Magnetic phase transitions in Nd7Rh3 single crystal

Magnetic phase transitions in rare earth intermetallic compound Nd₇Rh₃ have been investigated using a single crystal. Measurement results of magnetization, magnetic susceptibility, specific heat, and electrical resistivity reveal that Nd₇Rh₃ has two magnetic phase transitions at T_N=34 K, T_t2=9.1 K and a change of the magnetic feature at T_t1=6.8 K in the absence of an external magnetic field. Antiferromagnetic orderings exist in all the three magnetic states; a large magnetic anisotropy between the c-axis and the c-plane is observed. In the magnetic phase below T_t2, an irreversible field-induced magnetic phase transition takes place in the c-plane; after removing external magnetic field, a coexistence state of ferro- and antiferromagnetic ordering or a ferrimagnetic state having a remanent magnetization M_R is stabilized. The M_R decays to a certain value for several hours after the first process; a magnetic field cooling effect was also observed in the c-plane below T_t2. In the antiferromagentic state above T_t2, the irreversibility disappears and an ordinary antiferromagnetic state takes place. As the origin of this phenomenon, a kind of martensitic structural transition that is observed in Gd₅Ge₄ can be considered.     

Forced volume magnetostriction in Mn3.3Sn0.7C compound at room temperature

The negative volume magnetostriction in the external magnetic field for antiperovskite Mn_3.3Sn_0.7C compound is discovered. Its magnetic transition temperature from paramagnetism to ferrimagnetism is 348 K. The linear and volume magnetostrictions were investigated by measuring the change in length along the three-dimensional directions of the square samples at room temperature. Volume contraction was observed along all of the three directions throughout the whole magnetization. The value of volume magnetostriction is −44×10⁻⁶ at 1.5 T. The magnetization saturates basically at 1.5 T, however the volume magnetostriction should be higher with further increase in magnetic field.     

Magnetoelastic properties of Nd6Fe13Cu intermetallic compound

Magnetoelastic properties of Nd₆Fe₁₃Cu intermetallic compound are reported. To study the magnetoelastic behaviour of this compound, the thermal expansion as well as the longitudinal (λ_l) and transverse (λ_t) magnetostriction were measured by using the strain gauge method in the selected temperature range of 80-500 K under applied magnetic fields up to 1.5 T. An anomaly and invar-type effects are observed in the linear thermal expansion and α(T) curves at the Néel temperature. The linear spontaneous magnetostriction decreases sharply by approaching the Néel temperature and also shows the short-range magnetic ordering effects when antiferromagnetic-paramagnetic transition occurs. In the low field region, the absolute values of the anisotropic magnetostriction are small and then start to increase with applied magnetic field. Each isofield curve of the anisotropic magnetostriction passes through a minimum and then approaches to zero with increasing temperature. This magnetostriction compensation arises from the difference in the magnetoelastic coupling constants of the sublattices in this compound.     

High magnetostriction at low fields of (Tb1−xDyx)0.2Pr0.8(Fe0.4Co0.6)1.88C0.05 intermetallic compounds

The structure and magnetostriction of the (Tb_1−xDy_x)_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 intermetallic compounds (0≤x≤1) were studied by X-ray diffraction and magnetic measurements. The formation of an approximate single Laves phase with a MgCu2-type cubic structure was observed in this series of compounds. It was found that the Curie temperature and the saturation magnetization of the compounds would decrease with increase in the Dy content up to x=1. The magnetostriction λ_a (λ_a=λ_‖-λ_⊥) gently rises when x≤0.6, and follows with a precipitous fall when x exceeds 0.6, with the highest value of λ_a being reached in the compounds with x=0.6. The magnetostriction of all the samples was observed to approach their own saturation in the magnetic fields higher than 4 kOe. This indicates that the addition of a small amount of Dy could effectively improve the low-field magnetostriction of the Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compounds, which could become a kind of promising magnetostrictive material.     

Influence of low Co substitution on magnetoelastic properties of HoFe11Ti intermetallic compound

The thermal expansion and magnetostriction of HoFe_11−xCo_xTi (x=0, 0.3, 0.7 and 1) intermetallic compounds were measured, using the strain gauge method in the temperature range 77–590 K under applied magnetic fields up to 1.5 T. Results show that for samples with x=0 and 0.3, both linear thermal expansion and linear thermal expansion coefficient exhibit anomalies below the Curie temperature. Below room temperature, the spontaneous volume magnetostriction decreases with Co content. For all compounds studied, the anisotropic magnetostriction shows similar behaviour in the measured temperature range. The magnetostriction compensation occurs above room temperature in all samples. The volume magnetostriction shows a linear dependence on the applied field and by approaching the Curie temperature this trend changes to parastrictive behaviour. The results of the spontaneous magnetostriction are discussed based on the local magnetic moment model. The contribution of magnetostriction attributed to the magnetic sublattices R and T (Fe or Co) is discussed.     

Vortex states in mesoscopic single crystals Bi2Sr2CaCu2O8+δ in high magnetic fields

The c-axis resistivity measurements were performed in the vicinity of the ab-plane in order to investigate the interaction between Josephson vortices and pancake vortices in Bi₂Sr₂CaCu₂O_8+δ mesoscopic single crystals. It was found that the angular dependence of the c-axis resistivity drastically changes in high magnetic field regime. The vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region.     

Glassy-ferromagnetic behavior in Eu0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxide Eu_0.5Sr_0.5CoO₃ was prepared by the conventional solid-state reaction method. X-ray powder patterns indicated the prepared samples are pure, cubic perovskite structure (Pm3?m), and with no evidence of any secondary phases. The dc magnetization and ac susceptibility measurements were carried out to investigate the magnetic properties of the sample, and which indicated that cluster-glasses properties are suppressed with the increasing of the coercive field. We denied the possibility of spin-glasses and the existence of the Hopkinson effect in Eu_0.5Sr_0.5CoO₃ through the temperature-dependent ac susceptibility measurements, and explained the magnetic behavior of Eu_0.5Sr_0.5CoO₃ with the competition between magnetic anisotropy and the external magnetic field.     

Martensitic transformation and magnetic properties of ferromagnetic shape memory Fe66Pd30Rh4 alloys

This study mainly shows that in the Fe₆₆Pd₃₀Rh₄ (at%) alloys, the L1₀ phase plays an important role in magnetostriction due to the interplay of L1₀ martensitic twins with magnetic domains. The L1₀ martensitic twin structure exhibits a strong magnetocrystalline anisotropy energy constant (K_u=1.27-2.84×10⁶ (ergs/cm³)) along the tetragonal c axis direction. In addition, the L1₀ tetragonal martensitic twin structure shows both a perfect shape memory and a reversible shape memory effect; therefore, it is expected to be applicable in magneto-mechanical applications (such as microactuators or springs). However, in this study, we discover that solution treatment (ST) and aging heat treatments of Fe₆₆Pd₃₀Rh₄ ferromagnetic shape memory alloys influence the behavior of the martensitic transition, which is associated with the change in magnetic properties. The process of a thermoelastic L1₀+L1_m twin phase decomposition→non-thermoelastic L1₀+L1_m+α_bct structure in Fe₆₆Pd₃₀Rh₄ alloys during solution treatment and aging at 400-550 °C for various times is studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The relation of phase separation morphology to the magnetic property change is examined with a superconducting quantum interference device (SQUID) magnetometer, and magnetostriction measurement is performed with a strain gage method and magnetostrictive meter setup. The results indicate that the process of martensitic transformation during aging leads to an increase in coercivity and a decrease in magnetostriction, simultaneously.     

Structural, magnetic and electric properties of HoMnO3 films on SrTiO3(001)

HoMnO₃ films were grown on pure and Nb-doped SrTiO₃ (001) substrates by pulsed laser deposition. The films grew epitaxially with the c-axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn₃O₄ precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c-axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.     

Structure and magnetostriction of Tb0.2Pr0.8(Fe0.4Co0.6)1.93−xCx intermetallic compounds

The structure and magnetostriction of Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93−xC_x intermetallic compounds were studied by X-ray diffraction and magnetic measurements. Almost a single cubic Laves phase forms in the alloys for x ≤0.20, and a small amount of C can inhibit the formation of the 1:3 phase. The lattice parameter increases when 0≤x≤0.15, while the T_c and the spontaneous magnetization decreases with increasing x. The lattice parameter decreases slowly when 0.15≤x≤0.30, while the T_c decreases evidently with increasing x. The magnetostriction λ_a (=λ_∥-λ_⊥) is improved at low magnetic fields at room temperature for the compounds with 0.05≤x≤0.10, indicating that these C-containing compounds are promising magnetostrictive materials.     

Neutron scattering study on CeAgAs2

CeAgAs₂, an HfCuSi₂ like layered pnictide, orders antiferromagnetically at T_N=6.2(1) K. The ordering process was monitored in neutron diffraction experiments in the temperature range 10 K≥T≥3.5 K. At T=4 K the lattice parameters are a=5.7438(1) Å, b=5.7696(1) Å and c=21.0067(2) Å. The diffraction pattern of the antiferromagnetic phase with a propagation vector k=[0,0,0] point towards ferromagnetically ordered moments in Ce layers stacked along [001], the individual layers are coupled antiferromagnetically with a +− −+type sequence. The alignment of moments within the Ce layers cannot be determined reliably from the experimental data so that two different structure models are discussed. The proposed metamagnetic transition was confirmed by diffraction experiments applying an external magnetic field at T<T_N. In the interval 4 K≤T≤6 K, a relatively small field of μ₀H≈0.3 T suffices to fully suppress the antiferromagnetic ordering. The effect is completely reversible yet subject to hysteresis: After switching off the external field at any T<T_N the magnetic reflections gain their original intensity within several 10 min indicating the restoring of the antiferromagnetic phase.     

Field and temperature induced colossal strain in Gd5(SixGe1−x)4

Gd₅(Si_xGe_1−x)₄, known for its giant magnetocaloric effect, also exhibits a colossal strain of the order of 10,000 ppm for a single crystal near its coupled first-order magnetic-structural phase transition, which occurs near room temperature for the compositions 0.41≤x≤0.575. Such colossal strain can be utilised for both magnetic sensor and actuator applications. In this study, various measurements have been carried out on strain as a function of magnetic field strength and as a function of temperature on single crystal Gd₅Si₂Ge₂ (x=0.5), and polycrystalline Gd₅Si_1.95Ge_2.05 (x=0.487) and Gd₅Si_2.09Ge_1.91 (x=0.52). Additionally a giant magnetostriction/thermally induced strain of the order of 1800 ppm in polycrystalline Gd₅Si_2.09Ge_1.91 was observed at its first order phase transition on varying temperature using a Peltier cell without the use of bulky equipment such as cryostat or superconducting magnet.     

Growth of b-axis oriented VO2 thin films on glass substrates using ZnO buffer layer

VO₂ thin films are grown on glass substrates by pulsed laser deposition using vanadium metal as a target. In this study, a ZnO thin film was used as a buffer layer for the growth of VO₂ thin films on glass substrates. X-ray diffraction studies showed that the VO₂ thin film had b-axis preferential orientation on a c-axis oriented ZnO buffer layer. The thickness of the ZnO buffer layer and the oxygen pressure during VO₂ deposition were optimized to grow highly b-axis oriented VO₂ thin films. The metal-insulator transition properties of the VO₂ film samples were investigated in terms of infrared reflectance and electrical resistance with varying temperatures.     

Spontaneous magnetostriction in ThCo5

The spontaneous magnetostriction of the hexagonal compound ThCo₅ was studied by X-ray diffraction. Below the Curie-temperature a large positive spontaneous magnetostriction occurs, primarily along the c-axis.     

Changes of the magnetic interactions in TlCo2Se2 upon metal substitution

Various solid solutions TlCo_2−xMe_xSe₂ (Me=Fe, Ni and Cu) have been investigated by neutron powder diffraction, supplemented by magnetometry. The incommensurate spin-helix running along the c-axis in tetragonal TlCo₂Se₂ prevails for low concentrations of copper and iron but changes pitch. In the copper case, only cobalt carries a magnetic moment. On nickel substitution, however, collinear antiferromagnetic coupling between the ferromagnetic layers occurs. The magnetic moment distribution between the two transition metals in the solid solution TlCo_2−xNi_xSe₂ was tentatively probed with first principle calculations on fictive ordered TlCoNiSe₂, modelled by two types of superstructures. Also the ternary mother compounds, Pauli paramagnetic TlNi₂Se₂ and antiferromagnetic TlCo₂Se₂, were investigated with the same LMTO method.     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

Studies of the g factors and the local structure of the orthorhombic Ni center in KTaO3 crystal

The local structure and the g factor (g_x, g_y, and g_z) of the Ni⁺ center in KTaO₃ are theoretically studied using the perturbation formulas of the g factors for a 3d⁹ ion in orthorhombically elongated octahedra. The orthorhombic field parameters are determined from the superposition model and the local geometry of the system. In view of the covalency, the contributions from the ligand orbital and spin–orbit coupling interactions are taken into account from the cluster approach. In the calculations, the orthorhombic center is attributed to Ni⁺ occupying the host Ta⁵⁺ site, associated with the nearest-neighboring oxygen vacancy V_O along the c-axis. Furthermore, the planar Ni⁺–O²⁻ bonds are found to experience the relative variation ΔR (≈0.076 Å) along the a- and b-axis, respectively, due to the Jahn–Teller effect and the size mismatching substitution of Ta⁵⁺ by Ni⁺. Meanwhile, the effectively positive V_O can make the central Ni⁺ displace away from V_O along the c-axis by about 0.20 Å. The calculated g factors based on the above local distortions show good agreement with the experimental data.     

A neutron powder diffraction study of the helimagnetic structure of TlCo2Se2−xSx

The magnetic layer structure of TlCo₂Se_2−xS_x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition (x): the spins in TlCo₂Se₂ form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co-Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.     

Effect of grain-boundary flux pinning in MgB2 with columnar structure

We studied the flux pinning properties by grain boundaries in MgB₂ films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (J_cs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of J_c in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model.     

Magnetostriction of Fe81Ga19 oriented crystals

The effect of the orientation on the magnetostriction in Fe 81 Ga 19 alloy has been investigated experimentally and theoretically.The Fe 81 Ga 19 [001] and [110] oriented crystals were prepared and the magnetostriction was measured under different pre-stress.The saturation magnetostriction of the [001] oriented crystal increases from 170×10-6 to 330×10-6 under the pre-stress from 0 to 50 MPa.The [110] oriented crystal has a saturation magnetostriction from 20×10-6 to 140×10-6 with the compressive pre-stress from 0 to 40 MPa.The magnetostriction of [001] and [110] oriented crystals has been simulated based on the phenomenological theory.The domain rotation path has been determined and the resultant magnetostriction calculated under different pre-stress.The experimental and simulated results both show that the [001] oriented crystal exhibits better magnetostriction than [110] oriented crystal.The enhancement of the saturation magnetostriction by the compressive pre-stress in the [110] oriented crystal is higher than that in the [001] oriented crystal.