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In this letter, we experimentally validate the ability of a recently developed ferromagnetic hysteresis model to characterize magnetic after-effects in ferromagnetic materials. The modeling framework, which combines energy analysis at the lattice level with stochastic homogenization techniques to accommodate material, stress, and field nonhomogeneities, quantifies after-effects through a balance of the Gibbs and relative thermal energies. Attributes of the framework are illustrated through fits to experimental steel data.

T.R. Braun, R.C. Smith and M.J. Dapino, “Experimental validation of a homogenized energy model for magnetic after-effects,” Applied Physics Letters, Vol. 88, 122511, 2006.

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The characterization of a commercial Ni–Mn–Ga alloy for use as a deformation sensor is addressed. The experimental determination of flux density as a function of strain loading and unloading at various fixed magnetic fields gives the bias field needed for maximum recoverable flux density change. This bias field is shown to mark the transition from irreversible (quasiplastic) to reversible (pseudoelastic) stress-strain behavior. A reversible flux density change of 145mT is observed over a range of 5.8% strain and 4.4MPa stress at a bias field of 368kA∕m. The alloy investigated shows potential as a high-compliance, high-displacement deformation sensor.

N. SARAWATE and M.J. Dapino, “Experimental characterization of the sensor effect in ferromagnetic shape memory NiMnGa,” Applied Physics Letters, Vol. 88, 121923, 2006.

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Despite the huge magnetic-field-induced strain (MFIS) of up to 9.5% exhibited by certain Ni–Mn–Ga alloys, their usefulness in applications is severely hindered by the electromagnet device needed for driving the alloys with a magnetic field and orthogonal loading stress. In this paper we present macroscopic measurements obtained from a single crystal of Ni50Mn28.7Ga21.3 which demonstrate a large reversible MFIS of −4100ppm when the alloy is driven with quasistatic magnetic fields and fixed compressive stresses applied collinearly along the [001] axis. This collinear configuration marks a fundamental difference with prior research in the field and points to the existence in this alloy of stable bias or residual stresses—likely associated with pinning sites in the alloy—which provide the energy necessary to restore the original variant state when the field is reversed. We present macroscopic magnetomechanical measurements which show a decrease of the MFIS with increasing stress loading and a stiffening of the alloy with increasing dc fields. The latter behavior is phenomenologically similar to the ΔE effect in magnetostrictive materials. The large reversible MFIS and tunable stiffness properties exhibited by this alloy could enable practical Ni–Mn–Ga actuators for high-deflection, low-force applications which due to being driven by a solenoid transducer are more compact, energy efficient, and faster than their electromagnet counterpart. A thermodynamic model is presented which qualitatively characterizes the decay in MFIS with increasing compressive external load and provides a starting point for the characterization, design, and control of the proposed Ni–Mn–Ga devices.

A. MALLA, M.J. Dapino, T. Lograsso and D. Schlagel. “Large magnetically-induced strains in Ni50Mn28.7Ga21.3 driven with collinear field and stress,” Journal of Applied Physics, Vol. 99, No. 6, 063903, 2006.