Showing papers on "Curie–Weiss law published in 2010"

Solid-state transformation of [Co(NCS)2(pyridine)4] into [Co(NCS)2(pyridine)2]n: from Curie–Weiss paramagnetism to single chain magnetic behaviour

Abstract: Reaction of Co(NCS)2 with pyridine (pyr) in aqueous solution at room temperature leads to the formation of the pyridine-rich 1 : 4 compound of composition [Co(NCS)2(pyridine)4] (1) reported recently. On heating, the pyridine-rich 1 : 4 compound transforms into its corresponding pyridine-deficient 1 : 2 compound of composition [Co(NCS)2(pyridine)2]n (2), which decomposes on further heating. In the crystal structure of compound 2 the metal cations are coordinated by four N-atoms of two pyridine ligands and two N-bonded thiocyanato anions, each in mutually trans orientation, and by two S-atoms of two adjacent thiocyanato anions in a slightly distorted octahedral geometry. The thiocyanato anions bridge the metal cations forming one-dimensional polymeric chains. IR spectroscopic investigations on the pyridine-deficient 1 : 2 compound prepared in thermal decomposition are in accordance with bridging thiocyanato anions. Magnetic measurements of the pyridine-rich 1 : 4 compound and pyridine-deficient 1 : 2 compound reveal different behaviour with Curie–Weiss paramagnetism for compound 1 and single chain magnetic behaviour for compound 2, with a Mydosh-parameter φ = 0.12 and an effective energy barrier (−Ueff/kB) of 62.5 K for the spin relaxation.

Long-range magnetic ordering in magnetic ionic liquid: Emim[FeCl4].

Abstract: Up to now most of the magnetic ionic liquids containing tetrachloroferrate ion FeCl4 have evidenced a paramagnetic temperature dependence of the magnetic susceptibility, with only small deviations from the Curie law at low temperatures. However, we report on the physical properties of 1-ethyl-3-methylimidazolium tetrachloroferrate Emim[FeCl4], that clearly shows a long-range antiferromagnetic ordering below the Neel temperature TN≈3.8 K. In addition, the field dependence of the magnetization measured at 2 K is characterized by a linear behaviour up to around 40 kOe, while above this field the magnetization becomes saturated with a value of 4.3 μB/Fe, which is near the expected fully saturated value of 5 μB/Fe for an Fe3 + ion.

Structure and magnetic properties of nanocrystalline PrCo3

Abstract: The structure and magnetic properties of nanocrystalline PrCo$_3$ prepared by high-energy milling technique have been investigated by means of X-ray diffraction using the Rietveld method coupled to Curie temperature and magnetic measurements. The as-milled samples were subsequently annealed in temperature range from 750 to 1050 °C for 30 min to optimize the extrinsic properties. From x-ray studies of magnetic aligned samples, the magnetic anisotropy of this compounds is found uniaxial. The Curie temperature is 349 °K and no saturation reached at room temperature for applied field of 90 kOe. The coercive field of 55 kOe and 12 kOe measured at 10 and 293 K respectively is obtained after annealing at 750 °C for 30 min suggests that nanocrystalline PrCo$_3$ are interesting candidates in the field of permanent magnets. We have completed this experimental study by simulations in the micromagnetic framework in order to get a qualitative picture of the microstructure effect on the macroscopic magnetization curve. From this simple model calculation, we can suggest that the after annealing the system behaves as magnetically hard crystallites embedded in a weakly magnetized amorphous matrix. PACS : 75.50.Bb, 75.50.Tt, 76.80.+y

Thermodynamic fluctuations between magnetic states from first-principles phonon calculations: The case of bcc Fe

Abstract: The longstanding issue of magnetic thermodynamics containing anomalies can be resolved using a reliable model at finite temperatures: thermodynamic fluctuations among the competing collinear magnetic configurations (especially the low energy ones) in accordance with canonical partition function. Based on first-principles phonon calculations, we shed light on the magnetic materials (as exemplified in bcc Fe): the Schottky anomaly of heat capacity and the pressure-dependent Curie temperature stem from the magnetic configurational entropy due to the competition of various magnetic states.

Field-induced spin reorientation and giant spin-lattice coupling in EuFe 2 As 2

Abstract: We have studied a ${\text{EuFe}}_{2}{\text{As}}_{2}$ single crystal by neutron diffraction under magnetic fields up to 3.5 T and temperatures down to 2 K. A field-induced spin reorientation is observed in the presence of a magnetic field along both the $a$ and $c$ axes, respectively. Above critical field, the ground-state antiferromagnetic configuration of ${\text{Eu}}^{2+}$ moments transforms into a ferromagnetic structure with moments along the applied field direction. The magnetic phase diagram for Eu magnetic sublattice in ${\text{EuFe}}_{2}{\text{As}}_{2}$ is presented. A considerable strain $(\ensuremath{\sim}0.9\mathrm{%})$ is induced by the magnetic field caused by the realignment of the twinning structure. Furthermore, the realignment of the twinning structure is found to be reversible with the rebound of magnetic field, which suggested the existence of magnetic shape-memory effect. The Eu moment ordering exhibits close relationship with the twinning structure. We argue that the Zeeman energy in combined with magnetic anisotropy energy is responsible for the observed spin-lattice coupling.

Atomistic spin model simulation of magnetic reversal modes near the Curie point

Abstract: In order for the current increase in magnetic storage density to continue, one must overcome the so-called magnetic recording trilemma; namely, that smaller grains are required for higher data densities and to ensure their thermal stability, materials with a high anisotropy are required. The higher coercive field that this produces also becomes a limiting factor as the maximum field produced by the recording head is constrained by the saturation magnetization of the pole. One proposed solution to the trilemma is the use of heat assisted magnetic recording HAMR, which utilizes the temperature dependence of the anisotropy to enable writing of materials with a high coercivity. For the highest anisotropy media, this will require heating to the Curie temperature TC of the material. Close to TC, longitudinal fluctuations in the magnetization can have a significant impact on the expected energy barriers and therefore the relaxation time of the magnetization. These effects become especially important when attempting to minimize the time to reverse the magnetization state of the media that will be important at higher storage densities.

Magnetic thermodynamics of fcc Ni from first-principles partition function approach

Abstract: Exploration of longstanding issues in magnetic materials, for example the nature of Curie/Neel temperature and the Schottky anomaly of heat capacity, appeals to reliable models at finite temperatures. Based on first-principles calculations and partition function approach with the microstates being the collinear magnetic configurations, the magnetic thermodynamics of fcc Ni, including the heat capacity and the pressure-dependent Curie temperature, is predicted well and compared with the results from experiments and mean-field approach. As demonstrated in fcc Ni, it is found that the magnetic thermodynamics containing anomalies stems from the magnetic configurational entropy caused by the competition of various magnetic states.

Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy

Abstract: In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.

The Weiss Field Revisited

Abstract: In the model by P. Weiss the interaction between the molecules of a ferromagnetic substance is described by a uniform molecular field which is proportional to the current magnetization of the material. At each molecule the local field is the sum of the applied field and the molecular field. In this paper, the model by P. Weiss in a modified form is applied to ferroelectrics. More detailed investigations showed that based on the Weiss model we can simulate many typical ferroelectric properties, e. g. ferroelectric hystereses, the temperature dependence of the coercive field, the Curie-Weiss law for the electrical susceptibility and ‘butterfly’-curves for the electrical susceptibility vs applied field.

Temperature dependence of magnetic properties and change of specific heat in perovskite ErCrO3 chromites

Abstract: In this paper, the temperature dependence of magnetic properties and specific heat are systematically investigated for perovskite ErCrO3 chromites. The results show that there exists a strong temperature dependence of magnetic ordering and phase coexistence in the region of low temperature. Specifically, ErCrO3 possesses the long-range antiferromagnetic ordering and the appearance of weak ferromagnetism, occurring at T N =133 K. In the range of higher temperature, above 133.0 K, the reciprocal of magnetic susceptibility χ −1 behaves linearly, indicating a typical Curie–Weiss behavior fitted. The effective magnetic moment μ eff=10.57μ B and asymptotic paramagnetic Curie temperature T cw=−30 K, which suggests the predominance of antiferromagnetic interactions in ErCrO3 chromites. Around T SR≈22 K, ErCrO3 undergoes a spin reorientation from $\varGamma _{4}(G_{x},A_{y},F_{z};F^{R}_{z})$ to $\varGamma _{1}(A_{x},G_{y},C_{z};C^{R}_{z})$ or Γ 1(0). Also, the stability of the ferromagnetic Γ 4 phase increases with increasing applied field. Furthermore, the ac susceptibilities exhibit frequency-independent anomalies near 133 K and the coexistence of the magnetic configuration $\varGamma _{2}(F_{x},G_{y},C_{z};F^{R}_{x},C^{R}_{y})$ and Γ 4. Combining the magnetic properties and the specific-heat measurements, this current magnetization can be interpreted from the interaction between Cr3+–Cr3+, Cr3+–Er3+ and Er3+–Er3+.

Spin-polarized two-dimensional electron gas through electrostatic doping in LaAlO 3 / EuO heterostructures

Abstract: We carry out a first-principles study of stoichiometric heterostructures composed of polar oxide ${\text{LaAlO}}_{3}$ and ferromagnetic semiconductor EuO. We show that electrostatic doping achieved by an electric field in the polar oxide leads to a fully spin-polarized two dimensional electron gas at the interface. This mechanism contrasts with a previous calculation of the ${\text{LaAlO}}_{3}/\text{EuO}$ interface in which electron doping is introduced through a nonstoichiometric ${\text{LaAlO}}_{3}$ layer. Our system allows a low level of doping in EuO that is comparable to the Gd-doped EuO found in recently reported experiment, and is thickness controllable. We predict a change of magnetic moment of Eu and increase of the Curie temperature in the doped layers of EuO.

Anomalous thermal expansion and magnetic properties of Tm2Fe17-xCrx compounds

Abstract: The structural and magnetic properties of Tm2Fe17−xCrx (x=0, 05, 10, 15, and 20) compounds are investigated by means of x-ray diffraction and magnetization measurements The Tm2Fe17−xCrx compounds have a hexagonal Th2Zi17-type structure There exist a negative thermal expansion coefficient for Tm2Fe17−xCrx compounds near their Curie temperature and an anisotropic and strong spontaneous magnetostriction in the magnetic states of Tm2Fe17−xCrx compounds In Tm2Fe17−xCrx compounds, Cr atoms substituting for Fe atoms can increase obviously the Curie temperature and the easy-axis magnetocrystalline anisotropy The Curie temperature of Tm2Fe16Cr compound is about 160 K higher than that of the mother compound Tm2Fe17 The saturation magnetization of Tm2Fe17−xCrx compounds decreases with increasing Cr content

Nonequivalence of ensembles in the Curie?Weiss anisotropic quantum Heisenberg model

Abstract: The microcanonical entropy s(e, m) as a function of the energy e and the magnetization m is computed analytically for the anisotropic quantum Heisenberg model with Curie–Weiss-type interactions. The result shows a number of interesting properties which are peculiar to long-range interacting systems, including nonequivalence of ensembles and partial equivalence. Furthermore, from the shape of the entropy it follows that the Curie–Weiss Heisenberg model is indistinguishable from the Curie–Weiss Ising model in canonical thermodynamics, although they do in general differ in microcanonical thermodynamics. The possibility of experimentally realizing quantum spin models with long-range interactions in a microcanonical setting by means of cold dipolar gases in optical lattices is discussed.

Isotope effect in charge transport of LuB 12

Abstract: The galvanomagnetic properties of single-crystal samples with various isotopic boron compositions have been investigated for the first time for the normal state of superconductor LuB12 (T c ≈ 044 K) Precision measurements of the resistivity, Hall coefficient, and magnetic susceptibility have been performed over a wide temperature range of 2–300 K in magnetic fields up to 80 kOe A change of the charge transport regime in this nonmagnetic compound with metallic conduction is shown to occur near T* ≈ 50−70 K As a result, a sharp peak with significantly different amplitudes for Lu10B12 and Lu11B12 is recorded in the temperature dependences of the Hall coefficient R H(T) near T* A significant (about 10%) difference (in absolute value) of the Hall coefficients R H for the Lu10B12 and Lu11B12 compounds at helium and intermediate temperatures has been found and the patterns of behavior of the dependence R H(H) for T < T* in an external magnetic field H ≤ 80 kOe for Lu10B12 and Lu11B12 are shown to differ significantly Analysis of the Curie-Weiss contribution to the magnetic susceptibility χ(T) leads to the conclusion about the formation of magnetic moments μeff ≈ (013−019)μB in each unit cell of the fcc structure of LuB12 compounds with various isotopic compositions The possibility of the realization of an electronic topological 25-order transition near T* and the influence of correlation effects in the 5d-band on the formation of a spin polarization near the rare-earth ions in LuB12 is discussed

Magnetic anisotropy and ferromagnetic correlations above the Curie temperature in Eu2CuSi3 single crystals

Abstract: Magnetization M(T,B), specific heat cp(T), and electrical resistivity ρ(T,B) show ferromagnetic ordering in Eu2CuSi3 single crystals below the Curie temperature TC=34 K. Temperature T and magnetic field B-dependent M(T,B) data give evidence for a sizable magnetic anisotropy in the AlB2-derived hexagonal structure at T

Magnetic and thermodynamic properties of cobalt-doped iron pyrite: Griffiths phase in a magnetic semiconductor

Abstract: Doping of the band insulator FeS$_2$ with Co on the Fe site introduces a small density of itinerant carriers and magnetic moments. The lattice constant, AC and DC magnetic susceptibility, magnetization, and specific heat have been measured over the $0\le x\le 0.085$ range of Co concentration. The variation of the AC susceptibility with hydrostatic pressure has also been measured in a small number of our samples. All of these quantities show systematic variation with $x$ including a paramagnetic to disordered ferromagnetic transition at $x=0.007\pm 0.002$. A detailed analysis of the changes with temperature and magnetic field reveal small power law dependencies at low temperatures for samples near the critical concentration for magnetism, and just above the Curie temperature at higher $x$. In addition, the magnetic susceptibility and specific heat are non-analytic around H=0 displaying an extraordinarily sharp field dependence in this same temperature range. We interpret this behavior as due to the formation of Griffiths phases that result from the quenched disorder inherent in a doped semiconductor.

Non-Fermi-Liquid Behavior on an Iron-Based Itinerant Electron Magnet Fe3Mo3N

Abstract: We report magnetic, calorimetric, and transport properties of an iron-based itinerant electron magnet Fe 3 Mo 3 N. Magnetic susceptibility shows a Curie–Weiss behavior at high temperatures and takes a broad maximum at around 75 K. The absence of magnetic long range order was confirmed by 57 Fe-Mossbauer and neutron diffraction measurements. C / T shows a divergent behavior following -log T at low temperatures and reaches 128 mJ/(f.u.mol K 2 ) at 0.5 K. The deviation from the T 2 power law of resistivity also suggests a non-Fermi-liquid (NFL) behavior. The observed C / T and χ are enhanced compared with the values estimated from the theoretical density of states, suggesting a strong magnetic enhancement. The NFL behaviors indicate that Fe 3 Mo 3 N is one of the ideal systems located in the vicinity of the ferromagnetic quantum critical point.

Crystal growth, structure, and physical properties of Ln(Cu, Al)12 (Ln = Y, Ce, Pr, Sm, and Yb) and Ln(Cu, Ga)12 (Ln = Y, Gd―Er, and Yb)

Abstract: Single crystals of Ln(Cu,Al)12 and Ln(Cu,Ga)12 compounds (Ln = Y, Ce–Nd, Sm, Gd–Ho, and Yb for Al and Ln = Y, Gd–Er, Yb for Ga) have been grown by flux-growth methods and characterized by means of single-crystal x-ray diffraction, complemented with microprobe analysis, magnetic susceptibility, resistivity and heat capacity measurements. Ln(Cu,Ga)12 and Ln(Cu,Al)12 of the ThMn12 structure type crystallize in the tetragonal I4/mmm space group with lattice parameters a~8.59 A and c~5.15 A and a~8.75 A and c~5.13 A for Ga and Al containing compounds, respectively. For aluminium containing compounds, magnetic susceptibility data show Curie–Weiss paramagnetism in the Ce and Pr analogues down to 50 K with no magnetic ordering down to 3 K, whereas the Yb analogue shows a temperature-independent Pauli paramagnetism. Sm(Cu,Al)12 orders antiferromagnetically at TN~5 K and interestingly exhibits Curie–Weiss behaviour down to 10 K with no Van Vleck contribution to the susceptibility. Specific heat data show that Ce(Cu,Al)12 is a heavy fermion antiferromagnet with TN~2 K and with an electronic specific heat coefficient γ0 as large as 390 mJ K2 mol−1. In addition, this is the first report of Pr(Cu,Al)12 and Sm(Cu,Al)12 showing an enhanced mass (~80 and 120 mJ K2 mol−1). For Ga containing analogues, magnetic susceptibility data also show the expected Curie–Weiss behaviour from Gd to Er, with the Yb analogue being once again a Pauli paramagnet. The antiferromagnetic transition temperatures range over 12.5, 13.5, 6.7, and 3.4 K for Gd, Tb, Dy, and Er. Metallic behaviour is observed down to 3 K for all Ga and Al analogues. A large positive magnetoresistance up to 150% at 9 T is also observed for Dy(Cu,Ga)12. The structure, magnetic, and transport properties of these compounds will be discussed.