The magnetocaloric effect and critical behaviour of the Mn_0.94Ti_0.06CoGe alloy

Abstract

(2013). The magnetocaloric effect and critical behaviour of the Mn0.94Ti0.06CoGe alloy. Journal Of Physics-Condensed Matter, 25 (5), 1-7.The magnetocaloric effect and critical behaviour of the Mn0.94Ti0.06CoGe alloy AbstractStructural, magnetic and magnetocaloric properties of the Mn0.94Ti0.06CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected, at T-str = 235 K and T-C = 270 K. A giant magnetocaloric effect has been obtained at around Tstr associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni2In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, at T-str, the magnetic entropy change, -Delta S-M reached a maximum value of 14.8 J kg(-1) K-1 under a magnetic field of 5 T, which is much higher than that previously reported for the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around T-C = 270 K from the ferromagnetic to the paramagnetic state, we performed a detailed critical exponent study. The critical components gamma, beta and delta determined using the Kouvel-Fisher method, the modified Arrott plot and the critical isotherm analysis agree well. The values deduced for the critical exponents are close to the theoretical prediction from the mean-field model, indicating that the magnetic interactions are long range. On the basis of these critical exponents, the magnetization, field and temperature data around T-C collapse onto two curves obeying the single scaling equation M(H, epsilon) = epsilon(beta)f +/-(H/ epsilon(beta+gamma)). AbstractStructural, magnetic and magnetocaloric properties of the Mn 0.94 Ti 0.06 CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected at T str = 235 K and T C =270 K, respectively. A giant magnetocaloric effect has been obtained around T str associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni 2 In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, T str , the magnetic entropy change, -∆S M reached a maximum value of 14.8 Jkg -1 K -1 under a magnetic field of 5T which is much higher than that previously reported on the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around T C =270 K from ferromagnetic to paramagnetic state, we performed a detailed critical exponent study. The critical components, γ, β and δ determined using the Kouvel-Fisher method, the modified Arrott plot as well as the critical isotherm analysis agree well. The values deduced for the critical exponents are close to the theoretical prediction of the mean-field model, indicating that the magnetic interactions are long range. Based on these critical exponents, the magnetizat...

The magneto-structural transition in Mn1−xFexCoGe

“…For the Mn0.96Fe0.04CoGe sample, two changes are observed in the magnetisation curve ( [25]. In the case of Mn0.94Ti0.06CoGe, the transition temperatures were TC hex ~270 K and TM ~235 K. In both samples, substitution for Mn drives the martensitic transformation temperature TM below the Curie temperature of the hexagonal structure TC hex .…”

“…Importantly, the martensitic transformation temperature TM can be tuned easily [12,[22][23][24]. Slight compositional changes, achieved via doping for example [25,26], can stabilise the hexagonal phase to low temperature with a ferromagnetic transition occurring in the hexagonal phase at the Curie temperature TC hex ~275 K [27]. Therefore, if TM is lowered to within the temperature range between TC orth and TC hex , a magneto-structural transition can occur, with potential for an enhanced MCE.…”

Large entropy change accompanying two successive magnetic phase transitions in TbMn2Si2 for magnetic refrigeration

“…In the last two decades, the magnetic materials with large/giant MCE have been extensively studied experimentally and theoretically due to the high efficiency and eco-friendliness of magnetic refrigeration compared to the commonly used gas compression technique. [1][2][3][4][5][6][7][8] The isothermal magnetic entropy change (ÀDS M ) or adiabatic temperature change (DT ad ) under a varying magnetic field is the key performance indicator of magnetic cooling materials. Searching for magnetic materials with large values of ÀDS M /DT ad has been considered as the main task in this field during the last few years.…”

Magnetocaloric effect and magnetostructural coupling in Mn0.92Fe0.08CoGe compound

“…Importantly, the value of T str depends strongly on several factors including: external pressure, 8 the vacancies in the Co and Mn sites, as well as variation in chemical environment due to element substitution on Mn, Co, or Ge sites. [9][10][11][12][13][14][15][16][17] Recently, MnCoGebased compounds have attracted significant attention; [9][10][11][12][13][14][15][16][17] this is due mainly to the strong coupling between magnetism and structure in these materials. This coupling in turn can lead to MnCoGe-based materials, which exhibit large entropy changes and associated magnetocaloric effects, thereby offering scope for magnetic refrigeration around room temperature.…”

“…In order to achieve coincidence of magnetic and structural transitions which differ by around 300 K in the parent MnCoGe compound, several approaches to modify the Mn environments have been adopted. [9][10][11][12][13][14][15][16][17] Several studies concerning substitution of other elements for Mn, Co, or Ge have been reported in order to shift and thereby control the transition temperatures T str and T C . [9][10][11][12][13][14] The general aim of such studies is to cause the magnetic and structural transitions to coincide or overlap closely and thereby capitalise on both the magnetic and structural entropies at the resultant magnetostructural transition.…”