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ABSTRACT: Using small-angle neutron scattering (SANS), we have investigated the orientational order of iron nanoparticles dispersed in cyclohexanone. The particles have rodlike shape and size distributions with an average length of 200 nm and an average diameter of 25 nm. SANS shows an anisotropy, which is a measure of orientational order, in magnetic dispersions with a volume fraction of 3.2% and 3.9% iron particles in shear flow and/or magnetic field. The scattering anisotropy can be fitted by a model assuming an Onsager distribution of the orientation of the particles in shear flow. The orientational distribution of particles oriented by a magnetic field can be described by a different model assuming the Maier-Saupe orientational distribution for uniaxial ferromagnetic particles. The orientational distribution parameter m for the Maier-Saupe distribution or alpha for the Onsager distribution and the orientational order parameter S have been determined at shear rates gamma[over ] of to 0-4000 s(-1) and in magnetic fields of 0-18 mT. The S values indicate that the particles start to orient either in a shear flow of 100 s(-1) or in a magnetic field of 6 mT. Applying only shear results in an orientational order, with the dispersion returning to the disordered state when the shear rate is decreased to zero. In sharp contrast, application of magnetic fields greater than 6 mT results in orientational order in the field-increasing cycle, and two-thirds of the orientational order remains when the field is decreased to zero. This shows that the order in a magnetic field is different from the order in a shear flow, the action of magnetizing the particles along a certain direction is irreversible, and the orientational order parameter exhibits hysteresis.
Physical Review E 04/2008; 77(3 Pt 1):031403. · 2.26 Impact Factor
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ABSTRACT: We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.
Physical Review Letters 04/2007; 98(12):126403. · 7.37 Impact Factor
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ABSTRACT: Measurements of lattice-matched antiferromagnetic/ferromagnetic films which are ideal layered systems to study exchange bias are reported. Epitaxial films of FePt3 have two kinds of antiferromagnetic ordering. The spin ordering phase with wave vector Q1 = (½ ½ 0) has a Néel temperature TN = 160 K and that with wave vector Q2 = (½ 0 0) has TN = 100 K. Neutron diffraction confirmed the presence of Q2 = (½ 0 0) antiferromagnetic ordering in 200 nm Fe25Pt75 grown on MgO(100). The loop shift and coercivity of a trilayer film of CoPt3/FePt3/CoPt3 decrease with increasing temperature, consistent with the observed Néel temperature of FePt3. The x-ray diffraction rocking curve widths of films grown on MgO(100) and Al2O3(110) are compared and related to the loop shifts that are observed in the films with lattice-matched antiferromagnetic/ferromagnetic interfaces.
Journal of Applied Physics 04/2006; 99(8):08C109-08C109-3. · 2.17 Impact Factor
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ABSTRACT: Fe <sub>x</sub> Pt <sub>100-x</sub> films were grown on MgO(111) by co-sputtering Fe and Pt. Composition of the films was determined by Rutherford backscattering spectrometry with an accuracy of 1%. Epitaxy and alloy ordering were quantified by x-ray diffraction and the order parameter was determined to be 0.97 for a film with x=30 and 0.99 for a film with x=25 . Neutron diffraction measurements established the presence of an antiferromagnetic phase at T=100 K in 500 nm FePt <sub>3</sub> samples grown on MgO(111). Since FePt <sub>3</sub> can be grown as an ordered antiferromagnet and a disordered ferromagnet, these films provide a pathway to grow lattice matched interfaces for exchange bias studies.
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 08/2005; · 1.25 Impact Factor
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ABSTRACT: Recently a novel media structure for thermally assisted magnetic recording was proposed consisting of a layer of FePt exchange coupled to a FeRh layer. The FePt forms a high magnetocrystalline anisotropy, high coercivity ferromagnetic layer. The FeRh layer is antiferromagnetic at room temperature, but upon heating above a transition temperature becomes ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy. The coupled ferromagnetic FePt and FeRh layers form an exchange-spring system significantly lowering the coercive field of the composite system compared to a single layer of FePt. This feature opens intriguing possibilities for media applications for thermally assisted magnetic recording where the ferromagnetic phase of FeRh is exploited to help write the media while the low-temperature antiferromagnetic phase supports the long-term stability. Here temperature-dependent structural and magnetic measurements of undoped and doped FeRh single layer and FePt-FeRh bilayer films are presented and the promises and challenges of the exchange spring media structure are discussed.
IEEE Transactions on Magnetics 08/2004; · 1.36 Impact Factor
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ABSTRACT: The layered A-type antiferromagnet Pr <sub>0.5</sub> Sr <sub>0.5</sub> MnO <sub>3</sub> has been investigated by neutron scattering. Antiferromagnetic ordering is observed with a T<sub>N</sub> of 152.3 K and an exponent β of 0.148±0.01 that suggests 2d Ising behavior. The spin-wave excitations, measured for the neutron momentum transfer q perpendicular to the ferromagnetic planes at 2 K, show a gap of 2.3±0.2 meV at the zone center [0.5 0 0.5]. The dispersion near zone center yielded the spin-wave stiffness coefficient D<sub> SW </sub> of ∼100±10 meV Å<sup>2</sup> which is twice the value found in the isostructural A-type antiferromagnet LaMnO <sub>3</sub>. The double-exchange interaction, introduced by hole doping, enhances D<sub> SW </sub> in Pr <sub>0.5</sub> Sr <sub>0.5</sub> MnO <sub>3</sub>. The spin wave dispersion at low q fits the dispersion of the Heisenberg model with nearest neighbor exchange interaction and single ion anisotropy. © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; · 2.17 Impact Factor
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Physical Review B 01/2001; 63:134426. · 3.69 Impact Factor
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ABSTRACT: The magnetism of antiferromagnetic FexPt1-x thin films (x=0.27 and 0.30) epitaxially grown onto MgO (110) and a-axis sapphire (α-Al2O3) substrates has been studied by elastic neutron and x-ray scattering. Bulk chemically ordered FePt3 exhibits an antiferromagnetic spin structure with a wave vector Q1=2π/a(1/21/20) below TN1∼160K. For slightly Fe-rich alloys (x≳0.26) a spin-reorientation transition to a second antiferromagnetic phase with a wave vector Q2=2π/a(1/200) occurs below TN2∼100K at the expense of Q1. For increased Fe content (x∼0.30) the Q1 phase is strongly suppressed with a dominant Q2 phase. For (111)-oriented films grown on a-axis sapphire the spin structure is the same as that found in the bulk. The x=0.27 film exhibits transitions at TN1 and TN2. The film with x=0.30 exhibits an almost completely suppressed Q1 phase and a dominant Q2 phase with an enhanced ordering temperature of TN2∼140K. In contrast FePt3 (110) films grown onto MgO (110) exhibit only the Q1 phase for both compositions x=0.27 and x=0.30 with the onset of the Q2 phase suppressed. The distinct behavior of the films grown onto MgO from those grown onto a-axis sapphire and bulk FePt3 may be explained by higher strain and defect densities incorporated in the films grown onto MgO.
Phys. Rev. B. 63(13).
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ABSTRACT: Inelastic neutron scattering is used to investigate the spin dynamics in the layered A-type antiferromagnetic state of Pr0.5Sr0.5MnO3 at 20 K. The spin wave dispersion, observed between 2 and 12 meV, for neutron momentum transfer, Q, parallel to the ferromagnetic planes is found to be much steeper than the dispersion when Q is perpendicular to the ferromagnetic planes, indicating strong anisotropy. Here we show that the three-dimensional Heisenberg model with nearest neighbor exchange interactions, an interlayer antiferromagnetic coupling, an intralayer ferromagnetic coupling, and a single ion anisotropy can account for these dispersions. A comparison of the ratio of exchange couplings in Pr0.5Sr0.5MnO3 and LaMnO3 shows that the anisotropy of exchange interactions strongly depends on the type of orbital ordering.
Phys. Rev. B. 73(6).
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ABSTRACT: Neutron diffraction has been used to investigate the critical behavior at the onset of antiferromagnetic phase transitions in a (111) oriented Pt73Fe27 film grown on an a-axis oriented sapphire (α‐Al2O3) substrate. As in the bulk, there is an antiferromagnetic reorientation transition from the Q1=2π∕a(1∕2,1∕2,0) phase to the Q2=2π∕a(1∕2,0,0) phase upon cooling. The temperature dependence of the integrated intensity of the (1∕2,1∕2,0) and the (1∕2,0,0) antiferromagnetic Bragg peaks yielded the Néeel temperature of 160.25±0.2 K and a reorientation transition temperature of 95±0.2 K. The magnetization critical exponent β is found to be 0.368±0.013 for the Q1 phase and 0.37±0.02 for the Q2 phase. These critical exponents are in excellent agreement with the predictions of the 3d Heisenberg universality class. A comparison of the transition temperatures and the exponents in the film and in single crystal at the same alloy composition is presented.
Phys. Rev. B. 70(2).
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ABSTRACT: The element-specific and shell-specific magnetism of Eu in the filled skutterudite ferrimagnet Eu0.95Fe4Sb12 has been investigated using Eu M4,5- and Eu L2,3-edge x-ray magnetic circular dichroism (XMCD) spectroscopy as a function of temperature. Eu L3-edge x-ray absorption spectroscopy shows that Eu is mostly in the divalent state. Eu M5-edge x-ray absorption spectroscopy, measured by electron yield, shows nearly equal fractions of Eu2+ and Eu3+ states because it probes a significant portion of the surface volume which is dominated by the Eu3+ impurity state. Sum-rule analysis of the Eu M4,5-edge XMCD spectrum measured at 4.9 K yielded the 4f spin moment of (7.15±0.3)μB per Eu2+ ion. By comparing the Eu L2,3-edge XMCD spectrum in the ferrimagnetic state of Eu0.95Fe4Sb12 to that of a divalent Eu reference compound, the clathrate Eu8Ga16Ge30, we show that the 5d spin polarization of Eu in the skutterudite is strongly enhanced by the exchange coupling with the 3d band electrons of Fe, which were shown to have an ordered moment of −0.21μB/Fe in our earlier Fe L2,3 XMCD measurements. The temperature dependence of the magnetic order parameter, determined from the Eu L3-edge XMCD intensity, yields a mean-field-like exponent (∼0.52) in the skutterudite and a three-dimensional Heisenberg-type (∼0.36) exponent in the clathrate.
Phys. Rev. B. 79(1).
