Showing papers on "Metamagnetism published in 1999"

Itinerant electron metamagnetic transition in La(FexSi1−x)13 intermetallic compounds

Abstract: A first-order transition above the Curie temperature for ferromagnetic La(FexSi1−x)13 (x=0.86 and 0.88) compounds has been confirmed by applying a magnetic field. The magnetic state changes from the paramagnetic to the ferromagnetic state and the transition field increases with temperature, indicating an itinerant electron metamagnetic (IEM) transition. The IEM transition is broad in x=0.86 and becomes clearer in x=0.88, which takes a negative slope of the Arrott plot. The volume change just above the Curie temperature for x=0.88 is huge, about 1.5%, which is caused by a large magnetic moment induced by the IEM transition.

Giant volume magnetostriction due to the itinerant electron metamagnetic transition in La(Fe-Si)/sub 13/ compounds

Abstract: Ferromagnetic La(Fe/sub x/Si/sub 1-x/)/sub 13/ compounds with x/spl ges/0.87 show an itinerant metamagnetic transition above the Curie temperature T/sub c/. Since a large volume change of about 1.5% is caused by the transition, the present compounds have a possibility for practical applications as giant magnetostrictive materials. To adjust T/sub c/ around room temperature, the effect of the substitution by other magnetic elements on the Curie temperature was examined. The Curie temperature of La(Fe/sub 0.88/Co/sub y/Si/sub 0.12-y/)/sub 13/ compounds increases with increasing y and the itinerant electron metamagnetic transition appears above T/sub c/. For y=0.03 and 0.04, a huge volume change of 0.6/spl sim/0.9%, or a linear change of 0.2/spl sim/0.3%, due to the itinerant metamagnetic transition is observed around 250 K.

Magnetic Properties of Terbium B12 Icosahedral Boron-Rich Compounds

Abstract: Static magnetic susceptibility of the new B 12 icosahedral higher boride compound TbB 50 was measured and a large change in behavior around 17 K was observed, indicating the existence of an antiferromagnetic-like transition At high fields a metamagnetic transition was observed to occur and the magnetic phase diagram was determined This is the first report of a magnetic transition being observed in the higher boride compounds An interesting feature of this behavior is that it is in contrast to the typical Curie-Weiss behavior of the more terbium rich compound TbB 25 and also the more boron rich compound TbB 66 which were also synthesized and measured in this work This is considered to be an aspect of the unique structure of the REB 50 (RE=rare-earth) compounds recently discovered

Investigation of the magnetoelectric effect in BiFeO3-BaTiO3 solid solutions

Abstract: BiFeO3 forms solid solutions with BaTiO3, over the entire compositional range, with different crystal symmetries. Magnetoelectric (ME) measurements carried out with a superimposed alternating-current magnetic field, together with a time-varying direct-current magnetic field in isothermal conditions, indicated non-linearity. The peak observed coincided with a metamagnetic transition in the magnetization data. Rather than the spin flop reported earlier, it is a gradual reorientation of spins towards the field direction that destroys the spiral spin arrangement. With increasing content of BaTiO3, a quadratic signal was observed, indicating the structural dependence of the magnetoelectric effect. The temperature variation of the ME output in the investigation carried out for x = 0.75 indicates magnetic transitions.

High-field magnetization of Sr1-xCaxRuO3

Abstract: Magnetic properties of Sr 1- x Ca x RuO 3 have been investigated at temperatures from 4.2 up to 300 K and in magnetic fields up to 44 Tesla. The results of high-field magnetization process are consistent with the interpretation that this system ranges from an intermediate itinerant ferromagnetic metal to a strongly exchange-enhanced Pauli paramagnet (a nearly ferromagnetic metal) with increase of x . Although all the samples have indicated very large high-field magnetic susceptibility, no symptoms of the occurrence of itinerant-electron metamagnetism, which is seen in some nearly ferromagnetic metals like YCo 2 , have been observed.

Metamagnetic Transition in a Heavy Fermion Superconductor URu2Si2.

Abstract: We have carried out the magnetization measurements in high magnetic fields up to 60 T and in the wide temperature range from 1.3 to 77 K for a heavy femion superconductor URu 2 Si 2 . A sharp metamagnetic transition with three steps is found below 14 K for the field along the tetragonal [001] direction. This transition is changed into a sharp metamagnetic transition with two steps in a narrow temperature range from about 14 to 17 K and furthermore into a broad metamagnetic transition with one step above the Neel temperature T N =17.5 K. The broad transition persists up to a characteristic temperature T χ m a x (=55 K) where the magnetic susceptibility has a maximum in the temperature dependence. These results are discussed on the basis of a change of the electronic state of 5 f -electrons from itinerant to localized and the crystalline electric field model.

Chapter 3 Formation of 3d-moments and spin fluctuations in some rare-earth-cobalt compounds

Abstract: Publisher Summary This chapter discusses the formation of 3D-moments and spin fluctuations in some rare-earth-cobalt compounds. In combination with large magnetovolume effects, these compounds exhibit a number of characteristic properties that make them suitable for checking various physical theories. The chapter focuses on Co-based Laves phase compounds and describes the itinerant-electron metamagnetism (IEMM) in various materials. The chapter discusses the magnetic properties of the systems consisting of both itinerant electrons and localized spins are formulated based on the s–d model. The effects of spin fluctuations on the magnetic behavior, heat capacity, and magnetoresistance are also reviewed in the chapter. The 4f–3d exchange interaction and its effect on the induced Co-metamagnetic behavior is presented for the magnetic rare-earth-Co 2 compounds. The role of the magnetic R-atom and the influence of the 3d electrons affecting the character of the magnetic phase transitions and spin fluctuation scattering are discussed.

Chapter 171 Itinerant electron metamagnetism of co sublattice in the lanthanide-cobalt intermetallics

Abstract: Publisher Summary This chapter discusses itinerant electron metamagnetism of Co sublattice in the lanthanide-cobalt intermetallics. The first-order transition (FOT) is observed in several lanthanide-cobalt (R-Co) intermetallics under the effect of varying external parameters and internal parameters. This transition occurs between a nonmagnetic or low-induced moment state and a ferromagnetic state. This phenomenon is called itinerant electron metamagnetism (IEM). The various models to describe IEM are discussed. At the beginning, the basic concepts of the band theory of metamagnetism describing the magnetically uniform states are presented. Then, the role of volume effects and spin fluctuation effects on the metamagnetic behavior are also briefly introduced. The magnetic properties of the systems consisting of both itinerant electrons and localized spins are formulated on the basis of the s-d model. The experimental overview of IEM is given. This section consists of three major parts. The role of the magnetic R-atom as well as the influence of the 3d electrons affecting the character of the magnetic phase transitions and the spin fluctuation scattering is discussed. High-field magnetization in the RCo 2 with one unstable magnetic sublattice is also discussed. IEM in the RCo 3 - and RCo 5 -based compounds is presented.

Itinerant-electron metamagnetism of

Abstract: Electronic structures of the pseudo-binary compound with the -type hexagonal structure are calculated in the self-consistent linear muffin-tin orbital method within the atomic sphere approximation, by taking into account the preferential occupation sites for Ni. The calculated bulk moment disappears abruptly near 60% Ni concentration. Above the critical concentration of Ni, a metamagnetic transition from the paramagnetic to the ferromagnetic state is found to occur. These calculated results are consistent with the recently observed ones. It has been shown that the anomalous magnetic behaviours of can be explained by the detailed shape of the local density-of-states curve of the 3d-transition-metal atoms at the 3g site near the Fermi level.

A Heavy-Fermion State with Singular Density of States

Abstract: The metamagnetic behavior of CeRu 2 Si 2 and the non Fermi-liquid behavior of CeNi 2 Ge 2 are described by the periodic Coqblin-Schrieffer model with anisotropic c - f exchange interaction in the mean-field theory. Only the Kramers doublet of M J =±5/2 for the 4 f states of Ce ions is considered, for the case of which the density of quasi-particle states has singularity of | ω| -1/2 around the effective f -level energy. In the asymmetric case applicable to CeRu 2 Si 2 , the chemical potential reaches the singularity at a finite magnetic field, at which the magnetization shows a metamagnetic-like increase. On the other hand, in the symmetric case applicable to CeNi 2 Ge 2 , the chemical potential is on the singularity at zero magnetic field, and non-Fermi-liquid-like behaviors emerge in the susceptibility and the specific heat.

Itinerant-electron metamagnetism of MnSi at high pressure

Abstract: Metamagnetic behaviors observed recently for MnSi at high pressure are discussed by using the electronic structures calculated in a self-consistent linear muffin-tin orbital method within the atomic-sphere approximation. It is shown that the ferromagnetic state with the moment larger than about $0.4{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Mn}$ is stable at large lattice constants, while the paramagnetic state becomes stable at smaller lattice constants. By the fixed-spin-moment method the difference $\ensuremath{\Delta}E(M)$ between the total energies of the ferromagnetic and paramagnetic states is calculated as a function of magnetic moment M near the critical lattice constant. The calculated $\ensuremath{\Delta}E(M)$ is fitted to a form of the power series of ${M}^{2}$ up to the term of ${M}^{10}.$ Pressure dependences of the magnetization and susceptibility observed for MnSi are compared with the calculated results. Temperature dependences of the metamagnetic transition and susceptibility are also discussed by taking into account the effects of spin fluctuations and by using the expansion coefficients in the calculated $\ensuremath{\Delta}E(M).$ Metamagnetic behaviors observed for MnSi at high pressure are shown to be described very well by the present model.

Metamagnetic Transition in NdB6 with a Small Magnetic Anisotropy in Low Magnetic Fields.

Abstract: The high-field magnetization measurement of the antiferro (AF) magnetic compound NdB 6 , which has a small magnetic anisotropy in low magnetic fields, revealed an unexpected metamagnetic transition This is observed only for H ∥ , while the magnetization curve for H ∥ and is normal, as usually observed in an AF magnet with a small magnetic anisotropy To our knowledge, this is the first example of the anisotropic magnetic field induced metamagnetic transition for a compound with a small magnetic anisotropy in low magnetic fields

New heavy fermion metamagnet CeFe2Ge2

Abstract: We have investigated high field magnetization on single-crystalline CeFe 2 Ge 2 which is classified as a heavy fermion with a non-magnetic ground state. A clear metamagnetic anomaly, which resembles those reported in CeRu 2 Si 2 , was observed around 300 kOe for the field along the c -axis. The T 3/2 dependence of the electrical resistivity in a wide temperature range from around 15 K down to 2 K, followed by T 2 dependence at lower temperatures, is apparently similar to that in CeNi 2 Ge 2 which is close to a quantum critical point.

Metamagnetism of antiferromagnetic XXZ quantum spin chains

Abstract: The magnetization process of the one-dimensional antiferromagnetic Heisenberg model with the Ising-like anisotropic exchange interaction is studied by the exact diagonalization technique. It results in the evidence of the first-order spin-flop transition with a finite magnetization jump in the N\'eel ordered phase for $Sg~1.$ It implies that the $S=\frac{1}{2}$ chain is an exceptional case where the metamagnetic transition becomes second order due to large quantum fluctuations.

Specific-Heat Anomaly around Metamagnetic Transition in UCoAl

Abstract: We have measured the specific heat and magnetocaloric effect of an UCoAl single crystal down to ∼0.1 K in fields applied along the c -axis to investigate the thermodynamic aspects of the metamagnetic behavior. Hysteresis behavior observed in both physical quantities below ∼3 K confirms the first order character of the metamagnetic transition. We have found that an apparent decrease of the electronic part of specific heat C e / T by 20% at 0.25 K with increasing field across the metamagnetic transition field of ∼1 T, evidencing the change in the density-of-states at the Fermi energy. C e / T gradually decreases with increasing temperature in zero field. This anomalous temperature dependence can be explained by taking into account a spin fluctuation contribution in the low-field state.

Magnetic instabilities in CeRu2Si2 compounds

Abstract: Neutron scattering experiments were recently performed in compounds of the CeRu2Si2 family. The topic addressed is the proximity of magnetic instabilities. The first one is the proximity of a T=0 K quantum phase transition experimentally achieved with lanthanum doping. The second kind of instability is linked to the competition between antiferromagnetic and ferromagnetic interactions in these materials. The pseudo-metamagnetic transition of CeRu2Si2 and the occurrence of an antiferromagnetic phase in the ferromagnetic compound CeRu2Ge2 were studied. Comparisons with macroscopic measurements are made using a simple analysis of the data.

Alloying and pressure-induced transitions between 5f-band metamagnetism and ferromagnetism

Abstract: Effects of external hydrostatic pressure up to 1.2 GPa and the Al-Ga alloying on the magnetism in ${\mathrm{UCoAl}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}$ compounds have been studied. Ga substitutions for Al in UCoAl lead to gradual transformation of a $5f$-band metamagnetism towards ferromagnetism (for $xg~0.2$). The application of pressure yields reverse effects clearly seen in compounds with $xl~0.4.$ The spontaneous magnetization ${\ensuremath{\mu}}_{s}$ and the Curie temperature ${T}_{C}$ of ferromagnetic compounds are gradually suppressed with increasing pressure. The values of $d\mathrm{ln}{T}_{C}/dp$ and $d\mathrm{ln}{\ensuremath{\mu}}_{s}/dp$ slowly increase with x decreasing down to 0.4, whereas for $xl0.4$ an abrupt growth of pressure parameters is observed with further lowering Ga content. This is a fingerprint of a pressure-induced collapse of ferromagnetism and a simultaneous reentrance of the ``UCoAl-type'' metamagnetism in compounds with lower Ga content $(xl~0.3).$ The observed evolution of magnetism is discussed in terms of pressure and alloying-induced variation of the $5f$-ligand hybridization that determines both the stability of the U magnetic moment and the strength of U-U exchange interactions.

On the T Bloch law in magnets with fourth-order exchange interaction

Abstract: For the ferromagnets EuS and GdMg, in which fourth-order exchange interactions (i.e. biquadratic, three-spin and four-spin interactions) have been identified, the deviation of the spontaneous magnetization with respect to the T =0 value is shown to follow a T2 law instead of the famous T3/2 law expected for a Heisenberg ferromagnet. Moreover, the observed T2 law holds for temperatures as large as 0.8TC and the extrapolated magnetization value for does not conform to ferromagnetic saturation. This is because the fourth-order exchange interactions generate a second order-parameter which is assumed to govern the order of the transverse moment components. These moment components have a finite expectation value for at the expense of the Heisenberg order parameter. Like the spontaneous magnetization, the critical field curves B c ( T ) of the metamagnet EuSe and the antiferromagnet EuTe also start decreasing with a T2 term for . It is argued that the T2 law is a consequence of the fourth-order exchange interactions. This is shown experimentally by a study of the critical field curves [0pt] pertinent to the longitudinal (Heisenberg) order-parameter in the diamagnetically diluted antiferromagnets EuxSr1-xTe. In this solid solution series a particular composition of x c =0.85 exists at which the different fourth-order interaction processes compensate each other in the high temperature average. As a consequence, an EuxSr1-xTe sample with x =0.85 meets the requirements of a Heisenberg antiferromagnet at least if a quantity is considered for which the high-temperature average over all fourth-order interactions is decisive. This seems to be the case for the critical field curve [0pt] which gives the phase boundary to the paramagnetic phase. In fact, a crossover from a T2 to a T3/2 law is observed for [0pt] on approaching xc. This, we believe, shows the frequently observed T2 law is caused by the fourth-order interactions.

Electronic structure and the metamagnetic transition of FeSi at extremely high magnetic fields

Abstract: A field-induced moment for the narrow-band-gap semiconductor FeSi with a B20-type cubic structure is calculated by a self-consistent linear muffin-tin orbital method within the atomic sphere approximation, by introducing empty spheres in a unit cell. The fixed-spin-moment calculations give two successive metamagnetic transitions. It is found that the first metamagnetic transition will occur at a magnetic field of about 350 T from the nonmagnetic semiconductor to a metallic ferromagnet with a small moment of about 0.1 µB/Fe. This is consistent with the observed anomaly in the electrical conductivity at about 390 T. The second metamagnetic transition takes place from the small-moment state to a large-moment one with about 1 µB/Fe at about 700 T. The critical field of the first metamagnetic transition is shown to depend strongly on the value of the band-gap where the Fermi level lies.

Magnetoresistivity and metamagnetism of the Nd33Fe50Al17 alloy

Abstract: A ternary phase has been identified in the rare-earth transition metal Nd–Fe–Al system This phase has a composition close to Nd5(Fe3Al)12 and is antiferromagnetic with a Neel temperature of approximately 260 K A clear step appears in magnetization curves of the isotropic ribbon at temperatures below 140 K, indicating metamagnetism Magnetoresistivity (MR) has been observed in this compound MR increases with decreasing temperature and is estimated to be 72% at 42 K This compound exhibits MR of 1% in the paramagnetic state at room temperature