C. C. Becerra

The low-field remanent magnetization of the disordered antiferromagnets K2Fe1-xInxCl5.H2O and K2Fe(Cl1-xBrx)5.H2O

Magnetization measurements on single crystals of the site-diluted easy-axis antiferromagnet K2Fe1-xInxCl5.H2O were carried out at very low magnetic fields applied along the easy axis. The data reveal that a remanent magnetization Mr develops below the Neel temperature TN. This remanent magnetization is parallel or nearly parallel to the easy axis. For all concentrations x studied, 0.03<or=x<or=0.14, the sign and magnitude of Mr(T) in a given sample and for a given temperature are governed only by the axial field (Haxial) present when cooling through TN. The remanent magnetization is observed in fields as low as 10-3 Oe. For a given Haxial, Mr increases with decreasing T. At a given T, Mr versus Haxial is very nearly saturated even at approximately 1 Oe. The magnitude of the saturated remanent moment increases with x in the range of x studied. The normalized remanent magnetization Mr(t)/Mr(0), where t=T/TN is the reduced temperature, follows a universal curve, i.e., it is independent of Haxial for a given sample (in fields up to several oersted), and it is also the same for all K2Fe1-xInxCl5.H2O samples. Moreover, the r dependence of Mr(t)/Mr(0) in K2Fe1-xInxCl5.H2O is the same as in Mn1-xZnxF2. Values for the effective critical exponent beta r, obtained from the temperature dependence of Mr close to TN, are close to 0.4. (The extrapolated values at TN are between 0.35 and 0.39.) Temperature cycles following cooling through TN in zero field, and isothermal field cycles below TN, suggest that a domain-like structure is obtained when the sample is cooled in zero field through TN. Magnetization measurements were also carried out on the random-bond system K2Fe(Cl1-xBrx)5.H2O with X equivalent to 0.25. A remanent moment also develops in this case, but it is two orders of magnitude smaller than in the site-diluted system. The temperature dependence of Mr is also distinct in this case: with decreasing T, Mr first increases but then decreases. The mechanism that causes the low-field remanent magnetization is yet to be fully identified. The effect cannot be attributed to random fields (i.e., the excess magnetization of random-field-induced domains) because Mr saturates at very low fields. Domains that exist even in the absence of random fields are a likely source for the low-field remanent magnetization, but the details are still unclear. An explanation based on the volume effect, due to the statistical imbalance between the numbers of up and down spins in each of the domains, which exist even in the absence of random fields, seems to fail. Domain-wall magnetization, and domain magnetization due to the piezomagnetic effect, are other possibilities that remain to be explored. Domain formation may be facilitated by non-magnetic cations that act as vacancies in the magnetic lattice; this may be the reason why Mr in K2Fe1-xInxCl5.H2O is much larger than that in K2Fe1-xInxCl5.H2O.

Magnetic properties of UT2Si2 (T = Ni, Cu and Au) compounds

Abstract We have performed a study of the magnetic properties of UT2Si2 (T = Ni, Cu and Au) compounds by means of measurements of the temperature, T, dependence of the low-frequency ac magnetic susceptibility χac and electrical resistivity ρ. The χac data clearly shows that (1) a magnetic first-order transition takes place near 97 K in UNi2Si2; (2) a weak magnetic anomaly near 104 K precedes the onset of a much stronger ferromagnetic transition at ≈ 97 K in UCu2Si2; and (3) two magnetic transitions occur in UAu2Si2, one with onset at 44 K and the other with onset at 18 K. The ρ versus T data reveals (1) a small drop near 104 K for UCu2Si2 followed by a faster decline with T below 100 K; and (2) the transition near 44 K does not affect the behavior of ρ versus T, while the transition at 18 K is accompanied by a sharp increase in ρ with decreasing T followed by a sharp depression below 18 K.

Weak ferromagnetism in manganese tartrate dihydrate MnC4H4O6·2H2O

Magnetization and differential susceptibility of manganese tartrate dihydrate were measured in a single crystal from room temperature down to 0.4 K and in the presence of magnetic fields up to 70 kOe. Nearly antiferromagnetic order occurs below TN = 1.83 K, with spins oriented close to the c axis. Sharp peaks in the susceptibility and the presence of a remanent magnetization in the plane perpendicular to the c axis suggest a weak ferromagnetic structure of spins with a canting angle 0.6°. The magnetic phase diagram below TN was determined. The exchange and anisotropy field were obtained respectively as HE = 19.4 kOe and HA = 0.6 kOe.

Anomalous magnetic phase diagrams in the site-diluted Heisenberg antiferromagnets, A2Fe1-xInxCl5.H2O (A = Rb, K)

The effect of the substitution of diamagnetic ions for paramagnetic ones in the magnetic phase diagrams of the low-anisotropy antiferromagnets A2Fe1-xInxCl5.H2O (A = Rb,K) is investigated. In the region where the spin-flop (SF) transition occurs, the consequences of dilution are manifested as the appearance of a structure of secondary transition lines and a substantial enhancement of the transition width. In the SF region a multiple-peak structure is observed in the ac susceptibility measurements which is associated with the secondary transition lines. This behaviour is discussed in terms of several mechanisms proposed previously. When the sample is cooled in applied fields below HSF(T) we observe the presence of a remanent magnetization (Mr) in the antiferromagnetic (AF) phase. Such magnetization was previously found in these solid solutions at very low fields (a few Oe). Here we also find that Mr follows a temperature dependence that is independent of the concentration x and is the same for the K and Rb derivatives.

Low-field remanent magnetization in site-diluted easy-axis antiferromagnets. A universal behavior

Abstract The magnetization of the systems, K 2 Fe 1− x In x Cl 5 · H 2 O, Rb 2 Fe 1− x In x Cl 5 · H 2 O and Mn 1− x Zn x F 2 , is studied in low magnetic fields applied along the easy axis ( H axial M r always appears below the Neel temperature T N when cooling in a fixed axial field H axial > 10 −3 Oe. M r increases with decreasing T . At a fixed T , M r increases with H axial , but M r is practically saturated in fields as low as ∼ 1 Oe. The normalized remanent magnetization is a universal function of the reduced temperature t = T / T N . This function is: 1) independent of H axial for a given sample, 2) independent of x for a given system, and 3) the same for the three site-diluted systems. The effective critical exponent β r , which describes the T -dependence of M r , is between 0.35 and 0.4.

Magnetic‐phase diagram of a Mn0.9Co0.1P single‐crystal. Irreversible behavior in a magnetic field

The magnetic‐phase diagram of a Mn0.9Co0.1P single crystal was studied by ac susceptibility with a magnetic field (H) applied along its orthorhombic b axis. This system orders ferromagnetically at 197.5 K and below 37 K it shows a helimagnetic structure in H=0. The H‐T phase diagram above 37 K is similar to the one found in pure MnP, including the presence of a Lifshitz multicritical point (HL =11.0 kOe, TL =99±1 K) at the confluence of the paramagnetic, ferromagnetic, and modulated fanlike phase. It was found that: (1) the hysteresis at the first‐order transitions between the ordered phases is much larger than in MnP; (2) hysteretic behavior is observed inside the whole region of the phase diagram corresponding to the fanlike phase; (3) the second‐order fan‐para transition fades at low temperatures and an irreversible behavior is observed with the irreversibility field sharply increasing at lower temperatures. The last feature is in marked contrast with what is observed in pure MnP, where the second‐orde...

Magnetization jumps in the hysteresis curves of the diluted antiferromagnets A2Fe1−xInxCl5 · H2O (A K, Rb)

Abstract Single crystals of dilute samples of the antiferromagnets A 2 Fe 1− x In x Cl 5 · H 2 O (A  K, Rb) exhibit a net remanent magnetization M r below the ordering temperature ( T N ). This M r saturates in fields of few Oe. In this work we focus on the hysteresis loops M versus H obtained at several temperatures below T N . The loops were obtained after field cooling (FC) the samples in a field of ∼ 1 Oe applied along the easy axis. The loops were asymmetric and showed a series of well defined jumps in M versus H curves for T T N . This behavior suggests that a domain structure is present, and that the jumps are associated with abrupt changes in the domain structure.

Random field effects at the spin‐flop transition of diluted and mixed (K1−xRbx)2Fe1−yIny(Cl1−zBrz)5⋅H2O

The magnetization of the diluted and mixed low anisotropy antiferromagnets (K1−xRbx)2Fe1−yIny(Cl1−zBrz)5H2O was measured as a function of an applied magnetic field 0≤H≤4.0 T in the temperature range 1.5≤T≤15 K. The focus of this work is on the effects that random fields, generated by an external field applied along the easy axis, have on the spin‐flop transition. The sharp first order transition that occurs in K2FeCl5H2O has its width increased by almost an order of magnitude when substitution is introduced in any of the K, Fe, or Cl sites. In K2Fe1−yInyCl5H2O and K2Fe(Cl1−zBrz)5H2O hysteseris is also observed. Hysteresis is not observed in (K1−xRbx)2FeCl5H2O.

Magnetic properties and critical behavior of the pure and diluted two-dimensional weak ferromagnet (CH3NH3)2Mn1−xCdxCl4

The compound (CH3NH3)2MnCl4 had for a long time been considered a good example of a quasi-two-dimensional Heisenberg antiferromagnet, however, it is a weak ferromagnet with a nearly quadratic layer structure. In this work we measure the magnetization of (CH3NH3)2MnCl4 and the diluted with cadmium compound (CH3NH3)2Mn0.95Cd0.05Cl4. The Neel temperature for the pure sample was determined as TN=44.75 K and a weak-ferromagnetic moment was observed within the nearly quadratic layer (perpendicular to the easy axis). From the measured weak magnetic moment, we estimate a canting angle for the sublattices magnetizations θ∼0.07° from the easy axis. The overall temperature dependence of the weak magnetic moment is the same as that previously measured for the sublattice magnetization in neutron diffraction experiments (as expected). However, close to TN the critical exponent β of the magnetization changes from β1=0.17±0.02 [0.01<(1−T/TN)<0.1] to β2=0.23±0.02 [0.001<(1−T/TN)<0.01] in the immediate neighborhood of TN. ...

Spin waves in the 3d Heisenberg antiferromagnetic systems K2Fe1−xInxCl5·D2O (x=0.0 and 0.05)

Abstract The magnetic excitation spectra of the low anisotropy Heisenberg antiferromagnetic S= 5 2 systems K 2 Fe 1− x In x Cl 5 ·D 2 O ( x =0.0 and 0.05) are studied by means of inelastic neutron scattering experiments. A set of four exchange constants and the anisotropy energy have been determined from the analysis of the dispersion curves using a four-sublattice model. No indication of magnon– fracton crossover is observed from our results.