3.014 Materials Laboratory November 13th – 18th, 2006 Lab week 3 Phase Transitions Module γ-2: VSM study of Curie Temperatures 1 Instructor: Silvija Gradečak Objectives: a) Understand magnetic and thermal effects associate with the ferromagnetic/paramagnetic transition at the Curie temperature. b) Appreciate how a first-order structural phase transformation (in a ferromagnet) may affect magnetic properties and heat content. Summary of tasks:1) Use vibration sample magnometer (VSM) to measure the magnetization process, M vs. H, for Ni-Mn-Ga crystal below and above Curie temperature. Measure magnetization M(T) in Ni-Mn-Ga crystal on increasing and decreasing temperature between 20 and 120oC at 500 Oe and 10 kOe. Lessons to be learned: The Curie temperature defines a second-order magnetic transition between the low-temperature ferromagnetic state (long-range ordering of magnetic moments) and the high-temperature paramagnetic state (long-range disorder of magnetic moments). The other transition observed is a first-order structural transformation between the low-temperature, lower-crystal-symmetry martensite phase and high-temperature, higher-symmetry austenite phase. The latter transformation shows hysteresis because it is a first-order transformation; it is reflected in the magnetic susceptibility because of the change in magnetic anisotropythat occurs with the change in crystal symmetry. 2) Use differential scanning calorimetry (DSC) to study the martensite transformation and Curie transition in Ni-Mn-Ga between 20 and 120oC. Calculate the heat of transformation in each case. Lessons to be learned: Considerable heat is associated with the first-order transformation, less with the second-order transition. Distinguish endothermal and exothermal transitions. 1 Acknowledgement: Lab Notes courtesy of Dr. Bob O’Handley with minor modifications. 1Materials needed Small Ni-Mn-Ga crystal, suitable for VSM measurements. The martensite transformation temperature, Tm ≈ 30 - 60oC, depends strongly on alloy composition; the Curie temperature, TC ≈ 80 - 90oC, less so. Equipment to be usedVibrating sample magnetometer (VSM) and differential scanning magnetometer (DSC). Background Ni-Mn-Ga: Ni2MnGa is a member of a series of Fig. 1 Structure ofHeusler alloys (related to the DO3 austenitic and martensitic intermetallic compound Fe3Al) having interesting electrical, magnetic, thermal and optical properties. Alloys of Ni-Mn-Ga close to the intermetallic compound Ni2MnGa are chemically ordered below about 800oC in the L21 ordering (Strukturbericht notation, with Fm3m symmetry, as pictured in Fig. 1. Te composition you will study is close to the stoichiometric compound and is ferromagnetic with a Curie temperature of about 80oC. These compositions also have a structural transformation from a high-temperature cubic phase (austenite) to a low-temperature tetragonal phase (martensite); this transformation temperature falls between about 30 and 60oC. In the cubic state Ni-Mn-Ga has low coercivity and an almost-reversible M-H curve that saturates in a relatively-small applied field. In the tetragonal Austenite Fm3m Martensite I4/mmm Ni-Mn-Ga. 2 http://cst-www.nrl.navy.mil/lattice/struk/),phase, there is a significant hysteresis and a larger field is required to saturate the magnetization. The term martensitic transformation applied originally to the structural change that occurs between α-Fe (BCC) and its metastable, carbon-containing, tetragonal phase, martensite. The term has since come to describe any structural transformation that occurs without diffusion and by means of a local atomic displacement. On cooling below the martensite temperature, the structure undergoes a first-order transformation that contracts one of the cubic axes and enlarges the other two. The low-temperature phase is body centered tetragonal with I4/mmm symmetry (it is rotated about the contracted c axis by 45o relative to the parent phase and has a = b = 0.707 × the lattice constant of the parent). (Martensitic Ni-Mn-Ga alloys can be highly twinned because the tetragonal distortion can occur along any of the three cubic <100> axes and the twin-boundary energy is small. They are technically important because application of a magnetic field to the martensitic phase can give rise to strains of 6% through field-induced motion of twin boundaries.) Measuring M-H: The M-H curves are measured with a vibrating sample magnetometer (VSM). There are many instruments that can measure magnetization of a material. Most of them make use of Faraday's law of induction: E ! dl = "##t$B ! dA = "$#%#t(1) It says that a voltage, E ! dl", is generated in a path that encloses a time-changing magnetic flux, ∂φ/∂t. The sense of the voltage is consistent with Lenz's law as shown in Fig. 2. 3Figure removed due to copyright restrictions. Fig. 2 A decrease in flux through a coil results in a voltage in that coil whose sense is suchthat its current would create a field opposing the initial change. The flux density or magnetic induction inside a sample depends on the applied field and the sample magnetization, B = φ/A = µo(H+M). Outside the sample (M = 0) the induction, B = µoH, comes from the applied field and the H field due to the dipole moment of the sample. When the flux density around a magnetic sample is changed (by either moving the sample or the pickup coil, or by varying the sample magnetization with a small AC field), a voltage is induced in a nearby pickup coil. Integration of that voltage with time gives the flux change due to the sample. The sample may be magnetized by an electromagnet, which generates a magnetic field by passing a current through a copper coil as shown in Fig. 3 and 4. Figure removed due to copyright restrictions. Fig. 3. Direction of magnetic B field about a current-carrying solenoid is given by the right-hand rule. We will use a vibrating sample magnetometer in which a sample is vibrated (± 1 mm at about 88 Hz) to induce a voltage in a set of carefully designed pickup coils. The sample is magnetized by the field of the electromagnet. The magnetic flux forms a