Coexistence of ferro- and antiferromagnetic order in Mn-doped Ni2_2MnGa

Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy showing large magnetic field induced strains. We present here results for the magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and theory. Experimental trends for the composition dependence of the magnetization are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up to several tesla and at low temperatures. The saturation magnetization has a maximum near the stoichiometric composition and it decreases with increasing Mn content. This unexpected behaviour is interpreted via first-principles calculations within the density-functional theory. We show that extra Mn atoms are antiferromagnetically aligned to the other moments, which explains the dependence of the magnetization on composition. In addition, the effect of Mn doping on the stabilization of the structural phases and on the magnetic anisotropy energy is demonstrated.Comment: 4 pages, 3 figure

A two dimensional model for ferromagnetic martensites

We consider a recently introduced 2-D square-to-rectangle martensite model that explains several unusual features of martensites to study ferromagnetic martensites. The strain order parameter is coupled to the magnetic order parameter through a 4-state clock model. Studies are carried out for several combinations of the ordering of the Curie temperatures of the austenite and martensite phases and, the martensite transformation temperature. We find that the orientation of the magnetic order which generally points along the short axis of the rectangular variant, changes as one crosses the twin or the martensite-austenite interface. The model shows the possibility of a subtle interplay between the growth of strain and magnetic order parameters as the temperature is decreased. In some cases, this leads to qualitatively different magnetization curves from those predicted by earlier mean field models. Further, we find that strain morphology can be substantially altered by the magnetic order. We have also studied the dynamic hysteresis behavior. The corresponding dissipation during the forward and reverse cycles has features similar to the Barkhausen's noise.Comment: 9 pages, 11 figure

Adaptive modulations of martensites

Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape memory alloys. To understand the origin of these phases, we review and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase and tetragonal martensite in Ni-Mn-Ga magnetic shape memory alloy epitaxial films. The modulated martensite can be constructed from nanotwinned variants of a tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes phase stability, the sequence of 6M-10M-NM intermartensitic transitions, and magnetocrystalline anisotropy.Comment: 4 pages manuscript, 8 pages supplemen

Modulated Martensite: Why it forms and why it deforms easily

Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale. Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could follow from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows retaining modulated martensite as a metastable phase at room temperature. In this metastable state elastic energy is released by the formation of a 'twins within twins' microstructure which can be observed from the nanometre to millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries which are diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.Comment: 34 pages, 8 figure

Candidiasis, Bacterial Vaginosis, Trichomoniasis and Other Vaginal Conditions Affecting the Vulva

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Magnetic Domain Structure and Magnetically-Induced Reorientation in Ni-Mn-Ga Magnetic Shape Memory Alloy

Magnetization process during magnetically induced reorientation and related magnetic domains of cuboid Ni50Mn28.5Ga21.5 single crystal with {100} faces was investigated. Magnetic domains were visualized using magneto-optical indicator. The domains pattern is determined by strong uniaxial magnetic anisotropy of NiMn-Ga martensite. Thanks to magnetically induced reorientation the domains arrangements for all three crystal orientations could be obtained and we showed that the size of domains scales with square root of the thickness and penetrates through whole crystal. Visualization of magnetic domains on all faces of cuboid provides the 3D model of magnetic domains

Relation between structure, magnetization process and magnetic shape memory effect of various martensites occurring in Ni-Mn-Ga alloys

Magnetic properties and magnetic shape memory effect of three Ni-Mn-Ga alloys representing three different martensitic phases; five-layered tetragonal (5M), seven-layered orthorhombic (7M), and non-modulated tetragonal (NM) martensite were studied at room temperature. Magnetization process occurs mostly by magnetization rotation. Magnetic anisotropy energy is high for all three phases ranging from 1.1 to $1.7\times10^5$ J/m$^3$. The direction of short lattice constant is always a direction of easy magnetization. It is demonstrated that the high magnetic anisotropy and low twinning stress render a MSM effect possible in 5M and 7M martensite. The SM martensite exhibits more than 6% field induced strain. The giant strain is followed by large magnetization jump. Simultaneous measurement of magnetization changes and field-induced strain as a function of magnetic field demonstrates the nature of the magnetic shape memory effect and validates the proposed mechanism

High magnetic field study of the Dy2Fe17Hx compounds with x=0–3.8

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Neutron diffraction studies of magnetic shape memory Ni Mn Ga single crystal

Neutron diffraction of single crystal of the typical example of magnetic shape memory MSM alloy Ni49.7Mn29.3Ga21 was carried out with a 2D position sensitive detector. The quality and inhomogeneity of the single crystal and martensite variant distribution was studied using amp; 969; scan of selected nuclear Bragg reflections. The neutron diffraction reveals split of the 2 0 0 reflection of major martensite variant and large structural inhomogeneities in martensite phase. Using measurement in reciprocal space, we recorded a set of reflections that appear due to structural modulation 5 M of the martensite, however, the set seems to be incomplete with missing or very weak reflections of second order compared with X ray diffraction. The line of the magnetic reflection arising from the supposed antiferromagnetic ordering of the excess Mn atoms was very weak and it is difficult to discern from the background