F. C. Montenegro

Evidence for a spin‐glass behavior in the diluted antiferromagnet FexZn1−xF2

We report dc susceptibility ( χ) measurements in the diluted antiferromagnet Fex Zn1−x F2 with concentrations x=0.10, 0.25, 0.31, and 0.46. The field and temperature dependencies of χ in the x=0.10 and 0.25 samples reveal that a spin‐glass (SG) phase appears at low temperatures. For x=0.31, which is slightly above the percolation limit (xp=0.24), the SG phase is also present at low temperatures, but there is a clear indication of long‐range antiferromagnetic (AF) ordering at intermediate temperatures. Finally, in the sample with x=0.46, AF ordering with random‐field effects determines the characteristic behavior. These studies will help in the understanding of the crossover from random field to SG behavior in dilute antiferromagnets.

Crossover from random field to spin-glass behavior in FexZn1-xF2

It is well known that the diluted antiferromagnet FexZn1−xF2 under an external field is a prototype random field Ising model system (RFIM) when the concentration x is larger than ≂0.4. Here we show that as x decreases there is a crossover to spin‐glass (SG) behavior. For x=0.25 the dc magnetization has irreversibility, metastability, and remanence typical of a canonical SG whereas for x=0.48 they are characteristic of a RFIM system. The ac susceptibility also shows completely diverse behavior for x=0.25 and 0.48, characteristic of SG and RFIM system, respectively. The SG nature of the x=0.25 sample is confirmed by the critical behavior of the nonlinear dc susceptibility above the SG freezing temperature T≂10 K. Finally, for x=0.31 there is a temperature dependent crossover from RFIM to SG behavior.

Giant transversal magnetoimpedance and Hall‐effect measurements in Co70.4Fe4.6Si15B10

We report room‐temperature transversal magnetoimpedance (TMI) and Hall‐effect measurements performed in ribbons of the zero‐magnetostriction soft‐ferromagnet Co70.4Fe4.6Si15B10 alloy annealed at 587 K for 15 min at a dc magnetic field of 2 kOe. The annealing showed little effect on the magnetostriction of the samples. The frequency f and the amplitude Iac of the current used in the measurements, and the measuring magnetic field H were varied in the intervals 10≤f≤105 Hz, 2.5≤Iac≤25 mA, and −15≤H≤15 kOe, respectively. The magnetic field dependence of the TMI shows a peak which is strongly dependent on f and on Iac. It reaches the giant value of 28% at H=350 Oe for f=100 kHz and Iac=25 mA. This TMI giant value is of the same order of the giant longitudinal magnetoimpedance (GLMI) but its peak is positioned in a magnetic field two orders of magnitude larger than the value obtained for the GLMI. The Hall effect yielded a value of RS=0.43 μΩ cm kOe−1. Peaks which are also frequency dependent were observed at t...

Spin‐glass behavior in the Al:Mn quasicrystalline alloys (abstract)

Despite the large manganese concentration present in the quasicrystalline Al:Mn system (typically of the order of 20%), only a small fraction of unusually large magnetic moments are detected.1 Recently2 we performed a scaling analysis of the nonlinear magnetic susceptibility of the decagonal T‐Al78Mn22 quasicrystalline alloy. The critical exponents obtained (β=0.6 and γ=4.4) are identical to those obtained for canonical spin glasses.3 In this work, we present new scaling results for the icosahedral I‐Al80Mn20 alloy. A discussion on the dynamical scaling of the quasicrystalline alloys is also presented.

Ordering in the dilute weakly anisotropic antiferromagnet Mn{sub 0.35}Zn{sub 0.65}F{sub 2}

The highly diluted antiferromagnet Mn{sub 0.35}Zn{sub 0.65}F{sub 2} has been investigated by neutron scattering in zero field. The Bragg peaks observed below the Neel temperature (T{sub N}{approx_equal}10.9 K) indicate stable antiferromagnetic (AF) long-range ordering at low temperature. The critical behavior is compatible with random-exchange Ising model critical exponents ({nu}{approx_equal}0.69 and {gamma}{approx_equal}1.31), as reported for Mn{sub x}Zn{sub 1-x}F{sub 2} with higher x and for the isostructural compound Fe{sub x}Zn{sub 1-x}F{sub 2}. However, in addition to the Bragg peaks, unusual scattering behavior appears around the (1 0 0) AF Bragg peak below a glassy temperature T{sub g}{approx_equal}7.0 K. The glassy region T<T{sub g} corresponds to that of noticeable frequency dependence in earlier zero-field ac susceptibility measurements on this sample. These results indicate that long-range order coexists with short-range nonequilibrium clusters in this highly diluted magnet. (c) 2000 The American Physical Society.

d=3 random field behavior near percolation

The highly diluted antiferromagnet Mn0.35Zn0.65F2 has been investigated by neutron scattering for H>0. A low-temperature (T<11 K), low-field (H<1 T) pseudophase transition boundary separates a partially antiferromagnetically ordered phase from the paramagnetic one. For 1<H<7 T at low temperatures, a region of antiferromagnetic order is field induced but is not enclosed within a transition boundary.

Specific heat of the dilute antiferromagnetic system FexZn1-xF2

The specific heat (cp) of the dilute antiferromagnet FexZn1-xF2 has been measured in absence of external magnetic fields for x = 0, 0.26, 0.31, 0.34, 0.36, 0.38, 0.41, 0.45, 0.56, 0.88, 0.97, and 1.0. For x > 0.45, a sharp peak associated to the antiferromagnetic (AF) phase transition at TN(x) is the only observed feature. For 0.31 ≤ x ≤ 0.41, this peak becomes smaller with decreasing x and a rounded bump appears at higher temperatures T. Closer to the percolation concentration (xp = 0.24), the peak characteristic of the AF phase transition disappears and the rounded bump becomes the only observed feature. The low-T cp behavior confirms a crossover from AF long range order at large x to a spin glass behavior close to xp. At intermediate x, the low-T joint signature of both phenomena indicates a coexistence of AF order and a cluster-glass phase. The x-dependence of the Neel temperature was accounted for using a simple phenomenological model.

Random‐exchange to random‐field crossover breaking in Mn0.35Zn0.65F2

The critical phase boundary Tc(H) of Mn0.35Zn0.65F2 has been mapped in a (H,T) phase diagram using dc magnetization measurements. Our data show that Tc(H) is governed by a scaling law TN−Tc(H)∼H2/φ, with φ=3.4±0.2. This value of φ exponent is in clear departure with the universal random‐exchange Ising model to random‐field Ising model (REIM‐RFIM) crossover exponent φ≂1.4, found in weakly diluted samples of MnxZn1−xF2 and FexZn1−xF2. This observation and results from previous birefringence studies for samples of MnxZn1−xF2 with x≥0.4 suggest that a REIM‐RFIM crossover breaking occurs somewhere in the interval 0.35

ac susceptibility on the dilute antiferromagnet MnxZn1−xF2 close to the percolation threshold (abstract)

ac susceptibility measurements have been performed on the dilute antiferromagnet Mn0.35Zn0.65F2 in the absence of an external uniform field. For a driving field of amplitude Hac∼1.0 Oe applied parallel to the easy [001] direction, a frequency‐dependent susceptibility has been observed at low temperatures. When the driving field is applied perpendicular to this direction, a departure from the Curie–Weiss behavior indicates that some spin clusters are spatially oriented at low temperatures. The presence of these randomly oriented magnetic moments may cause the random‐exchange Ising model to be inappropriate1 in explaining the zero‐field behavior near to the percolation concentration xp. A random‐anisotropy model may be useful to explain the magnetic features of MnxZn1−xF2 close to xp.