J. Mattsson

Energy barrier distribution of a noninteracting nano-sized magnetic particle system

Abstract AC-susceptibility and magnetic relaxation data are analysed to determine the distribution of energy barriers ( E b ) of an assembly of small magnetic particles. The assembly, a dilute frozen ferrofluid, consists of spherical nano-sized maghemite ( γ -Fe 2 O 3 ) particles with a median diameter of 70 A. Assuming a direct proportionality between E b and particle volume, the volume distribution is found to be described by a normalised gamma distribution. The system is not adequately described by the commonly adopted log-normal distribution function, which comparably should yield an excess amount of large particles sizes. From the analyses, accurate values of the Arrhenius law prefactor ( τ 0 = 4 × 10 −10 s) and the magnetic anisotropy constant are derived.

Linear Response in Spin Glasses

The general requirements for linear response of the magnetic relaxation in spin glasses have been investigated experimentally. Especially, the behaviour in superimposed external d.c. magnetic fields has been considered. The experiments were performed on the amorphous metallic spin glass (Fe0.15Ni0.85)75P16B6Al3. It is found that the response to a field change ± Δh is linear for small enough field changes, irrespective of the magnitude of the superimposed external field, H. The linear response can be expressed by the response function p(H, t), which in general depends on H, but is found to be independent of H for equilibrium dynamics.

Determination of the critical exponent β from measurements of a weak spontaneous magnetisation in the 3d Ising antiferromagnet FeF2

Abstract The critical exponent for the order parameter, β, has been determined from the temperature dependence of a weak spontaneous magnetisation that appears below the Neel temperature in the prototypical 3d Ising antiferromagnet FeF2. The derived value of β = 0.325 ± 0.002 agrees with the theoretically calculated value for β ; of a 3d Ising system. The very occurrence of a spontaneous magnetisation in pure FeF2 is assumed to be of piezomagnetic origin.

Remanent magnetization in the diluted Ising antiferromagnet Fe0.6Zn0.4F2

The field, H≤5 T, and temperature, T≥2 K, dependence of the remanent magnetization μ of the diluted Ising antiferromagnet Fe0.6Zn0.4F2 has been investigated utilizing a superconducting quantum interference device magnetometer. The results at low temperatures, where there is no growth of random field domains after the field removal, are analyzed according to μ=AHx(T ln t/t0)−Ψ+B, where the two terms are domain wall and volume contributions to the remanent magnetization, respectively. It is found that Ψ decreases continuously with decreasing temperature, and reaches a value of ≊0.15 at 3 K. The field exponent x is found to decrease with increasing temperature, attaining values between 2 and 3. The results show a more complex behavior of the low‐temperature remanent magnetization than a model proposed by Nattermann and Vilfan which predicts constant values of the exponents: Ψ≊0.4 and x≊2.

Aging in a two-dimensional CuMn spin glass

Utilizing zero-field cooled magnetization measurements the nonequilibrium dynamics in a 2D spin glass film is investigated. The experiments show that aging is an inherent property of 2D spin glass systems with qualitatively similar features as in 3D systems.

2d dynamic exponent derived from CuMn spin glass films

Abstract The slowing down of the magnetic relaxation of thin multilayered CuMn spin glass films on approaching, T = 0 K, is investigated using SQUID magnetometry. In 2d spin glasses the slowing down follows a generalised Arrhenius law, log( t / t 0 ) α T −(1+ ψv ) , with a spin glass transition at 0 K. Magnetically decoupled films of thickness ≤ 30 A are found to exhibit 2d critical behaviour and the dynamical exponent ψv = 0.9 is determined.

Low field excess magnetisation in Fe0.7Mg0.3Cl2

Abstract The low field excess magnetisation ΔM ( H , T , t ) has been measured in the diluted Ising antiferromagnet Fe 0.7 Mg 0.3 Cl 2 . The observed behaviour is remarkably different from what is expected for a random field induced domain state. The decay of ΔM on approaching the Neel temperature is described by ΔM α (1 – T / T N ) β r with β r ≈ 0.4.

Relaxation of the field cooled magnetisation in 2D Cu(Mn) films

Abstract The relaxation of the field cooled magnetisation of a multilayered 2D Cu(Mn) spin glass film has been investigated in a SQUID-magnetometer. The sample is cooled in a constant magnetic field (10 Oe) from a high temperature, where the magnetisation is at equilibrium, to a temperature below the freezing temperature, T ƒ . At constant temperature, the magnetisation is then found to increase with time at temperatures near T ƒ , showing an initial decrease followed by an increase at long time scales at intermediate temperatures and only decreases with time at lower temperatures. The behaviour, including the surprising downward relaxation away from the thermodynamic equilibrium magnetisation, is qualitatively interpreted in terms of a spin glass domain model.

Monte Carlo evidence for the existance of an overlap length in Ising spin glass systems

Abstract Monte Carlo simulations of an Ising spin glass model have been performed. The spin system was qurnched from a random initial condition, equilibrated for a time tw1, whereupon the system was subjected to a field perturbation for a time tw2. After waiting for a time tw3, a weak magnetic field h was applied. The susceptibility χ(t)=(M+(t)−M-(t))/2h, where M±( t) is the magnetisation due to a positive/negative field h, was studied as function. of time. The time dependence of the susceptibility shows behaviour similar to results of temperature cycling experiments. We show, for the first time, that the effects of an overlap length are seen also in field cyclings.

Nonlinear susceptibility of 2D spin glass films

Abstract A careful study of the nonlinear susceptibility of multilayered two-dimensional Cu(14 at% Mn) spin glass layers separated by thick interlayers of Cu is reported. The experiments were performed in SQUID magnetometers using an ac susceptibility technique in the field range 0–0.5 kG and dc magnetization measurements in the field range 0–10 kG. The static 2D critical exponents have been determined from scaling analysis.