Atomic short-range order and its relation to the magnetic behavior of noble-metal based spin-glass alloys

We have investigated the spatial distribution of magnetic atoms (Mn or Fe) by determining the short range order parameters for the archetypal spin-glass alloys: $\text{Cu}$Mn, $\text{Au}$Mn, $\text{Au}$Fe and $\text{Pt}$Mn in the dilute limit. All four systems possess atomic short-range order (ASRO) such as to enhance local ferromagnetic regions. In the latter three systems the ASRO is relatively strong and initiates long-range structures which develop with increasing concentration and which eventually destroy the spin-glass state.     

Spin-glass in the spinel-type CuCrTiS4

It is difficult to find a typical compound to understand the spin-glass (SG). Only a few model compounds for the SG have been investigated; Fe_0.50Mn_0.50TiO₃ as a short-range interacting three-dimensional (3D) Ising SG, Dy_0.103Y_0.897Ru₂Si₂ as a long-range 3D Ising SG, and AgMn as a long-range 3D Heisenberg SG. A spinel-type compound CuCrTiS₄ will be provided in this study as a realistic candidate for a short-range interacting 3D Heisenberg SG. This CuCrTiS₄ can cause a SG phase below T _g?=?7.91?K, which is obtained by a random non-magnetic 50% substitution of Ti for Cr on B-sites in the ferromagnetic mother spinel CuCr₂S₄. The localized magnetic moment of CuCrTiS₄ comes from Cr³⁺ with S?=?3/2, where a valence state of Cu⁺Cr³⁺Ti⁴⁺S₄ ^2? is built up. Experimental data will be focused only on the static properties of dc magnetization M(T) under various magnetic fields, without any dynamical magnetic feature. The spin-glass phase transition T _g is well defined at low field, nevertheless hardly in higher fields than approximately 40 Oe. An appearance of the field dependence of M(T) of CuCrTiS₄ is similar to that of Fe_0.50Mn_0.50TiO₃ with the short-range Ising SG, and also these of AgMn and CuMn with the long-range interacting Heisenberg SGs. This general nature in M(T) seems to originate from the common mechanism under magnetic fields for all the spin-glasses.     

High resolution diffraction and small angle scattering neutron investigations of LaCo<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>O<Subscript>3+</Subscript><Subscript>δ</Subscript>: effect of oxygen content

We have investigated LaCo_0.5Mn_0.5O_{3+ δ} compounds with different oxygen content by means of magnetization, high resolution and small-angle neutron diffraction measurements. Oxygen content decrease down to stoichiometric composition leads to an essential increase of T_C and magnetic moment while Co/Mn ionic ordering degree is kept almost constant. It is assumed that upon oxygen reduction Co³⁺ ions change their valence state down to 2+ one that leads to dominating of Co²⁺-Mn⁴⁺ ferromagnetic interactions as well as T_C increase. Magnetic properties can be explained in terms of coexistence of long-range ferromagnetic order and short-range clusters with antiferromagnetic interactions prevailing. Size distribution of the mentioned short-range magnetic inhomogeneities is rather mild within the samples but it is strongly temperature dependent.     

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni₃Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.Tomsk Engineering-Construction Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1992.     

Ab initio molecular-dynamics simulations of short-range order in liquid Al80Mn20 and Al80Ni20 alloys

Atomic structures of liquid Al80Mn20 and Al80Ni20 have been calculated by first-principles molecular-dynamics simulations. For both liquid alloys, the local structure is characterized by a strong Al-TM (transition metal) affinity, which leads to a well-pronounced chemical short-range order. However, we show that the occurrence of magnetic moments localized on Mn atoms plays a key role in determining the short-range arrangement of Mn atoms which is also interpreted on the basis of the local fivefold symmetry.     

Possible quantum critical point in La(2/3)Ca(1/3)Mn(1-x)Ga x O3

We study the magnetic ground state in La(2/3)Ca(1/3)Mn(1-x)Ga x O3 manganites, where a quantum critical point (QCP) has been theoretically predicted. The metallic ferromagnetic ground state for low Ga doping breaks down for x > or = 0.11, an insulating state being established at low temperatures. Long-range ferromagnetism coexists with short-range magnetic correlations in the concentration range 0.11 < or = x < or = 0.145 while only the short-range correlations survive for x > or = 0.16. We discuss the implications of such a QCP to the physics of manganites and compare to other QCP systems.     

Inhomogeneous magnetic state in the electron-doped Sr<Subscript>0.98</Subscript>La<Subscript>0.02</Subscript>MnO<Subscript>3</Subscript> Manganite According to <Superscript>55</Superscript>Mn NMR data

⁵⁵Mn NMR spectra in the magnetically ordered state in Sr_0.98La_0.02MnO₃ manganite have been obtained and the magnetic susceptibility has been measured. It has been shown that the microscopic phase separation into the antiferromagnetic matrix and ferromagnetic clusters, which can be presented as magnetic polarons, is observed in the long-range magnetic order region.     

The influence of magnetic sublattice dilution on magnetic order in CeNiGe3 and UNiSi2

Polycrystalline samples of the Y-diluted antiferromagnet CeNiGe(3) (T(N)?=?5.5?K) and Th-diluted ferromagnet UNiSi(2) (T(C)?=?95?K) were studied by means of x-ray powder diffraction, magnetization and specific heat measurements performed in a wide temperature range. The lattice parameters of the Ce(1-x)Y(x)NiGe(3) alloys decrease linearly with increasing Y content, while the unit cell volume of U(1-x)Th(x)NiSi(2) increases linearly with increasing Th content. The ordering temperatures of the systems decrease monotonically with increasing x down to about 1.2?K in Ce(0.4)Y(0.6)NiGe(3) and 26?K in U(0.3)Th(0.7)NiSi(2), forming a dome of long-range magnetic order on their magnetic phase diagrams. The suppression of the magnetic order is associated with distinct broadening of the anomalies at T(N,C) due to crystallographic disorder being a consequence of the alloying. Below the magnetic percolation threshold x(c) of about 0.68 and 0.75 in the Ce- and U-based alloys, respectively, the long-range magnetic order smoothly evolves into a short-range one, forming a tail on the magnetic phase diagrams. The observed behaviour of Ce(1-x)Y(x)NiGe(3) and U(1-x)Th(x)NiSi(2) is characteristic of diluted magnetic alloys.     

Magnetic ground state and transition of a quantum multiferroic LiCu2O2

Based on resonant soft x-ray magnetic scattering, we report that LiCu2O2 exhibits a large interchain coupling which suppresses quantum fluctuations along spin chains, and a quasi-2D short-range magnetic order prevails at temperatures above the magnetic transition. These observations unravel the fact that the ground state of LiCu2O2 possesses long-range 2D-like incommensurate magnetic order rather than being a gapped spin liquid as expected from the nature of quantum spin-1/2 chains. In addition, the spin coupling along the c axis is found to be essential for inducing electric polarization.     

(Sr,Mn)TiO3: a magnetoelectric multiglass

By close analogy with multiferroic materials with coexisting long-range electric and magnetic orders a "multiglass" scenario of two different glassy states is observed in Sr(0.98)Mn(0.02)TiO(3) ceramics. Sr-site substituted Mn2+ ions are at the origin of both a polar and a spin glass with glass temperatures T(g) approximately equal to 38 K and < or =34 K, respectively. The structural freezing triggers that of the spins, and both glassy systems show individual memory effects. Thanks to strong spin-phonon interaction within the incipient ferroelectric host crystal SrTiO3, large higher order magnetoelectric coupling occurs between both glass systems.     

低掺杂La1-xSrxMnO3结构相变对其电磁特性的影响

高温高压条件下，低掺杂的La_1-xSr_xMnO₃化合物发生从菱方相(R3c)到立方相的转变，该相变使其从低温时的绝缘态(极化子有序)转变为金属态.同时，随温度的降低，局域锰离子的自旋也从铁磁长程有序变成了只有短程序结构的自旋玻璃态.这种转变是由于相变引起的Mn—O—Mn键角及Mn—O键长的改变所引起. 关键词：     

A high temperature X-ray study of ordering in iron-aluminium alloys

The variation with temperature of long-range order in iron-aluminium solid solutions was studied using a high temperature diffractometer, and short-range order was studied at room temperature by measuring diffusely scattered X-rays. Compositions in the range 19–50 at. per cent^† aluminium were investigated. The atomic configurations discovered by Bradley and Jay were largely confirmed, including the structural discontinuity at the stoichiometric Fe₃Al composition. The Fe₃Al-type superlattice exists over the range 23–35 per cent Al approximately, and the order disappears gradually during heating, with no sudden drop at the critical temperature T_c. The degree of order at a given temperature below T_c depends on the thermal history by which that temperature was approached. This anomaly is explained in terms of a competition between directional order, of ferromagnetic origin, and long-range order; this model is also used to interpret certain anomalous magnetic properties at high temperature.

Alloys with less than 23 per cent aluminium possess various kinds of short-range order. An alloy containing 20.5 per cent aluminium has a structure which is unaffected by heat treatment, but all the other alloys investigated are sensitive to heat treatment. A tentative equilibrium diagram is proposed.     

Orientational correlations and order in A3C60

A tight-binding method which has been previously applied to study the effect of uncorrelated orientational disorder on conduction-band properties is extended here to the case of systems with long-range order and/or short-range correlations. The density of states and conductivity are not highly sensitive to the specific short-range correlations, so long as the system is not too close to being fully ordered. Hence the strong effects of disorder found previously appear to be robust and should play an important role in the interpretation of normal-state properties of A₃C₆₀.     

Hyperfine fields and chemical order in Fe-Ge

Films of Fe_0.79Ge_0.21 were synthesized by ion beam sputtering, and were annealed at temperatures from 150 to 500 °C. The as-prepared materials comprised chemically disordered bcc crystallites, but short-range and long-range D0₃ chemical order developed upon annealing. The⁵⁷Fe hyperfine magnetic field distribution can be understood approximately with a model of magnetic response, so the hyperfine magnetic field distribution can be used to follow the changes in chemical short-range ordering. Large local isomer shifts were observed at⁵⁷Fe atoms near Ge atoms.     

Evolution of short-range, mesoscopic, and long-range orders in magnesium-zinc ferrites

The Mössbauer and X-ray diffraction techniques are used to study the evolution of structure and chemical inhomogeneities in magnesium-zinc ferrites. These are the octahedral short-range environment of a magnetic Fe³⁺ ion, mesoscopic quasi-planar regions of size 100–200 Å (clusters) with locally changed chemical composition and crystallographic order, and the general long-range order of the ferrite matrix. In each of these three inhomogeneities, the high-temperature long-term firing of the Mg-Zn ferrite (T=1280°C, τ=0–8 h) induces transitions (magnetic in the short-range order, concentration in the clusters, and structural in the matrix) within the time period τ=1–2 h. Subsequently, the structure stabilizes with the initial three-scale hierarchy retained.     

The variation of electrical properties of Ni3Mn films as a function of the degree of order

The conclusion, based on a comparison of experimental results concerning the resistivity, the saturation magnetization, and temperature studies of the Hall and Nernst — Ettingshausen effects, is made that the d-electrons in the ordered state of the compound Ni₃Mn take part in transport phenomena. In the disordered phase, the magnetic electrons are localized at ionic sites of the-fcc lattice. Research is performed on film samples of Ni₃Mn with various degrees of long-range order which are attained by a variation of the condensation temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 46–51, September, 1975.The authors thank Yu. B. Glukhov for a discussion of the results of this work.     

Short-range order in spin density wave antiferromagnets

The model of the short-range order state for itinerant antiferromagnets with SDW is developed. The thermodynamic transverse spin density fluctuation are shown to influence essentially on the formation of the long-range magnetic order. In the wide temperature range these occurs the shortprange, magnetic order regim. The properties of the low-frequency transverse excitations of the spin density are analized. The existence of a fully diffusive mode at small wave vectors is predicted.     

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S = 12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at T(c) = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.     

Effect of Oxygen Nonstoichiometry on the Magnetic Phase Transitions in Frustrated YBaCo<Subscript>4</Subscript>O<Subscript>7 + <Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 0.1, 0.2) Cobaltites

The structural and elastic characteristics of YBaCo₄O_{7 + x} (x = 0, 0.1, 0.2) cobaltites synthesized by various technologies and having various excess oxygen contents x are experimentally studied. The distortion of the crystal structure in stoichiometric samples is found to remove frustrations and to bring about a longrange magnetic order in the cobalt subsystem, which is accompanied by well-pronounced anomalies in the elastic properties in the temperature range of a magnetic phase transition T_N. At a weak deviation from oxygen stoichiometry, the structure distortion disappears, frustrations are retained, and the further development of a long-range magnetic order is hindered. As a result of an absent long-range magnetic order, the anomalies of the elastic characteristics at T_N smooth rapidly and disappear. This finding points to the suppression of structural and magnetic transitions in nonstoichiometric samples and to the conservation of only short-range correlations of order parameter. It is found that nonstoichiometric samples can be separated into two phases depending on the ceramic synthesis conditions.