Spinels (AB(2)O(4)) with magnetic ions occupying only the octahedral B sites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we ...report on the magnetic properties of the tetragonal spinel Zn0.8Cu0.2FeMnO4, the tetragonality resulting from the Jahn-Teller active Mn3+ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn-0.8(2+) Cu-0.2(2+)) A Fe-0.4(2+) Fe-0.6(3+) Mn3+(B)O4-delta. Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (chi' and chi''), dc susceptibility (chi), heat capacity C-p, and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of chi vs. T fits the Curie-Weiss law: chi = C/(T - theta) from T = 250 K to 400 K with theta similar or equal to 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J/k(B) = 17 K, and C = 3.29 emu K mol(-1) Oe(-1) yielding an effective magnetic moment mu(eff) = 5.13 mu B resulting from the high-spin states of Cu2+ (A site) and Fe2+ (B site), while the B site trivalent ions Mn3+ and Fe3+ are in their low-spin states. The extrapolated saturation magnetization obtained from the M vs. H data at T = 2 K is explained using the spin arrangement (Cu2+down arrow)(A)Fe2+down arrow, Fe3+down arrow, Mn3+down arrow(B) leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(chi T)/ dT shows the onset of ferrimagnetism below similar to 100 K and peaks near 47 K and 24 K. The relaxation time t obtained from temperature and frequency dependence of chi'' when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperature T-SG (H) follows the equation: T-SG (H) = T-SG (0) 1- AH(2/phi) with T-SG (0) = 46.6 K, A = 8.6 x 10(-3) Oe(-0.593) and phi = 3.37. The temperature dependence of hysteresis loops yields coercivity H-C similar to 3.8 kOe at 2 K without exchange-bias, but HC decreases with increase in T becoming zero above 24 K, the T-SG(H) for H = 800 Oe. Variations of C-p vs. T from 2 K to 200 K in H = 0 and H = 90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. For T < 9 K, Cp varies as T-2; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetization MTRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn0.8Cu0.2FeMnO4, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state at T-SG(0) = 46.6 K, followed by SL behavior below 9 K.
Strain, magnetization, dielectric relaxation, and unpolarized and polarized neutron diffraction measurements were performed to study the magnetic and structural properties of spinel ...Co_{1-x}V_{2+x}O_{4}. The strain measurement indicates that, upon cooling, ΔL/L in the order of ∼10^{-4} starts increasing below T_{C}, becomes maximum at T_{max}, and then decreases and changes its sign at T^{*}. Neutron measurements indicate that a collinear ferrimagnetic order develops below T_{C} and upon further cooling noncollinear ferrimagnetic ordering occurs below T_{max}. At low temperatures, the dielectric constant exhibits a frequency dependence, indicating slow dynamics. These results indicate the existence of an orbital glassy state at low temperatures in this nearly metallic frustrated magnet.
Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel Ti
Mn
(Fe
Co
)
O
has been reported by means of neutron diffraction and magnetization studies. Two distinct ...sets of compositions (i)
(Ti) = 0.20 and
(Fe) = 0.18; (ii)
(Ti) = 0.40 and
(Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti
Mn
Fe
Co
O
exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperature
= 258 K in comparison to the end compound MnCo
O
(
∼ 180 K) with a characteristic ratio
/√2
of 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti
Mn
Fe
Co
O
with higher
-site compositions, the presence of Jahn-Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion thereby ceasing the lattice-spin-orbital coupling. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility,
(
) reveals a cluster spin-glass state with Gabay-Toulouse (GT) like mixed phases behaviour below
. Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence of
(
) follows the weak anisotropic GT-line behaviour with crossover exponent Φ lies in the range 1.38-1.52 on the
-
plane which is in contrast to the
-site Ti substituted MnCo
O
spinel that appears to follow irreversible non-mean-field AT-line behaviour (Φ ∼ 3 +
). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition (
< 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime.
Low-temperature neutron diffraction and NMR studies of field-induced phases in linarite are presented for magnetic fields H∥b axis. A two-step spin-flop transition is observed, as well as a ...transition transforming a helical magnetic ground state into an unusual magnetic phase with sine-wave-modulated moments ∥H. An effective Jover ˜_{1}-Jover ˜_{2} single-chain model with a magnetization-dependent frustration ratio α_{eff}=-Jover ˜_{2}/Jover ˜_{1} is proposed. The latter is governed by skew interchain couplings and shifted to the vicinity of the ferromagnetic critical point. It explains qualitatively the observation of a rich variety of exotic longitudinal collinear spin-density wave, SDW_{p}, states (9≥p≥2).
We present a combined experimental and theoretical study of the mineral atacamite Cu2Cl(OH)3. Density-functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D ...connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateaulike magnetization at about Msat/2. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.
The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te ...exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors. PUBLICATION ABSTRACT
A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1−xMnxCo2O4 in the temperature interval 1.6 K T 300 K. Our ...neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1−xMnxCo2O4 system for all the Jahn-Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d1, 3d4 and 3d6, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet-Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length dc(TB-O) is longer than the equatorial bond length dab(TB-O) due to the splitting of the eg level of Mn3+ ions into and . For the composition x = 0.4, the distortion in the TBO6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at TCOMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of 110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below TCOMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.
The study of crystal structure temperature evolution of the ferroelectric composites (1 − х)NaNO
2
+ (x)KNO
2
at х = 5, 10 и 20 vol. % has been performed by neutron diffraction. The temperature ...dependencies of the ferroelectric order parameter η(т) for sodium nitrite have been determined in the composites and pure NaNO
2
. They coincide on heating and cooling and do not depend on KNO
2
concentration. The increasing of KNO
2
structural phase transition temperature (∼10-15 K) has been revealed in the composites in comparison with pure KNO
2
.
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