Range Of Exchange Interaction Research Articles

Magnetocaloric effect and critical magnetic behavior in Ni-rich Ni–Mn–Sn full Heusler alloy

In search of low-cost, hysteresis-free, broad working temperature magnetocaloric materials, Ni2.24Mn0.76Sn Heusler alloy has been prepared by the conventional arc melting method. The XRD pattern reveals that the sample is crystallized in Fm $$\overline{3}$$ m space group in L21 cubic symmetry. The Maxwell's equation is employed on magnetic isotherms to determine the change in magnetic entropy. This alloy gives a reasonable magnetocaloric effect ( $$\Delta S_{M}$$ = 1.25 J/Kg-K and RCP = 83.87 J/Kg at 3 T) with a large working temperature (67 K at 3 T) near the room temperature. Critical exponents have been calculated using modified Arrott plot, Kouvel–Fisher plot, and critical isotherm analysis to understand the nature of phase transition and exchange interaction, causing magnetic ordering in the system. The values of critical exponents (β = 0.454 ± 0.015, γ = 1.145 ± 0.035, δ = 3.522 ± 0.009) and range of exchange interaction (σ = 1.698) fall in between long-range mean-field theory and short-range 3D Heisenberg model, indicating the complex nature of exchange interaction.

Accelerating the switching of magnetic nanoclusters by anisotropy-driven magnetization dynamics

In this work the magnetization dynamics of clusters supported on non-magnetic substrates is shown to exhibit an unprecedented complex response when subjected to external magnetic fields. The field-driven magnetization reversal of small Co clusters deposited on a Cu(111) surface has been studied by means of first-principles calculations and atomistic spin dynamics simulations. For applied fields ranging from 1 Tesla to 10 Tesla, we observe a coherent magnetization reversal with switching times in the range of several tenths of picoseconds to several nanoseconds, depending on the field strength. We find a non-monotonous dependence of the switching times with respect to the strength of the applied field, which we prove to have its origin in the complex magnetic anisotropy landscape of these low dimensional systems. This effect is shown to be stable for temperatures around 10 K, and is possible to realize over a range of exchange interactions and anisotropy landscapes. Possible experimental routes to achieve this unique switching behaviour are discussed.

Low temperature magnetization behavior in Co 36Fe 36Si 3Al 1Nb 4B 20 (at%) nanostructured alloy

The investigation addresses low temperature magnetization behavior in Co 36Fe 36Si 3Al 1Nb 4B 20 alloy ribbons in their as-spun as well as annealed state. Optimum heat treatment at 875 K led to nanocrystallization whereby bcc-(FeCo)SiAl nanoparticles were dispersed in an amorphous matrix as evidenced from transmission electron microscopy. Low temperature magnetization studies were carried out in the range 77–300 K. Using the method of mathematical fittings, magnetization extrapolated to 0 K was obtained. The dependence of the magnetization with respect to temperature of BT 3/2 was used to determine the Bloch coefficient “ B” and spin wave stiffness constant “ D”. Magnetic softening revealed by lowering in the coercivity in the optimum nanostructured state was also the cause of a drop in the stiffness constant. The range of exchange interaction given by D/ T C was higher in the nanostructured state compared to the as-spun amorphous state. The effect of nanocrystallization and the resulting ferromagnetic coupling was further evidenced by low temperature magnetization studies.

Magnetic properties of amorphous Fe 73.5Cu 1Mo 3Si 12.5Al 1B 9 alloy

The FINEMET type Fe 73.5Cu 1Mo 3Si 12.5Al 1B 9 (numbers indicate at.%) alloy has been synthesized using single wheel melt spinning technique. The samples have been thermally annealed at about 40 K less than the crystallization temperature for various durations. The samples show increase in Curie temperature of the amorphous phase with annealing. To get a better insight into their magnetic behaviour we have done spin wave analysis on the as-spun and thermally annealed samples and extracted the spin wave stiffness constant and mean square range of exchange interaction. Low temperature thermomagnetic measurements from 10 to 300 K were performed with an applied field of 0.5 T. The data were fitted using the Bloch equation. The value of the C/ B ratio and the mean square range of exchange interaction were found to be characteristic of amorphous ferromagnets. The variation of spin wave stiffness constant is correlated to the changes in Curie temperature and the nature of the exchange interaction existing in this alloy is determined.

MAGNETIC PROPERTIES AND MAGNETOIMPEDANCE EFFECT IN TM70Cr5Si10Bl5 (TM = Fe, Co, Ni) ALLOYS

Amorphous TM70Cr5Sil0B15 (TM = Fe, Co, Ni) alloys were prepared by rapid quenching technique. Saturation magnetization of the amorphous ribbons was investigated by using SQUID and vibrating sample magnetometer from 5 to 800 K. For these two ferromagnetic alloys, the saturation magnetization up to 0.4 Tc can be described by the Bloch relation, Ms(T) = Ms(0)[1 - BT3/2 - CT5/2]. The spin wave stiffness constants and the range of exchange interaction were analyzed from the magnetization behaviour. And the temperature dependence of the magnetoimpedance effect was investigated in conjecture with their magnetic properties from 10 to 300 K.

On the Range of Exchange Interaction in Fe-Type Diluted Magnetic Semiconductors

On the Range of Exchange Interaction in Fe-Type Diluted Magnetic Semiconductors

Low-temperature magnetization of amorphous and nanocrystalline Fe 76.5− xU xCu 1Si 13.5B 9 alloys

The low-temperature magnetization of amorphous and nanocrystalline Fe 76.5− x U x Cu 1Si 13.5B 9 alloys ( x = 0–11) has been studied in the 4.2–300 K range. All examined magnetic parameters (exchange integral J, range of exchange interactions 〈 r 2〉, magnetic moment per Fe atom, μ Fe, spin wave stiffness constant D) are markedly influenced by the uranium substitution for iron. Both the μ Fe and D are found to increase with decreasing Fe content in amorphous alloys for x = 0 up to about 4. The noncollinear spin structures as a result of competing interactions are supposed to explain this behaviour. Some samples (with U contents of 2–6 at%) annealed for 1 h at 813 K appear to be nanocrystalline. Uranium seems to inhibit grain growth in the investigated FeUCuSiB system, just as Nb does in FeNbCuSiB nanocrystalline alloys.

Temperature dependence of the magnetization of glassy and nanocrystalline Fe/sub 82/M/sub 7/B/sub 10/Cu/sub 1/ [M=Zr,Hf,Nb] alloys with high permeability at high frequencies

We report the salient features of the magnetic properties of glassy and nanocrystalline Fe/sub 82/M/sub 7/B/sub 10/Cu/sub 1/ [M=Zr,Hf,Nb] alloys. By controlled annealing, fine 4-6 nm grain size bcc-Fe is formed in a ductile glassy matrix, which gives good soft magnetic properties like permeability and low coercive field. From the low temperature magnetic data and ferromagnetic resonance studies we determine and compare the changes in the spin wave exchange stiffness constants as well as the range of exchange interactions in the glassy and nanocrystalline states.

Magnetic properties of amorphous (Fe−1xNix)75Si10B15 alloys

Abstract Amorphous (Fe1−xNix)75Si10B15 (0.1 ⩽ x ⩽ 1, the step of x is 0.1) alloys were preparedby the rapid quenching technique and their magnetization measurements have been performed by utilizing a vibrating sample magnetometer from 77 K up to 800 K. The Curie temperature, Bloch coefficient, and range of exchange interaction were calculated from the temperature-dependent saturation magnetization curves. The low temperature dependence of magnetization is in good agreement with Bloch's relation, M s (T) = M s (0) (1 - BT 3 2 - CT 5 2 ) . The range of exchange interaction becomes shorter with the addition of nickel concentration. The observed mean square ranges of exchange interaction shows that this alloy system loses the ferromagnetic ordering when x is larger than 0.9.

Superexchange in insulators and semiconductors: II. The range of the exchange interaction

In this paper we discuss the range of exchange interactions in magnetic semiconductors. It is shown that the range depends strongly on the energy required to excite an electron from the fully occupied valence band to the empty magnetic d orbitals. The range of the exchange interaction and of the spin-density distribution is the same when this excitation energy is large. It is found that kinetic exchange and correlation exchange are the dominant contributions. The usual perturbation theory breaks down when the dp excitation energy becomes of the order of the band width. In this case one has to calculate the exchange interaction in a more rigorous way. This limit is discussed using two-particle Green functions. It is found that these results deviate strongly from the ones obtained using perturbation theory. We derive a path model from an analysis of the effective transfer between two magnetic orbitals. Such a model can explain the long-range exchange interactions reported in the last decades in a large number of magnetic semiconductors and semimagnetic semiconductors. This path model can explain the anisotropy of long-range spin-density distributions found in doped semiconductors.

57Fe,61Ni and91Zr hyperfine field distributions in magnetic Tm-Zr amorphous alloys

Pulsed NMR measurements were performed on [Tm1 (Tm2)]90+xZr10−x (Tm=Fe, Co, Ni) amorphous alloys utilizing a newly developed low temperature wide band probehead-preamplifier system. A 11,6 T broad57Fe hyperfine field distribution around the average of 23.6 T together with a less intensive peak around 11 T has been found in Fe90.6Zr9.4, in excellent agreement with Mossbauer studies. A rather flat distribution around the average value of 14.8 T characterizes the transferred hyperfine field at the Zr sites in the same alloy. Both features provide strong evidence for a broad range of exchange interactions and variations in the spin density at the Fe sites in this material. A much narrower and symmetrical distribution at the Zr sites around 10 T in Co90Zr10 is evidence of the higher magnetic homogeneity of this alloy as compared to Fe90Zr10. In Fe60Ni30Zr10, in addition to the57Fe signal, a contribution from61Ni nuclei could be separated, corresponding to an average hyperfine field value of 15.3 T.

Analysis of high temperature susceptibility using a generalization of the Curie–Weiss law

The Curie–Weiss Law is generalized to take into account differences among lattices, spin values, and range of exchange interactions. Several forms of the generalization are given for the purpose of facilitating the abstraction of magnetic moments from susceptibility data. A parameter γp is introduced that describes exactly the behavior in the high temperature limit and correlates well with the behavior nearer to the critical temperature.

Mossbauer studies in sputtered A-Fe<inf>80</inf>B<inf>20</inf>foils

Mossbauer studies have been carried out on sputtered amorphous Fe 80 B 20 foils in the temperature range of 4.4 K to 300 K. The results have been analysed using Window's method. The coefficients B and C in Bloch's law are evaluated from the data. One finds a smaller value of B in sputtered foils in comparison to the liquid quenched ribbons. There are indications that the mean square range of exchange interaction is larger in these foils.

Proof of theT52term in the magnetization of Fe(BC) metallic glasses

The magnetization of ${\mathrm{Fe}}_{80}{\mathrm{B}}_{20\ensuremath{-}x}{\mathrm{C}}_{x} (0\ensuremath{\le}x\ensuremath{\le}8)$ ferromagnetic metallic glasses has been measured in the temperature range $0<T\ensuremath{\lesssim}180$ K (corresponding to $0<\frac{T}{{T}_{C}}\ensuremath{\lesssim}0.3$) and in magnetic fields up to 40 kOe. As in crystalline ferromagnets, the magnetization obeys the equation $\frac{\ensuremath{\Delta}\ensuremath{\sigma}(T)}{\ensuremath{\sigma}(0)}=\ensuremath{\beta}{T}^{\frac{3}{2}}+\ensuremath{\gamma}{T}^{\frac{5}{2}}$. For the first time a clear necessity for the coefficient $\ensuremath{\gamma}$ even at lowest temperatures has been found in an amorphous series through bulk magnetization measurements. The concentration ($x$) dependence of the spin-wave stiffness constant $D$ is discussed. The range of exchange interactions in this amorphous series is of the order of 2 atomic distances, i.e., less than that in crystalline nickel.

Optically detected magnetic resonance of strongly coupled donor-acceptor pairs in ZnS

By monitoring the self-activated emission from ZnS:Ga,Al crystals, ODMR signals have been observed which result from recombination between donor and A-center acceptors which are coupled by a range of exchange interactions. In contrast to earlier ODMR investigations, in which only weakly coupled donor-acceptor pairs were observed, the authors describe here signals obtained from the spin triplet state of strongly coupled pairs. For such pairs the spin exchange constant J is much greater than 0.1 cm-1 but the donor-acceptor separation is still in excess of 0.8 nm.

Hyperfine field distribution in disordered binary alloys

Mossbauer measurements were made on the disordered binary alloys Ni3Fe NiFe and FeCo at different temperatures and the distribution of hyperfine magnetic fields has been evaluated using a model-independent method of analysis proposed by Window (1971). The full width at half-maximum of the P(H) distribution is found to remain constant for Ni3Fe with respect to temperature. It increases for NiFe but decreases for FeCo with increase of temperature. The full width at half-maximum is correlated to the exchange interaction and the measurements show that the range of exchange interaction is shorter in NiFe than in Ni3Fe. In FeCo all the atoms are found to have the same magnetic ordering temperature. The reduced value of the average field for NiFe is fitted to the Brillouin function.

Effect of finiteness of the range of exchange interactions on the surface spin susceptibility of paramagnetic metals

It is shown using a simple variational technique in the case of an exchange interaction with a finite range eta that the surface ferromagnetic instability for values of the electron-electron interaction for which the bulk is still paramagnetic, which exists in the case of a zero-range interaction, may disappear for large enough values of the range eta .

Spin-Wave Theory of MnF2 in the Random Phase Approximation

In order to extend the validity of ferromagnetic spin-wave theory to higher temperatures, Englert used the random phase approximation (RPA), in which the spin-wave interactions are represented by a model in which the number of spin-wave excitations are related self-consistently to the magnetization through a sum rule. We have applied RPA to the two-sublattice antiferromagnet of arbitrary spin and range of exchange interaction, and have found a formal correspondence to the Green's function theory of ferromagnetism of Tahir-Kheli and ter Haar. Using this approximation, the sublattice magnetization is computed self-consistently for parameters appropriate to MnF2 and compared over a wide temperature range to the sublattice magnetization predicted by the NMR experiments of several investigators. We find that the theory represents the significant features of the sublattice magnetization at low temperatures, and exhibits features peculiar to the temperature dependence of the anisotropy in that the magnetization is double valued in the region of the Néel point.