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Mechanism of Thermal Hysteresis and Phase Transition in （Mn,Fe）2PGeSi

Author: DongMei
Tutor: SongLin
School: Inner Mongolia Normal
Course: Condensed Matter Physics
Keywords: MnFe(P,Ge,Si) compounds magnetocaloric effect Curie temperature thermal hysteresis magnetic-entropy change
CLC: O469
Type: Master's thesis
Year: 2011
Downloads: 56
Quote: 0
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Abstract


MnFe(P1-xAsx) compounds become one of the best candidate materials for room temperature magnetic refrigeration, because of their giant magnetocalotic effect (MCE). In order to avoid using As, a lot of work have been done to replace As. The results show that As can be replaced by Ge and Si. MnFeP0.75-xGexSi0.25 compounds have the "virgin effect", its Curie temperature TC will increase and thermal hysteresis will reduce with the increase of Ge content in compounds. MnFeP0.75-xGexSi0.25 compounds have an enormous magnetocaloric effect in the vicinity of TC, as the series of compounds have large thermal hysteresis, which affects its practical application.Add interstitial atom of B, further study on Magnetocaloric effect and phase have been carried out on a series of samples of the MnFeP0.63Ge0.12Si0.25Bx (x= 0,0.01,0.02,0.03) compounds in this paper. X-ray diffraction analysis shows that its phases mainly consist of the Fe2P-type hexagonal structure phase which is P-62m in the space group. It was found that the lattice parameter a increased, the lattice parameter c decreased with increasing B, and the volumes did not change.The magnetic measurement shows that the Curie temperature increased from 300 K to 347 K and thermal hysteresis decreased from 20 K to 6 K for x= 0 to 0.03. The maximum values of the magnetic entropy change are 11.4、11.26、11.8 J/kgK respectively, as field change from 0 to 1.5 T.MnFe1-xCrxPo.63Ge0.12Si0.25 (x= 0,0.01,0.02,0.03) compounds has Fe2P type hexagonal structure, space group P/62m, which has second phase of (Fe, Mn)3Si. The compounds stated that "virgin effect", A virgin effect, much lower phase transition temperature for first cooling, has been detected, which is a close relationship with internal stress in the MnFe(P,Ge,Si) compounds. The Curie temperature and thermal hysteresis decreases with increasing Cr content. The maximum values of the magnetic entropy change are 14.8,9.04, 5.82 and 4.06 J/(kg-K) respectively, as field change from 0 to 1.5 T.The substitution effect of Ti for Fe on the structure and magnetocaloric properties in the MnFe1-xTixP0.63Ge0.12Si0.25 (x= 0,0.01,0.02,0.03) compounds has been investigated by means of X-ray diffraction (XRD) and magnetic measurements. Analysis shows that the MnFe1-xTixP0.63Ge0.12Si0.25 (x=0,0.01,0.02,0.03) compounds mainly has the Fe2P-type hexagonal structure. It is found that the lattice parameter a increases and the lattice parameter c slightly decreases with increasing Ti, and the unit cell volume does not obviously change. The Curie temperature decreased and thermal hysteresis does not obviously change with the increasing of Ti content. Curie temperature of MnFeP0.63Ge0.12Si0.25 was 305 K and the maximum magnetic entropy was 14.8 J/(kg-K) for a field change from 0 to 1.5 T.The paper also uses the same technology and different preparation methods of MnFe(P,Si,Ge) series of compounds to study Magnetocaloric effect, the main preparation methods are as follows:Pressure machine press the powder sample into four different block parts with the pressure (one ton p<1.32×108 Pa, a ton of half P= 1.97×108 Pa, three tons P= 3.95×108 Pa, four tons of P=5.27×108Pa), the density of the samples are different when under different pressure. As different density of the same sample, whether the effects of different compounds, the Curie temperature, phase transformation and thermal hysteresis nature of the refrigeration material will be affect.Use first principles density functional theory to obtain hexagonal Fe2P optimization parameters. Fe2P band calculated show that the valence band and conduction band overlap occurs, showing metal features. Combined with theoretical calculated lattice ferromagnetic Fe2P PDos different distribution curve and the atomic magnetic moments of different sizes, we can find Fe2P magnetic mainly from Fe (3g) grid.

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