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Search Results: 1 - 10 of 200681 matches for " P. Nordblad "
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Dynamical studies of the response function in a Spin Glass
K. Jonason,P. Nordblad
Physics , 1999, DOI: 10.1016/S0921-4526(99)00791-7
Abstract: Experiments on the time dependence of the response function of a Ag(11 at%Mn) spin glass at a temperature below the zero field spin glass temperature are used to explore the non-equilibrium nature of the spin glass phase. It is found that the response function is only governed by the thermal history in the very neighbourhood of the actual measurement temperature. The thermal history outside this narrow region is irrelevant to the measured response. A result that implies that the thermal history during cooling (cooling rate, wait times etc.) is imprinted in the spin structure and is always retained when any higher temperature is recovered. The observations are discussed in the light of a real space droplet/domain phenomenology. The results also emphasise the importance of using controlled cooling procedures to acquire interpretable and reproducible experimental results on the non-equilibrium dynamics in spin glasses.
Reentrant spin glass transition in La$_{0.96-y}$Nd$_{y}$K$_{0.04}$MnO$_3$: origin and effects on the colossal magnetoresistivity
R. Mathieu,P. Svedlindh,P. Nordblad
Physics , 2000, DOI: 10.1209/epl/i2000-00457-7
Abstract: Magnetic, electric and structural properties of La$_{0.96-y}$Nd$_y$K$_{0.04}$MnO$_{3+\delta}$ with 0$\leq y \leq$0.4 have been studied experimentally. A disordered magnetic state is formed as La is substituted by Nd, reflecting the competition between ferromagnetic (FM) double exchange and antiferromagnetic superexchange interactions. Key structural parameters are identified and correlated with changes in magnetic and electric properties. By application of a large magnetic field, spin disorder scattering is removed, creating a new magnetoresistance peak at a temperature lower than the first near-$T_c$ peak. Time dependent zero-field-cooled magnetisation measurements have been performed for $y$=0.4 around this temperature. A reentrant spin glass (RSG) transition is evidenced, with low field ageing properties in both the RSG and FM phases, similar to those observed in archetypal spin glass materials.
Memory and rejuvenation in a spin glass
R. Mathieu,M. Hudl,P. Nordblad
Physics , 2010, DOI: 10.1209/0295-5075/90/67003
Abstract: The temperature dependence of the magnetisation of a Cu(Mn) spin glass ($T_g$ $\approx$ 57 K) has been investigated using weak probing magnetic fields ($H$ = 0.5 or 0 Oe) and specific thermal protocols. The behaviour of the zero-field cooled, thermoremanent and isothermal remanent magnetisation on (re-)cooling the system from a temperature (40 K) where the system has been aged is investigated. It is observed that the measured magnetisation is formed by two parts: (i) a temperature- and observation time-dependent thermally activated relaxational part governed by the age- and temperature-dependent response function and the (latest) field change made at a lower temperature, superposed on (ii) a weakly temperature-dependent frozen-in part. Interestingly we observe that the spin configuration that is imprinted during an elongated halt in the cooling, if it is accompanied by a field induced magnetisation, also includes a unidirectional excess magnetisation that is recovered on returning to the ageing temperature.
Relaxation of the field-cooled magnetization of an Ising spin glass
T. Jonsson,K. Jonason,P. Nordblad
Physics , 1998, DOI: 10.1103/PhysRevB.59.9402
Abstract: The time and temperature dependence of the field-cooled magnetization of a three dimensional Ising spin glass, Fe_{0.5}Mn_{0.5}TiO_{3}, has been investigated. The temperature and cooling rate dependence is found to exhibit memory phenomena that can be related to the memory behavior of the low frequency ac-susceptibility. The results add some further understanding on how to model the three dimensional Ising spin glass in real space.
Magnetic Relaxation Phenomena in a CuMn Spin Glass
C. Djurberg,K. Jonason,P. Nordblad
Physics , 1998, DOI: 10.1007/s100510050824
Abstract: Experiments on the temperature and time dependence of the response function and the field cooled magnetisation of a Cu(Mn) spin glass at temperatures below the zero field spin glass temperature are used to explore the non-equilibrium nature of the underlying spin configuration. The results imply that a certain spin configuration is imprinted on the system as the temperature is decreased at a constant cooling rate. The cooling rate governs the magnitude of the FC magnetisation (M_{FC}(H,T)). Any intermittent halt at a constant temperature, T_{i}, imprints an extended spin configuration, a process that is reflected e.g. in a downward relaxation of M_{FC}. On continued cooling at the same rate, the magnitude of M_{FC}(T) remains at a lower level than that of a continuous cooling curve. These results are put into the context of the corresponding behaviour of the response function as observed in measurements of the relaxation of the zero field cooled magnetisation.
Dynamics of the diluted Ising antiferromagnet Fe0.31Zn0.69F2
K. Jonason,P. Nordblad,F. C. Montenegro
Physics , 1999,
Abstract: The diluted Ising antiferromagnet, Fe_{0.31}Zn_{0.69}F_{2}, has been investigated by dynamic susceptibility measurements in zero and and finite applied dc-fields. In zero field, there is a para- to antiferromagnetic phase transition at $T_{N} \approx {20.0 K}$, followed by a dramatic slowing down of the dynamics at low temperatures. The latter described by a pure Arrhenius law. The effect of a superposed dc-field on the antiferromagnetic phase transition and on the low-temperature dynamics is investigated, and a comprehensive static and dynamic phase diagram in the H-T plane is derived. In agreement with earlier results on the same system, $T_{N}(H)$ follows a random-exchange Ising model to random-field Ising model crossover scaling for fields $H \leq 1.5 T$. A random-field induced glassy dynamics appears for higher values of H, where the antiferromagnetic phase transition is destroyed. The low-temperature dynamics shows striking similarities with the behavior observed in reentrant antiferromagnets.
Critical dynamics of an interacting magnetic nanoparticle system
M. F. Hansen,P. J?nsson,P. Nordblad,P. Svedlindh
Physics , 2000, DOI: 10.1088/0953-8984/14/19/314
Abstract: Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behavior due to dipolar interactions, only the 17 vol% sample displays critical behavior close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single particle dynamics.
Spin glass like transition in a highly concentrated Fe-C nanoparticle system
P. J?nsson,P. Svedlindh,P. Nordblad,M. F. Hansen
Physics , 2000, DOI: 10.1016/S0304-8853(00)00838-6
Abstract: A highly concentrated (17 vol.%) Fe-C nano-particle system, with a narrow size distribution $d = 5.4\pm 0.4$ nm, has been investigated using magnetic ac susceptibility measurements covering a wide range of frequencies (17 mHz - 170 Hz). A dynamic scaling analysis gives evidence for a phase transition to a low temperature spin-glass-like phase. The critical exponents associated with the transition are $z\nu = 10.5 \pm 2$ and $\beta = 1.1 \pm 0.2$. The reason why the scaling analysis works for this sample, while it may not work for other samples exhibiting collective behavior as evidenced by aging phenomena, is that the single particle contribution to $\chi''$ is vanishingly small for $T>T_g$ and hence all slow dynamics is due to collective behavior. This criterion can only be fulfilled for a highly concentrated nano-particle sample with a narrow size distribution.
Symmetrical Temperature-Chaos Effect with Positive and Negative Temperature Shifts in a Spin Glass
P. E. J?nsson,H. Yoshino,P. Nordblad
Physics , 2002, DOI: 10.1103/PhysRevLett.89.097201
Abstract: The aging in a Heisenberg-like spin glass Ag(11 at% Mn) is investigated by measurements of the zero field cooled magnetic relaxation at a constant temperature after small temperature shifts $|\Delta T/T_g| < 0.012$. A crossover from fully accumulative to non-accumulative aging is observed, and by converting time scales to length scales using the logarithmic growth law of the droplet model, we find a quantitative evidence that positive and negative temperature shifts cause an equivalent restart of aging (rejuvenation) in terms of dynamical length scales. This result supports the existence of a unique overlap length between a pair of equilibrium states in the spin glass system.
Non-equilibrium dynamics in an interacting nanoparticle system
P. J?nsson,M. F. Hansen,P. Nordblad
Physics , 1999, DOI: 10.1103/PhysRevB.61.1261
Abstract: Non-equilibrium dynamics in an interacting Fe-C nanoparticle sample, exhibiting a low temperature spin glass like phase, has been studied by low frequency ac-susceptibility and magnetic relaxation experiments. The non-equilibrium behavior shows characteristic spin glass features, but some qualitative differences exist. The nature of these differences is discussed.
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