane gap in NENP by far-infrared spectroscopy in high magnetic fields.

Competition between Spin-Peierls Phase and Three-Dimensional Antiferromagnetic Order in CuGe1-xSixO3