A Study of the Origin of Weak Ferromagnetism in Zn1−x Co x O

To study the origin of ferromagnetism in Zn_1?x Co _x O thin films, its thermal diffusivity, in addition to its magnetization measurements, were analyzed. Thin films of Zn_1?x Co _x O (x = 0.03) were deposited on Si (100) substrates through ultrasonic spray pyrolysis. Magnetization M(B) measurements at low temperature showed a hysteresis loop that indicated the existence of ferromagnetic ordering in Zn_0.97Co_0.03O. However, the magnetic moment per Co ion was much lower than expected. A comparison of M(T) measured at zero-field-cooled and field-cooled conditions showed a superparamagnetic-like behavior, with a blocking temperature of about 130 K. Temperature dependence on the thermal diffusivity of Zn_0.97Co_0.03O showed a pronounced lambda-shaped minimum at 130 K, which indicated the existence of a second-order phase transition at this temperature. The weak ferromagnetism in the Zn_0.97Co_0.03O with the Curie temperature of 130 K was ascribed to the uncompensated magnetic moment at the surface of CoO nanoclusters.     

Magnetic and anomalous transport properties in Fe2MnAl

This study investigated the magnetism and anomalous transport properties of the melt-spun ribbon Fe₂MnAl alloy. It is found that the alloy exhibits ferromagnetic order with the Curie temperature of ∼150 K, followed by another magnetic transition at T_R ∼ 51 K. It is suggested there is antiferromagnetism pinning effect on the ferromagnetic matrix at the temperature below T_R. A steep rise of the resistance below T_R has been observed, which can be understood in terms of antiferromagnetic scattering. An appearance of the resistance maximum near the Curie temperature and the negative temperature dependence of the resistance above Curie temperature are associated with the magnetic phase transition from ferromagnetism to paramagnetism. A negative magnetoresistance at low temperature arising from inhomogeneous magnetic scattering is reported.     

Magnetic and neutron diffraction studies on potassium uranate KUO3

The magnetic properties of KUO₃ with the cubic perovskite structure have been studied. It shows an antiferromagnetic-type of transition at ca. 17 K in the magnetic susceptibility vs. temperature curve. Magnetic hysteresis is observed at 4.5 K, but not observed at 20 K. Neutron diffraction measurements have been performed on the powdered KUO₃ below and above the transition temperature. The neutron diffraction pattern measured at 10 K shows no appreciable difference from that measured at 50 K. A small ordered magnetic moment of U⁵⁺ ion will make it very difficult to observe magnetic diffractions, even if a magnetic ordering occurs. If the magnetic spin alignment occurs in a small domain, no magnetic diffraction will be found in the neutron experiments. On the other hand, the magnetic phenomena of KUO₃ have been analyzed on the basis of an octahedral crystal field model.     

Thermal conductivity of electron-doped CaMnO3 perovskites: Local lattice distortions and optical phonon thermal excitation

The thermal transport properties of a series of electron-doped CaMnO₃ perovskites have been investigated. Throughout the temperature range 5–300 K, phonon thermal conductivity is dominant, and both electron and spin wave contributions are negligible. The short phonon mean free paths in this system result in the relatively low thermal conductivities. The strong phonon scatterings stem from the A-site mismatch and bond-length fluctuations induced by local distortions of MnO₆ octahedra. The thermal conductivity in the magnetically ordered state is enhanced as a result of the decrease in spin–phonon scattering. The results also indicate that above the magnetic ordering temperature, observable thermal excitation of optical phonons occurs. The contribution of optical phonons to thermal conductivity becomes non-negligible and is proposed to play an important role in the glass-like thermal transport behavior (i.e. positive temperature dependence of the thermal conductivity) in the paramagnetic state. These features can be understood in terms of an expression of thermal conductivity that includes both acoustic and optical phonon terms.     

Concentrational commensurate-incommensurate magnetic phase transition in Y1 − x Tb x Mn6Sn6

The magnetic state of Y_{1 ? x} Tb _x Mn₆Sn₆ (with x ≤ 0.25) alloy is studied at different temperatures and magnetic fields by neutron diffraction. The alloy with x _c = 0.22 exhibits an incommensurate-to-commensurate structure phase transition. The transition is accompanied by a decrease in the unit-cell volume. The weak dependence of the intensity of satellites on the x concentration and the monotonic displacement of their angular positions with increasing x allow us to conclude that the antiferromagnet → ferrimagnet transition occurs through the uniform magnetic state. The incommensurate phase in YMn₆Sn₆ exhibits four satellites, which are observed in the low-angle range of X-ray diffraction pattern and demonstrate different behavior with changing temperature. At 293 K, the antiferromagnet → ferromagnet transition in the Y_{1 ?x} Tb _x Mn₆Sn₆ compound can be induced by applied magnetic field, in particular, the field μ₀ H = 0.3 T induces the transition in Tb_0.15Y_0.85Mn₆Sn₆.     

Insight into the magnetism of a distorted Kagome lattice,Dy3Ru4Al12, based on polycrystalline studies

The layered compound with distorted Kagome nets, Dy₃Ru₄Al₁₂, was previously reported to undergo antiferromagnetic ordering below (T_N=) 7 K, based on investigations on single crystals. Here, we report the results of our investigation of AC and DC magnetic susceptibility (χ), isothermal remnant magnetization (M_IRM), heat-capacity, magnetocaloric effect and magnetoresistance measurements on polycrystals. The present results reveal that there is an additional magnetic anomaly around 20 K, as though the Néel order is preceded by the formation of ferromagnetic clusters. We attribute this feature to geometric frustration of magnetism. In view of the existence of this phase, the interpretation of the linear-term in the heat-capacity in terms of spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in the vicinity of T_N, AC χ shows a prominent frequency dependence and, below T_N, M_IRM exhibits a slow decay with time. This raises a question whether the antiferromagnetic structure in this compound is characterized by spin-glass-like dynamics. In contrast to what was reported earlier, there is a change in the sign of the magnetoresistance (MR) at the metamagnetic transition. A butter-fly-shaped (isothermal) MR loop (interestingly spanning over all the four quadrants) is observed at 2 K with distinct evidence for the magnetic phase co-existence phenomenon in zero field after travelling through metamagnetic transition field. The results on polycrystals thus provide additional information about the magnetism of this compound, revealing that the magnetism of this compound is more complex than what is believed, due to geometric frustration intrinsic to Kagome net.     

Magnetocaloric effect in melt-spun FePd ribbon alloy with second order phase transition

Magnetic entropy change and refrigerant capacity for an applied magnetic field variation of 1.5 T have been determined in the temperature range of 293-700 K for the as-quenched polycrystalline Fe_73.2Pd_26.8 melt-spun ribbons alloy. Samples show a major magnetic phase with the fct crystal structure and a Curie temperature of 553 K, at which a second order magnetic phase transition occurs. A maximum magnetic entropy change of 1.04 J kg⁻¹ K⁻¹ was achieved at 550 K, while the maximum refrigerant capacity of 108 J/kg is attained at the applied field of 1.5 T. The field dependence of the magnetic entropy change for this material follows the phenomenological universal curve and at the temperature of the peak corresponds to a large field independent exponent of n = 0.84. Extrapolated value of field depending RC_Area at 5 T reach above 300 J/kg, which is comparable to that of some of the best-known Fe-doped GdSiGe coolant compounds.     

Magnetic structure of Tb2CuIn3

The intermetallic compound Tb₂CuIn₃ has been studied by means of neutron diffraction at various temperatures ranging from 293 to 3.8 K. Data analysis of the high temperature region confirms the hexagonal crystal structure (space group P6/mmm) with Tb at 1a, Cu and In statistically distributed at 2d positions. The unit cell constants are a=4.702(1) Å and c=3.682(2) Å. Below 33 K, Tb₂CuIn₃ undergoes a magnetic phase transition into a collinear antiferromagnetic structure described in an orthorhombic unit cell with A=a, B=a√3, C=c. Terbium magnetic moments of 6.8 μ_B at 3.8 K are oriented along the c axis. Complementary susceptibility measurements confirm the transition temperature. The magnetic isotherm at 5 K in fields with μ_oH up to 7 T is far from saturation.     

Magnetic properties of the new rare earth intermetallic compound Pr5AgSn3

A detailed structural characterisation, by means of the powder X-ray diffraction method, of the new Pr₅AgSn₃ intermetallic compound has been performed and a.c., d.c. magnetic measurements in the 4-300 K temperature range and in applied magnetic fields up to 9 Tesla have been obtained and analysed. The compound orders ferromagnetically below 33.5 K and exhibits a pronounced hysteresis cycle at 5 K with a saturation magnetisation (μ_sat) of 1.84 μ_B/Pr atom. In the paramagnetic state the compound follows the Curie-Weiss law with μ_eff.=3.40 μ_B and θ_P=+32 K. The splitting of the peak in the a.c. susceptibility by applied low d.c. fields as well as the temperature dependence of the remanence (B_r) and of the coercive field (H_c) are tentatively explained in terms of domain dynamics.     

Specific features of magnetocaloric effect in isotropic antiferromagnets

The molecular-field method has been used to study the magnetocaloric effect and the dependence of the magnetic entropy on an external magnetic field in a two-sublattice isotropic antiferromagnet. It has been shown that, for an isotropic antiferromagnet, the magnetization of a canted phase that arises after a spin-flop transition in an applied magnetic field is temperature-independent. Therefore, the magnetic entropy of the phase is independent of the strength of an external field and no magnetocaloric effect is observed. The dependence of the magnetic entropy on the magnetic field and the normal magnetocaloric effect appear only after the spin-flip transition of the antiferromagnet into a magnetic-field-induced ferromagnetic phase. In moderate magnetic fields, this occurs in fact only in the temperature range of T ≥ T _N, where T _N is the Néel temperature.     

Low-temperature properties of the orderable Cu<Subscript>3</Subscript>Pd alloy

The effect of atomic ordering on the transport properties of the Cu₃Pd alloy in the low-temperature range (T < 70K) in strong magnetic fields (H ≤ 150 kOe) has been studied. The temperature dependence of the resistivity during alloy disordering has been found to have a minimum at T ～ 10 K. The Hall effect has a negative sign. The magnetoresistance, on the contrary, is positive; the longitudinal effect equals ～50% of the transverse one.     

Ferroelasticity in a metal–organic framework perovskite; towards a new class of multiferroics

A metal–organic framework perovskite, [(CH₂)₃NH₂][Mn(HCOO)₃], exhibits a weakly first order ferroelastic phase transition at ～272 K, from orthorhombic Pnma to monoclinic P2₁/n, and a further transition associated with antiferromagnetic ordering at ～8.5 K. The main structural changes, through the phase transition, are orientational ordering of the azetidium groups and associated changes in hydrogen bonding. In marked contrast to conventional improper ferroelastic oxide perovskites, the driving mechanism is associated with the X-point of the cubic Brillouin zone rather than being driven by R- and M-point octahedral tilting. The total ferroelastic shear strain of up to ～5% is substantially greater than found for typical oxide perovskites, and highlights the potential of the flexible framework to undergo large relaxations in response to local structural changes. Measurements of elastic and anelastic properties by resonant ultrasound spectroscopy show some of the characteristic features of ferroelastic materials. In particular, acoustic dissipation below the transition point can be understood in terms of mobility of twin walls under the influence of external stress with relaxation times on the order of ～10^?7 s. Elastic softening as the transition is approached from above is interpreted in terms of coupling between acoustic modes and dynamic local ordering of the azetidium groups. Subsequent stiffening with further temperature reduction is interpreted in terms of classical strain–order parameter coupling at an improper ferroelastic transition which is close to being tricritical. By way of contrast, there are no overt changes in elastic or anelastic properties near 9 K, implying that any coupling of the antiferromagnetic order parameter with strain is weak or negligible.     

High-field magnetization of a Dy2Fe14Si3 single crystal

The magnetization of a Dy₂Fe₁₄Si₃ single crystal was measured at 4.2 K in pulsed fields up to 51 T along the principal axes. The compound orders ferrimagnetically at 500 K, has a spontaneous magnetic moment of 8 μ_B/f.u. (at 4.2 K) and exhibits a very large magnetic anisotropy, 〈1 0 0〉 being the easy axis. In fields applied along the 〈1 0 0〉 and 〈1 2 0〉 axes, field-induced phase transitions are observed at 33 T and at 39 T, respectively. The c-axis magnetization curve crosses the easy-axis curve at 19 T. At higher fields, for all directions, the magnetization continues to increase due to further bending of the sublattice moments. Temperature evolution of magnetic anisotropy and magnetic hysteresis are discussed as well.     

Antiferromagnetic ordering below 1.7 K in PrIr2Ge2

We present our investigations on magnetic and transport properties of polycrystalline PrIr₂Ge₂ which forms in CaBe₂Ge₂-type primitive tetragonal structure (space group P4/nmm). The ac magnetic susceptibility data exhibit two well pronounced peaks at 2.08 K and 0.76 K due to the onset of magnetic order. The specific heat also exhibits a sharp λ-type anomaly at 1.7 K confirming the onset of bulk antiferromagnetic order. The temperature dependence of magnetic part of entropy suggests a quasi-triplet ground state in this compound. The onset of magnetic order is also confirmed by the electrical resistivity data.     

Magnetic field effects in decamethyl ferrocenium tetracyanoethanide

The specific heat C(H,T) of (DMeFc) (TCNE) in the temperature range of 2–12 K in the presence of applied magnetic fields is reported. Previous results had shown anomaly at 4.82 K corresponding to a transition at T_c from a high temperature one-dimensional (1D) magnet (above T_c) to a three-dimensional (3D) macroscopic ferromagnet (below T_c). The results show a dramatic behavior of the peak associated with the ferromagnetic transition. The peak height decreases with increasing magnetic field, i.e. magnetic fields smear the transition. For small values of applied magnetic fields the peak associated with the ferromagnetic transition is completely smeared. The results are indicative of anisotropic chains with enhanced 1D coherence and frustration in interchain coupling. The spin-wave spectrum is dominated by the 1D chains below T_c.     

Anisotropic magnetocaloric effect in TbNiAl

We present study of the anisotropic magnetocaloric effect in TbNiAl crystallizing in the hexagonal ZrNiAl-type structure. TbNiAl orders antiferromagnetically below T_N = 47 K and undergoes second magnetic phase transition to another antiferromagnetic structure at T₁ = 23 K. Magnetization and specific heat measurements on single crystal revealed strongly anisotropic magnetocaloric effect. The large effect occurs for field applied along the hexagonal c-axis whereas the entropy change is almost zero for the perpendicular field direction. Plateau-like character of the determined temperature change is observed between T_N and T₁.     

Step-like and first-order characters of Tb0.2Pr0.8(Fe0.4Co0.6)1.93 with low interstitial carbon content

In this study we report detailed magnetic property of the 4f-3d pseudo-quaternary Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compound by detailed magnetization measurements. Very sharp magnetization jumps across the antiferromagnetic–ferromagnetic transition are observed below 3.0 K, and the number of jump-like transitions increases with decreasing temperature. The time-dependent magnetic relaxation, field sweep rate and cooling field dependence of magnetization jumps resemble the martensitic scenario. The number and occurrence of magnetization jumps are mainly determined by the competitions between the thermal fluctuation energy, elastic energy and Zeeman energy, and the field-induced antiferromagnetic to ferromagnetic phase transition at low temperatures is of first-order in nature.     

Evolution of physical properties of amorphous Fe–Ni–Nb–B alloys with different Ni/Fe ratio upon thermal treatment

Amorphous ribbons (Fe–Ni)₈₁Nb₇B₁₂ with Ni/Fe = 0, 1/6, 1/3 and 1 were prepared by planar flow casting. Thermal treatment of samples was performed in vacuum at temperatures chosen to map the evolution of selected properties in the course of transformation from amorphous state. The coefficient of thermal dilatation exhibits changes at temperatures close to the glass transition, Curie and crystallization temperatures; these effects are enhanced or suppressed by cyclic thermal treatments up to the vicinity of these temperatures. The values of saturation magnetostriction λ_S allow to infer about processes taking place in the investigated materials, especially with respect to formation of new magnetic phases or magnetic anisotropy.Complex processes of structural transformations induced by thermal treatment are strongly affected by Ni percentage. A transitional, magnetically harder phase, which is formed at lower temperatures preferentially near surfaces of the Ni-richest alloy, produces characteristic hysteresis loop shape. This shape disappears after annealing at higher temperatures and enables the material to show the lowest coercivity of the whole alloy series. The saturation magnetic polarization reflects mainly the resulting Curie temperature, which falls with increasing Ni percentage. Magnetic hysteresis loops were also used in the study of dynamics of magnetic domains by MOKE. Domain shape evolution is shown in dependence on composition and thermal treatment as well as a function of applied magnetic field, ranging from remanent sample state to magnetic saturation.     

Electrical and magnetic properties of NdCuPb compound

Electrical and magnetic properties of NdCuPb compound were investigated by means of electrical resistivity, magnetization measurements in the temperature range 1.5–100 K. Low-field dc susceptibility goes through a maximum at T_N=13.2  K, indicating a paramagnetic to an antiferromagnetic transition and then follows a sharp peak at T=5.9  K. The susceptibility data exhibits a Curie–Weiss like behavior in the paramagnetic regime and the effective moment per neodium atom is found to be 3.62μ_B from the data at temperatures above 42 K. This value is exactly equal to that for Nd³⁺, while at lower temperatures, the data yields a little bit less than its free ion value. The ratio M/H versus temperature T curves for different values of the magnetic field split into multiple branches at about T=42  K due to crystalline field effects.In addition, electrical resistivity in a magnetic field up to 120 kOe was also measured in the same temperature range. The resistivity gives non-metallic behavior. The antiferromagnetic transition is clearly discern by a “Cr-like” anomaly at about 13 K, followed by a sharp increase in the resistivity (like a jump) at T=5.9  K where the susceptibility gives similar effect. On other hand, the magnetic contribution to the resistivity begins to decrease at T=42  K at which M/H curves merges. All these behavior may be attributed to crystal-field-splitting of neodium atoms’levels.     

Fabrication and thermoelectric properties of highly textured NaCo2O4 ceramic

Highly textured NaCo₂O₄ polycrystalline sample was fabricated by means of the cold high-pressure compacting followed by the solid-state reaction. X-ray diffraction and scanning electron microscope were employed to show that the plate-like grains within the sample are aligned along the pressing direction. The resistivity ρ and thermoelectric power S along the preferred {0 0 1} plane were measured in the whole temperature range from 15 to 973 K in air and the correlation between thermoelectric properties and texture was investigated. It was found that both ρ and S exhibit metallic behavior in the whole temperature range and the above sample exhibits lower ρ and higher S due to high texture and density. The power factor exhibits a steep rise above 400 K and reaches 761 μW m⁻¹ K⁻² at 973 K, suggesting a promising candidate for thermoelectric application at higher temperature. The change of slope in both resistivity and thermoelectric power curves at about 450 K might arise from the spin-state transition of Co ions in the CoO₂ blocks.