We report $alpha$-Cu$_2$V$_2$O$_7$ to be an improper multiferroic with the simultaneous development of electric polarization and magnetization below $T_C$ = 35 K. The observed spontaneous polarization of magnitude 0.55 $mu$Ccm$^{-2}$ is highest among
the copper based improper multiferroic materials. Our study demonstrates sizable amount of magneto-electric coupling below $T_C$ even with a low magnetic field. The theoretical calculations based on density functional theory (DFT) indicate magnetism in $alpha$-Cu$_2$V$_2$O$_7$ is a consequence of {em ferro-orbital} ordering driven by polar lattice distortion due to the unique pyramidal (CuO$_{5}$) environment of Cu. The spin orbit coupling (SOC) further stabilize orbital ordering and is crucial for magnetism. The calculations indicate that the origin of the giant ferroelectric polarization is primarily due to the symmetric exchange-striction mechanism and is corroborated by temperature dependent X-ray studies.
Both Ba$_4$Mn$_3$O$_{10}$ and Sr$_4$Mn$_3$O$_{10}$ crystallize in an orthorhombic crystal structure consisting of corrugated layers containing Mn$_3$O$_{12}$ polydedra. The thermal variation of magnetic susceptibility of the compositions consists of
a broad hump like feature indicating the presence of low dimensional magnetic correlation. We have systematically investigated the magnetic data of these compounds and found that the experimental results match quite well with the two dimensional Heisenberg model of spin-spin interaction. The two dimensional nature of the magnetic spin-spin interaction is supported by the low temperature heat capacity data of Ba$_4$Mn$_3$O$_{10}$. Interestingly, both the samples show dielectric anomaly near the magnetic ordering temperature indicating multiferroic behavior.
The development of spin glass like state in a geometrically frustrated (GF) magnet is a matter of great debate. We investigated the effect of magnetic (Mn) and nonmagnetic (Ga) doping at the Cr site of the layered GF antiferromagnetic compound LiCrO2
. 10% Ga doping at the Cr site does not invoke any metastability typical of a glassy magnetic state. However, similar amount of Mn doping certainly drives the system to a spin glass state which is particularly evident from the relaxation, magnetic memory and heat capacity studies. The onset of glassy state in 10% Mn doped sample is of reentrant type developing out of higher temperature antiferromagnetic state. The spin glass state in the Mn-doped sample shows a true reentry with the complete disappearance of the antiferromagnetic phase below the spin glass transition. Mn doping at the Cr site can invoke random ferromagnetic Cr-Mn bonds in the otherwise 120 degree antiferromagnetic triangular lattice leading to the non-ergodic spin frozen state. The lack of spin glass state on Ga doping indicates the importance of random ferromagnetic/antiferromagnetic bonds for the glassy ground state in LiCrO2. Spin glass state in GF system has been earlier observed even for small non-magnetic disorder, and our result indicates that the issue is quite nontrivial and depends strongly on the material system concerned.
The mixed-valent multiferroic compound CaMn7O12 is studied for its magnetic and electric properties. The compound undergoes magnetic ordering below 90 K with a helimagnetic structure followed by a low temperature magnetic anomaly observed around 43 K
. The present study shows that the magnetic anomaly at 43 K is associated with thermal hysteresis indicating first order nature of the transition. The compound also shows field-cooled magnetic memory and relaxation below 43 K, although no zero-field-cooled memory is present. Clear magnetic hysteresis loop is present in the magnetization versus field measurements signifying the presence of some ferromagnetic clusters in the system. We doped trivalent La at the cite of divalent Ca expecting to enhance the fraction of Mn$^{3+}$ ions. The La doped samples show reduced magnetization, although the temperatures associated with the magnetic anomalies remain almost unaltered. Interestingly, the spontaneous electrical polarization below 90 K increases drastically on La substitution. We propose that the ground state of the pure as well as the La doped compositions contain isolated superparamagnetic like clusters, which can give rise to metastability in the form of field-cooled memory and relaxation. The ground state is not certainly spin glass type as it is evident from the absence of zero-field-cooled memory and frequency shift in the ac suceptibility measurements.
The single layered manganite Pr$_{0.22}$Sr$_{1.78}$MnO$_4$ undergoes structural transition from high temperature tetragonal phase to low temperature orthorhombic phase below room temperature. The orthorhombic phase was reported to have two structural
variants with slightly different lattice parameters and Mn-3$d$ levels show orbital ordering within both the variants, albeit having mutually perpendicular ordering axis. In addition to orbital ordering, the orthorhombic variants also order antiferromagnetically with different Neel temperatures. Our magnetic investigation on the polycrystalline sample of Pr$_{0.22}$Sr$_{1.78}$MnO$_4$ shows large thermal hysteresis indicating the first order nature of the tetragonal to orthorhombic transition. We observe magnetic memory, large relaxation, frequency dependent ac susceptbility and aging effects at low temperature, which indicate spin glass like magnetic ground state in the sample. The glassy magnetic state presumably arises from the interfacial frustration of orthorhombic domains with orbital and spin orderings playing crucial role toward the competing magnetic interactions.
The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias
phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us an comprehensive view to identify the magnetic character of various Ni-Mn-based shape memory alloys with competing magnetic interactions.
Magneto-structural instability in the ferromagnetic shape memory alloy of composition Ni$_2$Mn$_{1.4}$Sn$_{0.6}$ is investigated by transport and magnetic measurements. Large negative magnetoresistance is observed around the martensitic transition te
mperature (90-210 K). Both magnetization and magnetoresistance data indicate that upon the application of an external magnetic field at a constant temperature, the sample attains a field-induced arrested state which persists even when the field is withdrawn. We observe an intriguing behavior of the arrested state that it can remember the last highest field it has experienced. The field-induced structural transition plays the key role for the observed anomaly and the observed irreversibility can be accounted by the Landau-type free energy model for the first order phase transition.
The exchange bias (EB) in LaMn_{0.7}Fe_{0.3}O_3 is observed by the negative shift and training effect of the hysteresis loops, while the sample was cooled in external magnetic field. The analysis of cooling field dependence of EB gives the size of th
e ferromagnetic (FM) cluster ~ 25 Angstrom, where the magnetic anisotropy of FM cluster is found two order of magnitude higher than the FM bulk manganites. We propose that the nanoscale FM clusters are embedded in the glassy magnetic host with EB at the FM/glassy magnetic interface.
