Long-range Antiferromagnetic State Research Articles

High entropy effect on double exchange interaction and charge ordering in half doped Nd0.5Sr0.5MnO3 manganite

The entropy stabilized oxides i.e., the so-called high-entropy oxides are of great interest in recent time due to their intriguing physical properties. This new category of advanced materials is stabilized by the high configurational entropy of randomly mixed elements, where a specific lattice site is populated by five or more elements. The present study is devoted to explore the high entropy effect in well-known charge ordered manganite Nd0.5Sr0.5MnO3 where a delicate balance exists between the ferromagnetic double exchange and the antiferromagnetic superexchange interactions. Powder X-ray diffraction analyses confirm phase purity of the samples. FE-SEM imaging revealed the microstructure and elemental mapping confirmed the chemical homogeneity of the samples as well as nominal cationic composition. Bond valence sum (BVS) calculation using refinement parameters supports the expected cationic valence in all the samples. Replacement of Nd3+ by five trivalent cations as in (La0.1Pr0.1Nd0.1Gd0.1Bi0.1)Sr0.5MnO3 with larger average A-site ionic radius,<rA>and smaller size variance, σ2 compared with Nd0.5Sr0.5MnO3 dramatically suppressed the ferromagnetic double exchange interaction. This observation is in contrast with the expected variation in<rA>and σ2. In (La0.1Nd0.1Gd0.1Bi0.1Y0.1)Sr0.5MnO3 with smaller<rA> and marginally higher σ2 the ferromagnetic state ceases to exist. In both the multicationic compositions, a long-range antiferromagnetic state is evolved around 200 K with the retention of charge ordering. In the high entropy (La1/6Nd1/6Dy1/6Ca1/6Ba1/6Sr1/6)MnO3 composition with nominal<rA> and larger σ2 values the high temperature long-range ordering is completely suppressed and a new antiferromagnetic state is evolved at 45 K without manifestation of any charge ordering. This composition also avoids magnetic frustration unlike the dopant variant with larger σ2. The present investigation thus highlights the role of local lattice distortion or high entropy effect apart from the<rA>and σ2 in manganites.

Magnetic phase diagrams and large magnetocaloric effects of the two-dimensional antiferromagnetic triangular lattice of Gd3+ ions in KBaGd(BO3)2

We report a detailed study of the magnetic properties of KBaGd(BO$_3$)$_2$, in which magnetic Gd$^{3+}$ ($S=7/2$) ions form into two-dimensional triangular layers. Magnetization, specific heat and magnetocaloric effect (MCE) measurements have been performed on KBaGd(BO$_3$)$_2$ single crystals. The results show that a long-range antiferromagnetic state is established below $T_{\rm N}=0.24$ K. In zero fields, only about half of the full entropy is released at $T_{\rm N}$, indicating that not all the magnetic moments are frozen below the ordering temperature, as expected from the geometrical frustration of the triangular spin lattice. Further studies under external fields were performed down to 50 mK, and the magnetic phase diagrams are established with magnetic fields applied both within and perpendicular to the triangular plane. KBaGd(BO$_3$)$_2$ serves as an example of a two-dimensional triangular lattice with large spin values ($S=7/2$) and can be directly compared with the iso-structure KBaR(BO$_3$)$_2$ (R = Dy-Yb) family of doublet ground states, which exhibit effective spins of $S=1/2$.

Magnetic, thermal, and magnetocaloric properties of the holmium trialuminide HoAl3 with polytypic phases

We investigate the magnetic, thermal, and magnetocaloric properties of the intermetallic HoAl3 compounds with two different crystal structures. The room-temperature equilibrium trigonal phase HoAl3 undergoes an antiferromagnetic (AFM) transition at the Néel temperature TNtri= 9.8 K. The AFM order of the compound is strong against the magnetic field, so that inverse magnetocaloric effect (MCE) is found even under a magnetic field change of 0–50 kOe. The high-pressure cubic phase HoAl3, prepared by a rapid-solidification process, is antiferromagnetically ordered below TNcub= 15 K. Magnetic and specific heat measurements reveal that this long-range AFM state becomes unstable as the temperature drops and then is replaced by a magnetic glassy state below a spin freezing temperature Tf= 11 K. The successive changes in magnetic state result in complicated temperature and field dependence of the MCE at low temperatures.

Combined effect of magnetic field and hydrostatic pressure on the phase transitions exhibited by Ni-Mn-In metamagnetic shape memory alloy

We present a systematic study of the magnetostructural and magnetic transitions in the prototype metamagnetic shape memory alloy Ni50Mn34.5In15.5 under hydrostatic pressure and combined pressure and magnetic field. Pressure extends the area of stability of the antiferromagnetic martensitic phase. At low magnetic field the pressure derivatives of the Curie temperatures of austenite, TCA, and martensite, TCM, show opposite signs. This fact is described in the framework of the Landau thermodynamic model as arising from a weak long-range antiferromagnetic state of martensite. Two volume magnetoelastic constants were estimated using the experimental values of the pressure derivatives of TCA and TCM. A correlation between the signs of the pressure shifts of TCA, and TCM and the distance between Mn-Mn nearest neighbours is established, which matches the empirical Castelliz-Kanomata diagram. The entropy change at martensitic transformation (MT), ΔSMT, grows up when the MT temperature, TM, is approaching TCA under the influence of pressure, but under constant non-zero pressure this dependence is inverse.

Long-Range Order in the Dipolar XY Antiferromagnet Er_{2}Sn_{2}O_{7}.

Er_{2}Sn_{2}O_{7} remains a puzzling case among the extensively studied frustrated compounds of the rare-earth pyrochlore family. Indeed, while a first-order transition towards a long-range antiferromagnetic state with the so-called Palmer-Chalker structure is theoretically predicted, it has not yet been observed, leaving the issue as to whether it is a spin-liquid candidate open. We report on neutron scattering and magnetization measurements which evidence a second-order transition towards this Palmer-Chalker ordered state around 108mK. Extreme care was taken to ensure a proper thermalization of the sample, which has proved to be crucial to successfully observe the magnetic Bragg peaks. At the transition, a gap opens in the excitations, superimposed on a strong quasielastic signal. The exchange parameters, refined from a spin-wave analysis in applied magnetic field, confirm that Er_{2}Sn_{2}O_{7} is a realization of the dipolar XY pyrochlore antiferromagnet. The proximity of competing phases and the strong XY anisotropy of the Er^{3+} magnetic moment might be at the origin of enhanced fluctuations, leading to the unexpected nature of the transition, the low ordering temperature, and the observed multiscale dynamics.

Magnetic Structures of LiMBO3 (M = Mn, Fe, Co) Lithiated Transition Metal Borates

The magnetic ordering within LiMBO3 compounds (M = Mn, Fe, and Co) has been explored by magnetization measurements and neutron powder diffraction. For all M, an incommensurately ordered magnetic phase is established on cooling, followed by a change to a commensurate long-range antiferromagnetic state below TN2 = 12(1) K for LiMnBO3, TN2 = 25(1) K for LiFeBO3, and TN2 = 12(1) K for LiCoBO3. For LiMnBO3, the magnetic ordering at T = 2 K exhibits a propagation vector k = (1, 0, 0) and consists of antiferromagnetic chains that are coupled antiferromagnetically to each other, the magnetic moments being oriented along the [001] direction. In contrast, the magnetic order at T = 2 K in LiFeBO3 and LiCoBO3 exhibits a propagation vector of k = (1/2, 1/2, 1/2) and consists of ferromagnetic chains that are antiferromagnetically coupled. The magnetic moments lie roughly along the [023] direction within the bc plane for LiFeBO3, and along the [301] direction within the ac plane for LiCoBO3. The moment orientations in both LiMnBO3 and LiFeBO3 suggest an Ising character arising from unquenched orbital momentum due to unusual trigonal bipyrimidal coordination environments. No evidence of Ising behavior is found in the case of LiCoBO3.

Spin dynamics of (Pr0.5-xCex)Ca0.5MnO3 (x = 0.05, 0.10, and 0.20) system studied by muon spin relaxation

Muon spin relaxation (μSR) measurements were performed on the (Pr0.5-xCex)Ca0.5MnO3 (x = 0.05, 0.10, and 0.20) manganite system to study the influence of Ce substitution on the spin dynamics. A long-range antiferromagnetic state at low Ce substitution levels (x = 0.05) transforms to a spin-glass state by a marginal increase in x to 0.10. The manganite with x = 0.05 shows simple exponential relaxation down to low temperatures, whereas the manganites with x = 0.10 and 0.20 show a coexistence of two distinct relaxation mechanisms below spin glass-like transition temperature (TG). The μSR data for x = 0.10 and 0.20 provide evidence for the existence of a component with a root-exponential relaxation below TG, suggesting a nondiffusive relaxation mechanism similar to that in a magnetic glass. Above TG, the relaxation follows a stretched exponential function with distributed time-scales up to ∼150 K. Although similar types of relaxation dynamics exist in these two manganites (x = 0.10 and 0.20), the temperature dependent behavior slightly differ. In the light of the present results, we construct the phase-diagram of the (Pr0.5-xCex)Ca0.5MnO3 manganite system which encompasses different structural and magnetic correlations evolving as a function of Ce substitution.

Neutron Scattering Study of the Long-Range Ordered State in CeRu2Al10

Elastic and inelastic neutron scattering measurements have been performed on powder and single-crystal samples of orthorhombic CeRu 2 Al 10 . The order forming below T 0 = 27 K was identified as a long-range antiferromagnetic state with the wave vector k = (1,0,0). The magnetic spectral response in the ordered phase, measured on powder, is characterized by a spin gap and a pronounced peak at 8 meV, whose Q dependence suggests a magnetic origin. Both features are suppressed when temperature is raised to T 0 , and a conventional relaxational behavior is observed at 40 K. This peculiar spin dynamics is discussed in connection with recent magnetization results for the same compound.

Magnetic Properties and Magnetic Structures of Cu3(OD)4XO4, X = Se or S: Cycloidal versus Collinear Antiferromagnetic Structure

We report a comparative study of the magnetic properties of synthetic Cu3(OH)4(SO4)x(SeO4)1-x and the magnetic structures of the parent compounds. All compounds are isostructural and belong to the orthorhombic class of parent compounds. They consist of 3-legged ribbons of edge-sharing copper octahedra connected by micro3-OH and XO4 (X=S or Se). XO4 acts both as one-atom and three-atom bridges to connect seven Cu atoms (six Cu(2) and one Cu(1)) belonging to three neighboring ribbons. The two end members behave as low-dimensional AF with a long-range antiferromagnetic state below 5 (X=S) and 8 K (X=Se); the former shows evidence of a canting. Analyses of the neutron powder diffraction data for X=S were shown to display an ordered magnetic state (k=0 0 0) where the moments of Cu(2) within the two outer legs are collinear and parallel within each leg but antiparallel from each other; the orientation of the moments of Cu(2) is the c axis. In contrast, for X=Se k=approximately 1/7 0 0 and the magnetic structure is cycloidal and transforms progressively from being incommensurate (T>3 K) to commensurate (T<or=3 K). The moments of Cu(2) of each leg are oriented antiparallel as for X=S, but they rotate about the b axis while propagating along the a axis. In both cases the moments of Cu(1) of the inner leg remain random. The magnetic entropy of 15.2+/-1 J/kmol for the end members, estimated from integrating the heat capacity/temperature, is close to that expected (3R ln 2) for three Cu2+ (S=1/2).

Synthesis and magnetic properties of the quasi-one-dimensional compound Ca 0.83(Cu 1− xCo x)O 2

A new solid solution of the quasi-one-dimensional composite crystal, Ca 0.83( Cu 1−x Co x) O 2 (0⩽x⩽0.30) , has been synthesized under 1 atm of O 2 at 830°C. The non-doped compound Ca 0.83CuO 2 consists of two interpenetrating monoclinic subsystems of the [Ca] atoms and the edge-shared square planar [CuO 2] chains. Upon increasing x, both the subsystems undergo a phase change from monoclinic to orthorhombic ( M– O). The M– O change occurs at x∼0.04 for the [(Cu,Co)O 2] subsystem, while such a change occurs at x∼0.17 for the [Ca] subsystem. Magnetic susceptibility measurements show an evolution from a short-range ordered state near x=0 to a long-range antiferromagnetic state (T N =10–15 K) for the samples with x⩾0.15. The effective magnetic moment μ eff is found to increase with increasing x from 2.1 μ B for x=0.10 to 3.0 μ B for x=0.30, suggesting that the solid solution can be regarded as Ca 0.83[Cu 0.66 2+Cu 0.34− x 3+Co x 3+]O 2, in which a mixed state of Cu 2+( S=1/2), Cu 3+( S=0) and high-spin Co 3+( S=2) ions is realized.

Transport, magnetic, specific heat, internal friction, and shear modulus in the charge ordered La0.25Ca0.75MnO3 manganite

The resistivity, magnetization, specific heat, shear modulus, and internal friction in the charge-ordered (CO) La0.25Ca0.75MnO3 were studied. The electronic conduction in the CO state was found to obey Mott's variable-range hopping model with a change in the localization length ξ near TN. A large decrease of the magnetization was observed between the Néer temperature TN and the charge ordering transition temperature TCO, which was attributed to the consequence of the orbital ordering. Drastic stiffening in shear modulus and an internal friction peak appeared just below TCO, and were explained as due to the coupling between the lattice strain and the orbital degree of freedom. A small upturn of the magnetization and a small softening of the ultrasound near TCA≈42 K suggest the evolution of spin canting from the matrix of long-range antiferromagnetic state.

Long-range and short-range magnetic order in NaVGe 2O 6

The metaloxide pyroxene compound NaVGe 2O 6 containing isolated edge-sharing VO 6 ( S=1) chains undergoes a transition into a long-range antiferromagnetic state at T N=16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T M=26 K indicates the low-dimensional character of magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Neel temperature.

Long-range and short-range magnetic order in new compound NaVGe2O6

New metal oxide pyroxene compound NaVGe2O6 containing isolated edge-sharing VO6 (S=1) chains undergoes transition into a long-range antiferromagnetic state at TN=16 K. The broad maximum in the temperature dependence of magnetic susceptibility at TM=26 K indicates the low-dimensional character of the magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Neel temperature.

Electron transport and magnetic studies of Cu100-xMnx binary alloys.

A detailed study of Mn-rich \ensuremath{\gamma}-phase ${\mathrm{Cu}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ with Mn concentration up to 83 at. % has been carried with use of ac-susceptibility, dc-magnetization, and electrical-resistivity measurements. All the concentrated alloys have shown the signature of the spin-glass phase below certain temperatures with antiferromagnetic short-range order. The alloys with more than 73 at. % of Mn have long-range antiferromagnetism at higher temperatures. An earlier neutron diffraction study has already proved the existence of long-range antiferromagnetism at low temperatures for alloys with Mn above 72 at. %. Thus we conclude that ${\mathrm{Cu}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ is a spin glass with short-range antiferromagnetically coupled clusters up to 73 at. % Mn and beyond that it exists in a mixed spin-glass and long-range antiferromagnetic state below certain temperatures. This may be called a reentrant antiferromagnetism since the high-temperature phase is a pure long-range antiferromagnetic phase. We have constructed the magnetic phase diagram for the whole range of composition on the basis of our magnetic study and the earlier neutron diffraction measurements. We have found a ${\mathit{T}}^{2\mathrm{\ensuremath{-}}}$${\mathit{T}}^{5/2}$-type of magnetic contribution to the electrical resistivity in the spin-glass regime for all the alloys. We propose a scheme to isolate the magnetic contribution to the resistivity for all temperatures and apply it to a nondilute system.