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  • 1
    Electronic Resource
    Electronic Resource
    Effects of fluctuations and domain formation on the dynamics of simple ferromagnets : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 6179-6181 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Starting at the Curie temperature Tc dynamical susceptibilities have been measured between 1 Hz and 20 GHz on nonmetallic crystals to determine the influence of progressive ordering on the magnetization dynamics. In uniaxial materials (GdCl3, LiTbF4), the low-frequency response, ω/2π〈50 MHz, is governed by the domain-wall relaxation, the rate Γd of which speeds up rapidly near Tc. This novel feature appears to be the first proper signature of linear walls predicted by Ginzburg and Bulaevskii for strong anisotropy, χ(parallel)(T) (approximately-greater-than) χ⊥. The faster intradomain relaxation, arising from spin–spin interaction, undergoes a thermodynamic slowing down, the kinetic coefficient of which determines the magnitude of the damping in the walls. In the cubic ferromagnets (EuS, EuO), χ(ω) displays an oscillatory behavior with resonance and damping frequencies lying in the GHz region. Their decrease near Tc is described by simple powers to the spontaneous magnetization. A comparison made to the well-understood critical dynamics above Tc suggests an association of these phenomena with dipolar-anisotropic magnetization fluctuations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348798
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  • 2
    Electronic Resource
    Electronic Resource
    Signature of linear (Bulaevskii–Ginzburg) domain walls near Tc of Ba-hexaferrite : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6263-6265 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The domain wall dynamics of a BaFe12O19 single crystal has been studied by measurements of the linear ac susceptibility parallel to the easy axis between 10 and 106 Hz. In the temperature interval 0.98 Tc≤T≤Tc=742.5 K, both dispersion and absorption reveal pure relaxational behavior, the damping rate ΓW of which exhibits a distinct minimum about T*=0.992 Tc. Based on previous results on low-temperature ferromagnets (Tc≤3 K) and recent theoretical work, the speeding up of ΓW∼(Tc−T)−0.7 toward Tc is related to the relaxation of linear domain walls proposed by Bulaevskii and Ginzburg. For the first time, additional support for this wall type is gained from the magnitude of the wall-reconstructuring temperature T*. The steep rise of ΓW below T* can be related to Bloch-wall damping, however, the shape of the crossover of ΓW about T* remains to be explained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352664
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  • 3
    Electronic Resource
    Electronic Resource
    Fast averaging data acquisition unit for pulse spectrometers (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 517-518 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A combined pulse generator and digitizer based on a RISC processor has been designed and tested by 59Co NMR. The pulse generator provides a 16-bit TTL-level output with 100 ns resolution and is programmable in the high-level language FORTH. The digitizer supports two channels, each with a 10 bit, 40 MSPS analog-digital converter and online averaging in a 24×1024 bit memory, which is directly accessible by the RISC processor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1145116
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  • 4
    Electronic Resource
    Electronic Resource
    Contact-free determination of ultralow resistances of micron-sized wires (2000)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 71 (2000), S. 583-584 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We apply a limiting case of recently developed general inversion schemes between complex magnetic susceptibilities and electrical resistances ρ to the case of copper whiskers and demonstrate the feasibility and high accuracy of this contactless method to measure ρ. The present data reveal strong effects of ideal surface scattering and Bloch–Grüneisen phonon scattering on ρ in these samples with very low residual bulk resistivity, ρb(0)=0.27 nΩ cm. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1150248
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  • 5
    Electronic Resource
    Electronic Resource
    Homogeneous magnetization dynamics below Tc of EuO: Effects of spin waves and anisotropy : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 4641-4643 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using a broadband reflectometer, the dynamic susceptibility χ(ω) of a EuO sphere has been measured from 0.3 to 20 GHz at temperatures ranging between 4.2 K and Tc=69.5 K. For fields applied parallel to the hard [100] direction one heavily damped Lorentzian is observed which for internal fields H larger and smaller than the anisotropy field arises from the dynamics of the homogeneous and the wall magnetization, respectively. For fields near HA applied along one of the easy [111] axes, both processes contribute to χ(ω) but with increased and decreased values of the resonances and damping frequencies of the homogeneous and wall processes, respectively. The homogeneous process is identified by the static susceptibility χz(0) which obeys the H−1/2 divergence due to spin waves. Its line shape is explained by a frequency variation of the kinetic coefficient of the relaxation damping, Lz(ω)=Lz(0)/[1+(iω/ωc)]. The characteristic frequency, ωc/2π=18 GHz, turns out to be independent of temperature and magnetic field, and is related to the relaxation rate of the long-wavelength longitudinal magnetization fluctuations. The field and temperature variation of the kinetic coefficient of the relaxation can be described by the same scaling function, Lz(χz(T,H)), as previously observed above Tc. Based on results of numerical mode-coupling work, this behavior is associated with the dipolar anisotropy of the magnetic fluctuations. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361690
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  • 6
    Electronic Resource
    Electronic Resource
    Scaling behavior of the homogeneous magnetization dynamics in the ferromagnetic state of EuS : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6054-6056 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using a broadband vectorial microwave reflectometer, we measured the longitudinal dynamic susceptibility of a EuS sphere between 0.1 and 20 GHz in the temperature interval 4.2 K≤T≤Tc=16.5 K. For low internal fields, the shape corresponds to a heavily overdamped Lorentzian resonance, χ−1z(ω)= χ−1z(0)iω/Lz−(ω/Ωz)2 where Lz≤Ωz. The field and temperature dependence of the intrinsic kinetic coefficient and resonance is fully explained by that of the static susceptibility χz(0). Surprisingly, the resulting scaling function, Lz[χz(0)], agrees exactly with that observed earlier above Tc, where Lz[χz(0)] could quantitatively be explained by a crossover from a van Hove behavior for large χz(0), due to dipolar anisotropic fluctuations, to a critical speeding-up for χz(0)〈1, where isotropic spin diffusion prevails. This finding suggests that the dipolar anisotropy dominates the magnetization dynamics also below Tc of EuS, which seems to be supported by the fact that the correlation frequency of the torques acting on the magnetization, ωc=Ω2z/Lz, agrees with the relaxation rate of the longitudinal magnetization fluctuations, δMq(parallel)q.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355508
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  • 7
    Electronic Resource
    Electronic Resource
    Temperature and field dependence of domain wall dynamics up to the Curie point of EuO : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 6054-6056 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using a broadband vectorial microwave reflectometer, the dynamic susceptibility has been measured on a EuO sphere in the range from 0.3 to 20 GHz. The data taken at zero field between 4.2 K and the Curie temperature Tc=69.5 K were analyzed using a phenomenological model of a damped harmonic oscillator for the domain walls, that has been successfully applied to the early investigations on ferrites. Based on the micromagnetic Landau–Lifshitz (LL) model, the temperature variation of the intrinsic resonance and damping frequencies are explained up to T=0.98 Tc. The resulting LL-kinetic coefficient, L⊥=1.1×1011 s−1, remains independent of temperature and is related to the dipolar induced relaxation rate of the longitudinal magnetization fluctuations. In fields along the easy [111] axis and the symmetric [100] direction this wall process prevails for magnetizations smaller than Ms/4 and Ms/(square root of)3, respectively. Above these thresholds, the characteristic frequencies of the dominant process decrease with field, possibly due to rotational dynamics, and for H(parallel)[111] a second dynamics associated with the paraprocess is contributing. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362089
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  • 8
    Electronic Resource
    Electronic Resource
    Strong thermal fluctuation effects on the dynamics of Bloch walls (abstract) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5958-5958 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Approaching the Curie temperature of the uniaxial Ba and Sr hexaferrites from below in zero magnetic field, a rapid decrease of the domain wall relaxation rate Γw is observed which attains a minimum at T*=0.99 Tc. Above T* the increase of the relaxation, Γw∼(Tc−T)−0.8, agrees with that observed previously on low-temperature ferromagnets1 and is assigned to the presence of linear domain walls. Accordingly, the speeding-up of Γw towards Tc is associated with the divergence of the correlation length of a 3-dimensional Ising-magnet determining the width of the linear wall. The temperature T* is found to be significantly larger than T*0 predicted by mean field work of Bulaevskii and Ginzburg2 for the second order transition from linear to Bloch domain walls, which is characterized by a continuously increasing transverse magnetization, mB ∼ (T*− T)βB, in the wall. Adjusting the temperature variation of the relaxation rate below T* to a recent kinetic theory for elliptic walls,3 one finds βB=0.10 to be close to the 2-dimensional Ising value. Along with the suppression of T* this constitutes the first signature of severe fluctuation effects in domain walls.4
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355525
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  • 9
    Electronic Resource
    Electronic Resource
    On the resistivity minimum in amorphous metallic spin-glasses (abstract) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6566-6566 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Though the occurrence of a low-temperature resistivity minimum is a rather general feature of magnetic metallic glasses, its origin—whether magnetic or structural—is still under debate. We have performed temperature and magnetic-field-dependent measurements of the resistivity ρ(T,B) and the magnetization M(T,B) on amorphous Ni80−xFexSi8B12 below and above the critical concentration for ferromagnetic order (xc=4). In the ferromagnetic and reentrant phases ρ(T→0) increases as (−T1/2), which can be related to a quantum effect from modified electron-electron interaction in the presence of structural disorder.1 In marked contrast, in the conventional spin glass (x=2.4) ρ(T→0,B=0) exhibits a linear increase below the freezing temperature Tf=6.2 K. A reanalysis of literature data reveals a linear upturn of ρ(T→0) to occur in other amorphous spin glasses, too, with a quasi-universal law: Δρ/ρ≈−4(2) 1E−3 T/Tf. This finding will be discussed in terms of elastic scattering from the spin-glass order parameter2 and, alternatively, in terms of quantum interference on closed diffusion paths in the presence of inelastic spin-flip scattering. Recently, for the latter case, a linear T-law has been predicted to evolve from a crossover from dynamic spin scattering to the static limit at zero temperature.3
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356920
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  • 10
    Electronic Resource
    Electronic Resource
    Critical magnetic equation of state of the “Long-Range” ferromagnet GdCl3 (1973)
    Springer
    The European physical journal 260 (1973), S. 395-402 
    Details
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The magnetic equation of stateM(H, T) near the ferromagnetic transition point (T c =2.211 K) of GdCl3, a system in which the long-range dipole-dipole and the exchange interactions are of the same order of magnitude, is investigated. For a considerableM-H-T-region a scaling functionh(x) does exist. Qualitative analyses ofh(x) using series expansions and empirical forms show that the scaling function parameters of GdCl3 differ from those of systems with dominating short-range interactions. Quantitative comparison with the mean field theory (infinite range of interaction) and Stanley's high temperature series for nearest-neighbour Heisenberg interaction do not give satisfactory agreement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01397964
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