Spring MVC, How to submit a form by AJAX and get in the controller using ModelAttribute @Valid

I have the following Form:

        <form:form method="POST" action="${root}/Rhindo/Gerente/AddCargo" commandName="formCargo" >
        <div class="modal-body">
            <div class="row">
                <div class="col-xs-12 col-sm-12 col-md-12 col-lg-12">
                    <label class="text-muted">Nome</label>
                    <form:input type="text" path="nome" cssClass="form-control"/>
                </div>
            </div>
            <form:errors path="nome" cssClass="text-danger" />
            <br /><br />
            <div class="row">
                <div class="col-xs-6 col-sm-6 col-md-6 col-lg-6">
                    <label class="text-muted">Salario</label>
                    <form:input path="salario" placeholder="Somente Números" cssClass="form-control" value=""/>
                </div>
            </div>
            <form:errors path="salario" cssClass="text-danger" />
            <br /><br />
            <div class="row">
                <div class="col-xs-12 col-sm-12 col-md-12 col-lg-12">
                    <label class="text-muted">Requisitos</label>
                    <form:textarea type="text" path="requisitos" cssClass="form-control" style="resize: none"/>
                </div>
            </div>
            <form:errors path="salario" cssClass="text-danger" />
            <br /><br />
            <div class="row">
                <div class="col-xs-6 col-sm-6 col-md-6 col-lg-6">
                    <label class="text-muted">Carga Mínima</label>
                    <form:input type="text" path="cargamin" cssClass="form-control" value=""/>
                    <form:errors path="cargamin" cssClass="text-danger" />
                </div>
                <div class="col-xs-6 col-sm-6 col-md-6 col-lg-6">
                    <label class="text-muted">Desconto Impostos</label>
                    <form:input type="text" path="descImpostos" cssClass="form-control" value=""/>
                    <form:errors path="descImpostos" cssClass="text-danger" />
                    <br />
                </div>
            </div>
        </div>
        <div class="modal-footer">
            <input type="button" class="btn btn-warning" value="Cancelar" data-dismiss="modal" />
            <input type="submit" class="btn btn-primary" value="Cadastrar Cargo" id="cadCargo" />
        </div>
    </form:form>

Class: `@Model public class CargoCadastroModel {

@NotEmpty(message = "Informe o nome!") @Pattern(regexp = "^\\p{L}+(?: \\p{L}+)*$", message = "Nome inválido")
private String nome;

@NotEmpty(message = "Informe o salário!") @Pattern(regexp = "[0-9]d+,d{0,2}$", message = "Valor inválido")
private String salario;

@NotEmpty(message = "Informe os Requisitos!")
private String requisitos;

@NotEmpty(message = "Informe a carga mínima!") @Pattern(regexp = "[0-9]+$", message = "Valor inválido, somente números")
private String cargamin;

@NotEmpty(message = "Informe o imposto!") @Pattern(regexp = "[0-9]d+,d{0,2}$", message = "Valor inválido somente números")
private String descImpostos;

public CargoCadastroModel() {
}`

Ajax script:

$(document).on('click', '#cadCargo', function (e) {
e.preventDefault();  // prevent standard form submission
var fdata = new FormData($('#formCargo')[0]);
debugger;
$.ajax({
    type: 'post',
    url: '/Rhindo/Gerente/AddCargo',
    data: fdata,
    async: false,
    cache: false,
    processData: false,
    dataType: "html",
    contentType: 'application/json',
    success: function (content) {
        debugger;
        $("#cadModal").html(content);
    },
});
return false;

});

and Controller:

    @RequestMapping(value = "/Gerente/AddCargo", method = RequestMethod.POST)
public String getFormCargo(@RequestBody @ModelAttribute("formCargo") @Valid CargoCadastroModel cargoModel, BindingResult bindingResult, Model model) {
        if (bindingResult.hasErrors()) {
            model.addAttribute(cargoModel);
            return "/gerente/cadastroCargo";
        }
        else{

        }
    return "/gerente/cadastroCargo";
}

But in the controller my model is always null; i just have tried with JSON.strignfy, serialize, and nothing work Please, help