The influence of the chemical magnetization of oceanic basalts on determining the geomagnetic field paleointensity by the Thellier method

Dredged samples of basalts from the Mid-Atlantic Ridge (MAR) and the Red Sea Rift that are 0.2 to 1 Ma in age are used to show the possibility of separating the thermoremanent and chemical remanent magnetization components in oceanic basalts by the conventional Thellier–Coe method. It is found that the decay rate of chemical remanent magnetization during exposure of basalt samples with Curie temperatures of T _c = 145 and 240°C at temperatures of 340–380°C in the air was less than the formation of partial thermoremanent magnetization during the experiments using the Thellier–Coe method by a factor of 4.5–5; in contrast, for thermoremanent magnetization these rates almost coincide. Using this fact, we estimated the contributions from chemical remanent and thermoremanent magnetization components into the natural magnetization of basalts and, based on the thermoremanent magnetization component, defined the paleointensity of the geomagnetic field for the epoch when these basalts formed. In basalts from the southern MAR that are 1 Ma in age, the contribution of chemical remanent magnetization to the natural remanent magnetization is 60% to 80%; in those of 0.35 Ma it was less than 50%; in those of 0.2 Ma it was less than 10%. The calculated values of the virtual magnetic dipole moment of the Earth were close to those that have been published in the literature.     

Ferromagnetic glass on the base of aggregates of Ni amorphous nanogranules

We have studied theoretically (including computer simulations) magnetic properties of aggregates of ferromagnetic amorphous nanogranules in the presence of direct exchange between the neighboring granules and random anisotropy fields. We show that such a system can be considered as ferromagnetic glass. We demonstrate (basing on analytical considerations as well as on the results of numerical simulations) that the system is decomposed to clusters or domains with nearly collinear orientation of magnetization. The size of the domains depends on the ratio of the exchange interaction and random anisotropy. For quasi-2D structures we predict that the dipole-dipole interactions between the granules lead to a formation of magnetic vortices. Moreover, the computer simulations also reproduce the puzzling increase of the thermoremanent magnetization observed experimentally, which is expected to be a result of a temperature-dependent decrease in the anisotropy (or a temperature-dependent increase in the exchange). We also consider the structures with weak intergranular exchange and show that they are characterized by the presence of two critical temperatures.     

Reentrant spin glass behaviour in Nd0.84K0.12MnO3

Polycrystalline Nd_0.84K_0.12MnO₃ was prepared in single phase form with Pbnm space group. The magnetic properties are studied from magnetization, linear and non-linear susceptibility, and thermoremanent magnetization measurements. The sample exhibits paramagnetic to ferromagnetic transition followed by low temperature spin glass like transition. From frequency variation of ac susceptibility measurements, the spin glass transition temperature is found to be 97.6±0.1 K with critical exponents zυ=1.13±0.06. The critical exponent γ corresponding to spin glass transition has been determined from the third harmonic susceptibility analysis and it is found to be 3.09±0.05. The effective number of spins blocked under frustration and their correlation length are determined from the analysis of thermoremanent magnetization.     

Thermoremanent magnetization in Rb2Mn(1-x)CrxCl4 spin-glass

Time dependence of thermoremanent magnetization (TRM) in a new spin-glass, Rb₂Mn_(1-x)Cr_xCl₄ is measured at temperatures below the freezing temperature. The result shows that long time behavior of the TRM obeys a power law, M(t) = Ct^?A. It is shown from the analysis of the experiment that there is a possibility of the presence of a critical temperature.     

Magnetic properties of multiferroics-semiconductors Eu(1-x)Ce(x)Mn2O5

Studies of magnetization, magnetoresistance, and magnetic oscillations in semiconductor-multiferroics Eu(1-x)Ce(x)Mn2O5 (x = 0.2-0.25) (ECMO) at temperatures ranging from 5 to 350 K in magnetic fields up to 6 T are presented. It is shown that phase separation and charge carrier self-organization in the crystals give rise to a layered superstructure perpendicular to the c axis. An effect of magnetic field cycling on the superstructure, magnetization, and magnetoresistance is demonstrated. X-ray diffraction studies of ECMO demonstrating the effect of magnetic field on the superstructure are presented. The de Haas-van Alphen magnetization oscillations in high magnetic fields and the temperature-induced magnetic oscillations in a fixed magnetic field are observed at low temperatures. Below 10 K the quantum corrections to magnetization due to the weak charge carrier localization in 2D superlattice layers occur. It is shown that at all the temperatures the Eu(1-x)Ce(x)Mn2O5 magnetic state is dictated by superparamagnetism of isolated ferromagnetic domains.     

Full aging in spin glasses

The discovery of dynamic memory effects in the magnetization decays of spin glasses in 1983 marked a turning point in the study of the highly disordered spin glass state. Detailed studies of the memory effects have led to much progress in understanding the qualitative features of the phase space. Even so, the exact nature of the magnetization decay functions has remained elusive, causing confusion. In this Letter, we report strong evidence that the thermoremanent magnetization decays scale with the waiting time t(w). By employing a series of cooling protocols, we demonstrate that the rate at which the sample is cooled to the measuring temperature plays a major role in the determination of scaling. As the effective cooling time t(eff)(c) decreases, t/t(w) scaling improves and for t(eff)(c)<20 s we find almost perfect t/t(w) scaling, i.e., full aging.     

Magnetization processes of CuMn below the freezing temperature Tf

A simple classical model [5] that is based on a blocking of magnetic clusters in a uniaxial anisotropy field explains characteristic properties of magnetization processes of Cu(5–15 at.%) Mn as transition states between the “zero point magnetization” M₀ = M(T → OK) and the thermal equilibrium magnetization M_∞ = M(t → ∞, T). Further it shows two facts the experimental confirmations of which we report in this paper: (a) a magnetization decrease with increasing temperature in high fields and (b) a superposition rule for the magnetization processes in small fields and at low temperatures.     

Collective effects in a system of Pearl vortices and the magnetization of a thin superconducting film

The magnetization curves of a thin superconducting monolayer are calculated. It is found that for sufficiently high fields and temperatures the magnetization of a monolayer exhibits the same features as the reversible magnetization of layered superconductors in high fields. It is shown that these features are due to instability with respect to the dissociation of pairs of Pearl vortices into a gas of free vortices and to collective effects in a system of free vortices. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 460–465 (25 September 1998)     

磁性单层膜磁学性质的Monte Carlo模拟

采用类Ising模型,利用Monte Carlo方法研究了磁性单层膜中退磁性偶极作用和铁磁性交换作用对系统磁学性质的影响.结果显示,随着偶极相互作用的增加,系统在低温下的磁化出现平台现象,此时磁化曲线可分为2个阶段,在低外场下,温度升高,系统易磁化,在高外场下则反之.这种新奇的磁化行为导致系统的磁熵变在低温低外场下出现大于零的反常行为.在模拟过程中,对长程力作用采用了比较精确的处理方法.     

First order magnetization process in Pr2Fe14B single crystal

High-field magnetization process in Pr₂Fe₁₄B single crystal has been studied in static magnetic fields up to 230 kOe. The spontaneous magnetization is along the [001] direction of the tetragonal structure down to 1.5 K. When the field is applied along the [100] and [110] directions at low temperatures, magnetization jumps are observed at about 130 kOe and 160 kOe, respectively, which are considered to correspond to the first order magnetization process (FOMP). Observed features of the magnetization curves including FOMP and their temperature dependence are well reproduced by the calculation based on a simplified Hamiltonian including the crystalline electric fields and the FePr     

Stresses induced in polymer crystals by the atomic-molecular internal dynamics

The temperature dependences of the dynamic tensile stresses on the chemical bonds of the skeleton of chain molecules and the bonds between molecules in poly(ethylene) and polyamide 6 crystals are measured using X-ray diffraction and Raman spectroscopy in the temperature range 5–500 K. The relations of the dynamic stresses to the quantum characteristics of the vibrational dynamics of polymer molecules are derived. The contributions from the zero-point and thermal vibrations of molecules to the stresses are determined. It is revealed that the dynamic tensile stresses on the intermolecular bonds at the melting temperatures of the crystals approach the tensile strength of these bonds. Original Russian Text ? V.I. Vettegren, A.I. Slutsker, V.B. Kulik, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 198–205.     

Origin of self-reversed thermoremanent magnetization

The ability of certain magnetic minerals to acquire a remanent magnetization that opposes the direction of the Earth's magnetic field has fascinated rock magnetists since its discovery in 1951. Here, we determine the origin of this phenomenon, which is termed self-reversed thermoremanent magnetization (SR-TRM). We present direct transmission electron microscopy observations of negative exchange coupling across antiphase domain boundaries (APBs) in ilmenite-hematite. This coupling is linked intrinsically to the origin of SR-TRM and is responsible for the formation of two new classes of magnetic domain wall at APBs. We present simulations of the chemical and magnetic structure of the APBs and show that SR-TRM is generated by coupling between strongly ferrimagnetic Ti-rich domains and weakly ferrimagnetic Fe-rich domains, which form during the transition from short- to long-range cation order.     

Paleointensity of the geomagnetic field in the last half-million years in regions of the Red Sea and south of the Mid-Atlantic Ridge

The paleointensity of a geomagnetic field is determined on the basis of the standard Thellier’s method using the remanent magnetization (H _a ) of basalts of the rift zone of the Red Sea (with an age of 0.5 million years) and south of the Mid-Atlantic Ridge (with an age of 0.1–0.3 million years). Samples were selected whose natural remanent magnetization (NRM) was mainly one-component and the measured Curie temperatures of the titanium-magnetite fraction (T _C = 205–250°C) were close to the calculated values. Samples with T _C = 580°C were also used. It was shown that NRM of basalts with both high and low T _C values have a thermoremanent nature. The studies have revealed that the law of independence and additivity of partial thermoremanent magnetizations (PTRMs) is satisfied for the collected samples with an error of no more than 6% and the error of determination of the field of TRM creation using the Arai-Nagata diagram is no more than 10%. The value H _a = 32 A/m determined from NRM of basalts south of the Mid-Atlantic Ridge appears to be approximately equal to the contemporary intensity of the geomagnetic field (H _c ) in this region. In the Red Sea region, H _a = 77.5 A/m, which is 2.5 times higher than the current value. It is suggested that during formation of the basalts of the Red Sea rift zone the geomagnetic pole was located in that region and the virtual dipole moment was 35% higher than its contemporary value.     

Zero—field—cooled and field—cooled magnetizations and magnetic susceptibility of itinerant ferromagnet SrRuO3

Zero-field-cooled(ZFC) magnetization,field-cooled(FC) magnetization,ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160K.An empirical model is proposed to calculate the measured ZFC magnetization.The result indicates that the calculated ZFC magnetization compares well with the measured one.Based on the generalized Preisach model.both the ZFC and FC curves are reproduced by numerical simulations.The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.     

Magnetic properties of the Ising dipole ferromagnet LiTbF<Subscript>4</Subscript>

The field and temperature dependences of the magnetization of a LiTbF₄ single crystal are measured at temperatures ranging from 2 to 300 K in magnetic fields (up to 5 T) directed parallel and perpendicular to the crystallographic axis c. It is revealed that the temperature dependence of the induced (van Vleck) transverse magnetization exhibits nonmonotonic behavior with a maximum in the vicinity of the liquid-nitrogen temperature. The results of magnetization measurements are used to determine the parameters of the crystal field and interionic magnetic interactions.     

Specific character of metamagnetic transitions in Fe2P

Temperature and field dependences of magnetization in the fields directed along the easy axis of magnetization are studied on Fe₂P single crystals under pressure. It is shown that the first-order magnetic transition from ferromagnetism to paramagnetism (FM-PM) usually observed in sufficiently strong fields can be resolved into two sequential transitions in weak fields (H ? 600 Oe): 1) from the PM state to the intermediate metamagnetic phase and 2) transition to the low-temperature magnetic phase. The temperatures of these transitions undergo a specific evolution under pressure. A theoretical model, in which the characteristic features of magnetic behaviour of Fe₂P are associated with the successive additional ordering of magnetic components of its composition, is proposed.     

Effects of Cu dilution in IrMn on the exchange bias of CoFe/IrMn bilayers

The effect of nonmagnetic dilution in metallic antiferromagnets (AFMs) on the exchange bias (EB) has been investigated from a structural, magnetic, and Monte Carlo simulation point of view in bilayers of CoFe/(IrMn)1-xCux. Dilution by Cu atoms throughout the volume of the AFM IrMn gives rise to an enhanced EB field (HEB) for 5 K相似文献   

Upper bounds for Ising model correlation functions

A Griffiths correlation inequality for Ising ferromagnets is refined and is used to obtain improved upper bounds for critical temperatures. It is shown that, for non-negative external fields, the mean field magnetization is an upper bound for the magnetization of Ising ferromagnets.On leave (1970–71) from Northwestern University, Evanston, Illinois 60201. Supported at IAS by a grant from the Alfred P. Sloan Foundation.     

Metamagnetic transition in Na 0.85 CoO2 single crystals

We report the magnetization, specific heat, and transport measurements of a high quality Na(0.85)CoO2 single crystal in applied magnetic fields up to 14 T. At high temperatures, the system is in a paramagnetic phase. It undergoes a magnetic phase transition below approximately 20 K. For the field H||c, the measurement data of magnetization, specific heat, and magnetoresistance reveal a metamagnetic transition from an antiferromagnetic state to a quasiferromagnetic state at about 8 T at low temperatures. However, no transition is observed in the magnetization measurements up to 14 T for H perpendicular c. The low temperature magnetic phase diagram of Na(0.85)CoO2 is determined.