Magnetic phase diagrams of Ho1−xDyxNi2B2C

We performed the magnetization measurement on Ho_1−xDy_xNi₂B₂C single crystals (x=0.1, 0.2, 0.3, 0.4, and 0.6) with magnetic field applied perpendicular and parallel to the c-axis. But only for the magnetic field perpendicular to the c-axis, the increase of Dy³⁺ concentration affects the magnetically ordered states of HoNi₂B₂C compound and makes the phase diagram more complicated. The antiferromagnetic ordering state attributed to Dy³⁺ sublattice starts to appear from a case of x=0.2 and finally the magnetic phase diagram becomes analogous to that of DyNi₂B₂C as x is increased which is consistent with the neutron scattering result.     

Exchange and hyperfine fields acting on dilute Eu In SmCo5, Sm2(Co1-xFex)17 and Sm2CoxMy( rm M = Fe, Cu, Zn, Zr)

Mössbauer source experiments of dilute ¹⁵³Eu and ⁵⁷Fe in SmCo₅ and ¹⁵³Eu in Sm₂(Co_1-xFe_x)₁₇ and Sm₂ Co_x M_y at 4.1 K were performed. After the ${}^{153}\text{Sm}\text{→β?}{}^{153}$ decay the Eu ion is trivalent and exhibits extremely large hyperfine interactions due to strong exchange fields. Since the exchange interactions are comparable to the Eu³⁺ spin-orbit coupling, we calculated the expectation values of the Eu³⁺ spin, magnetic hyperfine field and electric field gradient as a function of exchange field and second order crystalline field, by diagonalization of the full Hamiltonian of spin orbit, exchange and crystalline field. For SmCo₅ and Sm₂(Co_1-xFe_x)₁₇ the exchange and crystalline fields are known and thus allow us to analyze our experimental results and obtain the polarized conduction electron contributions to the magnetic hyperfine field. The contribution due to magnetic neighbour polarization of conduction electrons is found to be linear in exchange field. The experimental results together with the theoretical analysis yield the Eu electric field gradient 4f Sternheimer shielding factor R_Q to be 0.26±04. It is shown that Mössbauer studies of two probes (¹⁵⁵Gd and Eu³⁺) in magnetic systems yield directly the second order crystalline field, the exchange field and the various contributions to the hyperfine field acting on the Eu nucleus. From the experimentally measured magnetic hyperfine fields alone, approximate values for the exchange fields in the mixed systems Sm₂Co_xM_y were determined.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Magnetic and magnetocaloric properties of martensitic Ni2Mn1.4Sn0.6 Heusler alloy

The evolutions of magnetic properties at low temperatures and the influence of magnetic field on the temperature dependence of specific heat in martensitic Ni₂Mn_1.4Sn_0.6 Heusler alloy are studied. The frequency-dependent blocking temperature and considerable exchange bias below it are measured in the martensitic phase. From the analysis of the specific heat curves under magnetic field, a large inverse magnetocaloric effect manifested as the magnetic field induced rise of isothermal magnetic entropy and/or magnetic field induced adiabatic temperature decrease in the vicinity of the reverse magnetostructural transformation and a significant value of the conventional magnetocaloric effect at the Curie temperature are obtained. The Debye temperature and electronic coefficient equal to Θ_D=310±2 K and γ= 16.6±0.3 mJ/K²mol, respectively, do not depend on the magnetic field.     

High heat generation ability in AC magnetic field for nano-sized magnetic Y3Fe5O12 powder prepared by bead milling

Nano-sized magnetic Y₃Fe₅O₁₂ ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9 μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y₃Fe₅O₁₂ ferrites. The highest heat ability in the AC magnetic field was for the fine Y₃Fe₅O₁₂ powder with a 15-nm crystallite size (the samples were milled for 4 h using 0.1 mm? beads). The heat generation ability of the excessively milled Y₃Fe₅O₁₂ samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4 h using 0.1 mm? beads, the heat generation ability (W g⁻¹) was estimated using a 3.58×10⁻⁴ fH² frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which is the highest reported value of superparamagnetic materials.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Possible observation of the coexistence of superconductivity and long-range magnetic order in NdRh4B4

The ternary rare earth compound NdRh₄B₄ has been studied by means of critical field, low temperature heat capacity, and static magnetic susceptibility measurements. Features in the upper critical field and heat capacity data at 1.31 K and 0.89 K suggest the occurrence of long-range magnetic order in the superconducting state. The temperature dependence of the static magnetic susceptibility follows a Curie-Weiss law with an effective magnetic moment μ_eff = 3.58 ± 0.05 μ_B and a Curie-Weiss temperature θ_p = ?6.2 ± 1.0 K between 20 K and room temperature. However,, magnetization vs. applied magnetic field isotherms suggest the development of a ferromagnetic component in the Nd³⁺ magnetization at low temperatures.     

Mössbauer study of Fe2+ ions in trigonal sites of spinels: GeFe2O4, GeCo2O4, GeNi2O4

We discuss here the results and the interpretation in the crystalline-field approach of some Mössbauer experiments on Fe²⁺ ions in the spinels GeFe₂O₄, GeCo₂O₄ and GeNi₂O₄. Once the sign of the quadrupolar interaction e²qQ has been deduced from a magnetic spectrum, the thermal variation of e²qQ may be used for determining the electronic level scheme of the Fe²⁺ ion (including the energies and wavefunctions of the levels). Then we may predict the form and magnitude of the spin hamiltonian, of the magnetic anisotropy and of the hypefine field tensor. Below T_N the experimental results are expressed in terms of a molecular-field, the eventual variations of which have been studied in magnitude and in orientation; by using the same calculation for the three compounds, we obtained a reasonable agreement between the experimental and calculated values of the hyperfine field at 0°K.     

Champs hyperfins magnetiques sur les noyaux de 121Sb5+ dans Cr2O3

The magnetic hyperfine interactions of the impurity Sb⁵⁺ in Cr₂O₃ have been examined by the Mössbauer effect of ¹²¹Sb.The magnetic field on the nucleus of ¹²¹Sb⁵⁺ (Cr₂O₃) measured at 77° K, H(0) = 170 ± 15 koe.The comparison of the results obtained for the impurity ¹²¹Sb⁵⁺ with those for the ¹¹⁹Sn⁴⁺ ions, occuring in the same matrix, suggests the preponderant effect of the decoupling of the electron spins of 5s valency on the values of the internal fields observed for those ions.     

Critical behavior and magnetic entropy change in the La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3 perovskite

Critical behavior in the La_0.6Sr_0.4Mn_0.8Fe_0.1Cr_0.1O₃ ceramics was studied using magnetization methods. Results show that the paramagnetic–ferromagnetic transition is of second order. Based on the critical behavior analysis using the Banerjee criterion and the Kouvel–Fisher method, we find the critical exponents: β=0.395±0.010, γ=1.402±0.010, and δ=5.208±0.007, for which the magnetic interaction is satisfied within the three-dimensional Heisenberg model. Results indicate the presence of short-range interactions. The magnetic entropy change (−ΔS_M) reached maximum values of 1.75, 1.45, 1.15, 0.8 and 0.43 J Kg⁻¹ K⁻¹ under a magnetic field variation of 5, 4, 3, 2 and 1 T, respectively. Nevertheless, these (−ΔS_M) values are much low for any potential application at this moment. The nature of this phenomenon is discussed in relation to the characteristics of the magnetic phase transition and critical exponents.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Magnetic and ESR studies of Er3+ in lanthanum dihydride LaH2

Er³⁺ electron spin resonance ESR and magnetic susceptibility have been studied in metallic lanthanum dihydride host. The ESR spectrum contains a single asymmetrical line with g-factor g = 6.68 ± 0.05 close to that expected for Γ₇ as ground state. The experimental magnetic susceptibility was interpreted on the base of LLW cubic crystal field Hamiltonian. The best fit of the experimental data has been obtained for the following B₄ and B₆ crystal field parameters: B₄ = ?5.2 × 10^?3 K; B₆ = 3.8 × 10^?5 K which support the anionic-like character hydridic model of hydrogen atoms in this hydride.     

Field-induced structural transition and the related magnetic entropy change in Ni43Mn43Co3Sn11 alloy

Magnetic properties and magnetic entropy change ΔS were investigated in Heusler alloy Ni₄₃Mn₄₃Co₃Sn₁₁. With decreasing temperature this alloy undergoes a martensitic structural transition at T_M=188 K. The incorporation of Co atoms enhances ferromagnetic exchange for parent phases. Austenitic phase with cubic structure shows strong ferromagnetic behaviors with Curie temperature T_C^A at 346 K, while martensitic phase shows weak ferromagnetic properties. An external magnetic field can shift T_M to a lower temperature at a rate of 4.4 K/T, and a field-induced structural transition from martensitic to austenitic state takes place at temperatures near but below T_M. As a result, a great magnetic entropy change with positive sign appears. The size of ΔS reaches 33 J/kg K under 5 T magnetic field. More important is that the ΔS displays a table-like peak under 5 T, which is favorable for Ericsson-type refrigerators.     

Field annealing effect on amorphous (FexCo1-x)85Nb15 alloys

The magnetic anisotropy induced by magnetic field annealing was examined for amorphous (Fe_xCo_1-x)₈₅Nb₁₅ alloys produced by a sputtering technique. An appreciably large uniaxial anisotropy was observed even at x = 0. The anisotropy constant (K_u) obtained was 2.3x10³ erg/cm³ at x=0, and it increased with x, showing a maximum of about 5.4x10³ erg/cm³ near x = 0.2. This compositional change in K_u was explained in terms of the directional ordering of magnetic Fe-Co atom pairs under the negative nonmagnetic interaction energy leading to a precipitation.     

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb₃Fe₅O₁₂, the exchange effective field can be expressed as H_eff=−λ·M_Fe=−λχ_eff·H_e=−γ·H_e where γ is named as the exchange field parameter and H_e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter γ, the properties of exchange interaction in ytterbium iron garnet (Yb₃Fe₅O₁₂) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter γ is the function of the temperatures under different external magnetic fields, and γ=a+b·T+c·T², where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb₃Fe₅O₁₂ are revealed. Also, excellent fits to the available experiments are obtained.     

Emission Mössbauer studies of the magnetoresistive compound, La0.7Sr0.3MnO3

The magnetic behavior of the Sr_0.3 manganite is studied using a local microprobe, ⁵⁷Co. In contrast with Ca substituted manganites, a much larger fraction of the material exhibits short-range order with superparamagnetic-like behavior even at 80 K. The differences in behavior are attributed to the large mismatch between the ionic radii of La⁺³ and the divalent substituent Sr⁺², which introduces anharmonicity in local vibrations. In common with all other compounds exhibiting negative bulk magnetoresistivity, the Sr_0.3 compound also exhibits very marked softening of lattice as one approaches T_c from below. Application of an external magnetic field results in coalescing of nanosized magnetic clusters to form larger ones with better alignment of spins.     

Investigation of TmFe4Al8 and DyFe4Al8 by means of 169Tm and 161Dy Mössbauer spectroscopy

We have performed ¹⁶⁹Tm and ¹⁶¹Dy Mössbauer spectroscopy on TmFe₄Al₈ and DyFe₄Al₈. From the temperature dependence of the electric quadrupole splitting of the ¹⁶⁹Tm spectra of TmFe₄Al₈ we have determined the second order crystal field potential V⁰₂ = (100 ± 10) K and the exchange field term g_Jμ_BH_M = (1 ± 1) K. The temperature dependence of the hyperfine field of the ¹⁶¹Dy spectrum of DyFe₄Al₈ gives g_Jμ_BH_M = (15 ± 3) K. With these exchange fields magnetic transition temperatures of the rare earth sublattices were found, which are consistent with experiment. The relaxation behaviour of the Tm sublattice below T_N = 187 K is discussed.     

Effet Mössbauer sur les noyaux d'impuretes 119Sn4+ dans la matrice de VO2

The phase transition in VO₂ doped with 0.16 at.% Sn⁴⁺ was investigated by Mössbauer spectroscopy of ¹¹⁹Sn. Hyperfine magnetic fields were observed on ¹¹⁹Sn nuclei below the transition temperature. At 77 K the maximum hyperfine magnetic field was found to be H(0)_{77 K} = 120 ± 10 kOe The results obtained are discussed in terms of fine antiferromagnetic particles.     

Magnetic order in Y6(57Fe:Mn)23H26

The ferrimagnetic compound Y₆Mn₂₃ and its hydride Y₆Mn₂₃H₂₆, both doped with 0.5%⁵⁷Fe, have been investigated using the ⁵⁷Fe Mössbauer resonance and dc field magnetization measurements. For the hydride a small ⁵⁷Fe magnetic hyperfine field is observed to increase abruptly below 110 K whereas the bulk magnetization results suggest antiferromagnetic ordering at T_N≈ 180 K.     

Decoupled magnetic subsystems in the random mixture FepCo1-p(C5H5NO)6(ClO4)2

Specific heat data on the random mixtures Fe_pCo_1-pL₆(ClO₄)₂, where L = C₅H₅NO, are presented. The Fe and Co magnetic atoms have competing anisotropies since the pure Fe and Co compounds are known to be good examples of the simple cubic, $\text{S =}\text{1}\text{2}$, Ising and XY magnet, respectively. The experimental data show the two magnetic subsystems in the mixtures to be almost completely decoupled, which is a consequence of the fact that the crystal field anisotropies of the Fe²⁺ and Co²⁺ ions, yielding g_∥ ? g_⊥ and g_⊥ ? g_∥, respectively, are very strong compared to the magnetic exchange interactions. Consequently the two magnetic subsystems experience one another as nonmagnetic impurities. A model is presented which explains these results, as well as those previously found for related random mixtures, in terms of two interpenetrating percolation clusters.