Spin Flop Research Articles

Mixed valence and metamagnetism in a metal flux grown compound Eu2Pt3Si5

A new compound Eu2Pt3Si5 with plate shaped morphology has been grown from excess In flux. The compound crystallizes in the orthorhombic U2Co3Si5 structure type, Ibam space group and the lattice parameters are a=10.007(2)Å, b=11.666(2)Å and c=6.0011(12)Å. The crystal structure of this compound can be conceived as inter-twinned chains of [Pt2Si2] and [PtSi3] tetrahedra connected along [100] direction to give rise to a complex three dimensional [Pt3Si5] network. Temperature dependent magnetic susceptibility data suggests that Eu2Pt3Si5 undergoes a strong antiferromagnetic ordering (TN=19K) followed by a weak ferromagnetic transition (TC=5.5K). The effective magnetic moment/Eu obtained from susceptibility data is 6.78μB accounts mixed valent Eu with almost 85% divalent Eu, which is supported by X-ray absorption near edge spectroscopy. The compound undergoes a metamagnetic transition under applied magnetic field through a probable spin flop mechanism.

Magnetoelectric coupling tuned by competing anisotropies inMn1−xNixTiO3

A flop of electric polarization from $P$$\parallel$$c$ ($P_c$) to $P$$\parallel$$a$ ($P_a$) is observed in MnTiO$_3$ as a spin flop transition is triggered by a $c$-axis magnetic field, $H_{\|c}$=7 T. The critical magnetic field $H_{\|c}$ for $P_a$ is significantly reduced in Mn$_{1-x}$Ni$_x$TiO$_3$ (x=0.33). $P_a$ and $P_c$ have been observed with both $H_{\|c}$ and $H_{\|a}$. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and inter-planar neighbors. In the x=0.33 system, the uniaxial and planar anisotropies of Mn$^{2+}$ and Ni$^{2+}$ compete and give rise to a spin reorientation transition at $T_R$. A magnetic field, $H_{\|c}$, aligns the spins along $c$ for $T_R$$<$$T$$<$$T_N$. The rotation of the collinear spins away from the $c$-axis for $T$$<$$T_R$ alters the magnetic point symmetry and gives rise to a new ME susceptibility tensor form. Such linear ME response provides satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.

Magnetic phase diagram of the low-anisotropy antiferromagnet Cs2FeCl5·H2O

Abstract From magnetization and ac susceptibility measurements we obtain the complete magnetic phase diagram of single crystals of Cs 2 FeCl 5 ·H 2 O for magnetic field up to 15 T. The magnetic field was applied along the directions parallel and perpendicular to the easy axis and the magnetization measured to temperatures down to 0.5 K. At zero magnetic field the antiferromagnetic ordering occurs at T N =6.63 K. For the field applied parallel to the easy axis the antiferromagnetic (AF) to the spin-flop (SF) transition occurs for fields from 1.4 T to 1.1 T depending on the temperature. The low temperature transition from the (SF) to the paramagnetic (P) phase occurs at 13.15 T. In the perpendicular configuration this transition occurs at fields around 13.5 T. From the extrapolation of the transition fields to zero temperature, we obtain a ratio of the anisotropy field H A to exchange field H E , α = H A /H E =(1.4±0.2)×10 −2 . A comparison with the phase diagram measured for MnF 2 is included.

Low-temperature heat transport ofNd2−xCexCuO4single crystals

We report a study of the Ce doping effect on the thermal conductivity (\kappa) of Nd_{2-x}Ce_{x}CuO_{4} (NCCO) at low temperatures down to 0.3 K and in magnetic fields up to 14 T. It is found that with Ce doping, the electronic thermal conductivity increases; at the same time, the a-axis field induced changes in \kappa(H), associated with the spin flop and spin polarization of Nd^{3+} sublattice, and the spin flop of Cu^{2+} sublattice, gradually disappear. These are clearly due to the electron doping and the destruction of the antiferromagnetic orders. In the superconducting NCCO with x = 0.14 and 0.18, although the electronic thermal conductivity shows sizable field dependencies with H // c, the paramagnetic scattering of phonons is still playing the dominant role in the heat transport, which is different from many other cuprates. In the lightly doped samples (x = 0.03 and 0.06), the low-T \kappa(H) isotherms with H // c show a step-like anomaly and is likely related to the spin/charge stripes.

Magnetic-field-induced spin flop transition and magnetoelectric effect inCa2Fe2−xAlxO5

Magnetic-field-induced spin flop transition and magnetoelectric effect in noncentrosymmetric ${\text{Ca}}_{2}$${\text{Fe}}_{2\ensuremath{-}x}$${\text{Al}}_{x}$${\text{O}}_{5}$ (0.5 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 1) single crystals have been investigated. Temperature dependence of magnetic susceptibility shows that the compounds have antiferromagnetic transition temperatures ${T}_{N}$ above room temperature. The spin easy axis varies from the $c$ axis to the $a$ axis with increasing Al content $x$. When a magnetic field is applied along the spin easy axis, a spin flop transition takes place, accompanied by anomalies in the electric polarization and dielectric constant. The anomalies can be ascribed to a noncollinear spin arrangement in the domain walls between two magnetic phases and/or the spin direction dependent modulation of the metal-ligand hybridization. Such a polarization change with a spin flop transition in polar antiferromagnets may provide a route to realize large magnetoelectric coupling at high temperatures.

Spin Frustration and Field-Induced Transitions of Modified Pyrochlore Fluorides ACr2F6 (A = Rb and Cs)

We investigated the magnetic properties of the modified pyrochlore fluorides ACr2F6 (A = Rb and Cs), which consist of antiferromagnetically interacting Cr3+ and Cr2+. Both systems have Curie–Weiss temperatures of about −40 K and exhibit magnetic ordering at about 17 K. Below the ordering temperatures, RbCr2F6 has a small spontaneous magnetic moment but not CsCr2F6. In a magnetic field, they exhibit a change in the slope of their magnetization curves at about 3 T, which is likely to be due to a spin flop. In addition, we found field-induced magnetic transitions of RbCr2F6 at 25 T, where it has a magnetic moment of about 4 μB per formula unit. The spin structure at about 25 T is considered to be a collinear one, in which all Cr2+ ions and half of the Cr3+ ions are aligned parallel to the field and the other half of the Cr3+ ions are antiparallel.

Transition metal phosphite complexes: from one-dimensional chain, two-dimensional sheet, to three-dimensional architecture with unusual magnetic properties

Four new compounds, [Ni(HPO3)(4,4′-bpy)(H2O)3]·4H2O (1), [Co2(HPO3)2(4,4′-bpy)2(H2O)6]·9H2O (2), [Zn(HPO3)(4,4′-bpy)0.5]·H2O (3), and [Co3(PO3)2(4,4′-bpy)3(H2O)6]·3H2O (4), have been synthesized. Compounds 1 and 2 are linear chains and isostructural except for the distinction of the metal ion and the number of lattice water molecules. Compound 3 consists of a 2D sheet structure. Compound 4 is a 3D magnetic porous material with 1D channels along the c axis. Magnetic measurements indicate that compound 1 shows paramagnetic behavior, while compound 2 displays antiferromagnetic behavior. Compound 4 exhibits antiferromagnetism with a pronounced field-induced spin–flop transition. A critical field of 3 T at 2.0 K is determined from the derivative dM/dH curve. Meanwhile, compound 4 shows a strong spin frustration arising from the geometric frustration in the kagome lattice. The dehydrated phase 4a displays a characteristic of N2 adsorption with type II isotherm. We performed detailed magnetic measurements for 4a, which exhibits quite different magnetic properties from 4. Compound 4a displays a ferrimagnetic behavior with a lower transition field of 0.2 T, and a weaker spin frustration.

Evolution of incommensurate spin order with magnetic field and temperature in the itinerant antiferromagnet GdSi

GdSi exhibits spin-density-wave (SDW) order arising from the cooperative interplay of sizeable local moments and a partially nested Fermi sea of itinerant electrons. Using magnetotransport, magnetization, and nonresonant magnetic x-ray diffraction techniques, we determine the H-T phase diagrams of GdSi for magnetic fields up to 21 T, where antiferromagnetic order is no longer stable, and field directions along each of the three major crystal axes. While the incommensurate magnetic ordering vector that characterizes the SDW is robust under magnetic field, the multiple spin structures of this compound are highly flexible and rotate relative to the applied field via either canting or spin-flop processes. The antiferromagnetic spin densities always arrange themselves transverse to the applied magnetic field direction. The phase diagrams are delineated by two types of phase boundaries: one separates a collinear from a planar spin structure associated with a lattice structural transition, and the other defines a spin flop transition that is only weakly temperature dependent. The major features of the phase diagrams along each of the crystal axes can be explained by the combination of local moment and global Fermi surface physics at play.

K4Ni3O6 and KNa2Ni2O4, New Quasi One‐Dimensional ­Oxonickelates(II, III)

AbstractTwo new quasi one‐dimensional alkali metal oxonickelates, K4Ni3O6 and KNa2Ni2O4, were obtained by the azide/nitrate route starting from respective stoichiometric mixtures of KN3, KNO3, NaN3, NaNO3, and NiO. According to the X‐ray analyses of the crystal structures [K4Ni3O6: C2/m, Z = 2, a = 899, 53(3), b = 610, 24(2), c = 778.75(2) pm, β = 91.665(2)°, R1 = 0.030, wR2 = 0.088; KNa2Ni2O4: P21/n, Z = 4, a = 573.70(2), b = 1072.89(3), c = 844.93(2) pm, β = 102.916(2)°, R1 = 0.038, wR2 = 0.108], the main feature common to both crystal structures are undulating 1∞NiO2 chains consisting of planar, edge‐sharing NiO4 squares. The magnetic susceptibilities for both compounds indicate onset of antiferromagnetic ordering at about 5 K. Evaluations of the Curie‐Weiss fits result in calculated magnetic moments higher than expected for the respective spin only values, implying some orbital contribution. Below the Néel temperature spin flop or meta‐magnetism is observed for both compounds.

Anomalous magnetoresistance and hidden spin canting in (DIETSe)2MCl4(M=Fe, Ga)

The quasi-one-dimensional (Q1D) molecular conductors (DIETSe)${}_{2}$$M$Cl${}_{4}$ [$M=\text{Fe}$, Ga] undergo a spin density wave (SDW) transition below 12 K. The SDW ground state is suppressed by applying high pressure, recovering the Q1D Fermi surface which is confirmed by the appearance of Lebed resonance in the angle-dependent magnetoresistance (MR). Above the critical pressure of SDW, MR shows kink structures at high magnetic fields, reminiscent of field-induced spin density wave (FISDW) transition in both salts. The $\ensuremath{\pi}$-$d$ hybrid (DIETSe)${}_{2}$FeCl${}_{4}$ also exhibits an antiferromagnetic (AF) transition of $d$-electron spins at 2.5 K, below which the spin-flop-induced positive large MR are observed. The change in the interlayer MR reaches 130$%$ at 10.5 kbar. The resistance anomalies associated with spin flop are also observed in the angle-dependent MR at low magnetic fields below 5 T, associated with clear hysteresis. A polar plot of these anomalies reveals the presence of hidden spin canting. Two magnetic easy axes of $d$-electron spins are found to be tilted $\ifmmode\pm\else\textpm\fi{}$16 degrees from the $b$ axis towards the $c$ axis. The interplay between the SDW instability of Q1D $\ensuremath{\pi}$ electrons and the local moments of AF $d$-electron spins is considered as the origin of the anomalous transport behaviors.

Theoretical Understanding of Enhanced Photoelectrochemical Catalytic Activity of Sn-Doped Hematite: Anisotropic Catalysis and Effects of Morin Transition and Sn Doping

To investigate the influence of the Morin transition on the photoelectrochemical (PEC) activity of hematite, electronic properties in different magnetic phases were studied on the basis of the first-principles calculations within the GGA+U approximation. The results show that the effective electron mass in the (0001) plane changes remarkably due to the spin–flop transition, while the effective electron masses in other Miller planes are not sensitive to the spin orientation around irons. The electronic structure calculations of Sn-doped hematite predict that the improved PEC activities of Sn-doped hematite are proved to arise from a shrinking of the band gap, decreasing of the effective electron mass, and thus enhanced electronic conductivity. More interestingly, the heavier doping of Sn (≥16.7 atom %) in hematite would induce a new level between the valence band maximum (VBM) and Fermi level E_F, which facilitates its PEC activity of visible light water splitting.

Magnetic properties and anisotropy of orthorhombic DyMnO3 single crystal

An orthorhombic DyMnO3 single crystal has been studied in magnetic fields up to 14T and between 3K and room temperature. The field dependent ordering temperature of Dy moments is deduced. The paramagnetic Curie Weiss behavior is related mainly to the Dy3+sublattice whereas the Mn sublattice contribution plays a secondary role. DC magnetization measurements show marked anisotropic features, related to the anisotropic structure of a cubic system stretched along a body diagonal, with a magnetic easy axis parallel to the crystallographic b axis. A temperature and field dependent spin flop transition is observed below 9K, when relatively weak magnetocrystalline anisotropy is overcome by magnetic fields up to 1.6T.

Y3MnAu5: Three Distinctive d-Metal Functions in an Intergrown Cluster Phase

The new Y(3)MnAu(5) intermetallic phase is obtained from the arc-melted elements in virtually quantitative yields after annealing at 1000 °C for ~3 d. Its remarkable structure [rhombohedral, R3, Z = 6; a = 8.489(1) Å, c = 18.144(2) Å] consists of a 2:1 cubic-close-packed intergrowth between edge-shared Mn-centered Au rhombohedra (Mn@Au(8)) with gold-centered antiprismatic (Au@Y(6)) clusters via a common gold network. Magnetic susceptibility (χ) data for Y(3)MnAu(5) were fitted by a Curie-Weiss law. The Curie constant indicates a large effective moment corresponding to nearly localized Mn spins S = 5/2, and the Weiss temperature demonstrates the dominance of ferromagnetic (FM) interactions. An antiferromagnetic (AFM) transition at T(N) = 75 K and a possible spin reorientation transition at 65 K were observed. Analysis of the χ data for T < T(N) suggests a planar noncollinear helical AFM structure that arises from competing AFM interactions between FM-aligned layers of spins in the ab-plane with a turn angle of 69° between the spins along the helix c-axis. A magnetic field-induced spin flop transition is observed below T(N). Spin-polarized LMTO-LSDA calculations indicate an ~2 eV splitting of the Mn 3d states and a metallic ground state, and their COHP analyses demonstrate that ~81% of the total Hamilton populations originate from heteroatomic polar Y-Au and Mn-Au bonding. The Mn 3d, Y 4d, and Au 5d characteristics are remarkably diverse: localized and magnetically polarized for Mn; reducing and cationic for Y; and relativistically strongly bonded and oxidizing for Au, bonding of the latter two being broadly delocalized.

Magnetism and crystalline electric field effect in ThCr2Si2-type CeNi2As2

Millimeter sized ThCr$_2$Si$_2$-type CeNi$_2$As$_2$ single crystal was synthesized by NaAs flux method and its physical properties were investigated by magnetization, transport and specific heat measurements. In contrast to the previously reported CaBe$_2$Ge$_2$-type CeNi$_2$As$_2$, the ThCr$_2$Si$_2$-type CeNi$_2$As$_2$ is a highly anisotropic uniaxial antiferromagnet with the transition temperature $T_N$=4.8 K. A field induced spin flop transition was seen below $T_N$ when the applied $\textbf{B}$ is parallel to the $\textbf{c}$-axis, the magnetic easy axis, together with a huge frustration parameter $f=\theta_W/T_N$. A pronounced Schottky-like anomaly in specific heat was also found around 160 K, which could be attributed to the crystalline electric field effect with the excitation energies being fitted to $\Delta_1=$325 K and $\Delta_2=$520 K, respectively. Moreover, the in-plane resistivity anisotropy and low temperature X-ray diffractions suggest that this compound is a rare example exhibiting a possible structure distortion induced by the $4f$-electron magnetic frustration.

Low Temperature Magnetization of Yb2Pt2Pb with the Shastry–Sutherland Type Lattice and a High-Rank Multipole Interaction

We measured DC magnetization of the antiferromagnet Yb 2 Pt 2 Pb ( T N ∼2.1 K) with the Shastry–Sutherland lattice for two in-plane directions H ∥[100] and [110] down to 0.08 K. Multiple metamagnetic transitions are found to develop in the field dependence of the magnetization M ( H ) near the I–II phase boundary. Substantial anisotropy observed in M ( H ) is well explained on the basis of a two-sublattice model with strong Ising anisotropy along the orthogonal [110] and [1\bar10] directions arising from the wave function of the ground state Kramers doublet predominantly comprised of |±7/2>, as recently proposed by Ochiai et al. In the high-field region of phase II, however, M ( H ) increases nearly linear to H and the full saturation is reached by a second-order transition even at the base temperature of 0.08 K, just like a spin flop state of an ordinary antiferromagnet. The results are interpreted by a pseudospin-flop state, which can be stabilized by a high-rank multipole, possibly a rank-7 octacosahec...

Electronic and magnetic properties of CeCoGa

We report the magnetic and electronic transport properties of CeCoGa, using magnetic susceptibility, magnetization, specific heat and electrical resistivity investigations. We also present X-ray photoemission valence band spectra (VB-XPS) as well as ab-initio band structure calculations for the ferromagnetic and antiferromagnetic ordering. The dc and ac magnetic susceptibility data reveal a magnetic phase transition at TN = 4.3 K. For T < TN magnetization M vs. magnetic field B isoterms have two metamagnetic transitions. Above TN susceptibility shows evidence of spin-glass like behavior. The magnetic entropy is strongly reduced at TN due to the on-site Kondo coupling. For CeCoGa the Kondo temperature is ∼20 K, much larger than the temperature of the magnetic ordering. Our calculations show that within bare local spin density approximation (LSDA) the ground state of CeCoGa is weakly magnetic, however, the 4f correlations when they are included within LSDA + U approach lead to stable antiferromagnetic ground state. Calculations for moderate U parameter indicate that magnetic ordering is present not only at the Ce sublattice, but also for the Co sites, moreover small induced magnetic moment is also present at the Ga sites. All sublattices exhibit antiferromagnetic ordering with different values of local magnetic moments what explains the ferrimagnetic behavior of CeCoGa. The magnetization M vs. magnetic field B shows at T = 2 K two metamagnetic features at B = 0.1 T and B = 5 T, which well correlates with the calculated magnetic moments of Co and Ga. The first transition can be attributed to spin–flop within the magnetic moments induced in Ga sublattice, whereas the anomaly at B = 5 T is related to the spin–flop within the Co sublattice.

On superparamagnetism in quasi-one-dimensional semiconductor antiferromagnets TlFeS2 and TlFeSe2

The Mossbauer spectra of the quasi-one-dimensional semiconductor antiferromagnets TlFeS2 and TlFeSe2 are analyzed. It is shown that complete compensation of the magnetic moments is violated in the quasi-one-dimensional antiferromagnetic domains of TlFeS2 and TlFeSe2 due to spin fluctuation (spin flops). In this case, the domains behave in the vicinity of the one-dimensional Neel temperature (TN1D) as ultrafine ferrimagnetic particles in the superparamagnetic state. This feature in the vicinity of TN1D is characteristic for quasi-one-dimensional semiconductor antiferromagnets in which TN1D ≫ TN3D.

Magnetic order and spin-flop transitions in the cobalt-doped multiferroic Mn1−xCoxWO4

We present a comprehensive single crystal neutron diffraction investigation of the $\rm Mn_{1-x}Co_{x}WO_4$ with $0.02\leq x \leq0.30$. At lower concentration $x \leq 0.05$, the system is quickly driven into the multiferroic phase with spin structure forming an elliptical spiral order similar to the parent compound. The reduction of electric polarization is ascribed to the tilting of the spiral plane. For $x\sim 0.075$, the magnetic structure undergoes a spin flop transition that is characterized by a sudden rotation of the spin helix envelope into the $ac$ plane. This spin structure persists for concentration up to $x=0.15$, where additional competing magnetic orders appear at low temperature. For $0.17 \leq x \leq 0.30$, the system experiences another spin flop transition and recovers the low-$x$ spiral spin configuration. A simple commensurate spin structure with $\vec{q}=(0.5,0,0)$ is found to coexist with the incommensurate spiral order. The complex evolution of magnetic structure in Co doped $\rm MnWO_4$ contrasts sharply with other transition metal ion doped $\rm Mn_{1-x}A_xWO_4$ (A=Zn, Mg, Fe) where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of $\rm Mn_{1-x}Co_{x}WO_4 $ results from the interplay between magnetic frustration and spin anisotropy of the Co ions.

Antiferromagnetic Ordering of MII(TCNE)[C4(CN)8]1/2 (M = Mn, Fe; TCNE = Tetracyanoethylene)

The magnetic behaviors of MII(TCNE)[C4(CN)8]1/2 (M = Mn, Fe) prepared from a new synthetic route yielding purer magnetic materials are reported. Detailed DC magnetization and AC magnetic susceptibility measurements reveal that both compounds exhibit magnetic ordering with antiferromagnetic ground states arising from antiferromagnetically coupled ferrimagnetic layers with ordering temperatures of 69 and 84 K for polycrystalline M = Mn (Mn) and Fe (Fe), respectively. Because of the presence of two spin sites, these antiferromagnets are best described as compensated ferrimagnets. A spin flop transition between the antiferromagnetically coupled ferrimagnetic layers occurs at 19 500 Oe at 5 K for Mn, whereas Fe is a metamagnet. Fe also exhibits a constricted hysteretic behavior with a 5 K critical field of 12 600 Oe, and a coercive field and remanent magnetization of 4800 Oe and 1850 emu Oe/mol, respectively.

Isothermal switching of perpendicular exchange bias by pulsed high magnetic field

Isothermal switching of a perpendicular exchange bias by a strong pulsed magnetic field has been investigated using a Pt/Co/α-Cr2O3 thin film system. The switching of the perpendicular exchange bias is accompanied by the spin reversal of interfacial uncompensated antiferromagnetic Cr spins. We have also demonstrated that the switching of the exchange bias is reversible by changing the pulsed magnetic field direction. The mechanism of the demonstrated switching is discussed from the viewpoint of the spin flop transition of the α-Cr2O3 layer.