Magnetostriction of antiferromagnetic FeGe2

The anisotropic magnetostriction of FeGe₂ is measured for magnetic field along the [1 0 0] and [1 1 0] axes at temperature 4.2 K and along [1 0 0] from 77 to 300 K. The behaviour is consistent with spin reorientation in the basal plane. The saturation magnetostriction and the characteristic field required to produce saturation decrease with increasing temperature and approach zero at the lower transition temperature, $\text{T}{}_{\mathrm{K}}\text{? 265}$ K. This suggests that the spins flip from the basal plane into the direction of the tetragonal [0 0 1] axis at T_K.     

The effect of magnetic field heat treatment on magnetostriction of Terfenol-D 2-2 composites

Based on the analysis of the magnetostriction for Terfenol-D composites, Terfenol-D 2-2 magnetostrictive composites have been prepared with laminations perpendicular to [1 1 2] axes. Then one of the samples was annealed in the vacuum at 423 K for 15 min at the magnetic field of 240 kA/m, which is along the direction of laminations and vertical to the [1 1 2] axes of the specimen. The static magnetostriction λ and dynamic magnetostrictive coefficient d₃₃ of samples were measured under the compressive stress of 0, 2, 4, 6 and 8 MPa. Effects of the compressive stress and the magnetic field heat treatment on the magnetostriction λ have been investigated. It is found that the magnetostriction of 2-2 composites can be improved under the compressive stress when the magnetic field is larger than 20 kA/m. The magnetostriction of 2-2 composites with the magnetic field heat treatment increases under compressive stress, and it can reach 1390×10⁻⁶ at the magnetic field of 200 kA/m and under the compressive stress of 4 MPa, much larger than the value of 860×10⁻⁶ without the magnetic field heat treatment. The highest magnetostriction of the 2-2 composite with the magnetic field heat treatment can reach 1530×10⁻⁶. The dynamic magnetostrictive coefficient d₃₃ of 2-2 composites with the magnetic field heat treatment have been improved, compared with that without magnetic field heat treatment. The maximum value of d₃₃ of the sample with magnetic field heat treatment is 71% larger than that without magnetic field heat treatment.     

钙钛矿（La1-yTby)0.67Sr0.33MnO3的磁致伸缩效应

采用固相反应烧结法制备了（La_1-yTb_y)_0.67Sr_0.33MnO₃系列样品（y=0,0.05,0.15,0.20,0.25,0.33,0.40,0.50,0.60,1.00).X射线衍射表明,随着y值增大钙钛矿型晶体结构从菱面对称性向正交对称性转变.180K时,μ₀H=7T条件下,在y=0.40样品的巨磁电阻可达900％.μ₀H=1.7T时,y=0.20样品的室温磁致伸缩为-50×10^-6.210K时,y=0.33样品的磁致伸缩可达-130×10^-6. 关键词：     

A comparative study of the magnetoelastic properties of the YFe10V2 and NdFe10V2 compounds

The magnetoelastic properties of iron-rich REFe₁₀V₂ (RE=Nd, Y) compounds were studied via magnetostriction and thermal expansion measurements in the 5–300 K range of temperature in up to 6 T external fields. Results of thermal expansion analysis show that the spontaneous magnetostriction of the compounds mostly originates from itinerant magnetization. Besides, the small volume striction appearing in the thermal expansion of the Nd compound close to 50 K suggests the existence of a basal to conical spin re-orientation transition. The volume magnetostriction isotherms of both compounds take minimum values for external field corresponding to the anisotropy field. In addition, the anisotropic and the volume magnetostriction traces of the NdFe₁₀V₂ take marked maxima under low field, with a relatively large initial magnetostrictivity, again more pronounced at the conical–axial spin re-orientation transition (T_SR=130 K). Analysis of the anisotropic magnetostriction of the Nd compound leads to the conclusion that the contribution of Nd–Fe interactions is negligible. The temperature dependence of volume magnetostriction is in good agreement with prediction of a phenomenological model based upon a fluctuating local band theory. This analysis shows that the difference between the forced volume strictions of Y and Nd compounds below and above T_SR originates from the Nd sublattice magnetization.     

Magnetic properties of tetragonal uranium compounds I. the U2N2Z-ternaries

In the present work we report the magnetic behaviour of the tetragonal ternaries of the U₂N₂Z-type (Z=Sb, Bi and Te). Magnetic susceptibility measurements performed in a wide temperature range (4.2–1000 K) have shown, that they are ferromagnetically ordered with Curie temperatures 166, 154 and 71 K for U₂N₂Sb, U₂N₂Bi and U₂N₂Te, respectively. For all the investigated compounds the κ^-1_M(T) function in the temperature range above T_C can be expressed as (A/T + B)^-1 - λ.Magnetization measurements were carried out up to magnetic field strengths of 140 kOe in the temperature range from 4.2 to the respective T_C. It follows from these measurements that σ(T)_H and σ(H)_T for U₂N₂Sb and U₂ N₂Bi are typical as for normal ferromagnets. On the other hand, U₂N₂Te exhibits, unexpectedly, two distinct maxima on the σ(T) curves up to fields of 4.5 kOe; one at 45 K and the other one at 71 K. Previous neutron diffraction studies of this compound have shown that the magnetic moments of uranium atoms at 4.2 K are titled by about 20° from the basal plane.The results obtained are interpreted in terms of the crystal-field interaction of the 5f² electrons of the U⁴ ion. In consequence a γ_5t doublet is expected to be the ground-state crystal field level in the U₂N₂Z-type as well as in many other ternary uranium tetragonal compounds. However, in the case U₂N₂Te, the singlet-singlet-doublet “band” as a ground system is postulated.     

Magnetostrictive strains in some light rare earth-aluminum laves phase compounds

Magnetostriction measurements of some RAl₂ (R = Pr, Nd and Sm) compounds were carried out from 4.2 K up to above their magnetic ordering temperature (T_c) and in applied field up to 2.1 T. PrAl₂ exhibited a huge magnetostriction (≈10^?3) and well-defined saturation below 30 K and at about 0.8 T. For SmAl₂ no saturation was observed up to the highest available applied field. This implies that the material is highly anistropic. The strain for NdAl₂ exhibited a change is sign at higher fields. Variation of strain with temperature for PrAl₂ is found to be in good agreement with the single-ion magnetoelastic theory. Large volume strains are observed for the compounds studied. These strains are attributed to a volume dependence of the crystal field interaction.     

A note on exchange and crystal field interactions in R2Fe14B compounds: Yb2Fe14B

The R₂Fe₁₄B phase has been found to exist for R=Yb. The magnetic properties presented in this paper complete the characterization of the compounds in this series for which the Stevens α_J coefficient of the R³⁺ ion is positive. ⁵⁷Fe Mössbauer spectroscopy establishes the existence of a magnetization reorientation at 115 K of the type observed in Er and Tm compounds associated with a small Fe magnetization anisotropy. From the neutron diffraction measurements obtained at 4.2 K with and without an applied magnetic field, the easy direction of magnetization was found to be along the [100] direction, in the basal plane of the tetragonal structure. These results show that in all compounds where α_J>0 for the R³⁺ ion, the easy direction of magnetization in the plane is determined by the second order crystal field terms and rare earth-Fe exchange interactions and is independent of the sign of the 4th order crystal field terms.     

Magnetic anisotropy of Sm2Fe14B single crystal

Magnetization measurements have been made on a Sm₂Fe₁₄B single crystal in magnetic fields up to 140 kOe. The easy direction of the magnetization lies along [100] in the tetragonal structure P4₂/mnm. Magnetic anisotropy energies at 290K along [110] and [001] have been estimated to be 5.8×10⁶ and 1.1×10⁸ erg/cm³, respectively, both becoming much larger at lower temperature. No evidence of the spin canting of Nd₂Fe₁₄B type is observed even at 4.2K.     

Magnetic behaviour of NpAs2 from Mössbauer spectroscopy

The Mössbauer hyperfine spectra of the 60 keV resonance of ²³⁷Np in powder and single crystal absorbers of NpAs₂ were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs₂. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μ_B at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.     

237Np Mössbauer study of a novel intermetallic NpRh2Si2

NpRh₂Si₂ (ThCr₂Si₂ structure) has been studied by ²³⁷Np Mössbauer spectroscopy between 4.2 and 100 K. The isomer shift value suggest a Np⁴⁺ electronic configuration. A single site combined magnetic plus quadrupole pattern is observed up to the magnetic ordering temperature of T_c=73(1) K. A Np magnetic moment of 1.4μ_B is deduced from the hyperfine field measured at 4.2 K. The magnetic moments are estimated to make an angle of either 90° or 34° with the tetragonal axis.     

The forced magnetostriction of nickel at 4.2 K

The forced volume magnetostriction of polycrystalline nickel at 4.2 K has been determined with a relative accuracy of 2 × 10^?2. Combining our result with previous data on the forced magnetostriction, we derive for the forced magnetostriction constants: h'₀ = (40 ± 1) × 10^?8T^?1, h'₁ = (-95 ± 2) × 10^?8T^?1, h'₂ = (-19 ± 2) × 10 ^?8T^?1.     

Anisotropy in the magnetic susceptibility of 2H-TaSe2

The magnetic susceptibility of 2H-TaSe₂ is temperature dependent and anisotropic. The susceptibility with H parallel to the c axis is approximately 2.5 times that perpendicular. The decrease in susceptibility below the charge density wave transition at 122 K is also anisotropic with [χ₆(122 K) ? χ₆(4.2 K]?[χ_⊥(122 K) ? χ_⊥(4.2 K)] ≈ 4. Consequently, this change cannot be simply interpreted as a change in the density of states at the Fermi level, unless very anisotropic electron g values are assumed.     

Magnetostriction studies of magnetic phase transitions in the copper metaborate CuB2O4

The field dependences of the longitudinal and transversal magnetostriction of the copper metaborate CuB₂O₄ were measured at various temperatures below the Néel point in magnetic fields directed along the tetragonal axis or in the basal plane. Magnetostriction was found to exhibit jumps at magnetic-field-induced phase transitions to a commensurate weak ferromagnetic state, as well as to grow smoothly in fields above and below the critical level. The magnetostriction observed in a magnetic field directed along the tetragonal axis is shown to be primarily caused by volume dilatation of the crystal. The experimental data obtained were used to construct the magnetic phase diagram of copper metaborate magnetized along the tetragonal axis.     

Nuclear ferromagnetic resonance on Co2B and on some dilute alloys

The NMR of Co⁵⁹ at 4.2 K in the compound Co₂ B and in the series (Co_0.99X_0.01)₂B where X = Ti, V, Cr, Mn, Fe and Ni is developed in this paper. The NMR spectrum of Co₂B shows a single narrow line which indicates that the easy magnetization direction is parallel to the tetragonal axis. In a static external field the shift of the line indicates that the hyperfine field is negative.Spin-lattice relaxation time T₁ measurements show that the density of states at the Fermi level reaches a maximum in the vicinity of X = V, Cr, in agreement with previous magnetic, resistivity and specific heat investigations. These results are interpreted by the virtual bound state concept.     

The magnetization of GdAl2

The magnetization of a single crystal of GdAl₂ has been measured parallel to the easy direction as a function of field (maximum field 1.7 T) within the temperature range 4.2–300 K. The main emphasis was placed on the results obtained for the ferromagnetic phase. From an analysis based on molecular field theory it is deduced that the magnetic moment at 0 K is 7.2 μ_B per Gd ion and that the molecular field cannot be represented by a simpler polynomial than λ₁M + λ₂M³ + λ₃M⁵. The same data is analysed using spin-wave theory from which it is deduced that the spin-wave stiffness is 18 meV Ų and that the conduction band susceptibility is approximately 2.6 x 10^-6 emu g^-1. The conduction electron polarization, parallel to the Gd ion moment, amounting to 0.2 μ_B per Gd ion implies the presence of an internal field acting on the conduction electrons of approximately 200 T at 0 K.     

Low temperature thermal expansion and magnetostriction of TmCd

Thermal expansion measurements in various external magnetic fields confirm the recently found Jahn-Teller transition in TmCd at 3.16 K. The tetragonal strain ?₃ at T = 0 K is found to be 0.62 × 10^?3. From magnetostriction data we determine a coupling constant g₀² = 0.4 × 10^?3 K which is in good agreement with the value found from ultrasonic experiments. In order to explain our magnetostriction results in the cubic phase, g₀ has to be taken negative.     

Magnetic susceptibility and magnetic-field behavior of CuB2O4 copper metaborate

An experimental study is reported regarding the temperature dependence of the magnetic susceptibility of a CuB₂O₄ tetragonal single crystal within the 4.2–200-K range. It has been established that the magnetic susceptibility exhibits anomalies at 21 and 10 K and depends strongly on crystal orientation in the magnetic field. A study has been carried out of the field dependences of the magnetization of CuB₂O₄ at various temperatures and crystal orientations. It is shown that for T>21 K, the crystal is in a paramagnetic state determined by Cu²⁺ copper ions with an effective magnetic moment of 1.77 μ_B. Within the 10–21 K interval, the field dependence of the magnetization is typical of a weak ferromagnet with magnetic moments of the two antiferromagnetically coupled sublattices lying in the tetragonal plane of the crystal. The spontaneous weakly-ferromagnetic moment is 0.56 emu/g at 10 K. The canting angle of the sublattice magnetic moments, determined by the Dzyaloshinski-Moriya interaction, is 0.49°. It is believed that below 10 K, the CuB₂O₄ crystal retains its easy-plane magnetic structure, but with a zero spontaneous magnetic moment.     

Observation of paramagnetic relaxation in the NpCu4Al8 intermetallic compound

²³⁷Np Mössbauer spectroscopy between 77 and 4.2 K was carried out in connection with neutron diffraction studies between 300 and 2 K on the tetragonal intermetallic NpCu₄Al₈. The Mössbauer spectra give an isomer shift of + 14.3 mm/s vs. NpAl₂ indicating the non-Kramers Np³⁺(⁵I₄)-ion to be present. Below 45 K the onset of magnetic hyperfine splitting is observed. It develops into a fully resolved Zeeman pattern at 4.2 K with B_eff≈330 T. Neutron diffraction finds no evidence for magnetic order over the whole temperature range scanned. All these results can consistently be explained by paramagnetic relaxation phenomena involving a low lying Γ^t₅ doublet closely followed by a Γ^t₄ singlet as the tetragonal crystalline field states of the Np³⁺ ion.