Defect‐induced ferrimagnetism in the half‐metallic Co2CrAl and Co2CrSi compounds

Co₂CrAl and Co₂CrSi are amongst the most studied Heusler alloys due to their half‐metallic character. These compounds are also well‐known to present ferromagnetism with high Curie temperatures. We show using first‐principles calculations that the creation of Cr antisites (Cr atoms at the Co sites) induces ferrimagnetism in these compounds without destroying the half‐metallic character of these alloys. The reduction of the total spin moment causes lower external fields and thus smaller energy losses in realistic magnetoelectronic, also known as spintronic, devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co₂MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co_1−xMn_x]₂MnSi is of particular interest since Mn₃Si is known to crystallize in the Heusler L2₁ lattice structure of Co₂MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.     

Role of defects on the electronic and magnetic properties of CrAs, CrSe and CrSb zinc-blende compounds

We present an extended study of single impurity atoms and atomic swaps in half-metallic CrAs, CrSb and CrSe zinc-blende compounds. Although the perfect alloys present a rather large gap in the minority-spin band, all defects under study, with the exception of void impurities at Cr and sp sites and Cr impurities at sp sites (as long as no swap occurs), induce new states within the gap. The Fermi level can be pinned within these new minority states depending on the lattice constant used for the calculations and the electronegativity of the sp atoms. Although these impurity states are localized in space around the impurity atoms and very fast we regain the bulk behavior, their interaction can lead to wide bands within the gap and thus loss of the half-metallic character.     

Magnetic hyperfine fields for Sn atoms in rare-earth intermetallides: huge deviation from proportionality between Bhf and Sz

The magnetic hyperfine fields for ¹¹⁹Sn impurity atoms, localized in Ga sites of ferromagnetic intermetallic compounds RGa (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), were measured by the Mössbauer spectroscopy technique. At T=5 K, the hyperfine field value (B_hf) varies from 3.3 T in TmGa to 28.0 T in GdGa. Huge deviation from the proportionality between B_hf and the projection of the R³⁺ ion spin (S_z=(g−1)J) was found. As the atomic number of the R element increases, the B_hf/S_z ratio drastically decreases from 12.6 T for PrGa to 3.3 T for TmGa. This unexpected result can be explained by the strong dependency of B_hf value on the relationship between the Sn-R atomic separation (R_nn) and the radius of the magnetic 4f shell (R_4f). In the framework of this concept, the available experimental data for Sn atom in the rare-earth compounds with non-magnetic sp elements were considered. The data may be described by the universal dependency on the single parameter, λ=R_nn/R_4f.     

First-principles study of the thermal properties of half-metallic ferromagnetic systems

The origin of the half-metallicity is different in diluted magnetic semiconductors and Heusler alloys. I briefly review our earlier work on (GaMn)As and (GaMn)N focusing on the relation between the half-metallicity and the strength of the interatomic exchange interactions. This relation is governed by the properties of the valence-band holes. In Heusler alloys the factors determining the thermal behavior are distinct. Here the relation between half-metallicity and the longitudinal fluctuations of atomic moments is considered. The temperature dependence of the Ni magnetization in NiMnSb is studied.     

Temperature-dependent electronic structure of gadolinium

We investigate the electronic structure of the ferromagnetic 4f-metal Gadolinium by use of a many-body evaluation of a generalized model of magnetism, the one-particle part of which is derived from an ASW-LSDA bandstructure calculation. A striking temperature-dependence of the conduction band states is traced back to a 4f-(5d, 6s) interband exchange. The conduction electron polarization (0.63 _B atT=0) decreases forTT _c very similar to the 4f-magnetization. A red shift of the lower -band edge of about 0.25 eV appears upon cooling fromT=T _c toT=0. — The quasiparticle band-structure exibits a remarkable non uniform magnetic behaviour at different positions in the Brillouin zone, and in particular for different subbands. Weakly correlated (s-like) dispersions show a Stoner-likeT-dependence of the exchange splitting. On the other hand, stronger correlated (d-like) dispersions split belowT _c into four branches, two for each spin direction. TheirT-dependence mainly concerns the spectral weights of the quasiparticle peaks and not so much the energetic positions. An exchange caused splitting remains even forT<T _c .     

Huge positive hyperfine fields for Sn impurity atoms on R sites of R–T intermetallic compounds (R=rare-earth,T=Fe,Co)

The magnetic hyperfine field B_hf of the ¹¹⁹Sn impurity atom on the R site of the RFe₂ (R=Sm, Tb, Tm), TbCo₂, RCo₅ (R=Dy, Ho, Er), GdCo₃ and Gd₂Co₇ intermetallic compounds has been investigated by Mössbauer spectroscopy technique. At 5 K, very large hyperfine fields equal to 46–56 T were observed. The _Bhf$B_{\mathrm{hf}}$ values are several times larger than commonly observed for Sn in 3d-based magnetic hosts. The hyperfine fields are positive (that is parallel to the 3d magnetic moments direction). The results can be interpreted qualitatively in terms of the theory proposed for the impurity atoms in homogeneous ferromagnetic hosts [J. Kanamori, H. Katayama-Yoshida, K. Terakura, Hyperfine Interact. 8 (1981) 573; J. Kanamori, H. Katayama-Yoshida, K. Terakura, Hyperfine Interact. 9 (1981) 363; M. Akai, H. Akai, J. Kanamori, J. Phys. Soc. Jpn. 54 (1985) 4246; S. Blügel, H. Akai, R. Zeller, P.H. Dederichs, Phys. Rev. B 35 (1987) 3271], when it is considered that the splitting between bonding and antibonding hybrid states is strongly dependent on the interatomic distance. As the distance between the probe atom and neighboring magnetic atoms increases, the population of the antibonding states grows and, as a consequence, the corresponding positive contribution to the _Bhf$B_{\mathrm{hf}}$ increases sharply. For Sn atom the positive contribution to the _Bhf$B_{\mathrm{hf}}$ dominates when the interatomic distance exceeds 0.28–0.29 nm.     

Magnetic and magneto-transport investigations of Co37Ni34Al29 alloy

Ferromagnetic shape memory alloy with nominal composition Co₃₇Ni₃₄Al₂₉ is investigated by transport and magnetic measurements. The anomaly due to the martensitic transition is observed around 130-210 K. The thermal hysteresis, observed due to martensitic transition in the dc magnetization versus temperature data, gets suppressed at higher applied field. Below 50 K, magnetization varies with temperature perfectly as T^3/2, which signifies that spin wave excitations are largely responsible for thermal demagnetization. The sample shows small negative magneto-resistance, which varies non-monotonically with temperature showing largest value at around 200 K.     

Light induced magnetic exchange-coupling

We observe ferromagnetic exchange-coupling to be induced between thin Fe layers separated by an insulating SiO barrier upon light irradiation. Coupling can reversibly be switched on with photons of energies less than the bandgap of SiO. We conclude that charge carriers in localized states within the bandgap mediate the observed exchange-coupling. We furthermore report that the exchange-coupling can also be turned off by irradiation with light of higher intensity.     

Effect of interchain coupling on spin correlation functions of spin-1 Heisenberg antiferromagnet CsNiCl3

The inelastic neutron-scattering experiment on CsNiCl₃ gives much bigger total intensity of multi-particle continuum than the theoretical calculation by the (1+1$1+1$)-dimensional O(3) non-linear σ-model. Three-dimensional effect has to be considered. A scenario was proposed where the interchain interaction is treated as an effective staggered magnetic field. We propose another model to approximately include the effect of the interchain coupling, and use the large-N expansion to do the calculations. In this circumstance we find that the Néel temperature is about 5 K, and also that for a range of wave-vectors the single magnon is unstable and will decay into three magnons.     

Magnetotransport properties of (La0.7Pb0.3MnO3)1−xAgx composites

Magnetic and transport properties of (La_0.7Pb_0.3MnO₃)_1−xAg_x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution with increase of Ag content percentage. Clearly irreversible behavior in the zero-field cooling and field cooling curves at a low field caused by the competition between the magnetization and magnetic domain orientation processes has been observed as x increases. Saturation magnetization decreases as x increases, while ferromagnetic transition temperature remains around 346 K for all composites. The resistivity decreases significantly for (La_0.7Pb_0.3MnO₃)_1−xAg_x composites. It is suggested that introduction of Ag into the niche of grain boundaries forms artificial conducting network and improves the carriers to transport. However, enhancement of magnetoresistance has been observed for the system.     

Exchange‐coupled L10‐FePt/Fe composite patterns with perpendicular magnetization

L1₀‐FePt and exchange‐coupled L1₀‐FePt/Fe composite films are grown epitaxially on MgO(001) single crystal substrates and are subsequently large‐area patterned utilizing an electron beam lithography process with Ar⁺ ion etching. The patterning process of the continuous film system leads to a different demagnetization behavior resulting in an increase of the out‐of‐plane coercivity of the patterned samples. Due to exchange‐coupling between L1₀‐FePt and Fe the coercivity of the L1₀‐FePt/Fe composite patterns is reduced by 52% as compared to the coercivity of L1₀‐FePt patterns. From the analysis of the temperature dependence of the coercivity it follows that the dots include regions with reduced anisotropy. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetism and Hall effect of the Heusler alloy Co2ZrSn synthesized by melt-spinning process

Magnetization and Hall resistivity have been measured for the Heusler alloy Co₂ZrSn synthesized by the melt-spinning process. The temperature dependence of magnetization follows the spin-wave theory at a low temperature. Abnormal behaviors are observed both in resistance and Hall effect below 8 K. The present Hall resistivity measurement shows that the anomalous Hall effects coexist with normal Hall effects. The negative value of normal Hall coefficient over the whole temperature range reveals that the major charge carriers are electrons. The anomalous Hall coefficient is proportional to the zero-field resistivity, suggesting that magnetic skew scattering is the dominant mechanism in the ferromagnetic regime. The reason for the abnormity below 8 K during transport is discussed.     

Magnetic properties of half-metallic semi Heusler Co1−xCuxMnSb compounds

A study of the half-metallic character of the semi Heusler alloys Co_1−xCu_xMnSb (0?x?0.9) is presented. We investigated the saturation magnetization M_S at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature T_C. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μ_B for Mn³⁺ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with M_S (5 K)=3.78 μ_B/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above T_C, the χ⁻¹ vs T curves follow very well the Curie–Weiss law. The effective moment μ_eff and paramagnetic Curie temperature θ are derived. A comparison between the values of M_S at 5 K and μ_eff shows a transition from localized to itinerant spin system in these compounds.     

Magnetic and magnetocaloric results of magnetic field-induced transitions in La1−xCexMn2Si2 (x=0.35 and 0.45) compounds

The La_1−xCe_xMn₂Si₂ compounds (x=0.35 and 0.45) exhibit an antiferromagnetic-ferromagnetic transition caused by the changes in distance between Mn atoms due to temperature changes. A field-induced transition from antiferromagnetic state to ferromagnetic state at a critical field, which decreases with increase in temperature, can also be induced by applying a magnetic field. In this paper our aim is to study the magnetization and magnetocaloric effect, close to transition temperatures. Our subsidiary aim is to examine the temperature dependence of critical field and ferromagnetic fraction of compounds. The variation of magnetocaloric effect with temperature is correlated with the ferromagnetic-antiferromagnetic phase coexistence. Our final aim is to examine the harmony between magnetocaloric effect values calculated both by the Maxwell theory and by the Landau theory.     

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb₃Fe₅O₁₂, the exchange effective field can be expressed as H_eff=−λ·M_Fe=−λχ_eff·H_e=−γ·H_e where γ is named as the exchange field parameter and H_e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter γ, the properties of exchange interaction in ytterbium iron garnet (Yb₃Fe₅O₁₂) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter γ is the function of the temperatures under different external magnetic fields, and γ=a+b·T+c·T², where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb₃Fe₅O₁₂ are revealed. Also, excellent fits to the available experiments are obtained.     

Electric and magnetic signatures of martensitic and intermartensitic transformations in Ni-Mn-Ga crystal

Results are reported on the temperature dependence of resistivity (200≤T≤360 K) and low-field magnetization (5≤T≤350 K) for the off-stoichiometric Ni₄₉Mn₂₉Ga₂₂ single crystal. Measurements are made for both heating and cooling cycles. The resistivity data show two first-order (hysteretic) transformations centered at about 340 and 250 K. The magnetization data show the same two transformations as the resistivity data as well as a third centered at 285 K. The results are consistent with a martensite/austenite transformation near 340 K and two intermartensitic transformations centered at 285 and 250 K (three martensite phases).     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Influence of temperature on critical fields in ZnCr2Se4

The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr₂Se₄ spinel. An antiferromagnetic order with a Néel temperature T_N=20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature θ_CW=55.1 K were established. An increasing static magnetic field shifts T_N to lower temperatures while a susceptibility peak at T_m in the paramagnetic region—to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Néel and Curie-Weiss temperatures. Below T_N the magnetic field dependence of susceptibility, χ_ac(H), shows two peaks at critical fields H_c1 and H_c2. The values of H_c1 decrease slightly with temperature while the values of H_c2 drop rapidly with temperature. The strong changes of H_c2 temperature induced are mainly responsible for a spin frustration of the re-entrant type in the spinel under study.