• S N Kaul

      Articles written in Pramana – Journal of Physics

    • Is gadolinium a helical antiferromagnet or a collinear ferromagnet?

      S N Kaul

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      Controversial issues concerning the nature of magnetic ordering in gadolinium are briefly reviewed. The recent experimental results are shown to resolve most of such issues in that they rule out the possibility of a helical spin structure in Gd and clearly bring out the role of longrange dipolar interactions in stabilising collinear ferromagnetic order for temperatures between the spin-reorientation temperature and the Curie point

    • Effect of site disorder on the magnetic properties of weak itinerant ferromagnet Ni75Al25

      Anita Semwal S N Kaul

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      Detailed study of Ni75Al25 samples with varying degree of site disorder reveals that site disorder promotes magnetic excitations such as spin waves and local spin-density fluctuations and thereby reduces both spin-wave stiffness and Curie temperature. Irreversibility lines in theT-H phase diagram of the weak itinerant ferromagnet Ni75Al25 have been determined for the first time and the effect of site disorder on them has been ascertained

    • Site occupancy of Fe in ternary Ni75−xFex+yAl25−yalloys

      B Annie D'santhoshini S N Kaul

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      The results of a detailed structural and magnetic study clearly indicate that regardless of the thermal history of the samples, Fe has a strong preference for the Ni sites in Ni-poor (nonstoichiometric) Ni75Al25 alloys. Fe substitution has a profound effect on the nature of magnetism in Ni25Al25

    • Experimental observation of quantum corrections to electrical resistivity in nanocrystalline soft magnetic alloys

      K Balakrishnan Y Sundarayya M K Naidu S N Kaul

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      X-ray diffraction patterns of nanocrystalline Fe-Cu-Nb-Si-B (FINEMET) alloys reveal that bcc α-Fe/α-FeSi crystallites with the average grain size of 20(5) nm are dispersed in amorphous matrix. Enhanced electron—electron interaction (EEI) and quantum interference (QI) effects as well as electron-magnon (and/or electron-spin fluctuation) scattering turn out to be the main mechanisms that govern the temperature dependence of resistivity. Of all the inelastic scattering processes, inelastic electron-phonon scattering is the most effective mechanism to destroy phase coherence of electron wave functions. The diffusion constant, density of states at the Fermi level and the inelastic scattering time have been estimated, for the first time, for the alloys in question

    • Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder

      S N Kaul Anita Semwal

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      Isothermal remanent magnetization decay,Mr(t), and ‘in-field’ growth of zero-field-cooled magnetization,MZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25Tc to 1.25Tc (whereTc is the Curie temperature, determined previously for the same sample from static critical phenomena measurements) for a nearly ordered intermetallic compound Ni3Al, which is an experimental realization of a three-dimensional (d = 3) ferromagnet with weak quenched random-exchange disorder. None of the functional forms ofMr(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions$$M_r (t) = M_0 [M_1 \exp ( - t/\tau _1 ) + (t/\tau _2 )^{ - \alpha } ]$$ and$$M_{ZFC} (t) = M'_0 [1 - \{ M'_1 \exp ( - t/\tau '_1 ) + (t/\tau '_2 )^{ - \alpha '} \} ]$$ closely reproduce such data in the present case. The most striking features of magnetic relaxation in the system in question are as follows: Aging effects are absent in bothMrt andMZFC(t) at all temperatures in the temperature range covered in the present experiments. A cross-over in equilibrium dynamics from the one, characteristic of a pured = 3 ferromagnet with complete atomic ordering and prevalent at temperatures away from Tc, to that, typical of ad = 3 random-exchange ferromagnet, occurs asT → Tc. The relaxation times τ1(T)(τ1(T)) and τ2(T)(τ2(T)) exhibit logarithmic divergence at critical temperatures$$T_C^{\tau _1 } (T_C^{\tau '_1 } (H))$$ and$$T_C^{\tau _2 } (T_C^{\tau '_2 } (H))$$;$$T_C^{\tau '_1 } $$ and$$T_C^{\tau '_2 } $$ both increase with the external magnetic field strength,H, such that at any given field value,$$T_C^{\tau '_1 } = T_C^{\tau '_2 } $$. The exponent characterizing the logarithmic divergence in τ1(T) and τ2T possesses a field-independent value of ≃16 for both relaxation times. Of all the available theoretical models, the droplet fluctuation model alone provides a qualitative explanation for some aspects of the present magnetic relaxation data

    • Ferromagnetic resonance investigation of nanocrystalline FeCuNbSiB

      G A Basheed S N Kaul

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      An elaborate line-shape analysis of the ferromagnetic resonance (FMR) spectra taken in the temperature range 100 K to 350 K on amorphous FeCuNbSiB alloys before and after nanocrystallizing them reveals that in the nanocrystalline state, (i) spin wave stiffness (D) is enhanced while the saturation magnetization,MS, is reduced, (ii) both the ‘in-plane’ anisotropy field,HK, as well as the FMR line-width scale with MS, (iii) the single-ion anisotropy of spin-orbit plus crystal field origin dominates over the twoion anisotropy of dipolar origin and (iv) multi-magnon scattering contributions to FMR line-width become important in some cases

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