Temperature Dependence Of Sublattice Magnetization Research Articles

Static and dynamic magnetic properties of the spin- triangle lattice antiferromagnet Na3Fe(PO4)2 studied by 31P NMR

31P nuclear magnetic resonance (NMR) measurements have been carried out to investigate the magnetic properties and spin dynamics of Fe3+ (S = 5/2) spins in the two-dimensional triangular lattice (TL) compound Na3Fe(PO4)2. The temperature (T) dependence of nuclear spin-lattice relaxation rates () shows a clear peak around Néel temperature, K, corresponding to an antiferromagnetic (AFM) transition. From the temperature dependence of NMR shift (K) above , an exchange coupling between Fe3+ spins was estimated to be K using the spin-5/2 Heisenberg isotropic-TL model. The temperature dependence of divided by the magnetic susceptibility (χ), , above proves the AFM nature of spin fluctuations below in the paramagnetic state. In the magnetically ordered state below , the characteristic rectangular shape of the NMR spectra is observed, indicative of a commensurate AFM state in its ground state. The strong temperature dependence of 1/T 1 in the AFM state is well explained by the two-magnon (Raman) process of the spin waves in a 3D antiferromagnet with a spin-anisotropy energy gap of 5.7 K. The temperature dependence of sublattice magnetization is also well reproduced by the spin waves. Those results indicate that the magnetically ordered state of Na3Fe(PO4)2 is a conventional 3D AFM state, and no obvious spin frustration effects were detected in its ground state.

Tunable Exchange Bias and Bipolar Switching of Magnetization Near Room Temperature

Temperature-induced sign reversal of magnetization in the vicinity of room temperature is reported in a single-phase spinel compound of Ni(Cr0.70Fe0.30)2O4. The magnetic compensation temperature is 358 K. The mechanism of magnetization reversal is explained by considering the different temperature dependences of sublattice magnetization. Bipolar switching of magnetization at room temperature is demonstrated by just varying the magnetic field from + 200 to + 2050 Oe, i.e., without changing the direction of the magnetic field. Tunable positive and negative exchange bias fields are observed in the vicinity of room temperature with a maximum value of 1 kOe.

Magnetovolume effects and effect of pressure on the temperature dependence of sublattice magnetization in Ho(Co0.9Ga0.1)2 compounds

The temperature dependence of the magnetic moments of cobalt and holmium ions in the temperature range of 5–300 K at pressures of P = 0 and 5 kbar was determined based on neutron diffraction data on Ho(Co0.9Ga0.1)2 intermetallide. The temperature dependence of the lattice parameter was established, and the spontaneous bulk magnetostriction was determined. These quantities were calculated using the exchange striction model of a ferrimagnet, and good agreement with the experiment was obtained. The parameters of the exchange interaction between pairs of atoms of this compound and the magnetoelastic coupling constant were estimated. An original interpretation of the nature of the first-order magnetic phase transition in HoCo2 is proposed.

Negative magnetization and zero-field cooled exchange bias effect in Co0.8Cu0.2Cr2O4 ceramics

The negative magnetization and zero-field cooled exchange bias (ZFC EB) effect are observed in Co0.8Cu0.2Cr2O4 polycrystalline ceramics. 20% Cu substitution for Co in CoCr2O4 leads to the evident magnetization reversal at the compensation temperature (Tcomp ∼ 50 K) with applied magnetic field of 500 Oe. Besides, Tcomp decreases monotonously with increasing applied field, and the negative magnetization finally disappears when the field increases to 9000 Oe. Different temperature dependence of sublattice magnetization at different crystallographic sites is proved to induce the magnetization reversal. In addition, ZFC EB effect can be tuned by measuring temperature and presents the maximum of exchange bias field (HEB) with ∼2300 Oe at 50 K. This unconventional EB effect can be attributed to the coupling interaction between the two sublattices.

Unified approach to electronic, thermodynamical, and transport properties ofFe3SiandFe3Alalloys

The electronic, thermodynamical, and transport properties of ordered ${\mathrm{Fe}}_{3}X$ ($X=\text{Al},\mathrm{Si}$) alloys are studied from first principles. We present here a unified approach to the phase stability, the estimate of the Curie temperature, the temperature dependence of sublattice magnetizations, magnon spectra, the spin-stiffnesses, and residual resistivities. An important feature of the present study is that all calculated physical properties are determined in the framework of the same first-principles electronic structure model combined with the effective Ising and Heisenberg Hamiltonians used for study of the thermodynamical properties of alloys. Curie temperatures, spin-stiffnesses, and magnon spectra are determined using the same calculated exchange integrals. Finally, the transport properties are calculated using the linear-response theory. Our theoretical estimates compare well with available experimental data. In particular, calculations predict (in agreement with experiment) the ordered $D{0}_{3}$ phase as the ground-state alloy structure, demonstrate that a correct relation of Curie temperatures of ${\mathrm{Fe}}_{3}\mathrm{Al}/{\mathrm{Fe}}_{3}\mathrm{Si}$ alloys can be obtained only by going beyond a simple mean-field approximation, provide reasonable estimates of spin-stiffnesses, and give resistivities compatible with structural disorder observed in the experiment. Although the calculated temperature dependences of the Fe magnetization on different sublattices are similar, they nevertheless deviate more than in the experiment, and we discuss a possible origin.

Monte Carlo simulations of the Blume–Emery–Griffiths model with S=3/2 and S=5/2

The magnetic properties of the Blume–Emery–Griffiths(BEG) model [1] with fractional large spin(S=3/2 and S=5/2) on two-dimensional square lattice are discussed by making use of the Monte Carlo (MC) simulation for the magnetization 〈Sz〉, sublattice magnetizations 〈Sz(A)〉 and 〈Sz(B)〉, the magnetic specific heat CM and spin structures. The ground state phase diagram of these models has been obtained for exchange parameter J2/J1 and anisotropy parameter D/J1. The temperature dependence of sublattice magnetizations 〈Sz(A)〉 and 〈Sz(B)〉 is investigated for various values of the anisotropy D. The relations between the Curie temperature Tc and the critical temperature Ts at which sublattice magnetizations begin taking different values are also studied.

Sublattice magnetization in a two-sublattice ferrimagnetic with single-ion anisotropy in one of the sublattices

Temperature dependence of sublattice magnetization of an anisotropic ferrimagnetic is investigated. It is shown that the presence of even weak single-ion anisotropy results in a quantum reduction of the spin, and, consequently, changes the behavior of sublattice magnetization.

Monte Carlo simulations of the Spin-2 Blume-Emery-Griffiths model

The magnetic properties of the spin S = 2 Ising system with the bilinear exchange interaction J1SizSjz, the biquadratic exchange interaction J2Siz2Sjz2 and the single-ion anisotropy DSiz2are discussed by making use of the Monte Carlo (MC) simulation for the magnetization <Sz>, sub-lattice magnetizations <Sz(A)> and <Sz(B)>, the magnetic specific heat CM and spin structures. This Ising spin system of S = 2 with interactions J1 and J2 and with anisotropy D corresponds to the spin-2 Blume-Emery-Griffiths model. The phase diagram of this Ising spin system on a two-dimensional square lattice has been obtained for exchange parameter J2/J1 and anisotropy parameter D/J1. The shapes of the temperature dependence of sublattice magnetizations <Sz(A)> and <Sz(B)> are related with abnormal behavior of temperature dependence of <Sz> at low temperatures and affected significantly by the single-ion anisotropy D. The staggered quadrupolar (SQ) ordering turns out to be different largely between Ising systems with the single-ion anisotropy (D ≠ 0) and without the one (D = 0).

Localized magnetic moments in the Heusler alloy Rh2MnGe

X-ray magnetic circular dichroism (XMCD) of core-level absorption (x-ray absorption spectroscopy, XAS) spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Rh2MnGe at the Rh M3,2 and Mn L3,2 edges. The ratio of Rh and Mn spin moments amounts to 0.05 which is smaller than the ratio of 0.1 determined by a local density approximation electronic band structure calculation. We have found that the orbital moments of the Rh 4d and Mn 3d states are very small. The observed Rh 2p XAS spectrum can be understood on the basis of the Rh 3d partial density of unoccupied states as is typical for metals. The observed features of the Mn 2p XAS and XMCD spectra are dominated by final state multiplets as is typical for oxides. The comparison of experimental and ab initio calculated XAS/XMCD spectra reveals a strong narrowing of the Mn 3d bands, indicating strongly localized Mn moments. The magnetic moments are considerably more localized for Rh2MnGe in comparison with the isoelectronic compound Co2MnGe. In spite of the strong localization of the Mn moment, the temperature dependences of sublattice magnetization are equal for the Mn and Rh sublattices in contrast to the prediction by a Heisenberg model. This might be attributed to the remaining itinerant character of the Rh moment.

Néel temperature and magnetic phase boundary of the spin-12Heisenberg antiferromagnet in three dimensions

We obtained the eighth-order linked-cluster series for the free energy, the sublattice magnetization, and the staggered susceptibility of the spin-$\frac{1}{2}$ Heisenberg antiferromagnet on three-dimensional bipartite lattices. The series are analyzed using the standard extrapolation techniques with the application of a conformal transformation method to obtain the N\'eel temperature and the temperature dependence of sublattice magnetization. We obtain a more accurate estimate of the N\'eel temperature and the magnetic phase boundary for the body-centered cubic and the simple cubic lattices. The results are compared with related works.

Thermodynamics of quantum layered magnets with a weak easy-axis anisotropy

The O( N) model of quantum two-dimensional antiferro- and ferromagnets with a weak easy-axis anisotropy is considered. Analytical results for the temperature dependence of sublattice magnetization and ordering point are obtained. The temperature renormalization of the anisotropy parameter is investigated. The case where a small interlayer coupling is present is also discussed. The results are in good agreement with experimental data on the layered perovskites K 2NiF 4 and Rb 2NiF 4.

Magnetic dynamics inLa2CuO4with interlayer coupling and anisotropy gaps

The role of anisotropy gaps in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4--}$the in-plane gap of \ensuremath{\sim}10 meV, mainly due to the Dzyaloshinsky-Moriya antisymmetric exchange interaction, and out-of-plane anisotropy gap of \ensuremath{\sim}20 meV--together with interlayer coupling, is analyzed in the magnetic dynamics due to thermal excitation of spin waves, as revealed in temperature dependence of sublattice magnetization. By fitting with sublattice magnetization data for ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, obtained from a M\"ossbauer-spectroscopic study, and incorporating the quantum correction ${\mathit{Z}}_{\mathit{c}}$ to spin-wave energy, we obtain the average planar exchange energy, ${\mathit{J}}_{\mathit{p}}$=1380 K, the ratio of effective interplanar-to-planar exchange energy, ${\mathit{r}}^{2}$=4.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$, and the characteristic energy for spin waves propagating perpendicular to the plane, 2${\mathit{Z}}_{\mathit{c}}$${\mathit{J}}_{\mathit{p}}$r=21 K.

メスバウアー分光法によるＢｉ３Ｆｅ５Ｏ１２単結晶膜の磁気的性質の研究

Very recently, an artificial new material "bismuth iron garnet" was prepared for the first time with a reactive ion-beam-sputterring method. A magnetic characterization of this single crystalline garnet film was achieved by applying conversion electron Mössbauer spectroscopy (CEMS). To examine the temperature dependence of sublattice magnetization, CEMS measurments were made at 6, 78, 290, and 670 K, i.e. in both the ferrimagnetic and paramagnetic regions. The magnetic and quadrupole hyperfine interactions as compared with YIG indicated continuous effects of lattice expansion owing to the large spherical size of Bi3+ ions. The temperature dependence of sublattice magnetization has been explained by the two-sublattice molecular field model with a molecular field constant Nad slightly modified from that of YIG.

Conversion electron Mössbauer spectroscopy of a single crystalline Bi 3Fe 5O 12 film

A magnetic characterization of a single crystalline bismuth iron garnet film was done by applying conversion electron Mössbauer spectroscopy (CEMS). To examine the temperature dependence of sublattice magnetization, CEMS measurements were made at 6, 78, 290 and 670 K, i.e. in both the ferrimagnetic and paramgnetic regions. The magnetic and quadrupole hyperfine interactions as compared with those in other garnets like YIG indicated continuous effects of lattice expansion. The temperature dependence of the sublattice magnetization has been explained at the present stage by the two-sublattice molecular field model using parameters slightly modified from those of YIG.

Magnetic dynamics in copper-oxide-based antiferromagnets: The role of interlayer coupling.

It is shown that thermal excitation of spin waves in a highly anisotropic antiferromagnet results in a characteristic temperature dependence of sublattice magnetization with a crossover from a 3D to a quasi-2D behavior. The magnetic dynamics in several copper-oxide-based antiferromagnets is analyzed in this context in terms of subtle details of their structural characteristics, and the temperature dependence of the Cu moment is used to determine the planar and interplanar exchange energies.

NMR Study of Magnetic- Exchange Interactions and Anisotropy in Antiferromagnetic EuTe

The 151 Eu NMR signals which are coupled to the high- and low-frequency branches of the antiferromagnetic resonance in EuTe were observed using a single crystal by the cw method in magnetic fields up to 21 kOe below 4.2 K. From the zero-field NMR frequency of the high-frequency branch, the temperature dependence of sublattice magnetization was obtained. From the spin wave theory, the exchange parameters were determined as J 1 / k B =0.114±0.003 K and J 2 / k B =-0.231±0.001 K, in good agreement with those determined by Oliveira et at . With these values the zero-point spin reduction was calculated to be 2.05%, and the zero-field 151 Eu NMR frequency at absolute zero to be 266.864±0.003 MHz. The frequency of the NMR low-frequency branch exhibited the “frequency pulling”. The external field dependence of the frequency yielded the in-plane anisotropy field H a =12.4±1.4 Oe at 1.8K.

Magnetization of a chain-like antiferromagnet

The temperature dependence of sublattice magnetizations in a chain-like antiferromagnet is investigated by the spin-wave theory considering kinematical interaction and dynamical interaction. For the latter the biquadratic terms are treated in the mean field approximation. The theory for zero-point spin reduction by Herbert and Ishikawa and Oguchi is extended to finite temperatures following Thermo-Field Dynamics approach by Takahashi. Numerical calculations for KCuF 3 show that the kinematical interaction plays an important role for the spontaneous magnetization, and when the effect of dynamical interaction is taken into account, the results obtained come nearer to the experimental ones.

Neutron Diffraction Study of γ-Phase Mn-Ir Single Crystals

Neutron diffraction was measured on single crystals of the disordered γ-phase Mn 1- x Ir x ( x =0.128, 0.170, 0.204 and 0.256). Diffraction patterns were interpreted by an antiferromagnetic two-sublattice structure with tetragonal symmetry. The (110) magnetic reflection was measured up to about 800 K for the determination of the temperature dependence of sublattice magnetization and the Néel temperature. The critical index for magnetization was found to be 0.38 for the x =0.170 alloy. The Néel temperature increases remarkably with increasing Ir at a rate of about 9 K/at%Ir. The obtained magnetic form factors are well expressed by f =(1/0.9)( f 3d -0.1 f 4s ) constructed from the scattering functions calculated by Freeman and Watson. The average magnetic moment per Mn atom increases from 2.2µ B to 2.6µ B with increasing Ir from x =0 to x =0.204. Diffuse scattering due to partial atomic order of Mn 3 Ir was observed in the x =0.256 alloys.

Temperature dependence of the sublattice magnetization in the stable magnetic phase and the one-third-power law

It is shown that the temperature dependence of sublattice magnetizations in various ferro-, antiferro- and ferrimagnets fits a simple power law (1− T T c ) β in a range of about 0.5 T c < T < 0.99T c. Though the temperature exponent in general deviates slightly from one-third, in ferrimagnets different values are obtained for the two non-equivalent iron sublattices.

Localized Magnetic Moments in Mn4N

The temperature dependence of sublattice magnetizations and the specific heat due to magnetic ordering of Mn 4 N were measured in order to study the origin of the anomalous thermomagnetic curve of the compound. The observed magnetization-temperature curves of two sublattices are more convex upwards than those of normal magnetic crystals. The discontinuity of specific heat at the Curie temperature was found to be 7.4±1.0 cal/formula deg. which is much smaller than that expected on the assumption that the localized magnetic moments remain unchanged up to high temperatures. The average state density at the Fermi surface was estimated from the temperature independent magnetic susceptibility to be about 6/Mn eV. It was concluded from these facts that the high state density gives rise to large induced moments at low temperatures which depolarize with increasing temperature.