Large Magnetovolume Effect Research Articles

Temperature-induced metamagnetic transition in ErCo 3: influence of Fe and Ni substitutions for Co

The phenomenon of a temperature-induced itinerant metamagnetism in ErCo 3 (at 65 K) was investigated with respect to small substitutions of Fe and Ni for Co. Five percent of Fe or Ni substitutions change the transition temperature to 170 or 25 K, respectively. The transition is accompanied by a large magneto-volume effect, Δ V/ V reaches ∼0.5%, which allowed to relate the thermal expansion data with the magnetic state of the d-electrons. The change of the magnetic characteristics with substitutions is mainly ascribed to the shift of the position of the Fermi level in the hybridised d-electron energy band.

Magnetostriction in a simple trivalent manganese perovskite

We found an unusual negative thermal expansion in the simple perovskite ${\mathrm{MnF}}_{3}$ below the Neel point, where the spins are ordered in an A-type magnetic structure. The structure of ${\mathrm{MnF}}_{3}$ has been studied between 5 and 500 K using powder neutron and synchrotron diffraction. The structure remains monoclinic $C2/c$ over the entire temperature range. We have found that the interplay of spin and orbital ordering is sufficient to result in a large positive magnetovolume effect without charge ordering. This expansion is reminiscent of the Invar effect observed in certain alloys.

The Magnetovolume Effect of an Amorphous Magnet Gd67Ni33

We have observed a large magnetovolume effect in a melt spun Gd-rich amorphous magnet Gd 67 Ni 33 . The magnet shows the forced volume magnetostriction of a positive value of 3.76×10 -5 T -1 at 4.4...

Magnetic Properties in Fe-Ni Invar Alloys Irradiated by High-Energy Ions

ABSTRACTAnomalously large shift of the Curie temperature has recently been observed in Fe-Ni Invar alloys irradiated with high-energy heavy ions. This large effect can be attributed to the large positive magneto-volume effect essentially originated in the itinerant electron ferromagnetism in Fe-Ni Invar alloys. To investigate the mechanism of the large modification of the ferromagnetism and the structure of the modified portion, measurements of the beam energy dependence of AC-susceptibility-temperature curves have been made. It was found that the amount of the shift of the Curie temperature did not change by increasing the beam energy. On the contrary, the intensity of the susceptibility of the modified portion increased with increasing the ion beam energy. The high-density electronic excitation is considered to be responsible for the large modification of the ferromagnetism in Fe-Ni Invar alloys.

Magnetovolume effects of Y–Fe–Co–Ti intermetallics

Thermal expansion and magnetic properties of the YFe11−xCoxTi compounds have been investigated by various experimental methods. A large magnetovolume effect has been clearly observed below TC for the Fe-rich compounds. When the transition metal concentration changes from Fe to Co rich, the invar-like effect becomes less pronounced and almost disappears close to the pure-Co compound. The dependence of the spontaneous volume magnetostriction on temperature and Co concentration has been estimated from the thermal expansion measurements. It has been found that, for the YFe11−xCoxTi compounds, the largest spontaneous volume magnetostriction (about 2.1%) occurs at x=3, close to the concentration in which a maximum in saturation magnetization was found. The anomalous thermal expansion can be attributed to the volume dependence of the magnetic energy, which is very sensitive to the distance between first-neighbors transition-metal atoms for Fe-rich compounds.

Structural, electronic, and magnetic properties of thin Mn/Cu(100) films

The atomic structure, electronic, and magnetic properties of thin Mn films epitaxially grown on Cu(100) substrates have been investigated by ab initio density-functional studies. Because the local-density approximation leads to a rather poor description of the magnetostructural properties of bulk Mn, a detailed study of the effect of generalized gradient corrections (GGC) to the exchange-correlation functional on the structure and magnetism of Mn in three and two dimensions has been performed. For the bulk we find that the GGC's lift the almost-degeneracy between the competing magnetic configurations and lead to a large magnetovolume effect, in much better agreement with experiment. For free-standing Mn monolayers the effect of the GGC's is even more pronounced: the relative stability of square and hexagonal layers is inverted, antiferromagnetic ordering leads to a large increase of the equilibrium distances. Therefore all investigations of Mn films on Cu substrates have been performed in the generalized gradient approximation. The results demonstrate that homogeneous Mn overlayers are unstable against interdiffusion and the formation of ordered surface alloys. At a coverage of $\ensuremath{\Theta}=0.5$ Mn monolayers, an ordered ferromagnetic $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface alloy is formed. The same atomic structure is assumed at a coverage of $\ensuremath{\Theta}=1$ and leads to an antiferromagnetic coupling between the CuMn alloy layers. In both homogeneous alloy layers and in the surface alloys, Mn is in a high-spin state with a magnetic moment close to $4{\ensuremath{\mu}}_{B}.$ The large atomic volume of magnetic Mn leads an outward relaxation of the Mn atoms and a pronounced buckling of the surface. Detailed comparisons of the calculated atomic structure with low-energy electron diffraction and photoelectron diffraction experiments and of the electronic structure with photoemission and inverse photoemission spectroscopies are reported.

Magnetic ordering of GdMn 2

GdMn 2 is a geometrically frustrated antiferromagnet that orders below T N=105 K, through a first-order transition with a large magnetovolume effect (Δ V/ V≈1%). Elastic neutron scattering measurements were performed on powder and single-crystal samples using short-wavelength beam ( λ=0.5 Å). No magnetic contribution to the nuclear peaks was found excluding any K=[0 0 0] component. Antiferromagnetic peaks were observed, indexed with a propagation vector [ 2 3 2 3 0] and fitted with a non-collinear magnetic arrangement of both Mn and Gd sublattices.

Magnetism and magneto-structural effects in transition-metal sulphides

Recent density-functional studies of the structural and electronic properties of a wide range of transition-metal sulphides (Raybaud P, Kresse G, Hafner J and Toulhoat H 1997 J. Phys.: Condens. Matter 9 11 085, 11 107) are extended to consider the effect of magnetic ordering in sulphides formed by 3d transition metals. We find that CrS is well described as an itinerant antiferromagnet and that the magnetic ordering leads to a substantial increase of the equilibrium volume and a reduction in the axial ratio of the NiAs-type lattice. MnS(NaCl structure) is correctly described as a high-spin type-II antiferromagnet (AFM) with a very large magneto-volume effect, but the semiconducting gap is underestimated - probably due to the neglect of correlation effects. Correlation effects are also important for stabilizing the high-spin AFM type-III state of MnS2 over the low-spin state. The phase transitions between non-magnetic (NM) NiAs-type FeS and antiferromagnetic troilite are well described by spin-density-functional theory, but the formation of a semiconducting gap and the magnitude of the magnetic moments and exchange splitting can be explained only by postulating correlation effects of intermediate strength. FeS2(pyrite or marcasite) and CoS are predicted to be non-magnetic, while cubic CoS2 is well characterized as an itinerant weak ferromagnet. NiS and NiS2 are predicted to be non-magnetic by local spin-density theory, in contrast to experiment.

Magnetic and magnetoelastic behavior of mechanically alloyed FeRh compound

Mechanical milling and subsequent annealing have been applied in order to obtain FeRh compound, as an alternative to bulk sample preparation methods (plasma jet- and arc-melting, induction, etc). Structural and magnetic characterization of the as-milled sample show that heat treatment at the proper temperature is necessary in order to obtain the equilibrium ordered α′-bcc phase, in which a first order ferro–antiferromagnetic transition accompanied by a large magnetovolume effect takes place. In order to study the magnetoelastic behavior of the mechanically alloyed FeRh compound, magnetostriction measurements were performed using high pulsed magnetic fields up to 14.2 T within the temperature range 4–450 K. The large magnetostriction observed in the room temperature range has been related to the field instability of the rhodium magnetic moment within the bcc ordered phase.

Magnetically stabilized surface alloys

We investigate the interrelation of magnetism and stability of ultrathin magnetic films and come to the conclusion that under certain circumstances there should exist magnetically stabilized two-dimensional surface alloys. We present ab initio results for the formation energy and the interdiffusion energy of Cu(001)c(2 × 2)3d surface alloy films. Results based on the density functional theory in the local spin density approximation (LSDA) indicate the existence of a thermodynamically stable surface alloy: Cu(001)c(2 × 2) Mn. Although in detail, the results depend on the Cu substrate, the trends presented below have an universal character. Therefore, Cu(100)c(2 × 2) Mn is only an example of a much wider class of magnetic surface alloys. Recently, several of these surface alloys have been found experimentally, showing an unexpectingly large structural corrugation of the surface alloy atoms. This will be discussed as a result of a large magneto-volume effect. We found in addition a non-magnetic surface alloy Ti/Cu(001), which is not yet confirmed experimentally. Total energy calculations were carried out using the full-potential linearized augmented planewave (FLAPW) method in film geometry.

Volume dependence of magnetic ordering temperatures of R3(FeTi)29 (R = Pr, Nd, Gd and Tb) compounds

The large magnetovolume effects and Invar-like behavior observed in the novel R3(FeTi)29 (R = Pr, Nd, Gd and Tb) intermetallic compounds are reviewed. A large decrease in the Curie temperature TC under pressure was found to be inversely proportional to the TC value. We also report compressibility measurements performed on Nd3(FeTi)29 within a wide temperature range (100–550 K). These allowed us to determine the influence of the volume changes on TC and on the spin reorientation temperature Tsr.

Pressure-induced decoupling of the magnetic ordering of the Mn and Gd sublattices in

The thermal expansion of was measured under pressures up to 20 kbar. The large positive magnetovolume effect, caused by the onset of the Mn magnetism, is found to be shifted towards lower temperatures with pressure, and vanishes around 15 kbar. Above this pressure only the Gd sublattice shows long-range magnetic order. Between 6 and 15 kbar the Gd and Mn sublattices order separately. The pressure-induced splitting of the ordering temperatures is ascribed to the Mn magnetism instability, which is observed for certain compounds.

Magnetovolume effects and magnetic anisotropy in Ce3(FeM)29 compounds (M=Ti, Cr)

The magnetovolume effects and magnetic anisotropy of the new Ce3(FeTi)29 and Ce3(FeCr)29 compounds have been studied by measuring their linear thermal expansion (LTE), pressure dependence of the Curie temperature Tc, compressibility, and anisotropy field. The value of Tc, its pressure dependence dTc/dp, and anisotropy field HA for the Ce(FeCr)12 phase are also reported. The linear thermal expansion of these compounds exhibits Invar-like behavior with large positive values of the spontaneous magnetostriction at Tc [e.g., ωs(0)≊1.2% in Ce3(FeTi)29]. Different values of the compressibility have been observed in the ferromagnetic and paramagnetic states of Ce3(FeTi)29 (κf=1.25×10−3 kbar−1 and κp=1.12×10−3 kbar−1) in qualitative agreement with the large magnetovolume effect observed in the LTE. Tc for all the phases studied decreases under pressure with initial slopes dTc/dp=−3.75, −3.6, and −2.7 K/kbar for Ce3(FeTi)29, Ce3(FeCr)29, and Ce(FeCr)12, respectively. Two anisotropy fields have been detected in Ce3(FeTi)29 associated with saturation along the b (HA) and c (H′A) axes of the monoclinic structure. The temperature dependence of H′A for Ce3(FeCr)29 and HA for Ce(FeCr)12 is also reported.

Giant forced-volume and saturation magnetostrictions of amorphous La(FexAl1-x)13 alloys composed of icosahedral clusters

The dependences of saturation magnetostriction lambda s and forced-volume magnetostriction delta omega / delta H on the concentration and temperature have been investigated for amorphous La(FexAl1-x)13 alloys prepared by high-rate DC sputtering. The magnetic phase diagram obtained from the differential magnetic susceptibility dM/dH has been correlated with these magnetostrictions. The spin freezing temperature Tf is drastically decreased but the Curie temperature TC is slightly increased on application of a magnetic field. The temperature dependences of lambda s and delta omega / delta H exhibit a broad peak at the spin freezing temperature Tf. The temperature dependence of lambda s for the ferromagnetic alloys is explained by the two-ion model, reflecting the peculiar amorphous structure consisting of icosahedral clusters. The peak of delta omega / delta H at the Curie temperature becomes indistinct with increasing ferromagnetic state in contrast with the peak at the spin freezing temperature. The giant values of delta omega / delta H and its divergent behaviour at the spin freezing temperature are accounted for by the variable amplitude of the local magnetic moment in the itinerant spin glasses. The significant large delta omega / delta H is connected with the pronounced large magnetovolume and magnetoelastic effects such as the spontaneous volume magnetostriction omega s, compressibility kappa and high-field susceptibility chi hf which bring about various Invar anomalies.

Magnetoelastic properties of Laves phase compounds (Gd1−xYx)Co2

The magnetoelastic effect in Laves phase compounds (Gd1−xYx)Co2 in the ferrimagnetic state is very remarkable because the Co moment induced by the exchange field from Gd gives rise to a large magnetovolume effect. The pressure shift of the Curie temperature is about -2 K/kbar, being comparable with that of GdCo2. These results are consistent with the phase diagram associated with itinerant electron metamagnetism.

Effects of exchange frustrations in NdMn2

Magnetic ordering in NdMn2 has been studied by magnetization measurements and powder X-ray and neutron diffraction experiments. Giant magnetoelastic effects giving rise to both a large magnetovolume effect and a monoclinic crystal distortion at the magnetic ordering are found. A magnetic structure with two different propagation vectors and an unusual decoupling phenomenon is deduced. All these effects are discussed and shown to lead to reduction in the high frustration of the Mn magnetism in NdMn2. A reorientation process involving the Nd moments and only a quarter of the Mn moments is also found with decreasing temperature and is discussed in terms of the Mn and Nd anisotropy.

Thermal properties of γ-Mn (abstract)

Mn can exist in a fcc environment in many alloys with other elements of the 3d-transition series. Some of these alloys, which can contain more than 50 at. % Mn, are classified as Invar. They show large magnetovolume effects and at the same time have ferromagnetic, antiferromagnetic, or mixed magnetic ground states. In order to gain a better understanding of Invar systems incorporating Mn, a closer examination of this element in a fcc environment is necessary. The γ state of Mn (fcc or fct Mn) is relatively little known, since its stability range lies only between 1364 and 1410 K. However, it is possible to stabilize γ-Mn in a temperature interval ranging from low temperatures up to about 600 K by the addition of small amounts of Cu. We have measured the specific heat (300<T<600 K) and thermal expansion (4<T<600 K and 1100<T<1300 K) of three MnCu alloys containing 4.42, 6.75, and 8.55 at. % Cu. The specific heat shows a first-order transition at Tt, which is the temperature below which the lattice undergoes a tetragonal distortion. This distortion is accompanied by an abrupt appearance of antiferromagnetism. The same effect is seen in the thermal expansion which shows a volume increase associated with the lattice distortion and the magnetic ordering. The high-temperature value of the thermal expansion coefficient is the largest so far observed in any fcc 3d system (45×10−6 1/K). From this data we estimate the thermal expansion behavior of pure γ-Mn. We obtain the atomic volume in the nonmagnetic and antiferromagnetic state and compare it to theoretical calculations.

Magnetism at high-pressure in fcc Fe-based alloys

We present new studies of the pressure effect on the stability of the ferromagnetic (FM) state in fcc Fe-based alloys up to 8 GPa and at different temperatures (300-2 K) using the 57Fe high pressure Mössbauer technique. The selected systems include weak (Fe68.3Ni31.5) and strong (Fe72Pt28) itinerant ferromagnets. We find in both weak and strong ferromagnets a pressure-induced magnetic phase transition from the FM, high-spin (HS) state to a low-spin (LS) state at a critical pressure. The existence of two energetically close lying magnetic states (HS and LS) reveals a unique physical picture of ferromagnetism in fcc Fe-based systems that exhibit large magnetovolume effects. The results are compared with spin polarised band structure calculation of fcc Fe. Differences between the behavior of weak and strong ferromagnets are also discussed.

Magneto-volume effects in some selected heavy-fermion compounds

We present a review of recent magneto-volume experiments, such as thermal expansion and magnetostriction, performed on good-quality single crystals of heavy-fermion systems, like CeCu6, CeRu2Si2, URu2Si2, UBe13 and UPt3. All investigated compounds exhibit large magneto-volume effects at low temperatures, as is reflected by their huge Grüneisen parameters (at least two orders of magnitude larger than in ordinary metals). A combination of specific-heat and thermal-expansion data yields furthermore information on the pressure dependence of the characteristic temperatures, and allows, in particular cases, for a detailed separation of the Kondo, crystal-field, phonon or other contributions to the thermal properties. We also discuss the magneto-volume effect connected with the metamagnetic-like transitions observed at 20 and 8 T (at 4.2K) for UPt3 and CeRu2Si2, respectively.

Observation of a pressure-induced collapse of the Fe magnetic moment in the strong itinerant ferromagnet Fe72Pt28.

We observe a pressure-induced magnetic collapse from a ferromagnetic [high-moment, (HM)] state to a low-moment (LM) magnetic state in the strong itinerant ferromagnet ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ (ordered and disordered phases). Such an existence of energetically closely lying HM and LM states demonstrates a unique physical picture of ferromagnetism in 3d magnetic systems that exhibit anomalously large magnetovolume effects. We further show that atomic ordering in ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ stabilizes the ferromagnetic ground state against pressure and modifies the spin correlations of the pressure-induced LM magnetic ground state.