Eduard Obradó

PREMARTENSITIC TRANSITION DRIVEN BY MAGNETOELASTIC INTERACTION IN BCC FERROMAGNETIC NI2MNGA

We show that the magnetoelastic coupling between the magnetization and the amplitude of a short wavelength phonon enables the existence of a first order premartensitic transition from a bcc to a micromodulated phase in Ni2MnGa. Such a magnetoelastic coupling has been experimentally evidenced by ac susceptibility and ultrasonic measurements under an applied magnetic field. A latent heat around 9J ymol has been measured using a highly sensitive calorimeter. This value is in very good agreement with the value predicted by a proposed model. [S0031-9007(97)04508-0] Martensitic transitions (MT) are first order displacive structural phase transitions accompanied by a significant strain of the unit cell. In general, homogeneous and intracell (short-wavelength phonons) strains are necessary in order to describe the path followed by the atoms at the transformation. An interesting feature displayed by the systems undergoing this kind of first order transitions is the existence of precursor effects [1,2]. They reflect that, in a sense, the system prepares for the phase transition before it actually takes place. For instance, in bcc materials the TA2f110g phonon branch has low energy, the corresponding elastic constant (C 0 ) is very low and softening of all these phonons and C 0 occurs on cooling [2]. The Ni2MnGa Heusler alloy is investigated in the present Letter. At high temperature it is ferromagnetic (the Curie temperature is Tc › 381 K), displays a bcc structure with an L21 atomic order (space group Fm3m), and transforms martensitically at TM › 175 K. This alloy is unique in the sense that (i) it is the only known bcc ferromagnetic material undergoing a MT and (ii) the MT is preceded by a structural phase transition (intermediate transition) to a micromodulated phase (the cubic symmetry is preserved) resulting from the freezing of a q › 0.33TA2 phonon which becomes the intracell strain characterizing the new phase. Such a phase transition has been evidenced by neutron scattering [3], x-ray [4], electron microscopy [5], and ultrasonic measurements [6,7]. We have recently suggested that this transition to the premonitory (intermediate) phase is a consequence of the magnetoelastic interplay between the phonon and the magnetization [7]. At the MT the system transforms to a modulated structure with tetragonal symmetry (homogeneous strain) [8]. The modulation of the martensitic structure is different from that of the premartensitic phase. It has been argued [7] that the intermediate transition has to be first order because there is no complete softening of the frequency of the soft phonon; nevertheless, attempts in measuring a latent heat have not been successful [9] and a small thermal hysteresis has been detected [10] only in samples subjected to external stresses (which can result in a modification of the characteristics of the transition). In this Letter we present a phenomenological model for the intermediate transition based on a Landau expansion, which includes a magnetoelastic coupling. The primary order parameter is the amplitude h of a TA2 f110g phonon, and secondary order parameters are « ,a s110 df 110g homogeneous shear suitable to describe a cubic to tetragonal change of symmetry, and M, the magnetization (considered here to be a scalar). In terms of these three order parameters we assume the free energy function to have the following general form: F sh, «, Md › Fstr sh, «d 1 FmagsM d 1 Fmesh, «, Md ,

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

Abstract This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.

Quenching effects in Cu–Al–Mn shape memory alloy

Abstract In this paper the effect of quenching from different temperatures ( T q ) in a Cu–Al–Mn alloy is studied. This alloy system, which displays an L2 1 ordered structure, transforms martensitically at an intermediate temperature T M , and undergoes a spin freezing process at a lower temperature T f . Positron annihilation measurements have shown that after the quench, an excess of vacancies is retained in the system, depending on T q . In addition, both T M and T f have been found to be sensitive to T q . This has been attributed to frozen-in disorder induced by the quench. Experimental results have been interpreted in terms of the growth of magnetic clusters, quenched-in vacancies and atomic disorder.

Magnetoelasticity in the Heusler Ni2MnGa alloy

Abstract In this paper we present magnetic and elastic measurements on a NiMnGa single crystal. It is shown that the structural properties of this alloy are strongly influenced by a magnetoelastic interaction.

Magnetoelastic behavior of the Heusler Ni2MnGa alloy

In this work the effect of the interplay between magnetic and structural degrees of freedom in the structural transitions undergone by Ni2MnGa alloy is investigated. Elastic constant and magnetic susceptibility measurements in a magnetic field are presented. A simple phenomenological model is proposed to account for the experimental observations.

MAGNETIC HYSTERESIS IN THE CU-AL-MN INTERMETALLIC ALLOY : EXPERIMENTS AND MODELING

We study isothermal magnetization processes in the Cu-Al-Mn intermetallic alloy. Hysteresis is observed at temperatures below the spin-freezing of the system. The characteristics of the hysteresis cycles as a function of temperature and Mn content (magnetic element) are obtained. At low temperature

Premartensitic phase transformation in the Ni2MnGa shape memory alloy

(\ensuremath{\lesssim}5

Hysteresis and avalanches in the site-diluted Ising model: comparison with experimental results in Cu–Al–Mn alloys

K) a change from smooth to sharp cycles is observed with increasing Mn content, which is related to the decrease of configurational disorder. We also study a zero-temperature site-diluted Ising model, suitable for the description of this Cu-Al-Mn system. The model reproduces the main features of the hysteresis loops observed experimentally. It exhibits a disorder-induced critical line separating a disordered phase from an incipient ferromagnetic ground state. The comparison between the model and the experiments allows us to conclude that the observed change in the experimental hysteresis loops can be understood within the framework of the theory of disorder-induced criticality in fluctuationless first-order phase transitions.

Spin-glass phase in the intermetallic CuAlMn compound

Abstract We present a phenomenological model for the premartensitic phase transition in ferromagnetic NiMnGa alloys. The model contemplates the effects of an applied mechanical stress on the premartensitic transition. It is shown that the premartensitic transition is driven by a magnetoelastic coupling. The first order character of this transition is enhanced by the application of a mechanical stress. Experimental data are also presented which support the predictions of the proposed model.

Anomalies related to the TA2 phonon mode condensation in the Heusler Ni2MnGa alloy

Abstract We study the zero temperature properties of hysteresis in a site-diluted Ising model. The model exhibits a critical line separating a disordered from an incipient ferromagnetic ground state: the shape of the hysteresis loop changes from a smooth cycle (formed as a sequence of tiny avalanches) to a sharp cycle exhibiting a macroscopic reversal of the magnetization for a given value of the field (infinite avalanche). Criticality is characterized by power-law distribution of avalanche sizes. Model predictions are contrasted with experimental results obtained in Cu–Al–Mn alloys.