Experimental Magnetization Curves Research Articles

Magnetic and structural properties of nanocrystalline PrCo3

The structure and magnetic properties of nanocrystalline PrCo3 obtained from high energy milling technique are investigated by X-ray diffraction, Curie temperature determination and magnetic properties measurements are reported. The as-milled samples have been annealed in a temperature range of 1023 K to 1273 K for 30 mn to optimize the extrinsic properties. The Curie temperature is 349 K and coercive fields of 55 kOe at 10K and 12 kOe at 293 K are obtained on the samples annealed at 1023 K. A simulation of the magnetic properties in the framework of micromagnetism has been performed in order to investigate the influence of the nanoscale structure. A composite model with hard crystallites embedded in an amorphous matrix, corresponding to the as-milled material, leads to satisfying agreement with the experimental magnetization curve. [K. Younsi, V. Russier and L. Bessais, J. Appl. Phys. 107, 083916 (2010)]. The microscopic scale will also be considered from DFT based calculations of the electronic structure of RCox compounds, where R = (Y, Pr) and x = 2,3 and 5.

Determination of the anisotropies and reversal process in exchange-bias bilayers using a rotational magnetization curve approach

Rotational magnetization curves of the exchange-bias bilayers were investigated based on the Stoner-Wohlfarth model, which can be grouped into three cases according to the magnetization reversal process. The unidirectional anisotropic field HE = 41.4 Oe, the uniaxial anisotropic field Hk = 4.2 Oe and the accurate direction of the easy axis of our FeNi/FeMn exchange-bias bilayers were obtained by fitting their experimental rotational magnetization curves. During the rotational process the magnetization reversal of the bilayers is a coherent rotation with a critical magnetization reversal field H1 = 41.372 Oe.

Characteristics of magnetism in nanoporous carbon with palladium clusters

The magnetic properties of samples of nanoporous carbon with palladium clusters prepared from polycrystalline SiC have been studied over a wide range of temperatures. The specific magnetic moment rapidly decreases with increasing temperature. The obtained experimental magnetization curves have a nonlinear character but do not reach saturation even in a field of 10 kOe. Being constructed as functions of H/T, they lie on a universal curve, which suggests superparamagnetism of the system. The experimental curves have a hysteresis. The coercive force, first, decreases with increasing temperature and, then (in the range of 5–20 K), increases, thus forming a notch singularity. On the contrary, the residual magnetization rapidly decreases with increasing temperature. A conclusion on the prevailing contribution of superparamagnetic clusters to the magnetic subsystem is made.

Magnetization curve modelling of soft magnetic alloys

In this paper we present an application of the so called hyperbolic model of magnetization. The model was modified and it was applied for nine different soft magnetic alloys. The tested samples were electro-technical steels (FeSi alloys) and a permalloy (FeNi alloy) with strongly different magnetic properties. Among them there are top, medium and definitely poor quality soft magnetic materials as well. Their minor hysteresis loops and normal magnetization curves were measured by alternating current measurement. The hyperbolic model of magnetization was applied for the experimental normal magnetization curves. It was proved that the applied model is excellent for describing mathematically the experimental magnetization curves.

Magnetic interactions in spin-glasslikeGe1−x−ySnxMnyTedilute magnetic semiconductors

We investigated the nature of the magnetic phase transition in ${\text{Ge}}_{1\ensuremath{-}x\ensuremath{-}y}{\text{Sn}}_{x}{\text{Mn}}_{y}\text{Te}$ mixed crystals with chemical composition changing in the range of $0.083\ensuremath{\le}x\ensuremath{\le}0.142$ and $0.012\ensuremath{\le}y\ensuremath{\le}0.119$. The dc magnetization measurements performed in magnetic fields up to 90 kOe and temperature range 2--200 K showed that the magnetic ordering at temperatures below $T=50\text{ }\text{K}$ exhibits features characteristic for both spin-glass and ferromagnetic phases. The modified Sherrington-Southern model was applied to explain the observed transition temperatures. The calculations showed that the spin-glass state is preferred in the range of the experimental carrier concentrations and Mn contents. The value of the Mn hole exchange integral was estimated to be ${J}_{pd}=0.45\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{eV}$. The experimental magnetization vs temperature curves were reproduced satisfactorily using the noninteracting spin-wave theory with the exchange constant ${J}_{pd}$ values consistent with those calculated using modified Sherrington-Southern model. The magnetization vs magnetic field curves showed nonsaturating behavior at magnetic fields $B<90\text{ }\text{kOe}$ indicating the presence of strong magnetic frustration in the system. The experimental results were reproduced theoretically with good accuracy using the molecular-field-approximation-based model of a disordered ferromagnet with long-range Ruderman-Kittel-Kasuya-Yosida interaction.

Modelling of Normal Magnetization Curves of Soft Magnetic Alloys

In this paper, we present an application of the so-called hyperbolic model of magnetization. The model was modified and it was applied for nine different soft magnetic alloys. The tested samples were electro-technical steels (FeSi alloys) and a permalloy (FeNi alloy) with strongly different magnetic properties. Among them, there are top, medium and definitely poor quality soft magnetic materials as well. Their minor hysteresis loops and normal magnetization curves were measured by alternating current measurement. The hyperbolic model of magnetization was applied for the experimental normal magnetization curves. It was proved that the applied model is excellent for describing mathematically the experimental magnetization curves.

Magnetization process of the S=1/2 distorted diamond spin chain with the Dzyaloshinsky-Moriya interaction

The distorted diamond chain has attracted a lot of interest, since the compound Cu3(CO3)2(OH)2, so-called azurite, was revealed to be a good candidate for this system. Although several theoretical studies were performed, the strong anisotropy observed in the magnetization measurement of azurite has not been explained very well. To solve the mechanism of this anisotropy, we introduce the Dzyaloshinsky-Moriya (DM) interaction suitable for the crystal symmetry of azurite. The numerical diagonalization study on this realistic model indicates that the anisotropy in the experimental magnetization curve would be reproduced with sufficiently large DM interaction. We also propose a mechanism of the field-induced incommensurate order, which was observed above the 1/3 magnetization plateau by the NMR measurement.

Effects of the shape of spin gate elements on their magnetic and magnetoresistive characteristics

The effect of the geometric shape of elements of a spin-dependent tunneling gate on their magnetic characteristic is studied theoretically and experimentally. The hysteresis dependences of the magnetization and magnetoresistance of rectangular, hexagonal, and ringed elements on the magnetic field are calculated for different characteristically geometric parameters of the elements. In the studies of the magneto-optical Kerr effect, magnetic hysteresis loops are observed. The calculated and experimental magnetization curves are compared, and qualitative agreement between the curves is gained.

Nonequilibrium dynamical ferromagnetism of interacting single-molecule magnets

We propose a nonequilibrium Monte Carlo (MC) approach to explore nonequilibrium dynamical ferromagnetism of interacting single molecule magnets (SMMs). Both quantum spin tunneling and thermally activated spin reversal are implemented in the same MC simulation framework. Applied to a typical example, this simulation method satisfactorily reproduces experimental magnetization curves with experimental parameters. Our results show that both quantum and classical effects are essential to determine the hysteresis behaviors. This method is effective and reliable to gain deep insights into SMMs.

Accurate determination of the magnetic anisotropy in cluster-assembled nanostructures

The simultaneous triple adjustment of experimental magnetization curves under different conditions is shown to allow the unambiguous and consistent determination of both the magnetic particle size distribution and anisotropy for granular nanostructures of Co clusters embedded in protective matrices. The importance of interface effects resulting in magnetically dead layers is demonstrated.

Monte Carlo Study of Magnetostructural Phase Transitions in Ni<sub>50</sub>Mn<sub>25+x</sub>Sb<sub>25-x</sub> Heusler Alloys

On the basis of Monte Carlo simulations, we investigate the temperature dependence of magnetization of Ni-Mn-Sb Heusler alloys, in which the part of the Mn atoms interact antiferromagnetically. It is shown that this antiferromagnetic exchange is responsible for existence of several magnetic phase transitions in the Heusler alloys. For a certain set of parameters of the model Hamiltonian we obtain coupled martensitic and magnetic phase transitions. Results of simulations agree in an excellent way with the experimental magnetization curves.

Monte Carlo study of the influence of antiferromagnetic exchange interactions on the phase transitions of ferromagneticNi-Mn-Xalloys(X=In,Sn,Sb)

On the basis of Monte Carlo simulations using the Potts model Hamiltonian, we investigate the complex temperature dependence of magnetization of ferromagnetic $\text{Ni-Mn-}X$ $(X=\text{In},\text{Sn},\text{Sb})$ Heusler alloys, in which part of the Mn atoms, which occupy the $X$ sites, interacts antiferromagnetically with the Mn atoms on the Mn sublattice. It is shown that this antiferromagnetic exchange is responsible for metamagnetic behavior and a series of magnetic phase transitions in the Heusler alloys. For an optimal choice of parameters of the model Hamiltonian, which have partially been obtained from ab initio calculations, we are able to describe the physics associated with the coupled martensitic-magnetic phase transition. The simulations lead to a qualitative agreement with the experimental magnetization curves and their dependence on temperature and composition.

Magnetostriction in the rare-earth ferroborates RFe3(BO3)4, R=Pr and Tb

Recent experimental data for magnetostriction in the rare-earth (RE) ferroborates RFe 3 (BO 3 ) 4 with R=Pr and Tb are discussed from a theoretical point of view. Multipole moments of RE ions are calculated in the framework of a crystal-field model for the RE ion and the molecular-field approximation. Quadrupole approximation is shown to be sufficient for interpretation of data for longitudinal magnetostriction at the magnetic field along the trigonal axis. Parameters of PrFe 3 (BO 3 ) 4 are deduced when accounting for the experimental magnetization curves that manifest a spin–flop transition.

Magnetic properties of polydisperse ferrofluids: A critical comparison between experiment, theory, and computer simulation

Experimental magnetization curves for a polydisperse ferrofluid at various concentrations are examined using analytical theories and computer simulations with the aim of establishing a robust method for obtaining the magnetic-core diameter distribution function p(x). Theoretical expressions are fitted to the experimental data to yield the parameters of p(x). It is shown that the majority of available theories yield results that depend strongly on the ferrofluid concentration, even though the magnetic composition should be fixed. The sole exception is the second-order modified mean-field (MMF2) theory of Ivanov and Kuznetsova [Phys. Rev. E 64, 041405 (2001)] which yields consistent results over the full experimental range of ferrofluid concentration. To check for consistency, extensive molecular dynamics and Monte Carlo simulations are performed on systems with discretized versions of p(x) corresponding as closely as possible to that of the real ferrofluid. Essentially perfect agreement between experiment, theory, and computer simulation is demonstrated. In addition, the MMF2 theory provides excellent predictions for the initial susceptibility measured in simulations.

Numerical simulation of magnetization of 2D magnon crystals based on yttrium iron garnet films

A numerical approach based on disregarding variation in the magnetization level across the thickness of 2D ferromagnetic systems is used to calculate the hysteresis loops for the tangentially magnetized 2D magnon crystals with rhombic and square lattice symmetries obtained by means of etching cylindrical holes with a diameter of 32 μm in a 5.2-μm-thick yttrium iron garnet (YIG) film. It is demonstrated that, when the effects of the crystallographic and growth anisotropies of YIG are disregarded, the results of calculations differ from experimental magnetization curves by no more than 10%. In this case, the theoretical behavior of the lateral distribution of the magnetization with an increase in the tangential magnetic field is in qualitative agreement with the experimentally observed rearrangement of the domain structure.

Magnetic anisotropy of the Nd2Fe14B compound and its hydride Nd2Fe14BH4

Magnetization curves of single crystals of Nd2Fe14B and its hydride Nd2Fe14BH4 have been measured along their principal crystallographic directions in a temperature range of 4.2–280 K. The magnetic anisotropy constants, which allow one to describe experimental magnetization curves as well as the low-temperature “easy-cone”-type magnetic structure and field-induced first-order magnetic phase transitions, have been determined in terms of a collinear-ferrimagnetic-ordering model. The anisotropy constants were shown to decrease in magnitude upon hydrogenation. In this case, the ratios of the effective fourth-and sixth-order constants to the second-order constant increase, whereas the constants responsible for the basal-plane anisotropy exhibit a more than threefold decrease. At the same time, the spin-reorientation temperature and opening of the magnetization cone at 4.2 K remain virtually unchanged. The results obtained are discussed in terms of the single-ion-anisotropy theory.

Hybrid-density functional study of magnetism and ligand control in Ni9 complexes

Strong influences of bridging ligands L on the magnetism of two Ni 9 complexes, Ni 9 L 2 (O 2 CMe) 8 {(2-py) 2 CO 2 } 4 (L=OH − 1 and N 3 - 2 ) are demonstrated by magnetic interactions. Effective exchange integrals J for 1 and 2 are determined non-empirically using UB3LYP and UBLYP. The calculated magnetic susceptibility for 1 is wholly consistent with the experimental values. Although the ground spin state of 2 obtained using the calculated J values agrees with the experimental S = 9 state, fitting of the J values to the experimental magnetic susceptibility curve indicates weak antiferromagnetic interactions via the azido ligands.

Frustrated ferromagnetic spin-12chain in a magnetic field

We study the ground state properties of the Heisenberg spin-1/2 chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions using two approximate methods. One of them is the Jordan-Wigner mean-field theory and another approach based on the transformation of spin operators to bose-ones and on the variational treatment of bosonic Hamiltonian. Both approaches give close results for the ground state energy and the T=0 magnetization curve. It is shown that quantum fluctuations change the classical critical exponents in the vicinity of the transition point from the ferromagnetic to the singlet ground state. The magnetization process displays the different behavior in the regions near and far from the transition point. The relation of the obtained results to experimental magnetization curve in $Rb_{2}Cu_{2}Mo_{3}O_{12}$ is discussed.

The Mechanism of Magnetic Interaction in Spin‐Ladder Molecular Magnets: A First‐Principles, Bottom‐Up, Theoretical Study of the Magnetism in the Two‐Legged Spin‐Ladder Bis(2‐amino‐5‐nitropyridinium) Tetrabromocuprate Monohydrate

AbstractThe magnetic properties of the isotropic (Jrail ≈ Jrung), two‐legged spin‐ladder bis(2‐amino‐5‐nitropyridinium) tetrabromocuprate monohydrate, [(5NAP)2CuBr4·H2O], have been studied using a first‐principles, bottom‐up approach, which allows computation of macroscopic magnetic properties (for instance, the magnetic susceptibility) of a crystal from only a knowledge of its crystal packing. Evaluation of the JAB parameters, using the 163 K X‐ray structure of [(5NAP)2CuBr4·H2O], indicates that the magnetic topology of this crystal is a two‐legged antiferromagnetic spin‐ladder, with values of Jrail and Jrung of –22.2 cm–1 and –19.7 cm–1, respectively. These values are very close to those that best fit the experimental magnetic susceptibility curve, which are–13.59 cm–1 and –14.16 cm–1, respectively. Very weak diagonal interactions within each ladder [J(d3) = –0.9 cm–1] and between nearby ladders [J(d4) = –0.3 cm–1] are also found. The computed magnetic susceptibility curve obtained using the two‐legged spin‐ladder properly reproduces the experimental magnetic curve (a quantitative agreement is obtained by applying a linear scaling factor of about 0.75 to the energies). The singlet–triplet spin‐gap of this spin‐ladder was computed to be 17 K, in close agreement with the experimental result (11 K). The change of the spin‐gap/Jrail with the size (2xL) of the magnetic spin‐ladder model space was found to converge towards 0.5, while that for a single‐legged (1xL) spin‐ladder converges towards zero, both in good agreement with the known trends for these systems. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Influence of periodic and random local crystal fields on magnetic properties of Nd xY 1−xCo 5 compounds

The magnetization processes have been studied for single crystals of Nd x Y 1− x Co 5 compounds in magnetic fields up to 360 kOe at temperatures 4.2–300 K. The experimental results have been used for determination of parameters of crystal field (CF) splitting of the energy levels of the ground J multiplet of Nd 3+ ions. A single set of CF parameters was found for all x values and all temperatures from above-mentioned range allowing satisfactory description of all experimental magnetization curves. An alternative set of parameters was obtained assuming presence, in studied compounds, of local random CFs generated by impurities (for instance, absorbed hydrogen).