In Ni-Mn-X（X=In,Sn,Sb） ferromagnetic shape memory alloys,a ferromagnetic transition from paramagnetic to ferromagnetic austenite and a martensitic transformation from ferromagnetic austenite to weak magnetic martensite occur in some particular composition ranges,in which abundant physical properties have been observed by the abrupt change of magnetization and resistivity around their transition temperatures in these alloys.Therefore,tuning the martensitic transformation temperature（TM） and enlarging the workingtemperature interval for Ni-Mn-X（X=In,Sn,Sb） alloys,are of great importance.In the present paper,we will focus on the effect of external factors,including pre-deformation,annealing,and high pressure annealing,on the magnetic transitions and the related magnetocaloric properties in Ni-Mn-Co-Sn ferromagnetic shape memory alloys.Our approaches and the main results in this particular field will be reviewed.
Surface energies of five different surfaces of scheelite crystal were calculated using density functional theory （DFT）. Based on the calculation results, the predominantly exposed surfaces in the morphologies of scheelite crystals were predicted. {112} and {001} cleavage surfaces and {112} crystal surface are the commonly exposed surfaces, which are consistent with both previous literatures and the present experimental observations based on the XRD. Cleavage generates more easily along {112} surfaces than along {001} surfaces due to their different interlayer spacings. The surface roughness and appearance of different predominantly exposed surfaces were then investigated using AFM. The roughness of smooth {112} cleavage surface is the lowest among these three surfaces. On {001} cleavage surface, terraces are flat and separated by steps of about 10 nm in height. Subsequently, contact angle measurements were adopted to evaluate the wettability and surface energies of these surfaces. The surface energies evaluated directly correspond to the trend calculated with DFT.
The structure,specific heat,magnetic and electrical properties of MnTe1-xSbx（x=0,0.1,0.15,0.2 and 0.25） alloys have been investigated.The MnTe1-xSbx alloys crystallize in a hexagonal NiAs-type structure,and the impurity of MnSb phase appears when x≥0.15.The MnTe0.9Sb0.1 compound exhibits ferrimagnetic behavior with hysteresis loops even at 350 K,showing that the magnetic properties of MnTe compound are very sensitive to little compositional change.The ferromagnetism in the MnTe1-xSbx alloys with higher Sb contents may be attributed to the impurity of MnSb phase.Energy dispersive X-ray spectroscopy analysis on the MnTe0.9Sb0.1 compound indicates that Sb is very difficult to dope into the lattice of MnTe.So the anomaly of resistivity at 300 K of MnTe0.9Sb0.1 and the peak of specific heat around 304 K of all the alloys are thought to be related with the antiferromagnetic interactions of MnTe-based lattice.
Ferromagnetic Ni-Mn-Ga films were fabricated by depositing on MgO （001） substrates at temperatures from 673 K to 923 K. Microstructure, crystal structure, martensitic transformation behavior, and magnetic properties of the films were studied. With increasing deposition temperature, the surface morphology of the films transforms from granular to continu- ous. The martensitic transformation temperature is not dependent on deposition temperature; while transformation behavior is affected substantially by deposition temperature. X-ray analysis reveals that the film deposited at 873 K has a 7M marten- site phase, and its magnetization curve provides a typical step-increase, indicating the occurrence of magnetically induced reorientation （MIR）. In situ magnetic domain structure observation on the film deposited at 873 K reflects that the marten- sitic transformation could be divided into two periods： nucleation and growth, in the form of stripe domains. The MIR occurs at the temperature at which martensitic transformation starts, and the switching field increases with the decrease of temperature due to damped thermal activation. The magnetically induced martensitic transformation is related to the difference of magnetization between martensite and austenite. A shift of martensite temperature of dT/dH = 0.43 K/T is observed, consistent with the theoretical value, 0.41 K/T.
An overview of the mathematical structure of the three-dimensional（3D） Ising model is given from the points of view of topology,algebra,and geometry.By analyzing the relationships among transfer matrices of the 3D Ising model,Reidemeister moves in the knot theory,Yang-Baxter and tetrahedron equations,the following facts are illustrated for the 3D Ising model.1） The complex quaternion basis constructed for the 3D Ising model naturally represents the rotation in a（3＋1）-dimensional space-time as a relativistic quantum statistical mechanics model,which is consistent with the 4-fold integrand of the partition function obtained by taking the time average.2） A unitary transformation with a matrix that is a spin representation in 2 n·l·o-space corresponds to a rotation in 2n·l·o-space,which serves to smooth all the crossings in the transfer matrices and contributes the non-trivial topological part of the partition function of the 3D Ising model.3） A tetrahedron relationship would ensure the commutativity of the transfer matrices and the integrability of the 3D Ising model,and its existence is guaranteed by the Jordan algebra and the Jordan-von Neumann-Wigner procedures.4） The unitary transformation for smoothing the crossings in the transfer matrices changes the wave functions by complex phases φx,φy,and φz.The relationship with quantum field and gauge theories and the physical significance of the weight factors are discussed in detail.The conjectured exact solution is compared with numerical results,and the singularities at/near infinite temperature are inspected.The analyticity in β=1/（kBT） of both the hard-core and the Ising models has been proved only for β〉0,not for β=0.Thus the high-temperature series cannot serve as a standard for judging a putative exact solution of the 3D Ising model.
A large and reversible magnetocaloric effect is found in the compound DyB2, which is associated with two successive mag- netic transitions： a spin-reorientation-like transition followed by a ferromagnetic-paramagnetic transition. These two transitions appreciably enlarge the magnetic-refrigeration temperature window and yield a huge refrigeration capacity of 610 J kg^-1, with a maximum magnetic entropy change -ASmax of 17 J kg^-1K^-1, at a magnetic-field change of 5 T. The corresponding values for low magnetic-field change of 2 T are 193 J kg^-1 and 7.4 J kg-lK^-1, respectively.
This paper investigates the martensitic transformation and magnetocaloric effect in pre-deformed Ni-Mn-Co Sn ribbons. The experimental results show that the reverse martensitic transformation temperature TM increases with the increasing pre-pressure, suggesting that pre-deformation is another effective way to adjust TM in ferromagnetic shape memory alloys. Large magnetic entropy changes and refrigerant capacities are obtained in these ribbons as well. It also discusses the origin of the enhanced martensitic transformation temperature and magnetocaloric property in pre-deformed Ni-Mn Co-Sn ribbons.
The （Co0.35Mno.65）2P compound, prepared by a mechanical alloying plus solid sintering process, exhibits a second-order transition from a ferromagnetic state to a paramagnetic one at Curie temperature of about 320 K with no clear thermal hysteresis. A magnetic-entropy change （△SM） value above 1.6 J.kg-1.K-1 for a 5 T field change is obtained in the whole temperature range of 292.5-352.5 K and the maximum value of the △SM is 2.3 J.kg-1.K-1 at 337.5 K. The study on the magnetocaloric effect of the （Co0.35Mn0.65）2P compound may be helpful for exploring good candidates for room-temperature magnetic refrigeration.
The magnetic phase transition and magnetocaloric effects in Fe-doped MnNiGe alloys are investigated. The substitution of Fe for Ni decreases the structural transition temperature remarkably, resulting in the magnetostructural transition occurring between antiferromagnetic and ferromagnetic states in MnNil_χFexGe alloy. Owing to the enhanced ferromagnetic coupling induced by the substitution of Fe, metamagnetic behaviour is also observed in TiNiSi-type phase of MnNil-xFezGe alloys at temperature below the structural transition temperature.
