Temperature dependence of the magnetization of canted spin structures

Numerous studies of the low-temperature saturation magnetization of ferrimagnetic nanoparticles and diamagnetically substituted ferrites have shown an anomalous temperature dependence. It has been suggested that this is related to freezing of canted magnetic structures. We present models for the temperature dependence of the magnetization of a simple canted spin structure in which relaxation can take place at finite temperatures between spin configurations with different canting angles. We show that the saturation magnetization may either decrease or increase with decreasing temperature, depending on the ratio of the exchange coupling constants. This is in agreement with experimental observations.     

Parameter dependence of resonant spin torque magnetization reversal

We numerically study ultra fast resonant spin torque (ST) magnetization reversal in magnetic tunneling junctions (MTJ) driven by current pulses having a direct current (DC) and a resonant alternating current (AC) component. The precessional ST dynamics of the single domain MTJ free layer cell are modeled in the macro spin approximation. The energy efficiency, reversal time, and reversal reliability are investigated under variation of pulse parameters like direct and AC current amplitude, AC frequency and AC phase. We find a range of AC and direct current amplitudes where robust resonant ST reversal is obtained with faster switching time and reduced energy consumption per pulse compared to purely direct current ST reversal. However, for a certain range of AC and direct current amplitudes a strong dependence of the reversal properties on AC frequency and phase is found. Such regions of unreliable reversal must be avoided for ST memory applications.     

Zero temperature magnetization of a one-dimensional spin glass

We calculate the zero temperature magnetizationm of a one-dimensional Ising spin glass is a weak magnetic fieldh. We show thatmCh _x and give closed expressions for the constantC and the exponentx which both depend on the probability distribution of nearest-neighbor interactions.     

Magnetic-field dependence of the specific heat in planar spin chains

The specific heat of theXY model develops a characteristic double peak as a function of an applied magnetic field at relatively low temperatures. We argue that such a behavior is generic for planar spin chains, including chains with single-site anisotropy, and should be amenable to experimental observation. We also show that anharmonic corrections in the standard spin-wave theory exhibit certain infrared anomalies which limit the validity of perturbative calculations.     

Low temperature thermodynamics of quantum spin chains

Low temperature thermodynamic quantities of quantum spin chains are obtained by combining the high temperature expansion and an argument based on the spin wave theory. The method is tested in the XY chain and applied to the Heisenberg chain for which low temperature specific heat and antiferromagnetic susceptibility are calculated.     

The temperature dependence of magnetization of Gd-Tb alloys

It is shown that a mean field theory can describe very closely the variation of magnetization with temperature found in a series of Gd-Tb alloys. The proportionality of the exchange constants to the density of the states at the Fermi energy has been confirmed. The values for the densities of states for Gd and Tb required for satisfactory fitting of the experimental results are in agreement with values calculated from electronic specific heat measurements.     

The spin wave excitations and the temperature dependence of the magnetization in iron,cobalt, nickel and their alloys

A critical review is given on the literature concerning spin waves in the ferromagnetic transition metals and their alloys. It is shown that spin waves can account for the low temperature behaviour of the magnetization in metallic glasses as well as in nickel(-alloys). A systematic correlation between Curie-temperature and spin wave stiffness is worked out, holding for crystalline and amorphous alloys of iron, cobalt and nickel. The reduced magnetization versus temperature curves of (amorphous) alloys are interpreted without referring to disorder effects.     

The temperature dependence of the spontaneous magnetization of nickel and iron

An explanation of the apparent T² dependence of the magnetization of Fe and Ni and some ferromagnetic alloys at low temperatures is given in terms of an energy gap in the spin-wave spectrum. It is suggested that the use of only the first term in the series expansion of the Bose—Einstein integral function, rather than the Robinson expansion, gives a more accurate representation of the experimental data.     

Concentration dependence of the transition temperature in spin glasses

The transition temperature T_f of randomly distributed spins interacting via the RKKY interaction (spin glasses) has been calculated numerically within the spherical approximation. The dependence of T_f on the concentration c of magnetic impurities and on the damping of the interaction due to a finite mean free path of the conduction electrons is investigated for 10^?5 < c < 10^?1.     

Surface magnetization near the critical temperature and the temperature dependence of magnetic-electron scattering from NiO

The temperature dependence of low energy electrons scattered by the exchange interaction in antiferromagnetic NiO is obtained over the temperature range T_N?T?295 K. By doping the N₁O crystal with (less than 0.1 %) Li, the conductivity was increased sufficiently to obtain data at lower temperatures than previously reported.     

The temperature dependence of the magnetic susceptibility of spin glasses

The temperature dependence of the magnetic susceptibility of spin glasses is calculated exactly on the basis of a magnetic cluster model, following Néel. The susceptibility of a single cluster below its blocking temperature is taken to be negligibly small compared to its value above. A distribution of blocking temperatures may be obtained from observations on spin glasses. Conditions for the sharpness of the susceptibility peak are discussed and the low-temperature variation of the susceptibility is also obtained.     

Universal diffusive decay of correlations in gapped one-dimensional systems

We apply a semiclassical approach to express finite temperature dynamical correlation functions of gapped spin models analytically. We show that the approach of [á. Rapp, G. Zaránd, Phys. Rev. B 74, 014433 (2006)] can also be used for the S = 1 antiferromagnetic Heisenberg chain, whose lineshape can be measured experimentally. We generalize our calculations to O(N) quantum spin models and the sine-Gordon model in one dimension, and show that in all these models, the finite temperature decay of certain correlation functions is characterized by the same universal semiclassical relaxation function.     

Single-copy entanglement in a gapped quantum spin chain

The single-copy entanglement of a given many-body quantum system is defined [J. Eisert and M. Cramer, Phys. Rev. A 72, 042112 (2005)10.1103/PhysRevA.72.042112] as the maximal entanglement deterministically distillable from a bipartition of a single specimen of that system. For critical (gapless) spin chains, it was recently shown that this is exactly half the von Neumann entropy [R. Orús, J. I. Latorre, J. Eisert, and M. Cramer, Phys. Rev. A 73, 060303(R) (2006)], itself defined as the entanglement distillable in the asymptotic limit-i.e., given an infinite number of copies of the system. It is an open question as to what the equivalent behavior for gapped systems is. In this Letter, I show that for the paradigmatic spin-S Affleck-Kennedy-Lieb-Tasaki chain (the archetypal gapped chain), the single-copy entanglement is equal to the von Neumann entropy; i.e., all the entanglement present may be distilled from a single specimen.     

Universal scheme for finite-probability perfect transfer of arbitrary multispin states through spin chains

In combination with the theories of open system and quantum recovering measurement, we propose a quantum state transfer scheme using spin chains by performing two sequential operations: a projective measurement on the spins of ‘environment’ followed by suitably designed quantum recovering measurements on the spins of interest. The scheme allows perfect transfer of arbitrary multispin states through multiple parallel spin chains with finite probability. Our scheme is universal in the sense that it is state-independent and applicable to any model possessing spin–spin interactions. We also present possible methods to implement the required measurements taking into account the current experimental technologies. As applications, we consider two typical models for which the probabilities of perfect state transfer are found to be reasonably high at optimally chosen moments during the time evolution.     

On the frequency dependence of the transition temperature in spin glasses

For the first time, the frequency dependence of T_f (temperature of the maximum of the a.c. susceptibility of spin-glasses) is shown to obey a Fulcher law $\text{τ = τ}{}_{\mathrm{o}}\text{exp}\text{[}\text{E}{}_{\mathrm{a}}\text{k(}\text{T}{}_{\mathrm{f}}\text{?}\text{T}{}_{\mathrm{f}}\text{)}\text{]}$. This is observed as well in the case of dilute alloys (or R.K.K.Y. spin-glasses : $\text{Cu}$Mn, $\text{Au}$Fe, …) as for frustrated systems (Eu_1?xGd_xS, Eu_xSr_1?xS …). For R.K.K.Y. spin-glasses, only in the case of a very small amplitude, V_o of the R.K.K.Y. interaction, this time dependence approaches an Arrhenius law. In the case of “frustrated” spin-glasses the concentration is the main parameter to determine the kind of frequency dependence of T_f. These properties are evidence for a glass-like phase transition in spin-glasses. The scaling of the frequency dependence of T_f with V_o is justified for R.K.K.Y. spin-glasses from present data.     

Field and temperature dependence of magnetization of U3P4

Conclusions We conclude that the occurrence of a maximum in the magnetization versus temperature curve observed under certain circumstances on U₃P₄ is not connected with its intrinsic magnetic structure, but substantially depends on the form in which the material is studied (polycrystalline powder, single crystal, single crystal powders with various dimensions of grains). This phenomenon seems to be affected by quenching of the state from the region of low anisotropy at higher temperature in the small particles with a strong tendency to single domain behaviour and thus it may be a consequence of rotation magnetization process in the fields relatively small for U₃P₄. The assertion about transitions between magnetic ordering, sometimes proposed in the literature on the basis of observed anomalies in the temperature course of magnetization without simultaneous confirmation by neutron diffraction, should be accepted with some reservation, especially in the case of simple uranium compounds. However, another situation arises for pseudobinary alloys of ferro- and antiferromagnetic components (e.g. UP_0·75S_0·25) where such transitions were confirmed both by magnetic methods and by neutron diffraction studies [13].The authors would like to express their thanks to Ing. A.Meovský for careful preparation of samples.     

Chaotic temperature dependence in a model of spin glasses

We address the problem of chaotic temperature dependence in disordered glassy systems at equilibrium by following states of a random-energy random-entropy model in temperature; of particular interest are the crossings of the free-energies of these states. We find that this model exhibits strong, weak or no temperature chaos depending on the value of an exponent. This allows us to write a general criterion for temperature chaos in disordered systems, predicting the presence of temperature chaos in the Sherrington-Kirkpatrick and Edwards-Anderson spin glass models, albeit when the number of spins is large enough. The absence of chaos for smaller systems may justify why it is difficult to observe chaos with current simulations. We also illustrate our findings by studying temperature chaos in the naıve mean field equations for the Edwards-Anderson spin glass. Received 27 March 2002 Published online 19 July 2002     

The temperature dependence of the magnetization of Fe-P, Co-P and Ni-P alloys

Foils of Fe_{100 ? x}, Co_{100 ? x}P_x and Ni_{100 ? ×P×} and were prepares by Eletro-deposition with 0 < x < 28. Materials with x is purely microcrystalline, material with x > 15 is purely amorphous. So the effects of composition and structural order on the magnetic properties of these alloys could be separated. The decrease of the magnetization with increasing temperature was measured by means of a vibrating sample magnetometer. The results are discussed on the basis of the spin wave concept. To describe the deviations of the experimental results from exact T^{$\text{3}\text{2}$}-laws temperature dependent spin wave stiffnesses are favored. The ratios spin wave stiffness to Curie temperature proved to be characteristic constants for each of the three alloy systems. The magnetic properties of a series are mainly determined by the transition metal component. The series differ from each other in the same sense as the pure metals. The change of the properties with the phosphorus content is first of all an effect of composition and far less an effect of structural order or disorder.